Finding and Exploiting New Four-Electron Donor Ligands

This research project addresses the activation of small molecules by promoting unusual four-electron donation from eta-2 ligands to a single metal center. The goal is to build monomeric metal reagents with electronic and steric properties that favor four-electron donation from either both filled pi orbitals of triple bonds or from one lone pair and the original pi bond of aldehydes, ketones and imines, building from the hypothesis that metal fragments can be designed to promote unprecedented bonding interactions with small molecules. Architectural variations of these metal ligand fragments will probe the requirements for promoting four-electron donation from simple ligands. Selective binding of nitriles through the pi system will be explored, based on the known analogy between four-electron donor alkyne ligands and eta-2 nitrile ligands and focusing initially on the design of a fragment that will leave one vacant site in a d4 octahedron so that four electrons will be required to conform to the eighteen electron rule. Binding alkynes will provide a preliminary screen for metal reagents, and promising metal ligand combinations will be utilized to pursue pi-bound nitrile complexes. In addition to triple bonds, other potential four-electron donor ligands include eta-2-coordinated aldehydes, ketones, imines, ketenes and ketenimines. If the metal moiety seeks four electrons from a single ligand, is it possible to bring both the pi electrons of the original double bond and a lone pair on the heteroatom of the substrate into a bonding relationship with a single metal center? This will require substantial reorganization of the aldehye, ketone, or imine geometry upon coordination, but it promises to significantly alter the reactivity of these organic functional groups. The reactivity of these metal ligand moieties may differ dramatically from the reactivity displayed by simple Lewis acid adducts of these groups.

With this award, the Inorganic, Bioinorganic, and Organometallic Chemistry Program is supporting the research of Professor Maurice S. Brookhart, of the Department of Chemistry at the University of North Carolina at Chapel Hill. Professor Brookhart and his students are studying ways to design specific interactions between chosen organic molecules and metal centers that may effect catalytic transformations of these bound organic molecules. In addition to providing fundamental information about the nature of metal-organic bonding, these studies offer promise for the development of efficient methods for the catalytic conversion of organic molecules into more complex or value-added products.