A remarkable Lewis acid-base adduct: preparation and structure of [TaH 5(dmpe) 2][LiHBEt 3] (dmpe=1,2-bis(dimethylphosphino) ethane)

Abstract

Treatment of [TaCl 4(dmpe) 2] with Li[HBEt 3] yielded [TaH 5(dmpe) 2][LiHBEt 3].Thestructure of this molecule was determined at 130 K by a single crystal X-ray diffraction analysis. The molecule crystallised in the space group Pbca with a=16.430(1), b=12.153(1) and c=28.124(2) A ̊ . The structure was solved by Patterson methods for 4927 unique observed reflections with I ≥ 2.5 σ( I). The hydride ligands on tantalum were located from this difference Fourier and refined satisfactorily. Refinement on F o by full-matrix block-diagonal least-squim techniques with anisotropic thermal displacement parameters for the non-hydrogen atoms and one common isotropic thermal displacement parameter for the hydrogen atoms converged at R F =0.047. The lithium is located between the tantalum and boron atoms (LiTa 2.735(13), LiB 2.176(14) Å). It is bonded to the hydride of the triethylborohydride (LiH 1.70(7) Å) and also interacts with two of the tantalum hydrides and two hydrogens of the triethylborohydiide (one α-H each of two of the ethyl groups). The NMR spectroscopic data show shifts significantly different from the pentahydride [TaH 5(dmpe) 2] indicating that the adduct does not dissociate in toluene solution. The 1H NMR spectrum at room temperature shows a pentet for the tantalum hydrides. At −40°C this is split into two pentets at δ−0.71 and −1.50 ppm of relative intensity 1:4 and cooling to −90C produced three hydride signals of relative intensity 1:2:2.