Synthetic and catalytic studies of Group 11 N-heterocyclic carbene complexes

Abstract

This thesis presents the synthesis, structure and reactivity of two-coordinate Group 11 metal complexes, supported by N-heterocyclic carbene (NHC) ligands. The NHC ligand was found to stabilize monomeric, terminal fluoride complexes of copper, silver and gold, the last representing the first isolable gold(I) fluoride. These complexes were shown to be reactive synthons for new metal complexes. The ability of the NHC ligand to support unusual coordination environments for these metals inspired us to explore the chemistry of copper(I) bonded to various main group elements, leading to the development of new catalytic reactions. The first well-characterized copper(I) boryl complex was synthesized and shown to react with a variety of unsaturated organic substrates. This complex reacts rapidly with CO2 to form CO and a copper(I) borate complex. The boryl complex may be regenerated by treatment of the borate complex with the bis(pinacolato)diboron, (pin)B-B(pin), giving the stable byproduct (pin)B-O-B(pin). The use of a copper(I) alkoxide precatalyst and stoichiometric diboron reagent results in catalytic reduction of CO2, with high turnover numbers (1000 per Cu) and frequencies (100 per Cu in 1 hour) depending on supporting ligand and reaction conditions.

(cont.) Carbon dioxide also inserts into the Cu-Si bond of a copper silyl complex. The resulting complex evolves CO to give a copper siloxide complex. Mesitaldehyde inserts cleanly and selectively into the Cu-B bond of (NHC)CuB(pin), to form a B-O and a copper-carbon bond. This complex reacts with bis(pinacolato)diboron to regenerate (NHC)CuB(pin) and produce an aldehyde diboration product, in which a diboron reagent has been added across the C=O bond of mesitaldehyde. A copper boryl complex with a smaller NHC supporting ligand proved to be a much more effective diboration catalyst and a wide range of aldehydes react cleanly with bis(pinacolato)diboron. The insertion of alkenes into an (NHC)copper(I) boryl affords isolable -boroalkyl complexes in high yields; competition experiments using substituted styrenes show that electron-donating substituents slow the reaction. Although the insertion products are stable at ambient temperature, a P-hydride elimination/reinsertion sequence affords a rearranged a-boroalkyl complex on heating.