Steps along the path to dihydrogen activation at [FeFe] hydrogenase structural models: Dependence of the core geometry on electrocatalytic proton reduction

Differences in the rate of electrocatalytic proton reduction by Fe-2(mu-PPh2)(2)(CO)(6), DP, and the linked phosphido-bridged analogue Fe-2(mu,mu-PPh(CH2)(3)PPh)(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 angstrom). DFT calculations of the 3P, 3P(-), 3P(2-) 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 (H(2)3P), with more rapid dihydrogen evolution following further reduction of H(2)3P. 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, H(2)3P, and H2Fe2(mu,mu-S(CH2)(3)S)(CO)(6) (H(2)3S) are related to the DFT-calculated H-H distances [H(2)3S (1.880 angstrom) < H(2)3P (2.064 angstrom) < H2DP (3.100 angstrom)], and this suggests a common reaction path for the thiolato- and phosphido-bridged diiron carbonyl compounds.