Studies on Three-Coordinate [Co{N(SiMe3)2}2L] Complexes, L = N‑Heterocyclic Carbene

A series of three-coordinate N-heterocyclic carbene bis­(trimethylsilyl)­amide complexes of cobalt, [Co­{N­(SiMe3)2}2(NHC)] (NHC = N,N′-diarylimidazolin-2-ylidene, aryl = 2,6-diisopropylphenyl, SIPr (1), aryl = mesityl, SIMes (2); κ1-N,N′-diphosphanylimidazol-2-ylidene, PCP (5); κ1-N-phosphanyl-N′-2,6-diisopropylphenylimidazol-2-ylidene, PC (6); cyclic alkyl amino carbene :C­(Cy)­CH2CMe2N-2,6-Pri2C6H3, cAACCy (7)) were prepared by the reaction of [Co­{N­(SiMe3)2}2] with the corresponding NHCs. The complexes exhibited interesting transamination reactivity in reactions with bulky 2,6-diisopropylaniline (NH2(DiPP)), where adjustment of the stoichiometry and reaction conditions resulted in the substitution of one or two N­(SiMe3)2 ligands by the anilido ligand NH­(DiPP), giving [Co­{N­(SiMe3)2}­{NH­(DiPP)}­(NHC)] (NHC = N,N′-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene, IPr (8)) and [Co­{NH­(DiPP)}2(NHC)] (NHC = SIPr (9); IPr (10); PCP (11)). X-ray crystallography revealed trigonal-planar coordination geometry at Co for all of the new complexes and long Co–CNHC bond lengths; in the cases of 5, 8, 9, 10, and 11, intramolecular Co···H and/or H···P interactions may provide further stabilization of specific conformations. Magnetic and electron paramagnetic resonance studies showed that the three-coordinate Co­(II) centers in 1–7 behave as S = 3/2 spins with strong anisotropy arising from the low symmetry of the coordination site (C2v). The anisotropy results in large values of the zero-field splitting parameter D.