Joseph A. Heppert

norganic chemistry: catalysis, methathesis, polymers, stereospecific reactions, hydrogen bonding, liquid crystals, inorganic/organic hybrid materials and Science Education: effects of inquiry in science laboratory instruction, effect of research effectiveness of K-12 science instruction, attrition at the University/Community College transition.

Professor Heppert's research group concentrates on both the catalytic chemistry and potential materials application of complexes of group 4, 5 and 6 metals. Problems currently under investigation include catalytic asymmetric methodology, hydrocarbon and hydrogel polymer synthesis, a search for new transition metal derived materials and fuels chemistry.Students acquire a range of research skills from modern synthetic methods to techniques for the characterization and handling of novel materials.

Stable Ruthenium Carbides. Transition metal carbides are important intermediates in the production of synthetic hydrocarbons through the Fischer-Tropsch process. Fischer-Tropsch synthesis employs group 8 metal catalysts at high temperature to dissociate carbon monoxide, forming surface-bound carbide (C) units. The catalyst then promotes the hydrogenation of these units, resulting in the “polymerization” methylene units into high purity hydrocarbons. Homogeneous models of most of the intermediates in this process have been known for some time, and the chemical behavior of these intermediates has been studied.

One critical exception is terminal transition metal carbide complexes containing the M=C: functional group, which has only been isolated during the past two years.The Cummins and Templeton groups have both identified anionic group 6 carbides that appear to be strong Brønsted bases.

Our group has isolated and characterized the first class of stable neutral terminal metal carbides. These compounds, which could be an important link in understanding the Fischer-Tropsch process, exhibit an extensive chemistry beyond Brønsted basicity and are soluble in a range of non-polar solvents. We are currently investigating the mechanism of formation and reactivity of these compounds.

Terminal ruthenium carbides are formed through metathesis reactions involving ruthenium alkylidenes and methylene cyclopropenes. The initial products of this reaction are styrene and a ruthenium cyclopropylidene complex. The cyclopropylidene intermediate extrudes dimethylfumarate leaving the ruthenium carbide functional group. The ruthenium carbon triple bond is a characteristically short 1.66 Å, while the chemical shift of the carbide carbon is highly deshielded at about 490 ppm. Our group has prepared several classes of terminal carbides, including systems with two phosphine ligands and systems bearing a phosphine ligand and a dihydroimidazol ligand.

Systems bearing pyridine ligands do not react with methylene cyclopropenes to produce isolable carbide complexes. Rather, we have isolated a ruthenium vinylidene that results from the isomerization of a proposed intermediate in the cyclopropylidene cleavage reaction. We are still studying whether this product supports one proposed mechanism in which the cyclopropylidene decomposes via stepwise cleavage.

The carbide complexes can also form coordination complexes. (Cy3P)2Cl2Ru(≡C:) reacts with copper triflate to generate a Cu2Cl2 bridged ruthenium dimer. This is the first synthesis of a molecular copper carbide complex and represents one of the few rational routes for the preparation of binary transition metal carbides. The central chloride-bridged copper dimer provides a 3 Å spacer, alleviating steric contact between the tricyclohexyl phosphine ligands bonded to the opposing ruthenium centers. We are continuing to study the coordination chemistry of the carbide complexes.