Organometallics
Note
were performed with EA 1110-FISONS(CE) and ICP-MASS HP-4500
instruments, respectively.
RESULTS AND DISCUSSION
Synthesis and Characterization. Compound 1 was
obtained by adding dropwise a solution of CpTiCl3 in THF
to an equimolar solution of LH in THF at −78 °C, as outlined
in Scheme 1. Similarly, compounds 2 and 3 were prepared by
■
Synthesis of LH. The tetrazole ligand LH was synthesized by the
literature procedure.8
Synthesis of [CpTiLCl2] (1). A solution of CpTiCl3 (1.0 mmol,
0.22 g) in 20 mL of THF was added dropwise to a solution of LH (1.0
mmol, 0.16 g) in 20 mL of THF at −78 °C. The reaction mixture was
warmed to room temperature and stirred for 12 h. The reaction
mixture was then filtered through Celite. All volatiles were removed
under vacuum, and the residue was washed with 20 mL of cold n-
pentane. The desired product 1 was isolated as red crystals after the
toluene solution remained at −20 °C in a refrigerator for a few days
(0.28 g, 68%). 1H NMR (400.13 MHz, CDCl3, ppm): δ 8.63 (d, 1H, J
= 7.8 Hz), 7.38 (t, 1H, J = 7.8 Hz), 7.01 (t, 3H of η5-C5H5, J = 7.6 Hz),
6.88 (s, 1H), 6.84 (d, 1H, J = 8.3 Hz), 6.61 (s, 2H of η5-C5H5), 3.74 (t,
THF), 1.84 (t, THF). 13C{1H} NMR (100.63 MHz, CDCl3, ppm): δ
150.9, 135.2, 128.9, 123.4, 122.9, 122.7, 122.2, 117.3, 108.8, 68.0
(THF), 25.6 (THF). Anal. Calcd for C18H22Cl2N4O2.5Ti (1·1.5THF):
C, 47.41; H, 4.89; N, 12.36. Found: C, 47.65; H, 4.90; N, 12.63.
Synthesis of [TiLCl3(THF)] (2). A solution of TiCl4 (1.0 mmol, 1.0
mL, 1 M solution in CH2Cl2) in CH2Cl2 was added by syringe over 1
min to a cold (−78 °C) solution of LH (1.0 mmol, 0.16 g) in 30 mL of
THF. The reaction mixture was warmed to room temperature over 12
h. The reaction mixture was then filtered through Celite. After the
solvent was reduced, the crude product was recrystallized by cooling to
Scheme 1. Synthetic Routes to Complexes 1−3
1
−20 °C, affording red crystals in a yield of 72% (0.33 g). H NMR
(400.13 MHz, CDCl3, ppm): δ 7.85 (br s, 1H), 7.54 (t, 1H, J = 8.5
Hz), 7.18 (t, 1H, J = 7.6 Hz), 7.00 (d, 1H, J = 8.3 Hz), 4.16 (t, THF),
1.98 (t, THF). 13C{1H} NMR (100.63 MHz, CDCl3, ppm): δ 163.3,
150.3, 134.8, 127.3, 124.7, 116.5, 114.1, 71.6 (THF), 25.6 (THF).
Anal. Calcd for C15H21Cl3N4O3Ti (2·THF): C, 39.20; H, 4.61; N,
12.19. Found: C, 39.28; H, 4.59; N, 12.09.
Synthesis of [TiL2Cl2] (3). In a manner analogous to the procedure
for 2, 3 was prepared from a solution of TiCl4 (1.0 mmol, 1.0 mL, 1 M
solution in CH2Cl2) in THF and LH (2.0 mmol, 0.32 g) in a yield of
71% (0.48 g). 1H NMR (400.13 MHz, DMSO-d6, ppm): δ 10.99 (br s,
2H of NH of tetrazole), 7.95 (d, 2H, J = 9.5 Hz), 7.37 (t, 2H, J = 8.6
Hz), 7.09 (d, 2H, J = 9.1 Hz), 6.96 (t, 2H, J = 8.0 Hz), 3.55 (t, THF),
1.72 (t, THF). 13C{1H} NMR (100.63 MHz, CDCl3, ppm): δ 155.4,
151.7, 132.6, 129.0, 119.6, 116.4, 110.4, 67.0 (THF), 25.1 (THF).
Anal. Calcd for C22H26Cl2N8O4Ti (3·2THF): C, 45.15; H, 4.48; N,
19.15. Found: C, 45.33; H, 4.52; N, 18.96.
adding a solution of TiCl4 in CH2Cl2 to equimolar and 2 molar
amounts of LH in THF at −78 °C, respectively. Compounds
1−3 were obtained in high yield (68−72%) and were purified
by washing with n-hexane and recrystallized in THF. They were
remarkably stable in the solid state for a few days. They
precipitated out within a couple of hours in chloroform-d in
capped NMR tubes. Compounds 1−3 are soluble in toluene
and polar organic solvents such as THF, CH2Cl2, and acetone;
however, ligand LH is insoluble in most solvents such as
toluene, THF, water, and acetone.
1
Compounds 1−3 were characterized by H and 13C{1H}
NMR spectroscopy, elemental analysis, and single-crystal X-ray
crystallography. The 1H NMR spectra of complexes 1−3
display well-defined resonances with the expected integrations.
Also, well-defined resonances for the aromatic carbons are
observed in the 13C{1H} NMR spectrum, and there is no
indication of the presence of higher oligomers or isomers. In
comparison to the free LH ligand, all signals in 1−3 are shifted
downfield, a consequence of complexation with the Lewis
X-ray Structure Determination for 1−3. Reflection data for 1−
3 were collected at 130 K on a Bruker APEX II CCD area
diffractometer with graphite-monochromated Mo Kα radiation (λ =
́
0.7107 Å). Specimens of suitable quality and size were selected,
mounted, and centered in the X-ray beam by using a video camera.
The hemisphere of reflection data was collected as ω scan frames with
0.5°/frame and an exposure time of 10 s/frame. Cell parameters were
determined and refined by the SMART program.10 Data reduction was
performed using SAINT software.11 The data were corrected for
Lorentz and polarization effects. An empirical absorption correction
was applied using the SADABS program.12 The structures of the
compounds were solved by direct methods and refined by full-matrix
least-squares methods using the SHELXTL program package with
anisotropic thermal parameters for all non-hydrogen atoms.13 Further
details are given in the Supporting Information. X-ray crystal structures
were drawn by the Diamond program, version 2.1e.
1
acidic titanium metal. The most interesting point in the H
NMR spectra of complexes 1−3 is the existence of peaks
corresponding to the THF molecules. Complexes 1 and 3 have
two THF peaks at 3.74 and 1.84 ppm, similar to those of a free
THF molecule (free THF: 3.76, 1.85 ppm). In contrast,
complex 2 has broad THF peaks at 4.16 and 1.98 ppm that are
significantly shifted downfield from the signals for free THF.
This indicates that the titanium metal possesses direct and
strong bonding with the THF molecule in the structure of
complex 2, which was also confirmed by an X-ray structure
analysis. These interactions between the THF molecules and
tetrazole ligands are likely important main factors in the
increased solubility of 1−3. Due to their strong interactions,
THF molecules were not removed after evacuation and were
thus observed in the 1H NMR spectra of complexes 1−3. These
results are consistent with X-ray structures of 1−3 in the
solution phase.
Cycloaddition Tests of CO2 to Propylene Oxide. Cyclo-
addition of CO2 was carried out by charging a stirring bar, propylene
oxide, and titanium catalysts into a 20 mL stainless steel reactor in the
glovebox, which was then sealed and removed from the glovebox. The
reactor was pressurized to 22 bar of CO2 and was heated to the
reaction temperature (75 or 25 °C). After the contents were stirred for
4.5 h, the reactor was cooled and vented, and a small sample of the
1
mixture was taken for H NMR analysis.
B
dx.doi.org/10.1021/om400605k | Organometallics XXXX, XXX, XXX−XXX