Computational Exploration on the N-heterocyclic Carbene (NHC)-for-Phenylacetylene Ligand Exchange in IrAu12 Alloy Clusters

Abstract

N-heterocyclic carbene (NHC) and alkynyl ligands such as phenylacetylene (PA) are recently emerging ligands for the stabilization and functionalization of metal nanoclusters. However, the kinetic possibility for the ligand exchange between these two types of ligands is rarely explored. Inspired by recent progress in using strong acids to selectively remove the PA ligands from [IrAu12(dppe)5(PA)2]⁺ (dppe = 1,2-bis(diphenylphosphino)ethane) cluster to create open active sites, this study employs theoretical simulations to report on the feasibility of acid-induced NHC-for-PA substitution on Ir@Au12 and its derived alloy nanoclusters. By adjusting the properties of acids and central atom doping, the reaction pathways and energy barriers for PA removal in the model cluster [IrAu12(dppe)5(PA)2]⁺ were systematically studied. The results showed that the acid-induced PA removal is the rate-determining step, with the energy barrier strongly influenced by the central metal (Pt < Rh < Ir ~ Pd < Au), the acidity, size and the number of attackable hydrogen atoms in the attacking acid. After the selective PA removal, the incoming NHC ligand can spontaneously adsorb at the exposed Au sites, which is a barrierless process. Moreover, the strong electron-donating ability of NHC ligand increases the electron density on the cluster core, which widens the HOMO-LUMO gap and greatly enhances the electronic stability of NHC-modified clusters. The results of this study sheds light on the exciting opportunity of post-synthetic ligand exchange to create NHC-stabilized superatom nanoclusters, which expands the horizons of predesigned and controllable structure design of novel metal nanoclusters.