Triple decker sandwiches and related compounds of the first row transition metals with cyclopentadienyl and hexafluorobenzene rings: remarkable effects of fluorine substitution

Abstract

The complete series of Cp₂M₂(μ-C₆F₆) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures have been examined theoretically for comparison with their unsubstituted Cp₂M₂(μ-C₆H₆) analogues. The singlet triple decker sandwich titanium complex Cp₂Ti₂(η⁶,η⁶-C₆F₆) with a closed shell electronic structure and a non-planar C₆F₆ ring is preferred energetically by a wide margin (>20 kcal mol⁻¹) over other isomers and spin states. This is in contrast to the hydrogen analogue for which related triplet spin state structures are clearly preferred. A similar low-energy triple-decker sandwich Cp₂V₂(η⁶,η⁶-C₆F₆) structure is found for vanadium but with a quintet spin state. The later transition metals from Cr to Ni energetically prefer the so-called “rice-ball” cis-Cp₂M₂(μ-C₆F₆) structures with varying hapticities of metal-ring bonding, a range of formal orders of metal–metal bonding, and varying spin states depending on the metal atom. Thus the lowest energy Cp₂Cr₂(μ-C₆F₆) structures are triplet and quintet structures with pentahapto–trihapto η⁵,η³-μ-C₆F₆ rings and formal CrCr double bonds. This contrasts with the structure of Cp₂Cr₂(μ-C₆H₆) having a bis(tetrahapto) η⁴,η⁴-C₆H₆ ring and a formal Cr–Cr quadruple bond. The lowest energy Cp₂Mn₂(μ-C₆F₆) structures are trans and cis quintet spin state structures. This contrasts with Cp₂Mn₂(μ-C₆H₆) for which a closed-shell singlet triple decker sandwich structure is preferred. The lowest energy Cp₂Fe₂(μ-C₆F₆) structure is a triplet cis structure with a tetrahapto–dihapto η⁴,η²-μ-C₆F₆ ring and a formal Fe–Fe single bond. The lowest energy Cp₂Co₂(μ-C₆F₆) structures are singlet spin state structures with formal M–M single bonds and either bridging bis(trihapto) η³,η³-C₆F₆ or tetrahapto–dihapto η⁴,η²-C₆F₆ rings. For Cp₂Ni₂(μ-C₆F₆) low energy singlet cis and trans structures are both found. The singlet cis-Cp₂Ni₂(μ-C₆F₆) structure has a Ni–Ni single bond of length ∼2.5 Å and a bridging bis(dihapto) η²,η²-C₆F₆ ligand with an uncomplexed CC double bond. The singlet trans-Cp₂Ni₂(μ-C₆F₆) structure has a bis(trihapto) η³,η³-C₆F₆ ligand.