Pendant amine bases speed up proton transfers to metals by splitting the barriers
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 ((+)), which was isolated at room temperature. The structure of (+) with a semibridging CO and a vacant apical site was determined by X-ray crystallography. Complex (+) 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 (+) 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.Journal of the American Chemical Society 09/2013; 135(37). DOI:10.1021/ja408376t · 11.44 Impact Factor
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ABSTRACT: Generation of hydrogen by reduction of two protons by two electrons can be catalysed by molecular electrocatalysts. Determination of the thermodynamic driving force for elimination of H2 from molecular complexes is important for the rational design of molecular electrocatalysts, and allows the design of metal complexes of abundant, inexpensive metals rather than precious metals ("Cheap Metals for Noble Tasks"). The rate of H2 evolution can be dramatically accelerated by incorporating pendant amines into diphosphine ligands. These pendant amines in the second coordination sphere function as protons relays, accelerating intramolecular and intermolecular proton transfer reactions. The thermodynamics of hydride transfer from metal hydrides and the acidity of protonated pendant amines (pKa of N-H) contribute to the thermodynamics of elimination of H2; both of the hydricity and acidity can be systematically varied by changing the substituents on the ligands. A series of Ni(ii) electrocatalysts with pendant amines have been developed. In addition to the thermochemical considerations, the catalytic rate is strongly influenced by the ability to deliver protons to the correct location of the pendant amine. Protonation of the amine endo to the metal leads to the N-H being positioned appropriately to favor rapid heterocoupling with the M-H. Designing ligands that include proton relays that are properly positioned and thermodynamically tuned is a key principle for molecular electrocatalysts for H2 production as well as for other multi-proton, multi-electron reactions important for energy conversions.Chemical Communications 01/2014; 50(24). DOI:10.1039/c3cc46135a · 6.72 Impact Factor
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ABSTRACT: The ability to control proton translocation is essential for optimizing electrocatalytic reductions in acidic solutions. We have synthesized a series of new hangman iron porphyrins with hanging groups of differing proton-donating abilities and evaluated their electrocatalytic hydrogen-evolving ability using foot-of-the-wave analysis. In the presence of excess triphenylphosphine, iron porphyrins initiate proton reduction electrocatalysis upon reduction to Fe-I. By changing the proton-donating ability of the hanging group, we can affect the rate of catalysis by nearly 3 orders of magnitude. The presence of an acid/base moiety in the second coordination sphere results in a marked increase in turnover frequency when extrapolated to zero overpotential.Organometallics 09/2014; 33(18):4994-5001. DOI:10.1021/om500300e · 4.25 Impact Factor