METAL TETRAHYDRIDOBORATES
Metal tetrahydridoborates have been applied as single-source precursors for CVD of metal borides. An example is the deposition of ZrB2 and HfB2 thin films from the volatile borohydride complexes Zr(BH4)4 and Hf(BH4)4. [[i], [ii],730,731,753] Unfortunately, the ability to grow a various-metal MBx layers from metal tetrahydridoborates is limited by the scarcity of homoleptic M(BH4)n complexes. [101,[iii]]. In addition, most of them are polymeric or oligomeric and therefore non-volatile (such as alkali metal, Be(II), Ti (III), Th(IV), Pa, or U(IV) derivatives) [329] , and only five are monomers (Al(III), Zr(IV) [[iv]], Hf (IV) [[v]], Np(IV) [329e, [vi]] or Pu(IV) [329e] tetrahydridoborates)
The rarity of volatile M(BH4)n complexes has two main reasons: 1) the BH4- group is sterically small and 2) it is a strong reductant. The small size means that three or four BH4 groups are required to saturate the coordination spheres of most transition metals. (i.e. metal oxidation state must be +3 or +4). For many transition metals, however, these oxidation states are not stable in the presence of strongly reducing BH4- groups. [101]
Separate borohydride complexes of metals are discussed under corresponding metal chapters (Cr, Ti, Zr, Hf) of the current encyclopedia.
Zr(BH4)4 and Hf(BH4)4 for CVD of ZrB2 and HfB2
Volatile borohydride complexes Zr(BH4)4 and Hf(BH4)4 habe been applied for the deposition of high-quality ZrB2 and HfB2 thin films by CVD [10, 11]
[i] Wayda, A. L.; Schneemeyer, L. F.; Opila, R. L. Appl. Phys. Lett. 1988, 53, 361-363.
[ii] Tolle, J.; Roucka, R.; Tsong, I. S. T.; Ritter, C.; Crozier, P. A.; Chizmeshya, A. V. G.; Kouvetakis, J. Appl. Phys. Lett. 2003, 82, 2398-2400
[iii] a) Marynick, D. S.; Lipscomb, W. N. Inorg. Chem. 1972, 11, 820-823; b) Dain, C. J.; Downs, A. J.; Goode, M. J.; Evans, D. G.; Nicholls, K. T.;
b) Rankin, D. W. H.; Robertson, H. E. J. Chem. Soc., Dalton Trans. 1991, 967-977;
c) Hoekstra, H. R.; Katz, J. J. J. Am. Chem. Soc. 1949, 71, 2488-2492;
d) Banks, R. H.; Edelstein, N. M.; Rietz, R. R.; Templeton, D. H.; Zalkin, A. J. Am. Chem. Soc. 1978, 100, 1957-1958.
(e) Banks, R. H.; Edelstein, N. M. In Lanthanide and Actinide Chemistry and Spectroscopy; Edelstein, N. M., Ed.; ACS Symp. Series 131; ACS: Washington, DC, 1980; pp 331-348.
(f) Bernstein, E. R.; Hamilton, W. C.; Keiderling, T. A.; La Placa, S. J.;
g) Lippard, S. J.; Mayerle, J. J. Inorg. Chem. 1972, 11, 3009-3016.
(h Charpin, P.; Nierlich, M.; Vigner, D.; Lance, M.; Baudry, D. Acta Crystallogr. C 1987, 43, 1465-1467;
i) Aldridge, S.; Blake, A. J.; Downs, A. J.; Gould, R. O.; Parsons, S.; Pulham, C. R. J. Chem. Soc., Dalton. Trans. 1997, 1007-1012.
[iv] (a) Bird, P. H.; Churchill, M. R. J. Chem. Soc., Chem. Commun. 1967, 403. (b) Haaland, A.; Shorokhov, D. J.; Tutukin, A. V.; Volden, H. V.; Swang, O.; McGrady, G. S.; Kaltsoyannis, N.; Downs, A. J.; Tang, C. Y.; Turner, J. F. C. Inorg. Chem. 2002, 41, 6646-6655.
[v] a) Broach, R. W.; Chuang, I.-S.; Marks, T. J.; Williams, J. M. Inorg. Chem. 1983, 22, 1081-1084. (b) Borisenko, K. B.; Downs, A. J.; Robertson, H. E.; Rankin, D. W. H.; Tang, C. Y. Dalton Trans. 2004, 967-970
[vi] Banks, R. H.; Edelstein, N. M.; Spencer, B.; Templeton, D. H.; Zalkin, A., J. Am. Chem. Soc. 1980, 102, 620-623