Cobalt(II) and cobalt(III) complexes of thioether-containing hexadentate pyrazine amide ligands: C-S bond cleavage and cyclometallation reaction

Anaerobic reaction of Co(O2CMe)2.4H2O with the thioether-containing acyclic pyrazine amide hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2L1) (-CH2CH2- spacer between the two pyrazine amide tridentate coordination units) furnishes [CoII(L1)].MeOH (1a) having CoN2(pyrazine)N'2(amide)S2(thioether) coordination. It exhibits an eight-line EPR spectrum, attesting to a low-spin (S = 1/2) state of CoII. A similar reaction in air, however, furnishes [CoIII(L3a)(L3b)].2MeOH (2a) (S = 0), resulting from a C-S bond cleavage reaction triggered by an acetate ion as a base, having CoN2(pyrazine)N'2(amide)S(thioether)S'(thiolate) coordination. On the other hand, the reaction of Co(O2CMe)2.4H2O with 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,5-dithiopentane (H2) (-CH2CH2CH2- spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [CoII(L2)].MeOH (1b.MeOH) (S = 1/2); its structurally characterized variety has the composition 1b.C6H6. Interestingly, 1b.MeOH undergoes facile metal-centred oxidation by aerial O2-H2O2-[Fe(eta5-C5H5)2][PF6], which led to the isolation of the corresponding cobalt(iii) complex [CoIII(L2)][ClO4] (2b). When treated with methanolic KOH, 2b affords a low-spin (S = 0) organocobalt(III) complex [Co(III)((L2')] (3). Structures of all complexes, except 1a, have been authenticated by X-ray crystallography. A five-membered chelate-ring forming ligand L1(2-) effects C-S bond cleavage and a six-membered chelate-ring forming ligand L2(2-) gives rise to Co-C bond formation, in cobalt(III)-coordinated thioether functions due to alpha C-H bond activation by the base. A rationale has been provided for the observed difference in the reactivity properties. The spectroscopic properties of the complexes have also been investigated. Cyclic voltammetry experiments in MeCN-CH2Cl2 reveal facile metal-centred reversible-to-quasireversible CoIV-CoIII (or a ligand-centred redox process; 2a), CoIII-CoII (1a, 1b.MeOH, 2a, 2b and 3), CoII-CoI (1a, 1b.MeOH, 2aand 2b), and CoI-Co0 (1a, 1b.MeOH and 2b) redox processes.