Synthesis and reactivity of copper(i) complexes with an ethylene-bridged bis(imidazolin-2-imine) ligand

Abstract

A series of copper(I) complexes with a sterically hindered, bidentate ligand, BL^iPr, derived from an N-heterocyclic carbene precursor have been isolated, characterized and their reactivity studied. The ethylene-bridged bis(imidazolin-2-imine) ligand (BL^iPr) provides strongly donating N-donor atoms for the stabilization of a copper(I) metal center, priming it for reactivity. The complexes [(BL^iPr)Cu(XyNC)]PF₆ (4) and [(BL^iPr)CuCl] (5) were characterized by X-ray crystallography and exhibit trigonal coordination at the copper centers. The reactivity of [(BL^iPr)Cu]SbF₆ toward dioxygen was studied at low temperature, indicating formation of a thermally sensitive intermediate with intense UV/Vis features and an isotope-sensitive vibration at 625 cm⁻¹ (599 cm⁻¹ with ¹⁸O₂). The intermediate is assigned as containing the bis(µ-oxo)dicopper(III) core, [2](PF₆)₂, and the related, stable hydroxo form was crystallized as [{(BL^iPr)Cu}₂(µ-OH)₂](PF₆)₂, [3](PF₆)₂. The reactivity of 5 as a catalyst for the ATR polymerization of styrene was assessed in terms of reaction kinetics and polymer properties, with low PDI values achieved for polymers with molecular weights up to 30 000 g mol⁻¹.