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

Abstract

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.