5834 Organometallics, Vol. 24, No. 24, 2005
Lincoln et al.
moisture-sensitive compounds. Dichlorofluoromethane45 and
2-isopropylpyridine48 were synthesized according to the lit-
erature procedures. Dichlorofluoromethane, all pyridines used,
d5-chlorobenzene (Fisher), and d8-toluene were stirred over
calcium hydride for at least 6 h under a nitrogen blanket and
distilled and degassed prior to use. Tris(pentafluorophenyl)-
borane and trityl tetrakis(pentafluorophenyl)borate (Strem)
were used as delivered. All chemicals were obtained from
Aldrich unless otherwise noted.
Synthesis of (2-Phenylindenyl)2ZrMe2. The compound
was prepared following a slight modification of the literature
procedure reported by Coates and Resconi.1,49 Methyllithium
(24.0 mL, 1.6 M in diethyl ether, 38.4 mmol) was added
dropwise to a stirred solution of 2-phenylindene (3.85 g, 20.0
mmol) in 250 mL of diethyl ether at -78 °C. The resulting
dark brown solution was allowed to warm to room temperature
and stirred in the dark for 12 h. The solution was then
transferred to a suspension of zirconium tetrachloride (2.55
g, 11.0 mmol) in 50 mL of pentane. When the lighter brown
solution began to darken after 1 h, the solvent was removed
in vacuo, leaving a dark brown residue. The product was
dissolved in toluene, filtered through Celite, and concentrated
in vacuo to 50 mL. The product crystallized from this solution
at -50 °C as a pale yellow solid (3.3 g, 6.5 mmol, 65% yield).
1H NMR (C6D6, 500 MHz): δ (ppm) 7.31 (d, J ) 0.01 Hz, 4H),
7.20 (d, J ) 0.01 Hz, 4H), 7.10 (t, J ) 0.02 Hz, 2H), 7.04 (q, J
) 0.08 Hz, 4H), 6.94 (q, J ) 0.08 Hz, 4H), 6.90 (t, J ) 0.10 Hz,
4H), 5.96 (s, 4H, Cp), -1.05 (s, 6H, Zr-CH3).
correlation between the tert-butyl resonance and both
cyclopentadienyl resonances at room temperature,
whereas the tert-butyl resonance shows a correlation to
only the upfield cyclopentadienyl resonance at -40 °C
(Figure 3). At this temperature, a correlation analogous
to that observed in the low-temperature 6a NOESY
spectrum is observed between the tert-butyl protons and
the ortho protons of the 2-phenyl substituent, again
suggesting that the predominant syn isomer holds these
groups in close proximity.
Finally, to better characterize the [(2-PhInd)2ZrMe-
(2-iPr-pyridine)+][MeB(C6F5)3-] intermediate (6a′), 2-iso-
propylpyridine was added to 2a at -40 °C and a NOESY
experiment was performed prior to the elimination of
methane to generate 6a. This spectrum reveals a
correlation between the Zr-CH3 resonance at 0.20 ppm
and both cyclopentadienyl resonances, and a correlation
between both the isopropylpyridine methyl resonance
(δ ) 0.70 ppm) and methine resonance (δ ) 1.07 ppm)
and only the downfield cyclopentadienyl resonance(δ )
6.14 ppm) (Figure 3). Such behavior is also consistent
with the presence of only one syn conformer.
Thus, the low-temperature NOESY spectra of the
cations 4a, 6a, 6a′, and 7a reveal that 2-phenylindene
metallocene cations ligated by cyclometalated 2-alkyl
pyridines exist primarily as the achiral syn conformers
at -40 °C. These results imply that the achiral syn
isomers are accessible conformations not only in the
solid state1 but also in solution. Moreover these results
suggest that the nature of the accessible conformations
of these conformationally dynamic catalysts is a com-
plicated function of the nature of the arylindene ligands,
the cation/anion interactions, and the nature of the alkyl
group bound to the metallocene cation.
Preparation of Methyl Cations. Compounds 2a and 2b
were prepared for NMR analysis as follows. Under nitrogen,
both 1 and B(C6F5)3 or [Ph3C+][B(C6F5)4-] were weighed
directly into a 5 mm Teflon-capped (J. Young) NMR tube,
dissolved in C6D5Cl, and allowed to react for at least 20 min
prior to use in NMR experiments unless otherwise indicated.
The rate of decomposition of the methyl cations has prevented
their characterization by elemental analysis or X-ray crystal-
lography. The methyl cations have also proven insufficiently
stable toward appropriate solvents to permit characterization
1
by mass spectroscopy. In lieu of such analyses, the H NMR
Conclusions
spectra of 2a and 2b at room temperature and below are
included in the Supporting Information, and their 1H and 13
chemical shifts are recorded in Tables 3 and 4.
C
NMR investigations of conformationally dynamic un-
bridged zirconocenium methyl cations [(2-PhInd)2ZrMe+]-
[RB(C6F5)3-] (R ) Me, C6F5) reveal that association of
sterically hindered counterions [RB(C6F5)3-] does not
provide a sufficient steric barrier to inhibit the confor-
mational dynamics of the 2-arylindene ligands. No
evidence was found that suggests the association of the
anions with the (2-PhInd)2ZrMe+ cations leads to “locked”
conformations at temperatures above -40 °C. The
activation barrier for ion-pair separation of the zir-
Preparation of Zirconocenium Pyridyl Cations. The
o-substituted pyridyl cations 3a-8b were prepared for NMR
analysis as follows. Under nitrogen, stock solutions of pyridine
or 2-alkylpyridines (∼50 mg/1 mL C6D5Cl) were prepared, and
slightly less than 1 equiv was added by micropipet to a Teflon-
capped (J. Young) NMR tube containing the appropriate
methyl cation (2a or 2b) in C6D5Cl. The sample was allowed
to react for at least 20 min prior to use in NMR experiments
unless otherwise indicated. When preparing the pyridyl,
methylpyridyl, and ethylpyridyl complexes, slightly less than
1 equiv of the pyridine was used, as these complexes are prone
-
conocene cations and RB(C6F5)3 was shown to be
slightly higher but in the range observed for related
zirconocene methyl cations. The higher activation bar-
1
to coordination of a second equivalent of pyridine; in the H
-
riers for ion-pair separation of CH3B(C6F5)3 relative
NMR spectra, this slight imbalance is manifested in reso-
-
nances near -0.2 ppm due to [(2-PhInd)2ZrCH3+][MeB(C6F5)3
]
-
to B(C6F5)4 are consistent with polymerization data
and near -0.6 ppm due to [(2-PhInd)2Zr+- -CH3- --MeB(C6F5)3].
Attempts to isolate the pyridyl complexes produce oils from
which solvent cannot be removed completely; their character-
ization by elemental analysis or X-ray crystallography has thus
been prevented. The pyridyl cations have also proven insuf-
ficiently stable toward appropriate solvents to permit charac-
terization by mass spectroscopy. In lieu of such analyses, the
room-temperature 1H NMR chemical shifts of 3a-8a are
recorded in Table 3, and for the representative complexes 4a,
6a, and 7a, the room-temperature 13C NMR chemical shifts
-
that indicate the B(C6F5)4 anion coordinates more
-
weakly to the metal center than the CH3B(C6F5)3
anion, leading to higher productivities.
Cationic o-substituted pyridyl complexes were pre-
pared as steric analogues to active cationic species
ligated to a growing polymer chain. Low-temperature
NOESY spectra of the cyclometalated pyridine zir-
conocene cations provide the first evidence for the
presence of Cs-symmetric syn conformations in solution.
Experimental Section
(48) Pasquinet, E.; Rocca, P.; Marsais, F.; Godard, A.; Queguiner,
G. Tetrahedron 1998, 54, 8771-8782.
(49) Balboni, D.; Camurati, I.; Prini, G.; Resconi, L.; Galli, S.;
Mercandelli, P.; Sironi, A. Inorg. Chem. 2001, 40, 6588-6597.
General Procedures. Standard Schlenk techniques and
a Vacuum Atmospheres drybox were used to handle air- and
Lincoln, Alice L.
