Summary of Current Research Projects in the Tolman Group

Figure 3. A Cu(I)-ketocarboxylate complex reacts with dioxygen to hydroxylate the appended arene ring, and the calculated structure of an intermediate.

Synthetic Modeling of Copper Protein Active Sites: Dioxygen Activation

Numerous copper-containing proteins use the oxidizing power of dioxygen to perform chemically interesting and important transformations, including the selective hydroxylation of hydrocarbons. A goal of our research is to understand on a fundamental chemical level how these processes occur through the detailed study of small molecule analogs of the metalloprotein active sites using a combination of experimental and theoretical methods. The specific questions we ask include: What are the structures, physicochemical properties, and reactivities of species resulting from the interaction of Cu(I) complexes with dioxygen? How do supporting ligand electronic and structural features influence the course of the Cu(I)/dioxygen reactions, as well as the interconversions among the various types of resulting reactive species?

In previous work, we characterized a new type of Cu/dioxygen intermediate that contains a bis(oxo)dicopper(III,III) core, and showed that it can equlibrate with a side-on (peroxo)dicopper(II,II) isomer in a process that models how the dioxygen O-O bond may be broken and formed at dimetal active sites in biology and catalysis (Figure 1). More recently, we characterized monomeric 1:1 species (Figure 2) that model reactive intermediates postulated to be involved in dioxygen activation at single copper sites in proteins (e.g. dopamine beta-monooxygenase). Current work focuses on evaluating ligand effects on copper-oxygen reactivity and on targeting putative copper-oxo species proposed as reaction intermediates (Figure 3).

Funding: National Institutes of Health

Revelant References

Synthetic Modeling of Copper Protein Active Sites: Cu-Sulfur Centers

Copper-sulfur centers in biology include the binuclear, delocalized mixed-valence "CuA" and monocopper "Type 1" electron transfer sites, as well as a novel tetracopper-sulfide cluster in nitrous oxide reductase. The Type 1 and CuA  sites shuttle electrons via redox reactions that occur rapidly, often over long distances, and at widely varying potentials. As a consequence of their unusual structural and spectroscopic properties and their importance in biological electron transfer, we have been interested in preparing synthetic analogs in order to assess how their structures, spectral properties, and ET function are related. We are also actively probing N-donor ligand supported copper-sulfur chemistry, with the ultimate goal of understanding the role of sulfide in the structure and function of the nitrous oxide reductase.

We have prepared accurate models of the CuA center and several key members of the type 1 class of active sites (Figure 4). Detailed spectroscopic studies performed in collaboration with the Solomon group (Stanford) have provided new insights into the electronic structures of these molecules and their protein counterparts. We have prepared several new copper-sulfur complexes, including those with S-S bonds and a novel delocalized mixed valent tricopper-sulfide that reacts with nitrous oxide (Figure 5). Current work is focused on further exploring copper-sulfur chemistry with a wide variety of N-donor ligands.

Funding: National Institutes of Health

Revelant Reference

Figure 6. Routes to new polymers from renewable resource starting materials.

Figure 7. Synthesis of a triblock copolymer derived from renewable resources that behaves as a thermoplastic elastomer.

New Catalysts for the Synthesis of Polymers from Renewable Resources (Joint with M. Hillmyer)

The use of biorenewable resource starting  materials for the preparation of commodity polymers will lessen the undesirable environmental impact of the ubiquitous petroleum-based derivatives such as polyethylene and polypropylene. Thus, we seek new and innovative methods for the conversion of molecules provided by plants into compounds that can be catalytically transformed into new, useful, and sustainable polyesters. To address this objective, we combine efforts in monomer synthesis, catalyst design and development through detailed mechanistic study and computational modeling, and polymer chemistry. Mechanistic insights into the conversion of lactide (LA) to polylactide (PLA) by new, highly active zinc-alkoxide catalysts have been obtained. Using such catalysts, a variety of new polymers have been prepared from renewable cyclic esters, as summarized in Figure 6 . Current work is focused on further new catalyst development and expanding the repertoire of materials derived from bioderived cyclic esters (Figure 7).

Funding: National Science Foundation and the University of Minnesota Institute for Renewable Energy and the Environment

Revelant References