London Dispersion-Induced Contraction of the Cu─O Bond in Copper(I) Phenolates.

Conventional chemical wisdom holds that shorter bonds of a given type generally exhibit greater strength. Here, we reveal a counterintuitive bonding mode in copper(I) phenoxides, where bulky substituents induce Cu─O bond contraction without concomitant strengthening. X-ray crystallographic analysis shows that increasing steric bulk at ortho positions of the phenolate ligand can shorten Cu─O bonds and promote symmetrical molecular geometries. Combined structural and computational studies demonstrate that London dispersion (LD) between N-heterocyclic carbene ligands and ortho-tert-butyl phenolates plays a crucial role in driving this conformational reorganization. Local energy decomposition analysis quantifies substantial dispersion stabilization (by up to 11.2 kcal mol^-1). Notably, natural orbital analysis indicates that the compressed Cu─O bonds exhibit diminished σ-character despite enhanced π-interactions. This LD-induced bond contraction results in overall shorter yet weaker Cu─O bonds than those in less sterically bulk analogues, thereby establishing a different bonding paradigm from the conventional bond-length/strength correlation.