Cyclopentadienylmolybdenum(II) and -(III) Complexes Containing Diene and Allyl Ligands. 1. Isomeric Preferences and Isomerization Rates in a Pair of Redox-Related Organometallic Complexes

International audienceTreatment of CpMoCl2(η-C4H6) (1) with 1 equiv of allylmagnesium bromide yields the dinuclear complexes [CpMo(η-C4H6)(μ-Br)]2 (2, major) and Cp2Mo2(η-C4H6)2(μ-Br)(μ-Cl) (3, minor). A solid solution of compounds 2 and 3 adopts an anti geometry in the solid state, as shown by X-ray crystallography, whereas both anti and syn isomers are observed in benzene solution by 1H-NMR spectroscopy. The reaction of 1 with 2 equiv of allylmagnesium bromide yields [CpMo(η-C3H5)(η-C4H6)] as an equilibrium mixture of a major (98%, 4a) and a minor (2%, 4b) isomer. NOE−NMR studies indicate the CpMo(prone-C3H5)(supine-C4H6) orientation for the major isomer 4a, which is also found in the solid state by X-ray crystallography. The orientation of 4b is suggested by the 1H-NMR chemical shifts as CpMo(supine-C3H5)(supine-C4H6). Oxidation of 4a/b by ferrocenium hexafluorophosphate in dichloromethane gives the 17-electron compounds [CpMo(η-C3H5)(η-C4H6)][PF6] (5a/b). The green compound 5a converts into the more stable red-violet 5b with an estimated half-life of