Proton Shuttle Mediated by (SCH2)2P= O Moiety in [FeFe]-Hydrogenase Mimics: Electrochemical and DFT Studies

The synthesis, characterization, and protonation of [Fe2(CO)6{(μ-SCH2)2(Et)P= O}] (1) using the moderately strong acid CF3CO2H (pKaMeCN = 12.7) are reported. Digital simulations of the cyclic voltammetry of 1 in the presence of CF3CO2H and DFT calculations have allowed us to obtain a detailed mechanistic picture of the processes underlying the catalytic hydrogen evolution reaction (HER) that 1 can mediate. Moreover, DFT has shed light on the role of the P= O functionality in the whole catalytic cycle of proton reduction. The reductive behavior of 1 features a double electron transfer with potential inversion, which is associated with deep structural rearrangement of the catalyst. The double reduction appears also functional to the intramolecular proton transfer from the P= O group to the diiron core, a crucial process for the H+/H- heterocoupling yielding H2. The key intermediate for the H2 formation and release is predicted to be a 3H+/3e- species, in which P= O is perfectly poised to shuttle protons from solution to the Fe-H-Fe moiety. Therefore, the R-P= O bridgehead installed in a dithiolato linker of a diiron core proves a valid and versatile alternative to the natural nitrogen-based Fe2 strap.