Analysis of Metallocene−Methylalumoxane Methide Transfer Processes in Solution Using a 19F NMR Spectroscopic Probe
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Abstract
The metallocenes (C6F5C5H4)CpZrMe2 (1, Cp = C5H5) and (C6F5C5H4)2MMe2 (2a, M = Zr; 2b, M = Hf) were treated with B(C6F5)3 and, separately, with commercial methylalumoxane (MAO, depleted of excess Me3Al). The ensuing methide transfer reactions were followed by 19F NMR. Spectra of the “activated” species obtained using MAO and excess B(C6F5)3 were strikingly similar, suggesting that similar “cation-like” species are formed using either organo-Lewis acid, and the MAO-activated species were tentatively formulated as (C6F5C5H4)CpZrMe+ Me[AlMeO]n- and as (C6F5C5H4)2MMe+ Me[AlMeO]n- (M = Zr, Hf). The relative amount of L2MMe2 and [L2MMe]+ species observed in solution was measured as a function of [MAO] and as a function of [M] at constant [MAO]. Results are interpreted in terms of a canonical model in which MAO contains relatively few highly active Lewis acidic sites.
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