Structure of (tetrahydroborato)bis(tetrahydrofuran)uranium(IV): a model for hydrogen–uranium bonding

Synthesis and Characterization of Ether Adducts of Thorium Tetrahydroborate Th(BH4)4 and Chemical Vapor Deposition of Thorium Boride Thin Films

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 ThB_2.5 and contain some oxygen. Auger, XPS, XRD, and SEM studies of these films are reported.