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DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 125 Syntheses, characterization and facial–meridional isomerism of tungsten tricarbonyl diphosphine complexes Sodio C. N. Hsu and Wen-Yann Yeh * Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan The complexes fac-[W(CO)3(h2-dppf)(h1-dppm)] 4f, fac-[W(CO)3(h2-dppm)(h1-dppf)] 5f, fac-[W(CO)3(h2-dppf)- (h1-dppe)] 6f, fac-[W(CO)3(h2-dppe)(h1-dppf)] 7f, fac-[W(CO)3(h2-dppm)(h1-dppe)] 8f and fac-[W(CO)3(h2-dppe)- (h1-dppm)] 9f have been prepared by treating fac-[W(CO)3(h2-diphos)(NCMe)] 1–3 [diphos = 1,19-bis(diphenylphosphino) ferrocene (dppf), dppm (Ph2PCH2PPh2) or dppe (Ph2PCH2CH2PPh2)] with the corresponding diphosphines.The initially afforded facial isomers are converted into the meridional forms (4m–9m) in a subsequent, slow rearrangement process through opening of the chelated diphosphine ligand. A five-co-ordinate, square-pyramidal intermediate is presumed.In contrast, acid-assisted facial–meridional isomerization of 4f and 6f is likely via a seven-co-ordinate hydrido species. The new compounds have been characterized by elemental analyses and IR, mass and NMR spectroscopy. The d6 metal tricarbonyl complexes, M(CO)3L3, exhibit facial (fac) and meridional (mer) isomers.1–6 When L is a good s donor and poor p acceptor relative to CO the fac isomer is expected to be more stable electronically to achieve stronger M to CO back donation.On the other hand, the mer isomer is less sterically encumbered and is favored when L contains bulky groups.7 Several studies regarding their isomerization have been reported. For example, Rousche and Dobson8 studied the thermolysis of fac-[Mo(CO)3(h2-dppe){P(OPri)3}] (dppe = Ph2- PCH2CH2PPh2) at 120 8C leading to mer-[Mo(CO)3(h2-dppe)- {P(OPri)3}] via dissociation/association of the P(OPri)3 ligand. Darensbourg et al.9 observed that on prolonged standing a solution of fac-[W(CO)3(13CO)(h2-dppm)] (dppm = Ph2PCH2- PPh2) at room temperature afforded a mixture of facial and meridional isomers, presumably undergoing an intramolecular CO rearrangement process.Schenk and Hilpert 10 found an interesting reaction of fac-[Mo(CO)3(h2-dppm)(NCMe)] and dppe giving fac-[Mo(CO)3(h2-dppe)(h1-dppm)] with switch of the chelate ligand, but no subsequent isomerization of this compound was indicated. In contrast, Krishnamurthy and coworkers 11 revealed that treating fac-[Mo(CO)3(NCMe){h2-Ph2- PN(Pri)PPh(dmpz)}] (dmpz = 3,5-dimethylpyrazol-1-yl) with dppe produced fac- and mer-[Mo(CO)3{h2-Ph2PN(Pri)- PPh(dmpz)}(h1-dppe)], in which the four-membered diphosphazane ring was retained and the dppe was co-ordinated in an h1 fashion.In these reactions, however, a plausible isomerization mechanism involving opening the h2-diphosphine ring was ignored. We recently prepared the complexes fac-[W(CO)3(h2-dppf)- (PMe3)] and fac-[W(CO)3(h2-dppf)(PPh2X)] [dppf = 1,19- bis(diphenylphosphino)ferrocene, X = H or OH], which did not lead to the meridional isomers upon heating.12 Apparently their geometries are not sterically congested enough to surpass the electronic advantages. We then investigated the analogous complexes with bulky diphosphines. This paper presents results concerning the syntheses and facial–meridional isomerism of a series of complexes of the type [W(CO)3(h2-diphos)- (h1-diphos9)] (diphos = diphosphine).It appears that the chelated diphosphine ligand is opened up to facilitate the isomerization reaction. Experimental General methods Reactions were performed on a double-manifold Schlenk line under dried nitrogen. The complexes fac-[W(CO)3(h2-dppf)- (NCMe)] 12 1 and fac-[W(CO)3(h2-dppm)(NCMe)] 9 2 were prepared by literature methods. 1,19-Bis(diphenylphosphino)- ferrocene (dppf) was prepared from ferrocene and chlorodiphenylphosphine as described.13 Trimethylphosphine (1.0 M in toluene), methyldiphenylphosphine, triphenylphosphine, dppm, dppe and tetrafluoroboric acid (54% in Et2O) from Aldrich were used without further purification.Solvents were dried over appropriate agents under nitrogen and distilled before use. Thin-layer chromatographic (TLC) plates were prepared from silica gel (Merck). Infrared spectra were recorded with a 0.1 mm path CaF2 solution cell on a Hitachi I-2001 spectrometer, 1H and 31P NMR spectra on a Varian VXR-300 spectrometer at 300 and 121.4 MHz, respectively, and fast atom bombardment (FAB) mass spectra on a VG Blotch-5022 spectrometer. Elemental analyses were performed at the National Science Council Regional Instrumentation Center at the National Chung-Hsing University, Taichung, Taiwan.Preparation of fac-[W(CO)3(Á2-dppe)(NCMe)] 3 The complex [W(CO)3(NCMe)3] (500 mg, 1.28 mmol) and dppe (510 mg, 1.28 mmol) were placed in a Schlenk flask (50 cm3) containing a magnetic stirring bar. The flask was capped with a rubber septum, evacuated and backfilled with nitrogen.After adding dichloromethane (15 cm3), the solution was stirred at ambient temperature for 5 h, forming a yellow precipitate. The supernatant was removed via a cannula and the solids washed with diethyl ether (50 cm3) and then dried under vacuum. The complex fac-[W(CO)3(h2-dppe)(NCMe)] 3 was afforded in 86% yield (780 mg, 1.10 mmol). Mass spectrum (FAB): m/z 707 (M1, 184W), 666 (M1 2 CH3CN) and 666 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21): n(CO) 1932vs, 1838s and 1822s. 31P-{1H} NMR (CD2Cl2, 20 8C): d 46.25 (s; with 183W satellites, 1JWP = 226 Hz). 1H NMR (CD2Cl2, 20 8C): d 1.97 (s, 3 H, NCMe), 2.59 (m, 4 H, CH2) and 7.30–7.80 (m, 20 H, Ph). Reaction of complex 1 and dppm The complex fac-[W(CO)3(h2-dppf)(NCMe)] 1 (50 mg, 0.058 mmol) and dppm (25 mg, 0.065 mmol) were placed in a Schlenk flask (50 cm3) containing a magnetic stirring bar. The flask was fitted with a rubber septum, evacuated and backfilled with nitrogen.After adding dichloromethane (15 cm3), the solution was stirred at ambient temperature for 20 h. The mixture was dried under vacuum and the residue separated by TLC, eluting126 J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 with CH2Cl2–n-hexane (1 : 1, v/v). Three bands were obtained according to the order of appearance: yellow [W(CO)4- (h2-dppf)] (trace amount), yellow mer-[W(CO)3(h2-dppm)- (h1-dppf)] 5m (28 mg, 40%) and yellow fac-[W(CO)3(h2-dppf)- (h1-dppm)] 4f (30 mg, 43%).Complex 5m (Found: C, 61.23; H, 4.37. C62H50FeO3P4W requires C, 61.71; H, 4.18%): mass spectrum (FAB) m/z 1206 (M1); IR (1,2-C2H4Cl2, cm21) n(CO) 1966w, 1864s and 1838 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 225.0 (dd, JPP = 31 and 24; with 183W satellites, JWP = 180, dppm), 216.69 (s, dppf), 211.56 (dd, JPP = 31 and 67; with 183W satellites, JWP = 244, dppm) and 22.96 (dd, JPP = 24 and 67, with 183W satellites, JWP = 302 Hz, dppf); 1H NMR (CD2Cl2, 20 8C) d 3.75 (s, 2 H), 4.00 (s, 2 H), 4.26 (s, 2 H), 4.30 (s, 2 H, C5H4), 5.04 (t, 2 H, JPH = 9 Hz, CH2) and 7.00–7.70 (m, 40 H, C6H5).Complex 4f (Found: C, 61.50; H, 4.19. C62H50FeO3P4W requires C, 61.71; H, 4.18%): mass spectrum (FAB) m/z 1206 (M1), 822 (M1 2 dppm) and 822 2 28n (n = 1–3); IR (1,2- C2H4Cl2, cm21): n(CO) 1938s, 1842s and 1826 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 224.16 (d, JPP = 23, dppm), 8.17 (dt, JPP = 23, 26; with 183W satellites, JWP = 214, dppm) and 16.91 (d, JPP = 26; with 183W satellites, JWP = 230 Hz, dppf); 1H NMR (CD2Cl2, 20 8C) d 3.08 (s, 2 H), 3.95 (s, 2 H), 4.27 (s, 2 H), 4.46 (s, 2 H, C5H4), 4.68 (s, 2 H, CH2) and 6.90–7.50 (m, 40 H, C6H5).Reaction of complex 2 and dppf The complex fac-[W(CO)3(h2-dppm)(NCMe)] 2 (20 mg, 0.023 mmol) and dppf (50 mg, 0.09 mmol) and CD2Cl2 (2 cm3) were placed in a dry NMR tube under nitrogen. The reaction was carried out at ambient temperature and monitored by 31P NMR spectroscopy. The resonances corresponding to fac-[W(CO)3- (h2-dppm)(h1-dppf)] 5f were detected initially.However, only 5m (17 mg, 0.014 mmol, 61%) was obtained after separation by TLC. Attempts to isolate 5f in pure form have been unsuccessful. Complex 5f: 31P-{1H} NMR (CD2Cl2, 20 8C) d 222.46 (d, JPP = 22, dppm), 217.03 (s, dppf) and 12.96 (t, JPP = 22 Hz, dppf). Reaction of complex 1 and dppe The reaction of fac-[W(CO)3(h2-dppf)(NCMe)] 1 (50 mg, 0.058 mmol) and dppe (25 mg, 0.063 mmol) was carried out (5 h) and worked up in a fashion similar to that of 1 and dppm.Three products were obtained in order of appearance on the TLC plate: yellow [W(CO)4(h2-dppf)] (trace amount), yellow fac- [W(CO)3(h2-dppf)(h1-dppe)] 6f (38 mg, 54%) and yellow fac, fac-[{W(CO)3(h2-dppf)}2(h1 :h1-dppe)] 6ff (25 mg, 42%). Complex 6f (Found: C, 61.72; H, 4.39. C63H52FeO3P4W requires C, 61.99; H, 4.29%): mass spectrum (FAB) m/z 1220 (M1), 1220 2 28n (n = 1–3), 822 (M1 2 dppe) and 822 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1938s, 1842s and 1826 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 212.49 (d, JPP = 31, dppe), 9.14 (dt, JPP = 31, 27; with 183W satellites, JWP = 211, dppe) and 16.0 (d, JPP = 27; with 183W satellites, JWP = 227 Hz, dppf); 1H NMR (CD2Cl2, 20 8C) d 1.82 (m, 2 H), 2.35 (m, 2 H, C2H4), 3.91 (s, 2 H), 4.24 (s, 2 H), 4.45 (s, 2 H), 4.67 (s, 2 H, C5H4) and 6.90–7.50 (m, 40 H, C6H5).Complex 6ff (Found: C, 58.92; H, 3.87.C100H80Fe2O6P6W2 requires C, 58.79; H, 3.95%): mass spectrum (FAB) m/z 1220 [M1 2 W(CO)3(dppf)] and 822 [M1 2 W(CO)3(dppf)(dppe)]; IR (1,2-C2H4Cl2, cm21) n(CO) 1938s, 1842s and 1828 (sh); 31P- {1H} NMR (CD2Cl2, 20 8C) d 15.0 (t, JPP = 27; with 183W satellites, JWP = 232, dppe) and 17.33 (d, JPP = 27; with 183W satellites, JWP = 230 Hz, dppf); 1H NMR (CD2Cl2, 20 8C) d 2.48 (br, 4 H, C2H4), 3.97 (s, 4 H), 4.24 (s, 4 H), 4.49 (s, 4 H), 4.73 (s, 4 H, C5H4) and 6.80–7.50 (m, 60 H, C6H5).Reaction of complex 3 and dppf The complex fac-[W(CO)3(h2-dppe)(NCMe)] 3 (100 mg, 0.141 mmol) was treated with dppf (80 mg, 0.14 mmol) by the same method as described above (60 h). Three products were isolated: yellow [W(CO)4(h2-dppe)] (32 mg, 33%), yellow mer- [W(CO)3(h2-dppe)(h1-dppf)] 7m (45 mg, 26%) and yellow fac-[W(CO)3(h2-dppe)(h1-dppf)] 7f (50 mg, 29%). Complex 7m (Found: C, 61.82; H, 4.40. C63H52FeO3P4W requires C, 61.99; H, 4.29%): mass spectrum (FAB) m/z 1221 (M1) and 1221 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1965w and 1859s; 31P-{1H} NMR (CD2Cl2, 20 8C) d 47.10 (dd, JPP = 12, 69; with 183W satellites, JWP = 271, dppe), 38.79 (dd, JPP = 15, 12; with 183W satellites, JWP = 216, dppe), 18.32 (dd, JPP = 15, 69; with 183W satellites, JWP = 295 Hz, dppf) and 217.03 (s, dppf); 1H NMR (CD2Cl2, 20 8C) d 2.38 (br, 4 H, C2H4), 3.71 (s, 2 H), 3.95 (s, 2 H), 4.17 (s, 2 H), 4.23 (s, 2 H, C5H4) and 6.90–7.70 (m, 40 H, C6H5).Complex 7f (Found: C, 61.85; H, 4.22.C63H52FeO3P4W requires C, 61.99; H, 4.29%): mass spectrum (FAB) m/z 1221 (M1) and 1221 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1935s and 1840s; 31P-{1H} NMR (CD2Cl2, 20 8C) d 33.68 (d, JPP = 22; with 183W satellites, JWP = 220, dppe), 8.02 (t, JPP = 22; with 183W satellites, JWP = 213 Hz, dppf) and 217.26 (s, dppf); 1H NMR (CD2Cl2, 20 8C) d 2.48 (br, 4 H, C2H4), 3.83 (s, 2 H), 3.97 (s, 2 H), 4.27 (s, 2 H), 4.62 (s, 2 H, C5H4) and 6.60–7.70 (m, 40 H, C6H5).Reaction of complex 2 and dppe Complex fac-[W(CO)3(h2-dppm)(NCMe)] 2 (100 mg, 0.144 mmol) was treated with dppe (60 mg, 0.15 mmol) for 50 h at 28 8C by the same method as described above. Yellow mer- [W(CO)3(h2-dppe)(h1-dppm)] 9m (70 mg, 46%) and yellow fac- [W(CO)3(h2-dppe)(h1-dppm)] 9f (58 mg, 38%) were obtained by TLC. Two intermediates corresponding to fac-[W(CO)3(h2- dppm)(h1-dppe)] 8f and mer-[W(CO)3(h2-dppm)(h1-dppe)] 8m were detected by 31P NMR spectroscopy during the reaction.However, attempts to isolate them failed, leading only to 9m and 9f. Complex 8f: 31P-{1H} NMR (CD2Cl2, 20 8C) d 221.85 (d, JPP = 22, dppm), 212.53 (d, JPP = 36, dppe) and 15.08 (dt, JPP = 22, 36 Hz, dppe). Complex 8m: 31P-{1H} NMR (CD2Cl2, 20 8C) d 224.60 (dd, JPP = 23, 30, dppm), 212.35 (dd, JPP = 30, 66, dppm), 212.06 (d, JPP = 39, dppe) and 25.67 (ddd, JPP = 23, 39, 66 Hz, dppe). Complex 9m (Found: C, 61.67; H, 4.46. C54H46O3P4W requires C, 61.73; H, 4.41%): mass spectrum (FAB) m/z 1050 (M1) and 1050 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1966w and 1860s; 31P-{1H} NMR (CD2Cl2, 20 8C) d 47.85 (dd, JPP = 11, 69; with 183W satellites, JWP = 273, dppe), 40.18 (dd, JPP = 15, 11; with 183W satellites, JWP = 220, dppe), 16.47 (ddd, JPP = 15, 42, 69; with 183W satellites, JWP = 293, dppm) and 225.92 (d, JPP = 42 Hz, dppm); 1H NMR (CD2Cl2, 20 8C) d 2.41 (m, 4 H, C2H4), 3.02 (br, 2 H, CH2) and 6.90–7.70 (m, 40 H, C6H5).Complex 9f (Found: C, 61.65; H, 4.45.C54H46O3P4W requires C, 61.73; H, 4.41%): mass spectrum (FAB) m/z 1050 (M1) and 1050 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1938s and 1844s; 31P-{1H} NMR (CD2Cl2, 20 8C) d 36.99 (d, JPP = 22; with 183W satellites, JWP = 225, dppe), 6.89 (dt, JPP = 30, 22; with 183W satellites, JWP = 215, dppm) and 225.61 (d, JPP = 30 Hz, dppm); 1H NMR (CD2Cl2, 20 8C) d 2.55 (m, 4 H, C2H4), 2.79 (br, 2 H, CH2) and 6.70–7.70 (m, 40 H, C6H5). Reaction of complex 1 and PPh3 Complex fac-[W(CO)3(h2-dppf)(NCMe)] 1 (50 mg, 0.058 mmol) was treated with PPh3 (16 mg, 0.06 mmol) for 20 h by the same method as described above.Three products were isolated: yellow mer-[W(CO)3(h2-dppf)(PPh3)] 12f (2 mg, 3%), yellow [W(CO)4(h2-dppf)] (14 mg, 28%) and yellow fac-[W(CO)3- (h2-dppf)(PPh3)] 12m (16 mg, 26%).J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 127 Complex 12m (Found: C, 60.67, H, 4.45. C55H43FeO3P3W requires C, 60.91; H, 4.00%): IR (1,2-C2H4Cl2, cm21) n(CO) 1965w, 1856s and 1840 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 15.16 (dd, JPP = 23, 11; with 183W satellites, JWP = 224), 20.72 (dd, JPP = 23, 66; with 183W satellites, JWP = 286) and 28.66 (dd, JPP = 66, 11; with 183W satellites, JWP = 282 Hz); 1H NMR (CD2Cl2, 20 8C) d 3.98–4.40 (m, 8 H, C5H4) and 6.80–7.60 (m, 35 H, C6H5).Complex 12f (Found: C, 60.99; H, 3.92. C55H43FeO3P3W requires C, 60.91; H, 4.00%): mass spectrum (FAB) m/z 1084 (M1), 822 (M1 2 PPh3) and 822 2 28n (n = 1–3); IR (1,2- C2H4Cl2, cm21) n(CO) 1940s, 1846s and 1828 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 16.26 (d, JPP = 28; with 183W satellites, JWP = 224) and 19.05 (t, JPP = 28; with 183W satellites, JWP = 210 Hz); 1H NMR (CD2Cl2, 20 8C) d 3.90 (s, 2 H), 4.26 (s, 2 H), 4.53 (s, 2 H), 4.80 (s, 2 H, C5H4) and 6.80–7.60 (m, 35 H, C6H5).Reaction of complex 1 and PPh2Me Complex 1 (50 mg, 0.058 mmol) was treated with PPh2Me (12 mg, 0.06 mmol) for 20 h by the method described above.Three products were isolated: yellow [W(CO)4(h2-dppf)] (2 mg, 4%), yellow mer-[W(CO)3(h2-dppf)(PPh2Me)] 13m (4 mg, 7%) and yellow fac-[W(CO)3(h2-dppf)(PPh2Me)] 13f (52 mg, 88%). Complex 13m (Found: C, 58.50; H, 4.23. C50H41FeO3P3W requires C, 58.75; H, 4.01%): mass spectrum (FAB) m/z 1022 (M1), 994 (M1 2 CO), 822 (M1 2 PPh2Me) and 822 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1960w, 1852s and 1842 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 1.50 (dd, JPP = 13, 65; with 183W satellites, JWP = 272), 17.62 (dd, JPP = 13, 23; with 183W satellites, JWP = 224) and 21.35 (dd, JPP = 23, 65; with 183W satellites, JWP = 292 Hz); 1H NMR (CD2Cl2, 20 8C) d 1.65 (d, 3 H, JPH = 7, Me), 4.16 (s, 2 H), 4.25 (br, 4 H), 4.28 (s, 2 H, C5H4) and 7.00–7.70 (m, 30 H, C6H5).Complex 13f (Found: C, 58.52; H, 4.13. C50H41FeO3P3W requires C, 58.75; H, 4.01%): mass spectrum (FAB) m/z 1022 (M1), 994 (M1 2 CO), 822 (M1 2 PPh2Me) and 822 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1938s, 1846s and 1830 (sh); 31P-{1H} NMR (CD2Cl2, 20 8C) d 23.71 (t, JPP = 27; with 183W satellites, JWP = 228) and 19.83 (d, JPP = 27; with 183W satellites, JWP = 216 Hz); 1H NMR (CD2Cl2, 20 8C) d 1.90 (d, 3 H, JPH = 5 Hz, Me), 3.99 (s, 2 H), 4.28 (s, 2 H), 4.50 (s, 2 H), 4.73 (s, 2 H, C5H4) and 7.00–7.60 (m, 30 H, C6H5).Isomerizations Complexes 4f and 7f. Typically, an NMR tube was charged with fac-[W(CO)3(h2-dppf)(h1-dppm)] 4f (15 mg) and CD2Cl2 (2 cm3) under nitrogen.The isomerization was carried out at room temperature and monitored by 31P NMR spectroscopy. It was complete after 72 h, showing only the resonances for 5m. Isomerization of complex 7f to give 7m takes 1 week to complete at room temperature, but only 3 h are required at 80 8C. Complexes 4m and 6m. A typical reaction: a Schlenk flask (50 cm3) was charged with mer-[W(CO)3(h2-dppf)(h1-dppm)] 4m (40 mg) and 1,2-dichloroethane (10 cm3). The flask was placed in an oil-bath at 80 8C and the reaction was monitored by IR spectroscopy in the CO stretching region. The spectra showed no further change after 6 h.The solvent was evaporated under vacuum and the residue separated by TLC, affording 5m in 90% yield. Isomerization of complex 6m was carried out at 80 8C, producing 7m in 71% yield. No reaction was observed at ambient temperature. Complex 4f in the presence of acid. The complex (18 mg, 0.015 mmol) and dichloromethane (10 cm3) were placed in a Schlenk flask (50 cm3) containing a magnetic stirring bar and a rubber septum.An ether solution of HBF4 (20 ml, excess) was added via a syringe. The mixture was stirred at ambient temperature for 2 h, and evacuated under vacuum. The residue was subjected to TLC, eluting with CH2Cl2–n-hexane (1 : 1, v/v). The material from the yellow band gave mer-[W(CO)3(h2- dppf)(h1-dppm)] 4m (14 mg, 78%) (Found: C, 61.42; H, 4.22. C62H50FeO3P4W requires C, 61.71; H, 4.18%): mass spectrum (FAB) m/z 1205 (M1), 1205 2 28n (n = 1–3) and 821 (M1 2 dppm); IR (1,2-C2H4Cl2, cm21) n(CO) 1961w and 1853s; 31P- {1H} NMR (CD2Cl2, 20 8C) d 21.66 (dd, JPP = 23, 66; with 183W satellites, JWP = 288, dppf), 16.83 (dd, JPP = 23, 33; with 183W satellites, JWP = 230, dppf), 14.66 (ddd, JPP = 33, 38, 66; with 183W satellites, JWP = 275, dppm) and 225.06 (d, JPP = 38 Hz, dppm); 1H NMR (CD2Cl2, 20 8C) d 3.23 (br, 2 H, CH2), 4.12 (br, 4 H, C5H4), 4.21 (br, 4 H, C5H4) and 6.80–7.70 (m, 40 H, C6H5).Complex 6f in the presence of acid. Isomerization of fac- [W(CO)3(h2-dppf)(h1-dppe)] 6f (20 mg, 0.016 mmol) in the presence of an excess amount of HBF4 was carried out and worked up in a fashion identical to that above. Yellow mer- [W(CO)3(h2-dppf)(h1-dppe)] 6m (12 mg, 60%) was obtained as the major product. Mass spectrum (FAB): m/z 1221 (M1), 823 (M1 2 dppe) and 823 2 28n (n = 1–3); IR (1,2-C2H4Cl2, cm21) n(CO) 1960w and 1854s; 31P-{1H} NMR (CD2Cl2, 20 8C) d 21.53 (dd, JPP = 23, 66; with 183W satellites, JWP = 291, dppf), 18.27 (ddd, JPP = 33, 34, 66; with 183W satellites, JWP = 273, dppe), 16.26 (dd, JPP = 23, 34; with 183W satellites, JWP = 225, dppf) and 213.45 (d, JPP = 34 Hz, dppe); 1H NMR (CD2Cl2, 20 8C) d 1.62 (m, 2 H), 2.36 (m, 2 H, C2H4), 4.12 (s, 2 H), 4.18 (s, 2 H), 4.22 (s, 2 H), 4.24 (s, 2 H, C5H4) and 6.60–7.80 (m, 40 H, C6H5).Reactions of complex 4f With dppe. The complex (23 mg, 0.019 mmol) and dppe (75 mg, 0.19 mmol) were placed in a Schlenk flask (50 cm3) containing a magnetic stirring bar.The flask was fitted with a rubber septum, evacuated, and backfilled with nitrogen. After introducing dichloromethane (15 cm3), the solution was stirred at ambient temperature for 48 h. The mixture was then dried under vacuum and the residue separated by TLC, eluting with CH2Cl2–n-hexane (1 : 1, v/v). Three products were isolated in order of appearance on the TLC plate: yellow mer-[W(CO)3- (h2-dppm)(h1-dppf)] 5m (11 mg, 48%), yellow fac-[W(CO)3- (h2-dppf)(h1-dppe)] 6f (60 mg, 25%) and yellow fac- [W(CO)3(h2-dppf)(h1-dppm)] 4f (50 mg, 21%).With PMe3. A Schlenk flask (50 cm3) was charged with fac- [W(CO)3(h2-dppf)(h1-dppm)] 4f (10 mg, 0.008 mmol) and 1,2- dichloroethane (10 cm3) under nitrogen. A toluene solution of PMe3 (12 ml, 0.012 mmol) was introduced by a microsyringe. The mixture was stirred at ambient temperature for 30 h. The solvents were removed under vacuum and the residue separated by TLC, eluting with n-hexane–dichloromethane (1: 1, v/v).The known complex fac-[W(CO)3(h2-dppf)(PMe3)] 12 10f (6 mg, 83%) and 5m (1 mg, 10%) were obtained. With CO. A two-necked flask (100 cm3) was equipped with a magnetic stir bar. One neck was connected to an oil bubbler and the other had an inlet tube for introduction of carbon monoxide gas into the solution. A solution of fac-[W(CO)3(h2- dppf)(h1-dppm)] 4f (20 mg, 0.017 mmol) in 1,2-dichloroethane (10 cm3) was transferred to the flask and saturated with CO gas.The mixture was stirred at ambient temperature for 48 h with slow bubbling of CO through the solution. The mixture was dried under vacuum and the residue separated by TLC. The known complex [W(CO)4(h2-dppf)] 14 11 (10 mg, 70%) and 5 (3 mg, 15%) were obtained. Results and Discussion Preparation of compound 3 The syntheses of fac-[W(CO)3(h2-dppf)(NCMe)] 12 1 and fac-128 J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 Scheme 1 2 Me C N P P C O C O O C W W C O O C O C P P P P Fc fast W C O OC O C P P P P Fc NC Me Fc P P C O C O O C W W C O OC O C P P P P Fc 1 4f 5m 4m H+ cat.W C O OC O C P P Fc P P heat 5f P P Fc slow P P [W(CO)3(h2-dppm)(NCMe)] 9 2 have been previously described. The analogous complex fac-[W(CO)3(h2-dppe)(NCMe)] 3 is prepared in a similar fashion by treating [W(CO)3(NCMe)3] with equimolar dppe at room temperature. The IR spectrum of 3 in the carbonyl region resembles those recorded for 1 and 2, indicating similar structures.The 31P NMR spectrum of 3 displays one resonance at d 46.25, which is significantly downfield from those recorded for 1 (d 22.4) and 2 (d 211.06). Scheme 2 L L Fc P P C O C O O C W P P Fc P P C O C O O C W 4f L = PMe3 10f CO 11 + P P Fc P P C O O C W 5m C O dppe 6f Preparation and isomerization of complexes 4f and 5f The complex fac-[W(CO)3(h2-dppf)(h1-dppm)] 4f is initially afforded from the reaction of fac-[W(CO)3(h2-dppf)(NCMe)] 1 and dppm at ambient temperature.Isomerization of 4f readily takes place at 25 8C to give mer-[W(CO)3(h2-dppm)(h1-dppf)] 5m together with a switch of the chelate ligand from dppf to dppm. Complete transformation from 4f to 5m takes 72 h at 25 8C, but only 4 h are required at 50 8C. In the presence of acid, however, mer-[W(CO)3(h2-dppf)(h1-dppm)] 4m is obtained as the sole product. Compound 4m is indefinitely stable at room temperature, but it slowly converts into 5m in hot 1,2-dichloroethane solution (80 8C).The other isomer fac-[W(CO)3(h2- dppm)(h1-dppf)] 5f is not isolable but only detected by 31P NMR spectroscopy as an intermediate during the reaction of fac-[W(CO)3(h2-dppm)(NCMe)] 2 and dppf, leading to 5m. The results are summarized in Scheme 1. It is obvious that complexes 4f and 5f are the kinetic products of the reactions, while 5m is most stable thermodynamically. However, when dppe, PMe3 or CO is present, 5m is obtained together with fac-[W(CO)3(h2-dppf)(h1-dppe)] 6f, fac- [W(CO)3(h2-dppf)(PMe3)] 10f and [W(CO)4(h2-dppf)] 11, respectively (Scheme 2).This indicates that substitution of the h1-diphosphine ligand is a competitive reaction. Nevertheless, once the meridional isomer is formed, it is quite resistant to ligand dissociation, such that 5m does not react with PMe3 at 50 8C. The different reactivity between complexes 4f and 5f can be rationalized in terms of the structures of parent compounds 1 and 2.Since the dppf ligand of 1 can modify its steric bite by twisting the cyclopentadienyl rings, the phenyl groups and the NCMe ligand are staggered.12 However, the four-membered dppm ring of 2 constrains the phenyl groups to being eclipsed to the axial NCMe and CO ligands.9 So compound 5f wouldJ. Chem. Soc., Dalton Trans., 1998, Pages 125–132 129 sustain stronger steric repulsions between phosphine ligands than 4f and is more susceptible to isomerization.Preparation and isomerization of complexes 6f and 7f Reactions of fac-[W(CO)3(h2-dppf)(NCMe)] 1 with dppe and its subsequent isomerization processes are summarized in Scheme 3 P C O C O O C W P P Fc P P Fc P P 7m W C O OC O C P 7f W C O O C O C P P N C Me 3 1 W C O O C P P Fc Me C N Fc P P P C O C O O C W 6f P P Fc P P C O C O O C W P Fc P P C O O C W 6ff P P C O P P Fc C O H+ cat. 6m P P Fc P P C O C O O C W W C O O C O C P P + C O Scheme 3. The results closely resemble the reactions with dppm shown above, except that a dimetallic complex fac, fac- [{W(CO)3(h2-dppf)}2(h1 :h1-dppe)] 6ff is produced and both 6f and 7f are isolable.Standing a solution of pure 6f at 25 8C yields 7m and 6ff, but under acidic conditions 6m is obtained solely without forming the dimetallic species. It is apparent that eliminating a dppe ligand from 6f is essential to give 6ff, while 6m is resistant to ligand dissociation. Furthermore, no isomerization of 6ff is evidenced.This is contrary to the reaction of fac-[Mo(CO)3(NCMe){h2-Ph2PN(Pri)PPh(dmpz)}] and dppe to produce mer,mer-[{Mo(CO)3[h2-Ph2PN(Pri)PPh(dmpz)]}2- (h1 :h1-dppe)].11 On the other hand, treating fac-[W(CO)3(h2-dppe)(NCMe)] 3 with dppf at room temperature produces [W(CO)4(h2-dppe)] (33%), fac-[W(CO)3(h2-dppe)(h1-dppf)] 7f (29%) and mer- [W(CO)3(h2-dppe)(h1-dppf)] 7m (26%). It is likely compound 7f is afforded initially, following by isomerization to give 7m as well as disproportionation to yield the W(CO)4 complex.Preparation and isomerization of complex 8f The acetonitrile ligand of fac-[W(CO)3(h2-dppm)(NCMe)] 2 is readily replaced by dppe, but leading to fac-[W(CO)3(h2- dppe)(h1-dppm)] 9f and mer-[W(CO)3(h2-dppe)(h1-dppm)] 9m (Scheme 4). Phosphorus-31 NMR spectroscopy has been applied to assess the progress of reaction. The spectrum of reaction mixture after 2 h shows the presence of fac- [W(CO)3(h2-dppm)(h1-dppe)] 8f along with small quantities of mer-[W(CO)3(h2-dppm)(h1-dppe)] 8m and 9f.The quantities of 8f, 8m and 9f are about equal in 24 h. After 3 d, 9f and 8m are present as the major products together with a minor amount of 8f and 9m. Although this transformation is slow, attempts to isolate all the components by TLC only afford 9f and 9m. It is probable that the conversion of 8 into 9 is accelerated by acid contaminant on the silica gel. Schenk and Hilpert 10 previously reported an analogous reaction of fac-[Mo(CO)3(h2-dppm)- (NCMe)] and dppe to yield fac-[Mo(CO)3(h2-dppe)(h1-dppm)], where the four-membered chelate ring of dppm is opened up to leave an h1-co-ordinated dppm while a more stable five-membered chelate ring is formed by dppe.The ultimate formation of 9m indicates the operation of ring strains in determining the products. Reaction of complex 1 and bulky monophosphine Treatment of fac-[W(CO)3(h2-dppf)(NCMe)] 1 with PPh3 and PPh2Me affords fac-[W(CO)3(h2-dppf)(PPh3)] 12f and fac- [W(CO)3(h2-dppf)(PPh2Me)] 13f, respectively, which then rearrange to give mer-[W(CO)3(h2-dppf)(PPh3)] 12m and mer- [W(CO)3(h2-dppf)(PPh2Me)] 13m.The analogous complexes fac-[W(CO)3(h2-dppf)(L)] (L = PMe3, PPh2H, or PPh2OH) are stable under similar conditions.12 Obviously, the energy barrier for fac æÆ mer transformation is reduced as the steric bulk of the ligands increases. Characterization of new compounds Compounds 5f, 8m and 8f cannot be isolated in a pure form and were characterized only by 31P NMR spectroscopy.The remaining complexes form crystalline solids and give satisfactory analyses (C, H). Their FAB mass spectra display the molecular ion and ions corresponding to successive loss of three carbonyls. The IR spectra in the carbonyl region for the facial isomers show three strong bands, presumably due to a disrupted C3 symmetry for the W(CO)3 group, while the meridional geometry is indicated by one weak and two medium to strong absorption bands.2–6,9,15 Identification of these new complexes is unambiguously substantiated by 31P NMR spectroscopy.Their spectra are assigned on the basis of chemical shifts and coupling patterns.2–4,10,16 In general, the facial isomers have a plane of symmetry and would130 J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 Scheme 4 2 W C O OC C P P N C Me P P P P P P C O C O O C W P P P P C O C O O W 8f W C O OC O C P P P P 9m 9f P P P C O C O O C W 8m O P C display two 31P resonances for the co-ordinated phosphines and one resonance for the pendant phosphine group, while the meridional isomers present four resonance signals. Typical spectra of 4f, 4m and 5m are illustrated in Fig. 1 for com- Fig. 1 The 31P-{1H} NMR spectra of (a) fac-[W(CO)3(h2-dppf)(h1- dppm)] 4f, (b) mer-[W(CO)3(h2-dppm)(h1-dppf)] 5m and (c) mer- [W(CO)3(h2-dppf)(h1-dppm)] 4m in CD2Cl2 solution parison. For 4f a doublet signal at d 16.91 and a doublet of doublets at d 8.17 accompanied by 183W satellites are assigned to the co-ordinated dppf and dppm moieties, respectively, and an upfield doublet resonance at d 224.16 without 183W satellites is assigned to the pendant dppm group.On the other hand, 4m and 5m display three complex resonances for the co-ordinated phosphines due to coupling between the inequivalent phosphorus atoms and to the 183W atom. The co-ordination shifts, D = dmonodentate 2 dfree ligand, and chelation shifts, DR = dchelate 2 dmonodentate, of the diphosphine ligands are of interest in these complexes.The co-ordination shifts for dppm, dppe and dppf are comparable, averaging 132, 124 and 128 ppm, respectively. In contrast, the chelation shifts are more diverse, where the four-membered dppm rings are shielded (230 ppm), the five-membered dppe rings are deshielded (125 ppm) and the dppf rings are only slightly affected (15 ppm). Although the theoretical aspects of the ring contribution remain elusive, Garrou17 has proposed the general utility of D and DR for structural assignments of transitionmetal phosphine complexes. Possible isomerization mechanism The facial–meridional isomerism may be reasonably accounted for by the reaction sequence shown in Scheme 5.Apparently, three bulky phosphines in a facial arrangement cause considerable steric repulsions, leading to ready dissociation of the h1-disphosphine ligand to give I, or opening up the h2- disphosphine ring to give II.The resulting square-pyramidal intermediates could then undergo site exchange, presumably through a trigonal-bipyramidal transient with one of the diphosphine ligands at the axial position and ring closing to yield the final products.18,19 Alternatively, an associative mechanism to generate a seven-co-ordinate, 20-electron intermediate [W(CO)3(h2-diphos)(h2-diphos9)] seems improbable based on the steric considerations. Since isomerization of complex 4f produces 5m solely without forming 4m, the site-exchange rate for II should be much faster than that for I.One possible explanation is the ring constraints of I increasing the energy separation between the more stable square-pyramidal and less stable trigonal-bipyramidal structures. Furthermore, adding PMe3 to a solution of 4f does not give [W(CO)3(h1-dppm)(h1-dppf)(PMe3)] or [W(CO)3(h2- dppm)(PMe3)]. This means that, once the intermediate II isJ. Chem. Soc., Dalton Trans., 1998, Pages 125–132 131 Scheme 5 W P P P P W P P P W P P P P W P P P P x y P x y y y x x W P P P P y x W P P P P y x W P P P y I II III W P P P P x y L W P P P P x y + L IV 8f 9f 4f 5m 6f 7m 5f 5m 7f 7m 4f 10f , 11 6f 6ff + P x 9f 9m formed, the subsequent site exchange and ring-closing processes must be very rapid.This mechanism is further supported by a kinetic study. The rate constants measured for the reaction of complex 4f with CO and PMe3 at 25 8C are comparable, being 2.17 × 1025 and 2.13 × 1025 s21, respectively.Furthermore, both reactions are observed to be first order in metal substrate and zero order in Scheme 6 W P P y W P P P P y x W P P P P y x 4m 5m 6m 7m 8m 9m P P x Scheme 7 W P P P P W P P P P H + W P P P P H + W P P P P x y x x x y y y 4f 4m 6f 6m + H+ – H+ PMe3 or CO concentration at a pressure close to 1 atm (ca. 101 325 Pa). This clearly indicates that the rate-determining step is fission of the W]dppm bond. Rearrangement of complex 4m to 5m, 6m to 7m and 8m to 9m probably occurs via the pathway shown in Scheme 6.It is apparent the ability of diphosphine ligands to form a chelate ring is dppe > dppm > dppf. The five-membered dppe ring is obviously preferable to the strained, four-membered dppm ring. On the other hand, though the chelated dppf ligand presents little ring strain, its large bite angle (ca. 988) 12 combined with the steric bulk of the phenyl groups could impose strong steric interactions with the adjacent ligands.This makes dppf less favorable as a chelate ligand than dppm. Vila and Shaw20 previously reported a trace of acid-induced rapid interconversion between the facial and meridional isomers of [W(CO)3(h2-dppm)(PEt3)] and a hydrido intermediate, [WH(CO)3(h2-dppm)(PEt3)]1, was evidenced.20 It is likely that protonation of complex 4f or 6f generates a fluxional seven-coordinate species, which then undergoes fast intramolecular rearrangement by placing the h1-diphosphine ligand trans to the h2-disphosphine group to reduce steric crowding.Subsequent deprotonation of this species could afford the meridional isomer (Scheme 7). A pseudo-three-fold rotation is likely involved to account for this facile rearrangement. It has been shown that the energy barrier for a ligand three-fold rotation decreases dramatically as the co-ordination number of the metal changes from six to seven.21 Acknowledgements We are grateful for support of this work by the National Science Council of Taiwan, Grant No.NSC 85-2113-M110-020. References 1 S. W. Kirtly, in Comprehensive Organometallic Chemistry, eds. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 1982, vol. 3, p. 783, and refs. therein. 2 A. Blagg, G. R. Cooper, P. G. Pringle, R. Robson and B. L. Shaw, J. Chem. Soc., Chem. Commun., 1984, 933; A. Blagg, T. Boddington, S. W. Carr, G. R. Cooper, I. D. Dobson, J. B. Gill, D. C. Goodall,132 J. Chem. Soc., Dalton Trans., 1998, Pages 125–132 B. L. Shaw and N. Taylor, J. Chem. Soc., Dalton Trans., 1985, 1213; A. Blagg and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1987, 221. 3 R. B. King and K. S. Raghuveer, Inorg. Chem., 1984, 23, 2482. 4 V. Zanotti, V. Rutar and R. J. Angelici, J. Organomet. Chem., 1991, 414, 177. 5 M. Cano, J. A. Campo, V. Pérez-García, E. Gutiérrez-Puebla and C. Alvarez-Ibarra, J. Organomet. Chem., 1990, 382, 397. 6 T. S. A. Hor and S.-M. Chee, J. Organomet. Chem., 1987, 331, 23; T. S. A. Hor, Inorg. Chim. Acta, 1987, 128, L3. 7 F. Basolo, Polyhedron, 1990, 9, 1503; D. M. P. Mingos, J. Organomet. Chem., 1979, 179, C29. 8 J.-C. Rousche and G. R. Dobson, Inorg. Chim. Acta, 1978, 28, L139. 9 D. J. Darensbourg, D. J. Zalewski, C. Plepys and C. Campana, Inorg. Chem., 1987, 26, 3727. 10 W. A. Schenk and G. H. J. Hilpert, Chem. Ber., 1991, 124, 433. 11 R. P. K. Babu, S. S. Krishnamurthy and M. Nethaji, Organometallics, 1995, 14, 2047. 12 S. C. N. Hsu, W.-Y. Yeh and M. Y. Chiang, J. Organomet. Chem., 1995, 492, 121. 13 J. J. Bishop, A. Davison, M. L. Katcher, D. W. Lichtenberg, R. E. Merrill and J. C. Smart, J. Organomet. Chem., 1971, 27, 241. 14 T. S. A. Hor and L.-T. Phang, J. Organomet. Chem., 1989, 373, 319. 15 F. A. Cotton, Inorg. Chem., 1964, 3, 702; R. Dobson and L. W. Houk, Inorg. Chim. Acta, 1967, 1, 287; J. M. Jenkins, J. R. Moss and B. L. Shaw, J. Chem. Soc. A, 1969, 2796; D. M. Adams, Metal- Ligand and Related Vibrations, Edward Arnold, London, 1967, p. 101. 16 E. E. Isaacs and W. A. G. Graham, Inorg. Chem., 1975, 14, 2560. 17 P. E. Garrou, Chem. Rev., 1981, 81, 229. 18 D. J. Darensbourg, Adv. Organomet. Chem., 1982, 21, 113. 19 G. R. Dobson, K. J. Asali, J. L. Marshall and C. R. McDaniel, jun., J. Am. Chem. Soc., 1977, 99, 8100; G. R. Dobson, Acc. Chem. Res., 1976, 8, 300. 20 J. M. Vila and B. L. Shaw, J. Chem. Soc., Chem. Commun., 1987, 1778. 21 J. R. Sowa, jun., V. Zanotti and R. J. Angelici, Inorg. Chem., 1993, 32, 848. Received 23rd June 1997; Paper 7/04416J

ISSN:1477-9226    

DOI:10.1039/a704416j

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J. Chem. Soc., Dalton Trans., 1997, Pages 129–135 129 DALTON Ruthenium-mediated selective activation of a C]H bond. Direct aromatic thiolation in the complexes [RuII{o-SC6H3(R)N] NC5H4N}2] (R = H, Me or Cl) Bidyut Kumar Santra and Goutam Kumar Lahiri* Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400076, India The reaction of the complexes ctc-[RuIIL2Cl2] (L = arylazopyridine, RC6H4N]] NC5H4N, where R = H, m-Me, p-Me or p-Cl; ctc = cis-trans-cis with respect to chlorides, pyridine and azo nitrogens respectively) with KS2COR9 (R9 = Me, Et, Prn, Bun or CH2Ph) in boiling dimethylformamide afforded [RuII{o-SC6H3(R)N]] NC5H4N}2] where the o-carbon atom of the pendant phenyl ring of both ligands L has been selectively and directly thiolated.The newly formed tridentate thiolated ligands are bound in a meridional fashion. When one methyl group is present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group.The molecular geometry of the complexes in solution has been determined by 1H NMR spectroscopy. This revealed the presence of an intimate mixture of the two isomers in solution in a 2 : 1 ratio. In the visible region the complexes exhibit two metalto- ligand charge-transfer transitions at ª700 and ª560 nm respectively and in the UV region intraligand (p–p*, n–p*) transitions.In acetonitrile solution the complexes exhibit one reversible ruthenium(II) ruthenium(III) oxidation couple near 0.4 V and an irreversible oxidative response near 1 V due to oxidation of the co-ordinated thiol group. Reduction of the co-ordinated azo groups occurs at ca. 20.8 and 21.4 V respectively. Coulometric oxidation of the complexes [RuII{o-SC6H3(R)N]] NC5H4N}2] at 0.6 V versus the saturated calomel electrode in dichloromethane produced unstable ruthenium(III) congeners. When R = p-Me, the presence of trivalent ruthenium in the oxidised solution was evidenced by a rhombic EPR spectrum having g1 = 2.359, g2 = 2.300 and g3 = 1.952. Metal-mediated activation of the carbon–hydrogen bond is a fundamentally important chemical reaction 1 which may lead to the formation of interesting new molecules otherwise difficult or even impossible to synthesize by conventional routes.We have recently observed an unusual reaction where the o-carbon atom of the pendant phenyl ring of a co-ordinated arylazopyridine ligand C6H4(R)N]] NC5H4N (L) in the complex [RuIIL2Cl2] has been regiospecifically and directly thiolated to give [RuII{o- SC6H3(R)N]] NC5H4N}2], via carbon–sulfur bond cleavage of the dithiocarbonate KS2COEt.Metal-assisted carbon–sulfur bond cleavage and concomitant formation of a new carbon– sulfur centre is very important in biological systems 2 as well as in industry.3 Preliminary synthetic aspects of this fascinating ruthenium-mediated selective and direct aromatic thiolation process (where R = H) have been communicated.4 Herein we report a detailed account including the substrate, solvent and reagent dependencies, 1H NMR spectroscopic characterisation of the final product, metal- and ligand-centred electroactivities and spectroelectrochemical correlations.Results and Discussion Synthesis The four substituted arylazopyridine ligands used are abbreviated as L1–L4. The ligand L binds to the metal ions in a bidentate N,N9 manner forming a five-membered chelate ring ML 1.The reaction of potassium O-ethyl dithiocarbonate with the starting complex ctc-[RuL1 2Cl2] 3a (ctc = cis-trans-cis with respect to chlorides, pyridine and azo nitrogens respectively) in a ratio of 2.5 : 1 in boiling dimethylformamide (dmf) for 3 h results in a red-brown solution (Scheme 1). Chromatographic purification of the red-brown solution on a silica gel column yields pure complex 4a in 70% yield, where the o-carbon atom of the pendant phenyl ring of both L1 ligands in 3a has been selectively thiolated (C6H4H æÆ C6H4SM).Thus through this C]H activation process the bidentate N,N9 form of the parent ligand has been selectively transformed into a tridentate N,N9,S ligand. The newly formed tridentate ligands are bound to the ruthenium centre in a meridional fashion. The crystal structure of 4a shows that during this conversion process (3a æÆ 4a) an overall internal geometrical reorganisation has taken place (in the starting complex 3 the pyridine and azo nitrogens are mutually trans and cis respectively whereas in 4a they are in reverse orientation).4 In the case of the starting complex 3b, in which one methyl group is present at the meta position of the each active phenyl ring, the reaction in Scheme 1 is very facile under identical experimental conditions, being complete in 1 h.In view of the presence of this methyl group, free rotation along the C]N bond can lead to the formation of three possible isomers 5–7. Indeed, an intimate mixture of isomers 5 and 6 has been detected in solution. Solution NMR study indicates that these two isomers exist in a 2 : 1 ratio (see below).All attempts to separate them either on a TLC plate or by column chromatography have failed. The para-substituted ligands (L3, L4) in starting complexes 3c Np N Na Na M Np R¢O C S S–K+ DAL 6/03955C/A1 L1 R = H L2 R = 8-Me L3 R = 9-Me L4 R = 9-Cl 1 ML 2 R¢ = Me, Et, Prn, Bun or PhCH2 1 2 3 4 5 6 7 8 9 10 R 11130 J.Chem. Soc., Dalton Trans., 1997, Pages 129–135 Table 1 Microanalytical a and electronic spectral data b Elemental analysis (%) Compound C H N S UV/VIS l/nm (e/dm3 mol21 cm21) 4a 4b 4c 49.85 (49.9) 51.75 (51.7) 51.8 (51.7) 3.10 (3.00) 3.55 (3.60) 3.50 (3.60) 15.89 (15.9) 15.15 (15.1) 15.15 (15.1) 12.15 (12.1) 11.4 (11.5) 11.4 (11.5) 710 (4980), 560 (13 220), 375 (26 990), 263 (37 960) 726 (4900), 562 (12 800), 384 (25 000), 260 (23 400) 706 (4600), 561 (12 700), 387 (26 900), 259 (21 800) a Calculated values are in parentheses. b In chloroform at 298 K.and 3d surprisingly do not readily undergo the thiolation reaction. Complex 3c, an electron-donating methyl group is present at the para position, reacts unexpectedly slowly and incompletely. Under identical reaction conditions to those in Scheme 1 more than 8 h were required to get only 10% pure 4c.Further increase in the reflux time did not improve the yield. When an electron-withdrawing chloride group is present at the para position the reaction was not observed. This behaviour is not understood at present, however the results clearly indicate the simultaneous influence of positional and electronic factors of the substituents in the active phenyl ring on the C]H activation process. The conversion of complex 3 into 4 is highly solvent dependent. In acetonitrile, benzene, dichloromethane, tetrahydrofuran, 2-methyltetrahydrofuran, ethanol, methanol and 2-methoxyethanol the reaction does not take place at all, whereas in dimethylformamide, methylformamide, dimethyl sulfoxide and hexamethylphosphoramide P(NMe2)3O it occurs.This implies that both the boiling point and relative permittivity of the solvents are important. Dimethylformamide appears to be the best choice for maximum yield in the minimum time. In the absence of compound 2, no change in the starting complex 3 is observed even under boiling.This may suggest the absence of direct participation of the solvent to form any solvent- Scheme 1 (i) Heat, dmf Na Cl Cl Na RuII Na S Na Np RuII Np Np Np S DAL 6/03955C/A2 + 2 (i ) + 2 KCL + ? 4a 4b 4c 4d 3a L = L1 3b L = L2 3c L = L3 3d L = L4 R R R R Me S Ru Na Np Np Na S Me S Ru Na Np Np Na S Me Me S Ru Na Np Np Na S DAL 6/03955C/A3 Me 6 Me 7 5 containing reactive intermediates prior to the activation process.The rate of the reaction is also dependent on the nature of the R9 group present in the dithiocarbonate 2. The progress of the reaction was monitored qualitatively to semiquantitatively by TLC as well as spectrophotometrically in dmf solvent for all three complexes 3a–3c using the different R9 groups in 2. The reactivity order was as follows: Me ª Et> Prn > Bun > benzyl. This indicates that the nature of the leaving group (R9) of the thiolating agent plays an important role in the kinetic stability of the reaction.In order to find other suitable thiolating agents, the reaction was tested with benzenethiol, carbon disulfide, S8, thiirane, dithiocarbamate, NaS2P(OEt)2 and NaS2PPh2 instead of 2 but these failed to give the desired product 4. The free L also did not undergo the transformation NC5H4N]] NC6H5 æÆ NC5H4N]] NC6H4SH. The microanalytical data of the products 4 (Table 1) are in good agreement with the calculated values and thus confirm the composition.Solid-state magnetic moment measurements at 298 K indicate that the complexes are uniformly diamagnetic (t6 2g, S = 0). In acetonitrile, dimethylformamide and methanol the complexes are non-conducting. The IR, electronic, 1H NMR and electrochemical behaviours of 4a are akin to those of the other complexes 4, therefore it is inferred that 4a–4c have very similar gross molecular structures. Infrared spectroscopy The IR spectra of complexes 4 display several intense bands in the region 4000–300 cm21.No attempt was made to assign all the bands. However, two strong bands near 1595 and 1585 cm21 are assigned to n(C]] C) and n(C]] N) stretching frequencies respectively, and the n(N]] N) stretching frequency of the ligand is observed near 1280 cm21. The n(N]] N) of free L appears at 1425 cm21,5 thus this frequency is appreciably lowered in complexes 4. This is attributed to the presence of strong dp (RuII) æÆ p*(L) back bonding in the ground state of ruthenium( II).The N]] N frequency of the thiolato ligand present in complexes 4 cannot be checked as the free form is not available. However, a 150 cm21 shift of n(N]] N) in 4 compared to that in free L strongly supports the p-acidic nature of the present tridentate ligand. The complexes display two Ru–S stretching bands at 360 and 340 cm21 as expected.6 Electronic spectra The solution electronic spectra of the complexes were studied in chloroform solvent in the region 300–900 nm.The spectral data are listed in Table 1 and spectra are shown in Fig. 1. The complexes display several absorption bands in the specified region which may be due to the presence of different donor and acceptor levels. All exhibit one moderately intense broad band in the region 706–726 nm and a strong relatively sharper band near 560 nm. The band near 700 nm is sensitive to the nature of the substituents present, while that at 560 nm remains more or less unaffected.Based on the intensities of these two allowed visible bands (Table 1) the transitions are assigned to be chargeJ. Chem. Soc., Dalton Trans., 1997, Pages 129–135 131 transfer in nature. Since in these complexes the ruthenium(II) is in the low-spin t6 2g state, the spectra clearly require the presence of low-lying ligand LUMO (lowest unoccupied molecular orbital) and the LUMO +1 orbitals above the metal HOMO (highest occupied molecular orbital) to generate two metal-toligand charge-transfer (m.l.c.t.) transitions.According to a quick extended-Hückel calculation the ligand LUMO involves the S and the azo group, and the LUMO +1 is mainly on the pyridine with some azo contribution (results provided by a referee). Thus the lowest-energy band near 700 nm is due to the dp (RuII) æÆ ligand LUMO transition. This explains the observed shifts in this transition with the substituent present in the activated phenyl rings (Table 1), the S and azo part of the molecule (which dominate the LUMO) being more affected by the Me substituents.For the starting complex 3 the dp (RuII) æÆ L(p*) (where L p* is dominated by the LUMO of the azoimine chromophore) the m.l.c.t. transition occurs at 580 nm.5 The charge-transfer transition energy is known to depend on the separation in potentials between the donor and acceptor levels.7 In the present complexes 4 the difference in potentials between the first reduction couple (]N]] N] reduction) and the reversible oxidation couple (RuII–RuIII) is ª1.2 V (Table 3) which is lower than that of the starting complex 3 (ª1.6 V).5 In accordance with the above fact the m.l.c.t.transition which Fig. 1 Electronic spectra of [RuII{o-SC6H3(Me-m)N]] NC5H4N}2] 4b (——) and [RuII{o-SC6H3(Me-p)N]] NC5H4N}2] 4c (– – – – –) in chloroform Fig. 2 Proton NMR spectra in CDCl3 of (a) [RuII(o-SC6H4N]] NC5- H4N)2] 4a and (b) [RuII{o-SC6H3(Me-p)N]] NC5H4N}2] 4c occurs at 580 nm for complex 3 is believed to take place near 700 nm for complexes 4.This lowering in m.l.c.t. transition energy on going from 3 to 4 implies that the filled ruthenium t2g level becomes destabilised in the present ligand environments compared to those of 3. The second m.l.c.t. band at 560 nm possibly originates from the dp (RuII) æÆ LUMO +1 orbital transition. In the UV region the complexes show two bands possibly because of intraligand p–p* and n–p* transitions involving energy levels higher than those of the ligand LUMO.At room temperature complexes 4 do not show any emission properties. 1H NMR spectra The 1H NMR spectra of all the complexes were recorded in CDCl3 solvent. The chemical shifts and the coupling constants are given in Table 2 and the spectra are displayed in Figs. 2 and 3. Complex 4a exhibits four doublets and four triplets having equal intensities [Fig. 2(a)], i.e. each half of the molecule is equivalent due to localised symmetry around the ruthenium centre. The individual proton resonances were assigned on the basis of their relative intensities, spin–spin structure and also from the effect of the substituents.8 In the case of complex 4b the aromatic region of the spectrum is complicated due to the presence of two isomers in solution, however the well resolved upfield methyl signals and direct comparisons of the individual methyl intensities with those of respective aromatic protons enabled us to reach reasonable conclusions.The presence of the methyl group at the meta position of the active phenyl ring in both ligands of 4b yields three isomers 5–7 through free rotation of the singly bonded metasubstituted phenyl rings. The 1H NMR spectrum of 4b displays two distinct methyl signals having unequal intensities. From the symmetry point of view one methyl signal is expected for each of isomers 5 and 6 and two equally intense peaks for 7. As the spectrum displays two unequally intense methyl peaks at d 2.32 and 2.37 respectively (Fig. 3), having intensity ratio 2 : 1, isomers 5 and 6 are predominant in solution. The downfield portion of the spectrum is overcrowded due to partial overlapping of the aromatic protons of isomers 5 and 6, which precluded unequivocal assignment of the signals as doublets or triplets. However, a tentative assignment can be made by comparing the spectrum of 4b with those of 4a and 4c. All pyridine protons Fig. 3 The 1H NMR spectrum of [RuII{o-SC6H3(Me-m)N]] NC5H4N}2] 4b in CDCl3.Peaks due to isomer 5 are indicated by unprimed numbers and for 6 by primed ones132 J. Chem. Soc., Dalton Trans., 1997, Pages 129–135 Table 2 Proton NMR spectral data in CDCl3 d (J/Hz) a Compound H(1) H(2) H(3) H(4) H(8) H(9) H(10) H(11) 4a 4b d Isomer 5 Isomer 6 4c 8.19 (8.8) b 8.14 b 8.0 b 8.07 (8.8) b 6.78 (6.7) c (6.9) 6.76 c 6.76 c 6.75 (6.9) c (6.9) 7.49 (7.0) c (8.0) 7.47 c 7.47 c 7.33 (8.3) c (7.6) 7.87 (8.2) b 7.84 b 7.84 b 7.81 (8.0) b 7.34 (8.6) b 2.32 (Me) 7.23 b 7.14 e 7.08 (6.9) c (7.3) 7.05 b 7.05 b 2.33 (Me) 6.98 (7.3) c (7.7) 6.93 c 2.37 (Me) 6.40 (8.4) b 7.73 (5.8) b 7.70 b 7.70 e 7.70 (5.5) b a Tetramethylsilane is the internal standard.b Doublet. c Triplet. d Owing to overlapping signals it does not seem possible to determine the J values for doublets or triplets unequivocally. e Singlet. Table 3 Electrochemical data at 298 K a RuIII–RuII couple Ligand oxidation Ligand reductions n& (m.l.c.t.)/cm21 Compound E8298/V (DEp/mV) Epa/V E8298/V (DEp/mV) DEb/V Obs.c Calc.d 4a 4b 4c 0.43 (70) 0.37 (60) 0.34 (70) 1.04 1.04 1.01 20.80 (60) 21.37 (80) 20.83 (60) 21.39 (80) 20.85 (60) 21.40 (70) 1.23 1.20 1.19 14 080 13 770 14 160 12 920 12 680 12 600 a Conditions: solvent, acetonitrile; supporting electrolyte, NEt4ClO4; reference electrode, SCE; solute concentration, 1023 mol dm23; working electrode, platinum.Cyclic voltammetric data: scan rate, 50 mV s21; E8298 = 0.5 (Epc + Epa) where Epc and Epa are the cathodic and anodic peak potentials respectively.b Calculated by using equation (5) of the text. c In CHCl3 solution. d Using equation (4) of the text. except H1 for the isomers 5 and 6 appear together. Phenyl-ring protons such as H10 (triplet) for isomer 5 and H8 (doublet) for isomer 6 appear separately, while H9 (doublet) for both isomers and H11 (doublet for 5 and singlet for 6) appear together (Fig. 3, Table 2).One methyl peak has been observed for complex 4c at d 2.33 as imposed symmetry makes the two ligands equivalent. All the seven aromatic proton signals are well resolved. Two doublets and two triplets from the pyridine ring and two doublets and one singlet from the phenyl ring are observed distinctly as expected [Fig. 2(b), Table 2]. Electron-transfer properties The electron-transfer properties of complexes 4 have been studied in acetonitrile solution by cyclic voltammetry (CV) using a platinum working electrode.The complexes are electroactive with respect to the metal as well as the ligand centres and dis- Fig. 4 Cyclic voltammograms and differential-pulse voltammograms (scan rate 50 mV s21) of a ª1023 mol dm23 solution of complex 4a in acetonitrile play the same four redox processes in the potential range ±1.5 V versus saturated calomel electrode (SCE) (tetraethylammonium perchlorate as electrolyte, 298 K). Representative voltammograms are shown in Fig. 4. The peak-to-peak separations of the couples lie in the range 60–80 mV. Reduction potential data are listed in Table 3. The assignments of the responses to specific couples are based on the following considerations. Ruthenium(II)–ruthenium(III) couple. All the complexes display one reversible wave with characteristic anodic (Epa) and cathodic (Epc) peak potentials near 0.4 V. The anodic and cathodic peak heights are equal and vary as the square root of the scan rate.This reversible oxidation process is assigned to the ruthenium(III)–ruthenium(II) couple, equation (1). Its one- [RuIIIL2]+ + e2 [RuIIL2] (1) electron nature was confirmed by constant-potential coulometry. The peak potentials (Epa and Epc) are virtually independent of the scan rate. The presence of trivalent ruthenium in the oxidised solution was confirmed by the characteristic rhombic EPR spectrum of the ruthenium(III) complex (Fig. 5). The formal potential of the couple varies depending on the R group present in the ligand as expected (Table 3).The ruthenium(II)– ruthenium(III) oxidation potential of the starting complex 3 appears near 1.1 V.5 Thus thiolation of the o-carbon atom of the pendant phenyl ring of L in 4 decreases the RuII–RuIII oxidation potential by ª0.7 V. The parent azopyridine ligand (L in 3) is known to stabilise low-valent metal complexes (bivalent in the case of ruthenium), due to its high p-acidic character and this is always reflected in the high RuII–RuIII oxidation potential. 9 The lowering of the oxidation potential in the present complexes 4 is due to the presence of the s-donor thiol group in the tridentate form of the azopyridine ligand in the complexes. Complexes 4 exhibit the lowest oxidation potential in an environment formed by the azopyridine moiety.J. Chem. Soc., Dalton Trans., 1997, Pages 129–135 133 Ligand oxidation. All complexes 4 exhibit a second irreversible oxidation response (anodic peak, Epa) near 1.0 V vs.SCE. No significant response on scan reversal in cyclic voltammetry is observed (Fig. 4, Table 3) for the complexes in this region. The oxidised complex thus decomposes rapidly on the cyclic voltammetric time-scale. Although the anodic current height (ipa) of this irreversible process is ª2.0 times that of the previous reversible ruthenium(II)–ruthenium(III) process, the differential pulse voltammogram shows the second oxidation wave to have the same height as that of the first, implying a one-electron process (Fig. 4). This irreversible oxidation process could be due to either RuIII æÆ RuIV oxidation or oxidation of the coordinated thiol group. Here the potential difference between the two successive oxidation processes is ª0.7 V. The average potential differences between the two successive redox processes of the ruthenium centre (RuII/III2RuIII/IV) in mononuclear complexes having C, N, O, thioether donor centres have been observed in many cases to be in the range 1.3–1.5 V.10 Therefore it seems reasonable to consider this irreversible response as due to oxidation of the co-ordinated thiol group.Ligand reduction. All the complexes display two successive reversible one-electron reductions near 20.8 and 21.3 V (Fig. 4, Table 3). The azopyridine ligand in 3 is known to act as a potential electron-transfer carrier.5 Each ligand can accommodate two electrons in one electrochemically accessible LUMO which is primarily azo in character. As two electroactive azo groups are present in complexes 4, four successive oneelectron reductions are expected for each complex.In practice two one-electron reductions are observed experimentally which are assigned to the reductions of the azo groups of the ligands as shown in equations (2) and (3). The other two reductions are [RuIIL2] + e2 [RuIIL(L~2)]2 (2) [RuIIL(L~2)]2 + e2 [RuIIL~2 2]22 (3) not detected, presumably due to solvent cut-off.Complexes 4 also exhibit the lowest reduction potentials for the azo function of the co-ordinated azopyridine.11 Spectroelectrochemical correlation The complexes display lowest m.l.c.t. transitions of the type t2 (Ru) æÆ ligand LUMO (where LUMO is dominated by the azo group and S of the ligand) near 700 nm (Table 1), reversible ruthenium(III)–ruthenium(II) reduction potentials near 0.4 V and first ligand (]N]] N]) reduction potentials near 20.8 V (Table 3). The m.l.c.t.transition involves excitation of the elec- Fig. 5 X-Band EPR spectrum of the coulometrically oxidised [RuIII{o-SC6H3(Me-p)N]] NC5H4N}2]+, in dichloromethane solution at 77 K. G = 1024 T tron from the filled t6 2g orbital of ruthenium to the p* orbital of the azo function (the first ligand reduction). The energy of this band can be predicted with the help of equations (4) and (5).12 n& (m.l.c.t.) = 8065(DE8) + 3000 (4) DE8 = E8298(RuIII–RuII) 2 E8298(L) (5) Here E8298(RuIII–RuII) is the formal potential (in V) of the reversible ruthenium(III)–ruthenium(II) couple, E8298(L) is the first ligand reduction and n& (m.l.c.t.) is the wavenumber of the charge-transfer band in cm21.The factor 8065 is used to convert the potential difference DE from V into cm21 and the term 3000 cm21 is of empirical origin. The calculated m.l.c.t. energies and experimentally observed m.l.c.t. transitions are given in Table 3, and there is a linear relationship between the n& (m.l.c.t.) and DE.The involvement of the sulfur in the LUMO along with the azo group may explain why the redox–charge transfer energy correlation gives errors that all lie outside those quoted by Chakravorty and co-workers 12 for other azopyridine ligand systems. Electrogeneration of the trivalent ruthenium congener Coulometric oxidation of complexes 4 in dichloromethane solution at 0.6 V versus SCE using a platinum-gauze working electrode produces a light red solution and the observed Coulomb count corresponds to a one-electron transfer (‘n’ values: 4a, 0.97; 4b, 1.02; 4c, 0.95; n = Q/Q9 where Q9 is the calculated Coulomb count for a one-electron transfer and Q that found after exhaustive electrolysis of 1022 mmol of solute).The resulting oxidised solution shows a cyclic voltammogram which is identical to that of the starting bivalent complex, [RuIIL2]; this may be due to the stereoretentive nature of the oxidation process.Although the complexes exhibit reasonably low ruthenium(II)–ruthenium(III) oxidation potentials, the oxidised solution is unstable. However, in one case (R = p-Me) we have succeeded in recording the X-band EPR spectrum of the oxidised species by quickly freezing the solution (liquid N2). The rhombic nature of the spectrum (Fig. 5) at 77 K (g1 = 2.359, g2 = 2.300, g3 = 1.952) is characteristic of trivalent ruthenium(III) in a distorted-octahedral environment (low-spin RuIII, t5 2g, S = ��� ).13 The mechanism of this ruthenium-mediated selective activation of the C]H bond of the phenyl ring of L is not yet clearly understood, primarily due to two reasons: (i) the reaction occurs under drastic conditions and (ii) it does not proceed with any tractable intermediate.In ruthenium chemistry cyclometallation of the pendant phenyl ring of azobenzene,14 azobenzene thioether15 and azophenol 16 ligands and cyclopalladation of azopyridine 17 are known. On the basis of the above evidence we assume that the reaction here may proceed through the orthometallated species (B) as reactive intermediate (Scheme 2).In the starting complex 3 (structure con- firmed crystallographically 18) the phenyl ring of both L units (which are active sites for the thiolation reaction) exists far away from the chlorides (the leaving groups) with the pyridine and azo nitrogens being mutually trans and cis respectively (A). In the final product C, however, the relative orientation of the respective nitrogens is exactly the opposite. Since A does not isomerise to the corresponding dichloro species of C (where the pyridine and azo nitrogens are cis and trans respectively) under similar reaction conditions but in the absence of compound 2, it may be considered that as a first step of the reaction the chlorides are replaced through formation of the four-membered orthometallated species B in the presence of 2.To facilitate the formation of B, proximity of the phenyl rings of L and the chloride ions is essential and can only be achieved via a geometrical reorientation possibly through a Bailar twist mechanism (Scheme 2).This satisfies the positional requirements134 J. Chem. Soc., Dalton Trans., 1997, Pages 129–135 needed for the first step of the activation process which eventually leads to the formation of C via the insertion of sulfur (generated by cleavage of the C–S bond of dithiocarbonate 2) into the reactive metal–carbon s bond.Bearing the proximity of these groups in complex 3 in mind, the reaction of 2 with the other stable isomers of 3 such as ttt (where both phenyl rings are close to chlorides) and ccc (where one phenyl ring is close to one chloride ion) were performed in dmf solvent. Instead of undergoing the direct thiolation reaction both isomers are rapidly isomerised to the ctc isomer (known to be the most stable isomer of the starting complex 35) which subsequently gives product 4.The invisibility of the proposed orthometallated intermediate B (if it exists) is evidently due to its extreme reactivity in the presence of the incoming sulfur group, which may originate from the presence of an unfavourable four-membered cyclometallated ring. Conclusion We have observed ruthenium-mediated intramolecular selective activation of a C]H bond of a phenyl ring. This process is highly dependent on the nature of the substrate, reagent and solvent.Suitably placed substituents in the active phenyl ring lead to the formation of isomeric products due to the specificity of the activation process. The newly formed thiolato derivative of the well known strongly p-acidic azopyridine ligand (L) destabilises the metal t2g orbital to a great extent which in turn reduces the ruthenium(III)–ruthenium(II) reduction potential by ª0.7 V and lowers the (dp) RuII æÆ ligand LUMO m.l.c.t. transition energy reasonably compared to the starting complex 3.The newly synthesized bis-chelated complexes 4 are susceptible to both metal- as well as ligand-based chemical and electrochemical transformations. The complexes can act as building blocks for the formation of homo- and hetero-nuclear polymeric species. Further investigations are in progress. Experimental Materials Commercial ruthenium trichloride (S. D. Fine Chemicals, Bombay, India) was converted into RuCl3?3H2O by repeated evaporation to dryness with concentrated hydrochloric acid.The ctc-[RuL2Cl2] complexes 3, KS2COR9 (R9 = Me, Et, Prn, Bun or CH2Ph), NaS2P(OEt)2 and NaS2PPh2 were prepared according to the reported procedures.5,13,19 Other chemicals and solvents were reagent grade and used as received. Silica gel (60–120 mesh) and alumina (neutral) used for chromatography were of BDH quality. For spectroscopic/electrochemical studies commercial acetonitrile was treated with CaH2 (overnight) followed by successive distillation over Li2CO3–KMnO4 and P4O10.The solvent was stored over molecular sieves (4 Å). Commercial tetraethylammonium bromide was converted into pure tetraethylammonium perchlorate by following an available procedure. 20 Dinitrogen gas was purified by successive bubbling through alkaline dithionite and concentrated sulfuric acid. Physical measurements Solution electrical conductivity was checked using a Systronic Scheme 2 305 conductivity bridge. Electronic spectra (900–200 nm) were recorded using a Shimadzu-UV-265 spectrophotometer, IR spectra on a Nicolet spectrophotometer with samples prepared as KBr pellets.Magnetic susceptibility was checked with a PAR vibrating-sample magnetometer. Proton NMR spectra were obtained with a 300 MHz Varian FT-NMR spectrometer. Cyclic voltammetric measurements were carried out using a PAR model 362 scanning-potentiostat electrochemistry system. A platinum-wire working electrode, a platinum-wire auxiliary electrode, and an aqueous saturated calomel reference electrode were used in a three-electrode configuration.The supporting electrolyte was NEt4ClO4 and the solute concentration ª1023 mol dm23. The half-wave potential E8298 was set equal to 0.5 (Epa + Epc), where Epa and Epc are the anodic and cathodic cyclic voltammetric peak potentials respectively. The scan rate was 50 mV s21. The coulometric experiments were done with a PAR model 370-4 electrochemistry apparatus incorporating a 179 digital coulometer. A platinum wire-gauze working electrode was used in coulometric experiments. All experiments were carried out under a dinitrogen atmosphere.All electrochemical data were collected at 298 K and are uncorrected for junction potentials. The EPR measurements were made with a Varian model 109C E-line X-band spectrometer fitted with a quartz Dewar for measurements at 77 K (liquid nitrogen). The spectrum was calibrated by using diphenylpicrylhydrazyl (dpph) (g = 2.0037). The elemental analyses were carried out with a Carlo Erba (Italy) elemental analyser.Solution emission properties were checked using a Shimadzu RF-540 fluorescence spectrophotometer. Preparation of complexes The starting complexes 3c and 3d were prepared for the first time following procedures reported for 3a and 3b.5 Complexes 4b and 4c were synthesized by the following procedures. [RuII{o-SC6H3(Me-m)N]] NC5H4N}2] 4b. The complex ctc- [RuL2 2Cl2] 3b (100 mg, 0.177 mmol) was dissolved in dmf (15 cm3) and heated to reflux for 5 min.To this boiling solution was added potassium O-ethyl dithiocarbonate (72 mg, 0.45 mmol). Heating was continued for 1 h. The initial blue colour of 3b gradually turned to red-brown. The progress of the reaction was monitored periodically by TLC. The solvent was removed under reduced pressure and the solid mass thus obtained was dried in vacuo over P4O10. The dried product was extracted into the minimum volume of dichloromethane and purified by using a silica gel column. With dichloromethane (as eluent) a slight amount of light yellow solution due to the excess of ligand was separated first and rejected.Using dichloromethane– acetonitrile (40 : 1) as eluent a deep red-brown band was separated. It was collected and evaporation of the solvent under reduced pressure afforded a crystalline solid. Finally the product was recrystallised from dichloromethane–hexane (1 : 5). Yield: 88 mg (90%).Complex 4c was prepared by following the above metcept for the reflux time. Approximately 8 h of heating were needed for the complete conversion of the starting blue complex 3c. After removal of solvent under reduced pressure the solid mass was dissolved in chloroform. The solution was filtered to remove any insoluble particles and subjected to chromatography on a silica gel column. A small red-brown band was eluted by chloroform–acetonitrile (5 : 1) leaving a dark band at the top of the column, which was not even moved by methanol.The solvent was evaporated under reduced pressure and the solid mass thus obtained was recrystallised from chloroform –light petroleum (b.p. 80–100 8C) (2 : 5). Yield: 9.8 mg (10%). Acknowledgements Financial support from the Department of Science and Tech-J. Chem. Soc., Dalton Trans., 1997, Pages 129–135 135 nology, New Dehli, India, is gratefully acknowledged. We are grateful to Professor A.Q. Contractor and Dr. R. Dabke, Indian Institute of Technology (IIT), Bombay, for providing electrochemical instrumental facility. Special acknowledgement is made to the Regional Sophisticated Instrumentation Centre, IIT, Bombay for providing the NMR facility. The referees comments at the revision stage were very helpful. References 1 E. M. Siegbahn, J. Am. Chem. Soc., 1996, 118, 1487; R. H. Schultz, A. A. Bengali, M. J. Tauber, B. H. Weiller, E. P. Wasserman, K. R. Kyle, C.B. Moore and R. G. Bergman, J. Am. Chem. Soc., 1994, 116, 7369 and refs. therein. 2 R. W. Hay, Bioinorganic Chemistry, Harwood, New York, 1984, p. 165; Y. M. Torchinsky, Sulfur in Proteins, Pergamon, Oxford, 1981; B. Jaun, Helv. Chim. Acta, 1990, 73, 2209; M. Schroder, Encyclopedia of Inorganic Chemistry, ed. R. B. King, Wiley, New York, 1994, vol. 7, p. 3593; H. Kawaguchi and K. Tatsumi, J. Am. Chem. Soc., 1995, 117, 3885. 3 A. W. Myers, W. D. Jones and S. M. McClements, J. Am. Chem.Soc., 1995, 117, 11704; D. A. Lesch, J. W. Richardson, jun., R. A. Jacobson and R. Angelici, J. Am. Chem. Soc., 1984, 106, 2901; R. Angelici, Acc. Chem. Res., 1988, 21, 387; P. G. Jessop, S. J. Retting, C. L. Lee and B. R. James, Inorg. Chem., 1991, 30, 4617; C. Bianchini, P. Frediani, V. Herrera, M. V. Jimenez, A. Meli, L. Rincon, R. S. Delgado and F. Vizza, J. Am. Chem. Soc., 1995, 117, 4333. 4 B. K. Santra, G. A. Thakur, P. Ghosh, A. Pramanik and G. K. Lahiri, Inorg.Chem., 1996, 35, 3550. 5 S. Goswami, A. R. Chakravarty and A. Chakravorty, Inorg. Chem., 1981, 20, 2246. 6 K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963, p. 214. 7 M. Haga, E. S. Dodsworth and A. B. P. Lever, Inorg. Chem., 1986, 25, 447; B. K. Ghosh and A. Chakravorty, Coord. Chem. Rev., 1989, 95, 239; E. S. Dodsworth and A. B. P. Lever, Chem. Phys. Lett., 1985, 119, 61; 1986, 124, 152; Y. H. Tse, P. R. Auburn and A. B. P. Lever, Can. J. Chem., 1992, 70, 1849; N. Bag, A. Pramanik, G. K. Lahiri and A. Chakravorty, Inorg. Chem., 1992, 31, 40. 8 G. K. Lahiri, S. Goswami, L. R. Falvello and A. Chakravorty, Inorg. Chem., 1987, 26, 3365. 9 G. K. Lahiri, S. Bhattacharya, S. Goswami and A. Chakravorty, J. Chem. Soc., Dalton Trans., 1990, 561. 10 B. M. Holligan, J. C. Jeffery, M. K. Norgett, E. Schatz and M. D. Ward, J. Chem. Soc., Dalton Trans., 1992, 3345; G. K. Lahiri, S. Bhattacharya, B. K. Ghosh and A. Chakravorty, Inorg. Chem., 1987, 26, 4324; N. Bag, G. K. Lahiri, S. Bhattacharya, L. R. Falvello and A. Chakravorty, Inorg. Chem., 1988, 27, 4396; P. Ghosh, A. Pramanik, N. Bag, G. K. Lahiri and A. Chakravorty, J. Organomet. Chem., 1993, 454, 273. 11 A. K. Deb and S. Goswami, J. Chem. Soc., Dalton Trans., 1989, 1635. 12 S. Goswami, R. N. Mukherjee and A. Chakravorty, Inorg. Chem., 1983, 22, 2825. 13 G. A. Thakur, K. Narayanaswamy and G. K. Lahiri, Indian J. Chem., Sect. A, 1996, 35, 379 and refs. therein. 14 J. D. Gilbert, D. Rose and G. Wilkinson, J. Chem. Soc. A, 1970, 2765. 15 A. K. Mahapatra, S. Dutta, S. Goswami, M. Mukherjee, A. K. Mukherjee and A. Chakravorty, Inorg. Chem., 1986, 25, 1715. 16 G. K. Lahiri, S. Bhattacharya, M. Mukherjee, A. K. Mukherjee and A. Chakravorty, Inorg. Chem., 1987, 26, 3359. 17 P. Bandyopadhyay, D. Bandyopadhyay, A. Chakravorty, F. A. Cotton, L. R. Falvello and S. Han, J. Am. Chem. Soc., 1983, 105, 6327. 18 A. Seal and S. Ray, Acta Crystallogr., Sect. C, 1984, 40, 929. 19 W. M. A. Higgins, P. W. Vogel and W. G. Craig, J. Am. Chem. Soc., 1955, 77, 1864. 20 D. T. Sawyer and J. L. Roberts, jun., Experimental Electrochemistry for Chemists, Wiley, New York, 1974, p. 167. Received 5th June 1996; Paper 6/03955C © Copyright 1997 by the Royal Society of Chemistry

ISSN:1477-9226    

DOI:10.1039/a603955c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 133–138 133 Chromatographic separation of stereoisomers of ligand-bridged diruthenium polypyridyl species Nicholas C. Fletcher, Peter C. Junk, David A. Reitsma and F. Richard Keene * School of Biomedical and Molecular Sciences, James Cook University of North Queensland, Townsville, Queensland 4811, Australia Cation-exchange chromatographic techniques have been developed to separate stereoisomers of polymetallic complexes, using SP Sephadex C-25 as support.Through the example of the ligand-bridged dinuclear cation [{Ru(dmbpy)2}2(m-bipym)]41 (dmbpy = 4,49-dimethyl-2,29-bipyridine, bipym = 2,29-bipyrimidine), the isolation and characterisation of the meso and rac diastereoisomers by elution with aqueous sodium toluene-4-sulfonate solution are demonstrated. The effects of variation in salt concentration and temperature on the efficacy of the separation are discussed. The enantiomeric pair of the rac diastereoisomer (DD and LL) was subsequently chromatographically resolved on SP Sephadex C-25, relying upon the inherent chirality of the support.Optical resolutions using eluents with chiral anions [aqueous sodium (1)-O,O9-dibenzoyl-D-tartrate and sodium (2)-O,O9-dibenzoyl-L-tartrate] were investigated, with the (2)-enantiomer demonstrating a positive [the (1)-enantiomer a negative] synergistic effect in combination with the Sephadex support. Crystals of the meso form were isolated, allowing an X-ray structural determination.The study of polymetallic ‘supramolecular’ assemblies is a rapidly expanding field of chemistry, largely because of their potential application in materials for such diverse purposes as photochemical molecular devices,1–4 and as photoprobes of structure and function of polynucleotides such as DNA.5–9 In particular, much attention has been focused upon the polypyridyl complexes of ruthenium and osmium as the basis for such assemblies as a result of their favourable photochemical and redox characteristics.1 When bidentate ligands are involved in such centres, stereoisomerism is inherent in the resultant assemblies, but surprisingly this problem has until recently received only tacit attention.In general, studies of chirality in octahedral tris(bidentate ligand)ruthenium(II) centres have been limited to mononuclear species, and isolation of enantiomers has relied on the sometimes inefficient technique of diastereoisomer formation using chiral auxiliary anions.10–13 Such complexes can then be used as chiral building blocks for larger assemblies,12 but this approach requires tedious synthetic procedures where care must be taken to preserve the chiral integrity at each metal centre during every reaction step.Additionally, in the vast majority of the targets studied, crystallisation has proved the most favourable method of diastereoisomeric discrimination. Our aim was to develop general chromatographic techniques for the separation of stereoisomers not only of mononuclear complexes, but also of oligomeric transition-metal assemblies containing polypyridyl ligands.In the latter case the simplest example is that of a dinuclear complex of the type [{Ru(pp)2}2(m-Lb)]n1 [pp is a symmetrical bidentate ligand, Lb is a bis(bidentate) bridging ligand]. There are the possibilities of the meso (LD) and the rac diastereoisomers, the rac form consisting of an enantiomeric pair (LL and DD), as illustrated in Fig. 1.In these studies, SP Sephadex C-25 was chosen as the cationexchange support, rather than the more commonly used polystyrene-based resins for which we had previously observed significant absorptions by some complexes containing polypyridyl ligands. This material has been extensively used in the separation of stereoisomers of various octahedral cobalt(III) species,14–16 but surprisingly it appears not to have been widely applied to ruthenium(II) complexes containing polypyridyl ligands until our recent studies.SP Sephadex is composed of a cross-linked dextran matrix, functionalised with strongly acidic propanesulfonate groups. The cations to be separated are adsorbed onto the anionic stationary phase, and separation is achieved as a result of differential ion-exchange equilibria involving the components of the mixture and the cations of the eluting electrolytic solution (typically a Na1 salt).13 To date we have successfully achieved the chromatographic separation of a wide variety of stereoisomers of mono-, di- and tri-metallic species using SP Sephadex C-25 as the support with aqueous sodium toluene-4-sulfonate solution as eluent.17–23 Further, chiral resolution of monomeric species such as [Ru- (tmbpy)(phen)(py)2]21 and [Ru(tmbpy)2(py)2]21 (where tmbpy = 4,49,5,59-tetramethyl-2,29-bipyridine, phen = 1,10-phenanthroline, py = pyridine) have been achieved by using an eluent containing the chiral anion (2)-di-4-toluoyl-L-tartrate.17,24 In addition, we have described the separation and isolation of the LL and DD forms of the dinuclear complexes [{Ru(dmbpy)2}2(mapy)] 41 (where dmbpy = 4,49-dimethyl-2,29-bipyridine, apy = 2,29-azopyridine),22 as well as the stereoisomers of the dinuclear species [{Ru(pp)2}2(m-hat)]41 (DL, DD and LL) and of the trinuclear complexes [{Ru(pp)2}2(m-hat)]61 (DDL and LLD, DDD and LLL) [where pp = bpy (2,29-bipyridine) or phen; hat = 1,4,5,8,9,12-hexaazatriphenylene].19 Results and Discussion Diastereoisomer synthesis, isolation and characterisation The ligand-bridged dinuclear complex [{Ru(dmbpy)2}2(m- Fig. 1 Schematic view of the isomeric possibilities for the general dinuclear species [{M(pp)2}2(m-Lb)]41134 J. Chem. Soc., Dalton Trans., 1998, Pages 133–138 bipym)]41 (where bipym = 2,29-bipyrimidine) was synthesized by the reaction under reflux of [Ru(dmbpy)2Cl2] with bipym in a 2 : 1 stoichiometric ratio for 10 min in a microwave oven.After initial chromatographic purification on SP Sephadex C-25 cation exchanger (eluted with 0.5 mol dm23 NaCl, discarding the first red monomeric fraction) and conversion into the hexa- fluorophosphate salt, the product was obtained in 90% yield. The diastereoisomeric separation of this complex was achieved by cation-exchange chromatography on the same support by using 0.25 mol dm23 sodium toluene-4-sulfonate solution as eluent, within a passage of 50 cm of the column length.The diastereoisomers were isolated from the eluted bands as the hexafluorophosphate salts. Since the complete removal of the toluene-4-sulfonate anion proved difficult, the complexes were converted into the chloride salts by passage through an anionexchange resin (Amberlite) for characterisation. The purity of the two forms was confirmed by 1H NMR spectroscopy: the differences between the two forms are clearly illustrated in Fig. 2. Assignment of the spectra (M-H6a and M-H5a) was achieved using 1H]1H correlation spectroscopy (COSY) techniques. The H6 and H5 protons of the dmbpy ligand situated over the bridge show the greatest differences in chemical shifts between the two isomers, as previously observed.22 From examination of molecular models, a greater anisotropic effect from the ring current of the adjacent dmbpy should be observed by the meso form, indicated by resonances M-H6a and M-H3a being shifted downfield in comparison to the corresponding protons of the rac diastereoisomer.Accordingly, the first fraction was assigned as the meso, and the second as the rac diastereoisomer, which were subsequently confirmed by other results (as described below). Similar synthetic procedures were used to obtain the analogous complexes [{Ru(tmbpy)2}2(m-bipym)]41, [{Ru(bpy)2}2(mbipym)] 41 and [{Ru(phen)2}2(m-bipym)]41.21 For all complexes the separation of the meso/rac diastereoisomers was possible by the same technique, although there was a variation in length of passage down the column before a definite resolution was observed.It was found that the identity of terminal ligands effected the isomeric separation; e.g. [{Ru(tmbpy)}2(m-bipym)]41 was resolved in approximately half the column distance required for [{Ru(dmbpy)}2(m-bipym)]41. Separation has been observed for other dinuclear species involving the bridge apy,22 and in the case of [{Ru(dmbpy)2}2(m-apy)]41 it was achieved in approximately a tenth of the distance required for [{Ru(dmbpy)}2(m-bipym)]41.Dinuclear complexes involving the bridges 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)- quinoxaline (dpq), 3,4-bis(2-pyridyl)-1,2,4-triazolate (bpt2) and hat 1 all demonstrate similar behaviour under the same condi- Fig. 2 Proton NMR spectra (500 MHz, 25 8C, D2O) of (a) DD/LLand (b) DL-[{Ru(dmbpy)2}2(m-bipym)]Cl4 tions, and in each case the meso isomer (as indicated by 1H NMR spectroscopy) was the fastest moving band on the column, despite the differing geometries.25 The evidence clearly indicates that this method for diastereoisomeric separation is general for a wide variety of dinuclear species of this type.A sample of the first major fraction (meso) of [{Ru- (dmbpy)2}2(m-bipym)]Cl4 was repurified by gel permeation chromatography on Sephadex LH20 support (methanol eluent) to remove inorganic impurities. Following removal of the solvent and redissolution in water, oily crystals were grown by slow evaporation from the aqueous solution which were suitable for X-ray analysis.To date, very few crystals of dinuclear ruthenium polypyridyl complexes have been obtained. Ward and co-workers 26,27 describe an alkoxide-bridged species, where only the rac form was isolated, the meso form being excluded on steric grounds. Two examples of dinuclear species containing unsymmetrical polypyridyl ligands have been reported, where the crystals have preferentially grown in the meso form.28,29 One of the problems in the growth of crystals of such species may well be the existence of a mixture of stereoisomers, and we present here for the first time the selective isolation of one of the various components prior to crystal growth.The crystals themselves proved to be of poor quality, crumbling easily and being extremely prone to solvent evaporation. However, a data set was obtained and the structure solved, illustrating the connectivity of the complex (Fig. 3). Assuming that the crystal is representative of the bulk solution, this first major band is confirmed as the meso (LD) form. The clefts between the bipyridine rings contained several water molecules, while there are planes of water and chloride anions between the predominantly organic layers of the complex (Fig. 4). These distinct layers are probably the cause of the extreme brittleness and solvation dependence of the material, and another possible reason for the extremely small number of structures of these complexes existing in the literature.The selected bond lengths and angles are given in Table 1. The molecule possesses crystallographic symmetry along the bipyrimidine bridge (the second half of the molecule being generated from the first). The average Ru]N bond lengths (2.04 Å) and N]Ru]N angles correlate with those published for previous structures.26–29 EVect of eluent concentration on the diastereoisomeric separation The variation of eluent concentration by a small amount can have a great influence on the observed separation. To illustrate this, a series of quantitative small-column experiments were undertaken using [{Ru(dmbpy)2}2(m-bipym)]41 as the target, with aqueous sodium toluene-4-sulfonate solutions as eluent at concentrations between 0.05 and 1.0 mol dm23: the relative Fig. 3 Thermal ellipsoid plot of meso-(LD)-[{Ru(dmbpy)2}2(mbipym)] 41, with ellipsoids drawn at 50% probability.All carbon atoms are isotropic and protons are omitted for clarityJ. Chem. Soc., Dalton Trans., 1998, Pages 133–138 135 rates of passage of the two bands down the column of SP Sephadex C-25 were recorded. To ensure consistency in the results, care was taken to preequilibrate the column in the desired eluent, overcoming the problems of contraction of the support upon change of the salt concentration. Further, care was taken to monitor the rate of flow by recording the volume of eluent passed and the time.In order to give comparable results from each column, where slight discrepancies in flow rate were observed the data were corrected by calculating the distance travelled (cm) by the passage of 1 cm3 of eluent down the column. A typical flow rate of 0.5 cm3 min21 was used in all cases to ensure comparable values, although the rate of flow was noted to have little effect on the column behaviour (in a range of 0.2–2.0 cm3 min21), provided an equilibrated system had been achieved.(Exact details are given in the Experimental section.) The relative rates of passage of the two diastereoisomers down the column using varying eluent concentrations are displayed in Fig. 5, giving the data for two forms (rac and meso) as well as those of the combined bands. At higher concentrations no resolution was observed over the length of column used in these experiments and consequently there are no data for the individual forms.Predictably, as the concentration increases the rate of travel of the two fractions increases. While the relative separation of the two bands did not change, the bands broadened with increased rate of passage due to a loss of the equi- Fig. 4 Packing diagram of meso-(LD)-[{Ru(dmbpy)2}2(m-bipym)]Cl4? 10H2O Table 1 Selected bond lengths (Å) and angles (8) for [{Ru- (dmbpy)2}2(m-bipym)]Cl4?10H2O Ru(1)]N(1) Ru(1)]N(2) Ru(1)]N(3) N(1)]Ru(1)]N(2) N(1)]Ru(1)]N(3) N(1)]Ru(1)]N(4) N(1)]Ru(1)]N(5) N(1)]Ru(1)]N(6) N(2)]Ru(1)]N(3) N(2)]Ru(1)]N(4) N(2)]Ru(1)]N(5) 2.094(2) 1.96(1) 2.09(2) 79.6(9) 94.7(8) 171.3(8) 97.2(8) 86.9(7) 89.1(10) 94.8(10) 175.8(9) Ru(1)]N(4) Ru(1)]N(5) Ru(1)]N(6) N(2)]Ru(1)]N(6) N(3)]Ru(1)]N(4) N(3)]Ru(1)]N(5) N(3)]Ru(1)]N(6) N(4)]Ru(1)]N(5) N(4)]Ru(1)]N(6) N(5)]Ru(1)]N(6) 2.00(2) 2.01(2) 2.11(2) 95.1(10) 78.5(9) 94.0(10) 175.7(10) 88.7(9) 100.2(8) 81.8(9) librium between the stationary phase and the mobile phase.As a consequence of this the separation of the bands takes a greater length of the column to be achieved.Further, with concentrations over 0.5 mol dm23, the rates of passage of the combined fractions did not increase significantly, implying that there is a maximum possible rate of passage of the compound down the column. From the observations made, it is apparent that the optimum concentration for efficient separation of the diastereoisomers of [{Ru(dmbpy)2}2(m-bipym)]41 appears to be ca. 0.25 mol dm23. EVect of temperature on the diastereoisomeric separation All previous stereoisomeric separations have been carried out at room temperature, and so investigations were made into the temperature dependence of the process, while maintaining the eluent concentration at 0.25 mol dm23. The small preequilibrated column described above was fitted with an external water-jacket, and the temperature of the SP Sephadex C-25 controlled using a circulating thermostatted bath.As illustrated in Fig. 6, the rate of passage of [{Ru- (dmbpy)2}2(m-bipym)]41 down the column decreases as the temperature increases. The data indicate a linear relationship, with the relative resolution of the two bands staying approximately constant. At lower temperature greater broadening of the bands was observed, associated with the faster passage through the support and loss of the equilibria. The association between the SP Sephadex C-25 support and the cationic substrate should be essentially temperature independent, being between a solid and solute phase.30 Since coulombic forces dominate the rate of passage of species down Fig. 5 Relative rate of travel of the diastereoisomers of [{Ru- (dmbpy)2}2(m-bipym)]41 down a 9 mm (inside diameter) column as a function of eluent concentration (aqueous sodium toluene-4-sulfonate) at 30 8C: rac (filled), meso (blank), combined forms (hatched) Fig. 6 Relative rate of travel of the diastereoisomers of [{Ru(dmbpy)2}2(m-bipym)]41 down a 9 mm (inside diameter) column as a function of temperature (eluent: aqueous 0.25 mol dm23 sodium toluene-4-sulfonate solution): meso (s), combined forms (n), rac (h)136 J.Chem. Soc., Dalton Trans., 1998, Pages 133–138 the SP Sephadex C-25 support,14 it can be assumed that the observed temperature dependence can be attributed to changes in the effective charges of the species travelling down the column. Tris(bipyridine)ruthenium(II)-type species associate with organic anions, and it is the differences in relative associations between the anion of the eluent and the various stereoisomers that lead to their separation by cation-exchange chromatography (the nature of these associations will be the subject of a subsequent publication).31 From the temperature-dependence studies it may be concluded that the degree of association is higher at lower temperatures.This stronger association effectively reduces the charge on the cations, lowering the affinity with the cation-exchange resin so that the rate of travel down the column is faster at lower temperatures.Chiral resolution of the racemic diastereoisomer with an achiral eluent Previously, we have established that the DD and LL enantiomers of the complex [{Ru(dmbpy)2}2(m-apy)]41 can be separated chromatographically using the chiral eluent sodium (2)- O,O9-di-4-toluoyl-L-tartrate.22 However, during our studies on the [{Ru(dmbpy)2}2(m-bipym)]41 system, the observation was made that there was considerable spreading of the slowermoving (rac) band on the column during the separation of the meso and rac diastereoisomers using sodium toluene-4- sulfonate as the eluent.To investigate whether chiral resolution could be achieved using this achiral salt, the slower-moving band from the diastereoisomer separation was reintroduced onto the top of the SP Sephadex C-25 column, and allowed to recycle several times down the length of the column (1 m).After it had travelled an effective column length (ECL) of ca. 2.5 m Fig. 7 Circular dichroism spectra of LL- (solid line) and DD- [{Ru(dmbpy)2}2(m-bipym)]41 (dashed line) Fig. 8 Relative rate of travel of the three stereoisomers of [{Ru(dmbpy)2}2(m-bipym)]41 down a 9 mm (inside diameter) column, eluted with aqueous 0.154 mol dm23 sodium (1 or 2)-dibenzoyltartrate solutions at 30 8C there was a clear separation of the two chiral forms of the rac diastereoisomer.These were collected and isolated as the hexa- fluorophosphate salts, and the resolution into the individual enantiomers confirmed by CD measurements (Fig. 7). By comparison with the CD enantiomer assignment for stereoselectively synthesized DD- and LL-[{Ru(bpy)2}2(mbipym)] 41 by Hua and von Zelewsky,12 the first band off the column was assigned as the DD isomer, followed by the LL form. Importantly, we have achieved for the first time a separation of the two enantiomers using an achiral eluent on the SP Sephadex C-25 cation exchanger.The inference is clearly that the inherent structure of the dextran support itself must provide the chiral environment responsible for this process. The individual units of SP Sephadex are composed of propanesulfonate- functionalised cross-linked a-D-glucopyranoside, and there are five chiral centres in each subunit. While coulombic forces dominate the cation-exchange chromatographic process, the charge densities or polarities of the cations 14 also exert an influence for species of the same charge.The use of a suitable counter anion in the eluent also has an effect on the second-sphere interactions between cation and anion.14,32 Since isomers may differ in their relative interactions with the counter anion, the resultant slight variations in the effective charge and polarity of the species facilitate separation on the column. However, with the chiral resolution observed with an achiral eluent, the effective charge and polarity of the two associated enantiomers must be the same.Hence the separation must be a consequence of the support material itself, where the mechanism has elements of exclusion on the basis of shape. The significance of this observation is that chiral eluents such as sodium (1)-O,O9-ditoluoyl-D-tartrate and sodium (2)-dibenzoyl-L-tartrate are not always necessary in the separation of simple enantiomeric pairs on the Sephadex support.It is however expected that the efficiency of such resolutions will be synergistically enhanced by the correct choice of chiral eluent. Chiral resolution of the racemic diastereoisomer with a chiral eluent Since the support material has such a significant effect on the two enantiomers, it can be assumed that the use of chiral counter anions in the eluent will either oppose or enhance the effect observed above. The rate of passage down the small preequilibrated column was therefore investigated to examine the effect of the eluents sodium (2)-dibenzoyl-L-tartrate and (1)- dibenzoyl-D-tartrate (0.154 mol dm23) on the rate of elution of the three individual stereoisomers down the column.The results are displayed in Fig. 8. Sodium (2)-O,O9-dibenzoyl-L-tartrate behaved in a similar fashion to sodium toluene-4-sulfonate as an eluent, in that the meso-DL form moved the fastest, with the rac-DD next and the rac-LL slowest on the column. The chiral resolution of the rac form was however achieved in a much shorter distance, demonstrating a positive synergistic effect between the chiral eluent with the Sephadex support.On the other hand, with sodium (1)-O,O9-dibenzoyl-D-tartrate as eluent, the order of travel of the two enantiomers was reversed with the LL form travelling faster than the DD. Similar behaviour has also been observed using simple monomers such as [Ru(bpy)3]21 and [Ru(phen)3]21 and is currently undergoing intense study to try to rationalise this observation.24 von Zelewsky and co-workers 33 have shown by X-ray crystallography that there is a specific association between L- [Ru(bpy)2(py)2]21 and (2)-O,O9-dibenzoyl-L-tartrate, and between D-[Ru(bpy)2(py)2]21 and (1)-O,O9-dibenzoyl-Dtartrate.While in this case the possible p-stacking interactions between the aromatic benzoyl groups and the ligand pyridyl groups cause selective crystallisation, the same type of associations appear to dictate the order of travel down the column.J. Chem.Soc., Dalton Trans., 1998, Pages 133–138 137 A comparison of the use of sodium (2)-O,O9-dibenzoyl-Ltartrate with sodium (1)-O,O9-dibenzoyl-D-tartrate as eluent indicates that there is much slower passage down the column for the latter electrolyte (1.25 times slower for the meso diastereoisomer). Additionally, the distance required to achieve chiral resolution between the enantiomeric pairs is larger with (1)- O,O9-dibenzoyl-D-tartrate.The implication is that the effect of the chirality of (1)-O,O9-dibenzoyl-D-tartrate opposes that of the Sephadex itself, slowing the rate of travel down the column and hindering the chiral resolution. Accordingly, care must be observed in the choice of chiral eluent to use this synergistic behaviour to enhance the separation. Conclusion In order to characterise simple ligand-bridged dinuclear polypyridyl species by standard techniques such as NMR spectroscopy the isolation of the individual diastereoisomers must first be achieved.Further, the isolation of stereochemically pure samples is likely to facilitate the growth of crystals appropriate for X-ray structural analysis. Such separations of diastereoisomers has been achieved using SP Sephadex C-25 cation exchanger and elution with sodium toluene-4-sulfonate. The variable-temperature experiments clearly indicate that there must be a significant degree of association of the eluent anion and cation which affects the interaction of the cation with the support.For the first time, chiral resolution of the racemic form has been achieved with the use of this achiral counter anion, rather than by using a chiral auxiliary. The respective bands from the column have been identified by a combination of a single-crystal X-ray determination and CD spectroscopy. Using the chiral auxiliaries (1)-O,O9-dibenzoyl-D-tartrate and (2)- O,O9-dibenzoyl-L-tartrate, it has become apparent that the choice of chirality of the counter anion becomes important, since it may either enhance or oppose the inherent chirality of the support material itself, and can prove critical in achieving a simple resolution of enantiomers.Experimental Instrumentation The NMR spectra were recorded on a Varian Unity Inova-500 spectrometer using the solvent as the internal reference, CD spectra in acetonitrile solution using a Jobin Yvon spectrophotometer (lmax/nm) and high-resolution mass spectra on a Bruker BioApex 47e ICR spectrometer with an electrospray source, using solutions ca. 2 mg cm23 in methanol. Microanalyses were carried out on a Carlo Erba EA 1108 CHNS analyser. For the column chromatography studies, preparative columns C16/ 100 and C26/100 were from Pharmacia Biotech. The semiquantitative column K9 (Pharmacia Biotech) was fitted with an external water-jacket, temperature regulated with a Talabo F10 circulatory thermostatted water-bath.Column flow rates were regulated with a Gilson minipulse 2 peristaltic pump. Materials The compounds 4,49-dimethyl-2,29-bipyridine (Aldrich), 2,29- bipyrimidine (Lancaster), 1,10-phenanthroline, 2,29-bipyridine and sodium toluene-4-sulfonate (Aldrich) and ruthenium trichloride hydrate (Strem) were used as received without further purification. Aqueous sodium (2)-O,O9-dibenzoyl-L-tartrate and sodium (1)-O,O9-dibenzoyl-D-tartrate solutions were prepared by the addition of sodium hydroxide solution to the corresponding acids (Fluka), until a pH of 8–9 was obtained.SP Sephadex C-25 and Sephadex LH20 in anhydrous form were from Pharmacia Biotech, Amberlite IRA 400 from Aldrich. The precursors [RuL2Cl2] (L = dmbpy, phen or bpy) were prepared according to the literature method.34 Laboratory-grade solvents were used unless otherwise specified. Complex syntheses [{Ru(dmbpy)2}2(Ï-bipym)][PF6]4. 2,29-Bipyrimidine (0.137 g, 0.86 mmol) and [Ru(dmbpy)2Cl2] (1.00 g, 1.73 mmol) in ethylene glycol (10 cm3) were heated on ‘medium heat’ for 10 min in a microwave oven, fitted with an external condenser.The crude mixture was diluted with water (200 cm3) and adsorbed on the top of a SP Sephadex C-25 column (dimensions 40 × 300 mm). On elution with 0.2 mol dm23 NaCl solution the first red band was removed and discarded. The second green band was eluted with 0.5 mol dm23 NaCl solution. The complex was precipitated with saturated KPF6 solution, the solid collected by vacuum filtration and washed with water (5 × 50 cm3).The dark green solid was dried in vacuo, yield 1.31 g (90%). A sample for elemental analysis was purified by passage through a short Sephadex LH20 column (eluent 50% methanol–acetone) (Found: C, 40.0; H, 3.5; N, 9.5. C56H54F24N12P4Ru2?2MeOH requires C, 40.0; H, 3.6; N, 9.6%). Further characterisation was made after diastereoisomeric isolation. Diastereoisomeric separation The diastereoisomeric mixture of [{Ru(dmbpy)2}2(m-bipym)]- [PF6]4 (250 mg) were converted into the chloride salts by metathesis with LiCl in acetone solution.The solid was collected by filtration through Celite“, and extracted with water. The resulting dark green solution (200 cm3) was introduced onto a SP Sephadex C-25 column (dimensions 26 × 1000 mm). Eluent flow was regulated by the use of a peristaltic pump. On elution with 0.25 mol dm23 sodium toluene-4-sulfonate solution the initial fast-moving pale red and green bands were rejected, while the first (meso) and second (rac) major dark green fractions were collected and the complexes precipitated by the addition of saturated aqueous KPF6 solution. The solids were extracted with dichloromethane, and the organic extracts dried with anhydrous Na2SO4.Following filtration, the solvent was evaporated and the residues dried in vacuo. Yields: meso, 80 mg, 32%; rac, 100 mg, 40%. These products were then converted into the chloride salt by passage of an aqueous solution down an Amberlite IRA 400 column for characterisation.meso: observed m/z 274.5663 (M41; most abundant isotope peak within cluster) (C56H54N12Ru2 requires 274.5675); 1H NMR d(D2O) 8.28 (4 H, s, dmbpy H3a and H3b), 8.12 (2 H, d, J 5.5, bipym H6), 7.84 (2 H, d, J 6.0, dmbpy H6a), 7.49 (2 H, d, J 6.0, dmbpy H6b), 7.38 (2 H, d, J 6.0, dmbpy H5a), 7.37 (2 H, d, J 6.0, bipym H5), 7.19 (2 H, d, J 6.0 Hz, dmbpy H5b), 2.47 [6 H, s, dmbpy CH3(4a)] and 2.43 [6 H, s, dmbpy CH3(4b)].rac: observed m/z 274.5663 (M41; most abundant isotope peak within cluster) (C56H54N12Ru2 requires 274.5675); 1H NMR d(D2O) 8.35 (2 H, s, bpy H3a), 8.31 (2 H, s, dmbpy H3b), 8.12 (2 H, d, J 5.5, bipym H6), 7.51 (2 H, d, J 6.0, dmbpy H6a), 7.43 (2 H, d, J 6.0, dmbpy H6b), 7.36 (2 H, d, J 6.0, bipym H5), 7.20 (2 H, d, J 6.0, dmbpy H5a), 7.16 (2 H, d, J 6.0 Hz, dmbpy H5b), 2.53 [6 H, s, dmbpy CH3(4a)] and 2.44 [6 H, s, dmbpy CH3(4b)].Resolution of the racemic form Using a similar method to that described above, [{Ru- (dmbpy)2}2(m-bipym)]Cl4 (ca. 50 mg) was introduced onto a column (dimensions 16 × 1000 mm). To increase the effective length of the column, once the Sephadex had equilibrated to the eluent a plunger was lowered onto the surface of the support and the system allowed to recycle. After the third passage down the column definite resolution had been achieved, and the two individual bands were collected and isolated as the hexafluorophosphate salts.Band 1, DD complex: CD lmax/ nm (CD3CN) 256 (De/dm3 mol21 cm21 165.4), 278 (2110.8), 300 (0.4), 318 (232.5), 392 (128.2) and 623 (24.3). Band 2, LL complex: CD lmax/nm (CD3CN) 256 (De/dm3 mol21 cm21 253.2), 278 (1109.1), 300 (7.1), 318 (132.5), 392 (222.6) and 607 (14.6).138 J. Chem. Soc., Dalton Trans., 1998, Pages 133–138 The complexes [{Ru(tmbpy)}2(m-bipym)]41, [{Ru(phen)2}2(mbipym)][ PF6]4 and [{Ru(bpy)2}2(m-bipym)][PF6]4 were prepared in an analogous fashion, and gave characterisations in accordance with the literature.12 Quantitative column techniques A small Perspex column (dimensions 9 × 600 mm), fitted with an insulated water-jacket connected to a circulating thermostatted water-bath, was set to a constant temperature (30 8C for all measurements, unless otherwise stated).SP Sephadex C-25 was equilibrated in the eluent solution [aqueous 0.1–1.0 mol dm23 sodium toluene-4-sulfonate for the variable-concentration data and 0.25 mol dm23 for the temperature-dependent measurements; 0.154 mol dm23 in the case of the investigations using sodium (1 or 2)-O,O9-dibenzoyl-(D or L)-tartrate solutions], and allowed to settle in the column at a constant eluent flow rate of approximately 0.5 cm3 min21.Once equilibration was obtained, the head of salt solution was reduced on the top of the support, and the compound to be separated was carefully introduced, dissolved in the eluent (0.25 mg of each isomer in 250 ml), so as not to disturb the Sephadex surface.With the sodium (1 or 2)-O,O9-dibenzoyl-(D or L)-tartrate eluents, each isomer/enantiomer was added separately, since separation down the column was not always possible. The rate of flow through the column was carefully monitored as was the rate of travel of the individual bands. To ensure standardised results, the data are given in the volume of solvent required to move the individual bands by unit length (cm3 cm21). Crystallography A sample of the first major fraction (meso) of [{Ru- (dmbpy)2}2(m-bipym)]41 was repurified by chromatography, using Sephadex LH20 as support with methanol as eluent to remove any excess of inorganic impurities.Following removal of the solvent, and redissolution of the residue in water, slow evaporation of the solution realised oily crystals which were suitable for X-ray determination. A unique room-temperature diffractometer data set (Enraf- Nonius CAD-4 diffractometer; T ª 295 K; monochromatic Mo-Ka radiation, l = 0.71073 Å; 2q–q scan mode) was measured, yielding No independent reflections, No with I > 3s(I) being considered ‘observed’ and used in the large-block leastsquares refinements.The crystal structure determination is of rather low precision as crystals of higher quality were elusive, and consequently the data were broad and weak from a poorly diffracting, decomposing specimen.Anisotropic thermal parameters were refined for Ru(1), Cl(1), Cl(2) and O(1) to O(5) only, due to limited data, and all other non-H atoms were refined isotropically. Hydrogen atoms were placed in calculated positions and not refined. Conventional residuals R, R9 on |F| are quoted, statistical weights derivative of s2(I) = s2(Idiff)1 0.0004s4(Idiff) being used. Neutral atom complex scattering factors were employed, and computation was by the XTAL 3.4 program system, implemented by S.R. Hall.35 Specific details are as follows: [{Ru(dmbpy)2}2(m-bipym)]Cl4?10H2O, C56H74Cl4N12O10Ru2, M 1419.24, monoclinic, space group P21/ n (no. 14), a = 10.87(1), b = 22.67(1), c = 13.88(1) Å, b = 111.37(9)8, U = 3187 Å3, Dc (Z = 2) 1.48 g cm23, F(000) 1460, 2qmax 458, N 4473, No 1446, R = 0.107, R9 = 0.106. CCDC reference number 186/767. Acknowledgements This work was supported by the Australian Research Council. We are grateful to Mr. Brian Foster for performing the microanalyses, and Dr.Ian Atkinson (James Cook University) and Mr. Rick Willis (Australian Institute of Marine Science) for undertaking the electrospray mass spectral measurements. Andreas Beyeler and Bruni Kolp (University of Fribourg, Switzerland) are thanked for their help with the acquisition of the CD spectra. References 1 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni, Chem. Rev., 1996, 96, 759 and ref. therein. 2 V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis Horwood, Chichester, 1991. 3 J.-P.Sauvage, J.-P. Collin, J. C. Chambron, S. Guillerez, C. Coudret, V. Balzani, F. Barigelletti, L. De Cola and L. Flamigni, Chem. Rev., 1994, 94, 993. 4 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304. 5 A. M. Pyle and J. K. Barton, Prog. Inorg. Chem., 1990, 38, 413. 6 A. Kirsch-De Mesmaeker, J. P. Lecomte and J. M. Kelly, Top. Curr. Chem., 1996, 177, 25. 7 E. D. A. Stemp, M. R. Arkin and J. K. Barton, J.Am. Chem. Soc., 1997, 2921. 8 P. Lincoln and B. Nordén, Chem. Commun., 1996, 2145. 9 P. Lincoln, A. Broo and B. Nordén, J. Am. Chem. Soc., 1996, 118, 2644. 10 J. K. Barton, A. Danishefsky and J. Goldberg, J. Am. Chem. Soc., 1984, 106, 2172. 11 C. Hiort, P. Lincoln and B. Nordén, J. Am. Chem. Soc., 1993, 115, 3448. 12 X. Hua and A. von Zelewsky, Inorg. Chem., 1995, 34, 5791. 13 T. J. Rutherford, M. G. Quagliotto and F. R. Keene, Inorg. Chem., 1995, 34, 3857. 14 G. H. Searle, Aust. J. Chem., 1977, 30, 2625. 15 H. Yoneda, J. Chromatogr., 1984, 313, 59. 16 Y. Yoshikawa and K. Yamasaki, Coord. Chem. Rev., 1979, 28, 205. 17 B. T. Patterson and F. R. Keene, 1997, Inorg. Chem., in the press. 18 T. J. Rutherford and F. R. Keene, Inorg. Chem., 1997, 36, 3580. 19 T. J. Rutherford, O. Van Gijte, A. Kirsch-De Mesmaeker and F. R. Keene, Inorg. Chem., 1997, 36, 4465. 20 T. J. Rutherford, D. A. Reitsma and F. R. Keene, J. Chem. Soc., Dalton Trans., 1994, 3659. 21 D. A. Reitsma and F. R. Keene, J. Chem. Soc., Dalton Trans., 1993, 2859. 22 L. S. Kelso, D. A. Reitsma and F. R. Keene, Inorg. Chem., 1996, 35, 5144. 23 F. R. Keene, Coord. Chem. Rev., 1997, in the press. 24 T. J. Rutherford and F. R. Keene, unpublished work. 25 D. A. Reitsma and F. R. Keene, unpublished work. 26 D. A. Bardwell, J. C. Jeffery, L. Joulié and M. D. Ward, J. Chem. Soc., Dalton Trans., 1993, 2255. 27 D. A. Bardwell, L. Horsburgh, J. C. Jeffery, L. F. Joulié, M. D. Ward, I. Webster and L. J. Yellowlees, J. Chem. Soc., Dalton Trans., 1996, 2527. 28 R. Hage, J. G. Haasnoot, H. A. Nieuwenhuis, J. Reedijk, D. J. A. De Ridder and J. G. Vos, J. Am. Chem. Soc., 1990, 112, 9245. 29 V. Balzani, D. A. Bardwell, F. Barigelletti, F. L. Cleary, M. Guardigli, J. C. Jeffery, T. Sovrani and M. D. Ward, J. Chem. Soc., Dalton Trans., 1995, 3601. 30 F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962. 31 N. C. Fletcher, P. C. Junk and F. R. Keene, unpublished work. 32 F. R. Keene and G. H. Searle, Inorg. Chem., 1974, 13, 2173. 33 B. Kolp, H. Viebrock, A. von Zelewsky and D. Abeln, personal communication. 34 P. A. Lay, A. M. Sargeson and H. Taube, Inorg. Synth., 1986, 24, 291. 35 S. R. Hall, G. S. D. King and J. M. Stewart, Xtal3.4 User’s Manual, University of Western Australia, Lamb, Perth, 1995. Received 13th August 1997; Paper 7/05947G

ISSN:1477-9226    

DOI:10.1039/a705947g

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 139–145 139 Low-spin octahedral cobalt(II) complexes of CoN6 and CoN4P2 chromophores. Synthesis, spectroscopic characterisation and electrontransfer properties † Bidyut Kumar Santra and Goutam Kumar Lahiri * Department of Chemistry, Indian Institute of Technology, Bombay Powai, Mumbai-400076, India The reaction of 2-(arylazo)pyridines (NC5H4)N]] NC6H4R L1–L7 (R = H, o-Me/Cl, m-Me/Cl, p-Me/Cl) with cobalt(II) perchlorate hexahydrate in absolute ethanol under anaerobic conditions afforded low-spin [CoIIL3]21 complexes, isolated as ClO4 2 salts.At room temperature the complexes are one-electron paramagnetic in nature, low-spin CoII, t2g 6eg 1, S = ��� and behave as 1 : 2 electrolytes in acetonitrile solvent. In acetonitrile solvent they show a ligand-to-metal charge-transfer (LMCT) band near 400 nm, an intraligand transition near 300 nm and ligand- field d–d transitions in the range 860–600 nm. The complexes exhibit quasi-reversible CoII]CoIII couples near 1 V and six sequential ligand reductions (N]] N groups) in the range 0.2 to 21.8 V versus saturated calomel electrode (SCE).At room temperature in the solid state they exhibit isotropic EPR spectra but at 77 K, both in the polycrystalline state and in the dichloromethane solution, display rhombic spectra. Reaction of [CoIIL3]21 with 2,29-bipyridine (bpy) and 1,10-phenanthroline (phen) resulted in complete ligand-exchanged products with concomitant metal oxidation, low-spin [CoIII(bpy)3]31 and low-spin [CoIII(phen)3]31 respectively. The reaction of PPh3 with the [CoII(L7)3]21 [L7 = 2-(p-chlorophenylazo)pyridine] yielded a partial ligand-exchanged product, low-spin [CoII(L7)2(PPh3)2]21, isolated as its ClO4 2 salt.The complex is one-electron paramagnet and a 1 : 2 electrolyte in acetonitrile solvent. It displays an LMCT band at 401 nm, an intraligand transition at 305 nm and four d–d transitions in the range 870–640 nm.It exhibits irreversible CoII to CoIII oxidation at 1.33 V (Epa) and four successive ligand reductions in the range 20.30 to 21.1 V versus SCE. At 77 K the complex displays an axial EPR spectrum. Cobalt(II) ion in octahedral complexes prefers to stabilise in high-spin configuration. Low-spin octahedral cobalt(II) species are rare.1 Most of the available octahedral cobalt(II) complexes exist either in high-spin or in high-spin/low-spin equilibrium.Few authentic examples of spin-paired octahedral cobalt(II) species are known.2 The present work originates from our interest to develop octahedral tris complexes of cobalt(II) in low-spin state of the CoN6 chromophoric class and to study their physicochemical properties. The other known [CoII(L]L)3] chromophoric complexes stabilise CoII either in high-spin state (e.g. L]L = 2,29- bipyridine, 1,10-phenanthroline or ethane-1,2-diamine) or in high-spin/low-spin equilibrium [e.g.L]L = CH3N]] C(CH3)- (CH3)C]] NCH3].3,4 As a part of our programme we have chosen the 2-(arylazo)pyridine ligands, (NC5H4)N]] NC6H4R (L). Their interaction with metal ions has been explored extensively in recent times. The complexes have shown various fascinating metal- as well as ligand-based chemical and electrochemical properties, such as metal-ion-mediated thiolation and hydroxylation of the pendant phenyl ring of L,5 metal-ion-assisted cleavage of the azo (N]] N) bond,6 isomerisation,7 use as chemical and electrochemical oxidants,8 oxo-transfer reaction,9 catalysis and electrocatalysis.8 The strong p-acidic property and asymmetric nature of the azopyridine ligands make them susceptible to chemical and electrochemical activities.Although the chemistry of many metal complexes of L have been emerged from the recent studies, those of cobalt have not been progressed so far. The richness of the ligating properties of 2- (arylazo)pyridine ligands has inspired us to study the hitherto unknown cobalt complexes and this study has led to the formation of very rare low-spin cobalt(II) tris chelates, [CoIIL3]21.To the best of our knowledge this work demonstrates the first authentic example of low-spin tris-octahedral cobalt(II) complexes of the CoN6 chromophoric class. Herein we report the † Non-SI units employed: mB ª 9.27 × 10224 J T21, G = 1024 T. synthesis, spectroscopic, electron-transfer properties and reactivities of a group of low-spin [CoIIL3]21 complexes.Results and Discussion Synthesis Seven substituted 2-(arylazo)pyridine ligands used for the present study are abbreviated as L1–L7 respectively. The ligand L binds to the metal ions in a bidentate Np, Na manner forming a five-membered chelate ring, ML. The brown complexes [CoIIL3]21 1–7 have been synthesized by stirring CoII(ClO4)2? 6H2O and the appropriate L1–L7 in a stoichiometric 1 : 3 ratio in dry ethanol under a dinitrogen atmosphere, equation (1), and CoII(ClO)4?6H2O 1 3L dry ethanol N2 [CoIIL3][ClO4]2?H2O (1) isolated as monohydrated perchlorate salts.The use of absolute ethanol is essential to get the pure product in the solid state. If 95% ethanol is used only an impure gummy product is obtained Np N Na R 2 3 4 5 6 7 8 9 10 11 12 Np Mn + Na ML L1 R = H L2 R = 8-Me L3 R = 8-Cl L4 R = 9-Me L5 R = 9-Cl L6 R = 10-Me L7 R = 10-Cl140 J. Chem. Soc., Dalton Trans., 1998, Pages 139–145 Table 1 Microanalytical,a magnetic moment,b conductivity c and IR d data Elemental analysis (%) IR (cm21) Compound 1 2 3 4 5 6 7 8 C 48.11 (48.0) 49.72 (49.83) 42.55 (42.65) 49.97 (49.83) 42.73 (42.65) 49.97 (49.83) 42.53 (42.65) 59.27 (59.19) H 3.46 (3.51) 4.13 (4.04) 2.76 (2.80) 3.98 (4.04) 2.84 (2.80) 4.07 (4.04) 2.82 (2.80) 3.68 (3.78) N 15.18 (15.27) 14.59 (14.53) 13.68 (13.57) 14.44 (14.53) 13.69 (13.57) 14.59 (14.53) 13.64 (13.57) 6.99 (6.90) meff/mB 1.98 2.10 2.12 1.95 1.99 2.09 2.13 2.05 LM/W21 cm21 mol21 280 290 300 285 297 305 302 304 n(N]] N) 1485 1455 1472 1453 1459 1479 1472 1448 n(ClO4) 1104, 630 1097, 630 1111, 637 1089, 624 1094, 617 1091, 637 1088, 635 1090, 620 a Calculated values are in parentheses. b In the solid state at 298 K.c In acetonitrile solution. d In KBr disc. after removal of solvent in vacuo, which is difficult to purify. For the ligands L1 and L4 pure tris complexes separated directly from the reaction mixture, but for the other ligands (L2, L3, L5– L7) no solid mass precipitated directly.However, the addition of an excess of aqueous NaClO4 to the above concentrated alcoholic solution resulted in an impure precipitate. Pure tris complexes 2, 3 and 5–7 were obtained by washing the above solid several times with hexane. The complexes can also be prepared starting from CoCl2? 6H2O in methanol. Here addition of an excess of aqueous NaClO4 to the concentrated initial brown solution yielded an impure solid mass, from which the pure tris complexes can be obtained by thorough washing with hexane.All our attempts to prepare mixed-ligand complexes such as [CoL2Cl2] by using different metal : ligand ratios have failed; in all the cases they ended up with the [CoL3]21 tris complex. Although identical products can be obtained from both routes, the use of the first method appears to be more facile from the yield point of view. The complexes are highly soluble in polar solvents such as acetonitrile, dimethylformamide (dmf) and dimethyl sulfoxide (dmso), moderately soluble in dichloromethane and chloroform and slightly soluble in benzene and water.The extent of solubility varies depending on the nature of the substituents present in the ligand framework. The microanalytical data (C, H, N) of the complexes listed in Table 1 are in good agreement with the calculated values, which confirms the gross composition of the tris chelates, [CoL3][ClO4]2?H2O.The complexes are 1 : 2 electrolytes in acetonitrile solution (Table 1). Solid-state magnetic moment measurements at 298 K established that they are uniformly one-electron paramagnets (Table 1), i.e. they possess the low-spin t2g 6eg 1 (S = ��� ) configuration. It is believed that the complexes have the sterically favourable meridional configuration (see later). Infrared spectra The Fourier-transform f the complexes were recorded as KBr discs in the range 4000–400 cm21.Selected band positions are depicted in Table 1. A very strong and broad vibration near 1100 cm21 and a strong and sharp vibration near 630 cm21 are observed for all the complexes due to the presence of ionic perchlorate. A strong and sharp band near 1600 cm21 is assigned to n(C]] C) 1 n(C]] N) stretchings. The n(N]] N) vibration is observed near 1450 cm21 as a sharp peak. The stretching vibration of the water of crystallisation appears near 3400 cm21 as a broad peak. The other expected vibrations are systematically present for all the complexes. Electronic spectra Solution electronic spectra of the complexes were recorded in acetonitrile solvent in the UV/VIS region (250–1100 nm).The data are listed in Table 2 and a representative spectrum is shown in Fig. 1. In the visible region the complexes exhibit one intense shoulder near 400 nm and a very intense sharp band near 300 nm. On the basis of their high intensities these two bands are assigned as charge transfer in nature.Since CoII in the complexes is in the low-spin t2g 6eg 1 configuration, the band near 400 nm may be due to a ligand-to-metal charge-transfer transition. 10 The very intense band near 300 nm is presumably due to an intra-ligand p–p*/or n–p* transition.5b Here both bands are sensitive to the nature of the substituents present in the ligand framework. In the lower-energy part of the visible region all the complexes systematically display four weak transitions (Table 2).Based on the low intensities of these bands they are considered to be possible d–d transitions. In view of the molecular asymmetry in the meridional complexes 1–7, the lifting of Fig. 1 Electronic spectrum of [CoII(L1)3][ClO4]2?H2O 1 in acetonitrile solvent. The inset shows low-energy d–d transitions Table 2 Electronic spectral data in acetonitrile at 298 K Coml/ nm (e/dm3 mol21 cm21) pound d–d Transitions Charge-transfer transitions 1 2 3 4 5 6 7 8 862 (322) 875 (390) 817 (128) 890 (220) 880 (205) 850 (460) 840 (200) 870 (1335) 776 (447) 782 (540) 760 (200) 770 (250) 782 (284) 770 (630) 760 (280) 760 (1770) 702 (456) 684 (633) 690 (290) 680 (300) 696 (442) 685 (700) 678 (300) 655 (1640) 650 (429) 638 (700) 615 (353) 610 (350) 592 (700) 600 (720) 612 (340) 644 (1273) 392 (18 356) 386 (9820) 387 (7020) 405 (13 550) 385 (10 100) 407 (23 947) 398 (14 370) 401 (4450) 318 (37 140) 305 (39 700) 314 (39 650) 322 (35 400) 318 (50 300) 327 (39 640) 324 (36 200) 305 (8140)J.Chem. Soc., Dalton Trans., 1998, Pages 139–145 141 orbital degeneracy of the states leads to a greater number of d–d transitions and some of them are actually distinctly resolved for all the complexes.11 The intensities of the d–d bands are found to be high. This is possibly due to the influence of nearby intense charge-transfer transitions. Electron-transfer properties Electron-transfer properties of the complexes have been studied in acetonitrile solution by cyclic voltammetry (CV) using a platinum working electrode at 298 K.The complexes are electroactive with respect to the metal as well as ligand centres and display six reversible redox processes in the potential range ±2 V versus the saturated calomel electrode (SCE). Tetraethylammonium perchlorate was used as electrolyte. Representative voltammograms are shown in Fig. 2 and reduction potential data are in Table 3. The responses are quasi-reversible, the peak-to-peak separations of the couples lying in the range 70–120 mV.The assignments of the responses to the specific couples I–VI in Table 3 were made based on the following considerations. The cobalt(III)–cobalt(II) couple. All the complexes display one quasi-reversible oxidative response near 1 V which is assigned to cobalt(II) to cobalt(III) oxidation, equation (2). The [CoIIIL3]31 1 e2 [CoIIL3]21 (2) Fig. 2 Cyclic voltammograms (scan rate 50 mV s21) of a ª1023 mol dm23 solution of complex 1 in acetonitrile at 298 K Table 3 Electrochemical data at 298 K a CoIII]CoII, Es 298/V (DEp/mV) Ligand reductions, Es 298/V (DEp/mV) Compound 1 2 3 4 5 6 7 8 Couple I 1.09 (120) 0.95 (100) 1.13 (100) 1.03 (110) 1.36 (120) 0.99 (100) 1.20 (120) 1.33 2 (Epa) b II 0.12 (80) 0.00 (100) 0.22 (110) 0.11 (70) 0.215 (100) 0.03 (100) 0.19 (110) 0.33 (60) III 20.42 (80) 20.46 (80) 20.40 (100) 20.45 (80) 20.32 (90) 20.48 (80) 20.31 (60) 20.67 (60) IV 20.84 (65) 20.90 (80) 20.83 (100) 20.86 (70) 20.68 (60) 20.88 (70) 20.69 (80) 21.07 (110) V 21.27 (100) 21.37 (90) 21.09 (100) 21.25 (90) 21.110 (90) 21.31 (80) 21.15 (100) VI 21.64 (70) 21.80 (70) 21.39 (100) 21.67 (70) 21.43 (80) 21.73 (80) 21.47 (80) a Conditions: solvent, acetonitrile; supporting electrolyte, NEt4ClO4; reference electrode, SCE; solute concentration, 1023 mol dm23, working electrode, platinum.Cyclic voltammetric data: scan rate, 50 mV s21; Es 298 = 0.5 (Epa 1 Epc) where Epc and Epa are the cathodic and anodic peak potentials respectively.b Considered due to irreversible nature of the voltammogram. one-electron nature of the couple, equation (2), is confirmed by constant-potential coulometry. Although the yellow solution of [CoL3]31 can be generated by coulometry, the oxidised solutions are unstable which has precluded the further characterisation of the oxidised trivalent [CoIIIL3]31 species. The formal potential of the couple [equation (2)] varies depending on the nature and the position of the substituents present in the ligand framework, e.g.the electron-donating Me group lowers the potential and the presence of the electron-withdrawing Cl2 makes the formal potential greater than for the unsubstituted ligand. The formal potential of the couple (2) follows the order 2 < 6 < 4 < 1 < 3 < 7 < 5 (Table 3). Under identical experimental conditions the oxidation of the [Co(bpy)3]21 tris chelate (bpy = 2,29-bipyridine) takes place at 0.2 V versus SCE, i.e.it is easily oxidisable to the cobalt(III) congener.12 Thus the azopyridine ligand (L) can act as a much better stabiliser of cobalt(II) ion compared to the bpy ligand, which is of course due to the stronger p-acidic property of L.13 This result is in accordance with the earlier observations on other metal complexes of L.14 Ligand reduction. The azopyridines (L) are well known electron-transfer centres.Thus each ligand can accommodate two electrons in one electrochemically accessible lowest unoccupied molecular orbital (LUMO) which is primarily azo in character.15 Since the complexes contain three electroactive ligands, six successive reductions are therefore expected for each complex in principle. All the six expected reductions are actually observed in careful cyclic voltammetric experiments, equations (3)–(8). The reduction potentials data are listed in [CoIIL3]21 1 e2 [CoIIL2L~2]1 (3) [CoIIL2L~2]1 1 e2 [CoIILL~2 2] (4) [CoIILL~2 2] 1 e2 [CoIIL~2 3]2 (5) [CoIIL~2 3]2 1 e2 [CoIIL~2 2L2~2]22 (6) [CoIIL~2 2L2~2]22 1 e2 [CoIIL~2L2~2 2]32 (7) [CoIIL~2L2~2 2]32 1 e2 [CoIIL2~2 3]42 (8) Table 3.The formal potential for the first reduction of L in cobalt complexes is uniformly more positive than that of free L, which is due to the positive charge of the metal ion. The observation of the complete set of six reductions in the tris chelate is rare. For [Co(bpy)3]21 only four of the six are detected experimentally.12 The one-electron nature of the first reduction [equation (3) and couple II in Fig. 2] is confirmed by constant-potential coulometry in acetonitrile solvent. The blue reduced solution is unstable, however we have managed to check the EPR spectrum by quickly freezing the reduced blue solution in liquid nitrogen (77 K). The starting [CoIIL3]21 complexes are one-electron paramagnets and EPR active (see later) but the one-electronreduced solutions are EPR silent. This indicates that the two unpaired electrons which are present in the reduced complexes [CoIIL2L~2]1 (one electron on the metal centre and the other on the ligand centre) are antiferromagnetically coupled.The extreme reactive nature of the other electrochemically reduced species did not allow us to study the reductions by spectroelectrochemical means. The one-electron nature of the other reductions [equations (4), (5) and (8); couples III, IV and VI in Fig. 2] is confirmed from current-height considerations.A direct comparison of the current height of couple V with those of the other couples suggests that V corresponds to a two-electron transfer and implies that the reductions corresponding to equations (6) and (7) have taken place simultaneously (see couple V, Fig. 2). Chemical reduction of the starting complex142 J. Chem. Soc., Dalton Trans., 1998, Pages 139–145 [CoIIL3]21 by hydrazine hydrate in acetonitrile solvent also generated the same blue unstable reduced solution, which is also EPR silent.Electron paramagnetic resonance spectra of [CoL3]21 complexes Consistent with the low-spin configuration, the [CoL3]21 complexes displayed EPR spectra both in solid and solution states. A representative spectrum for one complex (6) is shown in Fig. 3. The EPR spectra in the solid state for one representative complex (6) and in solution for all the complexes have been studied. At both room and liquid-nitrogen temperature (77 K) in the polycrystalline state, complex 6 displays an EPR spectrum consistent with low molecular symmetry and hyperfine coupling to the 59Co nucleus (I = 7 2 – , 100% abundant). As the three components of the g tensor overlap severely we have been unable to assign the spectrum and hence derive either the g values or hyperfine couplings. Dichloromethane solutions of all complexes at 77 K display similar but better resolved spectra (Fig. 3). The ‘crossing-point’ (g1 in Fig. 3) is found at g = 2.117 for 1, 2.079 for 2, 2.134 for 3, 2.162 for 4, 2.164 for 5, 2.166 for 6 and 2.167 for 7. Hyperfine coupling and the anisotropy of the g tensor leads to the signal being 618 G wide on average. There is also the possibility of superhyperfine coupling to N atoms of the ligands. Isomer preference Although the presence of asymmetric ligands in the [CoL3]21 tris chelates allows the possibility of two geometrical isomers, meridional A and facial B, only one isomer has been consistently obtained experimentally for all the complexes.Since the spectral features of all complexes 1–7 are very akin, we therefore logically assume that they have the same isomeric structure. The paramagnetic nature of the complexes has prevented the identification of the specific geometry by NMR techniques. However, angular-overlap considerations strongly favour meridional geometry for the low-spin d7, cobalt(II) case.16 Fig. 3 X-Band EPR spectrum of complex 6 in dichloromethane solution at 77 K. The inner scale indicates the edge-to-edge linewidth of the spectrum; tcne = tetracyanoethylene Sterically the meridional geometry is generally more favoured. Similar ruthenium and iron tris chelates ([RuL3]21, [FeL3]21) have also been isolated in meridional form. Thus the collective considerations of angular-overlap, steric factors, spectral features and the earlier ruthenium and iron cases lead us to believe the existence of a meridional geometry for the present complexes.Spin-state preference Octahedral cobalt(II) complexes are known to prefer high-spin configuration. Low-spin cobalt(II) octahedral complexes can be expected only in the presence of a sufficiently strong ligand field (Do > 15 000 cm21) which is required to get a 2E ground state, originating from the 2G state of the free ion.1b Owing to the non-availability of a sufficient number of ligand systems which can provide the minimum requirement of ligand-field strength, the low-spin configuration of CoII in octahedral arrangement is rare.The ligand-field strength of (arylazo)pyridine ligands (L) makes them appropriate candidates to facilitate the preferential formation of unusual low-spin octahedral cobalt(II) complexes. Reactions of [CoL3]21 with other strong �-acidic ligands (a) Complete ligand exchange with concomitant metal oxidation. Since unlike the azopyridine ligands (L) the other well known strong p-acidic ligands such as bipyridine and phenanthroline stabilise CoII in the high-spin state in the respective tris chelates, it is therefore interesting to prepare mixed-ligand complexes comprising two such ligands of type [CoIIL32xL9x]21 (L9 = bipyridine or phenanthroline, x = 1 or 2) to see the effect on magnetic and spectral features.Thus [CoL3]21 was treated with bpy and phen in alcoholic solution but surprisingly the [CoL3]21 complexes underwent complete ligand-exchange reaction and eventually yielded known low-spin [CoIII(bpy)3]31 and [CoIII(phen)3]31 complexes respectively.3a The ligand L is believed to have a greater ligand-field strength compared to bpy and phen and that is why [CoL3]21 complexes are stabilised in the low-spin state whereas the corresponding bpy and phen tris chelates are in the high-spin state.The driving force for the facile complete exchange of the strong p-acidic ligand L by the bpy and phen ligands is not clearly understood. The presence of cobalt(III) ion and the existence of a low-spin state in the tris-bpy and -phen complexes are understandable.A low CoII]CoIII oxidation potential (ª0.2 V versus SCE) is the driving force to stabilise the metal ion in the trivalent state and the low-spin state is the preferred configuration for the cobalt(III) octahedral complexes. (b) Partial ligand exchange without metal oxidation. The reaction of an excess of triphenylphosphine with [CoL3]21 in methanol solvent resulted in green mixed-ligand complexes of type [CoL2(PPh3)2]21, equation (9).Here one ligand L from the [CoL3]21 has been exchanged by the two monodentate phosphine ligands and the exchange reaction has taken place without any change in metal oxidation state and spin configuration. Complex 8 has been isolated as its perchlorate salt. Although all the tris complexes 1–7 react similarly with PPh3 and result in Na Np Na Np Np Na Na Na Na Np Np Np A BJ.Chem. Soc., Dalton Trans., 1998, Pages 139–145 143 similar green complexes a detailed study has been performed only for complex 7. Complex 8 is moderately soluble in non-polar solvents (CH2Cl2, CHCl3, benzene) and highly soluble in polar solvents (CH3CN, dmf and dmso). In CH3CN it is a 1 : 2 electrolyte. The one-electron paramagnetic nature of the complex has been established by solid-state magnetic moment measurement at 298 K (low-spin CoII, S = ��� ).Microanalytical data (C, H, N) (Table 1) for the complex support the composition [CoII(L7)2- (PPh3)2][ClO4]2. The Fourier-transform IR spectrum displays perchlorate vibrations at 1090 and 620 cm21 and phosphine vibrations at 720 and 510 cm21. The azo (N]] N) vibration appears at 1448 cm21 (Table 1). All other expected vibrations due to ligand L are systematically present. The 31P NMR spectrum in CDCl3 exhibits one sharp signal at d 29.2 which supports the trans configuration of the two PPh3 groups, as opposed to a cis arrangement.Such a configuration is expected from a steric point of view.17 In acetonitrile solvent complex 8 exhibits several bands in the UV/VIS region (250–1100 nm), Table 2, and the spectrum is shown in Fig. 4. The band at 401 nm is assigned to a ligand-tometal charge-transfer transition and that at 305 nm is believed to be due to a ligand-based transition. In addition four more transitions have been observed in the lower energy part of the visible region (Table 2, Fig. 4) as for the starting [CoIIL3]21 complexes but here the bands are much more intense (Table 2). These four bands could be due to possible low-energy d–d transitions which might have originated from the lifting of orbital degeneracy of the states in the tetragonally distorted low-spin cobalt(II) complex. The relatively high intensity of the bands is not clearly understandable, however the influence of the tail of the nearby charge-transfer transitions may be responsible for this.Electron-transfer properties of complex 8 have been studied in acetonitrile solvent using a platinum working electrode. Reduction potential values are given in Table 3 and the voltammograms are shown in Fig. 5. In acetonitrile solution the complex displays one ible oxidation process (anodic peak, Epa) at 1.33 V versus SCE (couple I). No significant Fig. 4 Electronic spectrum of [CoII(L7)2(PPh3)2][ClO4]2 8 in acetonitrile solvent.The inset shows low-energy d–d transitions Np Na Np Na CoII PPh3 PPh3 Cl Cl 2+ + L7 [CoII(L7)3][ClO4]2•H2O + PPh3 MeOH heat (9) 8 response on scan reversal in cyclic voltammetry is observed (Fig. 5). The oxidised complex thus decomposes rapidly on the cyclic voltammetric time-scale. This irreversible oxidative response is assigned to cobalt(II) to cobalt(III) oxidation [equation (10)]. Under identical experimental conditions the [CoII(L7)2(PPh3)2]21 1 e2 [CoIII(L7)2(PPh3)2]31 (10) Epa of the cobalt(II)–cobalt(III) couple of the corresponding [Co(L7)3]21 complex appears at 1.26 V.The observed 70 mV positive shift of the cobalt(II)–cobalt(III) oxidation potential on moving from complex 7 to 8 reveals that the (arylazo)pyridine ligand and phosphine together endow superior redox stability to cobalt(II). Complex 8 contains two electroactive ligands L having one azo group in each, therefore four one-electron ligand-based reductions are expected, equations (11)–(14).Cyclic voltam- [CoII(L7)2(PPh3)2]21 1 e2 [CoIIL7(L7)~2(PPh3)2]1 (11) [CoIIL7(L7)~2(PPh3)2]1 1 e2 [CoII(L7)~2 2(PPh3)2] (12) [CoII(L7)~2 2(PPh3)2] 1 e2 [CoII(L7)~2(L7)2~2(PPh3)2]2 (13) [CoII(L7)~2(L7)2~2(PPh3)2]2 1 e2 [CoII(L7)2~2 2(PPh3)2]22 (14) mograms of complex 8 exhibit three reversible reductions, couples II, III and IV (Fig. 5) at 20.33, 20.67 and 21.07 V (Table 3) versus SCE respectively. The one-electron nature of couples II and III and the two-electron stoichiometry of IV are established by cyclic voltammetric current-height considerations.Thus all the expected four ligand-based reductions are observed experimentally. Instead of getting all the four oneelectron reductions separately, the first two [equations (11) and (12)] appear distinctly (couples II and III) and the other two [equations (13) and (14)] are overlapped at 21.07 V (Fig. 5, couple IV). The EPR spectrum of complex 8 was recorded in a chloroform –toluene (1 : 1) glass at 77 K.It is much simpler than those found for 1–7 and a tentative assignment is possible. The spectrum appears to be axial, consistent with the molecular symmetry for 8 assuming trans-PPh3 ligands. A hyperfine pattern can be discerned on the g|| component (see Fig. 6). The parameters derived from this assignment are g|| = 2.009, g^ = 2.003, A|| = 6.78 G. These values would be consistent with the unpaired electron residing in the dx22y2 orbital. Conclusion We have thus observed that (arylazo)pyridine ligands are appropriate candidates to stabilise the cobalt ion preferentially Fig. 5 Cyclic voltammograms (scan rate 50 mV s21) of a ª1023 mol dm23 solution of complex 8 in acetonitrile at 298 K144 J. Chem. Soc., Dalton Trans., 1998, Pages 139–145 in the bivalent state and can facilitate the formation of unusual low-spin cobalt(II) complexes in octahedral arrangement. The (arylazo)pyridines alone or in combination with a phosphine generate high-potential cobalt(II) tris chelate or mixed-ligand complexes.The complexes have shown sequentially a complete set of electron-transfer processes which are not often observable. Experimental Materials Cobalt carbonate (Juhn Baker Inc. Colorado, USA) was converted into cobalt perchlorate by a standard method. Other chemicals and solvents were reagent grade and used as received. Silica gel (60–120 mesh) used for chromatography was of BDH quality. For spectroscopic/electrochemical studies HPLC grade solvents were used.Commercial tetraethylammonium bromide was converted into pure tetraethylammonium perchlorate by an available procedure.18 Dinitrogen gas was purified by successive bubbling through alkaline dithionite and concentrated sulfuric acid. Physical measurements Solution electrical conductivity was checked using a Systronic conductivity bridge-305. Electronic spectra (1100–250 nm) were recorded using a Shimadzu-UV-160A spectrophotometer, FTIR spectra on a Nicolet spectrophotometer with samples prepared as KBr pellets.Magnetic susceptibility was checked with a PAR vibrating-sample magnetometer. The 31P NMR spectra were obtained with a 300 MHz Varian Fouriertransform spectrometer. Cyclic voltammetry was carried out using a PAR model 362 scanning potentiostat electrochemistry system. A platinum-wire working electrode, a platinum-wire auxiliary electrode, and an aqueous saturated calomel reference electrode were used in a three-electrode configuration.A PAR model 279 digital coulometer was used for coulometry. The supporting electrolyte was NEt4ClO4 and the solute concentration was 1023 mol dm23. The half-wave potential Es 298 was set equal to 0.5 (Epa 1 Epc), where Epa and Epc are the anodic and cathodic cyclic voltammetric peak potentials respectively. The scan rate used was 50 mV s21. All the experiments were carried out under a dinitrogen atmosphere. Electrochemical data were collected at 298 K and are uncorrected for the junction potential.The EPR measurements were made with a Varian model 109C E-line X-band spectrometer fitted with a quartz dewar for measurements at 77 K (liquid nitrogen). Spectra were Fig. 6 X-Band EPR spectrum of complex 8 in a chloroform–toluene (1 : 1) glass at 77 K calibrated by using tetracyanoethylene (g = 2.0023). The elemental analyses (C, H, N) were carried out with a Carlo Erba (Italy) elemental analyser. Preparation of complexes Compounds L1–L7 were synthesized by condensing 2-aminopyridine with the appropriate nitrosobenzene following the available procedure.19 Complexes 1 and 4 were prepared by a general method, details are given for 1.Tris[2-(phenylazo)pyridine]cobalt(II) perchlorate monohydrate [CoII(L1)3][ClO4]2?H2O 1. 2-(Phenylazo)pyridine L1 (1.0 g, 0.005 mol) was taken in absolute ethanol (10 cm3) and stirred for 10 min under a dinitrogen atmosphere. Cobalt(II) perchlorate hexahydrate (0.65 g, 0.0017 mol) in absolute ethanol (10 cm3) was added to the solution and the mixture was stirred for 5 h under a dinitrogen atmosphere.The brown precipitate thus obtained was filtered off and washed with absolute ethanol. The solid product was then dried under vacuum over P4O10. Yield 1.6 g (71%). Complexes 2, 3 and 5–7 were prepared by following the above procedure but no solid mass was obtained from the reaction mixture. Therefore the volume of the solution was reduced to 5 cm3 under reduced pressure and an aqueous saturated solution of sodium perchlorate was added.The concentrated solution was kept in a refrigerator for 2 h. The brown precipitate thus obtained was filtered off and washed with a little (2 cm3) icecold water and dried under vacuum over P4O10. The complexes were further purified by washing several times with n-hexane and dried in vacuo over P4O10. Yield 60–70%. Bis[(2-p-chlorophenylazo)pyridine]bis(triphenylphosphine)- cobalt(II) perchlorate, [CoII(L7)2(PPh3)2][ClO4]2 8.To a methanolic solution (25 cm3) of the complex [Co(L7)3][ClO4]2?H2O 7 (0.2 g, 0.21 mmol) was added an excess of triphenylphosphine (0.17 g, 0.65 mmol). The mixture was heated to reflux for 6 h. The initial brown colour of 7 gradually changed to green. The progress of the reaction was monitored by TLC. The solvent was then evaporated to obtain a green gummy solid which was redissolved in acetonitrile (1 cm3). An aqueous solution of saturated sodium perchlorate was added and the mixture was kept in a refrigerator overnight.The green solid thus obtained was filtered off, washed with diethyl ether and ice-cold water and dried in vacuo over P4O10. The dried product was dissolved in a small volume of chloroform and subjected to chromatography on a silica gel (60–120 mesh) column. With benzene as the eluent, the light yellow solution due to liberated L7 separated first was rejected. Using a chloroform–acetonitrile (5 : 1) solution as eluent a green band separated.The green fraction was collected and evaporation of the solvents under reduced pressure yielded pure [CoII(L7)2(PPh3)2][ClO4]2 8. The yield was 215 mg, 82%. Acknowledgements Financial support received from the Department of Science and Technology, New Delhi, India, is gratefully acknowledged. We are grateful to Professor A. Q. Contractor, Indian Institute of Technology (I.I.T.) Bombay, for providing the electrochemical instrumental facility. Special acknowledgement is made to Regional Sophisticated Instrumental centre, I.I.T.for providing NMR and EPR facilities. The suggestions of the reviewers at the revision stage were very helpful. References 1 C. Ohrenberg, P. Ge, P. Schebler, C. G. Riordan, G. P. A. Yap and A. L. Rheingold, Inorg. Chem., 1996, 35, 749; F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 5th edn., 1988, p. 733; R. S. Drago, Physical Methods for Chemists, Saunders College Publishing, New York, 2nd edn., 1992, p. 448;J. Chem. Soc., Dalton Trans., 1998, Pages 139–145 145 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry, Harper Collins College Publishers, New York, 4th edn., 1993, p. 452. 2 W. N. Setzer, C. A. Ogle, C. S. Wilson and R. S. Glass, Inorg. Chem., 1983, 22, 266; J. R. Hartman, E. J. Hintsa and S. R. Cooper, J. Chem. Soc., Chem. Commun., 1984, 386; J. Am. Chem. Soc., 1986, 108, 1208; G.S. Wilson, D. D. Swanson and R. S. Glass, Inorg. Chem., 1986, 25, 3827; L. F. Warren and M. A. Bennett, J. Am. Chem. Soc., 1974, 96, 3340; R. C. Stoufer and D. H. Busch, J. Am. Chem. Soc., 1956, 78, 6016; Y. Nishida, K. Ida and S. Kida, Inorg. Chim. Acta, 1980, 38, 113; V. Cuttica and M. Paoletti, Gazz. Chim. Ital., 1922, 52, 279; F. Lions and K. V. Martin, J. Am. Chem. Soc., 1957, 79, 2733; 1958, 80, 3578. 3 F. H. Burstall and R. S. Nyholm, J. Chem. Soc., 1952, 3570; R.A. Palmer and M. C. L. Yang, Chem. Phys. Lett., 1975, 31, 492. 4 P. E. Figgins and D. H. Busch, J. Am. Chem. Soc., 1960, 82, 820. 5 B. K. Santra, G. A. Thakur, P. Ghosh, A. Pramanik and G. K. Lahiri, Inorg. Chem., 1996, 35, 3050; B. K. Santra and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1997, 129; P. Bandyopadhyay, D. Bandyopadhyay, A. Chakravorty, F. A. Cotton, L. R. Falvello and S. Han, J. Am. Chem. Soc., 1983, 105, 6327. 6 G. K. Lahiri, S. Goswami, L. R. Falvello and A. Chakravorty, Inorg. Chem., 1987, 26, 3365. 7 R. A. Krause and K. Krause, Inorg. Chem., 1980, 19, 2600; S. Goswami, A. R. Chakravarty and A. Chakravorty, Inorg. Chem., 1982, 21, 2737; B. K. Ghosh, A. Mukhopadhyay, S. Goswami, S. Ray and A. Chakravorty, Inorg. Chem., 1984, 23, 4633; T. Bao, K. Krause and R. A. Krause, Inorg. Chem., 1988, 27, 759. 8 G. K. Lahiri, S. Bhattacharya, S. Goswami and A. Chakravorty, J. Chem. Soc., Dalton Trans., 1990, 561. 9 S. Goswami, A. R. Chakravarty and A. Chakravorty, J. Chem. Soc., Chem. Commun., 1982, 1288. 10 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, New York, 1984, p. 724. 11 I. Bertini, G. Canti, C. Luchinat and A. Scozzafava, J. Am. Chem. Soc., 1978, 100, 4873. 12 N. Tanaka and Y. Sato, Bull. Chem. Soc. Jpn., 1968, 41, 2059. 13 P. S. Rao, G. A. Thakur and G. K. Lahiri, Indian J. Chem., Sect. A, 1996, 35, 946. 14 S. Bhattacharya, Polyhedron, 1993, 12, 235; S. Goswami, R. N. Mukherjee and A. Chakravorty, Inorg. Chem., 1983, 22, 2825. 15 N. Bag, A. Pramanik, G. K. Lahiri and A. Chakravorty, Inorg. Chem., 1992, 31, 40; B. K. Ghosh and A. Chakravorty, Coord. Chem. Rev., 1989, 95, 239. 16 J. K. Burdett, Adv. Inorg. Chem. Radiochem., 1978, 21, 113; Inorg. Chem., 1976, 15, 212; 1975, 14, 375. 17 A. Pramanik, N. Bag, D. Ray, G. K. Lahiri and A. Chakravorty, Inorg. Chem., 1991, 30, 410; G. K. Lahiri, S. Bhattacharya, M. Mukherjee, A. K. Mukherjee and A. Chakravorty, Inorg. Chem., 1987, 26, 3359. 18 B. K. Santra, M. Menon, C. K. Pal and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1997, 1387; D. T. Sawyer and J. L. Roberts, jun., Experimental Electrochemistry for Chemists, Wiley, New York, 1974, p. 167. 19 N. Campbell, A. W. Henderson and D. Taylor, J. Chem. Soc., 1953, 1281. Received 9th September 1997; Paper 7/06587F

ISSN:1477-9226    

DOI:10.1039/a706587f

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 147–151 147 Regioselective synthesis and electrochemical properties of �-conjugated cobaltacyclopentadiene oligomer and polymer complexes Isao Matsuoka,a Kunitsugu Aramakib and Hiroshi Nishihara *,a a Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Perfectly p-conjugated cobaltacyclopentadiene oligomer and polymer complexes were synthesized by polycondensation of a dihalogenated cobaltacyclopentadiene complex, [Co{C(C6H4I-4)]] CBunCBun]] C(C6H4I-4)}- (cp)(PPh3)] (cp = h5-C5H5), with [Ni(cod)2] (cod = cycloocta-1,5-diene).Their internuclear electronic interaction energy in the mixed-valence state, uOR, was estimated to be ca. 22 kJ mol21 by electrochemical analysis. Organometallic p-conjugated polymers involving transition metals, so called ‘conducting polymer complexes’, show unique properties due to electronic and magnetic interactions between the metal sites through the p-conjugated chain.1 We have previously reported a metallacycling polymerization (MCP), which involves a successive addition of diacetylenes to [Co(cp)(PPh3)2] 1 (cp = h5-C5H5), providing a new class of organometallic polymers consisting of cobaltacyclopentadiene units [Scheme 1, equation (1)].2 Endo et al.3 have reported similar MCP reactions independently. The complex polymers are structurally analogous to organic conducting polymers such as polypyrrole and polythiophene 4 which have five-membered aromatic heterocycles in the main chain.We have recently found that the oxidized forms of the polymer complexes are stable even at room temperature and thus reversible redox doping is possible when methyl groups are substituents on the cobaltacyclopentadiene rings.5 In these polymers internuclear electronic interaction between the cobalt atoms through the p-conjugated chain should be possible.Unfortunately, however, the complex polymers did not comprise sufficiently long and linear pconjugated chains, notwithstanding the high molecular weight (Mn > 105), because satisfactory regioselective polymerization does not take place in the MCP method, forming three binding modes in the polymers [equation (1)]. In this study we have succeeded in synthesizing perfectly linear p-conjugated cobaltacyclopentadiene oligomer and polymer complexes by the polycondensation of a dihalogenated cobaltacyclopentadiene complex, [Co{C(C6H4I-4)]] CBunCBun]] C(C6H4I-4)}(cp)(PPh3)] with [Ni(cod)2].This polymerization method involving polycondensation of dihalogenated compounds with zerovalent nickel complexes has been established by Yamamoto et al.6 for the synthesis of fully p-conjugated polymers such as polythiophene and poly(p-phenylene). The internuclear electronic interaction between the cobalt atoms is discussed based on the electrochemical properties of the oligomers thus obtained.Experimental All the reactions were carried out under a nitrogen atmosphere. The complex [Co(cp)(PPh3)2]?0.5C6H14 1 was prepared according to the literature method7 and recrystallized from toluene– hexane before use. Acetylenic compounds, 4,49-di(hex-1-ynyl)- biphenyl, 1-bromo-4-hexynylbenzene and 1-(hex-1-ynyl)-4- iodobenzene were prepared by the coupling of hex-1-yne with 4,49-diiodobiphenyl, 1-bromo-4-iodobenzene and 1,4-diiodobenzene, respectively, as described in the literature.8 The complex [Ni(cod)2] and anhydrous solvents were obtained from Kanto Chemicals.Tetra-n-butylammonium perchlorate was obtained from Tomiyama Chemicals as lithium battery grade. Dichloromethane used for electrochemical measurements was a HPLC grade chemical from Kanto Chemicals. The NMR and UV/VIS spectra were recorded with JEOL GX400 and JASCO V-570 spectrometers, respectively. Gel permeation chromatography (GPC) analysis of polymerization processes was carried out with a Shimadzu LC-4A HPLC apparatus (column, Tosoh TSKgel 45000HGX; detector, Shimadzu SPD2A UV spectrometer monitoring 254 nm).Separation and purification of polymers and oligomers were carried out with a JAI LC-908 recycling preparative HPLC apparatus with JAIGEL 2H and 3H columns. Synthesis of cobaltacyclopentadiene polymer 2 [Scheme 1, equation (2)] To a stirred solution of complex 1 (211 mg, 0.305 mmol) in toluene (5 cm3) was slowly added a solution of 4,49-di(but-1- ynyl)biphenyl (95.9 mg, 0.305 mmol) in toluene (2 cm3). After stirring for 24 h at room temperature the reaction mixture was chromatographed with a recycling preparative GPC apparatus.The solution was concentrated to ca. 1 cm3, to which hexane (15 cm3) was added. Orange powdery precipitates thus formed were collected by filtration, washed thoroughly with hexane and dried under vacuum.Yield 164 mg, 77% [Found: C, 79.72; H, 7.00. C44.30H43.75CoP0.85 (see Table 1) requires C, 80.43; H, 6.67%]. 1H NMR (C6D6): d 0.8–1.1 (6 H, m, CH3), 1.2–1.5 (4 H, m, CH3CH2CH2CH2), 1.6–1.9 (4 H, m, CH3CH2CH2CH2), 2.4– 2.7 (4 H, m, CH3CH2CH2CH2), 4.6–4.9 (5 H, m, C5H5) and 6.8– 7.8 (33 H, m, aryl). 13C NMR (C6D6): d 13.94, 14.35, 19.66, 21.60, 23.11, 23.66, 31.81, 32.41, 33.46, 38.96, 81.76, 89.91, 91.52, 123.35, 126.28, 126.73, 126.93, 126.99, 127.26, 128.67, 128.72, 129.02, 129.16, 129.72, 132.52, 132.63, 141.23, 153.73, 156.21, 160.76 and 161.00. 31P NMR (C6D6): d 52.42 (s, PPh3) and 56.17 (s, PPh3). Synthesis of dihalogenated cobaltacyclopentadiene monomers [Scheme 1, equation (3)] [Co{C(C6H4Br-4)]] CBunCBun]] C(C6H4Br-4)}(cp)(PPh3)] 3a. Complex 1 (1.08 g, 1.56 mmol) and BunC]] ] CC6H4Br-4 (0.74 g, 3.12 mmol) were dissolved in dry toluene (30 cm3) and the solution was stirred at room temperature for 8 h.The solvent was removed under reduced pressure, and the residue recrystal-148 J. Chem. Soc., Dalton Trans., 1998, Pages 147–151 Scheme 1 cp = h5-C5H5, cod = cycloocta-1,5-diene, bpy = 2,29-bipyridine, dmf = dimethylformamide Co PPh3 Co PPh3 R R R R Co A A A R R Co PPh3 Co PPh3 R R R R Co A A A R R X [Co(cp)(PPh3)2] + 2 BunCºC X Co PPh3 X Bun Bun X Co PPh3 X Bun Bun X Co PPh3 Bun X Bun Co Bun Bun X X 1 R = H, Me, COMe , A = m l (2) n-l-m R = Bun m 3a X = Br 3b X = I 1 (4) l 4 polymer 5 n = 2 6 n = 3 n n-l-m (3) [Co(cp)(PPh3)2] + RC-CºC-A-CºCR (1) A = 2 X = Br or I C6H5Me dmf Ni(cod)2, bpy cod lized from benzene–hexane twice to afford 3a as brown needles.Yield 187 mg, 14%. 1H NMR (CDCl3): d 0.74 (6 H, t, CH3, J = 6.93 Hz), 1.10–1.40 (8 H, m, CH3CH2CH2CH2), 2.01 (4 H, m, CH3CH2CH2CH2), 4.57 (5 H, s, C5H5), 6.40 (4 H, m, aryl), 6.68 (4 H, m, aryl) and 7.03–7.50 (15 H, m, PPh3). [Co{C(C6H4I-4)]] CBunCBun]] C(C6H4I-4)}(cp)(PPh3)] 3b. A mixture of complex 1 (557 mg, 1.56 mmol) and BunC]] ] CC6H4I-4 (457 mg, 3.12 mmol) dissolved in dry toluene (16 cm3) was stirred at room temperature for 12 h.The solution was concentrated to ca. 1 cm3 under reduced pressure and hexanes (5 cm3) was added slowly. The brown precipitates formed were collected and then recrystallized from benzene–MeOH to afford 3b as brown needles. Yield 244 mg, 32% (Found: C, 59.02; H, 4.99. C47H46CoI2P requires C, 59.11; H, 4.86%). 1H NMR (C6D6): d 0.84 (6 H, t, J = 6.52 Hz, CH3), 1.24–1.50 (8 H, m, CH3CH2- CH2CH2), 2.37 (4 H, m, CH3CH2CH2CH2), 4.54 (5 H, s, C5H5), 6.35 (4 H, m, aryl) and 6.79–7.63 (19 H, m, aryl). 13C NMR (C6D6): d 14.33, 23.37, 31.90, 34.25, 89.49, 128.44, 128.53, 128.79, 128.90, 129.76, 129.89, 130.35, 130.45, 131.11, 133.31, 133.41, 133.47, 133.57, 134.44, 134.53, 136.58, 153.25, 156.08, 156.11, 159.51 and 159.79. 31P NMR (C6D6): d 51.76 (s, PPh3). Polymerization of monomers with [Ni(cod)2] The reaction giving polymer 4 was carried out according to Scheme 1, equation (4).To a stirred solution of complex 3b (99 mg, 0.10 mmol) in dry dmf (4 cm3), 2,29-bipyridine (31 mg, 0.20 mmol), cycloocta-1,5-diene (22 mg, 0.20 mmol) and [Ni(cod)2] (55 mg, 0.20 mmol) were added. The mixture was stirred at 50 8C. The progress of the polymerization was monitored by GPC analysis. The change in GPC chromatram became negligible after 12 h of reaction. The solvent was removed under reduced pressure and the brown residue chromatographed with a recycling preparative GPC apparatus using toluene as eluent.The solution was concentrated to ca. 1 cm3 and hexane (10 cm3) slowly added. The brown precipitates thus formed were collected with a glass filter, washed well with hexane and dried in vacuum. Yield 40 mg, 55% [Found: C, 77.62; H, 6.82. (C47H46CoP)n requires C, 80.54; H, 6.62%]. 1H NMR (C6D6): d 0.8–1.0 (6 H, m, CH3), 1.1–1.8 (8 H, m, CH3CH2CH2CH2), 2.3–2.7 (4 H, m, CH3CH2CH2CH2) 4.7–4.9 (5 H, m, C5H5) and 6.9–7.8 (23 H, m, aryl). 13C NMR (C6D6): d 14.38, 23.55, 32.23, 34.55, 89.75, 125.80, 126.69, 127.77, 128.25, 128.46, 128.54, 128.90, 129.58, 131.60, 132.44, 133.77, 134.71, 136.94, 152.74 and 155.98. 31P NMR (C6D6): d 52.66 (s, PPh3). When the reaction was carried out under milder conditions, oligomers with nmax = 6 were obtained, where nmax refers the highest polymerization degree. The procedure to isolate dimer 5 and trimer 6 is given below. The complex [Ni(cod)2] (27 mg, 0.10 mmol), bpy (15 mg, 0.10 mmol) and cod (11 mg, 0.10 mmol) were dissolved in dry dmfJ.Chem. Soc., Dalton Trans., 1998, Pages 147–151 149 (2 cm3). The mixture was stirred for 0.5 h at room temperature to afford a purple solution of [Ni(bpy)(cod)] which was added to a solution of BunC]] ] CC6H4I-4 (80 mg, 0.084 mmol) in dmf (2 cm3). The mixture was stirred for 48 h at room temperature. The solution was condensed under reduced pressure and the residue chromatographed with a recycling preparative GPC apparatus, resulting in the isolation of dimer 5 and the trimer 6.Complex 5: yield 6.1 mg, 7.6% (Found: C, 68.50; H, 5.78. C94H92Co2I2P2 requires C, 68.20; H, 5.60%); 1H NMR (C6D6): d 0.87 (12 H, t, J = 7.22 Hz, CH3), 1.31–1.62 (16 H, m, CH3- CH2CH2CH2), 2.45 (8 H, m, CH3CH2CH2CH2), 4.69 (10 H, s, C5H5) and 6.4–7.4 (46 H, m, aryl); 13C NMR (C6D6): d 14.28, 21.89, 32.02, 34.38, 89.64, 122.59, 125.66, 125.83, 127.81, 128.19, 128.29, 128.40, 136.59 and 149.13; 31P NMR (C6D6): d 52.19 (s, PPh3).Complex 6: yield 5.8 mg, 7.2% (Found: C, 71.73; H, 6.20. C141H138Co3I2P3 requires C, 71.86; H, 5.91%); 1H NMR (C6D6): d 0.88 (18 H, t, CH3), 1.33–1.63 (24 H, m, CH3CH2CH2CH2), 2.45 (12 H, m, CH3CH2CH2CH2), 4.70 (10 H, s, C5H5 of end Co atoms), 4.83 (5 H, s, C5H5 of center Co atoms) and 6.46– 7.43 (69 H, m); 13C NMR (C6D6): d 14.33, 23.37, 31.90, 34.25, 89.49, 128.44, 128.53, 128.79, 128.90, 129.76, 129.89, 130.35, 130.45, 131.11, 133.31, 133.41, 133.47, 133.57, 134.44, 134.53, 136.58, 153.25, 156.08, 156.11, 159.51 and 159.79; 31P NMR (C6D6): d 52.31 (2 P, s, PPh3 of end Co atoms) and 52.86 (1 P, s, PPh3 of center Co atoms).Electrochemical measurements A glassy carbon rod (outside diameter 5 mm, Tokai Carbon GC-20) was embedded in Pyrex glass and the cross-section used as a working electrode. Cyclic voltammetry was carried out in a standard one-compartment cell under an argon atmosphere equipped with a platinum-wire counter electrode and a Ag–Ag1 reference electrode (10 mmol dm23 AgClO4 in 0.1 mol dm23 NBu4ClO4–MeCN) with a BAS CV-50W voltammetric analyzer.The computer software DIGISIM 2 (BAS) of Rudolph et al.9 was used for simulation of cyclic voltammograms. Results and Discussion Regioselective synthesis of cobaltacyclopentadiene oligomer and polymer An MCP reaction between [Co(cp)(PPh3)2] 1 and the diacetyl- Fig. 1 The GPC spectra for the MCP reaction between [Co(cp)- (PPh3)2] and BuC]] ] CC6H4C6H4C]] ] CBu (a), and polycondensation of complex 3b and [Ni(cod)2] at 50 8C (b) and at room temperature (c) ene BuC]] ] CC6H4C6H4C]] ]CBu was carried out in order to obtain more information on the regioselectivity [Scheme 1, equation (2)]. The synthetic procedure was similar to that we have previously reported.5 The polymer obtained was more soluble than the corresponding methyl-substituted polymer and could be purified readily with a GPC apparatus.The GPC spectrum of compound 2 is shown in Fig. 1(a). Its number-average molecular weight, Mn, was 2.7 × 105 (Mw/Mn = 5.2). In the 1H NMR spectrum three singlet signals for cyclopentadienyl groups were observed clearly at d 4.9, 4.8 and 4.6 [Fig. 2(a)]. The 13C NMR spectrum also showed three signals for cyclopentadienyl groups at d 91.52, 89.91 and 81.76. By comparison with related monomeric complexes, they are attributed to 2,4-diaryl-3,5-di-n-butylcobaltacyclopentadiene, 2,5- diaryl-3,4-di-n-butylcobaltacyclopentadiene and (h4-cyclobutadiene) cobalt complexes, respectively.The 31P NMR spectrum showed two signals at d 52.42 and 56.17 for triphenylphosphine ligands due to the presence of two geometric isomers of cobaltacyclopentadiene. The ratio of the three units was calculated by integration of the values for the cyclopentadienyl rings and the results are in Table 1. As for the regioselectivity of metallacyclization, Wakatsuki et al.10 have shown that the acetylenic carbon bearing a bulky group becomes the a-carbon of the metallacyclopentadiene.In the MCP reaction the regioselectivity with n-butyl groups is thus lower than that with methyl groups. We next attempted to synthesize a cobaltacyclopentadiene polymer comprising a single unit structure using a dihalogenated monomer. For this purpose compounds 3a and 3b were newly synthesized by double addition of acetylene BunC]] ] CC6H4X-4 (X = Br or I) to [Co(cp)(PPh3)2] 1.At the end of the reaction the presence of geometric isomers of 3a or 3b was Fig. 2 Proton NMR spectra of compounds 2 (a) and 4 (b) in C6D6 Table 1 Regioselectivity in the MCP reactions * R Me Bun l/n 0.70 0.57 m/n 0.15 0.28 * The values of l and m are the numbers of cobaltacyclopentadiene units binding at the 2,5 and 2,4 positions, respectively, as shown in Scheme 1, equation (1); n is the degree of polymerization.150 J. Chem. Soc., Dalton Trans., 1998, Pages 147–151 found by NMR measurements, but they could be removed by recrystallization.The method of polymerization was similar to that for linear p-conjugated polymers reported by Yamamoto et al.6 using [Ni(cod)2] as a C]C coupling reagent and dmf as solvent. The dibromo-derivative caused no reaction even at 50 8C for 2 d. On the contrary, the diiodo-derivative, 3b, reacted readily and the progress of the polymerization was monitored by GPC analysis. When the reaction of 3b was carried out with an excess amount of [Ni(cod)2] (2.0 equivalents) at 50 8C, the molecular weight, Mn, reached 2.0 × 105 (Mw/Mn = 2.8) after 12 h [Fig. 1(b)]. When the reaction was carried out with an equimolar amount of [Ni(cod)2] at room temperature oligomers up to a hexamer were obtained. The polymer and oligomer could be purified with a recycling preparative GPC method. A dimer 5 and a trimer 6 were isolated and used as samples for investigating physical properties in detail.They are soluble in common organic solvents such as toluene, dichloromethane and tetrahydrofuran. In the 1H and 13C NMR spectra of compound 4 only a signal for the cyclopentadienyl group appeared at d 4.8 [Fig. 2(b)] and 89.75, respectively. Its 31P NMR spectrum also showed one singlet at d 52.66 for PPh3. These results indicated that the cobaltacyclopentadiene polymer consists of only one unit structure. The UV/VIS absorption spectra of compounds 2, 3b and 4 are displayed in Fig. 3. The peak edge in the visible region for polymers 2 and 4 is shifted to longer wavelength than that of the monomer, 3b. This indicates the formation of a highly p-conjugated structure in the polymer complexes. The peak edge for 4 was further shifted to longer wavelength compared with 2, because the latter contains structures with unit-to-unit binding at the 2,4 positions of the cobaltacyclopentadiene, unfavorable for p conjugation. Electrochemistry of cobaltacyclopentadiene complexes Cyclic voltammograms of the cobaltacyclopentadiene monomer 3b and polymers 2 and 4 at a glassy carbon electrode in 0.1 mol dm23 NBu4ClO4–CH2Cl2 are shown in Fig. 4. The monomer undergoes a reversible oxidation at ca. 20.02 V and an irreversible oxidation at ca. 10.9 V vs. Ag–Ag1 at room temperature [Fig. 4(a)]. These results are similar to those for previously reported cobaltacyclopentadiene complexes with no halide groups.2,4,11,12 The E89 value and the reversibility of the oxidation reaction of the polymer were almost consistent with those of oligomers [Fig. 4(b), (c)]. These results support our previous conclusions 12 that the highest occupied molecular orbital (HOMO) based on the d orbital of the metal atoms in the polymer exists between the valence band (VB) and the conduction band (CB) derived from p conjugation, and that oxidation occurs at metal sites. For the polymer, 2 synthesized by the MCP reaction, a small irreversible oxidation wave was found at Ep,a = 10.5 V vs.Ag–Ag1, similar to the oxidation of [Co(cp)- (h4-C4(SiMe3)2(C6H4C6H4C]] ] CSiMe3)2)],13 indicating that 2 contains (h4-cyclobutadiene)cobalt complex moieties as noted above. Fig. 3 The UV/VIS spectra of compounds 2 (a), 3b (b) and 4 (c) in CH2Cl2 [CoIII 2] E891 [CoIIICoIV] E892 [CoIV 2] (5) 5 [CoIII 3] E891 [CoIIICoIVCoIII] E82 [CoIVCoIIICoIV] E893 [CoIV 3] 6 (6) Scheme 2 The oxidation processes of the dimer and the trimer are shown in Scheme 2, where CoIII and CoIV are in neutral and oxidized cobaltacyclopentadiene units, respectively.Although only a single oxidation peak is observed for the dimer and the trimer [see full lines in Fig. 5(a), (b)], the waves are broader compared with that of the monomer. This discrepancy suggests that the cobalt-(III) and -(IV) sites are weakly interacting and mixed-valence states CoIIICoIV, CoIIICoIVCoIII and CoIVCoIIICoIV are generated within a narrow potential range.On the basis of computer simulation, the oxidation potentials are calculated to be E891 = 275 and E892 = 22 mV vs. Ag–Ag1 for the dimer, and E891 = 281, E892 = 238 and E893 = 21 mV vs. Ag–Ag1 for the trimer. Aoki and Chen14 have reported a theoretical insight on the redox properties of a linearly combined multiredox system based on the electronic interaction energy between neighbouring redox sites. The parameters for evaluating the stability of the mixed-valence states, u1 and u2, can be evaluated from the redox potentials.When u1 = ��� (uRR 1 uOO) 2 uOR and u2 = ��� (uOO 2 uRR), where R and O refer to reduced and oxidized sites respectively the difference in redox potentials E893 2 E891 for a dimer corresponds to 2u1, and the differences E892 2 E891 and E893 2 E892 for a trimer correspond to 2u1 2 2u2 and 2u1 1 2u2, respectively. From our experimental results for the dimer, u1 is calculated to be 3.5 kJ mol21. Similarly for the trimer, u1 and u2 are estimated at 3.0 and 0.96 kJ mol21, respectively.These Fig. 4 Cyclic voltammograms of compounds 3b (a), 2 (b) and 4 (c) at a glassy carbon electrode in 0.1 mol dm23 NBu4ClO4–CH2Cl2 at a scan rate of 0.1 V s21J. Chem. Soc., Dalton Trans., 1998, Pages 147–151 151 values are reasonable because the difference in u1 between the dimer and the trimer is small. If the value of uRR is assumed to be zero since R is a neutral form and the interaction between neutral forms should be weak, uOR is estimated to be 22 kJ mol21 and is about one-fifth that for poly(1,19-dihexylferrocenylenes). 15 A possible rationale of the weak interaction noted above could be the significant difference in energy levels between the occupied p orbital of the phenylene moieties and the HOMO localized on cobalt sites. The energy level of this occupied p orbital should be much lower than that of the HOMO and thus the electronic interaction between the cobalt atoms through the phenylene groups is weak.The lack of coplanarity for the benzene and cobaltacyclopentadiene rings may also contribute to the weak internuclear interaction. This is supported by X-ray crystallographic analysis data that we have reported previously for a 2,4-diphenylcobaltacyclopentadiene complex,16 indicating that the dihedral angle between the benzene ring at the 2 position and the cobaltacyclopentadiene ring is 588, larger than the dihedral angle of benzene rings in poly(p-phenylene).17 Conclusion The polycondensation method involving a dihalogenated cobaltacyclopentadiene complex and [Ni(cod)2] has solved the prob- Fig. 5 Cyclic voltammograms of compounds 5 (a) and 6 (b) at a glassy carbon electrode in 0.1 mol dm23 NBu4ClO4–CH2Cl2 at a scan rate of 0.1 V s21 (full lines) and their simulation based on the open-boundary finite-diffusion model (broken lines) lems of preparing fully p-conjugated cobaltacyclopentadiene polymers which had not been obtained by the MCP method. It is found that the interaction between the cobalt sites in poly- (biphenylene cobaltacyclopentadienylene) is rather weak (uOR = ca. 22 kJ mol21). Acknowledgements This research was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas [‘Innovative Synthetic Reactions’ (No. 283) and ‘Electrochemistry of Ordered Interfaces’ (No. 09237101)] from the Ministry of Education, Science, Sports and Culture, Japan and The Sumitomo Foundation. References 1 H.Nishihara, Handbook of Organic Conductive Molecules and Polymers, ed. H. S. Nalwa, Wiley, New York, 1997, vol. 2, ch. 19, and refs. therein. 2 H. Nishihara, T. Shimura, A. Ohkubo, N. Matsuda and K. Aramaki, Adv. Mater., 1993, 5, 752; A. Ohkubo, K. Aramaki and H. Nishihara, Chem. Lett., 1993, 271. 3 I. Tomita, A. Nishio, T. Igarashi and T. Endo, Polym. Bull., 1993, 30, 179. 4 T. A. Skotheim (Editor), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 5 I. Matsuoka, K. Aramaki and H. Nishihara, Mol. Cryst. Liq. Cryst., 1996, 285, 199. 6 T. Yamamoto, T. Maruyama, Z. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takazoe, A. Fukuda and K. Kubota, J. Am. Chem. Soc., 1994, 116, 4832. 7 Y. Wakatsuki and H. Yamazaki, Inorg. Synth., 1989, 26, 191. 8 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975, 50, 4467. 9 M. Rudolph, D. P. Reddy and S. W. Feldberg, Anal. Chem., 1994, 66, 589A. 10 Y. Wakatsuki, O. Nomura, K. Kitaura, K. Morokuma and H. Yamazaki, J. Am. Chem. Soc., 1983, 105, 1907. 11 R. S. Kelly and W. E. Geiger, Organometallics, 1987, 6, 1432. 12 T. Shimura, A. Ohkubo, N. Matsuda, I. Matsuoka, K. Aramaki and H. Nishihara, Chem. Mater., 1996, 8, 1307. 13 T. Shimura, A. Ohkubo, K. Aramaki, H. Uekusa, T. Fujita, S. Ohba and H. Nishihara, Inorg. Chim. Acta, 1995, 230, 215. 14 K. Aoki and J. Chen, J. Electroanal. Chem. Interfacial Electrochem., 1995, 380, 35. 15 T. Hirao, M. Kurashina, K. Aramaki and H. Nishihara, J. Chem. Soc., Dalton Trans., 1996, 2929. 16 T. Fujita, H. Uekusa, A. Ohkubo, T. Shimura, K. Aramaki, H. Nishihara and S. Ohba, Acta Crystallogr., Sect. C, 1995, 51, 2265. 17 Y. Delugeard, J. Desuche and J. L. Baudour, Acta Crystallogr., Sect. B, 1976, 32, 172. Received 31st July 1997; Pa

ISSN:1477-9226    

DOI:10.1039/a705546c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 153 Co-ordination of copper(II) by amikacin. Complexation equilibria in solution and oxygen activation by the resulting complexes † Ma�gorzata Jez . owska-Bojczuk * and Wojciech Bal Faculty of Chemistry, University of Wroc�aw, Joliot-Curie 14, 50–383 Wroc�aw, Poland Protonation and copper(II) co-ordination properties of amikacin (A) were studied in solution by potentiometry, and NMR, UV/VIS, CD and EPR spectroscopies.Mononuclear, tetragonal and five-co-ordinate complexes of stoichiometries ranging from Cu(H3A) to CuH22A were found. The effects of amikacin on copper(II) binding by physiological copper(II) carriers, histidine and albumin, and facilitation of oxidation of 29-deoxyguanosine by copper(II)–amikacin complexes were also investigated. The results indicate that complexation of CuII by amikacin should not be expected to affect copper(II) homeostasis in blood, but may contribute to the intracellular activity of the drug.Amikacin is a semisynthetic aminoglycosidic antibiotic (a derivative of kanamycin A) active against Gram-negative bacteria, developed to combat gentamycin-resistant strains.1 It kills bacteria by inhibiting the translation step in microbial protein synthesis and subsequently damaging cytoplasmic membrane. 2,3 Amikacin has a broad spectrum of activity and is also widely used to prevent bacterial infections in cell cultures. A major disadvantage of amikacin, common to aminoglycosides, is its auditory- 4 and nephro-toxicity.5 These result in a narrow therapeutic window of blood plasma concentrations, thereby reducing amikacin’s use in therapy to life-threatening situations and requiring monitoring of the drug9s plasma level.6,7 Simple aminohexoses can bind copper(II) ions in the weakly acidic to neutral pH range.Primary binding occurs through the amino nitrogen. The effectiveness of the binding depends, however, on whether a five- or six-membered chelate ring can be formed with an appropriately located hydroxyl oxygen.If this is not possible a monodentate complex cannot withstand hydrolysis above pH 7.8,9 Otherwise, a range of chelate complexes is formed, with CuII bound through nitrogens and protonated or deprotonated sugar oxygens.8–15 Binding of CuII to aminoglycosides has been the subject of several studies,16–21 with kanamycin being the one most widely studied.16–19 Binding modes similar to those discovered for simple aminosugars, but not involving hydroxyl oxygen deprotonations, were proposed.The main interest of the authors was to selectively block particular functional groups for synthetic purposes. Complexation phenomena were therefore not studied systematically beyond the assumed acylation reaction conditions. Amikacin differs from kanamycin A by having the B1 amino group of the 2-deoxystreptamine moiety modified by acylation with 4-amino-2-hydroxybutyric acid.This modification, designed to prevent inactivation of the antibiotic by bacterial enzymes, adds to the molecule a peptide bond, a hydroxyl group and a terminal amino group. These potential donor groups are arranged so that they can all participate in a metal binding site alternative to, and possibly stronger than, the aminohydroxyl binding characteristic of aminosugars. The first objective of this study was therefore quantitatively and structurally to describe the interaction of CuII with amikacin in order to find whether such interaction might be possible in vivo.The † Supplementary data available: chemical shifts and coupling constants. For direct electronic access see http://www.rsc.org/suppdata/dt/1998/ 153/, otherwise available from BLDSC (No. SUP 57312, 5 pp.) or the RSC Library. See Instructions for Authors, 1998, Issue 1 (http:// www.rsc.org/dalton). copper(II)–histidine 1 : 1 complex is probably a major component of the low-molecular-mass fraction of copper in blood plasma.22 It has a potential to form ternary complexes with aminosugar analogues, as was recently demonstrated,23 and such complexes ought to be taken into account in the analysis of speciation in vivo.Therefore we also studied the formation of ternary complexes between CuII, amikacin, and histidine. Copper( II) was reported to enhance antibacterial activity of two aminoglycosides, gentamycin and streptomycin, in an in vitro test.24 This finding does not seem to have been followed by detailed studies, but these preliminary results nevertheless serve another good reason for our study.Acidic pH and anaerobic conditions 25,26 suppressed the bactericidal activity of amikacin. These facts are believed to reflect diminished active transport of the antibiotic through the microbial cytoplasmic membrane, which results from the inhibition of bacterial metabolism under such conditions. However, interestingly, low pH is a factor discouraging copper(II) complexation with aminosugars and hypoxia would inhibit oxidative activity of such complexes.Hypoxia partially inhibited killing of the bacteria by amikacin even after transport had been restored.25 This might indicate the existence of an oxygendependent component in the mechanism of amikacin bactericidal action. We have recently discovered that copper(II) complexes of an iminosugar, 1-deoxynojirimycin, activate oxygen and hydrogen peroxide very efficiently, oxidising 29-deoxyguanosine (dG) to its 8-oxo derivative.23 It therefore seemed important to investigate whether copper(II)–amikacin complexes can catalyse oxidative processes that might contribute to their biological activity.Materials and Methods Amikacin was from Fluka, CuCl2, D2O, NaOD, DCl, KNO3, ethane-1,2-diol, methanol and 3-(trimethylsilyl)propionic acid sodium salt (tsp) from Aldrich. L-Histidine, bovine serum albumin (BSA, essentially fatty acid free), dG and H2O2, chelex 100 resin, sodium and potassium phosphates were from Sigma.The reference sample of 8-oxo-dG was a gift of Dr. K. S. Kasprzak (National Cancer Institute–Frederick Cancer Research and Development Center). Potentiometry Potentiometric titrations of binary and ternary complexes of Cu21 with amikacin and histidine in the presence of 0.1 M KNO3 were performed at 25 8C using pH-metric titrations over154 J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 Fig 1 The molecule of amikacin in the fully deprotonated form (A).Non-exchangeable protons are marked in bold O O HO HO OH HO NH2 O OH H2N HO NH A2 A3 A4 A6b A6a NH2 A1 O B5 B4 B2a B2e B1 C1 C6b C6a OH O NH2 OH a b1 b2 g1 g2 A5 C5 C2 B3 B6 C4 C3 the range pH 3–11.5 (Molspin automatic titrator) with NaOH as titrant. Changes in pH were monitored with a combined glass–calomel electrode (Russell) calibrated daily in hydrogen concentrations by HNO3 titrations.27 Sample volumes were 1.5 cm3.Concentrations of ligands were 1023 mol dm23. Ligand : Cu21 molar ratios between 2 : 1 and 1 : 1 (binary system) and 1:1:1 (ternary system) were used. The data were analysed using the SUPERQUAD program.28 Standard deviations computed thereby refer to random errors only. They give, however, a good measure of the importance of a given species in solution. Spectroscopy The CD spectra were recorded at 25 8C on a JASCO J-600 spectropolarimeter over the range 190–750 nm, using 1 and 0.1 cm cuvettes.For ligand spectra the amikacin concentration was 1023 mol dm23. For complexation experiments, samples with 2 : 1 and 1:1:1 ligand-to-metal ratios were used, with a Cu21 concentration of 1023 mol dm23. Additionally, the binary system was checked for aggregation/oligomerisation by recording the spectra at pH 7, 9 and 11 at 0.003 and 0.01 mol dm23. In the BSA competition experiment samples containing 7 × 1024 mol dm23 BSA and CuII in 0.15 mol dm23 NaCl were titrated with 0.06 mol dm23 histidine or 0.06 mol dm23 histidine 1 0.06 mol dm23 amikacin.These samples were adjusted to pH 7.4 with small amounts of concentrated HCl or NaOH. The removal of CuII from BSA by histidine 1 amikacin was followed by recording the CD spectra after each increment at titrant. The spectra are expressed in terms of De = el 2 er, where el and er are the mbsorption coefficients for left and right circularly polarised light, respectively. Electronic absorption (UV/VIS) spectra were recorded on a Beckman DU-650 spectrophotometer over the spectral range 190–1100 nm in 1 and 0.1 cm cuvettes, using the same samples as in the CD measurements.The EPR spectra were recorded at 120 K on a Bruker ESP 300E spectrometer at the X-band frequency (9.3 GHz). Ethane-1,2-diol–water (1 : 2) was used as a solvent in order to obtain homogeneity of frozen samples. Sample concentrations were similar to those used in CD measurements.Proton NMR spectra of 0.005 mol dm23 amikacin were recorded at 27 8C with a Varian VXR500S spectrometer at 499.84 MHz; tsp was used as internal standard. The pH* (pH reading of the electrode not corrected for isotope effects) of the samples was adjusted by adding small volumes of concentrated DCl or NaOD. The correlation (COSY) spectrum was run in the absolute value mode and processed using standard Varian software. 8-Oxo-dG formation Solutions of dG (1024 mol dm23) in 0.1 mol dm23 sodium phosphate buffer, pH 7.4, were incubated in triplicate for 24 h at 37 8C in the presence of combinations of amikacin, histidine, CuII (0 or 1024 mol dm23), and H2O2 (0 or 1023 mol dm23).All stock solutions, except for CuII and H2O2, were purified with Chelex 100 prior to use. After incubation, the dG samples were analysed by HPLC without any additional pretreatment on a Beckman Gold system using UV detection (254 nm). A reversed-phase Beckman Ultrasphere ODS C18 column (4.6 mm × 25 cm) was used. The mobile phase was 0.05 mol dm23 KH2PO4 solution in 12% aqueous methanol.Results were quantified with the use of standard solutions containing known amounts of dG and 8-oxo-dG. Results and Discussion Protonation and conformation of the ligand The amikacin molecule (Fig. 1) contains four amino groups. The protonation macroconstants were obtained from potentiometric titrations and are presented in Table 1. In order to correlate these values with particular protonation sites, onedimensional NMR spectra were recorded in unbuffered D2O solutions at various pH* (pH meter reading calibrated in water, uncorrected for D2O isotopic effect) between 2.6 and 10.69 and Fig. 2 Dependence of chemical shift (from tsp) of selected amikacin protons on pH*J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 155 titration curves for individual protons of amikacin molecule were generated (see Fig. 2 for examples). Data obtained from these spectra and from a COSY spectrum recorded at pH* 10.3 (Fig. 3) allowed us to assign the amikacin spin system. The assignments (Table 2) are in agreement with those reported previously for phosphate-buffered solutions and obtained with different two-dimensional NMR techniques.29 The existence of parallel deprotonations was investigated by calculating pK* (pK values calculated with uncorrected pH-meter readings in D2O) values from titration curves of individual protons of rings A, B and C with the use of the Hill equation.Protons of rings A and C yielded uniform titration curves, described by co-operativity coefficients of 0.8 ± 0.2 and 0.9 ± 0.1, respectively. The pK* values obtained in these calculations are numerically very close to corresponding potentiometric macroconstants (see Table 1). The behaviour of ring B protons was somewhat more complicated. The pH* dependence of d values for hydrogens B2a, B2e and B5/B6 can be very well described by a pK* of 6.98 and cooperativity coefficient of 0.9 ± 0.1.On the other hand, signals for protons B3, B4, and, in particular, B1 are additionally sensitive to the deprotonation of the aglycon chain. The resulting pH* profiles appear to produce a higher pK value of ca. 7.5 for B3 and B4 (and very high errors of calculation), but in fact they are superpositions of B ring and aglycon deprotonations. Fig. 2 Fig. 3 A COSY spectrum (500 MHz) of amikacin (0.005 mol dm23) at pH* 10.3 Table 1 Protonation of amikacin Species H4A H3A H2A HA log ba 33.433(2) 26.602(2) 18.791(2) 9.901(2) pK 6.831 7.811 8.890 9.901 pK* 6.98(7) b 7.75(2) c 9.08(6) d Location of deprotonating amino group Ring B Ring C Ring A Aglycon chain a b(HxA) = [HxA]/[A][H]x; standard deviations on the last digit are given in parentheses.b Calculated from B2a, B2e and B5/B6 signals. c Calculated from C2, C3 and C4 signals. d Calculated from A1, A5 and A6a signals. Table 2 Chemical shifts (d, tsp) of amikacin protons at pH* 7.78.For notation, see Fig. 1 Ring A Ring B Ring C Aglycon chain A1 A2 A3 A4 A5 A6a A6b 5.517 3.647 3.79 a 3.380 4.030 3.403 3.165 B1 B2a B2e B3 B4 B5 B6 4.030 1.518 2.001 3.094 3.500 3.76 * 3.76 * C1 C2 C3 C4 C5 C6 5.125 3.575 3.194 3.512 4.048 3.803 a b1 b2 g 4.271 1.977 2.178 3.179 * Overlapping multiplet. presents these profiles for B1 and B5, clearly showing this effect. The selective sensitivity of B1, B3 and B4 may indicate the location of the aglycon chain above the B ring, and an interaction between the B and aglycon amino group.However, our data indicate that the extent of parallel deprotonations for amino groups of these rings does not exceed 10%, and therefore protonation macroconstants can be assigned to particular amino groups, as presented in Table 1 [the pK and pK* values are numerically very close because deuterium isotopic effects on electrode readings (pH*) and on pK values tend mutually to cancel].30 The conformation of the amikacin molecule remains essentially unchanged throughout the investigated pH* range, despite the charge change between 14 and 0.This is seen in the NMR spectra in the values of JHH coupling constants, indicating chair conformations of all three rings. None of the J values varied by more than 0.5 Hz throughout the whole pH* range (see SUP 57312). Also, the magnitudes of the chemical shift changes of particular protons are determined solely by their distances from the deprotonating amino groups of their rings (SUP 57312).The CD spectrum of amikacin consists of two bands of opposite signs and similar magnitude (l = 216, De = 11.47; 194 nm, De = 22.0 dm3 mol21 cm21), and remains virtually unchanged between pH 2 and 11.5. The only chromophore present that can absorb above 190 nm is the amide connecting the B ring with the aglycon chain. The lack of change in the CD spectra therefore confirms that the conformation of this part of the molecule is not affected by deprotonations. The pK values found for the individual amino groups agree very well with those previously observed for similar moieties.The value for the aglycon chain is typical for aliphatic amines (around 10), the value for the C6]NH2 group of ring A is almost identical to that obtained for 6-amino-6-deoxyglucopyranose, 8.94,15 and the value for the C3]NH2 group of ring C is in the range typical for aminosugars, 7 to 8.23 The relatively low value found for the amino group of ring B is due to the high overall charge of the fully protonated amikacin molecule, and possibly to the inductive effect of the neighbouring hydroxyamide moiety.An electrostatic interaction with the charged aglycon amine, emerging from the NMR data, is also likely. The supplementary material contains lists of chemical shifts and JHH constants vs. pH* and a list of cross-peaks observed in the COSY spectrum. Copper(II) co-ordination Stability constants of the copper(II)–amikacin complexes calculated from potentiometric titrations are presented in Table 3.A speciation diagram calculated with these constants for concentrations used in spectroscopic experiments is shown in Fig. Fig. 4 Species distribution for amikacin complexes for concentrations used in spectroscopic studies (0.001 mol dm23 CuII, 0.002 mol dm23 amikacin)156 J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 Table 3 Potentiometric and spectral data (CD, UV/VIS, EPR; l/nm; e, De/dm3 mol21 cm21; A||/G; G = 1024 T) for the amikacin–copper(II) system EPR Species Cu(H3A) Cu(H2A) Cu(HA) CuA CuH21A CuH22A log ba 29.68(9) 23.94(1) 17.77(1) 10.20(1) 1.61(1) 27.83(1) UV/VIS l (e) Minor Minor 628 (59) 614 (70) 670 (45) 607 (71) 670 (45) 596 (67) CD l (De) Minor Minor 649 (20.41),b 362 (10.10),c 297 (sh) (20.81),d 267 (21.34) e 740 (10.03),f 615 (20.09),f 515 (10.02),f 372 (10.04),c 287 (21.43),d 272 (sh) (21.29) e >750 (<20.13),f 542 (10.09),f 357 (10.12),c 296 (sh) (20.43),d 265 (21.20) e 750 (20.23),f 615(sh) (10.25),f 547 (10.40),f 273 (12.38),g 229 (22.14) h A|| 130 130 187 162 150 Rhombic g|| 2.35 2.35 2.24 2.19 2.16 a b[M(HxA)] = [M(HxA)]/[M][H]x[A]; standard deviations on the last digit are given in parentheses. b d–d Electronic transition of CuII in a tetragonal complex.c OÆCuII CT transition. d NÆCuII CT transition. e NH2ÆCuII CT transition. f d–d Electronic transition of CuII in a lower-symmetry complex.g Mixture of O, N2 and NH2ÆCuII CT transitions. h Intraligand transition. Fig. 5 The CD spectra of copper(II)–amikacin complexes. (A) Binary system, amikacin :CuII = 2 : 1 at various pH: (a) 5.51, (b) 6.02, (c) 7.02, (d ) 7.41, (e) 8.11, ( f ) 8.88, ( g) 10.00 and (h) 11.50. (B) Ternary system, amikacin : histidine :CuII = 1:1:1 at various pH: (i) 5.62, ( j) 6.67, (k) 7.14, (l) 7.95, (m) 8.67, (n) 9.69 and (o) 11.2. For comparison, spectra of Copper–histidine complexes are overlaid: (p) CuL, (q) CuL2 and (r) CuH21L.The asterisk marks the unique CT band in the ternary system (see text) 4. Only 1 : 1 complexes were detected by potentiometry and EPR spectroscopy gave no indication of the formation of copper( II) dimers. The pH dependence of the absorption and CD spectra exhibited excellent consistency with potentiometric speciation, and thus allowed us to calculate spectroscopic parameters of particular complex species (Table 3).All bands located above 250 nm could be assigned to particular d–d or charge transfer (CT) transitions on the basis of previous studies of copper(II) complexation to aminosugars 8–11,23 and peptides.31–34 Examples of experimental CD spectra are presented in Fig. 5(A). No aggregation was found in the solutions studied. The concentration of the initial complex at low pH, Cu(H3A), is too low for this species to be characterised by absorption and CD spectra.However, a complex species could be seen in the parallel part of the EPR spectra beside the copper( II) aqua-ion at pH 5–6, where Cu(H3A) and Cu(H2A) are present. This species has parameters characteristic of one nitrogen donor co-ordinated to copper(II) in aminosugar systems (1N complexes).14,23 The Cu(H3A) complex releases two protons with pK values of 5.74 and 6.26, forming the Cu(HA) species. Spectroscopic parameters of this complex are consistent with two nitrogen (2N) co-ordination.In particular, its EPR parameters are very similar to those seen previously for CuL2 complexes of aminosugars (A|| of 170–190 G, g|| 2.23–2.24).9,14–16 Three further deprotonations are seen, each one lowering the effective symmetry of the complex and introducing profound changes in spectroscopic parameters. The stoichiometry of the final high-pH complex, CuH22A, indicates that in addition to four amino groups, two other donor groups deprotonate. These additional groups, described by negative indices for the hydrogen in the formula, must be bound to CuII because they would not deprotonate otherwise.The amino nitrogen of ring B is the most likely anchoring site for CuII in the amikacin molecule,J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 157 due to its lowest pK. This binding results in the formation of the Cu(H3A) complex. Inspection of molecular models indicates that such binding excludes a possibility of an involvement of the C ring amino group in co-ordination.Therefore the deprotonation pattern of the complex must include a proton dissociation from this amino group, without the participation of copper(II) binding. The pK for formation of the CuA complex, 7.57, is numerically closest to the protonation constant of the C-ring nitrogen in free amikacin. The formation of this complex, and of the further ones at higher pH, results, however, in significant rearrangements of the complex structure.It is difficult to see how a deprotonation of the amine group at least 6 Å away from the CuII might cause that. Therefore, the C-ring amino group must deprotonate at a lower pH. In our recent study of protonation of kanamycin B, an aminoglycoside with five amino groups, we found that the lowest pK is 5.74, identical to the pK for formation of the Cu(H2A) complex of amikacin from Cu(H3A).35 Both deprotonating species have the same overall electrostatic charge of 51 [in Cu(H3A) it is due to monodentate Cu21 co-ordination]. However, the exact reasons for such a low pK value remain unclear.As a result of the non-coordinative deprotonation in ring C, the Cu(H3A) and Cu(H2A) complexes have the same copper(II) binding site, and thus identical EPR spectra. This leads to the amplification of their spectral pattern, which becomes detectable by EPR spectroscopy. The molecular model indicates that both the amide nitrogen and the C5 oxygen of the B ring can easily complete sixmembered chelate rings when CuII is bound to the B amino group.The pK value corresponding to the formation of Cu(HA), 6.17, is closer to those found for amide binding, usually 5–6,31,32 than for deprotonated sugar oxygen co-ordination, 6.5–7.8–15,23 The d–d band in the CD spectrum of Cu(HA) is negative, as is always the case for 2N complexes of simple peptides, while copper(II) complexes of aminosugars with deprotonated oxygens invariably have a positive band between 700 and 600 nm.The chelate ring formed by binding to both nitrogens at ring B is six-membered, as opposed to the five-membered rings in typical peptide complexes. The literature data discussed in ref. 23 indicate, however, that the five- and six-membered rings in aminosugar complexes do not differ much in stability, due to a fixed conformation of donor atoms offered by the rigid ligand. Deoxystreptamine amide is conformationally similar to aminosugars (cf.the NMR results), and so we can safely conclude that in the Cu(HA) complex CuII is co-ordinated to amikacin through both nitrogens of ring B. The oxygen of the C5]OH group of this ring can easily occupy the apical position in the copper(II) co-ordination sphere in Cu(HA) (as defined by the 2N co-ordination), thus completing a set of three fused chelate rings. Alcoholic oxygen co-ordination, well known for aminosugar complexes, is manifested by a characteristic OÆCuII charge transfer (CT) band at ca. 360 nm (Table 3).36 This oxygen deprotonates while bound to CuII, yielding CuA. The pK value for this process, 7.53, is typical for analogous processes of formation of equally charged CuH22L2 complexes with aminosugars.8–15,23 Such strong apical co-ordination causes the decrease in complex symmetry, manifested by the appearance of three d–d bands in the CD spectrum of CuA and a slight rhombic distortion in its EPR spectrum (decrease of A||). The following two deprotonations, to CuH21A and CuH22A, correspond to the binding of the two remaining amino groups to CuII.They are accompanied by the appearance of a twoband envelope in the absorption spectrum, characteristic for distorted square-pyramidal copper(II) complexes,37,38 as well as the increased rhombicity of the EPR spectrum. Each of these events results in extensive changes in the CD spectra. The centre of gravity of the band envelope shifts towards shorter wavelengths upon formation of CuH21A, indicating an increase of in-plane ligand field.The pK value, 8.59, corresponds to deprotonation at ring A in free amikacin (8.89). The molecular model indicates that the A ring amine nitrogen binds trans to the amide nitrogen at CuII, creating a large (10-membered) macrochelate ring. Such binding redefines the co-ordination sphere by placing the B ring amine in the apical position. This explains the alterations of the signs of the d–d bands. The ligand-field effect of the A amine is stronger than from the B amine, as seen in the spectra [Fig. 5(A)]. There is little steric hindrance for the A ring binding, but also no additional effects that might enhance it, thus only a 0.3 log unit stability gain. The final remaining amino group, at the aglycon chain, can also bind only apically relative to the plane of CuA21H, again redefining the complex geometry. The absorption spectrum is insensitive to this rearrangement, but CD and EPR data clearly support it. The intensity of Cotton effects increases several-fold, indicating the increase of complex rigidity.The parallel EPR parameters cannot be extracted for the spectrum corresponding to this complex. This may be due to a heavy overlap of individual spectra in frozen samples. Another potential donor, the ahydroxy group of the aglycon chain, is excluded from coordination by the rigidity of the amide bond, placing it away from CuII. A seven-membered chelate ring is formed. Such rings offer little entropic stabilisation, and thus a small stability gain of 0.5 log units.Copper(II) competition experiments The CD spectra of equimolar mixtures of amikacin, histidine and CuII feature a negative band at 390 nm, the intensity of which increases with increasing pH [Fig. 5(B)]. Such a band is absent from either binary system, thus indicating the formation of ternary complexes. On the other hand, EPR spectra of ternary mixtures could be easily deconvoluted into the spectra seen in binary complexes.However, the relative abundances of the binary components were different from those expected on the basis of their stability constants. These apparent discrepancies might indicate a dimeric or oligomeric and diamagnetic nature of the ternary complexes. Various stoichiometries are possible a priori: Cu2AL, Cu2A2L or Cu2AL2 being the simplest, but not necessarily the most likely (A stands for amikacin, L for histidine).The SUPERQUAD calculations accepted all these stoichiometries, as well as a set of monomeric ternary complexes, with similarly good fits. The nature of ternary copper(II)– amikacin-histidine complexes remains therefore open and will be a subject of our further studies. Copper(II)–amikacin complexes are very stable: a simulation of the partition of CuII between amikacin and histidine in an equimolar system at pH 7.4, with binary complexes only, shows that amikacin binds as much as 35% of CuII at 1023 mol dm23 and 25% at 1024 mol dm23 of the components.The existence of ternary complexes should further increase the fraction of CuII in amikacin-containing species. We therefore tested whether amikacin could be capable of affecting copper(II) homeostasis in blood plasma by comparing the competitiveness of the 1 : 1 amikacin–histidine mixture vs. histidine alone for removal of CuII from the N-terminal binding site of BSA. Thus, 1 mol equivalent of CuII was added to BSA at pH 7.4 and the visible CD spectrum of the resulting complex was recorded.This BSA–CuII complex was then titrated with histidine, with the pH maintained at 7.4. The competition for CuII between BSA and His was followed by recording the CD spectra after each addition of His. The removal of CuII from BSA was quantified with the use of ellipticity at 475 nm. At this wavelength, which is the maximum for the CuII–BSA complex, Cotton effects from histidine, amikacin and their complexes at pH 7.4 are negligible.A curve of partition of CuII between BSA and histidine was thus generated. The same procedure was then repeated, using a 1 : 1 mixture of histidine and amikacin. Fig. 6 presents both curves. One can clearly see that the presence of amikacin did not affect the speciation of CuII between albumin and histidine in vitro, and is thus unlikely to do so in blood plasma.158 J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 Oxidative properties of copper(II)–amikacin complexes The compound 8-oxo-dG is an intermediate in the process of dG oxidation, important for its promutagenic properties.The reaction of its formation from dG also serves as a useful general indicator of the ability of a given system to exert oxidative damage. Results of the oxidation of dG by H2O2 in the presence of CuII, amikacin, or/and histidine, presented in Table 4, indicate that copper(II)–amikacin complexes are particularly specific in facilitating 8-oxo-dG formation by H2O2.As much as 8% of reacted dG was converted into 8-oxo-dG in 24 h, as opposed to 1% generated by copper(II)–histidine, with a similar extent of dG decomposition. The ternary system, more likely to occur in vivo than the binary one, was even more specific, with 22%. Oxygen metabolism generates appreciable amounts of H2O2 intracellularly. If even minute amounts of CuII are available, e.g. via degradation or damage of respiratory chain enzymes, then amikacin-induced oxidations may become an important element of cellular toxicity of the drug, by means of oxidative damage to DNA or other biomolecules.The results presented above also indicate a possibility that oxidative properties of copper(II) complexes of amikacin contribute to its bactericidal activity, because hypoxia was shown to help bacteria survive amikacin therapy.25 Conclusion Amikacin co-ordinates CuII at physiological pH through the amino and amide nitrogens and the C5 oxygen of the B (deoxystreptamine) ring [Cu(HA), CuA].At higher pH also ring A and the aglycon chain amino groups are involved in the binding (CuH21A and CuH22A complexes, respectively). Fig. 6 Effect of amikacin on the partition of CuII between bovine serum albumin and histidine. Titration of 7 × 1024 mol dm23 CuII–BSA at pH 7.4 and 25 8C with histidine (d), and 1 : 1 histidine 1 amikacin (s). The concentration of the complex was calculated from the ellipticity at 475 nm (see text for further details).Experimental error bars are shown on d only for clarity Table 4 Oxidation of dG by H2O2 facilitated by amikacin (A), histidine (His) and their copper(II) complexes. Concentrations (mol dm23): dG, CuII, His and A 1024; H2O2 1023; phosphate buffer (pH 7.4) 0.1; 24 h incubations at 37 8C Components Control c CuII His CuII 1 His A CuII 1 A CuII 1 A 1 His 8-oxo-dG yield (%) a 0.04 ± 0.01 0.17 ± 0.1 0.03 ± 0.01 0.33 ± 0.06 0.15 ± 0.03 3.5 ± 0.5 9.4 ± 0.2 dG decomposition (%) b 0 12 ± 1 03 1 ± 2 04 4 ± 4 42 ± 3 a Averages of five measurements ± standard errors, normalised to initial dG concentrations.b Detection limit = 4%. c dG 1 H2O2. The complexes are very strong, only slightly less stable than those of histidine. However, amikacin does not contribute to competition between histidine and serum albumin and thus should not be expected to affect copper homeostasis in blood plasma. 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Dis., 1992, 165, 676. 26 M. Maurin and D. Raoult, J. Infect. Dis., 1994, 169, 330. 27 H. Irving, M. G. Miles and L. D. Pettit, Anal. Chim. Acta, 1967, 38, 475. 28 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton Trans., 1985, 1195. 29 N. H. Andersen, H. L. Eaton, K. T. Nguyen, C. Hartzell, R. J. Nelson and J. H. Priest, Biochemistry, 1988, 27, 2782.J. Chem. Soc., Dalton Trans., 1998, Pages 153–159 159 30 W. U. Primrose, in NMR of Macromolecules, ed. G. C. K. Roberts, IRL Press, Oxford, 1993, ch. 2, pp. 22 and 23. 31 H. Sigel and R. B. Martin, Chem. Rev., 1982, 82, 385. 32 L. D. Pettit, S. Pyburn, W. Bal, H. Koz�owski and M. Bataille, J. Chem. Soc., Dalton Trans., 1990, 3565. 33 G. Formicka-Koz�owska, H. Koz�owski, I. Z. Siemion, K. Sobczyk and E. Nawrocka, J. Inorg. Biochem., 1981, 15, 201. 34 I. Z. Siemion, A. Kubik, M. Jez· owska-Bojczuk and H. Koz�owski, J. Inorg. Biochem., 1984, 22, 137. 35 M. Jez· owska-Bojczuk, W. Bal and H. Koz�owski, Inorg. Chim. Acta, in the press. 36 M. Jez· owska-Bojczuk, H. Koz�owski, S. Lamotte, P. Decock, A. Temeriusz, I. Zaja�czkowski and J. Ste�pin� ski, J. Chem. Soc., Dalton Trans., 1995, 2657. 37 A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd edn., Elsevier, Amsterdam, 1984, pp. 568 and 569. 38 M. Jez· owska-Bojczuk, T. Kiss, H. Koz�owski, P. Decock and J. Barycki, J. Chem. Soc., Dalton Trans., 1994, 811. Receiv

ISSN:1477-9226    

DOI:10.1039/a704834c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 161–169 161 Zinc(II) complexes of phosphonic acid analogues of aspartic acid and asparagine Ewa Matczak-Jon,*,a Barbara Kurzak,b Wanda Sawka-Dobrowolska,c Barbara Lejczak d and Pawe� Kafarski d a Institute of Inorganic Chemistry, Technical University, 50-370 Wroc�aw, Poland b Department of Chemistry, Pedagogical University, 08-110 Siedlce, Poland c Department of Chemistry, University of Wroc�aw, 50-383 Wroc�aw, Poland d Institute of Organic Chemistry, Biochemistry and Biotechnology, Technical University, 50-370 Wroc�aw, Poland An analogue of aspartic acid obtained by the replacement of the a-carboxylic group by a phosphonic acid group underwent chemical transformation upon the action of zinc(II) ions under neutral and slightly alkaline conditions yielding, most probably, a cyclic phosphonamidate composed of two ligand molecules.In contrast, an analogue obtained by the replacement of the b-carboxylic group by a phosphonic acid moiety did not undergo such a transformation.In order to determine which structural features are responsible for the observed cyclization a series of aminophosphonic acids was synthesized and their behaviour in zinc(II) solutions studied by means of NMR spectroscopy. Cyclization of 1-aminoalkylphosphonic acids upon the action of zinc(II) ions in neutral and slightly alkaline media seems to be a general property of these acids. The crystal structures of two of the studied compounds were also determined.Recently we have shown that the analogue of glutamic acid obtained by the replacement of the a-carboxylic by a phosphonic acid group undergoes chemical transformation upon the action of zinc(II) ions under neutral and slightly alkaline conditions yielding, most probably, a cyclic phosphonamidate composed of two ligand molecules.1 This reaction may be considered as a mimic of biologically important reactions of pentavalent phosphorus acids in which zinc acts either as a Lewis acid accepting a lone electron pair of oxygen from the P]] O bond and thus increasing the electrophilicity of the phosphorus atom, or by complexing the amine group of the molecule changing significantly its nucleophilicity.In order to determine which structural features are responsible for the ability of phosphonic acid analogues of acidic amino acids to undergo such a transformation, and especially for a better understanding of the role of the carboxylic group, we have studied the behaviour of analogues of aspartic acid and asparagine in aqueous solutions containing zinc(II) ions.Experimental Materials Phosphonic acid analogues of aspartic acid and asparagine as well as their esters were obtained according to previously described procedures.2 Other aminoalkylphosphonic acids were synthesized according to the standard procedure.3 All the compounds were used in their racemic forms and are shown in Table 1.The purities and exact concentrations of the solutions of the pro-ligands used for potentiometric studies were determined by the method of Gran.4 The concentration of zinc(II) chloride stock solution was standardized by complexometric titration with ethylenedinitrilotetraacetate (edta). Carbonate-free potassium hydroxide (the titrant) was prepared and standardized against a standard potassium hydrogenphthalate solution. The concentration of KOH was ca. 0.2 mol dm23.Infrared spectra were recorded on a Perkin-Elmer 2000 FT-IR spectrometer. Potentiometric measurements Acid dissociation constants of the pro-ligands were determined by pH-metric titration of 5 cm3 samples of concentration 4 × 1023 mol dm23. The ionic strength was adjusted to 0.2 mol dm23 with KCl in each case. The titrations were performed over the range pH 2–11.5 with KOH solution of known Table 1 Compounds used in this study Compound 1 Structure HO2C PO3H2 NH2 Name 3-Amino-3-phosphonopropionic acid 2 HO2C PO3H2 NH2 3-Amino-3-phosphonobutyric acid 3 MeO2C PO3H2 NH2 Methyl 3-amino-3-phosphonobutyrate 4 H2N PO3 H2 O NH2 3-Amino-3-phosphonopropionamide 5 MeO2C P Me NH2 O OH Methyl 3-amino-3-[hydroxy- (methyl)phosphinoyl]propionate 6 H2O3P CO2H NH2 2-Amino-3-phosphonopropionic acid 7 PO3H2 NH2 1-Aminobutylphosphonic acid 8 PO3H2 NH2 1-Amino-1-methylpentylphosphonic acid 9 PO3H2 NH2 1-Amino-1-ethylbutylphosphonic acid 10 PO3H2 H2N 1-Aminocyclobutylphosphonic acid162 J. Chem.Soc., Dalton Trans., 1998, Pages 161–169 concentration (ca. 0.3 mol dm23). All titration solutions were thermostatted at 25 ± 0.1 8C using a constant-temperature water-bath. The pH was measured with a MOLSPIN automatic titration system using a micro combination pH electrode (Mettler- Toledo, type U402-M6-57/100). The electrode system was calibrated by periodic titrations of HCl solution (3 × 1023 mol dm23 in KCl) against standard KOH solution.The resulting titration data were used to calculate the standard electrode potentials, E8, and the dissociation constant for water (pKw = 13.74). These values were then used to calculate the hydrogen-ion concentration [H1] from potential readings; 5 number of titrations, 3; calculations, SUPERQUAD computer program.6 NMR measurements Phosphorus and proton NMR spectra were recorded on a Bruker DRX spectrometer operating at 300.13 MHz for 1H and 121.50 MHz for 31P given in relation to SiMe4 and 85% H3PO4, respectively.All downfield shifts are denoted as positive. The Bruker WIN NMR DAISY software was applied to iterate ABCX (X = 31P) 1H NMR spectra. The JHP and JHH coupling constants obtained in this way were used to calculate rotamer populations. Samples for NMR studies were prepared in deuteriated water with a pro-ligand concentration of 0.02 or 0.01 mol dm23. A zinc(II) to pro-ligand molar ratio of 1 : 2 was applied in all cases using zinc nitrate hexahydrate (Aldrich) as a source of ZnII.The pH of the samples was measured using a Radiometer pHM83 instrument equipped with a 2401C combined electrode and given as meter readings, without correction for pD. The measurements were performed only for freshly prepared samples. This allowed us to avoid among other things extensive hydrolysis of the carboxylate ester functions of compounds 3 and 5. The hydrolytic cleavage of the CO2Me group was monitored by integration of the 1H NMR methyl resonances which corresponded to both CO2Me and to MeOH which was released upon its hydrolysis.This showed that under the conditions of the experiment the hydrolysis of compound 3 may be neglected, whereas the extent of hydrolysis of 5 increased from about 1% at neutral pH to about 20% at pH 11.6. Under more alkaline solutions only the spectra of 3-amino-3-[hydroxy- (methyl)phosphinoyl]butyric acid were recorded. The measurements in solutions containing zinc(II) ions were limited to a rather narrow range of pH (usually between 6–7 and 9.5) because of precipitation.1 Crystallography Crystals of compounds 4 and 10 were obtained by recrystallization from water and were used for the data collection.The space groups and approximate unit-cell dimensions were determined from rotation and Wessenberg photographs. The diffraction data were measured at 293(2) K on a KUMA KM4 computer-controlled four-circle diffractometer with graphitemonochromated Cu-Ka (l=1.5418 Å) and Mo-Ka (l= 0.710 69 Å) radiation respectively.7 Details of the diffraction experiments, crystal data collection and refinement are given in Table 3.The structures were solved by direct methods with SHELXS 868 and refined on F2 for compound 4 and F for 10 by full-matrix least-squares methods using SHELXL 939 with anisotropic thermal parameters for non-hydrogen atoms. During the refinement an extinction correction was applied but no absorption correction.For compound 4 all hydrogen atoms were found from a Fourierdifference synthesis and refined isotropically. For 10 carbonbound hydrogen atoms were initially placed in calculated positions (with the thermal parameters being 1.2 times Ueq of the parent carbon atom) and the remainder were found from the difference synthesis. Hydrogen atoms of amino groups were refined with isotropic thermal parameters being 1.5 times Ueq of the parent nom. In the case of compound 10 the absolute structure cannot be determined reliably since the value of the Flack parameter 10 was 0.31(12) and refinement of the inverted structure gave R = 0.0451 and wR = 0.1022.Scattering factors and real, as well as imaginary, components of anomalous dispersion were those incorporated with SHELXL 93. The ORTEP11 package was used to generate molecular drawings. CCDC reference number 186/755. Results Potentiometric studies of proton complexes Protonation constants were calculated assuming that ligand concentrations were as calculated from the titration curves and then allowing ligand and proton concentrations to ‘float’ in SUPERQUAD.It was found that the concentrations never changed by more than 1% and calculated constants remained constant within the calculated errors. This confirmed the purity of the compounds studied. With the exception of phosphinic acid 5 all the compounds in Table 1 have two common functional groups: amino and phosphonate moieties with some of them having additional groups, namely carboxylic (compounds 1 and 2), methyl carboxylate (3) or amide (4) functions.Compounds 2, 3 and 8 also possess methyl groups in a position in relation to the phosphonic moiety (Ca carbon atom). Since the methyl group is less electronegative than a hydrogen atom, the replacement of the latter by methyl changes the electron density of the Ca carbon atom. This density is partly transferred to the neighbouring nitrogen and phosphorus atoms and consequently the basicity of the NH3 1 and PO3H2 groups in compounds 2 and 7 are higher than those found for structurally related compounds 1 and 6 [for example, compare pK(NH3 1) values for these pairs of compounds in Table 2].Examination of the pK(NH3 1) values for compounds 1 and 8 clearly shows that the positioning of the CO2 2 group makes scarcely any difference. Quite oppositely, the presence of a strongly electron-withdrawing methyl carboxylate in b position to an amino moiety (compound 3) resulted in a significant decrease of pK(NH3 1).The highest acidity (among phosphonic acids) of the NH3 1 group observed in the case of compound 4 well illustrates the fact that the inductive effect of an amide group is much greater than that found for methyl carboxylate. NMR studies Proton NMR spectra were recorded to obtain populations of rotational isomers for those aspartic acid derivatives which exhibit four-spin ABCX (X = 31P) subspectra. The homo- Table 2 Deprotonation constants (pK) of the studied aminophosphonic acids at 25 8C and I = 0.2 mol dm23.Standard deviations are given in parentheses pK Compound 1 a 2 3 4 5 6 a 7 8 9 10 NH3 1 10.07(1) 10.82(6) 9.34(5) 9.05(5) 7.4 b 10.79(1) 10.03(1) 10.35(4) 10.51(4) 9.79(9) PO3H2 5.52 5.74 5.40 5.22 6.07 5.66 6.02 6.18 5.73 CO2H 3.44 3.10 2.40 a Taken from ref. 12. b From NMR measurements; pK(PO2H) ª 1.J. Chem. Soc., Dalton Trans., 1998, Pages 161–169 163 Table 3 Homo- (1H]1H) and hetero-nuclear (1H]31P) coupling constants (J/Hz) used in conformational analysis of compounds 4–6 and their zinc(II) solutions 4 ZnII–4 5 ZnII–5 6 ZnII–6 pH 0.73 3.27 7.01 11.12 5.02 6.20 9.80 1.03 6.86 11.60 6.58 9.86 1.12 3.12 9.05 12.82 6.68 9.64 1H]1H H1, H2 H1, H3 H2, H3 217.1 10.4 3.7 217.0 10.8 3.4 217.0 11.8 2.8 216.5 11.2 3.4 217.0 11.0 3.2 216.9 11.5 3.0 215.6 11.4 3.0 217.7 9.8 4.1 217.3 10.0 3.9 216.0 11.1 3.2 217.4 9.9 3.9 216.1 11.1 3.3 216.3 8.5 4.6 215.7 10.8 3.6 214.8 11.2 2.2 214.7 11.8 1.9 215.5 11.6 2.7 215.1 13.2 2.1 1H]31P H1, P H2, P H3, P 7.5 8.8 13.9 7.0 7.9 13.0 5.4 6.2 12.8 5.5 6.5 13.3 6.5 7.5 13.3 6.1 6.2 13.3 6.0 6.4 13.6 9.5 9.2 10.5 7.3 8.2 9.8 7.1 6.9 10.3 7.5 8.3 10.5 7.1 6.9 10.3 16.0 17.7 16.1 14.9 17.8 13.6 12.5 16.6 11.9 13.5 17.7 10.0 13.5 16.9 12.1 11.9 17.7 11.8 nuclear JHH and heteronuclear JHP coupling constants used in calculations were obtained by iteration of experimental spectra and representative results are given in Table 3.A conformational analysis of a-aminophosphonic derivatives was performed according to the notation given in Fig. 1 based on 3JHP coupling constants. We have taken two sets of data for calculations, namely Jt = 33.0 Hz and Jg = 4.2 Hz recommended for a-Asp(P) (3-amino-3-phosphonopropionic acid) 12 and the widely used Pachler 3JHH coupling constants (Jt = 13.6 and Jg = 2.6 Hz).13 Both sets gave, with tolerable precision, approximate values of the mole fraction for each rotamer, as well as showing analogous tendencies in rotamer population changes versus pH.Both 4 and 5 adopt, similarly to a-Asp(P),12 a predominant p2 conformation, i.e. that containing bulky groups located trans to each other (Fig. 2). Protonation effects seem to induce particular conformations to a much lesser extent than does steric hindrance (Fig. 2). The intramolecular hydrogen bonding between the protonated amino group and the carbonyl group of the amide, the existence of which was suggested by our crystallographic data, although presumably weaker in solution, may be an additional stabilizing factor which favours the p2 conformation of 4 over p2 of 5 at a pH <10. Among the aminophosphonic acids studied those which are lacking phosphonic and amino groups bonded to the same carbon atom (Ca atom), namely 5 and 2-amino-3-phosphonopropionic acid [b-Asp(P), 6] seem to belong to the ‘fast equilibrium system’ class which was discussed in our previous work.1 In their solutions zinc(II) complexation results only in (dependent on co-ordination) shifting and (dependent on chemical exchange rate) broadening of the single peak in the 31P NMR spectra.The 31P NMR chemical shift profiles of compound 6 and species formed upon complexation with zinc(II) ions are given in Fig. 3. These data differ significantly from those obtained for the ZnII–g-Glu(P) 1 : 2 system [g-Glu(P) = 2-amino-2-phosphonobutanoic acid] where bidentate chelation involving NH2 and CO2 2 was shown1 because b-Asp(P) seems to involve all possible donor groups in zinc(II) co-ordination.This results in five- and six-membered joined chelate rings and a stability Fig. 1 Newman projections for the three staggered rotamers relative to the c1[Ca]Cb] torsion angle (R1 = OH, R2 = PO3H2, compound 1; NH2, PO3H2, 4; OMe, PMeO2 2 5) C H1 H2 R1 O R2 NH3 + H3 C H1 H2 R1 O H3 R2 H3N C H1 H2 R1 O H3N H3 R2 p1 p2 p3 increase of ZnL and ZnL2 species versus the respective ZnII–g- Glu(P) species of the same stoichiometry.14 This finding also corresponds fairly well to our general observation that stepwise chelation of zinc(II) by phosphonate followed by complexation by an amino group results both in a gradual downfield shift of 31P resonances and in a significant (about 2–3 log units) decrease in pKNH3 1.Conformational studies of b-Asp(P) were performed using the notation given in Fig. 1 with the replacement of the CO(R1) group by a phosphonic one (then R2 corresponds to b-carboxylate). Significant discrepancies were achieved when fractional populations were calculated basing on previously described 3JHH and 3JHP pairs of trans and gauche coupling constants. As the application of a pair of Jt and Jg values given in this work for vicinal H]P coupling constants seem to be generally restricted to phosphonic derivatives containing a PCHCH2 molecular fragment,15 we used Pachler’s 3JHH trans and gauche coupling constants.13 Again, the most stable and sterically favoured conformation of the molecule has a torsion angle c1 of 1808 (p2 conformation).However, upon zinc(II) complexation it increased considerably at the expense of Fig. 2 Rotamer populations and 31P NMR chemical shifts as a function of pH for compounds 4 and 5 and their zinc(II) solutions [(a), (c) and (b), (d ) respectively]. (s) Free pro-ligands and (d) their zinc(II) solutions164 J.Chem. Soc., Dalton Trans., 1998, Pages 161–169 both p1 and p3. The considerable broadening of the 31P NMR peak in slightly acidic and neutral ZnII–5 solutions seem to reveal a weak complexation of zinc(II), presumably in a monodentate way via the POMe2 group. There is no indication of the involvement of the amino group in metal chelation. As a result no significant chemical shift changes in the 1H NMR spectra of the pro-ligand solutions upon addition of ZnII (Fig. 2) and invariant rotamer populations in respect of those of the metalfree aminophosphonic acid were observed. Single 31P NMR resonances were also detected in the ZnII–4 system at a measurable range of pH values. Extensive precipitation precluded potentiometric measurements at pH > 6.5. The calculations confined to a rather narrow pH range revealed the formation of Zn(HL) (log b = 11.84) as a main species at pH 5 with some ZnL (log b = 5.27) and ZnL2 (log b = 9.30) complexes which formed stepwise upon increase of pH.Owing to precipitation, NMR data were also restricted to the slightly acidic and alkaline pH regions only (see Fig. 2). Under these conditions distinct differences in chemical shifts of all proton signals versus respective free pro-ligand curves seem to reveal a rearrangement of the ligand in the complex where at least the phosphonate and amino groups are involved in co-ordination. Chelation, however, does not significantly influence the rotamer populations with p2 still being predominant.Extensive precipitation at pH > 6.5 seems to result from formation of a new complex(es) in which all the possible donor groups, including amidate, as described,16 may be co-ordinated. The above finding corresponds well to our observations considering the ZnII– Met(P) system [Met(P) = 1-amino-4-thiapentylphosphonic acid] where the involvement of thioether sulfur as a third donor group also resulted in formation of poorly soluble zinc(II) complex(es).17 The IR spectrum of the precipitate did not allow us to determine if this is a zinc(II) complex of unchanged ligand or a complex of its dimeric cyclic form (see Discussion).The 31P NMR spectra of zinc(II) solutions with compound 1 resemble those recorded for zinc(II) solutions with its homologue, a-Glu(P) [4-amino-4-phosphonobutanoic acid], and structurally related compounds recently studied in our laboratories. 1 In the presence of zinc(II) ions a pair of single resonances appears under neutral and slightly alkaline solutions. The first one, sharp and shifted downfield, remains almost intact with change in pH. The second, broader and appearing at higher field, moves gradually when the pH increases (Fig. 4). The details of these solution equilibria were discussed in our previous paper.1 A downfield signal was ascribed to a cyclic phosphonamidate formed from the respective pro- Fig. 3 The 31P NMR chemical shifts as a function of pH for compound 6 (s) and its zinc(II) solutions (d) ligand upon action of zinc(II).If so, similar effects should be observed in spectra of simple a-aminophosphonic acids. Indeed, nearly identical spectra were observed in zinc(II) solutions with compounds 7 and 10. The assumption of the formation of a cyclic phosphonamidate is strongly supported by the fact that 31P chemical shifts recorded for the new species are always in the range d 24–29, which is characteristic for phosphonamidates.18 The pH-dependent 31P resonance might reflect both deprotonation of the amino group and its involvement in stepwise chelation of metal ion.The apparent narrowing of the signal upon increasing temperature reveals an increasing exchange rate between zinc(II) species and proligand, in relatively fast equilibrium. The extent of broadening seems to depend on the structurally related flexibility of the side chain of the respective phosphonic acid and decreases in the following order: pro-ligand 10 @ 7 > 1.Precipitation of solid material from neutral or slightly alkaline solutions of ZnII–10 upon standing was observed. This resulted in disappearance of downfield resonances for the samples. Significant narrowing and upfield shifting (ª1.5 ppm) of the second 31P NMR signal which accompanied this process reflects the pH decrease arising from reestablishment of the equilibrium towards the pro-ligand.Examination of the IR spectrum of the precipitated material also supports the formation of a cyclic phosphonamidate: especially in the dNH region because three peaks (1638, 1615 and 1538 cm21) present in the spectrum of the pro-ligand are replaced by a single peak at 1590 cm21 in the spectrum of the precipitate. Thus, the precipitate most probably represents the complex of zinc(II) and a cyclic phosphonamidate. The 31P NMR spectra recorded for respective solutions containing zinc(II) complexes of compounds 2 and 3 contained quite complicated multiplets instead of the single additional downfield resonance characteristic for ZnII–a-Glu(P) and analogous systems.We assume that this feature arises from conformational strain induced by the methyl group in the a position and the rotational barrier at the Ca]Cb bond. In order to verify this assumption the spectra for zinc(II) complexes of compounds 8, also containing a methyl group at the a position, and 9, containing an ethyl group at this position, were recorded.Their spectra in neutral and slightly alkaline solutions were similar to those observed for 2 and 3. The extent of multiplet complexity seems to depend on the size of the alkyl substituent Fig. 4 The 31P NMR chemical shifts as a function of pH for zinc(II) solutions of compounds 1, 7 and 10J. Chem. Soc., Dalton Trans., 1998, Pages 161–169 165 and somewhat on the length of the aminophosphonic acid backbone (Fig. 5). On the other hand no influence of this type of substitution on upfield resonances was observed. These resonances arise from the complexes of unchanged pro-ligand with zinc(II) ions and their shapes and dP values reflect both chemical exchange rates and pH-dependent equilibria between zinc(II)-bonded species and the respective pro-ligand. An increase in temperature caused not only narrowing of upfield resonances but also changed the multiplicity of downfield 31P signals (Fig. 6). In the case of zinc(II) solutions containing proligands 2 and 3 which differ only in carboxylic (free or substituted) functions the signals merged into a single broad peak, whereas for two other a-aminophosphonic acids (pro-ligands 8 and 9) complete coalescence was not observed even at 350 K. It is worth emphasizing that this process was fully reversible as the 31P NMR spectra recorded after cooling of the parent samples to room temperatures were identical with those initially measured.Again, solid precipitated from ZnII–pro-ligand 9 solutions of initial pH ca. 7 upon standing and this resulted in the disappearance of downfield resonances for the remaining solutions. Similarly, as in the case of compound 10, the IR spectrum Fig. 5 Low-field 31P NMR resonances of ZnII: pro-ligand 1 : 2 solutions: (a) pro-ligand 2, pH 7.64; (b) 3, 7.36; (c) 8, 7.45 and (d ) 9, 7.47 Fig. 6 Low-field 31P NMR resonances of ZnII: pro-ligand 1 : 2 solutions versus temperature: (a) pro-ligand 2 at 350 K, pH 7.10; (b) at 300 K (after cooling); (c) 3 at 350 K, pH 7.36 of the precipitate strongly supports the suggestion that the downfield resonance represents a complex between zinc(II) and a cyclic phosphonamidate.Also in this case changes in the dNH region are the most striking since three peaks found in the proligand spectrum (1640, 1615 and 1537 cm21) are replaced by a single peak at 1579 cm21 in the spectrum of the precipitated complex. Crystal structures The molecular structure and atom numbering of compounds 4 and 10 are shown in Figs. 7 and 8 respectively. Selected bond distances and angles are given in Table 5. As can be seen in Fig. 7 the phosphonic acid group of compound 4 is ionized with the proton being transferred to the amino group which results in the formation of a zwitterion. The bond lengths and angles in the aminomethylphosphonic acid part of the molecule are in good agreement with those found earlier for [a-Asp(P), compound 1],15 and b-Asp(P), 6.19 In the present structure two of the three P]O bonds lengths of 1.495(1) and 1.506(1) Å are significantly shorter than the third [1.569(1) Å].Thus, as in the case of a- and b-Asp(P), the two shorter P]O bonds may have partially double-bond character, while the longer one corresponds to the P]OH bond. The P]C(1) distance of 1.845(2) Å is slightly longer than the P]CH2 bond in b-Asp(P) [1.809(4) Å] and similar to that found in a-Asp(P) [1.846(4) Å].Compound 4 is planar with the maximum deviation from the least-squares best plane through atoms P(1), C(1), C(2) and C(3) being only 0.004(1) Å, while N(1) and N(2) show deviations of 21.166(4) and 0.070(4) Å, respectively. The amide moiety is also planar; the maximum deviation from the best plane through N(2), C(3), O(4) and C(2) is 0.001 Å. The hydrogen atoms H(8) and H(9) lie close to this plane [0.01(2) and Fig. 7 Molecular structure of 3-amino-3-phosphonopropionamide showing the atomic numbering.Displacement ellipsoids are drawn at the 35% probability level. The intramolecular hydrogen bond is shown as a dashed line Fig. 8 Molecular structure of two 1-aminocyclobutylphosphonic acid molecules (A and B) in the asymmetric unit, showing the numbering scheme. Ellipsoids are at the 30% probability level. Hydrogen bonds are shown as dashed lines166 J. Chem. Soc., Dalton Trans., 1998, Pages 161–169 Table 4 Summary of crystal data, data collection and refinement conditions for compounds 4 and 10 Formula M Crystal symmetry Space group No.reflections (2q/8) a/Å b/Å c/Å b/8 U/Å3 Z Dc/g cm23 Dm/g cm23 b F(000) Crystal dimensions/mm Decay of standards (%) Reflections measured 2q Range/8 h,k,l Ranges Reflections observed [Fo > 4s(F )] m/mm21 Extinction correction No. parameters varied Weights (a, b, f )c Goodness of fit R1 = S(|Fo| 2 |Fc|)/|Fo| wR2 = [Sw(Fo 2 2 Fc 2)2/Sw(Fo 2)2]� �� Largest feature in final difference map/e Å23 4 C3H9N2O4P 168.09 Monoclinic P21/c 25 (25–37) 8.455(2) 9.600(2) 8.930(2) 110.64(3) 678.3(3) 4 1.646(1) 1.65 352 0.25 × 0.20 × 0.18 2 1521 10–161.0 0–10, 0–11, 211 to 10 1273 3.372 0.0030(8) 124 0.0803, 0.26, 1/3 1.065 0.0367 0.1076 0.35, 20.25 10 C4H10NO3P?H2O 169.12 Orthorhombic Pbc21 45 (20–29) 6.368(2) 20.093(2) 11.700(4) 1497(6) 8 1.501(1) 1.50 720 0.35 × 0.30 × 0.30 2.5 3554 5.0–62.0 0–8, 0–26, 215 to 15 2458 0.328 0.0050(12) 230 0.0, 1.05, 1/3 1.077 0.044 0.0991 0.35, 20.29 a Details in common: three standard reflections every 100; Lorentz-polarization correction; w–2q scans.b By flotation in chlorobenzene–carbon tetrachloride. c w = 1/[s2(Fo 2) 1 (aP)2 1 bP] where P = [ f?max (0 or Fo 2) 1 (1 2 f )Fc 2]. 0.06(3) Å]. The observed N(2)]C(3) value of 1.323(2) Å is intermediate between a double and a single bond and compares well to the values for analogous bonds in other compounds.20 The carbonyl C]] O double bond [1.234(3) Å] is significantly longer than the normal length (1.215 Å) and agrees well with the length expected when the O atoms take part in hydrogenbond formation.21 This is the case here since O (carbonyl) is an acceptor of two hydrogen bonds [2.756(2) and 2.788(2) Å] from two hydrogen atoms from the N(1) amino groups.The side-chain conformation is given by the P]C]C]C and N]C]C]C torsion angles (Table 6), 179.5(1) and 257.1(2)8. The same conformation was found for a-Asp(P), for which the corresponding angles are 176.5(3) and 260.0(4)8.The HO(1)] P(1)]C(1)]C(2) torsion angle is however of 279.8(1)8, differing by about 118 from that observed in a-Asp(P) [91.1(3)8]. An extensive hydrogen-bond network exists in the crystal structure of compound 4 (Table 7 and Fig. 9) involving all the phosphonic, amine and amide groups. The conformation of the molecule is stabilized by the formation of an intramolecular N(1)]H(3) ? ? ? O(4) hydrogen bond of 2.756(2) Å.In addition, Fig. 9 Crystal-packing arrangement of compound 4. Hydrogen bonds are shown as dashed lines H(3) participates in a second weak hydrogen bond [3.257(2) Å] involving atom O(3) of the phosphonic group from a neighbouring molecule. Furthermore, a slightly longer hydrogen bond [2.904(2) Å] occurs between N(2)]H(9) and O(2). On the other hand, adjacent molecules are held together by pairs of O(1)]H(1) ? ? ? O(3) [2.584(2) Å] and N(1)]H(5) ? ? ? O(2) [2.693(2) Å] hydrogen bonds related by two centres of symmetry from centrosymmetric dimers (Fig. 10). All dimers in the crystal are connected to each other via the linear N(2)] H(8) ? ? ? O(3) [2.993(2) Å] hydrogen-bond system in which the amide group acts as a donor and the phosphonic oxygen O(3) at x, y, 1 1 z is an acceptor. In the solid state compound 10, similarly to other aminophosphonates, exists as a zwitterion. The asymmetric unit of its hydrate which comprises two independent molecules (A and B) and two water molecules is shown in Fig. 8 with the atomic Fig. 10 View of dimers of compound 4 showing the atomic numbering. Displacement ellipsoids are at the 30% probability level. The intramolecular hydrogen bond is shown as a dashed line. Primed atoms (9, 0) and all other atoms in the same molecules are at equivalent positions 1 2 x, 1 2 y, 2z and 2x, 1 2 y, 2zJ. Chem. Soc., Dalton Trans., 1998, Pages 161–169 167 numbering schemes. The two molecules are rather similar, but not identical (Table 5).The bond lengths and angles of the phosphonic groups are very similar to each other in both structures. The main differences are found in the cyclobutyl rings. Bonds C(3)]C(2) and C(3)]C(4) appear to be shorter in molecule A and in B whereas the C(2)]C(3)]C(4) angles in the two molecules differ by 3.08. The calculated values of the bonds and angles in which atom C(3A) is involved should be taken, however, with some reservation, because of the larger displacement parameter found for this atom (Fig. 8). The four-membered ring of molecule A is in a different conformation from that of B. The amount of puckering defined as the angle between the planes C(2)]C(1)]C(4) and C(2)]C(3)] C(4) is 5.8(4)8 for A and 21.4(5)8 for B. This indicates that the cyclobutane ring of A has an approximately planar conformation. The angles of bonds around atoms C(1) vary from 87.1(4) to 115.4(3)8 (for A) and 89.1(4) to 116.6(4)8 (for B) and the geometry around C(1) in both molecules is typical for a four-membered ring.22 As in other aminophosphonic acids, compound 10 also manifests the characteristic feature of a network of intermolecular hydrogen bonds (Table 7).Hydrogen bonds exist between the crystallographically independent acid molecules. Thus, molecules A and B of the asymmetric unit are interconnected by two hydrogen bonds, which involve the hydrogen atoms of amino groups and the phosphonic oxygen atoms (Fig. 8). The phosphonic groups of the two forms of 10 participate in Table 5 Selected bond lengths (Å) and angles (8) for compounds 4 and 10 Compound 4 P]O(2) P]O(3) P]O(1) P]C(1) O(4)]C(3) O(2)]P]O(3) O(2)]P]O(1) O(3)]P]O(1) O(2)]P]C(1) O(3)]P]C(1) O(1)]P]C(1) N(1)]C(1)]C(2) 1.495(1) 1.506(1) 1.569(1) 1.845(2) 1.234(3) 117.5(1) 106.9(1) 112.0(1) 109.4(1) 107.6(1) 102.9(1) 110.8(1) N(1)]C(1) N(2)]C(3) C(1)]C(2) C(2)]C(3) N(1)]C(1)]P C(2)]C(1)]P C(3)]C(2)]C(1) O(4)]C(3)]N(2) O(4)]C(3)]C(2) N(2)]C(3)]C(2) 1.488(2) 1.323(2) 1.519(2) 1.517(2) 110.7(1) 111.0(1) 113.4(2) 123.5(2) 120.4(2) 116.1(2) Compound 10 Molecule A P(1A)]O(1A) P(1A)]O(2A) P(1A)]O(3A) P(1A)]C(1A) N(1A)]C(1A) C(1A)]C(2A) C(1A)]C(4A) C(2A)]C(3A) C(3A)]C(4A) O(1A)]P(1A)]O(2A) O(1A)]P(1A)]O(3A) O(1A)]P(1A)]C(1A) O(2A)]P(1A)]O(3A) O(2A)]P(1A)]C(1A) O(3A)]P(1A)]C(1A) P(1A)]C(1A)]N(1A) P(1A)]C(1A)]C(2A) P(1A)]C(1A)]C(4A) N(1A)]C(1A)]C(2A) N(1A)]C(1A)]C(4A) C(2A)]C(1A)]C(4A) C(1A)]C(2A)]C(3A) C(2A)]C(3A)]C(4A) C(1A)]C(4A)]C(3A) 1.579(3) 1.496(3) 1.495(3) 1.821(5) 1.481(6) 1.561(7) 1.548(7) 1.481(8) 1.467(9) 111.4(2) 106.0(2) 104.9(2) 118.9(2) 107.4(2) 107.4(2) 111.4(2) 115.4(3) 114.7(3) 113.5(4) 112.8(4) 87.1(4) 89.0(5) 93.4(6) 90.3(5) Molecule B P(1B)]O(1B) P(1B)]O(2B) P(1B)]O(3B) P(1B)]C(1B) N(1B)]C(1B) C(1B)]C(2B) C(1B)]C(4B) C(2B)]C(3B) C(3B)]C(4B) O(1B)]P(1B)]O(2B) O(1B)]P(1B)]O(3B) O(1B)]P(1B)]C(1B) O(2B)]P(1B)]O(3B) O(2B)]P(1B)]C(1B) O(3B)]P(1B)]C(1B) P(1B)]C(1B)]N(1B) P(1B)]C(1B)]C(2B) P(1B)]C(1B)]C(4B) N(1B)]C(1B)]C(2B) N(1B)]C(1B)]C(4B) C(1B)]C(4B) C(1B)]C(2B)]C(3B) C(1B)]C(3B)]C(4B) C(1B)]C(4B)]C(3B) 1.579(3) 1.489(3) 1.495(3) 1.802(6) 1.510(7) 1.531(7) 1.548(6) 1.531(9) 1.511(9) 111.7(2) 106.1(2) 103.9(2) 118.6(2) 107.1(2) 108.5(2) 110.5(3) 114.9(4) 116.6(4) 113.5(4) 110.7(4) 89.1(4) 88.2(5) 90.5(4) 88.3(4) an additional hydrogen bond with the phosphonic acid groups of the other molecules and with the water oxygen atoms, O(1W) and O(2W).Thus, water forms three hydrogen bonds: two as a donor with phosphonate atoms O(3) and O(2), and a very short one as an acceptor with O(1). Consequently, compound Table 6 Selected torsion angles (8) for compounds 4 and 10 Compound 4 O(2)]P]C(1)]N(1) O(3)]P]C(1)]N(1) O(1)]P]C(1)]N(1) O(2)]P]C(1)]C(2) O(3)]P]C(1)]C(2) O(1)]P]C(1)]C(2) N(1)]C(1)]C(2)]C(3) P]C(1)]C(2)]C(3) C(1)]C(2)]C(3)]O(4) C(1)]C(2)]C(3)]N(2) 289.9(1) 38.3(1) 156.7(1) 33.6(2) 161.8(1) 279.8(1) 257.1(2) 179.5(1) 3.3(3) 2176.7(2) Compound 10 O(2A)]P(1A)]C(1A)]N(1A) O(3A)]P(1A)]C(1A)]N(1A) O(1A)]P(1A)]C(1A)]N(1A) N(1A)]C(1A)]C(2A)]C(3A) N(1A)]C(1A)]C(4A)]C(3A) C(4A)]C(1A)]C(2A)]C(3A) C(1A)]C(2A)]C(3A)]C(4A) C(2A)]C(3A)]C(4A)]C(1A) C(2A)]C(1A)]C(4A)]C(3A) O(2B)]P(1B)]C(1B)]N(1B) O(3B)]P(1B)]C(1B)]N(1B) O(1B)]P(1B)]C(1B)]N(1B) C(4B)]C(1B)]C(2B)]C(3B) C(1B)]C(2B)]C(3B)]C(4B) C(2B)]C(3B)]C(4B)]C(1B) C(2B)]C(1B)]C(4B)]C(3B) N(1B)]C(1B)]C(4B)]C(3B) N(1B)]C(1B)]C(2B)]C(3B) 164.2(3) 35.2(3) 277.3(3) 2117.7(6) 118.4(7) 24.0(7) 4.2(7) 24.2(7) 4.0(7) 168.6(3) 39.5(3) 273.1(3) 214.8(5) 15.2(5) 215.0(5) 15.0(6) 129.9(5) 2127.1(5) Table 7 Hydrogen bond lengths (Å) and angles (8) for compounds 4 and 10 D]H? ? ?A D? ? ?A D]H H? ? ?A D]H? ? ?A Compound 4 O(1)]H(1) ? ? ? O(3I) N(1)]H(3) ? ? ? O(4) N(1)]H(3) ? ? ? O(3II) N(1)]H(4) ? ? ? O(4II) N(1)]H(5) ? ? ? O(2III) N(2)]H(8) ? ? ? O(3IV) N(2)]H(9) ? ? ? O(2V) 2.584(2) 2.756(2) 3.257(2) 2.788(2) 2.693(2) 2.993(2) 2.904(2) 0.71(3) 0.88(3) 0.88(3) 0.91(3) 0.98(3) 0.83(3) 0.88(3) 1.88(2) 2.06(3) 2.57(3) 1.90(3) 1.73(3) 2.17(3) 2.03(3) 172(2) 136(2) 136(2) 167(2) 169(2) 171(2) 172(2) Symmetry codes: I 1 2 x, 1 2 y, 2z; II x, ��� 2 y, 2��� 1 z; III 2x, ��� 2 y, 2��� 1 z; IV 2x, 1 2 y, 2z; V x, y, 1 1 z; VI x, ��� 2 y, ��� 1 z.Compound 10 Molecule A N(1A)]H(2A) ? ? ? O(3B) N(1A)]H(3A) ? ? ? O(1BI) N(1A)]H(4A) ? ? ? O(2AII) O(1W)]H(5) ? ? ? O(3BIII) O(1W)]H(6) ? ? ? O(2AIV) O(1A)]H(1A) ? ? ? O(1W) 2.706(5) 2.756(5) 2.800(5) 2.776(6) 2.813(5) 2.571(5) 0.84(6) 0.91(6) 0.86(6) 0.76(6) 0.93(6) 0.74(5) 1.87(6) 2.10(6) 1.95(6) 2.07(7) 1.87(6) 1.83(6) 173(5) 157(5) 176(6) 156(5) 173(6) 173(6) Molecule B N(1B)]H(2B) ? ? ? O(3AIV) N(1B)]H(3B) ? ? ? O(1A) N(1B)]H(4B) ? ? ? O(2BV) O(2W)]H(7) ? ? ? O(3AVI) O(2W)]H(8) ? ? ? O(2BIV) O(1B)]H(1B) ? ? ? O(2W) 2.752(5) 2.877(5) 2.813(5) 2.767(6) 2.778(5) 2.580(5) 0.96(6) 0.96(5) 0.81(6) 0.74(7) 0.83(6) 0.78(6) 1.83(6) 1.97(6) 2.02(6) 2.14(6) 1.97(6) 1.81(5) 159(5) 157(5) 167(5) 162(5) 166(5) 173(5) Symmetry codes: I x 2 1, y, z; II x, ��� 2 y, 2��� 1 z; III x, ��� 2 y, ��� 1 z; IV 1 1 x, y, z; V 1 2 x, 1 2 y, ��� 1 z; VI 1 2 x, 1 2 y, 2��� 1 z.168 J.Chem. Soc., Dalton Trans., 1998, Pages 161–169 10 forms a channel-like structure in which water molecules lie in hydrophilic channels surrounded by phosphonic groups. These hydrophilic regions are separated from each other by hydrophobic regions composed of a network of cyclobutyl rings (Fig. 11). Discussion In a previous study we found that the analogue 11 obtained by the replacement of the a-carboxylic group of glutamic acid by a phosphonic acid moiety underwent chemical transformation upon action of zinc(II) ions under neutral or slightly alkaline conditions, yielding most probably a cyclic phosphonamidate 12 composed of two ligand molecules.1 In order to determine which structural features are responsible for the formation of this cyclic product we have undertaken similar studies using phosphonic acid analogues of asparagine and aspartic acid as pro-ligands. They were chosen intentionally as shorter homologues of compound 11 which should provide sterically hindered cyclic phosphonamidates.As seen from NMR studies, phosphonic acid analogues of aspartic acid exhibit an identical pattern of reactivity upon action of zinc(II) ion in neutral and alkaline media. Thus, compound 6 obtained by the replacement of the b-carboxylic moiety in aspartic acid by a phosphonic acid group did not undergo this transformation as indicated by the existence of only one signal in the 13P NMR spectra of its solutions with zinc(II).Quite oppositely, the spectra of zinc(II) solutions of compound 1, obtained by replacement of the acarboxylate moiety of aspartic acid, resemble those recorded for zinc(II) solutions with its homologue (11) since a pair of single resonances appears in neutral and slightly alkaline solutions. Introduction of a methyl group in the a position of this compound afforded 2 which showed quite complex multiplets instead of the single additional downfield resonance characteristic for compounds 1 and 11.We assumed that this feature arises from conformational strain induced by the methyl group and the rotational barrier at the Ca]Cb bond resulting in the appearance of various stereoisomers in the 31P NMR spectra. Fig. 11 Crystal-packing arrangement of compound 10.Hydrogen bonds are shown as dashed lines HO2C PO3 H2 NH2 N P N P HO2C H OH O HO O H CO2H 12 11 Esterification of compound 2 gave 3 with 31P NMR spectra which are very similar to those obtained for its parent 2. This suggested that the carboxylic group is not an essential feature for cyclization of aminophosphonates upon action of zinc(II). In order to check this assumption we synthesized a series of simple 1-aminoalkylphosphonic acids 7–10 and studied their behaviour in aqueous solutions containing zinc(II).The 31P NMR spectra of compound 7 observed in zinc(II) solutions were nearly identical with those observed for 1, indicating that the formation of a cyclic phosphonamidate is independent of the presence of an additional carboxylate moiety in the aminophosphonate side-chain. Consequently the spectra of zinc(II) solutions of compounds 8 and 9 are very similar to those observed for 2 and 3. Thus, a complex pattern of downfield resonances, arising from the formation of cyclic compounds characterized by conformational strain induced by the substituent at the a position and the rotational barrier at the Ca]Cb bond, was found.Additional proof of the importance of this rotational barrier comes from the spectra of compound 10. This highly symmetric compound gave spectra identical with those recorded for the analogue 1, namely a pair of single resonances appears in neutral and slightly alkaline solutions.Among analogues in which an a-carboxylate moiety was replaced by a phosphonic acid group only compounds 4 and 5 gave only one peak in their 13P NMR spectra with zinc(II) ion. This indicates that the replacement of a phosphonate moiety by a methyl group completely suppresses the formation of the cyclic phosphonamidate. Compound 4, however, should undergo such a cyclization. It may be suppressed by a massive crystallization of its zinc(II) complex from the solutions at pH > 6.5.Acknowledgements We gratefully acknowledge Komitet Badan� Naukowych for financial support (grant no. 2P3 0305507). References 1 E. Matczak-Jon, B. Kurzak, W. Sawka-Dobrowolska, P. Kafarski and B. Lejczak, J. Chem. Soc., Dalton Trans., 1996, 3455. 2 M. Soroka and P. Mastalerz, Pol. J. Chem., 1976, 50, 661; J. Oleksyszyn, E. Gruszecka, P. Kafarski and P. Mastalerz, Monatsh. Chem., 1982, 113, 1138. 3 (a) M. Soroka, Liebigs Ann. Chem., 1990, 331; (b) M. Soroka, D.Jaworska and M. Szcze�sny, Liebigs Ann. Chem., 1990, 1153. 4 G. Gran, Acta Chem. Scand., 1959, 4, 599. 5 M. Molina, C. Melios, J. O. Tognalli, L. C. Luchiari and M. Jafelicci, jun., J. Electroanal. Chem., Interfacial Electrochem., 1979, 105, 237. 6 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton Trans., 1985, 1195. 7 Kuma KM4 software. User’s Guide, version 3.1, Kuma Diffraction, Wroc�aw, 1987. 8 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467. 9 G. M. Sheldrick, SHELXL 93,n, 1993. 10 H. D. Flack, Acta Crystallogr., Sect. A, 1983, 39, 876. 11 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. 12 Z. Siatecki and H. Koz�owski, Org. Magn. Reson., 1980, 5, 431; 7, 172. 13 K. G. R. Pachler, Spectrochim. Acta, 1964, 20, 581. 14 T. Kiss, E. Farkas and H. Koz�owski, Inorg. Chim. Acta, 1989, 155, 281. 15 W. Sawka-Dobrowolska, T. G�owiak, Z. Siatecki and M. Soroka, Acta Crystallogr., Sect. C, 1985, 41, 453. 16 F. S. Stephens, R. S. Vagg and P. A. Williams, Acta Crystallogr., Sect. B, 1977, 33, 433; V. Kh. Sabirov, M. A. Porai-Koshits and Yu. T. Struchkov, Koord. Khim., 1993, 19, 81; Ch. Vansent, H. O. Desseyn, V. Tangoulis, C. P. Raptopoulou, A. Terzis and S. P. Perlepes, Polyhedron, 1995, 15–16, 2115. 17 B. Kurzak, R. Matczak-Jon and M. Hoffmann, J. Coord. Chem., in the press.J. Chem. Soc., Dalton Trans., 1998, Pages 161–169 169 18 N. Sampson and P. A. Bartlett, J. Org. Chem., 1987, 53, 4500; J. Rahil and R. F. Pratt, Biochemistry, 1993, 32, 10 763; B. P. Morgan, D. R. Holland, B. W. Mathews and P. A. Bartlett, J. Am. Chem. Soc., 1994, 116, 3251; A. Mucha, P. Kafarski, H.-J. Cristau and F. Plenat, Tetrahedron, 1994, 50, 12 743; R. Hirschman, K. H. Jager, C. M. Taylor, W. Moore, P. A. Sprengler, J. Witherington, B. W. Philips and A. B. Smith III, J. Am. Chem. Soc., 1995, 117, 6370; W. P. Malachowski and J. K. Coward, J. Org. Chem., 1996, 59, 7616, 7625. 19 W. Sawka-Dobrowolska, T. G�owiak and J. Kowalik, Acta Crystallogr., Sect. C, 1992, 48, 286 and refs. therein. 20 W. Sawka-Dobrowolska, T. G�owiak and Z. Siatecki, Acta Crystallogr., Sect. C, 1987, 43, 1942. 21 G. A. Jeffrey, J. R. Ruble, R. K. McMullen, D. J. DeFress, J. S. Binkley and J. A. Pople, Acta Crystallogr., Sect. B, 1980, 36, 2292. 22 A. Stein, Ch. W. Lehman and P. Luger, J. Am. Chem. Soc., 1992, 114, 7684; Y.-L. Lam, L.-L. Koch and H.-H. Huang, Acta Crystallogr., Sect. C, 1996, 52, 397; B. Sheldrick, D. Akrigg, M. I. Page and G. Cox, Acta Crystallogr., Sect. C, 1987, 47, 595. Received 29th May 1997; Pape

ISSN:1477-9226    

DOI:10.1039/a703734a

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 171–176 171 Crystal structures and magnetic properties of mercury(II) bromide complexes with pyridyl-substituted N-oxyl N9-oxides (nitronyl nitroxides) Chin-Jhan Lee,a Chian-Hong Huang,a Ho-Hsiang Wei,*,a Yi-Hung Liu,b Gene-Hsiang Lee b and Yu Wang b a Department of Chemistry, Tamkang University, Tamsui, Taiwan b Instrumental Center, College of Science, National Taiwan University, Taipei, Taiwan The crystal structures have been determined and magnetic properties investigated for four novel HgBr2 complexes with pyridyl-substituted ‘nitronyl nitroxides’, 4,4,5,5-tetramethyl-2-(4-pyridyl)-(L1), -2-(2-pyridyl)-(L2), -2-(3- pyridyl)-(L3) and -2-(6-methyl-2-pyridyl)-2-imidazoline N1-oxyl N3-oxide (L4).Complex 1, [HgBr2L1], is mononuclear, in which mercury(II) has planar trigonal co-ordination core, from two bromide atoms and one nitrogen atom of the pyridyl group. In 2, [HgBr2L2] the HgII atom has distorted-tetrahedral four co-ordination involving two bromide atoms and chelating by oxygen and nitrogen atoms of the L2 ligand. Complex 3, [(HgBr)3L3 2], is a zigzag polymeric chain with a distorted T-shaped HgBr2L3 unit and self-assembly involving coordination by an oxygen atom of the nitroxide groups.Complex 4, [(HgBr2)3L4 2], is a quasi-linear chain with HgBr2L4 moieties and HgBr2 cores. Cryomagnetic susceptibility measurements (4–300 K) showed that 1 and 3 exhibit a weak intermolecular alternating one-dimensional antiferromagnetic exchange interaction, while 3 and 4 possess weak one-dimensional antiferromagnetic and ferromagnetic exchange interactions respectively. A simple spinpolarization model has been used to justify the observed ferromagnetic exchange interaction between the spins of the radical NO group in complex 4.The magnetic properties of a large number of stable organic Noxyl N9-oxides (nitronyl nitroxides) and their metal complexes have been studied, especially in order to understand the factors that influence the magnetic exchange interactions between the metal ion and the radical centers.1–4 Recently, several magnetic properties of pyridyl-substituted N-oxyl N9-oxide radicals 5–8 and their metal complexes 9–15 have been reported.These radicals are especially attractive due to their donor atoms and to assemble an extended co-ordination geometry with changing magnetic coupling. Several transition-metal complexes with 4,4,5,5-tetramethyl- 2-(4-pyridyl)-2-imidazoline N1-oxyl N3-oxide (L1) 8,9 and a few metal complexes with 4,4,5,5-tetramethyl-2-(2-pyridyl)-2- imidazole N1-oxyl N3-oxide (L2) 10 and 4,4,5,5-tetramethyl-2-(3- pyridyl)-2-imidazoline N1-oxyl N3-oxide (L3) 11 have been studied.Although the different magnetic behaviours have been previously observed in these systems, however systematic studies of diamagnetic metal complexes with paramagnetic L1–L3 radical ligands have been lacking.Accordingly, we report herein the structural characterization and magnetic properties of HgBr2 complexes with the four related pyridyl-substituted radical ligands L1–L3 and 4,4,5,5- tetramethyl-2-(6-methyl-2-pyridyl)-2-imidazoline N1-oxyl N3- oxide (L4). We will show that the shortest contacts involving the intermolecular hydrogen bonding and the bridging bromides of HgBr2 are appropriate crystalline design elements with which to control the crystal packing and also to generate antiferromagnetic or ferromagnetic exchange interactions and to propagate them along predetermined spatial directions. Experimental Syntheses The pyridyl-substituted radicals L1–L4 were prepared and puri- fied according to the methods reported.16,17 The complexes [HgBr2L1] 1, [HgBr2L2] 2, [(HgBr2)3L3 2] 3 and [(HgBr2)3L4 2] 4 were prepared in the same manner as follows.To a solution of HgBr2 (0.2 mmol) in ethanol (10 cm3) was added with stirring a solution of the radical (0.2 mmol) in ethanol (10 cm3). The solution was stirred for an additional period then allowed to stand at room temperature for several days.Crystals suitable for X-ray crystallographic analysis were obtained and filtered off (Found: C, 24.4; H, 2.8; N, 7.0. Calc. for C12H16Br2HgN3O2 1: C, 24.3; H, 2.7; N, 7.1. Found: C, 24.1; H, 2.7; N, 7.0. Calc for C12H16Br2HgN3O2 2: C, 24.2; H, 2.7; N, 7.1. Found: C, 18.5; H, 2.7; N, 5.4. Calc. for C24H32Br6Hg3N6O4 3: C, 18.6; H, 2.1; N, 5.4.Found: C, 23.2; H, 3.0; N, 6.0. Calc. for C26H36Br6Hg3N6O4 4: C, 23.4; H, 3.0; N, 6.0%). IR (KBr disc): n& NO/cm21 1369 (1), 1368 (2), 1366 (3) and 1365 (4). Physical measurements The IR spectra were recorded on a Bio-Rad FTS40 FTIR spectrophotometer as KBr pellets in the 400–4000 cm21 region, X-band EPR spectra at 300 K for the complexes as powders on a Bruker ESC-106 spectrometer. The temperature dependence of the magnetic susceptibility of the polycrystalline samples was measured between 4 and 300 K at field 1 T using a N N N+ Me Me Me Me O O– L1 N N N+ Me Me Me Me O O– L2 N N N+ Me Me Me Me O O– L3 N N N+ Me Me Me Me O O– L4 Me172 J.Chem. Soc., Dalton Trans., 1998, Pages 171–176 Quantum Design model MPMS computer-controlled SQUID magnetometer. Diamagnetic corrections were made using Pascal’s constants.18 Crystallography Crystal data. Complex 1 C12H16Br2HgN3O2. M = 594.67, monoclinic, space group C2/c, a = 11.3272(20), b = 14.174(3), c = 10.132(3) Å, b = 104.038(16)8, U = 1578.1(6) Å3, Z = 4, Dc = 2.503 g cm23, F(000) = 1088, m = 147.714 cm21, crystal size = 0.25 × 0.30 × 0.50 mm, 2qmax = 55.08, N = 1813, No = 1463, R = 0.031, R9 = 0.036.Complex 2. C12H16Br2HgN3O2, M = 594.67, monoclinic, space group P21/c, a = 10.5811(12), b = 22.038(4), c = 7.2584(9) Å, b = 103.858(13)8, U = 1643.3(5) Å3, Z = 4, Dc = 2.404 g cm23, F(000) = 1088, m = 141.854 cm21, crystal size = 0.20 × 0.20 × 0.40 mm, 2qmax = 50.08, N = 2895, No = 1788, R = 0.039, R9 = 0.039.Complex 3. C24H32Br6Hg3N6O4, M = 1551.75, orthorhombic, space group Pbca, a = 7.643(3), b = 20.886(5), c = 22.733(4) Å, U = 3629.2(16) Å3, Z = 4, Dc = 2.840 g cm23, F(000) = 2771, m = 128.466 cm21, crystal size = 0.15 × 0.20 × 0.50 mm, 2qmax = 50.08, N = 3184, No = 1969, R = 0.060, R9 = 0.055. Complex 4. C26H36Br6Hg3N6O4, M = 1577.79, monoclinic, space group P21/c, a = 10.958(6), b = 24.663(9), c = 7.339(4) Å, b = 102.29(5)8, U = 1937.9(16) Å3, Z = 2, Dc = 2.704 g cm23, F(000) = 1414, m = 120.316 cm21, crystal size = 0.05 × 0.30 × 0.40 mm, 2qmax = 45.08, N = 2581, No = 1542, R = 0.066, R9 = 0.067.The X-ray crystal data were collected at room temperature using an Enraf-Nonius CAD4 diffractometer equipped with graphite-monochromated Mo-Ka radiation (l = 0.7107 Å), 2q– q scan mode. The N independent reflections and No with I > 2.0s(I) were observed. The structures were solved by location of heavy atoms using a Patterson map and refined by a full-matrix least-squares method using the NRCVAX19 software package; the function minimized was Sw(|Fo| 2 |Fc|)2, where w = 1/s2(Fo).All non-hydrogen atoms were readily located and refined with anisotropic thermal parameters; R = S|Fo| 2 |Fc|/S|Fo| and R9 = (Sw|Fo 2 Fc|2/S|Fo|2)� �� . CCDC reference number 186/771. Results and Discussion IR and EPR spectra The most important infrared absorption bands of complexes 1–4 at 1369, 1368, 1366, and 1365 cm21 respectively have been assigned to the N]O stretching mode.The EPR (9.7–9.80 GHz) spectra at 300 K in benzene solution all consisted of five lines centred at g = 2.01, with nitrogen hyperfine coupling constants aN = 7.56, 7.48, 7.67, and 7.45 G respectively (G = 1024 T). Crystal structures The crystal structure of [HgBr2L1] 1 is illustrated in Fig. 1. The mercury atom has a distorted trigonal three-co-ordination comprised of two terminal bromide atoms and the pyridyl Fig. 1 Perspective view of complex 1 with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level nitrogen atom of L1 with Hg]Br 2.) Å, Br]Hg]Br 134.73(4)8, and Hg]N(1) 2.292(8) Å (Table 1). The fragment O]N(2)]C(4)]N(2)]O [N(2)]O 1.296(7) Å] is coplanar as expected due to orbital conjugation and as shown by the sum of the angles of the bonds around the N(2) and C(4) atoms. The unit cell (Fig. 2) contains two pairs of symmetrically related molecules of the [HgBr2L1] with tail-to-tail (units of I and II), head-to-tail (II and III), and tail-to-head (III and IV) alternating arrangements along the b axis.Thus the two different intermolecular NO groups of the L1 radicals are far apart: 4.623(10) (between units II and III) and 5.142(12) Å (between I and II). It is noteworthy that the shortest intermolecular (between II and III) contact is NO ? ? ? HC(19) 2.633 Å. The structure of [HgBr2L2] 2 is shown in Fig. 3. Four-coordination of HgII is formed by two terminal bromide atoms [Hg]Br(1) 2.4412(17), Hg]Br(2) 2.4397(18) Å, and Br(1)]Hg] Br(2) 162.05(7)8] and one oxygen and one nitrogen atom from the L2 ligand [Hg]O(1) 2.521(9), Hg]N(1) 2.666(11) Å, O(1)]Hg]N(1) 68.3(3)8] in a distorted tetrahedral geometry (Table 2). The fragment O(1)]N(2)]C(6)]N(3)]O(2) with the co-ordinated N(2)]O(1) 1.281(13) Å and the unco-ordinated N(3)]O(2) 1.268(14) Å is nearly planar but forms a dihedral angle of 38.1(4)8 with the plane of the pyridyl ring.From the unit-cell packing (Fig. 4), two pairs of symmetrically related molecules of [HgBr2L2] are arranged tail-to-tail and head-tohead and tail-to-tail along the b axis. The shortest intermolecular contact between the O(2) atom of the N(2)]O(2) group and the hydrogen atom, H(109), of the methyl group [C(109)H3] in neighbouring molecules forms a weak hydrogen bond N(2)O(2) ? ? ? H(10)C(19) 2.330(9) Å. The head-to-head intermolecular distance Hg ? ? ? Br(29) is 3.4340(20) Å.The shortest intermolecular contact O(2) ? ? ? O(29) between the NO groups is 3.661(3) Å. The structure of complex 3 consists of two crystallographically independent units, HgBr2 and [(HgBr2)2L3 2]. As shown in Fig. 5, each Hg(2) atom is co-ordinated by two bromide atoms [Hg(2)]Br(2) 2.4520(22), Hg(2)]Br(3) 2.4441(21) Å] and a nitrogen atom from a pyridyl moiety [Hg(2)]N(1) 2.452(13) Å] in a distorted fashion (Table 3).The self-assembly bonding with O(1) of the nitroxide group [Hg(2)]O(1) 2.655(13) Å] extends Fig. 2 View of the packing of the molecules in complex 1. Weak intermolecular hydrogen bonds are shown by dotted lines Table 1 Selected bond distances (Å) and angles (8) for [HgBr2L1] 1 Hg]Br N(2)]O Br]Hg]Br C(1)]N(1)]Hg C(4)]N(2)]O C(5)]N(2)]O 2.4666(9) 1.296(7) 134.73(4) 120.8(4) 125.4(6) 121.5(5) Hg]N(1) C(4)]N(2) Br]Hg]N(1) C(1)]N(1)]C(1) N(2)]C(4)]N(2) 2.292(8) 1.335(8) 112.64(3) 118.3(7) 109.0(7)J. Chem.Soc., Dalton Trans., 1998, Pages 171–176 173 the co-ordination number of Hg(2) to four, forming a zigzag polymeric chain. The O(1)]N(2)]C(6)]N(3)]O(2) moiety is as expected coplanar, but forms a dihedral angle of 33.8(6)8 with the plane of the pyridyl ring. The shortest contact distances O(29) ? ? ? O(29) and O(2) ? ? ? H(109) between atoms belonging to different L3 ligands are 3.8223(14) and 2.329(17) Å respectively, which can be considered as weak interactions.The crystal structure of complex 4 as shown in Fig. 6 consists of two co-ordination types for HgII. The Hg(2) atom has a distorted-tetrahedral co-ordination by virtue of chelating O(2) and N(1) atoms of ligand L4 and two terminal Br(3) and Br(2) atoms. Atom Hg(1), originally co-ordinated by two bromides, has a linear co-ordination [Br(1)]Hg(1)]Br(1) 179.98, Hg]Br(1) 2.446(3) Å], Table 4, and additional weak bonds to four bromide atoms, two Br(3) and two Br(2) atoms, from neighbouring Hg(2)Br2 moieties; thus, there is six-co-ordination about Hg(1), leading to a chain-like motif in the structure.The fragment O(1)]N(2)]C(7)]N(3)]O(2) is nearly planar, but forms a Fig. 3 Perspective view of complex 2. Details as in Fig. 1 Fig. 4 View of the packing of the molecules in complex 2. Details as in Fig. 2 Table 2 Selected bond distances (Å) and angles (8) for [HgBr2L2] 2 Hg]Br(1) Hg]O(1) N(2)]O(1) Br(1)]Hg]Br(2) O(1)]Hg]N(1) Br(2)]Hg]N(1) Hg]O(1)]N(2) Hg]N(1)]C(1) 2.4412(17) 2.521(9) 1.281(13) 162.05(7) 68.6(3) 97.0(3) 114.1(7) 117.1(9) Hg]Br(2) Hg]N(1) N(3)]O(2) Br(1)]Hg]O(1) Br(2)]Hg]O(1) Br(1)]Hg]N(1) Hg]N(1)]C(5) 2.4397(18) 2.666(11) 1.268(14) 97.35(21) 100.24(21) 92.69(24) 127.2(8) dihedral angle of 43.0(8)8 with the plane of the pyridyl ring.The shortest contact distances, C(12)H ? ? ? O(19) and O(1) ? ? ? O(19), belonging two different L4 radicals, are 2.663(21) and 3.7785(10) Å respectively. In summary, the shortest contact distances O ? ? ? H, belonging to two different radicals, in complexes 1–4 are 2.633(6), 2.330(9), 2.329(17), and 2.663(21) Å respectivley, which are close to the sum of the van der Waals radii (2.6 Å).Magnetic properties The cmT vs. T plots for complexes 1–4 are shown in Fig. 7 in the 4–300 K range. The cmT value at 300 K of 1, 0.36 cm3 K mol21, Fig. 5 Perspective view of complex 3 with the atom numbering scheme. Atoms Hg(1) and Br(1) are omitted for clarity.Thermal ellipsoids are drawn at the 30% probability level. Weak interligand hydrogen bonds are drawn as dotted lines Table 3 Selected bond distances (Å) and angles (8) for [(HgBr2)3L3 2] 3 Hg(2)]Br(2) Hg(2)]N(1) N(2)]O(1) Hg(1)]Br(1) Br(2)]Hg(2)]Br(3) Br(3)]Hg(2)]O(1) Br(2)]Hg(2)]O(1) C(6)]N(2)]O(1) 2.4520(22) 2.452(13) 1.263(19) 2.4156(21) 160.34(7) 86.7(3) 102.4(3) 127.0(16) Hg(2)]Br(3) Hg(2)]O(1) N(3)]O(2) Br(1)]Hg(1)]Br(1) Br(3)]Hg(2)]N(1) N(1)]Hg(2)]O(1) C(7)]N(2)]O(1) 2.4441(21) 2.655(13) 1.27(3) 180.0 96.4(4) 88.2(4) 120.8(13) Table 4 Selected bond distances (Å) and angles (8) for [(HgBr2)3L4 2] 4 Hg(1)]Br(1) Hg(2)]Br(3) Hg(2)]N(1) N(2)]O(1) Hg(1) ? ? ? Br(3) Br(1)]Hg(1)]Br(1) Br(2)]Hg(2)]O(2) Br(2)]Hg(2)]N(1) N(3)]O(2)]Hg(2) C(7)]N(3)]O(2) 2.446(3) 2.450(4) 2.628(17) 1.30(3) 3.280(4) 179.9 101.2(5) 97.5(4) 109.6(17) 129.4(22) Hg(2)]Br(2) Hg(2)]O(2) N(3)]O(2) Hg(1) ? ? ? Br(2) Br(2)]Hg(2)]Br(3) Br(3)]Hg(2)]N(1) Br(2)]Hg(2)]O(2) N(1)]Hg(2)]O(2) C(7)]N(2)]O(1) 2.448(3) 2.460(4) 1.27(3) 3.108(3) 164.38(13) 94.3(5) 92.0(5) 72.8(7) 125.8(23)174 J.Chem. Soc., Dalton Trans., 1998, Pages 171–176 Fig. 6 Perspective view of complex 4 with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. Weak intermolecular hydrogen bonds are drawn as dotted lines is close to the value expected for non-coupled spins S = ��� (0.375 cm3 K mol21), increases slowly with decreasing temperature reaching a maximum at about 50 K, then decreases rapidly on further lowering of temperature.This is characteristic of a weak antiferromagnetic exchange interaction in 1. Clearly, the magnetic interaction does not arise from the diamagnetic mercury(II) ions. The structural analysis of 1 shows that the unit cell contains alternating pairs of [HgBr2L1] radicals with two different intermolecular distances involving NO groups. Consequently, the magnetic data were analysed by use of the expression (1) for the exchange coupling in an alternating chain Fig. 7 Temperature dependence of cmT for complexes 1 (a), 2 (b), 3 (c), and 4 (d). The solid lines are calculated with the parameters reported in the text cm = (Ng2mB 2/kT)[(A 1 Bx 1 Cx2)/ (1 1 Dx 1 Ex2 1 Fx3)] (1) of S = ��� ion developed by Hall et al.20 where x = J/kT, A = 0.25, B = 0.125 87 1 0.227 52a, C = 0.019 111 2 0.133 07a 1 0.5967a2 2 1.3167a3 1 1.0081a4, D = 0.107 72 1 1.4192a, E = 20.002 852 1 2 0.4236a 1 22.1953a2 2 0.824 12a3, and F = 0.377 54 2 0.067 022a 1 0.9805a2 2 21.678a3 1 15.838a4.The alternation parameter a is defined by the zero-field spin Hamiltonian (2) where J and aJ are the exchange interaction H = 22J on/2 i = l (S2i?S2i2l 1 aS2i?S2i1l) (2) parameters associated with a particular intermolecular spin interaction of NO groups between the units II and III and units I and II in the unit cell respectively. Since the data concern organic radicals, calculations did not include a contributionmperature-independent paramagnetism.A tolerable best fit obtained (the solid line in Fig. 7) with this equation leads to J = 22.05 cm21, g = 2.01 (from EPR spectroscopy), a = 0.10 and R = 2.1 × 1025. The discrepancy R value is defined as S(cm obs 2 cm calc)2/(Scm obs)2. For compound 2 the cmT value at 280 K, 0.38 cm3 K mol21, is identical to that expected for non-coupled spins S = ��� . On lowering the temperature cmT decreases slowly to reach a value of 0.33 cm3 K mol21 at 25 K, and then decreases rapidly to 0.17 cm3 K mol21 at 4 K.This suggests that the free radicals, L2, in 2 have weak intermolecular antiferromagnetic coupling. On the basis of the structural results, the unit cell contains alternating differently spaced intermolecular nitroxide radicals, therefore the magnetic data were also fitted according to equation (1), giving J = 22.47 cm21, g = 2.01 (from EPR spectroscopy), aJ = 0.35 cm21, and R = 5.3 × 1025, where J and aJ are the exchange interaction constants associated with a particularJ.Chem. Soc., Dalton Trans., 1998, Pages 171–176 175 intermolecular spin exchange of NO groups between the tailto- tail and head-to-head aligned species respectively in the unit cell of 2. For compound 3 the cmT value at 300 K, 0.73 cm3 K mol21, is slightly lower than that expected for a non-coupled spin S = ��� system (0.75 cm3 K mol21). On lowering the temperature cmT decreases rapidly and approaches 0.07 cm3 K mol21 at 4 K.This behaviour is characteristic of an intrachain antiferromagnetic exchange interaction between two NO? radicals in this polymer chain. We have attempted to reproduce theoretically the experimental susceptibility in this zigzag polymeric regime by use of the published expression (3) for J < 0 calculated by Bonner and c = (Ng2mB 2/kT)(A/B) (3) Fischer 21 for a classical-spin Heisenberg array of S = ��� spins, where x = J/kT, A = 0.25 1 0.074 97x 1 0.075 235x2 and B = 1.0 1 0.9931x 1 0.172 135x2 1 0.747 825x3.A very close agreement with the experimental data is obtained (the solid line in Fig. 7) with J = 214.70 cm21, g = 2.01 (from EPR spectroscopy) and R = 2.0 × 1025. For compound 4 the cmT value at 300 K, 0.71 cm3 K mol21, is slightly lower than that expected for a non-coupled spin S = ��� system. On lowering the temperature cmT increases slowly and approaches 0.81 cm23 K mol21 at 4 K which is higher than 0.75 cm3 K mol21 for a non-coupled spin S = ��� system; thus it is indicative of a weak ferromagnetic exchange interaction between L4 radicals in complex 4.According to the structural results, this weak ferromagnetic behaviour is caused by an intermolecular interaction between two neighbouring NO? radicals through CH ? ? ? ON in this quasi-polymeric chain. To fit quantitatively the magnetic data for 4, we first considered the exchange interaction as the leading term with the corresponding two spins S1 = S2 = ��� of a Hamiltonian H = 22JS1S2.However, fitting the data with this model gave very poor results. Thus we used the empirical expression of the magnetic susceptibility proposed by Baker et al.22 to fit ferromagnetic onedimensional isotropic Heisenberg S = ��� chains, equation (4) c = [Ng2mB 2/4k(T 2 q)](N/D) 2–3 (4) where x = J/2kT, N = 1.0 1 5.797 991 6x 1 16.902 653x2 1 29.376 885x3 1 29.832 959x4 1 14.036 918x5 and D = 1.0 1 2.797 991 6x 1 7.008 678 0x2 1 8.653 864 4x3 1 4.574 311 4x4.The solid curve in Fig. 7 represents the best fit of the experimental data obtained with J = 0.24 cm21, g = 2.01 (from EPR spectroscopy), q = 0.145 K, and R = 3 × 1024. Since the fitted magnetic data of complex 4 provide evidence for a weak intermolecular ferromagnetic interaction, one must consider a possible mechanism for exchange interaction between the nearest-neighbour L4 radicals. From the structural and the magnetic results discussed above, there is a weak intermolecular contact between the NO groups [O(1) ? ? ? O(19) 3.7785(10) Å].Such a NO ? ? ?ON9 contact means a direct intermolecular exchange interaction between the magnetic orbitals, however it is usually considered responsible for weak antiferromagnetic coupling in molecular solids.23 An acceptable mechanism suggested by McConnell24 for interpretation of the ferromagnetic behaviour of the N-oxyl N9-oxides has been extensively discussed.25 According to this model a spin distribution arising from intramolecular spin polarization of the adjacent atoms leads to alternating positive and negative spin density on the carbon backbone of the radical ligands.In addition, in the radicals, the unpaired electron is known to be localized over the two NO groups with the same spin density on each.26 As mentioned above, complex 4 has a shortest intermolecular distance of 2.663(21) Å for NO ? ? ? HC between the neighbouring L4 radicals.Since these two atoms carry significant spin densities of opposite sign which alternate along the chain, the requirements of the McConnell mechanism are fulfilled. Semiempirical molecular orbital calculations revealed a strong electronic polarization of NO and CH or OH bonds, indicating the NO groups could act as acceptors and the CH or the OH group as a donor in the hydrogen bonds.27 A schematic representation of the alternating spin densities for 4 is given in Fig. 8, which also shows the geometrical features relevant to the interaction between the sites carrying the spin density.25 The positive spin density on the NO(1) sites induces negative spin density on the hydrogen atom of the neighbouring HC(109) due to a spin polarization, which in turn induces positive spin density on the NO sites of the adjacent molecules caused by the orbital overlap between 1s(H) and p*(NO?), thus we have ­O(1)]N(2)­] ØC(7)]­O(2)N(3)­]ØC(9)]­C(12)]H(12)Ø ? ? ?­O(19)]N(29) and parallel spin alignments of NO sites in 4.From the investigations of the structures and cryomagnetic susceptibilities described above, complexes 1–3 display a weak antiferromagnetic and 4 a weak ferromagnetic exchange interaction through a superexchange mechanism with intra- and inter-molecular dipole–dipole interaction or spin polarization. The picture emerging from these studies is that the number of HgBr2 moieties, the relative positions of the pyridyl groups, and the intermolecular CH ? ? ? ON contacts in this family of free radicals have a notable influence on their crystal structure packing and structural dimension, thus leading to an important role in the magnetic exchange interactions in the crystals.However, more experimental instances and theoretical calculations are necessary to evaluate more distant interactions. Acknowledgements This work is supported by a grant from the National Science Council of Taiwan (NSC86-2113-M032-005).References 1 A. Caneschi, D. Gatteschi and P. Rey, Prog. Inorg., 1991, 39, 331. 2 A. Caneschi, P. Chiesi, L. David, F. Ferraro, D. Gatteschi and R. Sessoli, Inorg. Chem., 1993, 32, 1445. 3 K. Inoue and H. Iwamura, J. Am. Chem. Soc., 1994, 116, 3173; Angew. Chem., Int. Ed. Engl., 1995, 34, 927. 4 V. I. Orcharenko, F. L. de Panthou, N. V. Reznikov, P. Rey and R. Z. Sagdeev, Inorg. Chem., 1995, 34, 2263. 5 K. Awaga, T. Inabe and Y. Maruyama, Synth.Met., 1993, 55–57, 3323; Chem. Phys. Lett., 1990, 190, 349. 6 K. Inoue and H. Iwamura, Chem. Phys. Lett., 1993, 207, 551. 7 T. Okuno, T. Otsuka and K. Awaga, J. Chem. Soc., Chem. Commun., 1995, 827. 8 A. Caneschi, F. Ferraro, D. Gatteschi, P. Rey and R. Sessoli, Inorg. Chem., 1990, 29, 1756, 4217; 1991, 30, 3162. 9 C. Banelli, A. Caneschi, D. Gatteschi and L. Pardi, Inorg. Chem., 1992, 31, 741. 10 D. Luneau, G. Risoan, P. Rey, A. Grand, A. Caneschi, D. Gatteschi and J.Laugier, Inorg. Chem., 1993, 32, 5616. Fig. 8 Schematic drawing of complex 4 showing the alternating spin densities responsible for the ferromagnetic coupling176 J. Chem. Soc., Dalton Trans., 1998, Pages 171–176 11 A. Caneschi D. Gatteschi and R. Sessoli, Inorg. Chim. Acta, 1991, 184, 67. 12 F. L. de Panthou, E. Belorizky, R. Calemczuk, D. Luneau, C. Marcenant, E. Ressouche, P. Turek and P. Rey, J. Am.e Panthou, D. Luneau, J. Laugier and P. Rey, J. Am. Chem. Soc., 1993, 115, 9095. 14 H. O. Stumpf, L. Ouahab, Y. Pei, D. Grandjean and O. Kahn, Science, 1993, 261, 447. 15 A. Yamaguchi, T. Okuno and K. Awaga, Bull. Chem. Soc. Jpn., 1996, 69, 875. 16 D. Lanchem and T. W. Wittag, J. Chem. Soc. C, 1966, 2300. 17 E. F. Ullmann, L. Call and J. H. Osiecki, J. Org. Chem., 1970, 35, 3623. 18 O. Kahn, Molecular Magnetism, VCH, New York, 1993, p. 3. 19 E. J. Gate, Y. de Page, J. P. Cherland, F. L. Lee and P. S. White, J. Appl. Crystallogr., 1989, 22, 384. 20 J. W. Hall, W. E. Marsh, R. R. Weller and W. E. Hatfield, Inorg. Chem., 1981, 20, 1033. 21 J. C. Bonner and M. E. Fischer, Phys. Rev. A, 1964, 145, 640. 22 G. A. Baker, jun., G. S. Rushbrooke and H. E. Gibert, Phys. Rev., 1964, 135, A1272. 23 K. Yamaguchi, M. Okumura, J. Maki, T. Noro, H. Namimoto, M. Nakano, T. Fueno and K. Nakasuji, Chem. Phys. Lett., 1992, 190, 353. 24 H. M. McConnell, J. Chem. Phys., 1963, 39, 1910. 25 J. Cirujeda, M. Mas, E. Molins, F. L. de Panthou, J. Laugier, J. G. Park, C. Paulson, P. Rey, C. Rovira and J. Veciana, J. Chem. Soc., Chem. Commun., 1995, 709; J. Veciana, J. Cirujeda, C. Rovira and J. Vidal-Gancedo, Adv. Mater., 1995, 7, 221; K. Togashi, R. Imachi, K. Tomioka, H. Tsuboi, Y. Ishida, T. Nogami, N. Takeda and M. Ishikawa, Bull. Chem. Soc. Jpn., 1996, 69, 2821. 26 K. Awaga, T. Okuno, A. Yamaguchi, M. Hasegawa, T. Inabe, Y. Maruyama and N. Wada, Phys. Rev. B, 1994, 49, 3975. 27 E. Hernandez, M. Mas, E. Molins, C. Rovira and J. Veciana, Angew. Chem., Int. Ed. Engl., 1993, 32, 882. Received 14th July 1997; Paper 7/04999D

ISSN:1477-9226    

DOI:10.1039/a704999d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 177–184 177 Co-ordination chemistry and molecular mechanics study of the magnesium(II) and calcium(II) complexes of trisubstituted 1,4,7- triazacyclononane derivatives Jurriaan Huskens a and A. Dean Sherry *,†,a,b a University of Texas at Dallas, Department of Chemistry, PO Box 830688, Richardson, Texas 75083-0688, USA b University of Texas Southwestern Medical Center, Department of Radiology, the Rogers Magnetic Resonance Center, 5801 Forest Park Road, Dallas, Texas 75235-9085, USA The affinities of 1,4,7-tris(2-hydroxyalkyl)-1,4,7-triazacyclononane derivatives for MgII and CaII were found to differ greatly.Whereas 1,4,7-tris(2-hydroxyethyl)- (L1), 1,4,7-tris(2-hydroxy-2-methylpropyl)- (L2) and the unsymmetrical 1,4,7-tris(2-hydroxypropyl)-1,4,7-triazacyclononane derivative L3b barely discriminated between MgII and CaII, the symmetrical isomer L3a was more than 500 times more selective for MgII than for CaII.Similar selectivity differences were observed between the diastereomers of 1,4,7-tris(2-hydroxy-2-phenylethyl)- (L4) and 1,4,7-tris(2-hydroxydodecyl)-1,4,7-triazacyclononane (L5). These selectivities were related to the structure of the magnesium complexes as shown by molecular mechanics (MMX) calculations. Only those ligands favoring the formation of a magnesium complex with a large twist angle between the planes of co-ordinating oxygens and ring nitrogens resulting in a small, tight cavity showed a large preference for MgII over CaII.The MMX calculations predicted large twist angle structures for phosphinate derivatives, and for a phosphonate monoester derivative, and these ligands were found to have a high selectivity for MgII. Similarly, the calculated preference for the smaller twist angle correctly predicted the lack of MgII/CaII selectivity for acetate and amide derivatives and for a phosphonate diester derivative. Equilibration of the complexes of the ligands in the presence of both MgII and CaII was slow, as shown for example by kd,0 = 2.1 × 1024 s21 for CaL3a.These dissociation rates were a factor of 100 times larger at the boundary of a two-phase (water–chloroform) system. In the development of ligand systems for the assessment of free, ionized magnesium(II) in biological systems, the binding selectivity for MgII over CaII is of utmost importance. Generally, ligands bind CaII preferentially due to the lower charge density and, consequently, smaller solvation heats for this ion compared to the smaller MgII.1 As observed before,2 1,4,7-triazacyclononane derivatives are especially promising because they provide a small cavity and can saturate the first co-ordination sphere of MgII.Recently, we described several 1,4,7-triazacyclononane [9]aneN3 derivatives that showed a selectivity for MgII over CaII of, in some cases, more than two orders of magnitude.3–5 Although this macrocyclic structure guarantees a higher MgII/CaII selectivity (Ksel = KMgL/KCaL) over most other ligand systems,2 there are significant selectivity differences between 1,4,7-triazacyclononane derivatives. 1,4,7-Triazacyclononane- 1,4,7-triacetate (nota), for example,6 has a Ksel of only 5.9 while for 1,4,7-triazacyclononane-1,4,7-triyltrimethylene tris(methylphosphinate), Ksel = 170.5 This study extends the material presented before in a preliminary communication 7 in which we discussed some remarkable selectivity differences between 1,4,7-tris(2-hydroxyalkyl)- 1,4,7-triazacyclononane derivatives.Here, we report the preparation and selectivity studies of these and several other [9]aneN3 derivatives with amide, phosphinate ester, and phosphonate mono- and di-ester side-chains. These selectivity differences observed by 13C NMR spectroscopy are correlated to structural differences between the complexes of MgII and CaII, as predicted by molecular mechanics.Kinetic details on complexation equilibria are also provided. † E-Mail: sherry@utdallas.edu Experimental General All chemicals were purchased from Aldrich and used as received. Proton (500 MHz) and 13C (125 MHz) NMR spectra were recorded at 298 K on a General Electric GN500 spectrometer using a 5 mm 13C/1H probe. Deuteriated acetonitrile was used as a (co-)solvent; residual CHD2CN was used as the 1H internal chemical shift standard (d 1.93), while the CD3 resonance (d 1.3) was used as a reference for the 13C chemical shifts.Although proton decoupling was usually applied, coupled spectra were recorded occasionally for assignment purposes. Resonance areas were determined by peak integration using standard GE software. No corrections for differences in nuclear Overhauser effects (NOEs) or T1 relaxation rates were made since it was shown in a previous study5 that these did not change upon complexation with MgII. Typically, samples contained 0.2 M ligand and magnesium or calcium perchlorate was added in aliquots of 0.25 equivalent to yield concentrations of 0–0.3 M.In the selectivity experiments (see below), 1.5 equivalents of Mg(ClO4)2 were added to the CaL sample and Ca(ClO4)2 to the MgL sample. Both were monitored as a function of time until equilibrium was reached. In a few cases, the amount of CaII was increased further to 30 equivalents in order to obtain an accurate selectivity value. The dissociation rates of MgL5 and MgL8 at a CDCl3– water bilayer were determined as follows.The complex (5 cm3, 40 mM) was prepared in CDCl3 and was brought into contact with an aqueous solution (5 cm3, 10% D2O) containing 40 mM ethylenedinitrilotetraacetate (edta), buffered with 80 mM NaHCO3 and additional NaOH to pH 7.0. These systems were178 J. Chem. Soc., Dalton Trans., 1998, Pages 177–184 stirred vigorously and 0.5 cm3 samples were collected from both phases after 5, 15, 30 min, 1 and 3 h.The organic phase samples were studied by 13C NMR spectroscopy, using CDCl3 as the internal standard (d 77.0), while ButOH was added to the aqueous samples as an internal standard (CH3, d 31.2). Synthesis of the ligands 1,4,7-Tris(2-hydroxyethyl)-1,4,7-triazacyclononane L1. All tris(2-hydroxyalkyl) derivatives were prepared by the reaction of [9]aneN3 with the corresponding epoxide, using a procedure similar to that described previously.8 Compound L1 was prepared using [9]aneN3 (263 mg, 2.04 mmol) and ethylene oxide (280 mg, 6.36 mmol) in water (3 cm3) at 0 8C.The temperature was gradually raised to 25 8C. After 1 h the solvent was evaporated and the residue dissolved in an equimolar mixture of diethyl ether and CH2Cl2. The solution was dried over Na2SO4, filtered, and evaporated yielding 412 mg (1.58 mmol, 77%) of L1 isolated as a white solid, judged pure by 1H and 13C NMR spectroscopy. 1H NMR (CD3CN): L1, d 4.86 (3 H, OH, s), 3.45 (6 H, CH2CH2OH, t, 3JHH = 5.5 Hz), 2.64 (6 H, CH2CH2OH, t) and 2.58 (12 H, ring CH2, s); MgL1, d 6.64 (3 H, OH, s), 3.88 (6 H, CH2CH2OH, t, 3JHH = 6.1 Hz), 3.00 (6 H, CH2CH2OH, t) and 2.97–2.73 (12 H, ring CH2, m); CaL1, d 5.06 (3 H, OH, s), 3.76 (6 H, CH2CH2OH, t, 3JHH = 5.3 Hz), 2.77 (6 H, CH2CH2OH, t) and 2.80–2.69 (12 H, ring CH2, m). 13C NMR (CD3CN): L1, d 60.78, 60.21 (CH2CH2OH) and 54.63 (ring CH2); MgL1, d 60.50, 58.71 (CH2CH2OH) and 53.90 (ring CH2); CaL1, d 59.64, 59.48 (CH2CH2OH) and 52.73 (ring CH2). 1,4,7-Tris(2-hydroxy-2-methylpropyl)-1,4,7-triazacyclononane L2.Compound L2 was prepared from [9]aneN3 (245 mg, 1.90 mmol) and isobutylene oxide (2,2-dimethyloxirane) (557 mg, 7.74 mmol) in water (3 cm3) at 25 8C. After 15 h the mixture was evaporated and dried in high vacuum yielding 576 mg (1.67 mmol, 88%) of L2 isolated as a white solid, judged pure by 1H and 13C NMR spectroscopy. 1H NMR (CD3CN–CDCl3, 3:1 v/v): L2, d 3.89 (3 H, OH, s), 2.87 [6 H, CH2C(CH3)2OH, s], 2.47 (12 H, ring CH2, s) and 1.08 (18 H, CH3, s); MgL2, d 6.19 (3 H, OH, s), 2.90 [6 H, CH2C(CH3)2OH, s], 2.84 (12 H, ring CH2, s) and 1.38 (18 H, CH3, s); CaL2, d 5.05 (3 H, OH, s), 2.90 [6 H, CH2C(CH3)2OH, s], 2.72 (12 H, ring CH2, s) and 1.31 (18 H, CH3, s). 13C NMR (CD3CN–CDCl3, 3 : 1 v/v): L2, d 72.10 [CH2C(CH3)2OH], 70.92 [C(CH3)2OH], 61.21 (ring CH2) and 28.45 (CH3); MgL2, d 73.68 [C(CH3)2OH], 66.46 [CH2C- (CH3)2OH], 53.10 (ring CH2) and 30.67 (CH3); CaL2, d 72.98 [C(CH3)2OH], 69.17 [CH2C(CH3)2OH], 54.33 (ring CH2) and 30.75 (CH3). 1,4,7-Tris[(2S)-2-hydroxypropyl]-1,4,7-triazacyclononane L3a.Pure compound L3a was prepared similarly to that described previously,9 from [9]aneN3 (238 mg, 1.84 mmol) and (S)- propylene oxide (436 mg, 7.51 mmol) in chloroform (3 cm3), while warming the sample from 0 to 25 8C during the first 1 h. After 3 d the mixture was evaporated in high vacuum yielding 531 mg (1.75 mmol, 95%) of L3a isolated as a white solid, judged pure by 1H and 13C NMR spectroscopy. 1H NMR (CD3CN): L3a, d 5.52 (3 H, OH, s), 3.76 (3 H, CH, m), 2.76, 2.41 (12 H, ring CH2, two d), 2.57, 2.30 [6 H, CH2CH(CH3)OH, two dd, 2JHH = 213.5, 3JHH = 2.6, 9.5] and 0.99 (9 H, CH3, d, 3JHH = 6.2 Hz); MgL3a, d 6.52 (3 H, OH, s), 4.24 (3 H, CH, m), 2.86, 2.78 [6 H, CH2CH(CH3)OH, two dd, 2JHH = 215.1, 3JHH = 6.2, 7.0], 3.02–2.64 (12 H, ring CH2, m) and 1.23 (9 H, CH3, d, 3JHH = 6.0 Hz); CaL3a, d 5.01 (3 H, OH, s), 4.14 (3 H, CH, m), 2.90–2.72 (12 H, ring CH2, m), 2.66, 2.58 [6 H, CH2CH(CH3)OH, two dd, 2JHH = 212.2, 3JHH = 3.7, 8.5] and 1.17 (9 H, CH3, d, 3JHH = 6.2 Hz). 13C NMR (CD3CN): L3a, d 67.50 [CH2CH(CH3)OH], 64.48 (CH), 53.96 (ring CH2) and 20.60 (CH3); MgL3a, d 68.48 [CH2CH(CH3)OH], 64.96 (CH), 57.26, 51.17 (ring CH2) and 19.94 (CH3); CaL3a, d 67.34 [CH2CH(CH3)OH], 66.33 (CH), 57.00, 51.87 (ring CH2) and 21.22 (CH3). 1,4,7-Tris(2-hydroxypropyl)-1,4,7-triazacyclononane (L3a : L3b 5 1 : 3). Analogous to the procedure for compound L3a, a 1 : 3 mixture of L3a and L3b was prepared from [9]aneN3 (258 mg, 2.00 mmol) and racemic propylene oxide (415 mg, 7.16 mmol) yielding 555 mg (1.83 mmol, 92%) of the diastereomeric mixture, isolated as a white solid. 1H NMR (CD3CN): L3b, d 5.22 (3 H, OH, s), 3.75 (3 H, CH, m), 2.80–2.25 [18 H, ring CH2 and CH2CH(CH3)OH, m] and 1.00 (9 H, CH3, m); MgL3b, d 6.42, 6.12 [3 H, OH, two s (2 : 1)], 4.10 (3 H, CH, m), 3.10–2.05 [18 H, ring CH2 and CH2CH(CH3)OH, m] and 1.20 (9 H, CH3, m); CaL3b, d 5.08, 4.82, 4.71 (3 H, OH, three s), 4.15 (3 H, CH, m), 2.80–2.42 [18 H, ring CH2 and CH2CH(CH3)OH, m] and 1.17 (9 H, CH3, m). 13C NMR (CD3CN): L3b, d 68.41, 67.21 [1 : 2, CH2CH(CH3)OH], 65.36, 64.76 (1 : 2, CH), 55.24, 54.64, 54.39 (ring CH2), 20.75, 20.60 (1 :2, CH3); MgL3b, d 68.16, 67.63, 66.89 [CH2CH(CH3)OH], 66.16, 64.78, 64.34 (CH), 57.33, 55.23, 53.36, 52.51, 51.66, 50.86 (ring CH2), 19.95, 19.54, 19.03 (CH3); CaL3b, d 67.48, 67.05, 66.59 [CH2CH(CH3)OH], 66.12, 65.32, 64.59 (CH), 56.34, 53.77, 53.48, 52.22, 52.00, 51.03 (ring CH2), 21.02, 20.75 (2 :1, CH3). 1,4,7-Tris[(2S)-2-hydroxy-2-phenylethyl]-1,4,7-triazacyclononane L4a. Compound L4a was prepared from [9]aneN3 (161 mg, 1.25 mmol) and (S)-styrene oxide (606 mg, 5.04 mmol) in chloroform (3 cm3) at 25 8C. After 2 d the solvent was evaporated and the residue dissolved in MeOH. The product was separated from excess of epoxide and an impurity (likely the adduct of an extra epoxide with L4a at a hydroxy site) by column chromatography (silica, using 0–10% concentrated ammonia in MeOH as the eluent).Thus, 128 mg (0.262 mmol, 21%) of L4a were obtained as an off-white solid, judged pure by 1H and 13C NMR spectroscopy. 1H NMR (CD3CN): L4a, d 7.41 (6 H, o-H of Ph, d, 3JHH = 7.3), 7.34 (6 H, m-H of Ph, t), 7.25 (3 H, p-H of Ph, t, 3JHH = 7.4), 6.28 (3 H, OH, s), 4.86 (3 H, CH, dd, 3JHH = 2.6, 11.7), 2.95, 2.58 (12 H, ring CH2, two d), 2.84, 2.66 [6 H, CH2CH(Ph)OH, two dd, 2JHH = 213.0 Hz]; MgL4a, d 7.54 (6 H, o-H of Ph, d, 3JHH = 7.4), 7.41 (6 H, m-H of Ph, t), 7.38 (3 H, p-H of Ph, t, 3JHH = 7.4), 7.03 (3 H, OH, s), 5.28 (3 H, CH, dd, 3JHH = 3.7, 12.3), 3.14, 3.06 [6 H, CH2CH(Ph)OH, two dd, 2J = 215.0 Hz], 3.26–2.95 (12 H, ring CH2, m); CaL4a, d 7.47 (6 H, o-H of Ph, d, 3JHH = 7.3), 7.41 (6 H, m-H of Ph, t), 7.34 (3 H, p-H of Ph, t, 3JHH = 7.5), 5.96 (3 H, OH, s), 5.09 (3 H, CH, dd, 3JHH = 3.8, 9.2), 3.04, 2.91 [6 H, CH2CH(Ph)OH, two dd, 2JHH = 212.9 Hz], 2.93–2.63 (12 H, ring CH2, m). 13C NMR (CD3CN): L4a, d 144.11 (ipso-C), 129.10, 126.87 (o/m-C of Ph), 128.02 (p-C of Ph), 71.43 (CH), 68.17 [CH2CH(Ph)OH] and 53.41 (ring CH2); MgL4a, d 138.66 (ipso-C), 129.01, 127.80 (o/ m-C of Ph), 130.11 (p-C of Ph), 74.28 (CH), 65.55 [CH2CH- (Ph)OH], 57.16, 51.35 (ring CH2); CaL4a, d 142.03 (ipso-C), 129.54, 126.89 (o/m-C of Ph), 129.13 (p-C of Ph), 73.06 (CH), 66.55 [CH2CH(Ph)OH], 56.68, 51.89 (ring CH2).(±)-1,4-Bis[(2R)-2-hydroxy-2-phenylethyl]-7-[(2S)-2- hydroxy-2-phenylethyl]-1,4,7-triazacyclononane L4b. Analogous to the procedure for compound L4a, 143 mg (0.292 mmol, 15%) pure L4b was recovered as an off-white solid, judged pure by 1H and 13C NMR spectroscopy, from the reaction between [9]aneN3 (256 mg, 1.98 mmol) and racemic styrene oxide (996 mg, 8.30 mmol). 1H NMR (CD3CN): L4b, d 7.41 (6 H, o-H of Ph, d), 7.34 (6 H, m-H of Ph, t), 7.26 (3 H, p-H of Ph, t), 6.05 (3 H, OH, s), 4.83 (3 H, CH, m), 2.95–2.60 [18 H, ring CH2 and CH2CH(Ph)OH, m]; MgL4b, d 7.35–7.55 (15 H, Ph, m), 7.13, 6.91, 6.86 (3 H, OH, three s), 5.20 (3 H, CH, m), 3.25–2.75 [18 H, ring CH2 and CH2CH(Ph)OH, m]; CaL4b, d 7.30–7.50 (15 H, Ph, m), 5.90, 5.50, 5.39 (3 H, OH, three s), 5.09 (3 H, CH, m), 3.20–2.65 [18 H, ring CH2 and CH2CH(Ph)OH, m]. 13C NMR (CD3CN): L4b, d 144.26, 144.12 (1 : 2, ipso-C), 129.21, 129.03, 126.97, 126.79 (o/m-C of Ph), 128.14, 127.96 (p-C of Ph), 72.08, 71.58 (1 : 2, CH), 69.10, 67.87 [1 : 2, CH2CH(Ph)OH], 54.71, 54.11, 53.82 (ring CH2); MgL4b, d 138.97, 138.64, 138.36J.Chem. Soc., Dalton Trans., 1998, Pages 177–184 179 (ipso-C), 130.67, 130.53, 130.48, 130.17, 129.79, 127.70 (o/m/p- C of Ph), 73.96, 73.60, 72.29 (CH), 66.44, 65.35, 64.24 [CH2CH(Ph)OH], 57.18, 55.34, 53.63, 52.55, 51.74, 50.96 (ring CH2); CaL4b, d 141.75, 141.62, 141.34 (ipso-C), 129.68, 129.50, 127.38, 127.27, 127.19, 127.08 (o/m/p-C of Ph), 73.26, 72.89, 72.43 (CH), 67.74, 65.59, 64.89 [CH2CH(Ph)OH], 56.37, 53.69, 53.32, 52.07, 51.78, 50.88 (ring CH2). 1,4,7-Tris(2-hydroxydodecyl)-1,4,7-triazacyclononane (L5a:L5b 5 1 : 3). A 1 : 3 mixture of compounds L5a and L5b was prepared from [9]aneN3 (255 mg, 1.98 mmol) and racemic 1,2- epoxydodecane (1.684 g, 9.14 mmol) at 80 8C without additional solvent. After 15 h the mixture was evaporated in high vacuum. After addition of MeOH an oily layer separated. After evaporation in high vacuum this appeared to be 290 mg (0.426 mmol, 22%) of the pure diastereomeric mixture isolated as a waxy solid. 1H NMR (CD3CN–CDCl3, 2 : 1 v/v): L5a:L5b 1 : 3, d 5.50 (4 H, OH, s), 3.60 (4 H, CH, m), 2.80–2.30 [24 H, ring CH2 and CH2CH(OH)R, m], 1.90 (8 H, m), 1.40–1.10 (64 H, m) and 0.85 (12 H, t) (C10H21); MgL5a and MgL5b, d 6.13, 6.07, 5.88, 5.81 (4 H, OH, s), 4.05 (4 H, CH, m), 3.10–2.55 [24 H, ring CH2 and CH2CH(OH)R, m], 1.50–1.20 (72 H, m) and 0.85 (12 H, t) (C10H21); CaL5a and CaL5b, d 4.87, 4.84, 4.65, 4.53 (4 H, OH, s), 3.95 (4 H, CH, m), 3.00–2.45 [24 H, ring CH2 and CH2CH(OH)R, m], 1.50–1.20 (72 H, m) and 0.85 (12 H, t) (C10H21). 13C NMR (CD3CN–CDCl3, 2 : 1 v/v): L5a (*) and L5b, d 69.10, 68.40,* 67.98 [1:2:1, CH2CH(C10H21)OH], 66.85, 65.85, 65.66 * (1:1:2, CH), 54.84, 54.27, 53.99, 53.39 * (ring CH2), 35.53, 35.45 * (1 :3, CHOHCH2CH2R), 26.22 (CHOHCH2- CH2R), 32.39, 30.24, 30.11, 29.82, 23.13, 14.35 (CHOHCH2- CH2R); MgL5a (*) and MgL5b, d 72.08,* 71.76, 71.36, 70.61 [CH2CH(OH)R], 64.60, 63.31,* 63.13, 62.87 (CH), 57.02, 56.76,* 55.07, 53.38, 52.48, 51.49, 50.95,* 50.73 (ring CH2), 34.82,* 34.60, 34.28, 34.06 (CHOHCH2CH2R), 25.30, 25.10, 25.00, 24.75 (CHOHCH2CH2R), 32.34, 30.05, 30.01, 29.92, 29.79, 23.11, 14.32 (CHOHCH2CH2R); CaL5a (*) and CaL5b, d 71.13,* 70.62, 70.23, 69.61 [CH2CH(OH)R], 65.65, 64.56,* 63.37, 62.76 (CH), 56.70,* 56.22, 53.58, 53.15, 51.88, 51.77, 51.62,* 50.75 (ring CH2), 35.78,* 35.69, 35.38 (1:2:1, CHOHCH2CH2R), 25.59, 25.40, 25.25 (1:2:1, CHOHCH2CH2R), 32.39, 30.08, 29.83, 23.15, 14.34 (CHOHCH2CH2R). 1,4,7-Tris(diethylcarbamoylmethyl)-1,4,7-triazacyclononane L6. The compound [9]aneN3 (254 mg, 1.97 mmol), 2-chloro- N,N-diethylacetamide (1.070 g, 7.15 mmol) and NaHCO3 (675 mg, 8.04 mmol) were refluxed in MeOH (10 cm3) under N2, similarly as described before for the analogous 1,4,7,10- tetraazacyclododecane derivative.10 After 4 h the mixture was cooled to room temperature, NaHCO3 and NaCl were filtered off, and the solvent was evaporated yielding 1.358 g of the crude product.The residue was dissolved in water and washed with diethyl ether thus removing excess alkylating agent. The product was extracted into CH2Cl2, which was dried over Na2SO4. After filtration and evaporation, 500 mg (1.07 mmol, 54%) of L6 were recovered as a clear oil, judged pure by 1H and 13C NMR spectroscopy. 1H NMR (CD3CN): L6, d 3.36 (6 H, CH2CO, s), 3.36, 3.27 (12 H, CH2CH3, two q, 3JHH = 7.0 Hz), 2.80 (12 H, ring CH2, s), 1.11, 1.02 (18 H, CH3, two t); MgL6, d 3.81 (6 H, CH2CO, s), 3.42, 3.33 (12 H, CH2CH3, two q, 3JHH = 7.0 Hz), 2.90, 2.68 (12 H, ring CH2, m), 1.15, 1.11 (18 H, CH3, two t); CaL6, lines too broad to assign. 13C NMR (CD3CN): L6, d 170.17 (CO), 60.22 (CH2CO), 55.68 (ring CH2), 41.92, 40.25 (CH2CH3), 14.52, 13.30 (CH3); MgL6, d 173.52 (CO), 58.59 (CH2CO), 52.01 (ring CH2), 43.13, 42.95 (CH2CH3), 13.88, 12.70 (CH3); CaL6, d 172.95 (CO), 61.18 (CH2CO), 53.74 (ring CH2), 43.15, 42.44 (CH2CH3), 14.40, 13.16 (CH3). 1,4,7-Triazacyclononane-1,4,7-triyltrimethylenetris(methylphosphinic acid ethyl ester) L7. The triethyl ester L7 was prepared as described before (this study also includes 1H NMR data).5 13C NMR (CD3CN–CDCl3, 1 : 1 v/v): L7, d 60.42 (OCH2, 2JPC = 6.1), 58.09 (NCH2P, 1JPC = 114), 58.08 (ring CH2), 16.90 (OCH2CH3) and 13.40 (PCH3, 1JPC = 90.5 Hz); MgL7, d 64.52 (2JPC = 6.6), 64.29 and 64.13 (2JPC = 8.5) (1:2:1, OCH2), 58.0–54.0 (ring CH2 and NCH2P), 16.47, 16.40, 16.32 (2:1:1, OCH2CH3), 13.94 (1JPC = 95.9), 13.68 (1JPC = 94.5) and 13.43 (1JPC = 90.0 Hz) (1:2:1, PCH3); CaL7, d 62.91 (OCH2, br), 54.2–59.5 (ring CH2 and NCH2P), 16.51, 16.43 (2:2, OCH2CH3), 14.06 (1JPC = 86.3), 13.58 (1JPC = 91.7), 13.37 (1JPC = 87.6) and 13.13 (1JPC = 89.6 Hz) (PCH3). 1,4,7-Triazacyclononane-1,4,7-triyltrimethylenetris(phosphonic acid dibutyl ester) L8. The compound [9]aneN3 (247 mg, 1.91 mmol), (CH2O)n (178 mg, 5.94 mmol) and P(OBun)3 (1.580 g, 6.32 mmol) were mixed at 25 8C without any additional solvent. After 15 h the mixture was evaporated in high vacuum, yielding 1.558 g of L8, judged >90% pure by 13C NMR spectroscopy, which was used without further purification. 13C NMR (CD3CN): L8, d 65.98 (OCH2, 2JPC = 6.4), 57.53 (ring CH2, 3JPC = 5.7), 53.98 (NCH2P, 1JPC = 160 Hz), 33.39 (OCH2CH2), 19.51 (CH2CH3) and 13.94 (CH3); MgL8, d 70.01 (OCH2, 2JPC = 5.9), 55.35 (ring CH2, 3JPC = 8.3), 54.11 (NCH2P, 1JPC = 151 Hz), 32.69 (OCH2CH2), 19.19 (CH2CH3) and 13.77 (CH3); CaL8, d 68.42 (OCH2, 2JPC = 6.8), 55.20 (ring CH2, 3JPC = 7.0), 54.38 (NCH2P, 1JPC = 156 Hz), 32.86 (OCH2CH2), 19.29 (CH2CH3) and 13.83 (CH3).Trisodium 1,4,7-triazacyclononane-1,4,7-triyltrimethylene tris(phosphonate monobutyl ester) Na3L10. Compound L8 (750 mg, 0.92 mmol) was refluxed in a mixture of water (10 cm3), 1,4-dioxane (10 cm3), EtOH (5 cm3) and 40% aqueous NaOH (5 cm3) for 2 d. After evaporation, a concentrated aqueous phase and a gummy residue were obtained.The aqueous phase was decanted and the residue dissolved in CH2Cl2, which was dried over Na2SO4. After filtration and evaporation, 392 mg (0.62 mmol, 68%) of L10 (trisodium salt) was obtained, judged >90% pure by 13C NMR spectroscopy, which was used without further purification. 13C NMR (CD3CN–CDCl3, 1 : 1 v/v): L10, d 64.57 (OCH2, br), 54.47 (ring CH2, br), 56.30 (NCH2P, 1JPC = 149 Hz), 33.49 (OCH2CH2), 19.42 (CH2CH3) and 14.12 (CH3); MgL10, d 64.84 (OCH2, 2JPC = 4.1), 58.47 (3JPC = 8.3) and 52.90 (ring CH2), 56.59 (NCH2P, 1JPC = 148), 33.33 (OCH2- CH2, 3JPC = 5.4 Hz), 19.36 (CH2CH3) and 14.07 (CH3); CaL10, d 65.65 (OCH2, br), 56.13 (ring CH2 and NCH2P, br), 33.31 (OCH2CH2), 19.45 (CH2CH3) and 14.09 (CH3).Molecular mechanics calculations The geometries of the magnesium and calcium complexes of compounds L1, L2, L3a, L3b, nota, L7a, L7b, notpde (L89, R = R9 = OEt), L9 and notpme (L109, R = OEt) were optimized using the MMX force field of HYPERCHEM 4.5 on a 486DX, 66 MHz personal computer. The built-in force field parameters for MgII and CaII were used as such. Side-chains were positioned roughly into modes A, B or C (Scheme 3) before optimization, and the positions of the OH protons in L1–L3, and of the residual alkyl groups in L3, L7–L10 were varied systematically for observation of local energy minima.Since it was more practical to keep the 1,4,7-triazacyclononane ring fixed in a single conformation (D), all statistical possibilities for the stereocenters present in the side-chains or occurring after complexation on the phosphorus or coordinating oxygen nuclei were investigated.For nota, only local minima with all acetates in either A or C were observed. The side-chain orientations of the structures with the lowest energy are reported in Table 2. The energy values given in Table 2 are those directly obtained from the molecular mechanics calculations.180 J.Chem. Soc., Dalton Trans., 1998, Pages 177–184 Results and Discussion Binding selectivities for MgII versus CaII All tris(2-hydroxyalkyl) derivatives L1–L5 were prepared by the reaction of 1,4,7-triazacyclononane with an excess of the corresponding epoxide (Scheme 1), as described for L1 and L3a.8,9 If R � R9, two diastereomers of each derivative were formed when racemic epoxide was used. The ratio between the (RRR/SSS) (a) and the (RRS/SSR) (b) forms of L3–L5 was in all cases 1 : 3, as statistically expected.Optically pure L3a and L4a were prepared from the corresponding chiral (S)-epoxides. We were able to isolate pure L4b from the reaction mixture of L4a and L4b by column chromatography. The diastereomeric mixtures of L3a and L3b and of L5a and L5b were used without further separation in the selectivity experiments described below. The tris(N,N-diethylamide) derivative of nota, L6 (Scheme 1), was prepared analogously to the corresponding tetrasubstituted 1,4,7,10-tetraazacyclododecane derivative.10 The phosphorus derivatives L7 and L8 were prepared from the corresponding phosphine and paraformaldehyde (Scheme 1), as described before for the synthesis of L9.5 The trisodium salt of L10 was prepared by the hydrolysis of L8, using an excess of NaOH (Scheme 1).All ligands and their magnesium and calcium complexes were studied by 1H and 13C NMR spectroscopy at room temperature using CD3CN as the solvent.This solvent provided good solubility for most of the ligands (for a few, additional CDCl3 was added) and of magnesium and calcium perchlorates. Since the 1H resonances of the complexes appeared to overlap consider- Scheme 1 N N N H H H N N N HO R¢ R R¢ R HO OH R R¢ * * * N N N O O O N N N N N N P O P O P O R¢ R R R¢ R R¢ N N N P O P O P O O –Na+ R R O –Na+ R O –Na+ N O Cl NaHCO3/ MeOH O R R¢ CHCl3 or H2O L1 = R = R¢ = H L2 = R = R¢ = Me L3 = R = Me, R¢ = H L4 = R = Ph, R¢ = H L5 = R = n-C10H21, R¢ = H L3–L5, L7 a = ( RRR/SSS) b = ( RRS/SSR) NaOH dioxane/EtOH/H2O L6 L7 = R = Me, R¢ = OEt L8 = R = R¢ = OBun Na3L9 R = Me Na3L10 R = OBun PR(R¢)2 + (CH2O) n * ably, attention was focused mainly on the 13C NMR experiments.The 13C NMR spectra of compounds L1, L2, L3 and L4a showed single sharp resonances for the ring carbons, and for all side-chain nuclei. The unsymmetrical isomer L4b, however, showed three separate ring resonances and three resonances for each side-chain nucleus, of which some were overlapping.Consequently, the diastereomeric mixtures of L3a and L3b and of L5a and L5b showed four resonances for the ring carbons which had equal intensity since the single line for L3a corresponded to six nuclei while the three lines for the more abundant (3×) isomer L3b corresponded to two nuclei each. The amide derivative, L6, showed a single sharp resonance for ring carbons, but two lines for each of the ethyl nuclei due to slow rotation around the amide bondhe phosphorylated compounds L7 and L8 showed similar behavior, although 31P couplings complicated the spectra. For L10, broader lines were observed, especially for the ring nuclei. Upon addition of small amounts of Mg(ClO4)2 the resonances of L1–L6 started to shift considerably, while also a separate set of resonances appeared. The latter set appeared to be the 1 : 1 complex, ML, as it was the only species present when 1 equivalent or more of the metal ion was added to the ligand solution.This 1 : 1 complex was attributed to an inner-cage complex in which all ring nitrogens are involved in coordination and, likely, all three side-chain oxygens. The shift of the ligand resonances at lower metal-to-ligand ratios can probably be attributed to fast exchange species in which only the oxygens are involved in metal co-ordination (see Scheme 2). For L7 and L8, no shift of the free ligand resonances was observed, only the formation of the (slow-exchange) 1 : 1 complexes.For L10, line broadening was so severe that only the spectra at excess of metal were useful. For ML1 and ML2, single resonances were observed for all ring carbons and for all equivalent nuclei from the side-chains, while for ML3a and ML4a the ring carbons showed two resonances upon metal complexation. The latter can be attributed to restriction of the side-chain rotation upon formation of the coordinative chelate rings: the R groups then point preferentially to one side of the chelate ring which renders the two ring carbons attached to the nitrogen non-equivalent.Consequently, all six ring carbons of ML4b showed separate resonances, while for the diastereomeric mixtures of L3 and L5 eight ring carbon resonances were detected for both MgL and CaL: two arose from the symmetrical (RRR/SSS) isomer, while six belonged to the unsymmetrical (RRS/SSR) isomer. Similar to those of the free ligands, these lines had equal intensity.Analogously, the resonances of the side-chain nuclei appeared as four equally intense lines. For L5a and L5b the two carbons of the residual C10H21 closest to the CHOH group were thus resolved into four Scheme 2 Species observed in solution for MgL1 and CaL1, and the rates of their equilibria relative to the NMR time-scale N N HO N OH OH N N HO N OH OH M N N OH N HO HO HO M OH OH N N N M N N N OH HO OH slow fast fast ML2 ML L + L + MJ. Chem.Soc., Dalton Trans., 1998, Pages 177–184 181 Table 1 Selectivities, Ksel (= KMgL/KCaL), of the macrocyclic ligands (Scheme 1) as obtained from 13C NMR competition experiments, preferred sidechain co-ordination modes (Scheme 3) as obtained from MMX calculations (Table 2), and indications of the dissociation rates of the complexes Structure MgL CaL L L1 L2 L3a L3b L4a L4b L5a L5b nota L6 L7c L7b L8 L9 L10 Ksel 0.18 3.0 590 0.25 16 0.06 >60b 0.28 5.9 c 8.8 >25b >25b 1.6 170 d >25 Mode BBB BBB/CCC AAA ABB CCC 2 AAA/CCC AAA/CCC CCC 2 AAA AAA/CCC a/8 14 21/233 36 27 53/245 46 55 53/245 Mode BBB BBB BBB ABB CCC BBB BBB BBB BBB BBB a/8 9 18 9 221 7 12 14 14 Dissociation a Slow Slow Slow Slow Slow Slow Slow Slow Slow Fast Fast Fast Slow a In organic solvent; slow, t2� 1 = 1–8 h; fast, t2� 1 < 5 min.b Observed only for the diastereomeric mixture. c In aqueous solution, ref. 6. d In aqueous solution, ref. 5. lines.whereas the remaining nuclei appeared as partly overlapping, single lines. Identification of the diastereomers of L3 and their complexes was accomplished by comparison with the spectra of optically pure L3a, while for L5 the spectra were compared to the data obtained for L3. Similar observations were made for L6–L8. For L10 the resonances of the magnesium complex were sharp and well resolved, while the calcium complex showed considerable line broadening. The MgII/CaII selectivities of the ligands were determined by 13C NMR spectroscopy on samples with both MgII and CaII in excess, prepared by adding MgII to the sample containing CaL and by adding CaII to the MgL sample.Chemical equilibrium Fig. 1 The 13C NMR spectrum of a sample with L3a:L3b:Mg: Ca = 1:3:6:6 (bottom) and of a sample with L3a:Mg:Ca = 2:3:50; *, MgL3a; o, CaL3a; 1, MgL3b; #, CaL3b (methyl resonances not shown) was assumed when both samples indicated the same metal-ion selectivity value.For a sample containing L3a:L3b:MgII : CaII = 1:3:6:6, the 13C NMR spectrum (Fig. 1, bottom) showed that both MgII and CaII formed complexes with L3b (Ksel = 0.25), while L3a only bound MgII. Selectivity experiments with pure L3a (Fig. 1, top) indicated that Ksel = 590 for this ligand. This led to the conclusion that the difference in orientation of a single methyl group in these ligands results in a MgII/CaII selectivity difference of more than three orders of magnitude.For the tris(2-hydroxyalkyl) derivatives, only L3a, L4a and L5a exhibited large selectivities for MgII (Table 1), while L1, L2, L3b, L4b and L5b barely discriminated between MgII and CaII. The amide derivative L6, prepared earlier and tested as a carrier for alkali- and alkaline-earth-metal ions,11 showed a low selectivity, comparable to the selectivity of nota as observed in aqueous solution.6 The triethyl ester L7 showed a good selectivity for MgII for both diastereomers, as shown from an absence of the calcium complex for a sample containing L7a:L7b:MgII : CaII = 1:3:6:6.The same was observed for L10, but the tris- (phosphonate diester) derivative L8 showed no selectivity at all. It should be noted that L10 might form diastereomeric complexes in solution since the oxygens in a phosphonate side-chain will become inequivalent upon complexation of one of them, as described for L9.5 The observation of a single sharp set of resonances for MgL10, indicating C3 symmetry, seems to suggest only one isomer in solution, whereas the calcium complex, exhibiting considerable line broadening, might be present as a mixture of isomers.This contrasts with the behavior of L9 in aqueous solution, where the 31P resonance for the magnesium complex was broader than for the free ligand. This was attributed to the presence of both diastereomeric complexes in solution.5 Structure of the complexes of MgII and CaII To investigate further the striking selectivity differences between very closely related compounds such as L3a and L3b, we used molecular mechanics calculations to find the optimum structures of the magnesium and calcium complexes of L1, L2, L3a, L3b, nota, L7a, L7b, notpde (L89, R = R9 = OEt), L9 and notpme (L109, R = OEt). Local energy minima were observed for structures in which the side-chains adopted one of three possible co-ordination modes (Scheme 3), and the occurrence of these minima was attributed to a (low-energy) staggered conformation of the C]C bond in the side-chains for all co-182 J.Chem. Soc., Dalton Trans., 1998, Pages 177–184 Table 2 Local energy minima (in kcal mol21, ca. 4.184 kJ mol21; n.m. = no minimum observed; lowest per complex italicized) of the macrocyclic ligand complexes of MgII and CaII as a function of the orientation of the side-chains (Scheme 3; macrocyclic ring in D conformation), as determined by MMX calculations MgL CaL L L1 a L2 a RRR-L3a a SSS-L3a a L3b nota RRR-L7a SSS-L7a L8 RRR-L9 SSS-L9 RRS-L9 RSS-L9 RRR-L10 SSS-L10 AAA 42.30, 43.08 68.33, 65.54 61.06, n.m. 44.24, 47.25 46.26 b 33.31 107.69 104.08 139.49 104.59 101.23 103.39 102.28 105.08 107.18 BBB 41.96, 43.79 65.79, 63.54 44.87, 46.29 50.86, 57.49 45.50 c d 108.70 114.60 112.98 106.30 109.90 107.45 108.65 111.77 109.15 CCC 46.58, 46.72 63.64, 66.58 54.03, 46.85 49.04, 53.11 32.15 106.31 103.91 109.35 102.40 102.57 102.40 102.43 105.15 107.32 AAA 37.78, 38.27 n.m., 61.83 51.44, 51.74 40.74, 41.96 40.01 b 30.88 106.02 99.77 138.55 102.96 98.11 101.35 99.75 100.59 104.60 BBB 36.90, 36.13 56.64, 54.81 40.43, 39.24 n.m., 48.25 39.52 c d 96.92 104.51 102.36 95.18 97.70 95.93 96.79 101.58 97.88 CCC n.m., n.m. 59.35, 63.29 53.32, n.m. 47.40, n.m. 30.36 106.56 102.56 107.81 100.81 100.32 100.63 100.47 103.67 106.56 a Values for RRR and SSS conformations on the hydroxyl oxygens, respectively. b (SSR)-AAB structure (see text). c (SRR)-ABB structure (see text).d Not possible; see text. ordination m. Only for the sterically crowded complexes with L2 and L8, local minima were observed for which this bond was not fully staggered. Owing to the planarity of the C-CO2 group in nota, this acetate side-chain has one degree of freedom less, which resulted in minimized structures for modes A and C only. Table 2 shows the energies of the local minima of the complexes. Since only the D conformation of the triazacyclononane ring was used in the calculations, the stereochemistry of the sidechains was varied systematically.In the complexes with L9 and L10 the P atoms become chiral upon co-ordination. In an analogous manner, the co-ordinating oxygens of L1, L2, L3a and L3b become chiral upon co-ordination so that also the position of the hydroxyl protons was varied systematically. For L1, L2, L3a, nota, L7a and L8, only C3 symmetric structures were investigated. The complexes of L3a showed the lowest energies with R sidechains in the B conformation and S side-chains in the A conformation.All C conformations had significantly higher energies. In practice, this indicates that the Mg(SSS-L3a) complex exists in the D-AAA conformation, while the corresponding calcium complex occurs as the L-BBB conformer. The MgL3a complex showed the lowest energy for the AAA structure (see Table 2), while for the calcium complex the BBB structure appeared to be favored. Owing to these results, we tested only the structures of ML3b in which the R side-chains were positioned in the B conformation and the S groups in the A conformation, while the hydroxyl protons were varied system- Scheme 3 Front and top views of the possible co-ordination modes of the side-chains of the metal complexes of L1–L3 for which energy minima were observed by molecular mechanics calculations, and the resulting twist angles, a, of the plane of co-ordinating oxygens relative to the plane of macrocyclic ring nitrogens R O { N } M R¢ N } { O R R¢ M R O { N } M R¢ O N N N M O O O N N N M O O N N N M O O O a C B A atically.The AAA or BBB structures appeared to be not stable for these asymmetric complexes. For L3b it appeared that ABB (with the S side-chain in the A, and the R side-chain in the B conformation) was the most stable conformation for both the magnesium and the calcium complexes. Analogously to ML3a, the symmetrical forms of ML9 show a preference for BBB in the D-RRR complex and for AAA in the D-SSS complex.The lower energy for the AAA form of the DSSS complex compared to the D-RRR complex was confirmed by the crystal structures of several ML9 (R = Ph) complexes, which appeared to exist in the same D-SSS (or L-RRR) conformation. 12 The energy of the CCC conformations for these systems was close to the values for the AAA structures, which can be attributed to the comparable twist angles for these phosphonate side-chain containing ligands (see below).The energies of the unsymmetrical RRS and RSS forms were intermediate between the values for the RRR and SSS structures. The same appeared to be true for the complexes with L7 and L10, so that Table 2 only lists the extremes for the symmetrical cases. For all these ligands, BBB appeared to be the favored structure for the calcium complexes, while AAA or a combination with CCC was preferred by the magnesium complexes (see Table 2). In ML10 the apparent reversal of the energies between RRR and SSS compared to L7 and L9 is due to a change in the priorities on the phosphorus stereocenters. In the C3 symmetric complexes the co-ordination modes AAA, BBB and CCC give rise to different ‘twist angles’, a, i.e.the rotation of the plane of co-ordinating oxygen atoms compared to the plane of ring nitrogens (Scheme 3). Owing to the non-planarity of the macrocyclic ring (D/L conformations), these compounds contain an inherent twist of the plane of the side-chain carbon atoms directly connected to the ring nitrogens.Here, we define a positive a as a twist of the plane of coordinating oxygens in the same direction as that of the plane of side-chain carbons relative to the ring nitrogens. According to this definition, a trigonal prismatic structure has a twist angle of 08, while an octahedron has a = 1608 (or 2608). The twist a was large and positive for the AAA configuration, smaller (but positive) for BBB, and negative for CCC.Angles for the lowestenergy structures of the C3-symmetrical complexes are given in Table 1. For the unsymmetrical complexes, these angles differed slightly for each side-chain, such as for ML7b, or the side-chains even adopted different co-ordination modes within the same complexes with concomitant large angle differences, as observed for ML3b (see below). Owing to the larger P]C and P]O bond lengths compared to C]C and C]O, respectively,J.Chem. Soc., Dalton Trans., 1998, Pages 177–184 183 complexes of L7–L10 generally showed larger twist angles, especially for the AAA and CCC structures (in BBB, both longer bonds counteract so that the effect on a is minimal). The preferred structure appears to depend on the size of the metal ion. Crystal structures of some transition-metal complexes showed that the zinc complex of L3a adopted the BBB conformation (a = 4.58),13 while the smaller cations CoIII (a = 49.88) 14 and CrIII (a = 458) 15 preferred the AAA structure.For nota, the complexes of NiII (a=45.08),16,17 NiIII (a = 53.18) 17 and CrIII (a = 49.08) 16 adopted the AAA conformation, while the larger CuII (a = 226.68) and FeIII (a = 225.28) preferred CCC.16 It can be easily argued that a larger twist angle provides a smaller cavity for the metal ion. Our calculations showed that particularly AAA provided a smaller cavity than either BBB or CCC. In a particular case, however, the structure of the ligand may also be the driving force for the adoption of the AAA structure as nicely demonstrated for the trimethylenetris( phenylphosphinate) derivative L99 (R = Ph), which adopts the AAA structure for both divalent (CoII, NiII, ZnII and CuII) and trivalent (CoIII, FeIII, GaIII and InIII) metal ions (a = 48–528).12 The methyl groups in all optimized structures of L3a and L3b were pointing out from the center, causing all side-chains to be in the same orientation for L3a, while for L3b two different sidechain orientations were present in each optimized structure.This driving force seemed to favor a large twist angle for MgL3a (AAA). This resulted for MgL3b, however, in considerable steric hindrance between methyl groups when these had the A coordination mode. Thus, ABB appeared to provide the least steric hindrance. The absence of such a driving force, as found in MgL1, did not result in a preference for AAA. Therefore, we concluded that a large a (AAA) is only favored when there is a driving force on the side-chains to put a residual R group away from the center and is only possible when any resulting steric hindrance between two side-chains is absent.As a result, MgL1 adopted the BBB conformation because there is no driving force, while MgL2 and MgL3b did not adopt AAA or AAB due to resulting steric hindrance between methyl groups of neighboring side-chains. For MgL3a (and for MgL4a and MgL5a), all factors were optimal for adopting the AAA conformation .The large twist (AAA) makes the cavity for the metal ion quite small, so that this conformation is not optimal for the larger cation CaII, which always preferred the BBB (or ABB) structure. Similar to the case of L1, the absence of a driving force also explains the low twist angle observed for both Mg(nota) and Ca(nota), here expressed in a CCC structure since BBB is impossible (see above). This contrasts with the complexes of NiII, NiIII and CrIII,16,17 but in these cases the metal ion probably provides the driving force for the formation of AAA since this is closest to an octahedral arrangement.For L9 (R = Me) the AAA structure was favored for both the symmetrical and unsymmetrical magnesium complex. This ligand can be regarded as a structural analog of L2 with the difference that the longer P]C and P]O bonds reduce the steric hindrance between the P]] O oxygens and P]CH3 groups of neighboring side-chains, so that AAA is favored in this case.Similarly, AAA is favored for MgL7a, MgL7b and MgL10. The steric hindrance in the AAA form of MgL8, however, is too large, similar to that observed for MgL2. The existence of the driving force for formation of the AAA structure is clearly demonstrated for the derivative L99 (R = Ph),12 as discussed above. Here, we show that a methyl group in L3a and L9 is enough for adoption of the AAA structure in the corresponding magnesium complexes. Also, both present calculations and earlier NMR data 5 on L9 suggest that MgII/CaII selectivities are large for both the symmetrical and unsymmetrical complexes, whereas for larger R groups only the symmetrical one exists, both in the solid state (ML99, for various MII and MIII, see above) 12 and in solution (MgL10, see above).As becomes apparent from Table 1, a ligand showed a large MgII/CaII selectivity when a large twist angle structure, AAA, was favored for its magnesium complex.For the phosphonate side-chain containing ligands L7, L9 and L10, also the CCC structure had a relatively small cavity (large negative a) resulting in almost equal energies and, therefore, possible contributions to the equilibrating structures in solution. Only when large twist angle structures were favored, the cavity of the ligand was significantly smaller than for the calcium complex, thus favoring the formation of the magnesium complex. The forces that govern whether a magnesium complex adopts the AAA structure or not depend upon the nature of the sidechains, and appear to constitute a delicate balance: the different orientation of a single methyl group in L3a and L3b causes MgL3a to favor the small cavity structure AAA, while the steric hindrance between the side-chains in the analogous structure for MgL3b is too large.This resulted in a MgII/CaII selectivity difference of a factor of 2000. Dissociation rates of the complexes Upon addition of MgII to a solution of CaL3a in CD3CN (see above), equilibration to MgL3a took place over a period of several hours (see Fig. 2). In this process the dissociation of CaL3a is the rate-determining step since the formation of MgL3a is very rapid, as observed for the addition of MgII to free L3a by which MgL3a is formed instantaneously. The dissociation of CaL3a can be either (i) spontaneous, (ii) H1 catalysed or (iii) MgII catalysed (see Scheme 4). Pathway (ii) has been observed for aqueous systems,18 but cannot occur under the aprotic conditions employed here.Since the concentration decrease of CaL3a could be fitted with a simple exponential decay curve even though the concentration of free MgII changed considerably, pathway (iii) was also ruled out. Therefore, we concluded that the dissociation rate of CaL3a was determined entirely by its spontaneous dissociation [pathway (i)]. From Fig. 2, a dissociation half-time, t2� 1 , of about 1 h was deduced (kd,0 = 2.1 × 1024 s21).Similar, slow behavior was observed for the equilibration of L1–L6 and L10 in the presence of an excess of MgII and CaII. The dissociation of CaL was generally somewhat faster than of the Fig. 2 Relative concentrations of the MgL3a and CaL3a versus time upon the addition of 1.5 equivalents of Mg(ClO4)2 to a solution of CaL3a (with a 0.5 equivalent excess of CaII), measured by 13C NMR spectroscopy in CD3CN at 25 8C Scheme 4 CaL CaII + L MgL kd, 0, i + MgII – CaII + H + CaII + HL MgL ii + MgII – H +/CaII + MgII CaII + MgL iii184 J.Chem. Soc., Dalton Trans., 1998, Pages 177–184 corresponding magnesium complex. For example, for the dissociation of MgL3b t2� 1 ª 5 h. Although the dissociation of the magnesium complexes forming the AAA structure was difficult to study due to their high stability relative to the corresponding calcium complexes, their dissociation rates did not differ dramatically as witnessed by the equilibration of MgII/CaII/L3a in large excess of CaII (Fig. 1, top) which was complete in about 20 h. The equilibration of L7 and L8 was instantaneous (t2� 1 < 1 min), probably due to their smaller complex stabilities. In order to investigate whether these compounds might have a potential use as ionophores in MgII-selective electrodes, we studied the dissociation rate of the magnesium complexes of L5 and L8 in a two-phase system. The complex of L8 was prepared in CDCl3 and brought into contact with a buffered aqueous phase containing an equal amount of edta to scavenge aqueous free MgII. Vigorous stirring was applied, and 13C NMR spectra of the aqueous phase showed the formation of Mg(edta).As expected, all MgII appeared to be released from MgL8 in the organic phase within 5 min. For L5 (1 : 3 mixture of L5a and L5b), however, 50% of the bound MgII was released after 5 min and complete dissociation was observed only after 30 min.This led to the conclusion that the dissociation of MgL5 is about 100 times faster in the two-phase system than in organic solvent alone. This rate is actually about the same as the dissociation rate of MgL9 in water as studied extensively before.18 Therefore, we believe that in the dissociation experiment in the two-phase system the polar ring unit with the MgII is positioned at the phase boundary while the long alkyl chains remain in the organic phase. Conclusion We have found that 1,4,7-triazacyclononane derivatives that are able to accommodate MgII in a small, tight cavity (large a) form complexes with structures (AAA) different from those of CaII (BBB). This structural difference appears to be the origin of the large MgII/CaII selectivity seen for these ligands.The forces that govern whether a magnesium complex favors the adoption of the AAA structure or not appeared to be quite delicate, since the different position of a single methyl group causes the selectivity to reverse when comparing L3a and L3b.In general, AAA is favored only when the residual R groups are all positioned out of the center, where steric hindrance between the side-chains is minimal. Noteworthy is also that nota, originally presented as a ligand with a (relatively) high selectivity for MgII,2 now appears to be less selective than many other [9]aneN3 derivatives. This can be attributed to the preference for the small-twist-angle structure (CCC) for Mg(nota).From the two-phase system dissociation rates it becomes clear that the performance of [9]aneN3 derivatives in electrode membranes will depend on the position and rates of three equilibria: (i) the equilibrium between the magnesium complex in the bulk organic phase and the phase boundary (with the polar ring unit sticking into the aqueous phase); (ii) the release of MgII from the ring unit into the aqueous phase; (iii) the equilibrium between the free ligand in the bulk organic phase and the phase boundary. Especially (i) and (ii) lie far to the right for derivative L5. Future research will focus on mixed side-chain derivatives, likely with two phosphonate monoester side-chains and one 2-hydroxyalkyl so that the magnesium complex is uncharged, thus facilitating accommodation in the organic phase. Acknowledgements This research was supported in part by grants from the Robert A. Welch Foundation (AT-584) and the National Institutes of Health Biotechnology Research Program (P41-RR02584). References 1 A. E. Martell, R. M. Smith and R. J. Motekaitis, NIST Critical Stability Constants of Metal Complexes Database, NIST Standard Reference Database 46, NIST Standard Reference Data, Gaithersburg, MD, 1993. 2 M. J. van der Merwe, J. C. A. Boeyens and R. D. Hancock, Inorg. Chem., 1985, 24, 1208. 3 R. Ramasamy, I. Lazar, E. Brücher, A. D. Sherry and C. R. Malloy, FEBS Lett., 1991, 280, 121. 4 J. van Haveren, L. DeLeon, R. Ramasamy, J. van Westrenen and A. D. Sherry, NMR Biomed., 1995, 8, 197. 5 J. Huskens and A. D. Sherry, J. Am. Chem. Soc., 1996, 118, 4396. 6 A. Bevilacqua, R. I. Gelb, W. B. Hebard and L. J. Zompa, Inorg. Chem., 1987, 26, 2699. 7 J. Huskens and A. D. Sherry, Chem. Commun., 1997, 845. 8 B. A. Sayer, J. P. Michael and R. D. Hancock, Inorg. Chim. Acta, 1983, 77, L63. 9 J. Robb and R. D. Peacock, Inorg. Chim. Acta, 1986, 121, L15. 10 J. H. Forsberg, R. M. Delaney, Q. Zhao, G. Harakas and R. Chandran, Inorg. Che95, 34, 3705. 11 H. Tsukube, H. Adachi and S. Morosawa, J. Chem. Soc., Perkin Trans. 1, 1989, 1537; J. Org. Chem., 1991, 56, 7102. 12 E. Cole, R. C. B. Copley, J. A. K. Howard, D. Parker, G. Ferguson, J. F. Gallagher, B. Kaitner, A. Harrison and L. Royle, J. Chem. Soc., Dalton Trans., 1994, 1619. 13 I. Fallis, L. J. Farrugia, N. M. Macdonald and R. D. Peacock, Inorg. Chem., 1993, 32, 779. 14 A. A. Belal, L. J. Farrugia, R. D. Peacock and J. Robb, J. Chem. Soc., Dalton Trans., 1989, 931. 15 L. J. Farrugia, N. M. Macdonald, R. D. Peacock and J. Robb, Polyhedron, 1995, 4, 541. 16 K. Wieghardt, U. Bossek, P. Chaudhuri, W. Herrmann, B. C. Menke and J. Weiss, Inorg. Chem., 1982, 21, 4308. 17 M. J. van der Merwe, J. C. A. Boeyens and R. D. Hancock, Inorg. Chem., 1983, 22, 3490. 18 J. Huskens and A. D. Sherry, Inorg. Chem., 1996, 35, 5137. Received 27th May 1997; Paper 7/03640J

ISSN:1477-9226    

DOI:10.1039/a703640j

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 185–192 185 Spectroelectrochemical studies and excited-state resonance-Raman spectroscopy of some mononuclear rhenium(I) polypyridyl bridging ligand complexes. Crystal structure determination of tricarbonylchloro[ 2,3-di(2-pyridyl)quinoxaline]rhenium(I) Mark R. Waterland,a Timothy J. Simpson,a Keith C. Gordon *,a and Anthony K. Burrell *,b a Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand b Department of Chemistry, Massey University, Private Bag 11122, Palmerston North, New Zealand A number of mononuclear rhenium(I) complexes have been prepared and their physical properties and excitedstate and spectroelectrochemical resonance-Raman spectra studied.These compounds have the general formula [Re(CO)3Cl(L)], where L can be 2,3-di(2-pyridyl)quinoxaline (dpq), 2-(2-pyridyl)quinoxaline (pq) or 5-methyl- 2,3-di(2-pyridyl)quinoxaline (mdpq). The structure of [Re(CO)Cl(dpq)] was determined by single-crystal X-ray diffraction. The NMR data for complexes with dpq and mdpq suggest the unbound pyridyl is shielding protons on the bound pyridyl moiety.The resonance-Raman spectra of the reduced complexes show some polarisation of electron density towards the bound pyridyl ring. The excited states have very similar spectral features to those of the reduced complexes. This suggests that electrochemically prepared redox states model the metal-to-ligand charge-transfer state well, for these systems.The photophysical and electrochemical properties of d6 metal polypyridyl complexes are of interest because of their use in solar energy conversion schemes and molecular devices.1 We have examined a series of mononuclear rhenium(I) complexes in which the ligands are a variation on the commonly used bridging ligand 2,3-bis(2-pyridyl)quinoxaline, dpq. We have undertaken this study with three aims in mind. (1) We have found that binuclear rhenium(I) complexes, with dpq bridging ligands and related systems with substituents at the quinoxaline ring, show similar spectral signatures when reduced.2 This implies the reducing electron occupies a similar molecular orbital (MO) in each case and that the electron is localised on the pyridyl ring systems.The mononuclear rhenium(I) complexes offer an opportunity to test if this localisation is extended to smaller ligand systems bound to a single Re(CO)3Cl unit. In the case of the 2-(2-pyridyl)quinoxaline (pq) complex only one pyridyl unit is present. The extent of the MO occupied by the reducing electron has implications as to the ability of the bridging ligand to facilitate electronic communication.3 (2) Ruthenium(II) diimine complexes with polypyridyl bridging ligands similar to dpq are of considerable interest as building blocks in photoactive supramolecular assemblies.4 The excited-state chemistry of such complexes is dominated by metal-to-ligand charge-transfer (MLCT) excited states, in which the metal is formally oxidised and the ligand reduced.The spectroelectrochemistry of these systems can offer spectral signatures for each part of this excitation. The oxidised complex mimics the formal metal oxidation and the reduction of the complex mimics the radical anion formation at the lowestenergy ligand site, for these systems the bridging ligand L. However attempts to measure the resonance-Raman spectra of radical anion species in [Ru(bpy)2L]21 (bpy = 2,29-bipyridine) have been frustrated by the strong scattering observed from the bpy ligands through the RuÆbpy MLCT chromophore.5 The rhenium(I) complexes do not possess visible chromophores based on the spectator CO and Cl ligands.Hence the resonance- Raman spectra of the reduced complexes should provide the desired spectral signatures for the L radical anions. (3) We have used pulsed laser excitation to measure the resonance-Raman spectra of the excited state of [Re(CO)3- Cl(dpq)] and [Re(CO)3Cl(mdpq)] [mdpq = 5-methyl-2,3-bis(2- pyridyl)quinoxaline]. This confirms that the redox state, as formed in the electrochemical studies, and the excited state have the same spectral signature and thus similar structure. Experimental Synthesis The compounds dpq, mdpq and pq were synthesized by literature methods.6,7 Mononuclear rhenium(I) complexes of them were prepared by addition of 1 mmol of ligand to an equimolar amount of [Re(CO)5Cl] in methanol (100 cm3) under reflux conditions and a nitrogen gas atmosphere, for 18 h.The resulting red solution was filtered while hot to remove impurities. Upon cooling, the desired complex precipitated and was isolated by filtration. N N N N Re C C C O O O Cl N N N Re C C C O O O Cl N N N N Re C C C O O O Cl 1 4 5 6 7 8 9 1¢ 3¢ 4¢ 5¢ 6¢ 1¢¢ 3¢¢ 4¢¢ 5¢¢ 6¢¢ 1 1 4 4 5 5 6 6 7 7 8 1¢ 3¢ 4¢ 5¢ 6¢ 6¢ 5¢ 4¢ 3¢ 1¢ 1¢¢ 3¢¢ 4¢¢ 5¢¢ 6¢¢ [Re(CO)3Cl(dpq)] [Re(CO)3Cl(pq)] [Re(CO)3Cl(mdpq)]186 J.Chem. Soc., Dalton Trans., 1998, Pages 185–192 Table 1 Electronic absorption data for ligands and complexes in CH2Cl2 at room temperature Compound lmax/nm (1023 e/M21 cm21) dpq pq mdpq 1 2 3 246 (32.5) 253 (14.5) 253 (27.4) 253 (24.9) 269 (18.6) 275 (9.5) 275 (22) 260 (18.2) 274 (18.5) 280 (29.8) 334 (9.5) 334 (6.5) 335 (7.6) 353 (13.1) 373 (7.8) 370 (16.3) 368 (11.3) 448 (2.6) 449 (3.4) 439 (4.3) dpq: dH(300 MHz, solvent CDCl3, standard SiMe4) 8.38 (d, 2 H, H69, H60), 8.23 (q, 2 H, H5, H8), 7.97 (d, 2 H, H39, H30), 7.81 (m, 4 H, H49, H40, H6, H7) and 7.23 (m, 2 H, H59, H50) (Found: C, 75.8; H, 4.0; N, 19.9.Calc.: C, 76.1; H, 4.2; N, 19.7%); yield 61%. pq: dH(300 MHz, CDCl3, SiMe4) 9.97 (s, 1 H, H3), 8.79 (dd, 1 H, H69), 8.61 (dd, 1 H, H39), 8.17 (m, 2 H, H5, H6), 7.90 (td, 1 H, H49), 7.80 (m, 2 H, H7, H8) and 7.41 (td, 1 H, H59) (Found: C, 75.42; H, 4.57; N, 20.51. Calc.: C, 75.33; H, 4.38; N, 20.28%); yield 31%.mdpq: dH(300 MHz, CDCl3, SiMe4) 8.3 (dd, 2 H, H69, H60), 8.14 (d, 1 H, H39), 8.10 (dd, 1 H, H6), 7.96 (d, 1 H, H30), 7.82 (m, 2 H, H49, H40), 7.7 (m, 2 H, H5, H7), 7.23 (m, 2 H, H59, H50) and 2.88 (s, 1 H, H9) (Found: C, 64.47; H, 4.01; N, 16.06. Calc. for mdpq?3H2O: C, 64.74; H, 4.01; N, 15.91%); yield 81%. [Re(CO)3Cl(dpq)] 1: dH(300 MHz, CDCl3, SiMe4) 9.16 (d, 1 H, H60), 8.85 (m, 2 H, H69, H5), 8.32 (m, 1 H, H8), 8.01 (m, 3 H, H39, H6, H7), 7.89 (td, 1 H, H40), 7.69 (td, 1 H, H49), 7.49 (m, 2 H, H59, H50) and 7.26 (d, 1 H, H30) (Found: C, 42.48; H, 2.05; N, 9.34.Calc.: C, 42.75; H, 2.05; N. 9.50%); yield 47%. [Re(CO)3Cl(pq)] 2: dH(300 MHz, CDCl3, SiMe4) 9.75 (s, 1 H, H3), 9.23 (d, 1 H, H69), 8.86 (dd, 1 H, H8), 8.57 (d, 1 H, H5), 8.26 (m, 2 H, H49, H6), 8.08 (m, 2 H, H39, H7) and 7.70 (m, 1 H, H59) (Found: C, 37.40; H, 1.66; N, 8.39. Calc.: C, 37.47; H, 1.77; N, 8.19%); yield 77%. [Re(CO)3Cl(mdpq)] 3: dH(300 MHz, CDCl3, SiMe4) 9.13 (d, 1 H, H69), 8.71 (s, 1 H, H9), 8.68 (m, 1 H, H60), 8.15 (d, 1 H, H30), 8.01 (td, 1 H, H40), 7.91 (m, 2 H, H7, H8) and 7.71 (td, 1 H, H49), 7.51 (m, 2 H, H59, H50), 7.25 (d, 1 H, H39) and 2.88 (s, 3 H, H9) (Found: C, 43.76; H, 1.98; N, 9.61.Calc.: C, 43.75; H, 2.34; N, 9.28%); yield 48%. Physical measurements Infrared absorption spectra were recorded on a Bio-Rad FTS- 60 FTIR spectrometer, electronic absorption spectra on a Perkin-Elmer Lambda-19 spectrophotometer. Cyclic voltammograms were obtained from nitrogen-degassed dichloromethane solutions containing 0.1 M NBu4ClO4 as supporting electrolyte and complex at 1 mM concentration.The measurements were carried out using an EG & G PAR 273A potentiostat, with model 270 software, referenced to a saturated calomel electrode (SCE). The NMR spectra were recorded using a Varian 200 MHz instrument. Resonance-Raman measurements used a Spectra-Physics model 166 argon-ion laser to generate Raman scattering.Scattering was collected in a 1358 backscattering geometry and imaged using a two-lens arrangement8 into a Spex 750M spectrograph. Raman photons were detected using a Princeton Instruments liquid-nitrogen-cooled 1152- EUV charge-coupled device. Rayleigh and Mie scattering from the sample was attenuated using a Notch filter (Kaiser Optical Systems Inc.) of appropriate wavelength. Silver sols were prepared by a standard method.9 Spectroelectrochemical Raman and electronic absorption measurements were facilitated with a thin-layer electrochemical cell.10 For the excited-state resonance-Raman measurements, 448.3 nm pulsed excitation was employed.This was generated by stimulated Raman scattering 11 through acetonitrile, contained in a 10 cm cell, using the 354.7 nm third harmonic of the Nd:YAG pulsed laser. It was found that with 50 mJ per pulse of 354.7 nm light, 3.5 mJ per pulse of 448.3 nm was obtained. The pulse energy of the 448.3 nm beam was attenuated by lowering the 354.7 nm pump energy.Crystallography Single crystals of complex 1 were grown by the slow diffusion of diethyl ether into a solution of the complex dissolved in dichloromethane. A red rod-shaped crystal with approximate dimensions 0.35 × 0.81 × 0.34 mm was secured to the end of a glass fibre with cyanoacrylate glue. Crystallographic data are summarised in Table 4. All other relevant data are available as supporting material. Intensity data were collected using an Enraf-Nonius CAD-4 diffractometer (293 K, Mo-Ka X-radiation, graphite monochromator, l = 0.710 73 Å) in the range 4 < 2q < 64 by the q–2q scan mode with index ranges 211 < h < 10, 0 < k < 30, 0 < l < 19.A total of 7131 reflections were collected, of which 6884 were unique (Rint = 0.0097). The data were corrected for Lorentz-polarisation, and X-ray absorption effects, the last by an empirical method based on azimuthal scan data (Tmax :Tmin = 0.812 : 0.514).12 No correction for extinction was applied.Scattering factors are included in SHELXL 93.13 Systematic monitoring of three check reflections showed no systematic crystal decay and no correction was applied. The position of the Re atom was determined from a Patterson synthesis. Calculations were carried out using an IBM compatible 486 computer and SHELXL 93. The remaining non-hydrogen atoms were located by application of a series alternating least-squares cycles and Fourier-difference maps. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included in the structure factor calculations at idealised positions but were not subsequently refined.Final least-squares refinement of 271 parameters resulted in residuals R(Fo) of 0.0308 and R9(Fo 2) of 0.0745 [Fo > 4s(Fo)]. After convergence the quality of fit on Fo 2 was 1.005 and the highest peak in the final difference map was 1.275 e Å23 located close to the Re atom.Selected bond lengths and angles are given in Table 5. CCDC reference number 186/753. Results The infrared absorption spectra of complexes 1–3 in CH2Cl2 solution show three bands in the carbonyl stretching region. For all complexes these lie at 2025, 1925 and 1905 cm21. This pattern corresponds to 3 CO units in a fac isomer arrangement. They are assigned as the A9(1), A0 and A9(2) vibrations respectively. 14 The wavenumbers of the CO stretches are very sensitive to the dp electron density (oxidation state) of the rhenium centre.15 The constancy of the wavenumbers indicates the ligands have an equivalent perturbation on the metal centre.Electronic absorption data for complexes are presented in Table 1. The three ligands show p æÆ p* transitions at about 250, 275 and 334 nm. These are shifted upon complexation to the Re(CO)3Cl unit. Each complex has two MLCT bands at ca. 370 and 440 nm. Electrochemical data are presented in Table 2.Oxidation waves are irreversible for all three complexes; in these cases the data reported are for the anodic peak in the cyclic voltam-J. Chem. Soc., Dalton Trans., 1998, Pages 185–192 187 mogram. This is common for [Re(CO)3Cl(L)] complexes and is assigned as a metal-centred oxidation from rhenium-(I) to -(II).16 The first reduction for all three complexes is reversible, in that it shows a peak separation between anodic and cathodic peaks close to 60 mV and in the spectroelectrochemical experiments it is possible completely to regenerate the starting material.The reduction is assigned as ligand-centred, based on comparison with similar complexes.17 The resonance-Raman spectra of the complexes and the corresponding reduced species, generated using 488 nm excitation, are shown in Fig. 1. Band positions are in Table 3. All bands are polarised, consistent with an A-term scattering mechanism.18 All the complexes emit in CH2Cl2 solution making resonance-Raman spectra noisy, particularly in the higher-wavenumber region.An excitation wavelength of 488 nm is resonant with the MLCT transitions of each of the complexes. The strong enhancement of the CO vibration is consistent with this. The MLCT excited state formally oxidises ReI æÆ ReII and the CO bond length and wavenumber position are perturbed by this. Other than the carbonyl stretch there are a number of ligandbased vibrational modes which are enhanced in the resonance- Raman spectra (Fig. 1, Table 3). Most of these features lie between 1600 and 600 cm21. A number of the ligand modes are strongly enhanced; these lie at 1476 and 1326 cm21 for 1 and 1470 and 1335 cm21 for 3. For 2 a strong band is observed at 1483 cm21 with a weaker feature at 1339 cm21. The band positions observed in the Raman spectra of the ligands are shown in Table 3. The strong luminescence of pq and mdpq required their Raman spectra to be measured using a silver sol to quench emission and increase signal intensity, through the surface-enhanced Raman scattering (SERS) effect.19 The spectra of dpq and mdpq are similar with strong bands at 1390 and in the 1470 cm21 region and with weak features in the 1600–1550 cm21 region.The SERS spectrum of pq shows strong features at 1371 cm21 with bands at 1476, 1551 and 1576 cm21. Electronic absorption spectra of the reduced complexes are shown in Fig. 2. Reduction of 1 leads to bleaching of the 373 nm MLCT band and a new band appears at 390–400 nm; 12 absorbs across much of the visible region and the absorbance at 448 nm is barely changed upon reduction.A similar pattern of changes is observed for 2 and 3. In both cases a feature at 390– 400 nm is present. The spectra observed in the optically transparent thin-layer electrode (OTTLE) experiments were stepped through a series of potentials across the respective reduction wave. Spectral changes observed occurred smoothly showing isosbestic points at 244, 314, 342, 390 and 429 nm for 1, 235, 288 and 322 nm for 2 and 321, 352, 389, 421 and 489 nm for 3. Resonance-Raman spectra, generated at 488 nm, for the reduction products are shown in Fig. 1. Band positions for reduced complexes are presented in Table 3. The reductions of 1 and 2 are characterised by the complete bleaching of the CO band at ca. 2026 cm21. Therefore, the other observed features in the resonance-Raman spectra are entirely due to the reduced species; no residual parent complex is present in the irradiated volume.The situation is more complex for the reduction of 3 to Table 2 Electrochemical data for complexes in CH2Cl2 at room temperature Compound 1 2 3 Epa a/V Oxidation 1.62 1.59 1.63 E89 b/V vs. SCE Reduction 20.75 20.75 20.81 a The oxidations are irreversible and the values given are for the anodic peak in the cyclic voltammogram. b E89 = ��� (Epa 1 Epc), the value between the anodic and cathodic peaks of the cyclic voltammogram. 32.When a reducing potential is applied to 3 new features grow in but the parent bands are not bleached. The intensity of the parent species CO band indicates about 15% residual 3 in the reduced solution. These residual ground-state features may be subtracted out of the spectrum to produce a pure reducedspecies spectrum. This is shown in Fig. 1(c), lower trace. In each case the original parent-complex spectrum was regenerated by reoxidation of the sample. Hence, under the conditions of these experiments, the reductions were chemically reversible.A weak CO feature is observed to lower wavenumbers for each of the reduced species. The small enhancement of this band suggests the resonansition for the reduced species at Fig. 1 Resonance-Raman spectra (lexc = 488 nm, 10 mW at sample) of 5 mM solutions of complexes in CH2Cl2 measured in a spectroelectrochemical cell with 0.1 M NBu4ClO4 supporting electrolyte: (a) upper trace 1, lower trace 12; (b) upper trace 2, lower trace 22; (c) upper trace 3, middle trace partly reduced sample of 32, lower trace 32.S indicates solvent band188 J. Chem. Soc., Dalton Trans., 1998, Pages 185–192 Table 3 Wavenumbers (cm21) of observed Raman bands for ligands, complexes and reduced species in CH2Cl2 solution dpqa 1591 1475 1441 1398 1324 1 b 2026s 1605w 1572 1530w 1476s 1406 1360 1326 1261w 12b 2008w 1604 1586 1565 1478 1351 1326 1263 pqc 1576 1551 1476 1371s 1356 1320 2b 2026s 1603w 1575w 1545 1483s 1368 1357 1339 1301w 1260w 22b 2007w 1604 (sh) 1590 1571 1560w 1487 1444 1361 1353 1305 1280 1258 mdpqc 1599w 1533w 1472 1390s 3b 2025s 1605w 1569 1525w 1487 (sh) 1470s 1388s 1355w 1335s 1312w 1299w 1286w 1263w 32b 2007 1586 1567 1542w 1478 1351 1266 s = Strong; w = weak, sh = shoulder.a lexc = 632.8 nm. b lexc = 488 nm, Fig. 1. c lexc = 457.9 nm, SERS spectrum. Fig. 2 Electronic absorption spectra of complexes measured in a spectroelectrochemical cell at varying potentials: changes in the electronic absorption spectrum (CH2Cl2 solution with 0.1 M NBu4ClO4 supporting electrolyte) upon electrochemical reduction (arrows indicate changes on stepping through 0.0 V to 21.2 V applied potential versus Ag1–AgCl) for (a) 1, (b) 2 and (c) 3 488 nm is not metal-centred but probably a ligand-centred p æÆ p* transition.20 The reduction 1 æÆ 12 results in the following spectral changes.(1) The CO band at 2026 cm21, which is very strongly enhanced in the spectrum of 1, is completely absent from the spectrum of 12.A weak feature is observed at 2008 cm21. (2) A group of intense bands at 1604, 1586 and 1565 cm21 appear when 1 is reduced. The spectrum of 1 shows a number of weak bands at 1605, 1572 and 1530 cm21. (3) A strong band at 1476 cm21 for 1 is slightly shifted upon reduction to 1478 cm21. This band retains intensity in the spectrum of 12. (4) A number of weak features are also present at 1351, 1326 and 1263 cm21.The 1326 cm21 band is a strong feature of 1. The resonance-Raman spectrum of complex 22 [Fig. 1(b)] shows features at 2007 cm21, in the 1550 to 1610 cm21 region, as well as some bands in the 1240 to 1310 cm21 region. The strongly enhanced ligand vibrational transition at 1483 cm21 of 2 appears shifted to 1487 cm21 for 22. The resonance-Raman spectrum of complex 32 [Fig. 1(c), lower trace] is similar to that of 12. A weak CO band is observed at 2007 cm21 and a group of intense bands appear at 1586 and 1567 cm21.The spectrum of 3 has a strong band at 1470 cm21 which shifts to 1478 cm21 upon reduction. Features appear at 1351 and 1266 cm21. All of these features are wavenumber coincident with those of 12. The resonance-Raman spectra of complexes 1–3 were measured using pulsed excitation. With 448 nm pulses (7 ns duration) it was possible to measure the spectrum of the MLCT excited state. Figs. 3 and 4 show the spectra of 1 and 3 respectively as a function of increasing pulse energy. As the pulse Fig. 3 Resonance-Raman spectra of complex 1, CH2Cl2 solution, with: (a) 457.9 nm continuous-wave excitation, (b) 448.2 nm pulsed excitation, 0.7 mJ per pulse, (c) 1.0 mJ per pulse and (d ) 1.8 mJ per pulse. Beam diameter at sample for the pulsed studies = 400 mmJ. Chem. Soc., Dalton Trans., 1998, Pages 185–192 189 energy is increased the excited-state features grow in. A plot of ln E, where E is the pulse energy, vs.ln I, where I is the intensity of the band, shows a gradient >1. This is indicative of an excited-state band.21 For complex 1 (Fig. 3) the excited-state bands appear at 1597 and 1569 cm21 at the highest photon flux used. The groundstate band at 1530 cm21 is completely bleached, however ground-state features at 1472 and 1325 cm21 persist. This may be because they are also excited-state features. The spectrum of complex 2 generated using 448 nm excitation was noisy and no excited-state features could be observed.That of 3 (Fig. 4) shows the growth of a band at 1569 cm21 (ln E vs. ln I gradient = 1.8). The ground-state features at 1334 and 1526 cm21 show much reduced intensity in the high- flux spectra. One complication of these measurements was that the mononuclear complexes showed some decomposition with prolonged pulsed irradiation. In a typical experiment about 5–10% of the complex decomposed as measured by UV/VIS spectra.The decomposition product led to the appearance of a band at 1985 cm21 (not shown in the spectra). However, ground-state resonance-Raman spectra of solutions after prolonged pulsed excitation retained the 1985 cm21 band only; no other features observed in the pulsed spectra were detected in the spectra of these solutions. Therefore the features reported above are only present with pulsed excitation. Discussion A perspective drawing of complex 1 is depicted in Fig. 5. The Fig. 4 Resonance-Raman spectra of complex 3, CH2Cl2 solution. Details as in Fig. 3 Fig. 5 An ORTEP22 drawing of complex 1 with the atomic numbering scheme. Thermal ellipsoids are shown at the 30% probability level co-ordination geometry at the Re atom is a distorted octahedron with three carbonyl ligands arranged in the facial fashion. The N(12)]Re]N(1) angle being 73.7(1)8 is signifi- cantly smaller than 908, resulting from the small bite angle of the polypyridyl ligand.The rhenium–carbonyl bond lengths do not show any significant differences [1.910(4), 1.915(4) and 1.930(5) Å] but are consistent with those observed in similar complexes.23 The two Re–N bonds although chemically distinct are similar and the distances are within the range expected for such complexes.23 The ligand is distorted from planarity by steric interactions between the pyridyl rings. The free pyridyl ligand [C(17)]N(18)]C(19)]C(20)]C(21)]C(22)] is rotated, at an angle of 53.0(1)8, with respect to the co-ordinated pyridyl ligand [C(11)]N(12)]C(13)]C(14)]C(15)]C(16)] to minimise the steric interactions with the hydrogen atom on C(16).The co-ordinated pyridine is slightly twisted [15.3(4)8] with respect to the pyrazine. In contrast the free pyridine is significantly more twisted at 33.1(4)8, with respect to the pyrazine. Interestingly, the distortion of the free pyridine, with respect to the coordinated portion of the ligand, in [Ru(bpy)2(dpq)]21 is much greater than that observed for 1, at 668.23q As the free pyridine is distant from the site of co-ordination in all of the structurally characterised 23p–r mononuclear complexes of dpq, and related ligands, there is no strong steric reason for significant deviations from the geometry observed in the free ligand.23r The signifi- cance of these distortions and the variations observed between different complexes are yet to be explained.Electrochemical data (Table 2) show that complex 3 is signifi- Table 4 Crystal data for complex 1 Formula M Crystal system Space group T/8C a/Å b/Å c/Å b/8 U/Å3 Z m/mm21 Dc/g cm23 R9(Fo 2) R(Fo) C21H12ClN4O3Re 590 Monoclinic P21/c (no. 14) 20 7.4480(14) 20.273(8) 13.397(5) 99.83(2) 1993.2(11) 4 6.262 1.966 0.0745 * 0.0308 * R9(Fo 2) = [Sw(Fo 2 2 Fc 2)2/Sw(Fo 2)2]� �� where w21 = [s2(Fo 2) 1 (aP)2 1 bP] {a = 0.0361, b = 1.4198 and P = [max(Fo 2, 0) 1 2Fc 2]/3}.The structure was refined on Fo 2 using all data; the R(Fo) value is given for comparison with older refinements based on Fo with a threshold of F > 4s(F) and R(Fo) = S||Fo| 2 |Fc||/S|Fo|; R factors based on F2 are statistically about twice as large as those based on F.Table 5 Selected bond lengths (Å) and angles (8) for complex 1 Re]C(200) Re]C(300) Re]C(400) Re]N(12) Re]N(1) Re]Cl N(1)]C(2) N(1)]C(9) N(4)]C(3) C(200)]Re]C(300) C(200)]Re]C(400) C(300)]Re]C(400) C(200)]Re]N(12) C(300)]Re]N(12) C(400)]Re]N(12) C(200)]Re]N(1) C(300)]Re]N(1) 1.910(4) 1.915(4) 1.930(5) 2.160(3) 2.222(3) 2.462(1) 1.336(4) 1.384(4) 1.315(5) 87.0(2) 91.2(2) ) 94.6(1) 176.2(2) 91.7(2) 167.1(1) 104.5(1) N(4)]C(10) N(12)]C(11) N(12)]C(13) N(18)]C(17) N(18)]C(19) C(200)]O(2) C(300)]O(3) C(400)]O(4) C(400)]Re]N(1) N(12)]Re]N(1) C(200)]Re]Cl C(300)]Re]Cl C(400)]Re]Cl N(12)]Re]Cl N(1)]Re]Cl 1.364(5) 1.335(5) 1.353(5) 1.337(5) 1.339(5) 1.141(5) 1.151(5) 1.105(6) 94.3(2) 73.7(1) 92.4(2) 89.2(2) 176.3(1) 87.33(9) 81.99(8)190 J.Chem. Soc., Dalton Trans., 1998, Pages 185–192 cantly more difficult to reduce than 2 or 1, consistent with the higher energy of the MLCT transition of 3. The reduction potentials for 1 and 2 are similar suggesting that the removal of the second pyridyl ring has little effect on the electron-accepting redox orbital involved in the reduction process. The first oxidation potentials are all irreversible, however the oxidation potential of 2 is significantly higher than for 1 or 3.The NMR spectrum of complex 1 in comparison to the ligand, dpq, and its binuclear analogue [{Re(CO)3Cl}2dpq], is complex. The chemical shifts of the 39 and 30 protons on going from the dpq ligand to 1 are particularly interesting. The H39 resonance is practically unshifted upon chelation of the Re(CO)3Cl moiety. The H30 resonance shifts 0.71 ppm upfield. This is a surprising finding in view of the fact that the rhenium should deshield the protons of the chelating pyridine ring.24 For [{Re(CO)3Cl}2dpq] 4 the H39 and H30 resonances lie at d 8.48.The NMR spectra of mdpq and 3 show similar shift patterns, with the H30 shifting upfield by 0.72 ppm and the H60 shifting 0.79 ppm downfield, upon chelation.2 It is interesting that the H60 and H30 shifts for pq to 2, where there is no unbound pyridyl ring, are significantly smaller, 0.45 and 20.54 ppm respectively. An analysis of the shift patterns on going from 2,3-dipyridylpyrazine (dpp) to [Re(CO)3Cl(dpp)] carried out by Guarr and co-workers 24 reveals similar effects.Notably, the resonances of H a to the chelating N atoms (akin to H60 for 1–3) are shifted 0.80 ppm downfield with chelation; H30 is shifted 0.71 ppm upfield. The most plausible explanation for this finding is that the unbound pyridine ring is interacting with the H30 of the bound ring. This could occur by interaction of the N atom of the unbound ring [N(18) in Fig. 5] with H30 (H from C16 in 1–3) or by ring-current effects from the unbound ring affecting H30.Kaizu and co-workers 25 have reported the latter effect in the NMR spectrum of [Ru(CN)4(bpy)]22. They found that the resonances of the a-H were deshielded by ring currents of the CN moiety. For [Ru(bpy)3]21 the resonances lie at d 7.73, whereas for [Ru(CN)(bpy)4]22 they lie at d 9.42. The single-crystal structure of complex 1 shows a N(4) to H39 distance of 2.57 Å and a N(18) to H30 distance of 2.46 Å.If the nitrogen is directly interacting with the hydrogen to shield it, then this effect should be very similar for H39 and H30. However, the resonances for these protons lie at d 8.01 and 7.26, respectively. A through-space interaction from the N atoms is therefore ruled out. If one considers the ring current of the unbound pyridyl ring as being the source of shielding of H39 then the crystal structure clearly shows that no shielding can occur. A cylinder projected normal to the unbound pyridyl ring does not encapsulate H30.This implies that the solution-phase structure is rather different from the crystal structure. The rotation of the unbound pyridyl ring out of the plane of the quinoxaline is consistent with the electrochemistry findings in which the reduction potentials for 1 and 2 are very similar. Electronic spectra of the reduced species show absorption throughout the visible region with a band at 400 nm for each of the complexes. The long-wavelength absorptions are ligandcentred transitions involving the redox orbital and higher-lying ligand p* orbitals.The more intense band at 400 nm may be a MLCT transition. The energy of the transition is altered by the modification to the electron configuration of the ligand caused by reduction. The electronic absorption spectra for 12–32 appear similar, however the spectrum of 12 differs significantly from that of 42. For the binuclear complex the reduced species absorbs out to 550 nm;2 the mononuclear species clearly do not absorb to that extent in the red.Their radical anion transitions are significantly blue-shifted. This would suggest the reducing electron in the radical anion occupies an MO more extensively spread out than for the mononuclear systems. The crystal structure determination reveals a non-planar ligand configuration. It is unlikely the pyridine and quinoxaline ring systems can be planar.26 The crystallographic data on 1 and dpq27 show that the C]C and C]N bond lengths are very similar in the free and bound ligand.Any shifts in wavenumber for the ligand vibrational normal modes are, therefore, primarily a consequence of changes in the reduced mass of the vibration, due to the presence of the rhenium centre. In the normal coordinate analysis of [Ru(bpy)3]21 higher-wavenumber vibrations have modest contributions from the metal centre motion.28 One can therefore interpret the spectra of the complexes in terms of the ligand moieties.Previous studies into binuclear ruthenium complexes of dpq29 reveal that many of the features observed in the Raman spectra are quinoxaline based. The resonance-Raman spectra of the reported complexes all possess a strong transition at ca. 1470 cm21. This feature also turns up in the ligand spectra and is assigned as a vibration associated with the quinoxaline ring system. Vibrational modes at 1600 to 1500 cm21 are also observed for each of the three complexes.These bands are weakly enhanced in comparison to the 1470 cm21 band. The spectra of 1 and 3 are almost identical in this region. The highest-frequency quinoxaline vibrational mode which is primarily C]C and C]N based is at 1575 cm21.25 The observed bands at 1605 and 1582 cm21 cannot therefore be quinoxaline based. However, 2-substituted pyridines show strong Raman features at 1620–1570, n8a, and 1580–1560 cm21, n8b.30 We assign these high-frequency modes as pyridine-based vibrations.The other notable feature is at ca. 1330 cm21. At this frequency it is possible the mode is quinoxaline based or the C]C stretch of the inter-ring linkage. Inter-ring vibrations for polypyridyl ligands in metal complexes are common at this frequency31 and we assign this band to that vibrational mode. For 1 and 3 strong bands are observed at 1326 and 1335 cm21 respectively. In the case of 2 where only one inter-ring bond is present a feature at 1339 cm21 is observed.If these assignments are correct then the strong enhancement of the quinoxaline mode in comparison to the pyridyl modes suggests the optical electron in the MLCT transition will occupy a MO with greater wavefunction amplitude on the quinoxaline ring system. A similar observation was made for the binuclear rhenium(I) complexes.2 In regard to the nature of the redox MO, a number of limiting situations are possible. It may be delocalised over the entire ligand or localised predominantly on a single ring system (the pyridyl or quinoxaline rings).Analysis of the resonance-Raman spectra of the reduced complexes with respect to each other and the related reduced binuclear complex, 42, offer some insight into the nature of the MO. There are significant changes in the resonance-Raman spectra of the complexes upon reduction. Despite the reduced enhancement in the CO region it is possible to observe carbonyl bands for the reduced species.For all of the complexes the CO band is at lower frequency for the reduced species than for the ground state. This is due to the population of the p* ligand orbital which feeds electron density through the dp orbitals to the p* MO of the carbonyl ligands. The resulting increase in antibonding character for the CO linkage results in downshift in the frequency of vibration.32 All three reduced complexes show strong bands at ca. 1590 and 1570 cm21 with a weaker shoulder at ca. 1605 cm21. The wavenumber values for these bands correspond very closely to those for the binuclear system, 42, and its analogues.2 These factors suggest that the high-wavenumber modes are predominantly pyridyl in nature. The similarity in wavenumber for the modes also suggests that the pyridyl unit has a similar structure in each of the reduced complexes. This is consistent with a redox MO localised on the pyridyl function. Furthermore, the fact that the spectrum of 22 is similar to that for the other two reduced complexes indicates that the pendant pyridyl ring has little effect on the redox MO.J.Chem. Soc., Dalton Trans., 1998, Pages 185–192 191 A strong band is observed at 1478 cm21 for 12 and 32; for 22 this lies at 1487 cm21. In the spectrum of the related 42 no features are observed in the 1470 cm21 region. Coupled with the fact that in the parent species there is a strong quinoxaline based vibration in this region suggests that the 1478 cm21 band is quinoxaline based.There are two ways in which a quinoxaline-based band may be enhanced in the spectrum of the reduced complexes. If the redox MO is entirely pyridyl ring localised then a quinoxaline vibration may be enhanced if the radical anion transition populates a MO with amplitude at the quinoxaline ring. The other possible mode of enhancement would be if the redox MO had amplitude on the quinoxaline ring itself. It is unlikely that the 1478 cm21 band is a neutral quinoxaline mode because it shows no shift in wavenumber between 12 and 32. The quinoxaline mode for 1 and 3 lies at 1476 and 1470 cm21 respectively.This suggests there is some amplitude of the redox MO on the quinoxaline ring. The fact that for 22 this band lies at 1487 cm21 may be explained in terms of the reduced mass of the quinoxaline ring by removal of the pendant pyridyl. There are a number of bands in the 1300 cm21 region which show distinct signatures for the three reduced complexes.This suggests the redox MO has some amplitude on the quinoxaline ring. The resonance-Raman data suggest that the redox MOs in complexes 12–32 have significant pyridyl character with some contribution of the MO on the quinoxaline ring. The rather modest spectral differences between 12 and 22 suggest that the pendant pyridyl ring has little effect on the nature of the redox MO. This is consistent with the electrochemistry, which shows that 1 and 2 have equivalent E89 values, crystal structure, which shows the pendant pyridyl to be at 508 to the quinoxaline ring, and NMR data, which suggest the non-planarity of the rings are maintained in solution.The resonance-Raman spectra for the excited states of complexes 1 and 3 are significant in that they show spectral signatures very like those of the corresponding 12 and 32 species. Close examination of the high-flux spectrum of 1 shows two distinct bands at 1597 and 1569 cm21.These correspond very closely to the 1604, 1586 and 1565 cm21 bands of 12. The greater bandwidth of the 448 nm pulsed beam will correspondingly broaden the Raman scattering generated. Therefore the bands will be broadened out. This strongly suggests that the MLCT state, 1*, has a radical anion species almost identical in nature to that of 12. The excited-state resonance-Raman spectrum of 3 shows almost complete bleaching of ground-state features at 1335 and 1388 cm21 at the highest flux density.This suggests that the high-pulse power spectrum is almost all excited state. This shows features at 1569 and 1473 cm21 which closely correspond to 32 bands. The experiments on 1 and 3 were carried out with the same pulse energies and absorbed photon: molecule ratios. The observation of greater groundstate depletion in 3 than 1 would suggest the excited-state lifetime of 3 is the longer of the two. This would be consistent with the higher energy gap for 3.The blue-shifting of the MLCT band and more negative E89 value for reduction is consistent with this. Attempts to measure the excited-state lifetimes by emission techniques were unsuccessful due to low signal intensities. The fact that under pulsed excitation significant ground state is present suggests lifetimes of the order of 10 ns or less. Conclusion The following conclusions may be made from this study. (1) The differing ligands have little effect on the electrochemical and infrared properties of the complexes.(2) Crystal structure studies reveal that the unbound pyridine unit in complex 1 is not planar with the quinoxaline ring. Furthermore the NMR spectra suggest this non-planarity is retained in solution. (3) Changes in the electronic spectra on reducing the mononuclear complexes show that all of the reduced species absorb at 470 nm. This signature is unlike that for 42, suggesting differences between the redox MOs for each. (4) Comparison of the resonance-Raman spectra of complexes 12 and 42 confirms conclusion (3).The spectral signature for 42 shows no bands that can be related to the quinoxaline ring system. The spectrum of 12 shows a band at 1478 cm21 which is quinoxaline-based. This suggests that the redox MO in 12 is spread over the pyridyl and quinoxaline rings. (5) Spectral comparison of species 12–32 suggests that the high-wavenumber modes, which are common to all three systems, are associated with the bound pyridyl ring.(6) The excited-state resonance-Raman spectra of complexes 1 and 3 show that the MLCT excited states are very similar to the reduced complexes. This suggests the use of spectroelectrochemistry to model the excited state is a valid approach. (7) The features for species 12–32 provide spectral marker bands for the radical anions of the ligands pq, dpq and mdpq. Analysis of the previously studied [Ru(bpy)2L]21 complexes reveal that no radical anion features are present in the resonance-Raman spectra of the reduced complexes, even as weak features.We are currently studying ruthenium(II) complexes in which the 2,29-bipyridine ligand is replaced by a different bidentate polypyridyl ligand which retains the electronic properties of bpy but has a much lower scattering cross-section. Acknowledgements Support from the New Zealand Lottery Commission and the University of Otago Research Committee for the purchase of the Raman spectrometer is gratefully acknowledged.M. R. W. thanks the John Edmond postgraduate scholarship and Shirtcliffe fellowship for support for Ph.D. research. We also thank the University of Otago Chemistry Department for the award of a Ph.D. scholarship (to T. J. S.). We wish to thank the Massey University Research Fund for funding toward crystallographic data collection. 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Dollish, W. G. Fateley and F. F. Bentley, Characteristic Raman Frequencies of Organic Compounds, Wiley, New York, 1974. 31 K. C. Gordon, A. H. R. Al-Obaidi, P. M. Jayaweera, J. J. McGarvey, J. F. Malone and S. E. J. Bell, J. Chem. Soc., Dalton Trans., 1996, 1591. 32 M. W. George, F. P. A. Johnson, J. J. Turner and J. R. Westwell, J. Chem. Soc., Dalton Trans., 1995, 2711. Received 21st July 1997; Paper 7/05194H © Copyright 1998 by the Royal Society of Chemistry

ISSN:1477-9226    

DOI:10.1039/a705194h

出版商:RSC    

年代:1998

数据来源: RSC