Computational Characterization of Redox Non-Innocence in Cobalt-Bis(Diaryldithiolene)-Catalyzed Proton Reduction

Localized orbital bonding analysis (LOBA) was employed to probe the oxidation state in cobalt-bis(diaryldithiolene)-catalyzed proton reduction in nonaqueous media. LOBA calculations provide both the oxidation state and chemically intuitive views of bonding in cobalt-bis(diaryldithiolene) species and therefore allow characterization of the role of the redox non-innocent dithiolene ligand. LOBA results show that the reduction of the monoanion species [1Br](-) is metal-centered and gives a cobalt(II) ion species, [1Br](2-), coordinated to two dianionic ene-1,2-dithiolates. This electronic configuration is in agreement with the solution magnetic moment observed for the analogous salt [1F](2-) (μeff = 2.39 μB). Protonation of [1Br](2-) yields the cobalt(III)-hydride [1Br(CoH)](-) species in which the Co-H bond is computed to be highly covalent (Löwdin populations close to 0.50 on cobalt and hydrogen atoms). Further reduction of [1Br(CoH)](-) forms a more basic cobalt(II)-H intermediate [1Br(CoH)](2-) (S = 0) from which protonation at sulfur gives a S-H bond syn to the Co-H bond. Formation of a cobalt-dihydrogen [1Br(CoH2)](-) intermediate is calculated to occur via a homocoupling (H(•) + H(•) → H2) step with a free energy of activation of 5.9 kcal/mol in solution (via C-PCM approach).