Major differences between trifluorophosphine and carbonyl ligands in binuclear cyclopentadienyliron complexes

Abstract

The cyclopentadienyliron trifluorophosphine hydride CpFe(PF₃)₂H, in contrast to CpFe(CO)₂H, is a stable compound that can be synthesized by reacting Fe(PF₃)₅ with cyclopentadiene. Theoretical studies on the binuclear Cp₂Fe₂(PF₃)_n (n = 5, 4, 3, 2) derivatives derived from CpFe(PF₃)₂H indicate the absence of viable structures having PF₃ ligands bridging Fe–Fe bonds solely through the phosphorus atom. This contrasts with the analogous Cp₂Fe₂(CO)_n systems for which the lowest energy structures have two (for n = 4 and 2) or three (for n = 3) CO groups bridging an iron–iron bond. Higher energy singlet Cp₂Fe₂(PF₃)₃ structures have a novel four-electron donor bridging η²-μ-PF₃ ligand bonded to one iron atom through its phosphorus atom and to the other iron atom through a fluorine atom. Other higher energy triplet and singlet Cp₂Fe₂(PF₃)₂ structures are of the Cp₂Fe₂F₂(μ-PF₂)₂ type having terminal fluorine atoms and bridging μ-PF₂ ligands. The lowest energy Cp₂Fe₂(PF₃)₅ structure is actually Cp₂Fe₂(PF₃)₃(PF₄)(μ-PF₂) with a bridging PF₂ group and a terminal PF₄ group. Such structures are derived from a Cp₂Fe₂(PF₃)₄(μ-PF₃) precursor by migration of a fluorine atom from the bridging PF₃ group to a terminal PF₃ group with a low activation energy barrier.