LIGAND-EXCHANGE IN TAX(ETA-4-NAPHTHALENE)(ME2PC2H4PME2)2 - PENTAGONAL BIPYRAMID TO MONOCAPPED TRIGONAL PRISM TRAVERSE

Treatment of TaCl4(dmpeh)2 (dmpe = 1, 2-bis(dimethylphosphino)ethane) with sodium naphthalene affords TaCl(η4-C10H8)(dmpe)2, which is reduced by additional sodium naphthalene to an unisolated anion, Na[Ta(η4-C10H8)(dmpe)2]. Protonation or methylation of the anion gives TaH(η4-C10H8)(dmpe)2 or Ta(CH3)(774-C10H8)(dmpe)2, respectively. Related procedures yield the analogous 1, 3-cyclohexadiene adduct, TaCl(η-C6H8)(dmpe)2. Exchange experiments suggest that the naphthalene unit and the bidentate phosphine are of similar lability, both being relatively inert. TaH(η4-C10H8)(dmpe)2) is a poor hydrogénation catalyst. Crystals of the TaCl(η4-C10H8)(dmpe)2 compound are monoclinic, space group P21/c, with a = 10.343 (3) Å, b = 10.281 (3) Å, c = 24.642 (7) Å, β = 97.39°. Full-matrix least-squares refinement of positional and thermal parameters for all nonhydrogen atoms (including fixed H atom contributions) with 4019 data for which F > 3.92σ(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, Cl, and the midpoints of the η4-olefin bonds forming the pentagonal plane (deviations <~0.2 Å). 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 pseudorotation exchanging axial and equatorial sites in the pentagonal bipyramid in a pairwise fashion. Activation parameters for the process in Ta(CH3)(η4-C10H8)(dmpe)2 are ΔH≠ = 13.0 ± 0.2 kcal/mol and ΔS≠ = 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. © 1979, American Chemical Society. All rights reserved.