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

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.