Electrochemical Synthesis of Mono‐ and Disubstituted Diiron Dithiolate Complexes as Models for the Active Site of Iron‐Only Hydrogenases

Abstract

International audience[Fe2(S2C3H6)(CO)5{P(OMe)3}] (2) and [Fe2(S2C3H6)(CO)4{P(OMe)3}2] (3) were selectively prepared by the electrochemical reduction of [Fe2(S2C3H6)(CO)6] (1) in the presence of trimethyl phosphite ligand. Electrochemical data indicate a CO-displacement reaction catalyzed by electron transfer. Complexes 2 and 3 were characterized by X-ray crystallography, which shows that in the solid state, the trimethyl phosphite ligands lie in the apical configuration. NMR suggests that multiple isomers exist in solution. Both 2 and 3 exhibit a primary reduction step that involves two electrons. No stable hydride derivatives could be isolated by the reaction of 2 or 3 with strong acid. However, for both 2 and 3, proton reduction catalysis was detected at a potential less negative than that of the reduction of the diiron complex working as catalyst. The proposed mechanism involves a protonation of the diiron complex in the vicinity of the electrode, which is triggered by the more facile reduction of the protonated form (CE mechanism). The protonation and reduction steps are followed by a bimolecular reaction that regenerates the starting complex

Kinetic and thermodynamic aspects of the electrocatalysis of acid reduction in organic solvent using molecular diiron-dithiolate compounds

“…The negative shift of 0.23 V observed upon replacement of one CO by one P(OMe) 3 ligand is consistent with the potential shifts reported for other diiron-dithiolate compounds [38]. It is reasonable to assume that these effect is due to the more electron donor ability of P(OMe) 3 ligand as compared to that of carbonyl ligand.…”

Review of electrochemical studies of complexes containing the Fe2S2 core characteristic of [FeFe]-hydrogenases including catalysis by these complexes of the reduction of acids to form dihydrogen

“…Though X-ray crystallographic structures are often available, it is known that isomerization occurs in solution so the electrochemistry is sampling a mixture of isomers that are probably rapidly interconverting [44,[92][93][94]]. Another recent example involves phosphite complexes 56 and 57 [37]. Replacement of two CO ligands by the bidentate ligand, Ph 2 PCH 2 CH 2 PPh 2 (apical, basal on one Fe) (86), has very nearly the same effect as using two PMePh 2 monophosphine ligands, one on each Fe (54).…”

Non-innocent bma ligand in a dissymetrically disubstituted diiron dithiolate related to the active site of the [FeFe] hydrogenases

“…The IR spectrum of 2 in CH 2 Cl 2 displays three typical bands in the v CO region at 2029, 1960 and 1922 cm − 1 . The shift of the v CO bands is a useful indicator of the variation of the electron density in diiron models of the hydrogenase active site [41][42][43]. The average value of the IR bands in the carbonyl stretching region is lowered by 65 cm − 1 for the bma derivative 2 with respect to the all-CO parent compound (Table 2).…”