N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles.

Abstract

N-heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nano-materials chemistry as highly stabilizing ligands enabling formation of strong and covalent C-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists in the reduction of a preformed NHC-Au(I) complex with a large excess of a reducing agent rendering the particle size difficult to control. In this paper, we report on the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by reacting a commercially-available gold(I) precursor with an easy-to-synthesize NHC-BH3reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2Cl as a byproduct. This strategy garners efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation in the range of 4.9 ± 0.9 nm to 10.0 ± 2.7 nm. Our strategy also allows the unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without engaging extensive purification or size selectivity techniques and are stable over months.