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: 6.83). 03/2012; 48(37):4450-2. DOI: 10.1039/c2cc00044j
Source: PubMed


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: The behavior of [Fe(2)(CO)(4)(κ(2)-PNP(R))(μ-pdt)] (PNP(R) =(Ph(2)PCH(2))(2)NR, R=Me (1), Ph (2); pdt=S(CH(2))(3)S) in the presence of acids is investigated experimentally and theoretically (using density functional theory) in order to determine the mechanisms of the proton reduction steps supported by these complexes, and to assess the role of the PNP(R) appended base in these processes for different redox states of the metal centers. The nature of the R substituent of the nitrogen base does not substantially affect the course of the protonation of the neutral complex by CF(3)SO(3)H or CH(3)SO(3)H; the cation with a bridging hydride ligand, 1 μH(+) (R=Me) or 2 μH(+) (R=Ph) is obtained rapidly. Only 1 μH(+) can be protonated at the nitrogen atom of the PNP chelate by HBF(4)·Et(2)O or CF(3)SO(3)H, which results in a positive shift of the proton reduction by approximately 0.15 V. The theoretical study demonstrates that in this process, dihydrogen can be released from a η(2)-H(2) species in the Fe(I)Fe(II) state. When R=Ph, the bridging hydride cation 2 μH(+) cannot be protonated at the amine function by HBF(4)·Et(2)O or CF(3)SO(3)H, and protonation at the N atom of the one-electron reduced analogue is also less favored than that of a S atom of the partially de-coordinated dithiolate bridge. In this situation, proton reduction occurs at the potential of the bridging hydride cation, 2 μH(+). The rate constants of the overall proton reduction processes are small for both complexes 1 and 2 (k(obs) ≈4-7 s(-1)) because of the slow intramolecular proton migration and H(2) release steps identified by the theoretical study.
    No preview · Article · Jul 2012 · Chemistry - A European Journal
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    ABSTRACT: Four different pathways for deprotonation of [(μ-pdt){Fe(CO)3}{Fe(CO)(κ(2)-Me2PCH2N(Me)CH2PMe2)}] (pdt = propane-1,3-dithiolate) [Hμ](1+) were examined, including (1) the "Direct" deprotonation; (2) the "Indirect" deprotonation via the pendant amine N; (3) the "Indirect" deprotonation via the distal metal Fe; and (4) the "Indirect" deprotonation via the dithiolate S. Only deprotonation of the "Indirect" pathway via the pendant amine N is feasible at room temperature. The most favorable migration destination for the bridging hydride in [Hμ](1+) is the pendant amine N (activation energy barrier 16.1 kcal mol(-1)). Migrations to the other two possible sites including the distal metal Fe (34.6 kcal mol(-1)) and the S in the dithiolate group (41.5 kcal mol(-1)) were hindered by high proton shuttling barriers. Once the migration barriers of those three "Indirect" pathways are overcome, the following deprotonations from all three positions including the distal atom Fe, the dithiolate S and the pendant amine N, are all feasible. The results also demonstrate a large difference for deprotonation of the hydride from the terminal and bridging sites. The low energy of the virtual orbital associated with the antibonding M-H interaction of [HFe](1+) implies the high activity for the interaction with aniline.
    No preview · Article · Apr 2013 · Dalton Transactions
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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.
    No preview · Article · Jul 2013 · Dalton Transactions
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