Hidden in plain sight: commonly used copper N-heterocyclic carbene catalysts gain stabilization from anagostic Cu⋯H–C interactions

Abstract

The ubiquity in organic chemistry of copper catalysts supported by N-heterocyclic carbene (NHC) ligands is often attributed to the ability of sterically bulky and electronically donating NHCs to stabilize low-coordinate intermediates. Here, we show using meta-analysis of 893 solid-state Cu–NHC structures that nearly all Cu–NHC complexes contain at least one short Cu⋯H–C contact within the range expected for anagostic interactions, implying that Cu⋯H–C stabilization is prevalent in commonly used Cu–NHC catalysts. A quantum crystallography study of one example, I^tBuCuCl (1), provided evidence for four Cu⋯H–C anagostic interactions in the solid state. Based on experimental solid-state NMR spectroscopy and QTAIM analysis of theoretical charge density, evidence for four Cu⋯H–C anagostic interactions was also obtained for the solid-state structure of IPrCuCl (2), a ubiquitous complex in homogeneous Cu catalysis. The effect of these anagostic interactions on the solution-phase chemistry of 1 was probed by comparing catalytic activity of I^tBuCuCl with its deuterated analogue, d₁₈-I^tBuCuCl, revealing a secondary kinetic isotope effect (KIE) of k_H/k_D = 1.09 ± 0.01 for the hydrosilylation of a ketone at 315 K. Since these anagostic interactions were not previously assigned apart from selected supramolecular systems, knowledge of anagostic stabilization of Cu–NHC complexes is expected to guide future catalyst designs.