Joseph Topich
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Associate professor |
Education
B.A., Columbia University
Ph.D., Case Western Reserve University
Research interests
Molybdenum
The research program in our group is directed toward the synthesis, characterization and study of transition metal coordination complexes as bioinorganic models. Our current interests have focused on molybdenum. At present Mo is the only second-row transition metal known to be essential for living systems. Mo is at the active site of a number of important enzymes, including xanthine oxidase, sulfite oxidase, aldehyde oxidase and nitrate reductase. Biochemical studies indicate that there is a Mo-containing cofactor that is common to all of these enzymes. Characterization of the Mo-cofactor, particularly by Mo EXAFS spectroscopy, has shown that sulfite oxidase and nitrate reductase contain the Mo(VI)O2 and Mo(IV)O units in the oxidized and reduced forms, respectively. In addition there is evidence that at least two thiolate sulfurs are also coordinated to the Mo. Relevant bioinorganic models for the Mo-cofactor should 1) attempt to duplicate the cofactor ligand donor atom set and 2) carry out reactions with physiological or “pseudo”-physiological substrates.
Our work specifically involves coordination complexes of Mo in the +4, +5 and +6 oxidation states where ligands have been carefully designed to control chemical reactivity. Ligands are prepared that incorporate O, N and/or S donor atoms. In one study we have shown that stable cis-dioxomolybdenum(VI) coordination complexes can be prepared with tridentate Schiff base ligands. In DMF solution these Mo(VI) complexes undergo an oxygen atom transfer reaction when the substrate is an organophosphine. The reduced Mo(IV) complex and the oxidized organophosphine are initial products of this reaction. A kinetic study showed that the rate constants for the oxygen atom transfer reaction varied systematically with ligand variations. It was found that a delocalized ligand system including sulfur as a donor atom facilitated the Mo(VI) oxygen atom transfer. A careful FTIR examination of the Mo=O stretching vibrations in DMF solution shows a correlation between the Mo=O stretching frequency and the rate constant for oxygen atom transfer within a series of Mo complexes. This observation provides some insight into the mechanism of the oxygen atom transfer reaction.
In order to incorporate additional sulfurs as ligand donor atoms, we are preparing sterically bulky tripodal tetradentate ligands that will contain up to three sulfur atoms in addition to nitrogen. Tripodal tetradentate ligands are used to prepare Mo(VI) complexes and they enable us to carry out a systematic study of the effects on chemical properties of the different ligand donor atoms. In addition, the lability of ligand donor groups trans to an oxo oxygen allows competitive solvent or substrate binding. Cyclic voltammetry was performed on these Mo(VI) complexes in both DMF and CH2Cl2 solutions. The electrochemical results are consistent with that expected for a sulfur, oxygen, nitrogen ligand donor atom substitution pattern.
Much of the recent work on bioinorganic Mo models has been done in nonaqueous solvents. Mechanistic studies on the Mo cofactor show that water plays an important role in the reaction scheme. To more closely approximate the chemical environment of the Mo cofactor in model studies, we are preparing appropriately derivatized ligands and their corresponding Mo(VI) complexes that are water soluble. This allows us to examine the reactivity and electrochemistry of these Mo complexes in an aqueous environment. Under these experimental conditions, pH-dependent processes, such as those seen with sulfite oxidase and nitrate reductase, can be studied.
Publications
- J. Topich and J. O. Bachert III, "Solution IR Spectroscopic Studies of cis- Dioxomolybdenum(VI) Complexes," Inorg. Chem., 31, 511 (1992).
- C. J. Cook and J. Topich, "Synthesis and Characterization of cis- Dioxomolybdenum(VI) Complexes with Sterically Bulky Tripodal Tetradentate Ligands," Inorg. Chim. Acta, 144, 81 (1988).
- J. Topich and J. T. Lyon III, "Ligand Control of cis-Dioxomolybdenum(VI) Redox Chemistry: Kinetic and Activation Parameter Data for Oxygen Atom Transfer," Inorg. Chem., 23, 3202 (1984).
