Tuning the electronic properties of Fe2(μ-arenedithiolate) (CO)6-n(PMe3)n (n=0, 2) complexes related to the [Fe-Fe]-hydrogenase active site

Complexes [Fe-2(mu-S2Ar)(CO)(6)] (S2Ar) = benzene-1,2-dithiolate (1a) toluene-3,4-dithiolate (2a), 3,6-dichloro-1,2-benzenedithiolate (3a), quinoxaline-2,3-dithiolate (7a) have been prepared to investigate the electronic effect that different bridging arenedithiolate ligands have on the appended Fe-2(CO)(6) sites. Dinuclear complexes [Fe-2(mu-S2Ar)(CO)(4)(PMe3)(2)] (1-3,7)b and mononuclear complexes [Fe(S2Ar)(CO)(2)(PMe3)(2)] (1-3,7)c were synthesized from their parent hexacarbonyl complexes (1-3,7)a. IR spectroscopic, crystallographic and electrochemical analyses show that an increase of the electron-withdrawing character (where quinoxaline-2,3-dithiolate > 3,6-dichloro-1,2-benzenedithiolate > 1,2-benzenedithiolate > toluene-3,4-dithiolate) of the bridging ligand leads to a decreased electron density at the iron centers, which yield a milder reduction potential and higher eCO stretching frequencies. This effect is coherent for all of the investigated complexes. Electrocatalytic proton reduction by complex 3a (with trifluoromethanesulfonic acid) was evidenced by cyclic voltammetry. As a result of the milder reduction potential of 3a itself, proton reduction that is promoted by 3a proceeds at a potential that is milder than that for the 1a-catalyzed process.