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-Me2Cp)(2)ZrMe2 (P) and a number of Lewis acids (A) to form the methide-bridged (contact) ion pair (1,2-Me2Cp)(2)ZrMe(mu-Me)A (I; P + A --> I + Delta H-ipf). This is the first step in the activation of P to becoming an olefin polymerization catalyst. The calculated enthalpies of formation (Delta H-ipf) for I were as follows (A, Delta H-ipf in kcal/mol): B(C6F5)(3) (1a), -23.8; B(C6F5)(2)(C6H3F2) (1b), -21.5; B(C6F5)(2)(C6H5) (1c), -18,3; B(C6F5)(2)(C6H3(CH3)(2) (1d) -18.0; B(C6H5)(3) (1e), -6.7; B(C10F7)(3) (1f) -25.8; (MeAlO)(6) (2a), -15.9; (MeBO)(6) (2b), -22;3; AlMe3 (2c), -8.1; Al(C6F5)(3) (2d), -30.8. The charge separation between the (1,2-Me2Cp)(2)ZrMe+ and AMe(-) fragments in I was calculated for all A, and it was found that the charge separation as well as -Delta 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 le and 2c. Also considered was the complete dissociation in solution (toluene) of I into the counterions [(1,2-Me2Cp)(2)ZrMe](+) and AMe(-) with the dissociating enthalpy Delta H-ips as well as the formation from I of the solvent-separated ion pair (S = toluene) [(1,2-Me2Cp)(2)ZrMe](+)-S-[AMe](-) with the reaction enthalpy Delta 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, Delta H-ips and Delta 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-Me2Cp)(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 Delta H-ips and Delta H-ss for the ion pair [(1,2-Me2Cp)(2)ZrMe](+)[A](-) (III'); A(-) = B(C6F5)(4)(-) (3a), Al(C6F5)(4)(-) (3b), [(C2B9H11)(2)Co](-) (3c), and {tBuCH(2)CH[B(C6F5)(2)]H}(-) (3d)), where III' is formed from the reaction of P with the activator [CPh3+][A(-)]. Here the calculated values are as follows (A(-), Delta H-ips and Delta 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.