CYCLOPENTADIENYLMOLYBDENUM(II) AND CYCLOPENTADIENYLMOLYBDENUM(III) COMPLEXES CONTAINING DIENE AND ALLYL LIGANDS .1. ISOMERIC PREFERENCES AND ISOMERIZATION RATES IN A PAIR OF REDOX-RELATED ORGANOMETALLIC COMPLEXES

Treatment of CpMoCl2(eta-C4H6) (1) with 1 equiv of allylmagnesium brom ide yields the dinuclear complexes [CPMo(eta-C4H6)(mu-Br)](2) (2, majo r) and Cp2Mo2(eta-C4H6)(2)(mu-Br)(mu-Cl) (3, minor). A solid solution of compounds 2 and 3 adopts an anti geometry in the solid state, as sh own by X-ray crystallography, whereas both anti and syn isomers are ob served in benzene solution by H-1-NMR spectroscopy. The reaction of 1 with 2 equiv of allylmagnesium bromide yields [CpMo(eta-C3H5)(eta-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) or ientation for the major isomer 4a, which is also found in the solid st ate by X-ray crystallography. The orientation of 4b is suggested by th e 'H-NMR chemical shifts as CpMo(supine-C3H5)(supine-C4H6). Oxidation of 4a/b by ferrocenium hexafluorophosphate in dichloromethane gives th e 17-electron compounds [CpMo(eta-C3H5)(eta-C4H6)][PF6] (5a/b). The gr een compound 5a converts into the more stable red-violet 5b with an es timated half-life of <20 s in THF. It can be observed, however at low temperature by EPR spectroscopy. The [CpMo(supine-eta-C3H5)(supine-eta -C4H6)][PF6] configuration for 5b has been confirmed by X-ray diffract ion methods. Upon reduction with cobaltocene, 5b is converted selectiv ely to 4b, followed by slow equilibration (t(1/2) = 6.5 h) with 4a. Re fluxing or photolyzing a solution of 4a/b in benzene generates a third isomer, 4c, which adopts a CpMo(supine-C3K5)(s-trans-C4H6) configurat ion as confirmed by an X-ray analysis. The distribution of 4a and 4c a t equilibrium is approximately 1:1 by starting either from 4a/b or fro m pure 4c and independent of the equilibration method (thermal/photoch emical). Oxidation of 4c generates the corresponding 1-electron oxidat ion product 5c, which rapidly isomerizes to 5b. It can be observed, ho wever, by EPR in THF together with a fourth isomer, 5d (ca. 1:1), beli eved to differ from 5c only in the orientation of the allyl ligand. Eq uilibrium, rate, and electrochemical data allow most of the thermodyna mic and kinetic parameters related to the transformation of the differ ent compounds to be sorted out. The faster s-trans- to s-cis-butadiene isomerization for 5 relative to 4 indicates the easier accessibility of the unsaturated 15-electron vs 16-electron intermediate. Possible r easons for this trend are analyzed.