A Unified View of Ethylene Polymerization by d0 and d0fn Transition Metals. 1. Precursor Compounds and Olefin Uptake Energetics

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Abstract

A systematic study has been carried out on the complexation of ethylene to a number of d0 [L]MC2H50,+,2+ fragments [M = Sc(III), Y(III), La(III), Lu(III), Ti(IV), Zr(IV), Hf(IV), Ce(IV), Th(IV), and V(V); L = NH(CH)2NH2- (1), N(BH2)(CH)2(BH2)N2- (2), O(CH)3O- (3), Cp22- (4), NHSi(H2)C5H42- (5), [(oxo)(O(CH)3O)]3- (6), (NH2)22- (7), (OH)22- (8), (CH3)22- (9), and NH(CH2)3NH2- (10)], where a hydrogen on the β-carbon of the ethyl unit is bound to the metal in an agostic interaction (β-agostic bond). It is shown that the complexation energy of an ethylene molecule to a [L]MC2H5n+ precursor can be predicted to within ±20 kJ/mol by simple empirical rules, based on the accessible metal surface of the [L]MC2H5n+ fragment and its gross charge. Discussions are also given of the relative preference for frontside (ethylene syn to β-agostic bond) versus backside (ethylene anti to β-agostic bond) coordination by the olefin as a function of the central atom, the auxiliary ligand set L and the strength of the β-agostic bond. It is finally shown that the β-agostic bond strength in the [L]MC2H5n+ precursor follows the order Ti ≈ Zr > Th > Hf for [L]MC2H5+ and Sc ≈ Y ≥ La > Lu for [L]MC2H5 for L = 7−9, with agostic interactions for uncharged precursor complexes [L]MC2H5 generally being weaker than for charged precursor complexes.

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