Selective protonation of terminally bound η 3-allyls on (η 3-allyl) 2(μ 2-η 3-allyl) 2dimolybdenum(II) by β-diketones forming(μ 2-η 3a-allyl) 2(η 2-β-diketonate) 2 dimolybdenum(II) complexes

Abstract

The β-diketones, R(CO)CH 2(CO)R′ [where R = R′= Me (acac); R = Me, R′= CF 3 (tfac); or R = R′ = CF 3 (hfac)] selectively protonate the terminally bound η 3-allyls on Mo 2(μ 2-η 3-allyl) 2(η 3-allyl) 2 ( A) leaving the Mo 2(μ 2-η 3-allyl) 2 core intact. The molybdenum(II) dimers, Mo 2(μ 2-η 3-allyl) 2(η 2-L) 2[where L = acac ( B), tfac ( C) or hfac ( D)] are formed from this reaction. From 1H, 13C and 19F NMR data, it is evident that the two bridging allyls in each complex, B- D, are bound in a cis arrangement with endo and exo conformations, respectively. Thus, a plane of symmetry bisects the MoMo bond axis and the two bridging allyls in B and D, but is absent parallel to the MoMo bond axis and bisecting each of the chelating β-diketonate ligands. The complex, C, exists primarily as the isomer in which the bulky trifluoromethyl groups on the two tfac ligands are as far apart as possible from each other. As the electron-donating ability of the terminally bound ligands in A- D decreases, allyl > acac > tfac > hfac, an upfield shift in the proton resonances of the bridging allyls is observed.