Structural effects of one-electron oxidation on a cyclobutadieneiron derivative: X-ray crystal structures of [Fe(CO){P(OMe)3}2(η-C4Ph4)] and [Fe(CO){P(OMe)3}2(η-C4Ph4)]BF4

Abstract

Single-crystal X-ray diffraction studies on the tetraphenylcyclobutadiene iron complexes [Fe(CO){P(OMe)₃}₂(η-C₄Ph₄)](1) and [Fe(CO){P(OMe)₃}₂(η-C₄Ph₄)]⁺(2), the latter as its[BF₄]^– salt, show that significant variations in molecular geometry occur as a consequence of one-electron oxidation. The Fe–P distances increase by 0.115(5)Å and the Fe–CO distance by 0.075(8)Å, while cyclobutadiene ring carbon–iron lengths show a very small increase of 0.021(8)Å. No significant change in the square-planar geometry of the C₄ ring occurs on oxidation, but the Fe(CO){P(OMe)₃}₂ fragment adopts a different conformation relative to the C₄Ph₄ moiety in (1) as compared with (2). In one of the two distinct cations present in crystals of (2) the Fe(CO){P(OMe)₃}₂is strongly tilted so that the pseudo-three-fold axis of this conical fragment is not coincident with the four-fold axis of the C₄ ring. These structural observations have been analyzed in order to test current theories of the electronic structure of FeL₃(η-C₄R₄) complexes. The principal conclusion is that the highest occupied molecular orbital (1) is iron based and not strongly involved in bonding to cyclobutadiene. This is in full accord with the results of both photoelectron spectroscopic and generalized molecular orbital studies on [Fe(CO)₃(η-C₄H₄)]. The orbital depopulated on oxidation contributes to both Fe–P(OMe)₃ and Fe–CO π back bonding. Oxidation diminishes such π bonding and causes changes in P–O bond lengths consistent with P–O σ* orbitals contributing to the π-acceptor function of the P(OMe)₃ ligand.