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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: 6.38). 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) contain-ing a chelating diphosphine ligand with a pendant amine was readily oxidized by Fc + (Fc = Cp 2 Fe) to a Fe II Fe I 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. [FeFe]-hydrogenases ([FeFe]-H 2 ases) are enzymes that catalyze both proton reduction and H 2 oxidation. These reactions are closely related to energy storage by production of H 2 from water splitting and energy release by H−H bond cleavage in a fuel cell. With a strong desire to replace the commonly used Pt by earth-abundant metal-based catalysts, structural and functional mimicking of the [FeFe]-H 2 ase active site has attracted extensive attention since the crystal structure of [FeFe]-H 2 ases was unveiled. 1 The key structural factors of the [FeFe]-H 2 ase active site are a diiron dithiolate core, an amine cofactor in the S-to-S bridge, and a 4Fe4S cluster tethered to the diiron core through a cysteine residue (Figure 1). Direct biophysical evidence, experimental results, and theoretical calculations revealed that the amine cofactor acts as a shuttle for protons being transferred to and from the distal Fe and the 4Fe4S cluster functions as an electron transfer relay to complete the redox processes at the diiron core. The proton-coupled electron transfer process provides a low-energy pathway for H−H bond cleavage and formation at the diiron dithiolate core. This enables [FeFe]-H 2 ases to serve as highly active catalysts for both proton reduction and H 2 oxidation. 2 Over the past decade, studies of [FeFe]-H 2 ase mimics have been mostly concentrated on the H red state (Figure 1) for proton reduction 3 and H 2 activation under photolysis, 4 while modeling of the H ox state for H 2 oxidation has been paid less attention. The first experimental evidence for the formation of the Fe II Fe I bridging-CO complex was observed in situ from the one-electron oxidation of a diiron carbonyl cyanide precursor bearing a thioether group. 5 Later, two structurally characterized mixed-valence diiron models of H ox were reported by the groups of Darensbourg and Rauchfuss. 6 The H ox mimics reported to date are diiron dithiolate complexes either bearing the special σ ligands 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and cis-1,2-C 2 H 2 (PPh 2) 2 (dppv) or featuring a bulky bridge, 2,2-dimethyl-1,3-propanedithiolate (dmpdt). 7,8 There are only three examples of the activation of H 2 using Fe II Fe I complexes, reported by Rauchfuss and co-workers. The models for H ox containing an azadithiolate (adt) bridge react very slowly with H 2 under high pressure to give μ-H complexes, 9 while in the presence of a supplemental oxidant the activation of H 2 occurs under mild conditions at significantly higher rates. 10 Recently, a functional model of H ox containing both an internal amine and a redox-active unit, Cp*Fe(C 5 Me 4 CH 2 PEt 2) (FcP*) (Cp* = C 5 Me 5), 11 was reported to be active for the oxidation of 1 atm H 2 at 25 °C in the presence of excess FcBAr F 4 [Fc = Cp 2 Fe, Ar F = 3,5-(CF 3) 2 C 6 H 3 ] and P(o-tol) 3 , giving 0.4 turnover/h in 5 h. We previously reported a diiron complex bearing a pendant amine in a chelating diphosphine ligand, [(μ-pdt){Fe(CO) 3 }-{Fe(CO)(PNP)}] (1) [pdt = propane-1,3-dithiolate, PNP = Ph 2 PCH 2 N(nPr)CH 2 PPh 2 ], and its doubly protonated species [1(H N H μ)] 2+ (Scheme 1). 12 An interesting feature of this
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    ABSTRACT: [FeFe]-hydrogenases are enzymes in nature that catalyze the reduction of protons and the oxidation of H2 at neutral pH with remarkably high activities and incredibly low overpotential. Structural and functional biomimicking of the active site of [FeFe]-hydrogenases can provide helpful hints for elucidating the mechanism of H2 evolution and uptake at the [FeFe]-hydrogenase active site and for designing bioinspired catalysts to replace the expensive noble metal catalysts for H2 generation and uptake. This perspective focuses on the recent progress in the formation and reactivity of iron hydrides closely related to the processes of proton reduction and hydrogen oxidation mediated by diiron dithiolate complexes. The second section surveys the bridging and terminal hydride species formed from various diiron complexes as well as the intramolecular proton transfer. The very recent progress in H2 activation by diiron dithiolate models are reviewed in the third section. In the concluding remarks and outlook, the differences in structure and catalytic mechanism between the synthetic models and the native [FeFe]-H2ase active site are compared and analyzed, which may cause the need for a significantly larger driving force and may lead to lower activities of synthetic models than the [FeFe]-H2ases for H2 generation and uptake.
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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.
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Ying Wang