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

A systematic study has been carried out on the complexation of ethylene to a number of d 0 [L]MC 2 H 5 0,+,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) 2 NH 2 - ( 1 ), N(BH 2 )(CH) 2 (BH 2 )N 2 - ( 2 ), O(CH) 3 O - ( 3 ), Cp 2 2 - ( 4 ), NHSi(H 2 )C 5 H 4 2 - ( 5 ), [(oxo)(O(CH) 3 O)] 3 - ( 6 ), (NH 2 ) 2 2 - ( 7 ), (OH) 2 2 - ( 8 ), (CH 3 ) 2 2 - ( 9 ), and NH(CH 2 ) 3 NH 2 - ( 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]MC 2 H 5 n + precursor can be predicted to within ±20 kJ/mol by simple empirical rules, based on the accessible metal surface of the [L]MC 2 H 5 n + 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]MC 2 H 5 n + precursor follows the order Ti ≈ Zr > Th > Hf for [L]MC 2 H 5 + and Sc ≈ Y ≥ La > Lu for [L]MC 2 H 5 for L = 7 − 9, with agostic interactions for uncharged precursor complexes [L]MC 2 H 5 generally being weaker than for charged precursor complexes.