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

摘要

Simple dinuclear iron dithiolates such as (mu-SCH2CH2CH2S)[Fe(CO)(3)](2), (1) and (mu-SCH2CH2S)[Fe(CO)(3)](2) (2) are functional models for diiron-hydrogenases, [FeFe]-H(2)ases, that catalyze the reduction of protons to H-2. The mechanism of H-2 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 (pK(a)) and electrode reduction potentials (E degrees) are directly computed and compared to the measured pK(a) 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 H-2 via 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 H-2 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 2(2-)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.