Abstract
Treatment of TaCLt(dmpe)2 (dmpe = l,2-bis(dimethylphosphino)ethane) with sodium naphthalene affords TaCl(»/4-CioHs)(dmpe)2, which is reduced by additional sodium naphthalene to an unisolated anion, Na[Ta(7j4-CioHg)- (dmpe)2]. Protonation or methylation of the anion gives TaH(i;4-CioHg)(dmpe)2 or Ta(CH3)(?74-CioHg)(dmpe)2, respective- ly. Related procedures yield the analogous 1,3-cyclohexadiene adduct, TaCl(??-C6Hg)(dmpe)2. Exchange experiments suggest that the naphthalene unit and the bidentate phosphine are of similar lability, both being relatively inert. TaH(i)4-CioHg)- (dmpe)2 is a poor hydrogenation catalyst. Crystals of the TaCl(i?4-CioHg)(dmpe)2 compound are monoclinic, space group P2i/c, with a = 10.343 (3) A,b = 10.281 (3) Á, c = 24.642 (7) Á, ß = 97.39°. Full-matrix least-squares refinement of posi- tional and thermal parameters for all nonhydrogen atoms (including fixed H atom contributions) with 4019 data for which F > 3.92a{F) led to R = 0.028 and Rw = 0.040. The molecular structure of the complex may be considered to be an approximate pentagonal bipyramid, with Ta, 2P, pi, and the midpoints of the ?j4-olefin bonds forming the pentagonal plane (deviations <~0.2 Á). The dihedral angle between the coordinated and uncoordinated portions of naphthalene is 43.0°. The naphthalene complexes undergo 31P site exchange by a mechanism involving rotation of the diene unit. Formally, this corresponds to a pseu- dorotation exchanging axial and equatorial sites in the pentagonal bipyramid in a pairwise fashion. Activation parameters for the process in Ta(CH3)(i74-CiaHg)(dmpe)2 are AH* = 13.0 ± 0.2 kcal/mol and AS* = 1.0 ± 0.9 eu. These values are only approximate as they are based on the assumption that the rate of rotation of the diene over edges of the P4 plane is the same for all edges.