DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 2151–2157 2151 Synthesis, characterisation and polynuclearisation reaction of trans(S)-[Co(aminothiolate-N,S)2(en)]-type cobalt(III) complexes with 2-aminoethanethiolate, L-cysteinate and D-penicillaminate Toshiaki Yonemura,*a Zhi-Ping Bai,b Ken-ichi Okamoto,c Tomoharu Ama,a Hiroshi Kawaguchi,a Takaji Yasui a and Jinsai Hidaka d a Department of Material Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan. E-mail: yonemura@cc.kochi-u.ac.jp b Co-ordination Chemistry Institute, Nanjing University; State Key Laboratory of Co-ordination Chemistry of Nanjing University, Nanjing, 210093, China c Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan d Department of Industrial Chemistry, Kinki University in Kyushu, Iizuka, Fukuoka 820-8555, Japan Received 15th December 1998, Accepted 30th April 1999 The trans(S)-[Co(N)4(S)2]-type mononuclear complexes [Co(L)2(en)]1 or 2 (L = 2-aminoethanethiolate (aet) 1, L-cysteinate (L-cys) 3 or D-penicillaminate (3-sulfanyl-D-valinate) (D-pen) 5) were newly prepared by the reaction of trans-[CoCl2(en)2]1 with L at pH 8.5 and 25 8C.The crystal structure of 1 was determined by X-ray diVraction analysis. The geometry around the cobalt atom is approximately octahedral and two thiolate donor atoms in the aet ligands occupy trans positions to each other. Two Co–S bonds are lengthened by the double sulfur trans influence across the cobalt centre attributable to two thiolato sulfur donor atoms.The diastereomers of 3 and 5 were separated by the column chromatographic technique and characterised on the basis of their UV/Vis, CD and 13C NMR spectra. The LDD isomer of 5 (5ÀDD) is preferably formed by attractive (COO ? ? ? H–N–H) interactions. Though cis(S)- [Co(L)2(en)]1 or 2 (L = aet 2, L-cys 4 or D-pen 6) were also produced by the same preparative reaction, they could not be separated into the geometrical isomers because of very rapid isomerisation and/or polynuclearisation reaction during elution from the column.It is clear that all trans(S) complexes also show drastic and characteristic UV/Vis spectral changes with time in aqueous solutions and tend to isomerise to the cis(S) isomer, followed by the formation of S-bridged polynuclear complexes owing to both the double sulfur trans influence and high nucleophilicity of the thiolate donor atoms. The polynuclearisation reactions of the three trans(S) isomers proceed according to diVerent processes depending on their ligands.In the polynuclearisation reaction of 3, especially, the novel dinuclear complex LLLL-[Co{Co(L-cys-N,S)3}(L-cys-N,O,S)]22 7, which has not so far been identified in usual dinuclearisation reactions, was newly obtained. Introduction Metal thiolate co-ordination chemistry has experienced explosive development during the past two decades as a consequence of growing awareness of the occurrence of cysteine ligation in a variety of electron-transfer proteins, metalloenzymes and so on.However, only a few systematic studies have been reported for the properties of these complexes, because their syntheses are not generally trivial but complicated by diYculties inherent to thiolate co-ordination chemistry.1–3 Cobalt(III) complexes are appropriate to the investigation of the stereochemistry and spectroscopic properties of complexes with sulfur-containing aminocarboxylate ligands because cobalt(III) ions produce stable complexes with sulfur-containing ligands.2–8 So far, although a lot of cobalt(III) complexes with thiolate ligands have been reported with biochemical or structural interests, only one isomer has ever been preferentially isolated in many cases.The presence of two or more thiolate donor atoms in the co-ordination sphere induces an extreme specificity concerned with the formation of geometrical isomers.In general, cobalt(III) complexes containing two or three thiolate donor atoms take the cis(S) geometry as found in fac-[Co(L-N,S)3] [L = 2-aminoethanethiolate (aet), L-cysteinate (L-cys) or Dpenicillaminate (3-sulfanyl-D-valinate) (D-pen)] or bridge easily to other metal ions to form S-bridged polynuclear structures, which are stable enough in aqueous solution thus hampering the isolation of mononuclear species.2,3,9 However, the trans(S) isomers of [Co(N)4(S)2] and [Co(N)2(O)2(S)2] type were isolated by using a multidentate thiolate ligand such as endet = {2S(CH2)2N(CH3)CH2}2 in [Co(endet)(en)]1,10 an aromatic thiolate ligand such as 2-pyrimidinethiolate in[Co(pymt)2- (en)]1,11 a thioether and/or sulfinate ligand, such as L-methioninate in [Co(L-met)2],5 S-methyl-D-penicillaminate (smp) in [Co(D-smp)(D-psi)] (D-psi = 3-sulfino-D-valinate).12 It is diYcult to isolate the trans(S) isomer of the cobalt(III) complexes with two or three L-cysteine like aliphatic aminothiolate ligands.The first partial report of the crystal structure of the trans(S)- type cobalt(III) thiolato complex [Co(aet)2(en)]1 1 has been published as a preliminary communication.13 We report here the correct crystal structure of trans(S)-[Co(aet-N,S)2(en)]ClO4, and complete descriptions of the syntheses, separation of the diastereomers, structural characterisation and specific properties of trans(S)-[Co(aminothiolate-N,S)2(en)]-type complexes. The detailed investigations of these complexes will contribute significantly not only to our understanding of the mononuclear thiolato complexes but also to those of S-bridged di- and tri-nuclear complexes.Experimental Synthesis trans(S)- 1 and cis(S)-[Co(aet-N,S)2(en)]1 2. A solution of 2-aminoethanethiol hydrochloride (5.68 g, 50 mmol) in 20 cm32152 J. Chem. Soc., Dalton Trans., 1999, 2151–2157 of water was added to a solution of trans-[CoCl2(en)2]Cl14 (7.27 g, 25 mmol) in 50 cm3 of water.The mixed solution was adjusted to pH 8.5 and stirred at 25 8C for 1 h, whereupon it immediately turned from green to reddish brown. The reaction mixture was poured onto an SP-Sephadex C-25 column and separated into three bands, brownish violet (A-1), yellow (A-2) and greenish brown (A-3), in this elution order, by development with a 0.2 mol dm23 KCl aqueous solution. The formation ratio A-1 :A-2:A-3 was about 4:1:2. It was confirmed on the basis of the UV/Vis spectral data that the A-2 and A-3 bands contained [Co(aet)(en)2]2115 and trinuclear complex [Co3(aet-N,S)6]31,9 respectively.The A-1 band containing the desired complexes was circulated several times with the same eluent. It was separated into two bands, violet (1) and brown (2). The early violet eluate (1) was concentrated to a small volume with a rotary evaporator below 25 8C, and a large amount of methanol added to the concentrated solution to eliminate KCl. After the filtrate had again been concentrated to a small volume, a large amount of methanol was added to the concentrated solution to deposit a violet complex.The complex obtained as its chloride salt was dissolved in water and the resulting solution passed through a QAE-Sephadex column (ClO4 2 form) by elution with water in order to replace the Cl2 ion by ClO4 2. The eluate was concentrated to a small volume and allowed to stand in a refrigerator for a week. The resulting violet crystals were collected by filtration and dried in a silica gel desiccator.Yield: 0.2 g {Found: C, 19.57; H, 5.20; N, 15.14. Calc. for trans(S)-[Co(aet-N,S)2(en)]ClO4, C6H20ClCoN4O4S2: C, 19.44; H, 5.44; N, 15.11%}. NMR(13C, D2O): d 54.51 (CH2S), 47.63 (CH2NH2 of (en) and 30.15 (CH2NH2 of (aet). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 548 (230), 448 (sh) (160), 324 (23000), 244 (sh) (2800) and 216 (sh) (13000). The late brown eluate (2) was treated with the same procedure. The separation of the geometrical isomer of 2 was attempted by the same column chromatographic method as that used for 1.However, it was not successful because of the formation of [Co3(aet-N,S)6]319 during the elution. The 13C NMR spectral measurements of the brown eluate 2 indicated that the complex contained two isomers (C1-cis(S) and C2-cis(S) were distinguished by the peak intensities). Yield: 0.9 g {Found: C, 19.33; H, 5.23; N, 14.92. Calc. for cis(S)-[Co(aet-N,S)2(en)]- ClO4, C6H20ClCoN4O4S2: C, 19.44; H, 5.44; N, 15.11%}.NMR(13C, D2O): d 52.85, 51.48 (CH2S), 48.38, 46.09 (CH2NH2 of en), 33.11, 30.73 (CH2NH2 of aet) for C1 isomer and 52.94 (CH2S), 47.61 (CH2NH2 of en) and 30.61 (CH2NH2 of aet) for C2 isomer. UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 574 (190), 444 (sh) (310), 380 (sh) (750), 334 (sh) (2800), 284 (15500), 266 (sh) (15000) and 216 (sh) (11000). trans(S)- 3 and cis(S)-[Co(L-cys-N,S)2(en)]2 4. These complexes were prepared by a method similar to that used for the aet complex, using 3.02 g (25 mmol) of L-cysteine. The reaction was carried out at 25 8C for 20 min.The reaction mixture was poured onto a QAE-Sephadex A-25 column and then separated into two bands, brownish violet (B-1) and greenish brown (B-2), in this elution order, by development with a 0.2 mol dm23 KCl aqueous solution. The formation ratio B-1 : B-2 was about 1 : 3. It was confirmed on the basis of the UV/Vis spectral data that the B-2 band contained trinuclear complexes, DLLLDLLL-, DLLLLLLL- and LLLLLLLL-[Co3(L-cys-N,S)6]32.2 The B-1 band containing the desired complexes was circulated several times.It was separated into two bands, violet (3) and brown (4). The resolution of the diastereomers of 3 was carried out by the column chromatographic method. As the band of 3 was circulated more than two times, it was separated into two bands, brownish violet (1)555 CD-3ÀLL, which showed a positive CD sign at 555 nm, and violet (2)555 CD-3ƒLL, which showed a negative one, in this order.The formation ratio of (1)555 CD-3ÀLL and (2)555 CD-3ƒLL was 27 : 73. The early eluate ((1)555 CD-3ÀLL) was concentrated to a small volume with a rotary evaporator below 25 8C, and a large amount of methanol added to the concentrated solution to eliminate KCl. After the filtrate was again concentrated to a small volume, a large amount of methanol was added. The brownish violet complex was deposited from the solution and collected by filtration. The complex obtained as its chloride salt was dissolved in water and the resulting solution passed through a SP-Sephadex C-25 column (Cs1 form) by elution with water in order to replace the K1 ion by Cs1.The eluate was concentrated to a small volume and methanol–ethanol (1 : 1) added till crystals deposited. The resulting brownish violet complex was collected by filtration and dried in a silica gel desiccator. The late eluate ((2)555 CD-3ƒLL) was treated in the same manner except that the complex was obtained as its potassium salt.Yield: 0.1 (1)555 CD-3ÀLL and 0.4 g (2)555 CD-3ƒLL. Found for (1)555 CD-Cs3ÀLL: C, 18.81; H, 4.81; N, 10.77. Calc. for (1)555 CD-LLL-trans(S)-Cs[Co(L-cys-N,S)2(en)], C8H18CoCsN4O4- S2?2.5H2O: C, 17.95; H, 4.33; N, 10.47%. NMR(13C, D2O): d 179.67 (COO), 69.00 (CH2S), 47.46 (CH2NH2 of en) and 34.26 (CH2NH2 of L-cys). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 543 (190), 446 (sh) (230) and 326 (21900). CD (H2O): l/nm (De/dm3 mol21 cm21) 595 (11.50), 515 (11.85), 440 (sh) (10.5), 324 (221.20) and 214 (118.20).Found for (2)555 CDK3ƒLL: C, 21.72; H, 5.41; N, 12.55. Calc. for (2)555 CD-ƒLL-trans(S)- K[Co(L-cys-N,S)2(en)], C8H18CoKN4O4S2?3H2O: C, 21.33; H, 5.37; N, 12.44%. NMR(13C, D2O): d 69.28 (CH2S), 47.79 (CH2NH2 of en) and 34.05 (CH2NH2 of L-cys). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 543 (190), 446 (sh) (190) and 326 (20000). CD (H2O): l/nm (De/dm3 mol21 cm21) 555 (22.50), 377 (12.60), 307 (11.80), 260 (sh) (12.0) and 234 (16.00).The late brown eluate (4) by treated with the same procedure as that used for the early eluate. However, the desired complex could not be isolated as crystals because it was thermally unstable and readily isomerised to the fac-[Co(L-cys-N,S)3]32 and polynuclear complexes during the repeated elution. Thus, the brown eluate (4) was used intact for measurements of the UV/Vis, CD and 13C NMR spectra and the plasma emission spectral analysis.trans(S)- 5 and cis(S)-[Co(D-pen-N,S)2(en)]2 6. These complexes were prepared by a method similar to that used for the L-cys complex, using 3.78 g (25 mmol) of D-penicillamine. The reaction was carried out at 25 8C for 2 h. On QAE-Sephadex the mixture was separated into three bands, brownish violet (C-1), green (C-2) and brownish green (C-3), in this elution order, by development with a 0.2 mol dm23 KCl aqueous solution. The formation ratio C-1 :C-2: C-3, was about 4:1:1.It was con- firmed on the basis of the UV/Vis spectral data that the C-2 and C-3 bands contained mono- and tri-nuclear complexes, [Co(Dpen- N,S)3]32 and [Co3(D-pen-N,S)6]32, respectively.3 The C-1 band containing the desired complexes was circulated several times. It was separated into three bands, violet (5), brown and dark brown (6). The second brown eluate showed an identical UV/Vis spectrum to that of trans(N)-[Co(D-pen-N,O,S)2]2.6 The resolution of the diastereomers of 5 was carried out by the column chromatographic method.The band containing 5 was separated into two bands, violet (2)535 CD-5ƒDD and brownish violet (1)535 CD-3ÀDD, in this order, by circulation more than two times. The formation ratio of (2)535 CD-5ƒDD and (1)535 CD-5ÀDD was 20 : 80. The cesium salts of those complexes were obtained by the same method as that used for the L-cys complex. Yield: 0.2 g (2)535 CD- 5ƒDD and 1.1 g (1)535 CD-5ÀDD. Found for (2)535 CD-Cs5ƒDD: C, 24.41; H, 5.62; N, 9.32.Calc. for (2)535 CD-DDD-trans(S)-Cs[Co(D-pen- N,S)2(en)], C12H26CoCsN4O4S2?3H2O: C, 24.01; H, 5.37; N, 9.33%. NMR(13C, D2O): d 177.76 (COO), 76.12 (CH), 50.31 (CS), 48.5 (CH2 of en), 34.25, 31.53 (CH3). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 552(220), 392 (sh) (1600), 327 (22400) and 248 (sh) (6300). CD (H2O): l/nm (De/dm3 mol21 cm21) 535 (26.22), 420 (sh) (11.3), 322 (129.83), 270 (sh) (12.5), 240 (sh)J. Chem. Soc., Dalton Trans., 1999, 2151–2157 2153 (29.1) and 212 (225.22).Found for (1)535 CD-Cs5ÀDD: C, 23.63; H, 5.82; N, 9.03. Calc. for (1)535 CD-LDD-trans(S)-Cs[Co(D-pen- N,S)2(en)], C12H26CoCsN4O4S2?4H2O: C, 23.31; H, 5.54; N, 9.06%. NMR(13C, D2O): d 177.85 (COO), 77.89 (CH), 50.18 (CS), 47.7 (CH2 of en), 34.30, 31.93 (CH3). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 550 (280), 423 (sh) (320) and 326 (21400), 244 (sh) (7900). CD (H2O): l/nm (De/dm3 mol21 cm21) 570 (12.69), 540 (sh) (12.3), 470 (sh) (20.5), 378 (22.55), 314 (23.79) and 236 (220.31).The late eluate containing complex 6 was concentrated to a small volume with a rotary evaporator below 20 8C. After the deposited KCl had been filtered oV, a small amount of ethanol and a large amount of acetone were added to the filtrate in an ice-bath. The resulting dark brown complex was collected by filtration, washed with acetone and diethyl ether, and then dried in a vacuum dessicator. The separation of the geometrical isomer of 6 was attempted by the same column chromatographic method as that used for 5.However, it was not successful because of the isomerisation to trans(N)-[Co(D-pen-N,O,S)2]26 during the repeated elution. The 1H and 13C NMR spectral measurements of the eluate (6) indicated that 6 contained two isomers (LDD-C1 and LDD-C2) of the four possible (LDD-C1, DDD-C1, LDD-C2 and DDD-C2). Yield: 0.7 g. Found for K6: C, 28.57; H, 6.37; N, 10.43. Calc. for cis(S)-K[Co(D-pen-N,S)2- (en)], C12H26CoKN4O4S2?3H2O?0.2C4H10O?0.2KCl: C, 28.53; H, 6.39; N, 10.44%.NMR(13C, D2O): d 179.04, 178.11 (COO), 76.59, 75.14 (CH), 52.51, 49.12 (CS), 48.78, 46.38 (CH2 of en), 33.61, 33.58, 31.22, 30.71 (CH3) for LDD-C1 isomer and 177.78 (COO), 77.65 (CH), 50.23 (CS), 47.72 (CH2 of en), 34.31, 31.98 (CH3) for LDD-C2 isomer. Isomerisation and polynuclearisation reactions of trans(S)-type complexes in the solution Each solution of trans(S)-[Co(aminothiolate-N,S)2(en)]1 or 2 (aminothiolate-N,S = aet, L-cys or D-pen) was allowed to stand at 22 or 50 8C under a nitrogen atmosphere, and its change with time was monitored by UV/Vis and CD spectral measurements.After 2 weeks the solution was diluted 10 times with deoxygenated water and poured onto a QAE-Sephadex A-25 (Cl2 form, 4 × 100 cm) or an SP-Sephadex C-25 (K1 form, 4 × 100 cm) column. The adsorbed band was eluted with a degassed 0.05 mol dm23 KCl aqueous solution. The chromatographic behaviours and UV/Vis and CD spectral data of the resulting eluates corresponded to those of the eluates obtained in the preparation of the complexes.Namely, four bands for the aet complex, trans(S)-1, cis(S)-2, [Co(aet)(en)2]21 and [Co3(aet- N,S)6]31, and five bands for the D-pen complex, trans(S)-5, cis(S)-6, [Co(D-pen-N,O,S)2]2, LDDD-[Co(D-pen-N,S)3]32 and LDDDDDDD-[Co3(D-pen-N,S)6]32, were obtained. On column chromatography of the L-cys complex, especially, besides the three eluates trans(S)-3, cis(S)-4 and LLLLLLLL- and LLLLDLLL- [Co3(L-cys-N,S)6]32 obtained in the preparation of the complexes, a new brown band appeared between the B-1 and B-2 bands.The UV/Vis, CD and 13C NMR spectral behaviours indicated that the brown band contained a dinuclear complex, LLLL-[Co{Co(L-cys-N,S)3}(L-cys-N,O,S)]22 7. The eluate from the brown band was concentrated to a small volume and filtered to remove KCl. The brown complex 7 was obtained on addition of acetone to the filtrate.Yield: 0.3 g. Found for K27: C, 19.53; H, 4.18; N, 7.43. Calc. for K2[Co{Co(L-cys-N,S)3}(L-cys- N,O,S)], C12H20Co2K2N4O8S4?5H2O?0.2C3H6O: C, 19.55; H, 4.06; N, 7.24%. NMR(13C, D2O): d 186.27, 179.7–179.4 (COO), 66.90, 64.53, 63.54, 63.31 (CH), 40.04, 37.88, 37.79, 30.46 (CH2S). UV/Vis (H2O): l/nm (e/dm3 mol21 cm21) 617 (sh) (630), 495 (sh) (2500), 448 (4200), 348 (sh) (10000), 316 (14100) and 262 (19500). CD (H2O): l/nm (De/dm3 mol21 cm21) 642 (14.35), 552 (211.36), 482 (15.70), 460 (sh) (14.7), 398 (15.55), 356 (19.91), 320 (sh) (20.5), 268 (229.86) and 232 (217.96).For the L-cys complex, the concentration of each eluate obtained from the chromatographic separation was determined by plasma emission spectral analysis. The amounts of the four isomers, trans(S)-3, cis(S)-[Co(L-cys-N,S)2(en)]2 4, [Co{Co(Lcys- N,S)3}(L-cys-N,O,S)]22 7 and [Co3(L-cys-N,S)6]32 in the eluate were estimated from curve analyses of the UV/Vis spectra. The concentration of each component in solution was determined at given times by calculation using a NEC PC-9801 VM computer using a least-squares linear method.In the curve analyses the spectral data at 81 points in the region of 400–240 nm (intervals of 2 nm) were used. Measurements The UV/Vis spectra of the complexes were recorded on a JASCO UVIDEC-610C or 670 spectrophotometer and the CD spectra on a JASCO J-600 or 720 spectropolarimeter. All the measurements were carried out in aqueous solutions at room temperature. The elemental analyses were performed by the Analysis Centre of the University of Tsukuba.The concentrations of cobalt in the complexes were determined by plasma emission spectral analysis with a Nippon Jarrel-Ash ICAP-575 ICP spectrophotometer. The 1H and 13C NMR spectra were recorded on a Bruker AM-500 spectrometer at the probe temperature in D2O. Sodium 4,4-dimethyl-4-silapentane-1- sulfonate (DSS) was used as an internal reference.Crystal structure determination Single-crystal X-ray diVraction experiments were performed on an Enraf-Nonius CAD4 diVractometer with graphitemonochromatized Mo-Ka radiation (l = 0.71073 Å). Crystallographic data for trans(S)-[Co(aet)2(en)]1 1 are summarised in Table 1. Unit cell parameters for a single crystal (0.38 × 0.40 × 0.45 mm) of 1 were determined by a least-squares refinement of 25 reflections in the range of 20 < 2q < 22. The structure was solved by direct methods and refined by full-matrix least-squares treatment on F.The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions and refined as riding atoms (C–H = N–H 0.95 Å and B = 1.20 × value of riding atom). All the calculations were performed using the TEXSAN crystallographic software package.16 The largest parameter shifts were 0.02 times e.s.d. and |Dr|max in the final Fourier-diVerence maps were 0.3822, and 15 e Å23. CCDC reference number 186/1447.See http://www.rsc.org/suppdata/dt/1999/2151/ for crystallographic files in .cif format. Results and discussion Molecular structure Although we first considered the possibility of a monoclinic symmetry for complex 1 in the preliminary communication, Table 1 Crystallographic data for trans(S)-[Co(aet)2(en)]ClO4 1 Chemical formula M Crystal colour Crystal system Space group a/Å b/Å c/Å V/Å3 ZF (000) Dc/g cm23 m(Mo-Ka)/cm21 Reflections collected Observed reflections [I > 3sI ] RR9 C6H20ClCoN4O4S2 370.75 Purple Orthorhombic Fdd2 (no. 43) 10.5077(6) 17.108(1) 15.7022(6) 2822.7(2) 8 768 1.744 8.56 4312 3878 0.040 0.0642154 J. Chem. Soc., Dalton Trans., 1999, 2151–2157 further refinement showed that the orthorhombic space group Fdd2 fits much better to all observed reflections. A perspective drawing of the complex cation is given in Fig. 1, together with a numbering scheme. Selected bond distances and angles are listed in Table 2.The present complex adopts a six-co-ordinate structure and the co-ordination geometry around the Co atom is approximately octahedral CoN4S2. Two thiolato sulfur atoms of the aet ligands occupy trans positions to each other. Therefore, 1 is assigned to trans(S)-[Co(aet)2(en)]1. The Co–S1 distances (2.2871(4) Å) are longer than the Co–S distances of other complexes containing a co-ordinating aliphatic thiolate or thioether sulfur atom, such as 2.226 Å in [Co(aet)(en)2]21,15 2.267(10)Å in [Co(CH3S(CH2)2NH2)(en)2]31,17 2.239(1)Å in trans-[Co{CH3SCH(CH3)CO2}(tren)]21 (tren = tris(2-aminoethyl) amine),18 and 2.232(1)–2.244(2) Å in trans-[Co(ma or mta)(tren)]21 (ma = 2-sulfanyl-acetate and mta = 2-(methylsulfanyl) acetate).7,9 This indicates that the Co–S bonds are lengthened by the double sulfur trans influence due to two aliphatic thiolato-type sulfur donor atoms.As the other striking structural characters are almost the same as described previously, further detailed descriptions are omitted here.Structural assignment and properties Three geometrical isomers, trans(S), C2-cis(S) and C1-cis(S), are possible for [Co(aminothiolate-N,S)2(en)]-type complexes. The 13C NMR spectrum of 1 exhibits three peaks [d 54.51, 30.15 (aet) and 47.63 (en)] due to methylene carbons of each ligand. This indicates that 1 has a C2 symmetry. The UV/Vis spectral behaviour resembled those of other trans(S)-[Co(N)2(S)2(en)]- type complexes with pymt or endet ligands,10,11 giving a sharp d–d absorption band at (17–18) × 103 cm21 and a characteristic sulfur-to-cobalt charge transfer (SCCT) band at 31 × 103 cm21 (Fig. 2). These spectral behaviours provide useful information for structural assignment of the other trans(S)-type complexes. The UV/Vis spectra of 3 and 5 also show the d–d transition and intense SCCT bands in the same region as 1 does. The 13C NMR spectra revealed that 3 and 5 have C2 symmetry, because (2)555 CD-3ƒLL and (1)555 CD-3ÀLL exhibited four resonance lines due to the eight carbons, and (2)535 CD-5ƒDD and (1)535 CD-5ÀDD exhibited six Fig. 1 Molecular structure of trans(S)-[Co(aet)2(en)]ClO4 1. Hydrogen atoms have been omitted for clarity. Table 2 Selected bond distances (Å) and angles (8) for trans(S)- [Co(aet)2(en)]ClO4 1 Co–S(1) Co–N(1) Co–N(2) S(1)–C(1) S(1)–Co–S(1*) S(1)–Co–N(1) S(1)–Co–N(2) S(1)–Co–N(1*) S(1)–Co–N(2*) N(1)–Co–N(2) 2.2871(4) 1.975(2) 1.979(2) 1.817(2) 175.09(3) 90.77(4) 90.37(5) 85.81(4) 93.25(5) 91.73(7) N(2)–C(3) C(3)–C(3*) N(1)–C(2) C(1)–C(2) N(1)–Co–N(2*) N(2)–Co–N(2*) Co–N(1)–C(2) N(1)–Co–N(1*) Co–S(1)–C(1) Co–N(2)–C(3) 1.484(2) 1.509(4) 1.492(2) 1.508(3) 174.82(7) 84.98(10) 114.9(1) 91.8(1) 98.70(6) 109.3(1) resonance lines due to the twelve carbons.Therefore, these four complexes are assignable to trans(S)-[Co(aminothiolate-N,S)2- (en)]-type (aminothiolate = L-cys or D-pen) complexes. Two diastereomers, DLL (DDD) and LLL (LDD), are possible for each of 3 and 5.The CD spectrum of (2)535 CD-5ƒDD exhibits a negative band in the first absorption band region, while that of the (1)535 CD-5ÀDD has a positive band in that region (Fig. 2). Accordingly, (2)535 CD-5ƒDD and (1)535 CD-5ÀDD are assigned to (2)535 CD-DDDand (1)535 CD-LDD-trans(S)-[Co(D-pen)2(en)]2, respectively. In a similar manner, (1)555 CD-3ÀLL and (2)555 CD-3ƒLL are assigned to (1)555 CD-LLL- and (2)555 CD-DLL-trans(S)-[Co(L-cys)2(en)]2, respectively.In the first absorption band region of (2)555 CD-3ƒLL, two CD bands of the same 1 sign appeared. This agrees well with the pattern predicted from the CD spectra of the trans(N)- [Co(L-aminocarboxylate)2(ox)]2 and trans(O)-[Co(L-aminocarboxylato) 2(en)]1 complexes.20 On the contrary, (1)535 CD-5ÀLL shows only one CD band in the first absorption band region. This diVerence may be related to the UV/Vis spectral behaviour, which does not have the splitting in the first absorption band region, and attributed to the peculiar transition of the thiolate sulfur donor atom.The resonance peaks due to the methine and methylene carbons in isomers (2)555 CD-3ƒLL and (1)555 CD-3ÀLL showed almost the same chemical shift patterns. For the en moiety the resonance peaks due to the methylene carbons in (2)555 CD-3ƒLL and (1)555 CD- 3ÀLL have almost the same chemical shifts as those in 1. These facts suggest that (2)555 CD-3ƒLL and (1)555 CD-3ÀLL do not form any hydrogen bond in the solution.Thus the diVerence in the amounts between (2)555 CD-3ƒLL and (1)555 CD-3ÀLL (73 : 27) reflects the favourable equatorial orientation of the carboxylate and the steric repulsion between the methine and methylene groups on the L-cys ligands. The resonance peaks due to the methylene carbons in the en ligand in (2)535 CD-5ƒDD are shifted to lower magnetic field (ca. 1 ppm) compared to those in (1)535 CD-5ÀDD and the resonance peaks due to the methine carbons in (2)535 CD-5ƒDD are shifted to higher magnetic field (ca. 2 ppm) compared to those in (1)535 CD-5ÀDD. Similar trends are observed for the di- and trinuclear complexes [Co{Co(aminothiolate-N,S)3}(D-pen-N,O,S or dien)]0 or 22 and [Co3(aminothiolate-N,S)6]32 (aminothiolate- N,S = L-cys or D-pen).2,3 It is reasonable to consider that these shifts are aVected by the formation of the intramolecular hydrogen bond COO ? ? ? H–N–H. Molecular models of these Fig. 2 The UV/Vis and CD spectra of trans(S)-[Co(aet)2(en)]1 1 (——), DDD-trans(S)-[Co(D-pen-N,S)2(en)]2 5ƒDD (—-—) and LDDtrans( S)-[Co(D-pen-N,S)2(en)]2 5ÀDD (- - - - -).J.Chem. Soc., Dalton Trans., 1999, 2151–2157 2155 complexes suggest the formation of hydrogen bonds due to steric repulsion between the methyl protons on the D-pen ligands and the amine protons on the D-pen or en ligand. The formation ratio of (1)535 CD-5ÀDD : (2)535 CD-5ƒDD (80 : 20) slightly increased compared with (2)555 CD-3ƒLL : (1)555 CD-3ÀLL (73: 27) because of such structural stabilisation.On the other hand, the UV/Vis spectral pattern of complex 6 is quite similar to that of other cis(S)-[Co(N)2(S)2(en)]-type complexes, giving a characteristic broad SCCT band at 34 × 103 cm21.10 Further, the 13C NMR spectrum exhibits twelve intense resonance lines (d 30.71, 31.22, 33.58, 33.61, 46.38, 48.78, 49.12, 52.51, 75.14, 76.59, 178.11 and 179.04) and six weak ones (d 31.98, 34.31, 47.72, 50.23, 77.65 and 177.78). This suggests that 6 is a mixture of the C1-cis(S) (major) and C2-cis(S) (minor) isomers.The cis(S) isomers seem to take preferentially the absolute configuration LDD or DDD. The absolute configurations of the C1- and C2-cis(S) isomers containing 6 are more favourable to LDD configurations than DDD ones because of similar steric repulsion attributable to the methyl group in 5 and the repulsion between the unshared electron pairs on the sulfur donor atoms located in cis positions.3 The CD spectrum of 6 shows a relatively intense positive peak in the first absorption band region, suggesting that the configurations of the cis(S) isomers containing 6 are assignable to LDD.The C1-cis(S) isomer was formed in larger amount than the C2-cis(S) one. This fact may be explained as follows: in the C2-cis(S) isomer, the two sulfur donor atoms occupy trans positions to the two nitrogen atoms of the en ligand, so that the en ligand seems to become labilised by the structural trans eVect due to the sulfur donor atoms.This interpretation is consistent with the fact that cobalt(III) complexes with thiolate ligands prefer to the cis(S) geometry. The UV/Vis spectra of 2 and 4 are quite similar to that of cis(S)- [Co(D-pen-N,S)2(en)]2 6 over the whole region. They exhibit a broad SCCT band at 35 × 103 cm21 characteristic of cis(S)- [Co(N)2(S)2(en)]-type complexes.10 This suggests that 2 and 4 also adopt the cis(S) geometries of [Co(aet-N,S)2(en)]1 and [Co(L-cys-N,S)2(en)]2, respectively, contain two geometrical isomers, C1- and C2-cis(S), and each of the geometrical isomers in 4 consists of two diastereomers, LLL and DLL.Actually, the 13C NMR spectral measurements indicated that 2 and 4 contained two and four possible isomers, respectively. However, the desired complex could not be separated because it was thermally unstable and readily isomerised to the fac-[Co(L-cys- N,S)3]32 and polynuclear complexes during repeated elution.The UV/Vis spectrum of brown complex 7 shows d–d transition bands in the region (16–24) × 103 cm21 and intense bands due to the SCCT transition at ca. 38 × 103 cm21. Such spectral behaviour is quite similar to those of [Co{Co(aminothiolate- N,S)3}(D-pen-N,O,S or dien)]-type (aminothiolate = aet, L-cys or D-pen) dinuclear complexes.3 The 13C NMR shift pattern of 7 also resembles that of the latter complexes. Four resonance peaks due to the carboxylate carbon atoms of the L-cys ligands were observed in the d 186–179 region.The carbon resonance peaks appeared at d 186.27 and 179.7–179.4 for the co-ordinated and unco-ordinated carboxyl groups, respectively. A single resonance peak due to the methylene carbon atom is located relatively upfield (d 30.46) compared with those of the corresponding carbon atoms in the [Co(aminothiolate-N,S)3] moiety (d 40.04–37.79). These results suggest that three sulfur atoms in the L-cys ligands bridge two cobalt(III) ions, but a sulfur atom in one tridentate L-cys-N,O,S does not.The results from UV/Vis and NMR spectral measurements indicate that 7 is the dinuclear complex [Co{Co(L-cys-N,S)3}(L-cys-N,O,S)]22, having C1 symmetry. The CD spectral behaviour of 7 is similar to those of LLLL-[Co{Co(L-cys-N,S)3}(D-pen-N,O,S or dien)]22 or 0 and L-[Co{Co(aet-N,S)3}(D-pen-N,O,S)]1.3 The IR spectral measurement also supports this. Accordingly, it is suggested that 7 adopts a LLLL configuration.This complex containing L-cys- N,O,S as a tridentate ligand was not obtainable by the usual preparative method for the dinuclear complexes. Structural change of trans(S)-type complexes Fig. 3 shows the UV/Vis spectral changes of the trans(S) isomers in 0.1 mol dm23 KCl solution at 50 8C. The UV/Vis and CD spectra became approximately constant after 2 weeks. During this time course, clear-cut isosbestic and isodichroic points were not observed.The change in the absorption curve, in which the maximum of the SCCT band shifted to higher energy (from 327–324 to 291–284 nm), suggests that the cis(S) type isomers were formed. Moreover, Fig. 3 (a) and (b) show that the absorption intensities in the d–d transition region increased with time, accompanied by the production of di- and trinuclear complexes. These imply that the trans(S) isomer in the solution changes to several species, which were confirmed by the spectral data of the eluate from the column chromatographic separation [four eluates showing four diVerent spectra for aet and L-cys complexes and five eluates with five diVerent spectra for the D-pen complex] as described in the Experimental section.The observed and calculated curves of trans(S)-[Co(L-cys- N,S)2(en)]2 3 at 22 8C after 2 weeks were in good agreement. The calculated concentration–time dependencies are plotted in Fig. 4. The starting trans(S) isomer decreased during the monitored timescale, while the cis(S) isomer increased in concentration for the initial period and then (after ca. 250 h) the dinuclear and trinuclear complexes were formed. With time (after ca. 800 h) Fig. 3 The UV/Vis spectral changes of the complexes in 0.1 mol dm23 KCl solution at 50 8C measured at 12 or 6 h intervals: (a) trans(S)- [Co(aet)2(en)]1 1, (b) DLL-trans(S)-[Co(L-cys-N,S)2(en)]2 3ƒLL and (c) LDD-trans(S)-[Co(D-pen-N,S)2(en)]2 5ÀDD.2156 J. Chem. Soc., Dalton Trans., 1999, 2151–2157 the cis(S) isomer began to decrease and simultaneously the dinuclear and trinuclear complexes increased.The dinuclear complexes, LLLL-[Co{Co(L-cys-N,S)3}(L-cys-N,O,S)]22 7, finally became major species. In this case, the dinuclear complex 7 was selectively formed with only the LLLL configuration and the trinuclear complexes were formed with LLLLLLLL and LLLLDLLL (ca. 1 : 1) configurations; the reason for these selectivities is not clear at present.The isomerisation and polynuclearisation reactions of 3 are explained in Scheme 1. In the initial stage of the reaction (i), (ii) mononuclear complexes are formed. Path (i) is an equilibrium reaction because a small amount of trans(S) isomer was also obtained from the isomerisation reaction of the cis(S) one. The tris(L-cysteinato)cobalt(III) complex, fac-[Co- (L-cys-N,S)3]32, produced in path (ii) has high nucleophilicity, so in the final stage of the reaction the S-bridged di- and trinuclear complexes are mainly formed (iii), (iv).That is a reason why the pure trans(S) and cis(S) isomers were not isolated up to now. This is supported by a previous report that the S-bridged trinuclear complex [Co3(L-cys-N,S)6]32 was easily formed from fac-[Co(L-cys-N,S)3]32.2 Path (v) (the change between the di- and tri-nuclear complexes) did not occur under the present conditions. For the trans(S)-aet complex 1 a similar spectral change and column chromatographic separation were observed except that only trinuclear complex was formed in the polynuclearisation stage (of course the dinuclear complex is not formed because the aet ligand functions only as a didentate-N,S lig- Fig. 4 The changes of the isomer proportions in the isomerisation and polynuclearisation reactions of trans(S)-[Co(L-cys-N,S)2(en)]2 3 (0.05 mol dm23 KCl, 22 8C): trans(S)-[Co(L-cys-N,S)2(en)]2 (s), cis(S)-[Co- (L-cys-N,S)2(en)]2 (n), [Co{Co(L-cys-N,S)3}(L-cys-N,O,S)]22 (h), [Co3(L-cys-N,S)6]32 (d).and). In the isomerisation and polynuclearisation of trans(S)-Dpen complex 5, although the SCCT band shifts to higher energy, a drastic increase of the absorption intensities in the d–d transition region was not observed. These results suggest that the S-bridged di- and tri-nuclear complexes are diYcult to form, since the six methyl groups attached to the neighbouring carbon atoms of the co-ordinated sulfur atoms in the [Co- (D-pen-N,S)3] moiety are sterically bulky.Therefore, the mononuclear trans(N)-[Co(D-pen-N,O,S)2]2 complex, which has the minimum steric hindrance of all products, is produced as major species, and small amounts of LDDD-fac-[Co(D-pen-N,S)3]32 and LDDDDDDD-[Co3(D-pen-N,S)6]323 are formed in the final stage of this reaction. The result of the column chromatographic separation also supports this. Consequently, the isomerisation reactions of trans(S)-[Co(D-pen-N,S)2(en)]2 proceed through the paths shown in Scheme 2.UV/Vis Spectral behaviours of [Co(S)n(N)6-n]-type (n 5 1, 2 or 3) complexes Fig. 5 shows the UV/Vis spectra of mononuclear complexes, [Co(D-pen-N,S)(en)2]1,8 trans(S)-[Co(D-pen-N,S)2(en)]2 3, cis(S)-[Co(D-pen-N,S)2(en)]2 4 and fac-[Co(D-pen-N,S)3]32,3 which are of [Co(S)n(N)6-n] type (n = 1, 2 or 3). As the UV/Vis spectral patterns of the L-cys and D-pen complexes, trans(S)- or cis(S)-[Co(L-cys- or D-pen-N,S)2(en)]2 3–6, are also similar to one another, these can be compared with the same trans(S)- or cis(S)-[Co(N)4(S)2] system.It is known that the first absorption band (1A1g æÆ 1T1g transition of the complex for the Oh symmetry) splits clearly into two components for the trans(O)-[Co(N)4(O)2]- or trans(N)-[Co(N)2(O)4]-type complexes. This is the so-called Yamatera semiempirical rule which states that the position and shape of the d–d absorption band can be predicted from semiempirical calculation on the basis of molecular orbital theory.2 According to this treatment, the complexes adopting the same geometry are expected to show similar UV/Vis spectral behaviour.However, the present sulfur-atom containing trans(S)-[Co(D-pen- or L-cys-N,S)2(en)]2 complexes indicate a deviation from the rule, that is no clearly explicit splitting is observed for those complexes. The reason is that the splitting is so small because the ligand field strengths of nitrogen and sulfur are similar, and further it has not been clarified how the ligand field strength of thiolato sulfur compares with those of nitrogen and oxygen (N > S > O or S > N > O).3–10 Consequently, the geometrical structures of the complexes with thio- Scheme 1 Co N N N N S S COO– COO– Co S N N N N S COO– COO– Co S S N N N S COO– COO– –OOC Co S S S Co N N N –OOC COO– COO– Co S S S N N N COO– –OOC –OOC Co S N N N O S S N S Co COO– COO– COO– O trans(S) cis(S) fac(S) trinuclear complex (iii) (i) (iv) (v) dinuclear complex (ii)J.Chem. Soc., Dalton Trans., 1999, 2151–2157 2157 late sulfur-containing ligands cannot be determined from only the splitting patterns in their d–d absorption band region. The position and pattern of the SCCT bands commonly reflect the geometries such as cis(S) and trans(S) for cobalt(III) complexes containing two thiolate or thioether sulfur atoms. The cis(S) isomers of thiolato and thioether complexes exhibit intense broad SCCT bands in the region of (32–38) × 103 cm21 which are composed of more than two components.The trans(S) isomers exhibit an intense sharp band at lower energy (ca. 31 × 103 cm21). Of course, the present trans(S)-[Co(aet-, L-cys- or D-pen- N,S)2(en)]2 is also distinguishable from the cis(S) isomer on the basis of the UV/Vis spectral patterns in the SCCT band region (Fig. 5). Further, for the [Co(S)n(N)6-n]-type (n = 1–3) complexes (except the trans(S) isomer), we found that the SCCT band broadens (no explicit splitting), shifts to higher energy, and Fig. 5 The UV/Vis spectra of [Co(S)n(N)6-n]-type (n = 1–3) complexes: [Co(D-pen-N,S)(en)2]1 (——), trans(S)-[Co(D-pen-N,S)2(en)]2 (5) (– – –), cis(S)-[Co(D-pen-N,S)2(en)]2 6 (----) and fac-[Co(D-pen-N,S)3]32 (—-—). Scheme 2 Co N N N N S S –OOC Co S N N N N S –OOC –OOC Co N O S N O S O O –OOC trans(S) trans(N) cis(S) increases in intensity with n (1 to 3). This seems to depend on the number of the sulfur atoms co-ordinating to CoIII. Such spectral behaviours were also observed for all cobalt(III) complexes with the other thiolate ligands.Therefore, these facts suggest that the structures of the [Co(S)n(N)6-n]-type (n = 1–3) complexes are characterised by the position and intensity of the SCCT band in their UV/Vis spectra. Acknowledgements This work was supported by a Grant-in-Aid for Encouragement of Young Scientists (No. 08740522) from the Ministry of Education, Science, Sports and Culture, Japan. References 1 J.J. Mayerle, S. E. Denmark, B. V. DePamphilis, J. A. Ibers and R. H. Holm, J. Am. Chem. Soc., 1975, 97, 1032; R. W. Lane, J. A. Ibers, R. B. Frankel, G. C. Papaefthymiou and R. H. Holm, J. Am. Chem. Soc., 1977, 99, 84; P. de Meester and D. J. Hodgson, J. Am. Chem. Soc., 1977, 99, 101; H. M. Helis, P. de Meester and D. J. Hodgeson, J. Am. Chem. Soc., 1977, 99, 3309; G. J. Gainsford, W. G. Jackson and A. M. Sargeson, J. Am. Chem. 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