Activation Mechanism of Nickel(0) N-Heterocyclic Carbene Catalysts Stabilized by Fumarate Ligands

Nickel(0) catalysts of N-heterocyclic carbenes (NHCs) that are stabilized by electronic deficient alkenes possess desirable properties of air tolerance and ease of handling while also retaining high catalytic activities. Since catalyst stability often comes at the expense of catalytic activity, we have undertaken a detailed study of the activation mechanism of a new IMes-nickel(0) catalyst stabilized by di-(o-tolyl) fumarate that converts the stable pre-catalyst form into a catalytically active species. Computational evaluation provided evidence against a simple ligand exchange as the activation mechanism, and a stoichiometric activation process that covalently modifies the stabilizing ligand was identified. A detailed computational picture for the activation process was developed, with predictive insights that explain the catalyst features that lead to both active and inactive precatalysts.