Carbon- and Sulfur-Bridged Diiron Carbonyl Complexes Containing N,C,S-Tridentate Ligands Derived from Functionalized Dibenzothiophenes: Mimics of the [FeFe]-Hydrogenase Active Site

Abstract

Photochemical reactions of [Fe(CO)5] with dibenzothiophene (DBT) derivatives bearing a N-donor group produced a series of C,S-bridged diiron carbonyl complexes [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3], as previously reported for 4-(2′-pyridyl)dibenzothiophene (L1), where L′ represents the N,C,S-tridentate ligands L1′–L5′, formed by C–S bond cleavage of L1–L5, respectively. The DBT derivatives used in this study have Schiff base or oxazoline moieties at the 4-position: L2 = PhCH2N═CH-DBT, L3 = 2-MeOC6H4CH2N═CH-DBT, L4 = (S)-PhC(Me)HN═CH-DBT, L5 = (R)-4-isopropyl-2-oxazolinyl-DBT. The diiron complexes were characterized by NMR, absorption, and circular dichroism spectroscopy, and the dinuclear structures bridged by thiolate S and aryl C atoms were established by X-ray crystallography. The diiron complex [{Fe(μ-L′-κ3N,C,S)(CO)2}Fe(CO)3] consists of two units, Fe(L′)(CO)2 and Fe(CO)3: the latter unit is located on a thiolate-containing metallacycle in the former one. The chiral Schiff base ligand precursor L4 gave a 55:45 mixture of two diastereomers for [{Fe(μ-L4′-κ3N,C,S)(CO)2}Fe(CO)3], while chiral L5 with an (R)-4-isopropyl-2-oxazolinyl group afforded [{Fe(μ-L5′-κ3N,C,S)(CO)2}Fe(CO)3] in a 9:1 diastereomeric ratio. The diiron carbonyl complexes of the N,C,S-tridentate ligands (L1′—L5′) showed two reversible redox couples for [Fe2(μ-L′)(CO)5]0/– and [Fe2(μ-L′)(CO)5]−/2–. The two-electron-reduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile.