5372 Organometallics, Vol. 25, No. 22, 2006
Gonza´lez-Herna´ndez et al.
a Waters 150-C chromatograph eluting with 1,2,4-tricholorobenzene
at 135 °C. Narrow MWD polystyrene standards were used for GPC
calibration.
Conclusions
Zirconocene aluminohydride complexes are useful catalyst
precursors, which when activated by MAO offer advantages
with respect to both polymerization activity and polymer MW.
On the other hand activation with discrete Lewis acids such as
B(C6F5)3 is, unsurprisingly, complicated by the presence of alane
in these formulations. Thus, there is limited utility for these
catalyst precursors when compared with the corresponding
metallocene dialkyls and this discrete Lewis acid.
The multisite behavior exhibited by the B(C6F5)3-activated
Cp* catalyst at lower T can be viewed as arising from two
different catalysts. The model studies indicate that the initially
formed dinuclear ion-pair is thermally unstable, consistent with
multisite behavior. The ultimate decomposition product [Cp*2-
ZrH][µ-H2B(C6F5)2] formed in the absence of monomer is much
less active and thus cannot account for the multisite behavior.
Given the propensity of zirconocene hydride complexes to
engage in reversible C-H activation of the Cp* groups,23 it is
possible that the multisite behavior results from formation of
“tuck-in” complexes during propagation.
The (TMSCp)2ZrH3AlH2 complex gives rise to a more active
(and single-site) catalyst at elevated T when activated with
B(C6F5)3. It is reasonable to attribute this to the higher thermal
stability of the species partnered with the bridging HB(C6F5)3
anion. Certainly, discrete mononuclear complexes with this
bridging counterion are quite robust.11c Further, the increased
activity seen in the presence of excess B(C6F5)3 may be related
to activation of this otherwise dormant species, as seen with
other structurally characterized ion-pairs that feature a bridging
borohydride anion.11c
Given the tentative detection of a [(C6F5)3B(µ-H)B(C6F5)3]
anion in solution at low T in the present study, it is plausible to
invoke reversible, though transient, formation of an ion-pair
partnered with this counteranion as a possible mechanism for
this activation, and this will be the subject of future work.
Synthesis of Cp2ZrH3AlH2 Complexes [Cp ) Cp*, TMSCp,
nBuCp]. Complexes (TMSCp)2ZrH3AlH2 and (nBuCp)2ZrH3AlH2
were synthesized using the methods reported by Stephan and co-
workers,8b,c varying the solvent and temperature of reaction. A
solution of the corresponding metallocene dichloride (TMSCp)2ZrCl2
or (nBuCp)2ZrCl2 (4.57 mmol) in diethyl ether (25 mL) was stirred
at 0 °C, and 2.2 equiv of LiAlH4 (10.05 mmol) in diethyl ether
was added. The formation of a fine white powder was observed
when the mixture was allowed to warm to room temperature (30
min), and the solution was filtered through Celite. The diethyl ether
was evaporated to provide the aluminohydride complexes in
quantitative yield, which were dissolved in benzene, to obtain a
known concentration solution for each complex, which were stored
frozen at -30 °C.
The preparation of the Cp*2ZrH3AlH2 aluminohydride was
similar to the procedure described by Stephan in THF.8b,c Cp*2-
ZrCl2 (1.02 mmol) was dissolved in THF (6 mL), LiAlH4 (2.2
mmol, 2.0 M in THF) was added at room temperature, and the
solution was stirred for 1.5 h. The solvent was partially evaporated,
and the residue was extracted with hexane and filtered through
Celite. A white powder was obtained on concentrating the extracts
to dryness (98% crude yield) and was recrystallized from a mixture
of benzene and hexane at low temperature.
Synthesis of Cp*2ZrH(µ-H)2B(C6F5)2. Cp*2ZrH2 (0.18 g, 0.49
mmol) was added to a suspension of HB(C6F5)2 (0.17 g, 0.49 mmol)
in 5 mL of toluene a room temperature. After all the solids
dissolved, volatiles were removed in vacuo. A yellow solid was
obtained. Yield: 0.33 g, 95%. 1H NMR (300 MHz, 25 °C, C6D6):
δ 6.64 (s, 1H), 1.63 (s, 30 H), -0.73 (1:1:1:1 quartet, JBH ) 75
Hz, 2H). 19F NMR (288 MHz, 25 °C, C6D6): δ -130.3 (br d, o-F),
-157.6 (t, p-F), -163.6 (m, m-F). IR (Nujol, NaCl): 2139m,
2091m and 2018m (µ-H-B), 1639m, 1510s (Zr-H), 1377s, 1325s,
1096s, 1024m, 960s, 882m, 802m, 725m, 631m cm-1. Anal. Calcd
for C32H33F10BZr: C, 54.15; H, 4.65. Found: C, 54.01; H 4.48.
Formation of HAl(C6F5)2. R2AlCl (0.4 g, 0.1 mmol) and Cp2-
ZrHCl (0.26 g, 1 mmol) were mixed at room temperature in 20
mL of toluene. The mixture was stirred for 4 h, and all volatiles
were removed in a vacuum. Benzene (10 mL) was added and the
suspension filtered to remove Cp2ZrCl2. An oil was obtained after
the solvent was removed. 1H and 19F NMR spectra show a mixture
of HAl(C6F5)2 with residual zirconium impurities. By comparing
the intensity of 1H and 19F signals of a weighed amount of this oil
to those of a weighed amount of TFX in C6D6 the estimated purity
of this compound is about 95 mol %. 1H NMR (toluene-d8:/
bromobenzene-d5, 1:1, 300 MHz): δ 6.11 (br, Al-H). 19F NMR
(toluene-d8:/bromobenzene-d5, 1:1, 300 MHz): δ -122.3, -122.6,
-152.1, -152.4, -161.3, -161.8.
Experimental Section
General Data. All operations were carried out on a standard
high-vacuum line or in a drybox under inert atmosphere. Toluene,
diethyl ether, benzene, and bromobenzene were reagent grade,
distilled from the appropriate drying agents under Ar atmosphere.
Deuterated solvents were dried over P2O5 and distilled from
potassium and benzophenone. The B(C6F5)3 obtained from Aldrich
Co. was recrystallized from pentane or hexane solution at low T,
after being treated with a solution of BCl3 in heptane to remove
traces of water. Tri-isobutylaluminum (Aldrich), MAO (10%
toluene, Aldrich), and LiAlH4 (1M, Et2O, Aldrich) were used as
purchased. The compounds HB(C6F5)2,15 Cp*2ZrH2,14 ClAl(C6F5)2,18
(n-BuCp)2ZrCl2,24 and (TMSCp)2ZrCl225 were prepared by literature
methods.
The 19F NMR spectra and those obtained on the addition of a
small amount of Cp*2ZrH2 to this material are included as
Supporting Information.
Activation of Cp2ZrH3AlH2 Complexes [Cp ) Cp*, TMSCp,
nBuCp]. The activations with MAO were carried out dissolving
the aluminohydride in toluene, adding the prescribed amount of
MAO, and then the activated complexes were transferred to the
polymerization reactor by syringe. The activation of the alumino-
hydride complexes with B(C6F5)3 was carried out in toluene solution
at -30 °C or in bromobenzene/toluene (B/T) (1:1) mixtures at -50
°C. As for activations in toluene, the Cp′2ZrAlH5 complex was
sealed in a vial. The corresponding equivalents of the B(C6F5)3
dissolved in toluene were added at -30 °C. The reaction mixture
was allowed to warm at room temperature and added by syringe
to the polymerization reactor. In B/T mixtures, the Cp*2ZrAlH5,
dissolved in 1:1 bromobenzene/toluene, was added at -50 °C to
the corresponding equivalents of B(C6F5)3 in bromobenzene/toluene
The 1H and 11B NMR spectra were recorded on a Varian Inova
400 MHz instrument and were referenced to residual deuterated
solvent and external BF3‚Et2O, respectively. The 19F NMR spectra
were obtained on a Varian Inova 400 or Mercury 300 MHz
instrument using 2,3,5,6-tetrafluoroxylene (TFX) as an internal
reference at δ -145.69 relative to CFCl3. Polymer molecular
weights were determined by gel permeation chromatography using
(23) Bernskoetter, W. H.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J.
Organometallics 2006, 25, 1092-1100, and references therein.
(24) Davis, J. H.; Sun, H.-N.; Redfield, D.; Stucky, G. D. J. Magn. Reson.
1980, 37, 441-448.
(25) Lappert, M. F.; Riley, P. I.; Yarrow, P. I. W.; Atwood, J. L.; Hunter,
W. E.; Zaworotko, M. J. J. Chem. Soc., Dalton Trans. 1981, 814-821.
Gonzalez-Hernandez, Rebeca
