Reduction–oxidation properties of organotransition-metal complexes. Part 22. Stereospecific oxidative cyclopropane ring opening and reductive cyclobutane ring formation in polycyclic hydrocarbon complexes of iron; X-ray crystal structures of [Fe2(CO)6(η5:η′5-C16H16)][PF6]2·CH3NO2 and [Fe2(CO)6(η4:η′4-C16H16)]

Abstract

Electrochemical studies show that [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₆)](2) undergoes irreversible two-electron oxidation at a platinum electrode in CH₂Cl₂. Chemical oxidation with [Fe(η-C₅H₅)₂][PF₆] gives [Fe₂(CO)₆(η⁵:η′⁵-C₁₆H₁₆)][PF₆]₂(3), X-ray structural studies on the nitromethane solvate of which reveal the detailed stereochemistry of the polycyclic hydrocarbon ligand. Two cycloheptadienyl moieties, each η⁵-bonded to Fe(CO)₃ units, are fused to a central cyclohexene ring at C(2)–C(3) and at C(2′)–C(3′). The six-membered ring adopts a very flattened chair conformation with C(4) and C(4′)(bonded to iron as terminal members of pentadienyl units) in pseudo-axial sites, at 3.78 Å apart. Complex (3) is reduced by K[BH(CHMeEt)₃] to [Fe₂(CO)₆(η⁴:η′⁴-C₁₆H₁₆)](4), X-ray structural studies on which show stereospecific cyclobutane ring formation via the linking of atoms C(4) and C(4′) of (3). This linking is accompanied by ring inversion of the cyclohexene residue and by significant twisting of the ring double bond [C(2)–C(1)–C(1′)–C(2′) torsion angle –8.6(5)°]. The C(4)–C(4′) bond formed is the longest in the cyclobutane ring at 1.596(4)Å[cf. others, average 1.548(3)Å]. The ring inversion is required to bring C(4) and C(4′) into adjacent pseudo-equatorial sites and hence proximity [the C(4)–C(3)–C(3′)–C(4′) torsion angle is 159.7(4)° in (3) and 25.2(2)° in (4)]. Complex (4) is oxidised by [Fe(η-C₅H₅)₂][PF₆] to (3) which reacts with PPh₃ and with iodide ion to give [Fe₂(CO)₆{η⁴:η′⁴-C₁₆H₁₆(PPh₃)₂}][PF₆]₂(5) and [Fe₂l₂(CO)₄(η⁵:η′⁵-C₁₆H₁₆)](6), respectively.