Theoretical studies on the desulfurization of benzothiophene (thianaphthene) and thienothiophene (thiophthene) by carbon–sulfur bond cleavage: binuclear iron carbonyl intermediates

Abstract

Thiophene is known experimentally to be desulfurized by Fe₃(CO)₁₂ under mild conditions to give the tricarbonyl ferrole (η⁴,η²-C₄H₄)Fe₂(CO)₆. A similar reaction of benzothiophene (thianaphthene) with Fe₃(CO)₁₂ gives a (C₈H₆S)Fe₂(CO)₆ complex in which an iron carbonyl moiety has inserted into the thiophene ring to give a thiaferranaphthalene ligand. Density functional theory shows this experimental structure to be the lowest energy structure. Furthermore, the lowest energy structures of the diiron pentacarbonyl (C₈H₆S)Fe₂(CO)₅ are simply derived from this (C₈H₆S)Fe₂(CO)₆ by loss of a CO group retaining the thiaferranaphthalene ligand. However, a higher energy isomeric (η⁶,η²-C₈H₆S)Fe₂(CO)₅ structure retains the original benzothiophene ligand with the C₆ ring bonded to an Fe(CO)₂ moiety as a hexahapto ligand and the CC double bond of the C₄S ring bonded to an Fe(CO)₃ moiety as a dihapto ligand with an Fe→Fe dative bond between the iron atoms. Similar insertion of an iron atom into a thiophene ring to give a thiaferrabenzene ring is predicted to occur in the lowest energy (C₆H₄S₂)Fe₂(CO)₆ structure derived from either the anti or syn isomers of thienothiophene. However, the bonding of the exocyclic iron atom to the resulting thiaferrabenzothiophene ligand involves atoms in both rings in contrast to the (C₈H₆S)Fe₂(CO)₆ complex where the benzene ring is not involved in the ligand–iron bonding.