Binuclear iron carbonyl complexes of thialene

Abstract

Thialene (C₈H₆S) is an isomer of benzothiophene and related to azulene by replacement of a CC unit in the seven-membered ring by a sulfur atom. The geometries of binuclear iron carbonyl complexes of thialene (C₈H₆S)Fe₂(CO)_n (n = 6, 5, 4) have been investigated using density functional theory for comparison with the corresponding azulene derivatives. The lowest energy (thialene)Fe₂(CO)₆ structures have a bis(tetrahapto)-η⁴,η⁴-thialene ligand bonded to two separate Fe(CO)₃ units without involvement of the sulfur atom. This differs from the isomeric (benzothiophene)Fe₂(CO)₆ structure known experimentally in which an iron atom has inserted into a carbon–sulfur bond to give a ferrathianaphthalene system. The only low-energy (thialene)Fe₂(CO)₅ structure has a pentahapto-trihapto-η⁵,η³ thialene ligand bonded to an Fe₂(CO)₅ unit, again without involvement of the sulfur atom. This structure is related to the experimental structure of azulene diiron pentacarbonyl, (η⁵,η³-C₁₀H₈)Fe₂(CO)₅ by replacement of the uncomplexed CC double bond in the seven-membered ring with the sulfur atom in the thialene six-membered ring. The potential energy surface of the unsaturated (thialene)Fe₂(CO)₄ is very complicated but does not include any low-energy structures with formal FeFe double bonds. Thermochemical considerations suggest (thialene)Fe₂(CO)₅ as a realistic synthetic objective, which is potentially accessible from reactions of thialene with Fe₂(CO)₉ or (benzalacetone)Fe(CO)₃ under mild conditions.