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Photochemical Organic Oxidations and Dechlorinations with a μ-Oxo Bridged Heme/Non-Heme Diiron Complex

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journal contribution
posted on 27.12.2004, 00:00 by Ian M. Wasser, H. Christopher Fry, Paul G. Hoertz, Gerald J. Meyer, Kenneth D. Karlin
Steady state and laser flash photolysis studies of the heme/non-heme μ-oxo diiron complex [(6L)FeIIIOFeIIICl]+ (1) have been undertaken. The anaerobic photolysis of benzene solutions of 1 did not result in the buildup of any photoproduct. However, the addition of excess triphenylphosphine resulted in the quantitative photoreduction of 1 to [(6L)FeII···FeIICl]+ (2), with concomitant production by oxo-transfer of 1 equiv of triphenylphosphine oxide. Under aerobic conditions and excess triphenylphosphine, the reaction produces multiple turnovers (∼28) before the diiron complex is degraded. The anaerobic photolysis of tetrahydrofuran (THF) or toluene solutions of 1 likewise results in the buildup of 2. The oxidation products from these reactions included γ-butyrolactone (∼15%) for the reaction in THF and benzaldehyde (∼23%) from the reaction in toluene. In either case, the O-atom which is incorporated into the carbonyl product is derived from dioxygen present under workup or under aerobic photolysis conditions. Transient absorption measurements of low-temperature THF solutions of 1 revealed the presence of an (P)FeII-like {P = tetraaryl porphyrinate dianion} species suggesting that the reactive species is a formal (heme)FeII/FeIVO(non-heme) pair. The non-heme FeIVO is thus most likely responsible for CH bond cleavage and subsequent radical chemistry. The photolysis of 1 in chlorobenzene or 1,2-dichlorobenzene resulted in C−Cl cleavage reactions and the formation of {[(6L)FeIIICl···FeIIICl]2O}2+ (3), with chloride ligands that are derived from solvent dehalogenation chemistry. The resulting organic products are biphenyl trichlorides or biphenyl monochlorides, derived from dichlorobenzene and chlorobenzene, respectively. Similarly, product 3 is obtained by the photolysis of benzene−benzyl chloride solutions of 1; the organic product is benzaldehyde (∼70%). A brief discussion of the dehalogenation chemistry, along with relevant environmental perspectives, is included.