Steric nature of the bite angle. A closer and a broader look

Abstract

The bite angle (ligand–metal–ligand angle) is known to greatly influence the activity of catalytically active transition-metal complexes towards bond activation. Here, we have computationally explored how and why the bite angle has such effects in a wide range of prototypical C–X bonds and palladium complexes, using relativistic density functional theory at ZORA-BLYP/TZ2P. Our model reactions cover the substrates H₃C–X (with X = H, CH₃, Cl) and, among others, the model catalysts, Pd[PH₂(CH₂)_nPH₂] (with n = 2–6) and Pd[PR₂(CH₂)_nPR₂] (n = 2–4 and R = Me, Ph, t-Bu, Cl), Pd(PH₃)X⁻ (X = Cl, Br, I), as well as palladium complexes of chelating and non-chelating N-heterocyclic carbenes. The purpose is to elaborate on an earlier finding that bite-angle effects have a predominantly (although not exclusively) steric nature: a smaller bite angle makes more room for coordinating a substrate by bending away the ligands. Indeed, the present results further consolidate this steric picture by revealing its occurrence in this broader range of model reactions and by identifying and quantifying the exact working mechanism through activation strain analyses.