Our research is focused on two areas of light-induced chemistry: the photochemistry of metal-metal bonded organometallic compounds and the photo­chemistry of ferric carboxylate complexes. These studies are ulti­mately related to efforts to use the sun's energy to drive useful chemical processes.

We are studying the photochemistry of metal-metal bonded compounds, with a special focus on the generation of odd-electron (radical) organometallic intermediates using visible light. Radical intermediates are involved in a wide variety of important reactions, including polymerization and atom transfer. Our goal is to understand the factors affecting the ease of generation and reactivity of organometallic radicals. We have discovered a very efficient thiolate (RS) group transfer reaction in the photoreaction of (C5H5)2M2(CO)6 [M=Mo, W] with organic disulfides and thiols. In this research, we use a range of preparative and spectroscopic techniques. These include glove box and vacuum line use for air-sensitive compounds, various chromatographic methods for separation, along with multinuclear magnetic resonance, fourier-transform infrared, UV-VIS, and mass spectroscopic methods. Our continuous photolysis studies of the photoreaction with disulfides show that clean, efficient M–M bond cleavage reactions form metal thiolate complexes. These products are mononuclear and monodentate, in contrast to the bridging or chelating commonly found in the products of thermal reactions. The quantum yields are high, so the reactions are useful in synthetic work.

Cyclic group 14 carbene analogs appear to react with the same organometallic dimers by a different, more efficient pathway. Instead of mononuclear products, bridged M-E-M systems are formed. The quantum yields are nearly twice those seen in traditional radical trapping reactions of metal dimers. Heterometallic dimers give heterodimetallic products in larger amounts than would be expected from free radical intermediates.

In another project, we are looking at the chemistry of a-hydroxy polycarboxylate complexes of iron(III). Irradiation by near-UV light causes reduction of Fe(III) to Fe(II), coupled to a two-electron oxidative decarboxylation of a ligand, e.g., citric acid to acetone dicarboxylic acid. Uncomplexed aquo ion Fe³⁺(aq) is the predominant species in strongly acid solution (pH < 1.0). At slightly higher pH values, a 1:1 mononuclear iron–citrate (FeCit) complex dominates. At still higher values (pH 2–4) a dinuclear 1:1 complex is found and is responsible for the observed photo-reactivity. We are exploring the effect of ligand structure and conformation on the efficiency of this and related reactions.

Electronic structure calculations on these transition metal-containing systems are being done using Fenske-Hall and density functional methods. Both methods allow us better to understand ground-state properties and their trends in a series of related complexes. The latter method will also allow us to probe excited state properties, including transition energies and absorption cross-sections for their formation. We will be applying these methods to metal complexes to better understand the formation and partitioning of the different excited states formed by light absorption.

REPRESENTATIVE PUBLICATIONS