C-S Bond Cleavage, Redox Reactions, and Dioxygen Activation by Nonheme Dicobalt(II) Complexes

Abstract

Synthesis and reactivity of a series of thiolate/thiocarboxylate bridged dicobalt(II) complexes were investigated in comparison with their carboxylate bridged analogues bearing free thiol/hydroxyl groups. Upon one-electron oxidation, complexes [Co₂(N-Et-HPTB)(μ-SR¹)](BF₄)₂ (R¹ = Ph, 1a; Et, 1b; Py, 1c) and [Co₂(N-Et-HPTB)(μ-SCOR²)](BF₄)₂ (R² = Ph, 2a; Me, 2b) yielded [Co₂(N-Et-HPTB)(DMF)₂](BF₄)₃ (6) (DMF = dimethylformamide) along with the corresponding disulfides (where N-Et-HPTB is the anion of N,N,N',N'-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane). Unlike the inertness of carboxylate bridged complexes [Co₂(N-Et-HPTB)(μ-O₂C-R³-SH)](BF₄)₂ (R³ = Ph, 3a; CH₂CH₂, 3b) and [Co₂(N-Et-HPTB)(μ-O₂CR⁴)](BF₄)₂ (R⁴ = Ph, 4a; Me, 4b; CH₂CH₂CH₂OH, 5) toward O₂, the bridging ethanethiolate in 1b was oxidized to yield a sulfinate bridged complex, [Co₂(N-Et-HPTB)(μ-O₂SEt)](BF₄)₂ (10). Detailed investigation of the synthetic aspects of 1a-1c led to the discovery of a C-S bond cleavage reaction and yielded the dicobalt(II) complexes [Co₂(N-Et-HPTB)(SH)(H₂O)](BF₄)₂ (8a), [Co₂(N-CH₂Py-HPTB)(SH)(H₂O)](BF₄)₂ (8b) (where N-CH₂Py-HPTB is the anion of N,N,N',N'-tetrakis[2-(1-picolylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane)), and [Co₂(N-Et-HPTB)(μ-S)](BF₄) (9). Both 8a and 8b feature nonheme dinuclear Co(II) units containing a terminal hydrosulfide. The present study thus reports comparative redox reactions for a rare class of 16 dicobalt(II) complexes and introduces a selective synthetic strategy for the synthesis of unprecedented dicobalt(II) complexes featuring only one terminal hydrosulfide.