Controlling Nickel Catalyst Reactivity by Controlling Molecular Catalyst Speciation

Controlling molecular catalyst speciation under reaction conditions can be an effective approach for extending catalyst lifetimes and controlling selectivity. Anchoring molecular catalysts to solid supports, to generate hybrid catalysts, is a means for preventing bimolecular catalyst interactions and preventing the formation of catalyst species such as dimers and multimers. Here we compare a series of different anchoring motifs for molecular nickel catalysts bound to metal oxide supports. The catalysts are anchored to the supports through functional groups on the ligand framework, and carboxylate, ester, and silanol groups are compared in terms of synthetic ease, anchoring stability, catalyst loading on the surface, and catalytic behavior with respect to Suzuki–Miyaura cross-coupling. The results show that covalent bonds between the molecular catalysts and the oxide support lead to increased catalyst surface loadings and higher surface loading, which helps avoid mass transport limitations during catalysis. In addition, the metal–ester-bound catalysts exhibit support-dependent reactivity, which is unique and different from the carboxylate and silanol anchoring groups. Infrared and X-ray photoelectron spectroscopy are used to characterize the molecular nature of the catalysts, and reactivity trends show that the covalent bonding of the catalysts to the surface controls catalyst speciation with respect to geometry and valency, which influences catalytic activity.