Stabilization of a Cobalt Cobalt Bond by Two Cyclic Alkyl Amino Carbenes
Mondal, Kartik Chandra × Samuel, Prinson P Roesky, Herbert W Carl, Elena Herbst-Irmer, Regine Stalke, Dietmar Schwederski, Brigitte Kaim, Wolfgang Ungur, Liviu Chibotaru, Liviu Hermann, Markus Frenking, Gernot #
AMER CHEMICAL SOC
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY vol:136 issue:5 pages:1770-1773
(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.