The search for new catalysts for Ziegler-Natta olefin polymerization: Exploratory studies in unusual direction

The preparation of two tetravalent d2 chromium alkyls is revisited. Their ability to work both as starting materials for further functionalization and as Ziegler-Natta catalysts is investigated. The clarification of the reaction pathway through which a trivalent chromium salt may form a homoleptic tetravalent derivative is also considered. Finally, the elucidation of the nature of the Cr-Cr interaction in a unique [R2Cr] 4 tetranuclear cluster and evaluation of its stability is undertaken. This is followed by a reassessment of the stability of tetravalent vanadium alkyls. Wilkinson first reported, in the seventies, the preparation of homoleptic R4V species [R = C6H5, CH2Si(CH 3)3]. The full characterisation of a homoleptic R4V, by crystal structure, and testing the catalytic activity is reported. The alpha,alpha-diiminopyridine ligand {alpha,alpha' -[2,6-(i-Pr)2PhN=C(Me)]2(C 5H3N)} has recently attracted considerable attention because of its ability to form highly active polymerization catalysts. Recent work from this research group and others has shown that this alpha,alpha '-diiminopyridine ligand can be involved in the organometallic transformations of the metal center. The ability of the ligand to accept or to donate negative charge to the metal is central to the fine tuning of the redox potential of the metal, and of its Lewis acidity that in turn determines the catalytic behaviour. A clarification for the ability of this remarkable ligand system to accept negative charge is offered. Reductions of {alpha,alpha'-[2,6-(i-Pr) 2PhN=C(Me)]2(C5H3N)} with strong reductants such as Li and Li(naftalenide) and in the absence of transition metals were carried out to yield rare case trilithium organyls