ACS Award in Organometallic Chemistry

Sponsored by Dow Chemical Co. Foundation

Described by a colleague as "a passionate practitioner" of chemistry, Gerard F. R. Parkin, professor of chemistry at Columbia University, has focused his research career on synthetic, structural, mechanistic, and theoretical studies of organometallic complexes.

In particular, Parkin is noted for his recent work on equilibrium isotope effects (EIEs) for the oxidative addition of H₂ or D₂ to metal centers. Using both theoretical calculations and experiments to examine the temperature dependence of the effects, Parkin and postdoc Kevin E. Janak found an inverse EIE (K_H/K_D <1) at low temperatures that switched to a normal EIE (K_H/K_D >1) at higher temperatures; the EIE then approached unity as the temperature approached infinity. The unusual behavior is due to temperature dependence of the entropy term for the equilibrium.

"Parkin combined the experimental results with high-level theory to extract the most fundamental explanation for the observation of inverse versus normal isotope effects," says William D. Jones, a professor of chemistry at the University of Rochester. "This alone was an impressive accomplishment, but an even greater strength of Parkin's is his ability to communicate the essential features required to understand these effects in a clear and succinct fashion."

Separately, Parkin and coworkers have examined the effect of joining two cyclopentadienyl ligands through an ansa bridge. "It looks like a relatively innocent substituent, but it's much more than that," Parkin says. "It can really make a metal react in totally different ways compared to how it would react in the absence of the bridge." For example, permethylmolybdenocene complexes in which the ligands are joined through a [(CH₃)₂Si] ansa bridge, when photolysed to eliminate H₂, will activate C-H and C-C bonds of molecules such as benzene and acetonitrile, respectively. Without the bridge, the complexes undergo intramolecular deactivation.

That work, Parkin says, has led to a further study of molybdenum complexes to provide insight into molybdenum sulfide-based catalysts for hydrodesulfurization and hydrodenitrogenation reactions for purifying crude oil. "What we're trying to do is define the coordination chemistry of molybdenum with respect to the types of impurities that need to be removed, such as thiophenes," Parkin tells C&EN.

Current projects in the Parkin lab include developing catalysts for mercury detoxification. For example, targeting compounds like dimethylmercury, Parkin has developed a synthetic system to cleave Hg-C bonds and is aiming to develop this system further.

Parkin's work overall "is consistently marked by creativity, rigor, and insight," says John E. Bercaw, a professor of chemistry at California Institute of Technology and Parkin's postdoc adviser from 1985 to 1988. Rochester's Jones adds, "In every case one sees Parkin's intense ability to delve into the most fundamentally important aspects of the study."

Parkin received a B.A. degree in 1981, and M.A. and D.Phil. degrees in 1985, all from Queen's College at Oxford ??University.

In 1993, Parkin received the ACS Award in Pure Chemistry, in particular for a crystallographic study of cis-mer-MoOCl₂(P[CH₃]₂C₆H₅)₃. Researchers had previously thought the complex had two bond-stretch isomers, differing only by the length of the Mo=O bond. Parkin's group instead demonstrated that what had seemed to be a longer Mo=O bond in one isomer was in fact an error, due to contamination with MoCl₃(P[CH₃]₂C₆H₅)₃.

Parkin will present the award address before the Division of Inorganic Chemistry.