Sterically Demanding Ag and Cu N-Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity.

The synthesis and full characterization of new air-stable Ag and Cu complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag O afforded a collection of Ag complexes, and their first use as ligand transfer reagents led to novel isostructural Cu or Au complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new Cu complexes. The solid-state structures feature large N-C -N angles (118-128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%V ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%V =57.5) overcomes to other known erNHC-metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76-93 % product at the 0.5-2.5 mol % catalyst loading, proving the stability of Cu erNHC complexes at elevated temperatures (100 °C).