title: Steps along the path to dihydrogen activation at [FeFe] hydrogenase structural models: dependence of the core geometry on electrocatalytic proton reduction.
abstract: Differences in the rate of electrocatalytic proton reduction by Fe2(mu-PPh2)2(CO)6, DP, and the linked phosphido-bridged analogue Fe2(mu,mu-PPh(CH2)3PPh)(CO)6, 3P, suggest that dihydrogen elimination proceeds through a bridging hydride. The reaction path was examined using electrochemical, spectroscopic, and in silico studies where reduction of 3P gives a moderately stable monoanion [Kdisp(3P-) = 13] and a distorted dianion. The monomeric formulation of 3P- is supported by the form of the IR and EPR spectra. EXAFS analysis of solutions of 3P, 3P-, and 3P2- indicates a large increase in the Fe-Fe separation following reduction (from 2.63 to ca. 3.1-3.55 A). DFT calculations of the 3P, 3P-, 3P2- redox series satisfactorily reproduce the IR spectra in the nu(CO) region and the crystallographic (3P) and EXAFS-derived Fe-Fe distances. Digital simulation of the electrocatalytic response for proton reduction indicates a low rate of dihydrogen evolution from the two-electron, two-proton product of 3P (H23P), with more rapid dihydrogen evolution following further reduction of H23P. Because dihydrogen evolution is not observed upon formation of H2DP, dihydrogen evolution at the two-electron-reduced level does not involve protonation of a hydridic Fe-H ligand. The rates of dihydrogen elimination from H2DP, H23P, and H2Fe2(mu,mu-S(CH2)3S)(CO)6 (H23S) are related to the DFT-calculated H-H distances [H23S (1.880 A) < H23P (2.064 A) < H2DP (3.100 A)], and this suggests a common reaction path for the thiolato- and phosphido-bridged diiron carbonyl compounds.
authors.affiliation: School of Chemistry, University of Melbourne, Victoria, Australia.
authors.forename: Mun Hon
authors.initials: MH
authors.lastname: Cheah
authors.forename: Stacey J
authors.initials: SJ
authors.lastname: Borg
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Best
mesh_terms: D002384:Catalysis; D004563:Electrochemistry; D006859:Hydrogen / Q000737:chemistry*; D006864:Hydrogenase / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D011522:Protons; D012680:Sensitivity and Specificity; D013057:Spectrum Analysis / Q000379:methods
chemical_list: D007506:Iron-Sulfur Proteins; D011522:Protons; D006859:Hydrogen; C118830:iron hydrogenase; D006864:Hydrogenase