Chelating aryloxide ligands in the synthesis of titanium, niobium, and tantalum compounds: Electrochemical studies and styrene polymerization activities

The chelating trisphenol. ligands tris(2-hydroxyphenyl)amine (1H(3)) and tris(2-hydroxy-4,6-dimethylbenzyl)amine (2H(3)) proved to be excellent precursors for the chelating phenoxides, and the latter has been used to prepare a series of cyclopentadienylmetal derivatives of early transition metals. For niobium and tantalum, reactions with CpMCl4 lead to the compounds CpMCl(1) and CpMCl(2). An X-ray diffraction study of CpNbCl(1) establishes a pseudo-octahedral structure with a trans disposition of the eta (5)-cyclopentadienyl ring and the nitrogen atom of the chelating ligand. Similar reactions of CpTiCl3 lead to the CpTi(1) and CpTi(2) analogues. Electrochemical experiments provide useful information on the reduction potentials of the compounds, from which it is clear that ligand 2 is a stronger donor than is 1. At the same time, it appears that chelate ring size is important; while the reduction of complexes containing 1 are largely reversible, those of complexes containing 2 are irreversible. This is interpreted to mean that the six-membered rings in the latter are opening during reduction, a process involving formal loss of an aryloxide from the metal center. In an attempt to correlate this solution reactivity with catalytic efficiency in a bond-forming process, the compounds were screened for activity as styrene polymerization catalysts in the presence of methylaluminoxane cocatalyst. While the niobium and tantalum analogues were inactive, the titanium compounds of I showed high activity and appreciable selectivity for the preparation of syndiotactic polystyrene.