Role of Solvent Coordination on the Structure and Dynamics of ansa-Zirconocenium Ion Pairs in Aromatic Hydrocarbons

The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations (Me2Si-(2-methyl-4-phenyl-6-tert-butylindenyl)(2)ZrMe+, [1Me](-): Me2Si(2,4-dimethylindenyl)(2)ZrMe+, [2Me](+); Me2Si(indenyl)(2)ZrMe+, [3Me](+) paired with the weakly coordinating B(C6F5)(4)(-) anion have been investigated by NMR spectroscopy in aromatic hydrocarbons with different polarities (toluene, epsilon(r) = 2.38; chlorobenzene, epsilon(r) = 5.69; 1,2-difluorobenzene (ODFB), epsilon(r) = 13.38). These highly electrophilic cations are stabilized by solvent coordination, as evidenced by the unequivocal identification of [1Me center dot C7H8]B+(C6F5)(4-), a rare example of a toluene-stabilized ion pair that has been fully characterized in solution. Combining spectroscopic and DFT methods allowed us to evaluate how solvent coordination modulates the dynamic processes typical of this class of catalysts. An exchange between the two faces of coordinated toluene (face inversion, FI) occurs without solvent decomplexation (Delta H-FI(double dagger) = 14.6 kcal mol(-)(1); Delta S-FI(double dagger) = 3 cal mol(-1) K-1; Delta G(FI)(double dagger)(298) = 13.7 kcal mol(-1)) and is about 20 times faster than the exchange between coordinated and free solvent (solvent decomplexation, SD, Delta H-SD(double dagger), = 17.9 kcal mol(-1); Delta S-SD(double dagger) = 10 cal mol(-1) K-1; Delta G(SD)(double dagger) (298) = 14.9 kcal mol(-1)) and ion pair symmetrization (IPS, Delta H-IPS(double dagger) = 18.6 kcal mol(-1); Delta S-IPS(double dagger) = 12 cal mol(-1) K-1; Delta G(IPS)(double dagger)(298) = 14.9 kcal mol(-1)). For the ion pairs [1-3Me.solven]B+(C6F5)(4)(-), IPS rate constants (k(IPS)) do not correlate with the solvent polarity (k(IPS)(ODFB) > k(IPS)(toluene) > k(IPS)(chlorobenzene)). For the more coordinating toluene and chlorobenzene solvents, Delta G(IPS)(double dagger) nicely tracks with the amount of positive charge at the metal, increasing as the positive charge increases (a Azdvie,,cms; IMe2 > 3Me(2) > 2Me(2)). In contrast, in the less coordinating ODFB, differences in the IPS barriers for [I-3Me center dot ODFB](+)13(C6F5)(4)(-) appear to correlate better with steric parameters, as measured by the percent buried volume on the open quadrants (%V-Bur,V-open) for the corresponding neutral precursors (%V-B(ur,open) = 34.8, 35.5, 34.6% for 1Me(2), 2Me(2), and 3Me(2), respectively).

Automated summary

The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations paired with a weakly coordinating anion have been investigated in different solvents. Solvent coordination stabilizes the cations and affects their dynamic processes.