| Popis: |
NbCl5 reacts with NaBH4 and LiCp+ (Cp+ = EtMe4C5) dependent on the stoichiometry and temperature to give Cp+2NbCl2 (1), CP+2Nb2Cl2(B2H6)(2), CP+2NbBH4(3a) and Cp+ 2Nb2(B2H6)2 (4a). With a large excess of NaBH4, Cp+ 2NbCl2 gives 2 at 25-degrees-C, which reacts further with NaBH 4 to give 4a. The synthesis of the deep violet, very stable 4a can be generalised by using C5Me5Li and tBUC5H4Li, respectively. Compounds 1-4 are characterised by means of IR, H-1, and heteroatom NMR spectra. An X-ray structure analysis was carried out with 4a. Whereas 1 and 3 are analogous to already known complexes, in 2 and 4a the B2H, dianion (which is not stable in the free state) is present as a bridging ligand, oriented perpendicularly with respect to the Nb-Nb axis. So far, the highest nuclear magnetic Nb-93 shielding has been observed for compounds 4, extending the range of known delta(Nb-93) values by 2500 ppm. The temperature dependence of the linewidths of both the B-11 and the Nb-93 NMR signals of 4c allows evaluation of the magnitude of J((NbB)-Nb-93-B-11 = 50 +/- 10 Hz. |
