Niobium Organometallics

Abstract

This chapter deals with the progress in organometallic chemistry of niobium during 1993–2004, when the chemistry has been dominated by the thermodynamic driving force to attain the highest oxidation state. This is stabilized, in most cases, through the formation of Nb–O, Nb–N, or similar multiple bonds, although an appreciable number of complexes in the intermediate +3 oxidation state have also been described – particularly with cyclopentadienyl (Cp) or related ligands, such as tris-pyrazol-1-ylborates (Tp). New examples of complexes with η1-ligands, especially cyclopentadienyl-containing alkyl, silyl, and hydride derivatives, have been isolated, and, in some cases, unexpected structures (e.g., hypervalent situations for silicon in silyl-containing niobocenes) and different modes of reaction have been found. It is also worth noting the large number of examples reported in which insertion processes into Nb–alkyl bonds, especially with isocyanides as reagents, take place and mostly give rise to either η2-iminoacyl derivatives or different types of related niobacycle-containing species. A small group of structurally well-characterized η2-acyl complexes has also been reported during this period. In contrast, the development of the chemistry of alkylidene–niobium complexes has been more restricted, probably because much more is known about the olefin metathesis catalytic processes. New examples of π-bonded ligand-containing complexes, especially cyclopentadienyl olefin and -alkyne derivatives, have also been published, although in general, new types of reactivity, especially those based on insertion and C–C coupling reactions, have not been discovered with regard to those previously published in COMC (1995). The chemistry of π-ene and -enyl complexes (η3- to η7-complexes) has also developed markedly in this period and numerous new compounds have been prepared that are not covered in COMC (1995). For example, interesting systematic studies using the fragment Cp(η4-1,3-diene)Nb, which is isolobal to Cp2M (M = group 4 metal), in catalytic polymerizations of ethylene, norbornene, and methylmethacrylate have been carried out. The chemistry of cyclopentadienyl systems with different types of substituted Cp rings, namely, bis-cyclopentadienyl and monocyclopentadienyl derivatives with different kinds of ancillary ligands, has also been at the forefront in this area. In this respect, a large number of new classes of complexes – some of which exhibit rich reactivity – have been described in detail. Furthermore, the progress in the study of new classes of cyclopentadienyl-containing complexes is also noteworthy; for example, ansa-niobocene, a constrained geometry ansa-cyclopentadienyl and imido-cyclopentadienyl-containing niobium derivatives have opened up new possibilities in terms of reactivity studies. For example, the isolobal relationship between Cp(NR)Nb and Cp2M (M = group 4 metal) fragments has been described, and different olefin polymerization catalytic studies based on the cyclopentadienyl-imido niobium unit have been considered. As a continuation of previous studies compiled in COMC (1995), new examples of niobium complexes have proved beneficial in modeling and understanding some catalytic processes, including the previously mentioned olefin polymerizations. Finally, a number of complexes have been useful in different organic syntheses in the field of metal-mediated organic synthesis, especially in C–C bond formation processes based on the use of lower-valent niobium complexes.