Reduction–oxidation properties of organotransition-metal complexes. Part 20. Oxidative and thermolytic cyclopropane ring-opening reactions; X-ray crystal structure of [Fe2(CO)6(η4:η′4-C16H18)]

Abstract

Oxidation of [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₈)](1) with ferrocenium hexafluorophosphate results in regioselective cyclopropane ring-opening and the formation of [Fe₂(CO)₆(η⁵:η′⁵-C₁₆H₁₈)]²⁺(2), a derivative of trans-1,2-bis(cycloheptadienyl)ethylene. The same hydrocarbon ligand is formed when (1) is thermolysed to give the insoluble polymer [{Fe₂(CO)₂(µ-CO)₂(η⁵:η′⁵-C₁₆H₁₈)}_n](3). An X-ray diffraction study has shown that the regio- and stereo-specificity observed in the formation of (2) and (3) derives from the stereochemistry of (1). The molecule (1) as a whole has near two-fold symmetry, but this is not crystallographically required. Two ring systems comprising fused cyclopropane and cyclohepta-1,3-diene fragments are joined by a single bond between the two apical cyclopropane atoms. Four atoms of each C₇ ring are essentially co-planar, giving an ‘envelope’ conformation to the ring, and are bonded to an Fe(CO)₃ fragment; the remainder of the C₇ ring is also planar, and incorporates the two C atoms shared with the C₃ ring and the CH₂ group. The C₇ ring folds away from the metal atom, and there are further folds at the junction with the C₃ ring and again at the apex of the C₃ ring, all in the same sense. The C₂ symmetry thus confers an overall S shape on the dimeric species when viewed along the two-fold axis. Along the Fe ⋯ Fe vector, by contrast, the C₃ rings are seen nearly edgewise and the two C₇ rings are almost eclipsed. Crystals of [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₈)](1) are triclinic, space group P, and the structure has been refined to R 0.043 for 3468 reflections measured at 293 K. Complex (3) undergoes metal–metal bond cleavage with iodine to yield [Fe₂I₂(CO)₄(η⁵:η′⁵-C₁₆H₁₈)](4) which may also be prepared from the reaction of (2) and iodide ion. Triphenylphosphine adds to the terminal carbon atoms, adjacent to the CH₂ groups, of the dienyl units of (2), giving the bis(phosphonium) salt [Fe₂(CO)₆{η⁴:η′⁴-C₁₆H₁₈(PPh₃)₂}][PF₆]₂(5).