Electrochemical and theoretical investigations of the role of the appended base on the reduction of protons by [Fe2(CO)4(κ- ²-PNP^R)(μ-S(CH2)3Se] (PNP ^R={Ph2PCH2}2NR, R=Me, Ph)

The behavior of [Fe2(CO)4(κ2- PNPR)(μ-pdt)e] (PNPR=(Ph2PCH 2)2NR, R=Me (1), Ph (2); pdt=S(CH2) 3S) in the presence of acids is investigated experimentally and theoretically (using density functional theory) in order to determine the mechanisms of the proton reduction steps supported by these complexes, and to assess the role of the PNPR appended base in these processes for different redox states of the metal centers. The nature of the R substituent of the nitrogen base does not substantially affect the course of the protonation of the neutral complex by CF3SO3H or CH3SO 3H; the cation with a bridging hydride ligand, 1a μH+ (R=Me) or 2a μH+ (R=Ph) is obtained rapidly. Only 1a μH + can be protonated at the nitrogen atom of the PNP chelate by HBF4Et2O or CF3SO3H, which results in a positive shift of the proton reduction by approximately 0.15 V. The theoretical study demonstrates that in this process, dihydrogen can be released from a ·2-H2 species in the FeIFe II state. When R=Ph, the bridging hydride cation 2a μH+ cannot be protonated at the amine function by HBF4Et2O or CF3SO3H, and protonation at the N atom of the one-electron reduced analogue is also less favored than that of a S atom of the partially de-coordinated dithiolate bridge. In this situation, proton reduction occurs at the potential of the bridging hydride cation, 2a μH+. The rate constants of the overall proton reduction processes are small for both complexes 1 and 2 (kobs4-7 s-1) because of the slow intramolecular proton migration and H2 release steps identified by the theoretical study. Electrocatalytic proton reduction: The behavior of [Fe2(CO)4(2-PNPR)(μ-pdt)e] (PNPR=(Ph2PCH2)2NR, R=Me (1), Ph (2); pdt=S(CH2)3S) in the presence of acids is investigated both experimentally and theoretically to determine the proton reduction mechanisms. The rate constants of proton reduction are small for both complexes 1 and 2 (kobs 4-7 s-1) because of the slow intramolecular proton migration and H2 release steps identified by the DFT study. The transition state structures for the release of H2 at the {2H+/1e} and {2H+/2e} reduction states are shown in the figure. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.