A boron, nitrogen-containing heterocyclic carbene (BNC) as a redox active ligand: synthesis and characterization of a lithium BNC-aurate complex

Abstract

Stabilization of low oxidation gold anions as aurate or auride by organic ligands has long been a synthetic challenge, owing to the proneness of low-valent gold centres to cluster. Despite being the most electronegative metal, isolable gold(I) aurate complexes have only been obtained from a few σ-withdrawing organo- and organo-main group ligands. Stabilization of highly-reduced gold complexes by π-modulating redox active ligands has only been achieved by cyclic (amino)(alkyl)carbene (CAAC), which is limited to 1e⁻-reduction to form neutral gold(0) complexes. This work reports a simple modular synthesis of a boron, nitrogen-containing heterocyclic carbene (^ClBNC) at a gold(I) center through metal-assisted coupling between azadiboriridine and isocyanides. The anionic electrophilic ^ClBNC ligand in the gold(I) complex [(^ClBNC)AuPMe₃] (3a and 3b) allows a 2e⁻-reduction to form the first η¹-carbene aurate complex [(BNC)AuPMe₃]Li(DME) (5a, DME = dimethoxyethane). Single crystal crystallographic analysis and computational studies of these complexes revealed a highly π-withdrawing character of the neutral 4π B,N-heterocyclic carbene (BNC) moiety and a 6π weakly aromatic character with π-donating properties to the gold(I) fragment in its reduced form, showcasing the first cyclic carbene ligand that allows electronic tunability between π-withdrawing (Fischer-type)- and π-donating (Schrock-type) properties.