Transition-metal nanoclusters: fundamental studies of the factors controlling formation, stabilization, and subsequent catalytic activity

Abstract

Following a critical review of the relevant nanocluster stabilization literature, the research presented herein primarily studies prototype Ir(0)n nanoclusters, including: (i) an evaluation of nanoclusters putatively stabilized by solvents plus the effects of weakly coordinating anions, (ii) an evaluation of five common polymeric nanocluster stabilizers, (iii) the first explicity study of the halide series for their individual efficacies in the formation and stabilization of nanoclusters, (iv) an investigation into the true source of nanocluster stabilization in imidazolium-based ionic liquids, and (v) a demonstration that added imidazolium-based ionic liquids serve to poison catalytically active Ir(0)n nanoclusters in the test reaction of acetone hydrogenation at room temperature and mild H2 pressure. The broad focus of this dissertation concerns the formation and stabilization of transition-metal nanocluster, specifically Ir nanoclusters. The extant literature in the area of transition-metal nanocluster stabilizers is voluminous and in general not to the level of precision that modern science requires. The emphasis on compositionally well-defined nanoclusters and careful studies which follow aim to resolve some of the confusion in the nanocluster stabilization literature. Four detailed investigations on claimed stabilizers—solvents, polymers, halides, and imidazolium-based ionic liquids—have been performed with an emphasis on the formation of multiple alternative hypotheses as the most efficient method for scientific investigation. The approach of multiple alternative hypotheses led to the validation of the DLVO theory of colloidal stability for the specific case of BF4- in high dielectric constant solvents. Additionally, BF4- is shown to be important for nanocluster stabilization even in the presence of polymers or halides. The method of multiple alternative hypotheses also led to the identification of N-heterocyclic carbenes as the likely nanocluster stabilizer generated in ionic liquids. Finally, imidazolium-based ionic liquids have also been probed with regard to their role in catalytic acetone hydrogenation reactions, and it was found that these ionic liquids poison previously active Ir(0)n nanoclusters at room temperature and mild pressure.