Binuclear cyclooctatetraene–iron carbonyl complexes: examples of fluxionality and valence tautomerism

Abstract

Reactions of cyclooctatetraene with iron carbonyls under various conditions give not only the monomeric (η⁴-C₈H₈)Fe(CO)₃ but three C₈H₈Fe₂(CO)₆ isomers and one C₈H₈Fe₂(CO)₅ isomer. Density functional theory on the C₈H₈Fe₂(CO)₆ system shows the trans-(η⁴,η⁴-C₈H₈)Fe₂(CO)₆ isomer to be the lowest energy isomer. The cis-(η³,η³-C₈H₈)Fe₂(CO)₆ isomer with an Fe–Fe bond and an uncomplexed CC double bond in the C₈H₈ ring lying ∼11 kcal mol⁻¹ in energy above trans-(η⁴,η⁴-C₈H₈)Fe₂(CO)₆ appears to correspond to one of the metastable C₈H₈Fe₂(CO)₆ isomers obtained under relatively mild conditions. However, the cis-(η⁴,η⁴-C₈H₈)Fe₂(CO)₆ structure without an Fe–Fe bond suggested for the other metastable isomer appears to be a high-energy structure with a large imaginary vibrational frequency. Following the corresponding normal mode leads to cis-(η³,η³-C₈H₈)Fe₂(CO)₆. For C₈H₈Fe₂(CO)₅ the two lowest energy structures are singly bridged cis-C₈H₈Fe₂(CO)₄(μ-CO) structures differing only by a 22.5° rotation of the C₈H₈ ring around the central Fe₂ unit. One of these structures is the experimental C₈H₈Fe₂(CO)₅ structure. The closeness in energy of these two C₈H₈Fe₂(CO)₅ structures is consistent with the experimentally observed fluxionality of this molecule in the NMR spectrum at low temperatures. The unsaturated C₈H₈Fe₂(CO)_n (n = 4, 3) structures obtained by further decarbonylation of C₈H₈Fe₂(CO)₅ retain the bridging bis(tetrahapto) or bis(pentahapto) C₈H₈ rings of C₈H₈Fe₂(CO)₅ and provide examples of structures with formal FeFe double bonds with the lowest energy such structures having triplet rather than singlet spin states. Viable carbonyl-rich (η²,η²-C₈H₈)Fe₂(CO)₈ and (η⁴,η²-C₈H₈)Fe₂(CO)₇ structures represent possible intermediates in the formation of the various C₈H₈Fe₂(CO)_n (n = 6, 5) species from cyclooctatetraene and iron carbonyls.