American Chemical Society
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Protonolysis and Amide Exchange Reactions of a Three-Coordinate Cobalt Amide Complex Supported by an N‑Heterocyclic Carbene Ligand

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journal contribution
posted on 2015-05-18, 00:00 authored by Christopher B. Hansen, Richard F. Jordan, Gregory L. Hillhouse
A three-coordinate cobalt species, IPrCoCl­{N­(SiMe3)2} [1; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene], was synthesized by the reaction of {IPrCoCl2}2 with NaN­(SiMe3)2. Compound 1 is a useful starting material for low-coordinate (IPr)Co species. 1 reacts with 2,6-di-tert-butyl-4-methylphenol (BHT-H) via aminolysis of the Co–N bond to generate a three-coordinate phenoxide complex, IPrCoCl­(O-2,6-tBu2-4-MeC6H2) (2). The reaction of 1 with 2,6-diisopropylaniline (NH2DIPP) generates IPrCoCl­(NHDIPP) (4), which undergoes disproportionation to form a mixture of 4, {IPrCoCl2}2, and IPrCo­(NHDIPP)2 (3). The same product mixture is formed by the reaction of 1 with Li­[NH­(DIPP)], which unexpectedly proceeds by amide exchange. Compound 3 was synthesized independently by the reaction of {IPrCoCl2}2 with 4 equiv of Li­[NH­(DIPP)]. The reaction of 1 with the bulkier lithium 2,6-dimesitylanilide (LiNHDMP) also proceeds by amide exchange to generate IPrCoCl­(NHDMP) (5), which is stable toward disproportionation. Compounds 1 and 2 exhibit trigonal-planar geometries at cobalt in the solid state. The solid-state structure of 3 also contains a trigonal-planar cobalt center and exhibits close Co---H contacts involving the methine hydrogen atoms of the NH­(DIPP) groups in the axial positions. The solid-state structure of 5 features an interaction between cobalt and a flanking aryl group of the anilide ligand, resulting in pyramidalization of the cobalt center.