Date: January 24, 2007
Creator: Bonanno, Jeffrey B.; Henry, Thomas P.; Wolczanski, Peter T.; Pierpont, Aaron W. & Cundari, Thomas R., 1964-
Description: This article discusses strong tantalum-to-boron dative interactions. Abstract: Treatment of (silox)3 Ta (1, silox = tBu3SiO) with BH3·THF and BCI2Ph afforded (silox)3Ta(BH3) (2) and (silox)3Ta-(ɳ2-B,CI-BCI2Ph) (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 BH3-THF-d8 was noted via variable-temperature 1H NMR studies. Addition of BH3-THF to (silox)3TaH2 provided the borohydride-hydride (silox)3HTa(ɳ3-BH4) (5), and its thermolysis released H2 to generate 2. Exposure of 2 to D2 enabled the preparation of isotopologues (silox)3Ta(BH3-nDn) (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→BH3) or (silox)3Ta(ɳ2-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, ɳ2-B,H structures were favored, but for 2SiMe and 2SiBu, the dative structure proved to be similar in energy. The electronic and vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the ɳ2-B,H conformer of 2SiBu. In related computations pertaining to 3, small R ...
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