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

Treating ThCl 4 with LiBH 4 in various ethereal solvents affords the adducts Th(BH 4 ) 4 (Et 2 O) 2 , Th(BH 4 ) 4 (thf) 2 , and Th(BH 4 ) 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 Et 2 O and thf complexes adopt trans -octahedral coordination geometries, whereas the dme complex exhibits a cis -octahedral structure. All four BH 4 - 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(BH 4 ) 4 (thf) 2 are isomorphous with those of U(BH 4 ) 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 1 H and 11 B 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(BH 4 ) 4 (Et 2 O) 2 over glass, Si(100), and aluminum substrates heated to 350 °C yields amorphous films of approximate stoichiometry ThB 2 ; films deposited from Th(BH 4 ) 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.