Pendant amine bases speed up proton transfers to metals by splitting the barriers

Division of Theoretical Chemistry & Biology, School of Biotechnology, KTH Royal Institute of Technology, 106 91 Stockholm, Sweden.
Chemical Communications (Impact Factor: 6.72). 03/2012; 48(37):4450-2. DOI: 10.1039/c2cc00044j
Source: PubMed

ABSTRACT By using density functional theory on [FeFe]-hydrogenase mimics we deconvolute the function of pendant amine bases in proton transfer to and from the metal center. By dividing the high free energy barrier into one high enthalpy-low entropy barrier and one with a low enthalpy-high entropy, a lower free energy barrier is reached.

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    ABSTRACT: A [FeFe]-hydrogenase model (1) containing a chelating diphosphine ligand with a pendant amine was readily oxidized by Fc(+) (Fc = Cp2Fe) to a (FeFeI)-Fe-II complex ([1](+)), which was isolated at room temperature. The structure of [1](+) with a semibridging CO and a vacant apical site was determined by X-ray crystallography. Complex [1](+) catalytically activates H-2 at 1 atm at 25 C in the presence of excess Fc(+) and P(o-tol)(3). More interestingly, the catalytic activity of [1](+) for H-2 oxidation remains unchanged in the presence of ca. 2% CO. A computational study of the reaction mechanism showed that the most favorable activation free energy involves a rotation of the bridging CO to an apical position followed by activation of H-2 with the help of the internal amine to give a bridging hydride intermediate.
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Ying Wang