Non-heme FeIV(O) species are proposed to be important intermediates in the biological reduction of dioxygen to water, a reaction that is critical to strategies for long-term solar-energy storage.1 These non-heme FeIV(O) species have been shown capable of exchanging oxygen-atoms with H218O, a process that is central to their detection by mass spectrometry.2,3 In order to accurately detect these species, it is important to have an understanding of the mechanism of oxygen-atom exchange (OAE) between non-heme FeIV(O) centers and H218O. While a mechanism has been proposed for OAE between H218O and synthetic FeIV(O) model compounds, it has not been thoroughly evaluated for a range of FeIV(O) complexes supported by different ligand architectures.2,3In an effort to gain mechanistic insights, the kinetics of OAE between synthetic non-heme FeIV(O) complexes and H218O was studied by electrospray ionization mass spectrometry. These kinetic studies have revealed that the denticity of the supporting ligand has a profound effect on the overall mechanism. The rates of OAE with FeIV(O) complexes supported by tetradentate ligands show a linear dependence on [H218O], suggesting that binding of H218O to the FeIV(O) center is involved in the rate-determining step (rds). In contrast, rates of OAE with FeIV(O) complexes supported by pentadentate ligands display a non-linear dependence on [H218O], suggesting that H218O binding occurs prior to the rds. Eyring activation parameters determined for OAE reactions with FeIV(O) complexes supported by pentadentate ligands reveal minimal enthalpic but significant entropic contributions to the Gibbs energy, ultimately signifying that the transition state involved in the rds is heavily influenced by entropic factors, such as hydrogen-bonding from exogenous water. Eisenberg, et al. Inorg. Chem. 2005  , 44, 6799 � 6801.Seo, et al. Angew. Chem. 2004  , 116, 2471 � 2474.Company, et al. Chem. Eur. J. 2011  , 17, 1622 � 1634.

Mechanistic insights into the oxygen-atom exchange reaction between H218O and non-heme FeIV(O) complexes

248th National Meeting (2014)

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