Steps along the Path to Dihydrogen Activation at [FeFe] Hydrogenase Structural Models:  Dependence of the Core Geometry on Electrocatalytic Proton Reduction

• Published in: Inorganic chemistry 2007-03, Vol.46 (5), p.1741-1750
• Main Authors: Cheah, Mun Hon, Borg, Stacey J, Best, Stephen P
• Format: Article
• Language: English
• Subjects: Catalysis; Electrochemistry; Hydrogen - chemistry; Hydrogenase - chemistry; Iron-Sulfur Proteins - chemistry; Models, Chemical; Oxidation-Reduction; Protons; Sensitivity and Specificity; Spectrum Analysis - methods
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0623361

Differences in the rate of electrocatalytic proton reduction by Fe2(μ-PPh2)2(CO)6, DP, and the linked phosphido-bridged analogue Fe2(μ,μ-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 [K disp(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 3P 2- indicates a large increase in the Fe−Fe separation following reduction (from 2.63 to ca. 3.1−3.55 Å). DFT calculations of the 3P, 3P -, 3P 2- redox series satisfactorily reproduce the IR spectra in the ν(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 (H2 3P), with more rapid dihydrogen evolution following further reduction of H2 3P. Because dihydrogen evolution is not observed upon formation of H2 DP, dihydrogen evolution at the two-electron-reduced level does not involve protonation of a hydridic Fe−H ligand. The rates of dihydrogen elimination from H2 DP, H2 3P, and H2Fe2(μ,μ-S(CH2)3S)(CO)6 (H2 3S) are related to the DFT-calculated H−H distances [H2 3S (1.880 Å) < H2 3P (2.064 Å) < H2 DP (3.100 Å)], and this suggests a common reaction path for the thiolato- and phosphido-bridged diiron carbonyl compounds.