The research area of longest standing concerns complexes that contain metal-carbon multiple bonds, usually alkylidene complexes (M=CHR) where the metal is usually W or Mo in its highest possible oxidation state. The primary goal is to synthesize and characterize complexes that are catalysts for the metathesis of olefins and that have predictable and controllable activities. The most important present application is the catalytic synthesis of small enantiomerically pure organic molecules from racemic precursors, e.g., “desymmetrization” of a triene with an enantiomerically pure catalyst to give a cyclic product as one enantiomer. Asymmetric metathesis studies rely on the development of new catalysts that contain enantiomerically pure ligands such as 3,3′-disubstituted biphenolates and binaptholates, and facile new routes to them. He also is interested in the synthesis and development of new acetylene metathesis catalysts, and in designing new ligands and exploring new catalytic reactions involving organometallic species.
A second area of research concerns the reduction of dinitrogen in a well-defined manner, the ultimate goal being activation and reduction of dinitrogen using protons and electrons. Molybdenum complexes that contain a triamidoamine ligand with 3,5-[2,4,6-i-Pr3C6H2]2C6H3 (hexaisopropylterphenyl) groups on the amido nitrogens are of special interest since many derivatives that one might expect to be involved in a catalytic reduction of dinitrogen at a single metal center can be prepared and characterized. Moreover, it has now been shown for the first time that dinitrogen can reduced to ammonia catalytically under carefully controlled conditions with an efficiency in reducing equivalents of ~65%. This is the first time that dinitrogen has been reduced catalytically in a knowledgeable manner at room temperature and pressure. These results suggest that the single molybdenum center in FeMo nitrogenase may be the site of the reduction of dinitrogen to ammonia. A wide variety of related triamido/amine ligands for Mo have been prepared and new diamido/bisdonor ligands are being evaluated for vanadium dinitrogen chemistry.
A third area of research concerns the controlled synthesis of specialty polymers with alkylidene catalysts by either ring-opening metathesis polymerization (ROMP) of cyclic olefins (e.g., norbornenes or cyclobutenes) or by cyclopolymerization of 1,6-heptadiynes to yield polyenes. ROMP polymerizations are employed for the synthesis of elastomeric triblock copolymers, in particular polymerizations that involve bimetallic initiators. Cyclopolymerization of diethyldipropargylmalonate and related species yields polymers that contain five- or six-membered rings incorporated into polyene chains. These polyenes are relatively air stable and dark red to purple. The object is to design catalysts that will yield polymers that contain only five- or six-membered rings in a living fashion and with total control over chain length. It ultimately then will be possible to correlate polymer structure and chain length with physical and other properties characteristic of highly conjugated polyenes.