Dinuclear iron dithiolates such as (�-SCH2CH2S)[Fe(CO)3]2 are functional model catalysts for diiron-hydrogenases in the reduction of protons to H2.  The mechanism of H2 production 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 (E0) are directly computed and compared to the measured pKa and E0.  Computations show that in the strong acid, HOTs, the one-electron reduced species can be protonated to form a bridging hydride complex as the most stable structure.  This species can be reduced and protonated to form dihydrogen and regenerate the neutral catalyst, an ECEC process.  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 the monoanion, an E[CECE] process   When the weak acid, HOAc, is the proton source a more negative applied reduction potential is necessary, where two one-electron reductions yield the dianion before the first protonation on the thiolate.  Subsequent reduction and protonation form dihydrogen and regenerates the monoanion through an E[ECEC] process.  Results of density functional theory (DFT) calculations on the protonation of the neutral, but more basic complex, Fe2(�-pdt)(CO)4(PMe3)2 (pdt = propane-1,3-dithiolate), show that diiron bridging-hydride species are more stable than iron terminal-hydride, sulfur-hydride, or formyl isomers. Consistent with experimental observation, the transoid basal/basal forms are more stable than other �-H isomers. The favored reaction pathways involve transfer of the proton to an iron terminal site rather than directly to the bridging site. These kinetically favored terminal-hydride species isomerizes through a Ray-Dutt twist to produce the apical/ basal bridging-hydride isomer. This isomer rearranges through higher barrier Bailar twists to the cisoid and transoid basal/basal isomers, the former finally rearranging to the latter isomer.

Making hydrogen by adding protons and electrons to models for iron-iron hydrogenase

243rd National Meeting (2012)

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