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

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl
amino carbene, :C­(CH2)­(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:CoII(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 [(Me2-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.