Summary of Current Research Projects in the Tolman Group

 

Figure 1. Equilibrium between peroxo- and bisoxodicopper complexes.

  

Figure 2. 1:1 Cu-dioxygen adducts.

Figure 3. Example of the synthesis of novel heterobimetallic bis(oxo) complexes, here from a Cu(I)-Ge(II) precursor.

1. 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). Novel heterobimetallic bis(oxo) complexes also have been prepared (Figure 3). Current work is focused on further exploring the reactivity of the 1:1 adducts with organic and metal-containing substrates, generating new adducts with different structures and properties, and developing syntheses of multinuclear complexes as models of proposed intermediates in catalysis by the multicopper oxidases and oxygenases.

Revelant References

   Figure 4. Models of (left) type 1 and (right) CuA electron transfer sites.
 
 
Figure 5. Copper-sulfur complexes supported by N-donor ligands.

2. 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 (Figure 5). Current work is focused on further exploring copper-sulfur chemistry with a wide variety of N-donor ligands.

Revelant Reference

    Figure 6. Carboxylate ligands used to build novel nonheme iron complexes.

Figure 7. A diiron(II) complex that forms a dioxygen adduct that reacts with phosphines via a mechanism that involves substrate coordination.

3. Sterically Hindered Carboxylates for Modeling Nonheme Iron Biosites (Joint with L. Que)

Nonheme iron sites in proteins that feature substantial carboxylate ligation are of special interest due to the significance of the reactions they perform and the complexity of their structures and mechanisms of action. In a project jointly directed with Professor Larry Que, we have focused on using sterically hindered carboxylates (Figure 6) in which interligand steric interactions are relied upon to induce low coordination numbers and nuclearities in derived complexes, with hydrophobic shielding effects of the large organic substituents acting to stabilize reactive species and influence redox properties. In fact, by using these types of ligands, a number  of novel Fe(II) compounds have been isolated, some of which react with dioxygen to yield reactive intermediates with unusual spectroscopic features and/or reactivity (Figure 7). Current work is focused on extending the use of ligands already proven to be effective in order to model other types of metalloprotein active sites, as well on the development of new types of hindered ligand systems.

Revelant References

Figure 8. Routes to new polymers from renewable resource starting materials.

4. New Catalysts for the Polymerization of Cyclic Esters (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 8. Current work is focused on further new catalyst development and expanding the repertoire of materials derived from bioderived cyclic esters.

Revelant References