DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 135–140 135 Chemistry of ruthenium(II) complexes of the tridentate NNS donor methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone. Isolation and structural characterisation of a novel ruthenium(II) complex containing a co-ordinated imine of an ·-N heterocyclic ketone Milan Maji,a Madhumita Chatterjee,a Saktiprosad Ghosh,*a Shyamal Kumar Chattopadhyay,b Bo-Mu Wu c and Thomas C. W. Makc a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India b Department of Chemistry, Bengal Engineering College (Deemed University), Howrah 711103, India c Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 11th August 1998, Accepted 30th October 1998 A series of ruthenium(II) complexes of the NNS donor ligand methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL) has been synthesized using RuCl3?xH2O and [Ru(PPh3)3Cl2]: [Ru(HL)2][ClO4]2 1, [Ru(L)(PPh3)2Cl] 2, [Ru(HL)- (PPh3)2Cl]Cl 3, [Ru(HL)(PPh3)2Cl]PF6 4, [Ru(L)(PPh3)(bpy)]PF6 5, [Ru(L)(PPh3)(dppe)]PF6 6, [Ru(HL)(PPh3)- (pic)]PF6 7 and [Ru(HL)(PPh3)(mpi)]Cl2 8 [where bpy = 2,29-bipyridine, dppe = 1,2-bis(diphenylphosphino)ethane, Hpic = pyridine-2-carboxylic acid, mpi = methyl(2-pyridyl)methyleneimine].Chemical and electrochemical studies have been carried out. Structures of the compounds 3?CH2Cl2 3 and 8?CH2Cl2?3H2O have been determined by single crystal X-ray diVraction.The thione form of the ligand (HL) is chelated to the ruthenium centre through the pyridine nitrogen, imine nitrogen and the thione sulfur atom. The existence of a new unstable ligand methyl(2-pyridyl)- methyleneimine (mpi) co-ordinated to RuII through the pyridine and imine nitrogen atoms was confirmed from the crystal structure of compound 8. The chemistry of ruthenium bound to nitrogen–sulfur donor ligands has evinced considerable interest in recent years primarily due to their ability to form complexes with unusual stereochemistry,1 uncommon co-ordination number,2,3 interesting electronic structure and bonding situations and with intricate electron-transfer characteristics.4,5 Thiosemicarbazides and thiosemicarbazones constitute an important class of nitrogen– sulfur donor ligands, because of their highly interesting chemical 5–9 and biological properties.10 As a part of our programme to investigate ruthenium complexes of thiosemicarbazides and thiosemicarbazones in general, we undertook the study of ruthenium complexes of methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL).During the course of our investigations we came across two very interesting phenomena. (1) Under certain reaction conditions RuII-catalysed reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone moiety occurred. Such reductive cleavage is rather common in the molybdenum complexes of hydrazine,11 but never observed previously in the metal complexes of thiosemicarbazides and thiosemicarbazones.(2) We isolated and structurally characterised the first metal complex of the imine of a 2(N)-heterocyclic ketone, i.e. a ruthenium(II) complex of methyl(2-pyridyl)- methyleneimine formed during the process mentioned earlier. Most ketone imines (HN]] CR1R2) are unstable at room temperature 12 and are found to react with some metal ions as a monodentate ligand through the imine nitrogen or as an exobidentate ligand bridging two metal ion centres through its deprotonated iminate nitrogen.However, no report on a metal complex containing a co-ordinated imine of a 2(N)- heteroaromatic ketone has appeared previously. This paper reports the results of our studies on several ruthenium complexes involving HL as well as the complex [Ru(HL)(PPh3)- (mpi)]Cl2 containing methyl(2-pyridyl)methyleneimine (mpi). Structures of two complexes, [Ru(HL)(PPh3)2Cl]Cl?CH2Cl2 and [Ru(HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O, are described and discussed.Experimental Materials and instrumentation Elemental analysis were performed with a Perkin-Elmer 240 CHN analyser. Those of complexes 3 and 8 were before crystallisation. The IR and electronic spectra were recorded on a Perkin-Elmer 783 spectrophotometer (as KBr disks) and on a Shimadzu UV-VIS recording spectrophotometer respectively. Solution conductances were measured on a Systronics direct reading conductivity meter (model 304) and magnetic susceptibility (at room temperature) was determined with a PAR vibrating sample magnetometer using Hg[Co(SCN)4] as the calibrant.The NMR spectra were recorded on a Bruker 300 MHz spectrometer using SiMe4 as an internal standard. Electrochemical data were collected with a BAS CV-27 and a BAS model X-Y recorded at 298 K. Cyclic voltammetry experiments were carried out with platinum working and auxiliary electrodes and a Ag–AgCl reference electrode. The compound RuCl3?xH2O was obtained from Arora Matthey (Calcutta, India) and 2-acetylpyridine from Aldrich. 4-(4-Tolyl)thiosemicarbazide 10 and [Ru(PPh3)3Cl2] 13 were prepared according to published procedures. Acetonitrile (pure) obtained from E. Merck (India) was freshly distilled over calcium hydride for electrochemical experiments. Other reagents were used without further purification. Preparations Methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL). 4-(4-Tolyl)thiosemicarbazide (2.42 g, 0.02 mol) was dissolved in ethanol (100 cm3) by heating and 2-acetylpyridine (3.62 g,136 J.Chem. Soc., Dalton Trans., 1999, 135–140 0.02 mol) was added. The mixture was stirred for 45 min. Then acetic acid (2 cm3) was added and stirred again for 3 h. The product was filtered oV, washed with water and diethyl ether and dried. The yield was 90%. 1H NMR (CDCl3, room temperature): d 9.32 (s, 1 H, NH), 8.88 (s, 1 H, NH), 8.76 (d), 8.61 (d), 8.01 (d), 7.87 (t), 7.74 (t), 7.56 (t), 7.37 (t), 7.31 (t), 7.19 (t), 2.49 (s, 3 H, CH3) and 2.36 (s, 3 H, CH3).[Ru(HL)2][ClO4]2 1. CAUTION! perchlorate salts of metal complexes with organic ligands are potentially explosive. Only a small amount of compound should be prepared, and handled with caution. The ligand (HL) (568 g, 0.2 mmol) was suspended in methanol (30 cm3) and RuCl3?xH2O (261 mg, 0.2 mmol) dissolved in methanol (25 cm3) was added drop by drop. The mixture was stirred for 3 h.It was filtered and the filtrate concentrated to about 10 cm3 using a rotary evaporator. An aqueous solution of lithium perchlorate was added to the concentrated solution and the desired compound precipitated. It was washed with water followed by diethyl ether and dried over fused calcium chloride (Found: C, 41.03; H, 3.60; N, 12.90. Calc. for C30H32Cl2N8- O8RuS2: C, 41.47; H, 3.69; N, 12.90%). Conductance in CH3CN (LM): 230 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/ nm(103emax/M21 cm21)]: 650 (2.536), 382 (86.032), 255 (100.9) and 210 (151.58).[Ru(L)(PPh3)2Cl] 2, [Ru(HL)(PPh3)2Cl]Cl 3 and [Ru(HL)- (PPh3)2Cl]PF6 4. The ligand (HL) (71 mg, 0.25 mmol) was dissolved in ethanol (20 cm3) and [Ru(PPh3)3Cl2] (240 mg, 0.25 mmol) added. The mixture was refluxed for 4 h under dry nitrogen, then cooled. The solid product (2) was filtered oV, washed with ether and dried in a calcium chloride desiccator. The filtrate was concentrated in a rotary evaporator to about 10 mL.Compound 4 was isolated by adding saturated aqueous ammonium hexafluorophosphate to the concentrated solution. It was filtered oV, washed thoroughly with water and then with ether and finally dried over fused calcium chloride. Alternatively, the chloride compound 3 was obtained by concentrating the filtrate to about 10 mL and adding ether. The solid separated was filtered oV, washed thoroughly with ether and recrystallised from dichloromethane (Found: C, 64.63; H, 4.78; N, 6.01.Calc. for C51H45ClN4P2RuS 2: C, 64.86; H, 4.77; N, 5.93%). Conductance in CH3CN (LM): 15.4 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 580 (0.9007), 481 (2.256), 398 (9.760), 376 (11.192), 274 (11.870) and 215 (40.85) (Found: C, 62.67; H, 4.83; N, 5.59. Calc. for C51H46Cl2N4P2RuS 3: C, 62.45; H, 4.69; N, 5.71%). Conductance in CH3CN 123.09 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 478 (0.3218), 445 (9.381), 402 (10.67), 378 (12.58) and 210 (73.15). 1H NMR (CDCl3, room temperature): d 12.16 (s, 1 H, NH), 11.12 (s, 1 H, NH), 8.76 (d), 7.41 (m), 7.36 (t), 7.24 (m), 7.06 (t), 6.79 (t), 2.31 (s, 3 H, CH3) and 2.16 (s, 3 H, CH3) (Found: C, 56.27; H, 4.34; N, 5.19.Calc. for C51H46ClF6N4P3RuS 4: C, 56.17; H, 4.22; N, 5.14%). Conductance in CH3CN 153.24 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 478 (0.3712), 442 (11.37), 403 (16.15), 362 (12.37), 265 (35.41) and 212 (52.27).[Ru(L)(PPh3)(bpy)]PF6 5. The complex [Ru(L)(PPh3)2Cl] (119 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). 2,29-Bipyridine (219.5 mg, 0.12 mmol) followed by methanol (25 cm3) was added. The mixture was refluxed for 8 h. After cooling the solution was concentrated in a rotary evaporator to about 10 cm3. Compound 5 was isolated by adding saturated aqueous ammonium hexafluorophosphate to the concentrated solution.The precipitated compound was filtered oV, washed thoroughly with distilled water and dried over fused calcium chloride. It was then washed with ether and dried (Found: C, 54.93; H, 4.29; N, 9.1. Calc. for C43H38F6N6P2RuS: C, 54.49; H, 4.01; N, 8.87%). Conductance in CH3CN 125.21 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm (103emax/M21 cm21)]: 456 (8.01), 376 (15.59), 296 (28.57) and 209 (81.19). [Ru(L)(PPh3)(dppe)]PF6 6. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). 1,2-Bis(diphenylphosphino)ethane (49 mg, 0.12 mmol) was added followed by methanol (25 cm3). The mixture was refluxed for 8 h then concentrated in a rotary evaporator. The compound was isolated by adding an aqueous solution of ammonium hexafluorophosphate. It was filtered oV, washed with water and dried over fused calcium chloride. The dry compound was finally washed with ether and dried (Found: C, 60.13; H, 4.67; N, 4.59. Calc. for C59H54F6N4P4RuS: C, 59.54; H, 4.54; N, 4.71.Conductance in CH3CN 127.10 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 481 (0.820), 424 (5.94), 373 (11.91), 261 (21.75) and 216 (45.96). [Ru(HL)(PPh3)(pic)]PF6 7. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in dichloromethane (20 cm3). Pyridine-2-carboxylic acid (16 mg, 10.12 mmol) was added, followed by methanol (25 cm3). The mixture was refluxed for 8 h (Hpic) then concentrated in a rotary evaporator. Compound 7 was isolated by adding an aqueous solution of ammonium hexafluorophosphate. It was filtered oV, washed thoroughly with water and dried over calcium chloride.The dry compound was washed again with ether and dried (Found: C, 50.8; H, 4.03; N, 4.75. Calc. for C39H34F6N5O2P2RuS: C, 51.26; H, 3.72; N, 7.67%). Conductance in CH3CN 139.30 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/M21 cm21)]: 577 (0.811), 481 (4.82), 376 (21.93), 256 (23.09) and 220 (54.13).[Ru(HL)(PPh3)(mpi)]Cl2 8. The complex [Ru(L)(PPh3)2Cl] (120 mg, 0.12 mmol) was dissolved in ethanol (40 cm3) and a sixfold excess of ligand (HL) (0.72 mmol) added. The mixture was refluxed for 24 h then cooled and filtered. The filtrate was evaporated to dryness. The solid residue was stirred with n-hexane to wash out excess of ligand and triphenylphosphine, filtered oV, dried and recrystallised from a mixture of dichloromethane and n-hexane. The components in the filtrate were separated by column chromatography using neutral silica gel.The first component was triphenylphosphine eluted with light petroleum (bp 60–80 8C), the middle fraction was N-(p-tolyl)thiourea and the last fraction with the ligand (HL) eluted with 10% ethyl acetate in light petroleum (Found: C, 57.21; H, 4.62; N, 10.38. Calc. for C40H39Cl2N6PRuS 8: C, 57.28; H, 4.65; N, 10.02%). Conductance in CH3CN 210.00 W21 cm2 mol21. Electronic spectrum in CH3CN [lmax/nm(103emax/ M21 cm21)]: 464 (3.469), 378 (10.94), 317 (15.65), 260 (20.94) and 216 (45.96). 1H NMR (CDCl3, room temperature): d 12.25 (s, 1 H, NH), 8.14 (d), 8.04 (t), 7.87 (q), 7.6 (m), 7.3 (s), 7.05 (d), 6.66 (s), 2.31 (s, 3 H, CH3) and 2.35 (s, 3 H, CH3) (Found: C, 57.68; H, 5.93; N, 17.03. Calc. for N-(p-tolyl)thiourea (C8H10- N2S): C, 57.83; H, 6.02; N, 16.87%). IR in CHCl3: n(NH) 3400, 3380, n(SH) 2420, n(C–S) 840 cm21. 1H NMR (CDCl3, room temperature): d 8.62 (d, 1 H, Ph), 8.24 (d, 1 H, Ph), 7.77 (t, 1 H, Ph), 7.31 (t, 1 H, Ph) and 3.37 (s, 3 H, CH3).X-Ray crystallography Brown prismatic crystals were grown by the slow diVusion of n-hexane into dichloromethane solution of complexes 3 and 8 at room temperature. Single crystals 0.15 × 0.19 × 0.20 and 0.10 × 0.20 × 0.40 mm were chosen for diVraction study respectively. Crystal data are in Table 1. Intensity data were collected at 294 K on a MSC/Rigaku-IIC imaging plate diVractometer using graphite-monochromatized Mo-Ka (l = 0.71073J.Chem. Soc., Dalton Trans., 1999, 135–140 137 Å) radiation from a rotating anode generator. For 3 a total of 15190 reflections were collected, with 5221 independent reflections (Rint = 6.44%).14 For complex 8, 8178 (Rint = 0.00) unique data were collected. The intensities were corrected for Lorentzpolarisation eVects and absorption using the ABSCOR program. 15 The structure of 3 and 8 were solved by Patterson methods and direct methods respectively.All non-hydrogen atoms were refined anisotropically by full matrix least squares, with a riding model for hydrogen atoms, using the SHELXTL PLUS (PC Version) package.16 For compound 3 with 2966 [F > 6.0s(F)] observed reflections, refinement converged with Rf = 0.042 and R9 = 0.049. Largest diVerence peak and hole are 0.83 and 20.93 e Å23 respectively. For compound 8 with 6127 observed reflection (|Fo| � 6s|Fo|), refinement converged with Rf = 0.072 and R9 = 0.078.Largest diVerence peak and hole are 0.95 and 20.93 e Å23 respectively. Selected bond lengths and bond angles are given in Table 2. CCDC reference number 186/1231. Results and discussion Reaction of ruthenium chloride with HL aVords the bis chelate complex [Ru(HL)2][ClO4]2 1. The compound is diamagnetic and behaves as a 1 : 2 electrolyte in acetonitrile solution. Previous works 17–19 with thiosemicarbazones of 2-acetylpyridine have established that the ligand behaves as a planar NNS donor, co-ordinating through the pyridine nitrogen, the imine nitrogen and the thione sulfur atom.The IR spectrum of compound 1 indicates a similar co-ordination behaviour of the ligand. Thus 1 may be considered as a ruthenium(II) bis chelate, where each tridentate NNS ligand occupies a meridional plane. Reaction of (HL) with [Ru(PPh3)3Cl2] in refluxing ethanol leads to the isolation of the monochelates [Ru(L)(PPh3)2Cl] 2 and [Ru(HL)- (PPh3)2Cl]X [X = Cl 3 or PF6 4].The neutral complex 2 separated out from the reaction mixture, whereas the cationic complexes [3 and 4] were isolated from the mother-liquor by addition of the appropriate anion. Compounds 2 and 3/4 can easily be converted into each other by the addition of acid and base respectively. Crystal structure analysis of 3 established that in the distorted octahedral complex the two triphenylphosphine moieties are trans to each other, while the three NNS donor points of the ligand and the co-ordinated chloride constitute the equatorial square plane.The electronic spectrum of the bulk compound 3 in acetonitrile is identical to that of the crystals dissolved in the same solvent, indicating that the bulk compound is the trans isomer. The ready interconversion of 2 and 3 and very similar IR and electronic spectra suggest that the trans structure also prevails in 2. The steric repulsion between the two bulky triphenylphosphine moieties, as well as the p-tolyl moiety of the ligand leads to the formation of only the trans compounds.When compound 2 is dissolved in acetonitrile the coordinated chloride is solvolysed and the resulting solution behaves as a 1 : 1 electrolyte. However, compounds 3 and 4 did not suVer such a change. It is well known that RuII, a low spin d6 system, undergoes substitution by a dissociative mechanism. 20omplex 2 being neutral, can dissociate the chloride ion much more easily than 3 and 4 which are monocationic. Such an eVect of the overall charge of the complex unit on the dissociation of chloride ion is well documented.21 Compound 2 reacts with bidentate donors like bipyridine (bpy) and dppe to give [Ru(L)(PPh3)(bpy)]PF6 5 and [Ru(L)(PPh3)(dppe)]PF6 6 (Scheme 1).However, reaction with Hpic produces [Ru(HL)- (PPh3)(pic)]PF6 7 in which the ligand is present in its protonated form, picolinic acid displaying its usual behaviour by acting in the monoanionic bidentate fashion.The proton dissociated from picolinic acid appears to transform the deprotonated ligand into its protonated form. A very interesting reaction took place when compound 2 was refluxed with an excess of ligand HL. From the reaction medium it was possible to isolate the complex [Ru(HL)(PPh3)(mpi)]Cl2 8, in which the ligand HL is in its protonated form while the imine (mpi) retains its neutral (non-deprotonated) form. Though a number of diphenylmethyleneimine complexes are reported in the literature involving a variety of co-ordination modes, no [a(N)- heterocyclic]methyleneimine complexes have been reported to date.To our knowledge this is the first report of an [a(N)- heterocyclic]iminato complex which has been fully characterised by X-ray crystallography. The formation of the imine complex from the thiosemicarbazone may be visualised to proceed via a two-electron reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone by the ruthenium(II) acceptor centre in 2.The resulting ruthenium(IV) complex could be reduced subsequently by the triphenylphosphine or by the excess of ligand present in the system. If the ligand plays the role of reductant, it should be converted into the N-(4- tolyl)thiourea. The latter is actually isolated from the reaction medium and identified by its characteristic NMR and IR spectra and elemental analysis. The two-electron reductive cleavage of the N–N bond is one of the elementary reaction steps in the reduction of nitrogen to ammonia.Examples of such reductive cleavage are abundant in molybdenum complexes of hydrazine. 10 It is also known that diphenylmethyleneimine complexes may be generated by the reaction of an appropriate precursor metal complex and azines like Ph2C]] N–N]] CPh2. However, this is the first report of the generation of an imine complex by such cleavage of the N–N bond of a thiosemicarbazone coordinated to a metal centre.Structures of complexes 3 and 8 In both complexes 3 and 8 the ligand occupies a meridional plane co-ordinating through pyridine nitrogen [N(1)], imine nitrogen [N(2)] and the thiolate sulfur [S(1)] atom. Along with these three donor atoms a chlorine [Cl(1)] atom in 3 (Fig. 1) and an imine nitrogen [N(6)] in 8 (Fig. 2) complexes a square plane around the metal ion. The Ru–Cl(1) distance (2.459 Å) in 3 is somewhat long {cf. Ru–Cl distance of 2.387 Å in [Ru(PPh3)- Cl2] 13}. Two trans triphenylphosphine groups in 3 and one triphenylphosphine and one pyridine nitrogen [N(5)] of methyl- (2-pyridyl)methyleneimine ion 8 complete the octahedron.It is worthwhile to make a comparison of structures of 3 and 8 with that of [Ru(L9)(PPh3)2]ClO4 9; [L9 = monoanion of 2,6-diacetylpyridine 4-(4-tolyl)thiosemicarbazone].6 The trans triphenyl- Scheme 1138 J. Chem. Soc., Dalton Trans., 1999, 135–140 phosphine groups in 3 have identical Ru–P bond lengths (2.399 Å).These bonds are longer than reported (2.370, 2.373 Å) for trans-[Ru(L9)(PPh3)2]ClO4 but similar to that observed in [Ru(CO)(C2HN2S3)2(PPh3)2] 22 (2.397 Å, 2.399 Å). However, the Ru–P bond lengths in both 3 and 9 are longer than that observed in 8 (2.334 Å), the latter being on the shorter side of the range normally observed for Ru–P bonds.23 Again, in the 2-acetylpyridine SchiV base complexes 3 and 8, the Ru–N (py) distances are larger than the Ru–N (imine) distances, but in 2,6- Fig. 1 Perspective view of the [Ru(HL)(PPh3)2Cl]1 cation of [Ru- (HL)(PPh3)Cl]Cl?CH2Cl2 with atom labelling. Fig. 2 Perspective view of the [Ru(HL)(PPh3)(mpi)]21 cation of [Ru- (HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O with atom labelling. Table 1 Crystal data for [Ru(HL)(PPh3)2Cl]Cl?CH2Cl2 3 and [Ru- (HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O 8 Formula M Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 ZF (000) Dc/g cm23 RR 9 C52H48Cl4N4P2RuS 1065 Pnma 19.279(4) 15.947(3) 15.752(3) 4843(2) 4 2184 1.462 0.0418 0.0493 C41H47Cl4N6O3PRuS 977.7 P1� 11.286(1) 13.629(1) 16.109(2) 98.36 97.19 109.02(1) 2277.99(11) 2 1000 1.424 0.072 0.078 diacetylpyridine monothiosemicarbazone complex 9 the opposite trend is observed.6 In 9 the Ru–N (py) distance is appreciably shorter than the normally observed value and the Ru–N (py) distances increase in the order 9 < 3 < 8.The Ru–N(2) (imine) distances are slightly shorter than the Ru–N(1) (py) distances, but they follow the same order, e.g. 9 < 3 < 8. The Ru–S(1) distances are normal, but they follow an order opposite to that of the Ru–N(1) distances, e.g. 9 > 3 > 8. The C(8)–S(1) distances in all the three compounds are similar (1.69–1.71 Å) and close to the C]] S distances observed in the free thiosemicarbazides and thiosemicarbazones.24,25 Again, though the imine C(6)– N(2) distances are close to their expected values, both the C(8)– N(3) bond distances in the thiosemicarbazone moiety are appreciably shorter than the C–N single bond distance.The C(8)–N(4) distance in 8 is shorter than the C(8)–N(3) distance, but in complex 9 the opposite is true. Similarly the N(2)–N(3) distances in all the complexes are appreciably shorter than that reported for free thiosemicarbazide or for hydrazine.23,24 It has been suggested that in thiosemicarbazides and thiosemicarbazones there is an extensive p delocalisation over the entire chain, so that none of the bonds can be considered a true single or double bond.Rheingold and co-workers 9 proposed that, even in deprotonated thiosemicarbazones, the iminothiolate sulfur S(1) undergoes rehybridisation to sp2 and the lone pair on the p orbital can participate in conjugation with the imine moiety. Such extensive p delocalisation within the ligand moiety coupled with the p backbonding from the metal is responsible for the apparent anomalies in bond distances mentioned above. Electrochemistry The electrochemical data for the complexes are given in Table 3.The electrochemistry of the complexes is dominated by a reversible RuII–RuIII oxidation. Peak potential separations between anodic and cathodic peaks, Epa 2 Epc, vary between 60 and 90 mV and are virtually independent of scan rate. These peak separations, though larger than the ideal Nernstian value of 59 mV, are commonly observed for this type of com- Table 2 Selected bond distances (Å) and angles (8) for [Ru(HL)- (PPh3)2Cl]Cl?CH2Cl2 3 and [Ru(HL)(PPh3)(mpi)]Cl2?CH2Cl2?3H2O 8 Ru(1)–N(1) Ru(1)–N(2) Ru(1)–S(1) Ru(1)–P(1) Ru(1)–P(1A) Ru(1)–Cl(1) C(6)–N(2) N(2)–N(3) N(3)–C(8) C(8)–S(1) C(8)–N(4) C(5)–C(6) N(4)–C(9) N(1)–Ru(1)–N(2) N(1)–Ru(1)–S(1) N(1)–Ru(1)–Cl(1) N(2)–Ru(1)–Cl(1) N(2)–Ru(1)–S(1) Cl(1)–Ru(1)–S(1) P(1)–Ru(1)–N(1) P(1)–Ru(1)–N(2) P(1)–Ru(1)–S(1) P(1)–Ru(1)–Cl(1) P(1)–Ru(1)–P(1A) P(1A)–Ru(1)–N(1) P(1A)–Ru(1)–N(2) P(1A)–Ru(1)–S(1) P(1A)–Ru–Cl(1) C(5)–C(6)–N(2) 2.085(5) 1.984(5) 2.386(2) 2.399(1) 2.399(1) 2.459(2) 1.304(7) 1.383(7) 1.359(7) 1.707(6) 1.334(8) 1.462(9) 1.416(8) 77.4(2) 160.8(1) 99.0(1) 176.4(1) 83.3(1) 100.2(1) 91.9(1) 91.9(1) 88.7(1) 88.2(1) 175.2(1) 91.9(1) 91.9(1) 88.7(1) 88.2(1) 112.5(5) Ru(1)–N(1) Ru(1)–N(2) Ru(1)–S(1) Ru(1)–P(1) Ru(1)–N(5) Ru(1)–N(6) C(6)–N(2) N(2)–N(3) N(3)–C(8) C(8)–S(1) C(8)&ash;N(4) C(5)–C(6) N(4)–C(9) N(1)–Ru(1)–N(2) N(1)–Ru(1)–S(1) N(1)–Ru(1)–N(6) N(2)–Ru(1)–N(6) N(2)–Ru(1)–S(1) N(6)–Ru(1)–S(1) P(1)–Ru(1)–N(1) P(1)–Ru(1)–N(2) P(1)–Ru(1)–S(1) P(1)–Ru(1)–N(6) P(1)–Ru(1)–N(5) N(5)–Ru(1)–N(1) N(5)–Ru(1)–N(2) N(5)–Ru(1)–S(1) N(1)–Ru(1)–N(6) C(5)–C(6)–N(2) C(38)–C(39)–N(6) 2.092(6) 1.991(5) 2.358(2) 2.334(2) 2.110(6) 2.075(7) 1.336(8) 1.371(9) 1.347(8) 1.698(7) 1.344(11) 1.478(12) 1.418(9) 78.0(2) 161.1(2) 98.7(3) 169.5(2) 83.4(2) 98,9(2) 97.0(2) 92.0(2) 87.3(1) 98.3(2) 171.6(1) 90.4(2) 93.5(2) 87.0(2) 76.5(2) 110.8(6) 115.3(7)J.Chem. Soc., Dalton Trans., 1999, 135–140 139 plexes.6,26,27 In most of the cases no well defined peaks are observed at the cathodic side of the cyclic voltammograms. This is probably due to the reduction of the ligand followed by decomposition of the resultant complex.For complexes 5 and 8 a reductive couple observed around 21.5 V may be ascribed to a ligand (bipyridine/mpi) centered reduction.28 It may be noted that in this study we have extensively varied the co-ordination environment around the ruthenium(II) acceptor centre employing a variety of nitrogen, sulfur, phosphorus, oxygen and chloride donors.So, it is worthwhile to follow the trend in the variation of RuIII–RuII potential with the change of donor environment around the metal ion, particularly because such studies are rather scanty.29 It is well established that such potentials are aVected by both the nature of the donor sets as well as the overall charge of the complex, the latter being the dominating factor. So, a meaningful correlation is possible only when one compares complexes having identical charges.Thus, we may compare the ERu(III)/Ru(II) of Ru(bpy)3 21 (1.38 V, N6 donors) with that of [Ru(HL)2]21 (20.005 V, N4S2 donors) and conclude that replacement of two pyridine nitrogens by two thiocarbonyl sulfurs has stabilised the RuIII by 1.43 V. This may be rationalised by referring to the higher polarisability and poorer p-accepting capability of the thiocarbonyl sulfur compared to pyridyl nitrogen, and both the factors tend to stabilise the ruthenium(III) state.Again, we can compare the RuIII–RuII potential of the complex [Ru(L)(PPh3)2Cl] (0.36 V, N2P2SCl donors) with that of [Ru(bpy)2Cl2] (0.34 V, N4Cl2 donors); in this case the replacement of two pyridine nitrogens and a chloride by two phosphorus and a thiolato donor set keeps the potential almost unaltered. Though thiolato ligands are known to be eYcient in stabilising higher (III and IV) oxidation states of ruthenium, in the present case that eVect is compensated by the introduction of two phosphine donors, which are even more eYcient in stabilising ruthenium(II) than the pyridine nitrogens.Similarly one can compare the series of five monocationionic complexes [Ru(HL)(PPh3)2Cl]- Cl (0.65 V, N2P2SCl donors), [Ru(HL)(PPh3)2Cl]PF6 (0.70 V, N2P2SCl donors), [Ru(L)(PPh3)(bpy)]PF6 (0.65 V, N4PS9 donors), [Ru(L)(PPh3)(dppe)]PF6 (0.77 V, N2P3S9 donors) and [Ru(HL)(PPh3)(pic)]PF6 (0.47 V, N3PSO donors) and conclude that the presence of bipyridine nitrogen or imine nitrogen as well as phosphine donors tends to stabilise the ruthenium(II) state, whereas thiolato and carboxylato donors stabilise the ruthenium(III) state.One may also compare the ERu(III)/Ru(II) values of [Ru(bpy)2(SPh)2] (20.28 V, N4S2 2 donors) 29 and [Ru(bpy)2(pybt)]1 [0.32 V, N5S2 donors; pybt = 2-(2-pyridyl)benzenethiolate] 28 with those of thiolato complexes reported in this paper and conclude that the benzenethiolato group is more eYcient in stabilising RuIII than the iminethiolates described in this paper, a fact which correlates Table 3 Cyclic voltammetric data a of the complexes in acetonitrile at 298 K E2� 1 /V (DEp/mV) Compound 1 [Ru(HL)2][ClO4]2 2 [Ru(L)(PPh3)2Cl] 3 [Ru(HL)(PPh3)2Cl]Cl 4 [Ru(HL)(PPh3)2Cl]PF6 5 [Ru(L)(PPh3)(bpy)]PF6 6 [Ru(L)(PPh3)(dppe)]PF6 7 [Ru(HL)(PPh3)(pic)]PF6 8 [Ru(HL)(PPh3)(mpi)]Cl2 Oxidation 0.005(90) 0.36(75) 0.65(60) 0.70(60) 0.65(60) 0.77(80) 0.47(60) 0.67(60) Reduction 21.42(100) 21.56(80) Donor sites b N4S2 N2P2S9Cl N2P2SCl N2P2SCl N4PS9 N2P3S9 N3PSO N4PS a Conditions: supporting electrolyte, NEt4ClO4 (0.1 M); working electrode, platinum; reference electrode, Ag–AgCl; solute concentration, 1023 M.E2� 1 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials; DEp = Epa 2 Epc, Ipc /Ipa = 1, and scan rate = 50 mV s21. b S refers to thiocarbonyl sulfur and S9 to thiolato sulfur. well with the lower basicity of the latter as described by Rheingold and co-workers.9 Electronic spectra The electronic spectra of low-spin d6 complexes are generally dominated by metal to ligand charge transfer in the visible region.30–32 As most of the complexes discussed in this work are of Cs or lower symmetry all the d orbitals are non-degenerate.So, a number of MLCT transitions are expected. However, due to the small energy separation between some of these d orbitals, as well as poor overlap between them and the excited state orbitals, some of the expected MLCT transitions may not be resolved.In general, all the complexes exhibit two well resolved MLCT transitions around 420–480 (band I) and 373–378 nm (band II). When the energy of band I is plotted against E8Ru(III)/Ru(II) a nice linear correlation EMLCT = 1.18 E8Ru(III)/Ru(II) 1 2.42 is obtained (Fig. 3). Besides, for some of the complexes, there is an additional low energy band at 480–580 nm (band III). While bands I and III are substituent dependent, II is unaVected by substituents.The two highest energy bands at 260–290 and 210–220 nm are probably due to intraligand transitions. 1H NMR spectra The 1H NMR spectrum of the ligand (HL) exhibits signals at d 2.49 (3 H) and 2.35 (3 H) which are assigned to the CH3 group of the p-tolyl moiety and that attached to the imine moiety of the ligand. The signals at d 9.32 (1 H) and 8.9 (1 H) are due to NH protons and all aromatic protons exhibit signals in the region d 7.19–8.61.33 For complex 3 the signal of the CH3 group attached to the imine moiety was shifted upfield to d 2.16, whereas the CH3 proton signal of the p-tolyl moiety remains unaVected at d 2.31.The NH proton signals are at d 12.16 and 11.12. The aromatic protons are observed between d 6.8 and 8.76. Compound 8 exhibits three CH3 proton signals at d 2.31 (3 H), 2.35 (3 H) and 2.46 (3 H). The signal at d 2.35 is due to the CH3 group of the p-tolyl residue.One NH signal is observed at d 12.25. The broken organic fragment isolated from the reaction mixture [N-(p-tolyl)thiourea] exhibits a signal at d 3.37 due to the CH3 group33 of the tolyl part. The phenyl protons are observed between d 7.3 and 8.62. The NH proton signals are not observed and similar observations were reported 34 previously in the case of N-(p-nitrophenyl)thiourea. Conclusion This paper describes the ruthenium(II) complexes of methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone, in which the Fig. 3 Plot of EMLCT versus RuIII–RuII potential (Eobs, on NHE scale).140 J. Chem. Soc., Dalton Trans., 1999, 135–140 ligand behaves either as a monoanionic tridentate NNS donor (thioenol form) or as a neutral tridentate NNS donor (thione form). The pH-dependent interconversion of the compounds [Ru(L)(PPh3)2Cl] and [Ru(HL)(PPh3)2Cl]Cl is a manifestation of thione–thioenol tautomerisation of the co-ordinated ligand. Formation of the compound [Ru(HL)(PPh3)(mpi)]Cl2 from the complex [Ru(L)(PPh3)2Cl] is an extremely interesting manifestation of the unusual reactivity of the co-ordinated thiosemicarbazone moiety.The complex [Ru(HL)(PPh3)(mpi)]Cl2, produced through reductive cleavage of the hydrazinic N–N bond of the thiosemicarbazone ligand, is the first structurally characterised metal complex of an imine of a heterocyclic ketone. 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