Biomedical subjects

Nicholas G Stahl

Publications and source records attributed to Nicholas G Stahl.

3 recordsLinked to original sources

PubMed2005

B(C6F5)3- vs Al(C6F5)3-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences.

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.

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PubMed2004

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 [Cp(2)ZrMe](+)[MeB(C(6)F(5))(3)](-) (1), [(1,2-Me(2)Cp)(2)ZrMe](+)[MeB(C(6)F(5))(3)](-) (2), [(Me(2)SiCp(2))ZrMe](+)[MeB(C(6)F(5))(3)](-) (3), [Me(2)C(Fluorenyl)(Cp)ZrMe](+)[FPBA](-) (FPBA = tris(2,2',2' '-nonafluorobiphenyl)fluoroaluminate) (4), [rac-Et(Indenyl)(2)ZrMe](+)[FPBA](-) (5), [(Me(5)Cp)(2)ThMe](+)[B(C(6)F(5))(4)](-) (6), [(Me(2)SiCp(2))Zr(Me)(THF)](+)[MeB(C(6)F(5))(3)](-) (7), [(Me(2)SiCp(2))Zr(Me)(PPh(3))](+)[MeB(C(6)F(5))(3)](-) (8), [(Me(2)SiCp(2))Zr(Me)(THF)](+)[B(C(6)F(5))(4)](-) (9), [(Me(2)Si(Me(4)Cp)(t-BuN)Zr(Me)(solvent)](+)[B(C(6)F(5))(4)](-) (solvent = benzene, toluene) (10), [(Cp(2)ZrMe)(2)(mu-Me)](+)[MePBB](-) (PBB = tris(2,2',2"-nonafluorobiphenyl)borane) (11), and [(Cp(2)Zr)(2)(mu-CH(2))(mu-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 (1)H NOESY and (1)H,(19)F 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 Me(2)Si 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.

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PubMed2003

Metallocene polymerization catalyst ion-pair aggregation by cryoscopy and pulsed field gradient spin-echo NMR diffusion measurements.

Pulsed field gradient spin-echo (PGSE) NMR and cryoscopic measurements have been performed on a series of homogeneous metallocene polymerization catalyst ion-pairs to determine if aggregation is a significant phenomenon under typical polymerization conditions. Cryoscopic measurements on [(Me5Cp)2ZrMe]+[MeB(C6F5)3]- (1), [rac-Et(Indenyl)2ZrMe]+[MeB(C6F5)3]- (2), [(1,2-Me2Cp)2ZrCHTMS2]+[MeB(C6F5)3]- (3), [Me2Si(Me4Cp)(t-BuN)TiMe]+[MeB(C6F5)3]- (4), [Me2Si(Me4Cp)(t-BuN)ZrMe]+[MeB(C6F5)3]- (5), and [Me2C(Fluorenyl)(Cp)ZrMe]+[MeB(C6F5)3]- (6) were carried out in benzene in the 10-18 millimolal concentration range. PGSE measurements, using (p-tolyl)4Si as an internal standard, were also performed on catalyst ion-pairs 1, 4, 6, [(Me5Cp)2ThMe]+[B(C6F5)4]- (7), [(Me2SiCp2)ZrMe]+[MeB(C6F5)3]- (8), and [Cp2ZrMe]+[MeB(C6F5)3]- (9) in the 0.8-10.0 millimolar range. All results are consistent with a 1:1 ion-pair structural model and show little evidence for ion-quadruples or higher-order aggregates.

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