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Cationic Organoscandium β-Diketiminato Chemistry:  Arene Exchange Kinetics in Solvent Separated Ion Pairs

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posted on 18.04.2003, 00:00 by Paul G. Hayes, Warren E. Piers, Masood Parvez
Abstraction of methide from the β-diketiminato supported organoscandium complex [LScMe2]2 using the trityl borate activator [Ph3C][B(C6F5)4] in arene solvents gives solvent separated ion pairs in which the arene (C6H5Br, 1a; C6H6, 1b; C7H8, 1c; 1,3,5-Me3C6H3, 1d) is coordinated to the cationic scandium center in an η6 bonding mode. L1 incorporates methyl groups in the 2,4 positions of the ligand backbone and bulky 2,6-diisopropylphenyl groups on the nitrogen atoms. The relative binding strength of the arenes is C6H5Br ≪ C6H6 < 1,3,5-Me3C6H3 < C7H8. Ion pairs 1a and 1c have been characterized crystallographically, and the C6H5Br derivative is notable for its η6 bonding mode in preference to the more common η1 bonding mode via the halogen atom. The kinetics of displacement of mesitylene by toluene (1d1c) yield activation parameters of ΔH = 21.4(6) kcal mol-1 and ΔS = 6(1) cal mol-1 K-1. In combination with the observed lack of dependence of [toluene] on the rate of displacement, these data suggest a mechanism involving partial dissociation of the coordinated arene, followed by attack of the incoming arene. This chemistry has relevance to the role of these solvent separated ion pairs in olefin polymerization processes and presents a rare opportunity for the detailed study of these ephemeral species.