A titanium redox-switch enables reversible C–C bond forming and splitting reactions

Abstract

Using an Earth-abundant transition metal to mediate formation and splitting of C–C σ-bonds, in response to electrical stimuli, constitutes a promising strategy to construct complex organic skeletons. Here, we showcase how [ⁿBu₄N][N₃] reacts with an isocyanide adduct of a tetrahedral and high-spin Ti^II complex, [(Tp^tBu,Me)TiCl] (1), to enact N-atom transfer, C–N bond formation, and C–C coupling, to form a dinuclear complex, [(Tp^tBu,Me)Ti{AdN(N)C–C(N)NAd}Ti(Tp^tBu,Me)] (3), with two Ti^III ions bridged by a disubstituted oxalimidamide ligand (ⁿBu = n-butyl, Tp^tBu,Me = hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate, Ad = 1-adamantyl). Magnetic and computational studies reveal two magnetically isolated d¹ Ti^III ions, and electrochemical studies unravel a reversible two-electron oxidation at −0.87 V vs. [FeCp₂]^0/+. Despite these observations, chemical oxidation of 3, ultimately, leads to rupture of the oxalimidamide moiety with C–C bond splitting to form [(Tp^tBu,Me)Ti{1,3-μ₂-AdNCN}₂Ti(Tp^tBu,Me)][B(C₆F₅)₄]₂ (4), which displays an antiferromagnetically coupled Ti₂^III,III configuration, mediated by superexchange through its bridging carbodiimide ligands. A comparative reactivity study of isocyanide toward a transient vanadium nitride [(Tp^tBu,Me)VN(THF)] (5) gives further insight into the structure of putative intermediates involved in the coupling sequence.