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Redox Chemistry of Noninnocent Quinones Annulated to 2Fe2S Cores

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posted on 11.11.2013, 00:00 by Gabriel B. Hall, Jinzhu Chen, Charles A. Mebi, Noriko Okumura, Matthew T. Swenson, Stephanie E. Ossowski, Uzma I. Zakai, Gary S. Nichol, Dennis L. Lichtenberger, Dennis H. Evans, Richard S. Glass
Noninnocent ligands that are electronically coupled to active catalytic sites can influence the redox behavior of the catalysts. A series of (μ-dithiolato)­Fe2(CO)6 complexes, in which the sulfur atoms of the μ-dithiolato ligand are bridged by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-benzoquinones, 1,4-naphthoquinone, or 1,4-anthraquinone, have been synthesized and characterized. In addition, the bis-phosphine complex derived from the 1,4-naphthoquinone-bridged complex has also been prepared and characterized. Cyclic voltammetry of these complexes shows two reversible one-electron reductions at potentials 0.2 to 0.5 V less negative than their corresponding parent quinones. In the presence of acetic acid two-electron reductions of the complexes result in conversion of the quinones to hydroquinone moieties. EPR spectroscopic and computational studies of the one-electron-reduced complexes show electron delocalization from the semiquinones to the 2Fe2S moieties, thereby revealing the “noninnocent” behavior of these ligands with these catalysts.