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

The synthesis and full characterization of new air-stable Ag^I and Cu^I complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag₂ O afforded a collection of Ag^I complexes, and their first use as ligand transfer reagents led to novel isostructural Cu^I or Au^I complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new Cu^I complexes. The solid-state structures feature large N-C_NHC -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_bur ) values span easily in the 50-57 % range, and that one of (9-Dipp)CuBr complex (%V_bur =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^I erNHC complexes at elevated temperatures (100 °C).