This award in the Inorganic, Bioinorganic, and Organometallic Chemistry Program provides continued support to Dr. Robert R. Holmes of the Chemistry Dearptment, University of Massachusetts at Amherst, for research on the reactivity of phosphorus and silicon compounds. During substitution reactions, phosphorus and silicon accommodate five or six groups around the central atom. Highly reactive species of this type will be synthesized, characterized using NMR spectroscopy, and modeled using ab initio methods and the capabilities of the San Diego supercomputer Center. Cyclic pentaoxyphosphoranes and isoelectronic five-coordinate anionic silicates will be the focus of the investigation. Substituents with rings of varying sizes and a variety of donor atoms will be prepared. The phosphorus work is expected to give information on the mechanism of hydrolysis of cyclic adenosine monophosphate, cAMP, while the silicon work models the initial stages of silanol polymerization. In both cases, the goal is to explore the formation of six-coordinate intermediates as the source of the increased reactivity observed for five-coordinate phosphorus and silicon compounds. The reaction rates for hydrolysis and solvolysis of the higher coordinate derivatives will be determined and compared to those of the lower coordinate species. Nitrogen donor atoms will be used in order to mimic the functions of amino acid residues present at the enzymatic active site and assess their role in forming higher coordinate intermediates. This research will determine the role of an alternate reaction pathway in two important cases. The possibility of six-coordinate intermediates affecting both reactivity and geometry in the physiologically important reaction of phosphoryl transfer or in the commercially important polymerization of silanol has not been previously explored.
