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

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 sigma-withdrawing organo- and organo-main group ligands. Stabilization of highly-reduced gold complexes by pi-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 ((BNC)-B-Cl) at a gold(i) center through metalassisted coupling between azadiboriridine and isocyanides. The anionic electrophilic a (BNC)-B-Cl ligand in the gold(i) complex (((BNC)-B-Cl)AuPMe3] (3a and 3b) allows a 2e(-)-reduction to form the first eta(1)-carbene aurate complex [(BNC)AuPMe3]Li(DME) (5a, DME = dimethoxyethane). Single crystal crystallographic analysis and computational studies of these complexes revealed a highly pi-withdrawing character of the neutral 4 pi B, N heterocyclic carbene (BNC) moiety and a 6 pi weakly aromatic character with pi-donating properties to the gold(i) fragment in its reduced form, showcasing the first cyclic carbene ligand that allows electronic tunability between pi-withdrawing (Fischer-type)- and pi-donating (Schrock-type) properties.

Automated summary

Stabilization of low oxidation gold anions as aurate or auride has been a challenge in synthetic chemistry. This study reports a modular synthesis of a boron, nitrogen-containing heterocyclic carbene at a gold center, which allows electronic tunability.