Synthesis, Ion Aggregation, Alkyl Bonding Modes, and Dynamics of 14-Electron Metallocenium Ion Pairs [(SBI)MCH2SiMe3+···X-] (M = Zr, Hf): Inn
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- 作者：Fuquan Song ; Simon J. Lancaster ; Roderick D. Cannon ; Mark Schormann ; Simon M. Humphrey ; Cristiano Zuccaccia ; Alceo Macchioni ; and Manfred Bochmann
- 刊名：Organometallics
- 出版年：2005
- 出版时间：March 14, 2005
- 年：2005
- 卷：24
- 期：6
- 页码：1315 - 1328
- 全文大小：306K
- 年卷期：v.24,no.6(March 14, 2005)
- ISSN：1520-6041
文摘
The new mixed-alkyl metallocene complexes (SBI)M(Me)CH2SiMe3 (M = Zr, Hf) areaccessible by the successive treatment of (SBI)MCl2 with Me3SiCH2MgCl and MeMgCl intoluene (SBI = rac-Me2Si(1-Ind)2). Reaction with B(C6F5)3 or CPh3+[B(C6F5)4]- in toluene ortoluene/difluorobenzene affords (SBI)M
+(CH2SiMe3)(
-Me)B-(C6F5)3 and the ion pairs[(SBI)MCH2SiMe3+···B(C6F5)4-], respectively. Both types of compounds are thermally stablein aromatic solvents at ambient temperature. Whereas in the MeB(C6F5)3- complexes thealkyl ligand points away from the metal and tight anion coordination forms the familiarinner-sphere ion pair, in the B(C6F5)4- salts the alkyl ligand adopts a conformation thatenables agostic bonding to a 
-CH3 group. Here, and by implication in M-polymeryl speciesof similar steric requirements, agostic interactions are preferred over anion coordination,leading to an outer-sphere ion pair structure. This alkyl bonding mode retards the -SiMe3rotation, which for M = Hf is slow on the NMR time scale at -20 
C (at 300 MHz), while inthe zirconium analogue cooling to below -60 C is required. It was shown that chain swinginginvolves a 180 rotation of the alkyl ligand about the Zr-C bond. Measurements of diffusioncoefficients by pulsed field gradient spin-echo (PGSE) techniques suggest that while (SBI)Zr(CH2SiMe3)(-Me)B(C6F5)3 exists in solution as mononuclear zwitterions as expected,[(SBI)ZrCH2SiMe3+···B(C6F5)4-] forms ion quadruples ([Zr] 
2 mM), rising to hextuples athigher concentration. The relative positions of cations and anions depend on the ion pairconcentration; higher aggregates make it difficult to assign specific anion positions. Therate of ion pair symmetrization ("anion exchange" kex), as determined by variable-temperatureNMR spectroscopy, decreases with decreasing metallocene concentration. For [(SBI)ZrCH2SiMe3+···B(C6F5)4-] at 25 C and [Zr] = 2 mM, kex 500 ± 170 s-1; this value represents theupper limit of anion mobility expected under catalytic conditions where concentrations aretypically 100 times lower. Ion pair symmetrization rates are therefore at least 1 order ofmagnitude slower than the growth of the number-average molecular weight of polypropenechains (kp[M] 104 s-1 at [M] = 0.59 mol L-1) generated with tetraarylborate-based (SBI)Zrand other high-activity catalysts at identical temperatures. It is suggested that while forslower, inner-sphere ion pair catalysts the rate of 1-alkene consumption is commensuratewith kex ("continuous" chain propagation mechanism), high-activity catalysts may operateby a mechanism where the anion does not bind to the metal center and so does not limit therate of monomer enchainment. In such a situation, agostic metal-alkyl interactions formthe catalyst resting states in preference to anion coordination.
+(CH2SiMe3)(-Me)B-(C6F5)3 and the ion pairs[(SBI)MCH2SiMe3+···B(C6F5)4-], respectively. Both types of compounds are thermally stablein aromatic solvents at ambient temperature. Whereas in the MeB(C6F5)3- complexes thealkyl ligand points away from the metal and tight anion coordination forms the familiarinner-sphere ion pair, in the B(C6F5)4- salts the alkyl ligand adopts a conformation thatenables agostic bonding to a -CH3 group. Here, and by implication in M-polymeryl speciesof similar steric requirements, agostic interactions are preferred over anion coordination,leading to an outer-sphere ion pair structure. This alkyl bonding mode retards the -SiMe3rotation, which for M = Hf is slow on the NMR time scale at -20 C (at 300 MHz), while inthe zirconium analogue cooling to below -60 C is required. It was shown that chain swinginginvolves a 180 rotation of the alkyl ligand about the Zr-C bond. Measurements of diffusioncoefficients by pulsed field gradient spin-echo (PGSE) techniques suggest that while (SBI)Zr(CH2SiMe3)(-Me)B(C6F5)3 exists in solution as mononuclear zwitterions as expected,[(SBI)ZrCH2SiMe3+···B(C6F5)4-] forms ion quadruples ([Zr] 2 mM), rising to hextuples athigher concentration. The relative positions of cations and anions depend on the ion pairconcentration; higher aggregates make it difficult to assign specific anion positions. Therate of ion pair symmetrization ("anion exchange" kex), as determined by variable-temperatureNMR spectroscopy, decreases with decreasing metallocene concentration. For [(SBI)ZrCH2SiMe3+···B(C6F5)4-] at 25 C and [Zr] = 2 mM, kex 500 ± 170 s-1; this value represents theupper limit of anion mobility expected under catalytic conditions where concentrations aretypically 100 times lower. Ion pair symmetrization rates are therefore at least 1 order ofmagnitude slower than the growth of the number-average molecular weight of polypropenechains (kp[M] 104 s-1 at [M] = 0.59 mol L-1) generated with tetraarylborate-based (SBI)Zrand other high-activity catalysts at identical temperatures. It is suggested that while forslower, inner-sphere ion pair catalysts the rate of 1-alkene consumption is commensuratewith kex ("continuous" chain propagation mechanism), high-activity catalysts may operateby a mechanism where the anion does not bind to the metal center and so does not limit therate of monomer enchainment. In such a situation, agostic metal-alkyl interactions formthe catalyst resting states in preference to anion coordination.