Department of Chemistry

College of Science

---

Current Activities

---

The evolutionarily perfected catalytic sites in most hydrogenase enzymes are comprised of two base metals, bridged by sulfurs and buried deeply in the proteins, that function in hydrogen production from protons and electrons, or, according to the reverse reaction, to use H₂ as a fuel source. We strive to understand how such molecular constructions of nickel and iron, or two iron atoms, ligated by carbon monoxide and cyanide so as to be surprisingly familiar to organometallic chemists, are able to efficiently compete with platinum in fuel cell assemblies. It is our hypothesis that the [FeFe]H₂ase enzyme has the diiron organometallic trapped in an "entatic" high energy state which is rotated relative to analogous compounds on the chemist's benchtop. Such a rotated state is stabilized in non-symmetric compounds outfitted with electron-donating ligands. Computational studies further suggest the need for steric hindrance at the bridgehead of the S to S linker. Synthetic efforts are focused on such [Fe^IFe^I] compounds which are evaluated as electrocatalysts for H₂ production. At the Fe^IIFe^I/II redox level, H₂ uptake and activation is assayed by isotopic exchange in D₂O/H₂ mixtures. The ultimate goal is the synthesis of base metal, hydrogen-processing catalysts ultimately for replacement of platinum in fuel cell electrodes.

We also pursue the new paradigm for enzyme active site construction that has been discovered in the Acetyl CoA Synthase (ACS) enzyme active site and develop square planar NiN₂S₂ and other MN₂S₂ complexes as a new class of ligands that support organometallic chemistry. We posit that the binuclear makeup of these active sites may be interpreted in terms of metal-dithiolates as ligands to a second, catalytically active metal. This is the basis of design of ACS biomimetics and studies of fundamental ligand properties such as electronic, steric, and hemi-lability effects.

We have shown that the Merrifield synthesis of the small peptides can also produce resin-bound NiN₂S₂ which bind other metals at sulfur in imitation of the ACS enzyme. Our ultimate goal is to take advantage of the ligating properties of the evolutionarily perfected NiN₂S₂ ligands for the production of resin supported, site-isolated organometallic catalysts.

Our research is augmented by interface with Professor Michael Hall, a computational chemist who has interest in computing reaction mechanisms for enzymes, many times anchoring those calculations in spectroscopic properties of the small molecule analogues which we make. We also collaborate with Professor Manuel Soriaga, an electrochemist with interest in fuel cell development. Students in the MYD group become knowledgeable in inorganic and organometallic synthesis, bioorganometallic chemical processes, x-ray crystallography, electrochemical analysis and various spectroscopies.

---

Selected Publications

---

Computational Definition of a Mixed Valent Fe(II)Fe(I) Model of the [FeFe]Hydrogenase Active Site Resting State, Christine M. Thomas, Marcetta Y. Darensbourg, and Michael B. Hall, J. Biol. Inorg. Chem. 2007, 101, 1752-1757. (Ed Steifel Memorium)

Sulfur K-Edge XAS and DFT Studies on Ni^II Complexes with Oxidized Thiolate Ligands: Implications for the Roles of Oxidized Thiolates in the Active Sites of Fe and Co Nitrile Hydratase, Abhishek Dey, Stephen P. Jeffrey, Marcetta Darensbourg, Keith O. Hodgson, Britt Hedman, Edward I. Solomon Inorg. Chem. 2007, 46, 4989-4996.

Regioselective ¹²CO/¹³CO Exchange Activity of Mixed Valent Fe(II)Fe(I) Models of the H_ox State of [FeFe]-Hydrogenase, Christine M. Thomas, Tianbiao Liu, Michael B. Hall, and Marcetta Y. Darensbourg, Chem. Commun. 2008, 1563-1565.

Electronic Effects of (N₂S₂)M(NO) Complexes (M = Fe, Co) as Metallodithiolate Ligands, Jennifer L. Hess, Harold L. Conder, Kayla N. Green, Marcetta Y. Darensbourg, Inorg. Chem. 2008, 47, 2056-2063.

Refining the Active Site Structure of Iron-Iron Hydrogenase Using Computational Infrared Spectroscopy, Jesse W. Tye, Marcetta Y. Darensbourg, Michael B. Hall, Inorg. Chem. 2008, 47, 2380-2388.

Thiolate Bridging and Metal Exchange in Adducts of a Zinc Finger Model and Pt(II) Complexes: Biomimetic Studies of Protein/Pt/DNA Interactions, Elky Almaraz, Queite de Paula, Qin Liu, Joseph H. Reibenspies, Marcetta Y. Darensbourg, Nicholas Farrell J. Am. Chem. Soc. 2008, 130, 6272-6280.

Series of Mixed Valent Fe(II)Fe(I) Complexes That Model the H_ox State of [FeFe]Hydrogenase: Redox Properties, Density-Functional Theory Investigation, and Reactivities with Extrinsic CO, Christine M. Thomas, Tianbiao Liu, Michael B. Hall, and Marcetta Y. Darensbourg, Inorg. Chem. 2008, 47, 7009-7024.