SYNTHESIS AND STRUCTURE OF TRIS[DIHYDROBIS(PYRAZOLYL)BORATE] (TETRAHYDROFURAN)URANIUM(III), U[H(MU-H)BPZ(2)](3)(THF) - 3-CENTER B-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-U(III) INTERACTIONS IN THE PRESENCE OF COORDINATED THF LIGAND

Reaction of UI3(THF)(4) with three equivalents of KH(2)Bpz(2) gives the complex U[H(mu-H)Bpz(2)](3)(THF) (1). The molecular structure has been determined by single-crystal X-ray diffraction: monoclinic, space group P2(1)/c, a = 10.662(2), b = 13.586(3), c = 20.868(6) Angstrom, beta = 103.32(2)degrees, V = 2941(1) Angstrom(3) and Z = 4. The six nitrogen donor atoms of the chelating H(2)Bpz(2)(-) ligands are arranged in a slightly distorted trigonal prismatic geometry. Two of the H(2)Bpz(2)(-) ligands span triangular edges on a single square face of the trigonal prism, while the third spans the edge common to the remaining two square faces. The oxygen atom of the coordinated THF ligand caps one of the square faces. The remaining two rectangular faces and one of the two triangular faces of the trigonal prism are capped by three-center two-electron B-H...U bridge bonds, providing an effective ten-coordinate U(III) center. The observation of low frequency B-H stretching bands is in accord with the presence of these interactions. In solution the molecule is fluxional. Although the rearrangement is nearly stopped at -100 degrees C, its nature could not be elucidated. Repeated cycles of dissolution of 1 in toluene and solvent removal yield the THF-free complex U[H(mu-H)Bpz(2)](3) (2). The molecule is also fluxional. The activation energy for equilibration of the BH2 hydrogens is 51 kJ mol(-1).