Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes

(Me₂-cAAC:)₂Co₂ (2, where Me ₂-cAAC: = cyclic alkyl amino carbene,:C(CH₂)(CMe ₂)₂N-2,6-iPr₂C₆H₃)) was synthesized via the reduction of precursor (Me₂-cAAC:Co ^II(μ-Cl)Cl)₂ (1) with KC₈. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2^•- and 2 ^•+ ions. The latter was synthesized via reduction of 1 with required equivalents of KC₈ and characterized as [(Me ₂-cAAC:)₂Co₂]^•+OTf^- (2^•+OTf^-). Electron paramagnetic resonance spectroscopy of 2^•+ reveals the coupling of the electron spin with 2 equiv ⁵⁹Co isotopes, leading to a (Co^0.5) ₂ state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2^•+OTf^-, respectively. Theoretical investigation revealed that both 2 and 2^•+OTf ^- possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2^•+ is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2^•+ is caused by intermolecular interactions.