Stabilization of a Cobalt Cobalt Bond by Two Cyclic Alkyl Amino Carbenes

(Me-2-cAAC:)(2)Co-2 (2, where Me-2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)(2)N-2,6-iPr(2)C(6)H(3))) was synthesized via the reduction of precursor (Me-2-cAAC:Co-11(mu-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(center dot-) and 2(center dot+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me-2-cAAC:)(2)Co-2]center dot+OTf-(2(center dot+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(center dot+) reveals the coupling of the electron spin with 2 equiv Co-59 isotopes, leading to a (Co-0.5)(2) state. The experimental Co1-Co-2 bond distances are 2.6550(6) and 2.4610(6) angstrom for 2 and 2(center dot+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(center dot+)OTf(-) possess a Co-Co bond with an average value of 2.585 angstrom. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong pi-accepting property of cAAC. 2(center dot+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(center dot+) is caused by intermolecular interactions.