Metal-Alkyl Group Effects on the Thermodynamic Stability and Stereochemical Mobility of B(C₆F₅)₃-Derived Zr and Hf Metallocenium Ion-Pairs

Abstract

This paper reports significant dependence of ion-pair formation energetics and stereomutation rates upon the metal-bound alkyl substituent (R) and the solvent dielectric constant in the metallocenium series (1,2-Me2Cp)2MR+ MeB(C6F5)3 - where R = Me, CH2 t Bu, CH2TMS, CH(TMS)2, and M = Zr and Hf, as determined by reaction titration calorimetry and dynamic NMR spectroscopy. For the ion-pair forming reaction, (1,2-Me2Cp)2M(R)Me + B(C6F5)3 → (1,2-Me2Cp)2MR+ MeB(C6F5)3 -, enthalpies in toluene solution at 25 °C for M = Zr and R = Me, CH2TMS, and CH(TMS)2 are −24.6(0.8), −22.6(1.0), and −59.2(1.4) kcal/mol, respectively. Corresponding values for M = Hf and R = Me and CH2TMS are −20.8(0.5) and −31.1(1.6) kcal/mol, respectively. ΔH ‡ reorg values for the reorganization process that interchanges the ion-pair enantiotopic sites for M = Zr and R = Me, CH2 t Bu, CH2TMS, and CH(TMS)2 are 22(1), 18(1), 17(1), and 9(2) kcal/mol, respectively. Corresponding ΔH ‡ reorg values for M = Hf and R = Me, CH2 t Bu, and CH2TMS are >24, 12(3), and 15(2) kcal/mol, respectively. ΔH ‡ reorg values are highest in low dielectric constant solvents such as C6D12. Activation parameters for β-Me elimination in the complexes (1,2-Me2Cp)2MCH2 t Bu+ MeB(C6F5)3 - for M = Zr and Hf are found to be ΔH ‡ β - Me = 22.5(0.9) and 17.3(0.9) kcal/mol, and ΔS ‡ β - Me = 4.3(3.3) and −11.9(3.4) cal/mol·K, respectively.

The first thermally stable half-sandwich titanium zwitterionic complex

“…It can be suggested that the new set of distinct signals belonged to aggregated polynuclear species which were proposed to facilitate undesired side reactions [9]. The formation of polynuclear species was apparently due to an easier ion-pair separation in the more polar CD 2 Cl 2 solvent [10].…”

Formation and structures of cationic zirconium complexes in ternary systems (X=Cl, Me)

“…Filtering off the solid residues and the removal of volatiles gave an orange solid, which was washed with 10 ml of light petroleum and dried to afford the orange product 0.20 g (96%). 1 This complex displays a well-documented ion pair symmetrization [11b, 37,38] Complex IV was predominantly formed at Al/Zr ratios of 5-10 after 3-15 min shaking of the NMR tube with ðSBIÞZrCl 2 þ AlBu i 3 þ ½CPh 3 ½BðC 6 F 5 Þ 4 . IV formed a green or blue-green oil in the NMR tube.…”

Kinetic and mechanistic aspects of metallocene polymerisation catalysts

“…Scheme 3) and are rather poor models of the growing polymer chain. Bulkier alkyl ligands give much higher anion exchange rates, e.g., in the series (1,2-Me 2 Cp) 2 Zr(R)(l-Me)B(C 6 F 5 ) 3 (12) the reorganisation enthalpies decrease sharply with increasing bulkiness of the alkyl ligand, R = Me > CH 2 Bu t > CH 2 - [50]. For the ansa-zirconocenes 13, rates for R = CH 2 SiMe 3 are approximately four times higher than for R = Me, with values of ca.…”