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)center dot 4H(2)O 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 [Co-II(L-1)]center dot MeOH (1a) having CoN2(pyrazine)N'(2)(amide)S-2(thioether) coordination. It exhibits an eight-line EPR spectrum, attesting to a low-spin (S=1/2) state of Co-II. A similar reaction in air, however, furnishes [Co-III(L-3a)(L-3b)]center dot 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)center dot 4H(2)O with 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]1,5- dithiopentane (H2L2) (-CH2CH2CH2-spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [Co-II(L-2)]center dot MeOH (1b center dot MeOH) (S=1/2); its structurally characterized variety has the composition 1b center dot C6H6. Interestingly, 1b center dot MeOH undergoes facile metal-centred oxidation by aerial O-2-H2O2-[Fe(eta(5)-C5H5)(2)][PF6], which led to the isolation of the corresponding cobalt(III) complex [Co-III(L-2)][ClO4] (2b). When treated with methanolic KOH, 2b affords a low-spin (S=0) organocobalt(III) complex [Co-III((L-2')] (3). Structures of all complexes, except 1a, have been authenticated by X-ray crystallography. A five-membered chelate-ring forming ligand L-1(2-) effects C-S bond cleavage and a six-membered chelate-ring forming ligand L-2(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 Co-IV-Co-III (or a ligand-centred redox process; 2a), Co-III-Co-II (1a, 1b center dot MeOH, 2a, 2b and 3), Co-II-Co-I (1a, 1b center dot MeOH, 2a and 2b), and Co-I-Co-0 (1a, 1b center dot MeOH and 2b) redox processes.