Redox and Structural Properties of Mixed-Valence Models for the Active Site of the [FeFe]-Hydrogenase: Progress and Challenges

Abstract

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these lowmolecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2 (S 2 C 3 H 6 )(CO) 3 (L)(dppv) (L = CO, PMe 3 , Pi-Pr 3 ) oxidize at potentials ~180 mV milder than the related ethanedithiolates (Angew. Chem. Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. EPR spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2 (S 2 C 3 H 6 )(CO) 4 (dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6 ) (CO) 3 (PMe 3 )(dppv)]BF 4 , as seen previously for the ethanedithiolate [Fe 2 (S 2 C 2 H 4 )(CO) 3 (PMe 3 ) (dppv)]BF 4 . For [Fe 2 (S 2 C n H 2n )(CO) 3 (Pi-Pr 3 )(dppv)]BF 4 , however, the spin is localized on the Fe (CO) 2 (Pi-Pr 3 ) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and DFT-calculations, suggest, however, that the Fe(CO) 2 (PiPr 3 ) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2 (S 2 C 2 H 4 )(CN) (CO) 3 (dppv)] − resulted in 2e oxidation of 0.5 equiv to give the μ-cyano derivative [Fe I 2 (S 2 C 2 H 4 ) (CO) 3 (dppv)](μ-CN)[Fe II 2 (S 2 C 2 H 4 )(μ-CO)(CO) 2 (CN)(dppv)], which was characterized spectroscopically.

Back-clocking of Fe2+/Fe1+spin states in a H2-producing catalyst by advanced EPR

“…Analogous signals have been reported for a mononuclear Fe 1 + (S = 1/2)-SiP 3 complex where the + 1 state has been generated by reduction of the initial Fe 2 + (S = 2)-SiP 3 complex using H 2 gas [9]. Similar signals have been also reported is two cases of chemically reduced di-iron models of hydrogenase [10,12].…”

“…Overall the CW EPR simulation reveals that Fe 1 + (S = 1/2) spin is coupled with four 31 P nuclei. The 31 P hyperfine couplings found here are intermediate to those reported for the Fe 1 + -SiP 3 system [9] and the di-iron Fe 1 + complexes [10,12] (see Table 1). …”

“…In the case of Fe-catalysts, the pertinent catalytic species can be Fe 2 + , Fe 1 + and Fe 0 . While model systems have historically focused on Fe 0 and Fe 2 + systems, the coordination chemistry of Fe 1 + has more recently come into focus [9][10][11][12]. This is in part due to its suggested intermediacy in enzymatic hydrogenase activity.…”

Synthesis, characterization and electrocatalysis of diiron propanediselenolate derivatives as the active site models of [FeFe]-hydrogenases

“…In biomimetic study of [FeFe]Hases, phosphine ligands are often used to prepare the H-cluster model compounds [33,[52][53][54]. This is largely because (i) phosphine ligands are easily available, (ii) phosphine ligands have the electronic effects similar to those of CN ligand present in the H-cluster, and (iii) both electronic and steric effects of phosphine ligands can be readily regulated by changing the groups attached to their P atoms.…”

Hydrogenase enzymes: Recent structural studies and active site models

“…In related studies, Rauchfuss and coworkers have prepared a compelling model for the oxidized active site using a phosphine complex [95]. A recent contribution by Rauchfuss summarizes redox and structural properties of mixed valence model complexes [96].…”