A Density Functional Study of Ion-Pair Formation and Dissociation in the Reaction between Boron- and Aluminum-Based Lewis Acids with (1,2-Me₂Cp)₂ZrMe₂

Calculations have been carried out on the reaction between the olefin polymerization precatalyst (1,2-Me 2 Cp) 2 ZrMe 2 (P) and a number of Lewis acids (A) to form the methide-bridged (contact) ion pair (1,2-Me 2 Cp) 2 ZrMe(μ-Me)A ( I; P + A → I + Δ H ipf ). This is the first step in the activation of P to becoming an olefin polymerization catalyst. The calculated enthalpies of formation (Δ H ipf ) for I were as follows (A, Δ H ipf in kcal/mol): B(C 6 F 5 ) 3 ( 1a ), −23.8; B(C 6 F 5 ) 2 (C 6 H 3 F 2 ) ( 1b ), −21.5; B(C 6 F 5 ) 2 (C 6 H 5 ) ( 1c ), −18.3; B(C 6 F 5 ) 2 (C 6 H 3 (CH 3 ) 2 ( 1d ), −18.0; B(C 6 H 5 ) 3 ( 1e ), −6.7; B(C 10 F 7 ) 3 ( 1f ), −25.8; (MeAlO) 6 ( 2a ), −15.9; (MeBO) 6 ( 2b ), −22.3; AlMe 3 ( 2c ), −8.1; Al(C 6 F 5 ) 3 ( 2d ), −30.8. The charge separation between the (1,2-Me 2 Cp) 2 ZrMe + and AMe - fragments in I was calculated for all A, and it was found that the charge separation as well as −Δ H ipf increases through the series 1e, 1c, 1b, and 1a with the number of fluorine atoms. A good activating Lewis acid (A) has the equilibrium shifted strongly from P and A toward I, and this is the case for all A except 1e and 2c. Also considered was the complete dissociation in solution (toluene) of I into the counterions [(1,2-Me 2 Cp) 2 ZrMe] + and AMe - with the dissociating enthalpy Δ H ips as well as the formation from I of the solvent-separated ion pair (S = toluene) [(1,2-Me 2 Cp) 2 ZrMe] + −S−[AMe] - with the reaction enthalpy Δ H ss . The two types of separation processes have both been postulated as the second and final steps in the activation of P. The calculated values are as follows (A, Δ H ips and Δ H ss in kcal/mol): 1a, 38.0, 18.7; 1f, 43.6, 18.9; 2a, 57.0, 32.4; 2b, 46.9, 35.3; 2c, 69.2, 35.3; 2d, 48.3, 20.6. It is concluded that the formation of [(1,2-Me 2 Cp) 2 ZrMe] + −S−[AMe] - is the more likely separation process. Consideration has also been given to the influence of solvent polarity on the separation process, with S = toluene, chlorobenzene, and 1,2-dichlorobenzene. Finally discussed are Δ H ips and Δ H ss for the ion pair [(1,2-Me 2 Cp) 2 ZrMe] + [A] - ( III ‘); A - = B(C 6 F 5 ) 4 - ( 3a ), Al(C 6 F 5 ) 4 - (3b ), [(C 2 B 9 H 11 ) 2 Co] - ( 3c ), and { t BuCH 2 CH[B(C 6 F 5 ) 2 ]H} - ( 3d )), where III ‘ is formed from the reaction of P with the activator [CPh 3 + ][A - ]. Here the calculated values are as follows (A -, Δ H ips and Δ H ss in kcal/mol): 3a, 22.1, −4.2; 3b, 26.2, 0.7; 3c, 34.9, 7.5; 3d, 26.7, −0.5. It is found that III type ion pairs are easier to dissociate than I held together by a methide bridge.

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