Treatment of (silox)3Ta (1, silox = tBu 3SiO) with BH3·THF and BCl2Ph afforded (silox)3Ta(BH3) (2) and (silox)3Ta- (η2-B,Cl-BCl2Ph) (3), which are both remarkably stable Ta(III) compounds. NMe3 and ethylene failed to remove BH3 from 2, and no indication of BH3 exchange with BH 3·THF-d8 was noted via variable-temperature 1H NMR studies. Addition of BH3·THF to (silox) 3TaH2 provided the borohydride-hydride (silox) 3HTa(η3-BH4) (5), and its thermolysis released H2 to generate 2. Exposure of 2 to D2 enabled the preparation of isotopologues (silox)3Ta(BH3-nD n) (n = 0, 2; 1, 2-D; 2, 2-D2; 3, 2-D3) for isotopic perturbation of chemical shift studies, but these failed to distinguish between "inverse adduct" (i.e., (silox)3Ta→BH 3) or (silox)3Ta(η2-B,H-BH3) forms of 2. Computational models (RO)3Ta(BH3) (R = H, 2′; SiH3,2SiH SiMe3,2SiMe, and SitBu3, 2SiBu) were investigated to assess the relative importance of steric and electronic effects on structure and bonding. With small R, η2-B,H structures were favored, but for 2 SiMe and 2SiBu, the dative structure proved to be similar in energy. The electonic iand vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the η2-B,H conformer of 2SiBu. In related computations pertaining to 3, small R models favored the oxidative addition of a BCl bond, while with R = Si tBu3 (3SiBu), an excellent match with its X-ray crystal structure revealed the critical steric influence of the silox ligands.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry