David Goldberg (Johns Hopkins University)

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Location: B01 McCourtney Hall

Abstract:  Small molecule activation is at the heart of chemistry and biology. Some of the most important challenges in human health and disease, energy transduction and storage, and environmental sustainability, involve the transition metal-mediated redox activation of small molecules (e.g. O2, N2, CO2, NO, H2S, CH4). This seminar will focus on our laboratory’s recent efforts in the design and synthesis of new iron coordination complexes that bind and activate O2, and allow us access to key M/O2 intermediates. We employ ligand design, assisted by computational modeling, to construct both heme- and nonheme-related transition metal complexes in which the first and second coordination spheres are tuned for specific small molecule targets. Nature has devised a remarkable tool box of metalloenzymes to carry out reactions such as O2 activation, and part of our efforts have led to new information on the mechanistic principles by which these enzymes may operate. In one example, a heme-related, iron(II) corrole complex activated O2 to give a rare iron(III)-superoxo complex, which reacts with H-atom transfer and indole substrates via biomimetic oxidation pathways. In another example, a nonheme, sulfur-ligated iron(II) complex activates a single molecule of O2to give sequential intermediates (FeIII2(µ-O2), FeIV(O), and FeIII(OH) species) which were characterized by a range of methods (UV-vis, Mössbauer, EPR, resonance Raman, XAS). These results provide some insights regarding possible mechanisms for nonheme iron oxygenases. A new series of nonheme iron complexes that include the first cis-ligated (FeIII(OH)(X) (X = halide, thiolate) complexes is also described, and their preferential radical transfer reactivity (OH• versus X•) toward carbon radicals (R•) is discussed in light of the product-determining, radical rebound step for nonheme iron hydroxylases, halogenases and related enzymes. Our results also suggest strategies in certain cases for the design and construction of new iron oxidation catalysts.

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