Issue 12, 2010

Mechanism of electrocatalytic hydrogen production by a di-iron model of iron–iron hydrogenase: A density functional theory study of proton dissociation constants and electrodereduction potentials

Abstract

Simple dinuclear iron dithiolates such as (μ-SCH2CH2CH2S)[Fe(CO)3]2, (1) and (μ-SCH2CH2S)[Fe(CO)3]2 (2) are functional models for diiron-hydrogenases, [FeFe]-H2ases, that catalyze the reduction of protons to H2. The mechanism of H2 production with 2 as the catalyst and with both toluenesulfonic (HOTs) and acetic (HOAc) acids as the H+ source in CH3CN solvent has been examined by density functional theory (DFT). Proton dissociation constants (pKa) and electrode reduction potentials () are directly computed and compared to the measured pKa of HOTs and HOAc acids and the experimental reduction potentials. Computations show that when the strong acid, HOTs, is used as a proton source the one-electron reduced species 2 can be protonated to form a bridging hydride complex as the most stable structure. Then, this species can be reduced and protonated to form dihydrogen and regenerate 2. This cycle produces H2via an ECEC process at an applied potential of −1.8 V vs. Fc/Fc+. A second faster process opens for this system when the species produced at the ECEC step above is further reduced and H2 release returns the system to 2 rather than 2, an E[CECE] process. On the other hand, when the weak acid, HOAc, is the proton source a more negative applied reduction potential (−2.2 V vs. Fc/Fc+) is necessary. At this potential two one-electron reductions yield the dianion 22− before the first protonation, which in this case occurs on the thiolate. Subsequent reduction and protonation form dihydrogen and regenerate 2 through an E[ECEC] process.

Graphical abstract: Mechanism of electrocatalytic hydrogen production by a di-iron model of iron–iron hydrogenase: A density functional theory study of proton dissociation constants and electrode reduction potentials

Supplementary files

Article information

Article type
Paper
Submitted
01 Dec 2009
Accepted
18 Jan 2010
First published
23 Feb 2010

Dalton Trans., 2010,39, 3093-3104

Mechanism of electrocatalytic hydrogen production by a di-iron model of ironiron hydrogenase: A density functional theory study of proton dissociation constants and electrode reduction potentials

P. Surawatanawong, J. W. Tye, M. Y. Darensbourg and M. B. Hall, Dalton Trans., 2010, 39, 3093 DOI: 10.1039/B925262B

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