Title: Heme and Non-heme Transition Metal Complexes: Activation of O2 and NO, and the Reactivity of High-Valent M(O)/M(OH) Species
The activation of dioxygen, nitric oxide and other small molecules is critical for a wide range of life processes. Transition metals are uniquely suited to handle the challenges involved with the multielectron, multiproton transformations that are required for these processes. Metalloproteins have evolved to bind and stabilize transition metal ions in specific structural and electronic environments that are designed to facilitate these types of reactions. This presentation describes our ongoing efforts to construct small, synthetic analogs of both heme and nonheme metalloenzyme active sites, with a focus on iron and manganese chemistry. We examine the influence of both first- and second-coordination sphere elements on the structures and properties of Fe and Mn centers, concentrating on their reactivity toward O2 and NO. Questions regarding structure and function will be addressed, emphasizing the study of key, proposed bond-making and bond-breaking steps at the metal center. Heme-related systems include the development of ring-contracted porphyrinoid ligands (corroles, corrolazines) for the stabilization of high-valent metal-oxo and metal-hydroxo species (e.g. MnV(O), MnIV(OH), FeIV(O)(π-cation-radical), FeIV(OH)). These well-defined high-valent molecules mediate fundamental reactions of biological significance, including H-atom transfer (HAT), proton-coupled electron-transfer (PCET), oxygen-atom-transfer (OAT), and radical “rebound”. Nonheme-related systems include mononuclear, thiolate-ligated Fe complexes as models for sulfur-ligated Fe enzymes (thiol dioxygenases, isopenicillin N synthase, EgtB/OvoA). Some of these complexes are shown to activate O2 for selective S-oxygenation chemistry. Their spectroscopic features (Mössbauer, EPR, NMR, rR) provide benchmarks for analysis of enzyme data, and provide evidence to support a novel mechanism for cysteine dioxygenase (CDO), a mammalian thiol dioxygenase. Recent work will also be presented on nonheme Fe complexes that activate NO for oxidation and reduction, including a mononuclear species that reduces NO to N2O as seen in nitric oxide reductases. In parallel, the fundamental rules that govern the reactivity of these systems are being determined and employed in the design of synthetic oxidation and NOx reduction catalysts.