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

Abstract

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 [Co^II(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 =½) state of Co^II. A similar reaction in air, however, furnishes [Co^III(L^3a)(L^3b)]·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 (H₂L²) (-CH₂CH₂CH₂- spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [Co^II(L²)]·MeOH (1b·MeOH) (S =½); its structurally characterized variety has the composition 1b·C₆H₆. Interestingly, 1b·MeOH undergoes facile metal-centred oxidation by aerial O₂-H₂O₂-[Fe(η⁵-C₅H₅)₂][PF₆], which led to the isolation of the corresponding cobalt(III) complex [Co^III(L²)][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 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 α 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·MeOH, 2a, 2b and 3), Co^II-Co^I (1a, 1b·MeOH, 2a and 2b), and Co^I-Co⁰ (1a, 1b.MeOH and 2b) redox processes.