Treating ThCl4 with LiBH4 in various
ethereal
solvents affords the adducts Th(BH4)4(Et2O)2, Th(BH4)4(thf)2, and Th(BH4)4(dme) (thf = tetrahydrofuran
and dme = 1,2-dimethoxyethane). The structures of these three compounds
have been established by single-crystal X-ray diffraction: if the
tetrahydroborate groups are considered as occupying one coordination
site, the Et2O and thf complexes adopt trans-octahedral coordination geometries, whereas the dme complex exhibits
a cis-octahedral structure. All four BH4
– ligands in each compound are tridentate, rendering
each thorium center 14-coordinate. The Th···B distances
range from 2.64 to 2.67 Å, and the Th–O bond lengths are
2.47–2.52 Å. We propose that crystals of Th(BH4)4(thf)2 are isomorphous with those of U(BH4)4(thf)2, but owing to pseudosymmetry
the latter was reported in a unit cell that was too small by a factor
of 2. IR spectra and 1H and 11B NMR data are
reported as well. All three adducts are volatile, subliming readily
at 60 °C and 10–4 Torr, making them potentially
useful as precursors for the chemical vapor deposition (CVD) of thin
films of thorium boride. Passage of Th(BH4)4(Et2O)2 over glass, Si(100), and aluminum substrates
heated to 350 °C yields amorphous films of approximate stoichiometry
ThB2; films deposited from Th(BH4)4(thf)2 have stoichiometries closer to ThB2.5 and contain some oxygen. Auger, XPS, XRD, and SEM studies of these
films are reported.