Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl Complexes: A Systematic Study

Abstract

Five novel NHC copper­(I) boryl complexes were synthesized by B–B activation via σ-bond metathesis of symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino diborane(4) derivatives. Despite their low stability, the NHC copper boryl complexes were thoroughly characterized spectroscopically and structurally. Variation of the NHC ligand (ItBu or Me2IiPr) as well as of the boryl ligand (Bpin, Bdmab, or BiPrEn) allowed, for the first time systematically, a study in such complexes of the dependence on steric encumbrance. For sterically more demanding ligand combinations, mononuclear linear complexes were obtained, while with less demanding ligand combinations, dimeric dinuclear complexes with two bridging μ-boryl ligands were obtained, exhibiting extremely short Cu···Cu distances (<2.26 Å). The decomposition of all these complexes was found to proceed via a common pathway, leading ultimately to elemental copper, the free NHC ligand, and the respective symmetrical diborane(4) derivative. The rate of decomposition depended strongly on the steric encumbrance of the individual complex. Two apparently low-valent copper clusters were observed and suggested to be relevant species with respect to the reductive decomposition of the copper­(I) boryl complexes