Binuclear hexafluorocyclopentadiene iron carbonyls: bis(dihapto) versus trihapto–monohapto bonding in iron–iron bonded structures

Abstract

Binuclear organometallic systems incorporating free cyclopentadiene (C₅H₆) as a ligand rather than the ubiquitous cyclopentadienyl ligand (C₅H₅) have never been reported in the literature. However, the hexafluorocyclopentadiene ligand (C₅F₆) is known to form the stable binuclear carbonyl derivatives (η²,η²-C₅F₆)Fe₂(CO)₈ and trans-(η³,η¹-C₅F₆)Co₂(CO)₇. The related hexafluorocyclopentadiene iron carbonyl systems (C₅F₆)Fe(CO)_n (n = 4, 3, 2) and (C₅F₆)Fe₂(CO)_n (n = 8, 7, 6, 5) have now been explored using density functional theory. The hexafluorocyclopentadiene ligand is found to use one or both of its CC double bonds to bind either as a dihapto η²-C₅F₆ monoolefin or a tetrahapto η⁴-C₅F₆ diolefin in mononuclear (C₅F₆)Fe(CO)_n structures. In binuclear iron carbonyl structures lacking direct iron–iron bonds, such as the lowest energy trans- and cis-(η²,η²-C₅F₆)Fe₂(CO)₈ structures as well as higher energy trans-(η²,η²-C₅F₆)Fe₂(CO)_n (n = 7, 6, 5) structures, the hexafluorocyclopentadiene ligand binds as a bis(dihapto) diolefin. However, the lowest energy (C₅F₆)Fe₂(CO)_n (n = 7, 6, 5) structures have cis stereochemistries with direct iron–iron bonds of orders ranging from one to three. In such lowest energy structures the hexafluorocyclopentadiene ligand is bonded to the Fe₂ unit as a trihapto–monohapto η³,η¹-C₅F₆ ligand.