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

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 ^<i>t</i> BuCuCl (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 ^<i>t</i> BuCuCl with its deuterated analogue, <i>d</i> ₁₈-I ^<i>t</i> BuCuCl, revealing a secondary kinetic isotope effect (KIE) of <i>k</i> _H/<i>k</i> _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.