Rigid vs Spacer Ligand Design: Modulating Cooperativity in Pd-NHC Catalysis

Abstract

Unsymmetrical bis-N-heterocyclic carbene (NHC) ligand (bearing 1,2,3-triazolylidene (1,2,3-tzNHC) and benzimidazolylidene (bzNHC) donors)-supported homobimetallic Pd(II) complexes, with (2) or without (1) a phenylene spacer, were synthesized and evaluated in demanding Suzuki–Miyaura couplings of chloroarenes. The spacer-free complex 1 exhibited superior activity to that of 2, despite showing Pd–Pd cooperativity, attributed to conformational rather than electronic effects, as supported by experimental and computational studies. In contrast, electronic effects dictate the activity trends among the spacer-free complexes 1, 3, and 4, which differ in the identity of one of the NHC donors (1,2,3-triazolylidene, imidazolylidene, and 1,2,4-triazolylidene, respectively).