NOE and PGSE NMR Spectroscopic Studies of Solution Structure and Aggregation in Metallocenium Ion-Pairs

The solution structures of the metallocenium homogeneous polymerization catalyst ion-pairs [Cp2ZrMe]+[MeB(C6F5)3]- (1), [(1,2-Me2Cp)2ZrMe]+[MeB(C6F5)3]- (2), [(Me2SiCp2)ZrMe]+[MeB(C6F5)3]- (3), [Me2C(Fluorenyl)(Cp)ZrMe]+[FPBA]- (FPBA = tris(2,2‘,2‘ ‘-nonafluorobiphenyl)fluoroaluminate) (4), [rac-Et(Indenyl)2ZrMe]+[FPBA]- (5), [(Me5Cp)2ThMe]+[B(C6F5)4]- (6), [(Me2SiCp2)Zr(Me)(THF)]+[MeB(C6F5)3]- (7), [(Me2SiCp2)Zr(Me)(PPh3)]+[MeB(C6F5)3]- (8), [(Me2SiCp2)Zr(Me)(THF)]+[B(C6F5)4]- (9), [(Me2Si(Me4Cp)(t-BuN)Zr(Me)(solvent)]+[B(C6F5)4]- (solvent = benzene, toluene) (10), [(Cp2ZrMe)2(μ-Me)]+[MePBB]- (PBB = tris(2,2‘,2”-nonafluorobiphenyl)borane) (11), and [(Cp2Zr)2(μ-CH2)(μ-Me)]+[MePBB]- (12), having the counteranion in the inner (1, 3, 4, 5, and 6) or outer (7, 8, 9, 10, 11, and 12) coordination sphere, have been investigated for the first time in solvents with low relative permittivity such as benzene or toluene by 1H NOESY and 1H,19F HOESY NMR spectroscopy. It is found that the average interionic solution structures of the inner sphere contact ion-pairs are similar to those in the solid state with the anion B-Me (1, 3) or Al-F (5) vectors oriented toward the free zirconium coordination site. The HOESY spectrum of complex 6 is in agreement with the reported solid-state structure. In contrast, in outer sphere contact ion-pairs 7, 8, 9, and 10, the anion is located far from the Zr-Me+ moiety and much nearer to the Me2Si bridge than in 3. The interionic structure of 8 is concentration-dependent, and for concentrations greater than 2 mM, a loss of structural localization is observed. PGSE NMR measurements as a function of concentration (0.1−5.0 mM) indicate that the tendency to form aggregates of nuclearity higher than simple ion-pairs is dependent on whether the anion is in the inner or outer coordination sphere of the metallocenium cation. Complexes 2, 3, 4, 5, and 6 show no evidence of aggregation up to 5 mM (well above concentrations typically used in catalysis) or at the limit of saturated solutions (complexes 3 and 6), while concentration-dependent behavior is observed for complexes 7, 8, 10, and 11. These outer sphere ion-pairs begin to exhibit significant evidence for ion-quadruples in solutions having concentrations greater than 0.5 mM with the tendency to aggregate being a function of metal ligation and anion structure. Above 2 mM, compound 8 exists as higher aggregates that are probably responsible for the loss of interionic structural specificity.