Reduction-oxidation properties of organotransition-metal complexes. Part 12. Formation of carbon–carbon bonds via the oxidative dimerisation of [Fe(CO)3(η4-C8H8)] and the reduction of [Fe2(CO)6(η5:η′5-C16H16)]2+; X-ray crystal structures of [Fe2(CO)4{P(OPh)3}2(η5:η′5-C16H16)][PF6]2 and [Fe(CO)3(η4-C16H16)]

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Abstract
The complexes [Fe(CO)3 –nLn(η4-cot)][1: cot = cyclo-octatetraene; n= 0–3, L = P(OMe)3; n= 1, L = P(NMe2)3, PPh3, P(OCH2)3CMe, or P(OPh)3] undergo chemically irreversible one-electron oxidation in CH2Cl2 at a platinum electrode. Chemical generation of the highly reactive radical cation [Fe(CO)2L(C8H8)]+[2; L = CO or P(OPh)3], by oxidation of (1) with silver(I) salts or [N(C6H4Br-p)3][PF6], is followed by isomerisation and dimerisation, via C–C bond formation, to give [Fe2(CO)6 –nLn(η5:η′5-C16H16)]2+[3; n= 0 or 2, L = P(OPh)3]. The crystal structure of [3; n= 2, L = P(OPh)3], as the [PF6]– salt, reveals the presence of a dimeric C16H16 unit comprising two fused ring systems bonded to one another across a two-fold crystallographic axis of symmetry. The five unbridged carbon atoms of the C7 ring are coplanar and η5-bonded to the iron atom, which in turn is orthogonally co-ordinated to the two CO ligands and the P(OPh)3 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 dimeric ligand thus has an overall S shape when viewed down the two-fold axis. Along the Fe ⋯ Fe vector, by contrast, the C3 rings are seen edgewise and the two C7 rings are almost eclipsed. Complex (3; n= 0) reacts with Na[BH4] to give [Fe2(CO)6(η4:η′4-C16H18)](4). With halide ion, however, (3; n= 0) affords [Fe(CO)3(η4-C16H16)](5)via reductive C–C coupling. X-Ray analysis shows that in complex (5) a further link between the two halves of the dimer has been formed, in that the fifth C atom of the C7 ring which in (3) was bonded to the metal atom is now bonded to its counterpart in the other half of the dimer, forming an additional central C6 ring. As might be expected, the C7 ring which is not co-ordinated to iron is more nearly planar and the C–C bonds are more localised. With [Fe2(CO)9], complex (5) yields [Fe2(CO)6(η4:η′4-C16H16)](6), but with NMe3O·2H2O in refluxing benzene, ring detachment results in the isolation of the free polycyclic hydrocarbon, C16H16(7).

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