Density functional study of the thermodynamics of hydrogen production by tetrairon hexathiolate, Fe4[MeC(CH2S)3]2(CO)8, a hydrogenase model.
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abstract
The tetranuclear iron complex Fe(4)[MeC(CH(2)S)(3)](2)(CO)(8) (1) functions like a hydrogenase to catalyze proton reduction to H(2) in the presence of 2,6-dimethylpyridinium acid (LutH(+)). Experimentally, at the first reduction potential (-1.22 V vs Fc/Fc(+)), the concentration of LutH(+) decreases slowly, while at the second reduction potential, which is sufficient to reduce 1(-) (-1.58 V vs Fc/Fc(+)), the concentration of LutH(+) decreases more rapidly. Here, density functional theory predicts both reduction potentials (E(0)) and proton-transfer free energies relative to LutH(+) for numerous intermediates and several important transition states as a basis for developing thermodynamics cycles for routes to hydrogen production by 1. At the less negative potential, one-electron reduction of 1 is followed by protonation to form a bridging hydride complex; then, a second one-electron reduction is followed by a second protonation, an ECEC process. This doubly reduced and doubly protonated species has a structure with bridging hydrides between both outer Fe-Fe pairs and can produce H(2) and regenerate 1 only by bringing the two hydrogens into proximity through a high-energy intermediate or transition state, a result consistent with the experimentally slow uptake of LutH(+) at this potential. In contrast, at the more negative (lower) reduction potential the two-electron-reduced species, 1(2-), which has bridging carbonyls between both Fe-Fe pairs, is protonated at a terminal Fe position to form a species that produces H(2) by rapidly picking up a second proton and regenerating 1 in an EECC process. Thus, the latter route avoids the high-energy intermediates and transition states necessarily accessed by the former route, a result that explains the more rapid uptake of LutH(+) at the second more negative potential. Although both routes arrive at a doubly reduced, singly protonated species in the third step of these processes, the calculations predict that a high barrier prevents the rapid interconversion of these two nearly isoenergetic species. The calculations confirm the importance of terminal metal hydrides, rather than bridging hydrides, for rapid H(2) production and show in detail how the bridging CO maintains the terminal hydride structure at the lower reduction potential even though the bridging hydride conformation is more stable. These results provide clues for designing new biomimic electrocatalysts and further evidence for the terminal Fe-H mechanism in [FeFe]-hydrogenase. The calculations also predict that at even lower reduction potentials, new more highly reduced intermediate species can be accessed that could lead to alternative routes to H(2).
