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

Abstract

(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 (Co0.5) 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. © 2014 American Chemical Society.