Facile synthesis of a dimeric titanium(iv) complex with terminal TiO moieties and its application as a catalyst for the cycloaddition reaction of CO2 to epoxides

Article abstract of DOI:10.1039/c6ra23255h

In this report, facile and exclusive synthesis of dimeric titanium(iv) complex with a terminal TiO moiety from the reaction between novel pyridine-based tridentate ligand (LH₂) and Ti(O-i-Pr)₄ under the bubbling of wet air is presented. Alternatively, the same dimeric Ti complex was obtained via wet air bubbling of monomeric LTi(O-i-Pr)₂ or addition of the same equiv. of H₂O into LTi(O-i-Pr)₂. All compounds including LH₂ and two titanium complexes were characterized by single crystal X-ray analyses. Newly synthesized terminal oxo-titanium compound is the first example of structurally characterized dimeric terminal oxo-titanium compound having no TiO→Ti bonds. Two titanium complexes were used as effective catalysts for the cycloaddition of CO₂ to propylene oxide in the presence of various kinds of cocatalysts such as n-Bu₄PBr, n-Bu₄NI, n-Bu₄NBr, n-Bu₄NCl, PPNCl, and DMAP.

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DOI: 10.1039/C6RA23255H
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PAPER
Facile Synthesis of Dimeric Titanium (IV) Complex with Terminal Ti=O
Moiety and Its Application as a Catalyst for Cycloaddition Reaction of
CO2 to Epoxides†
Hyejin Kim,‡a Sung Ho Choi,‡a Duseong Ahn,a Yoseph Kim,a Ji Yeon Ryu,b Junseong Lee*b and Youngjo Kim*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/C6RA00000x
In this report, facile and exclusive synthesis of dimeric titanium (IV) complex with a terminal Ti=O
moiety from the reaction between novel pyridineꢀbased tridentate ligand (LH2) and Ti(OꢀiꢀPr)4 under the
bubbling of wet air is presented. Alternatively, the same dimeric Ti complex was obtained via wet air
10 bubbling of monomeric LTi(OꢀiꢀPr)2 or addition of the same equiv of H2O into LTi(OꢀiꢀPr)2. All
compounds including LH2 and two titanium complexes were characterized by single crystal Xꢀray
analyses. Newly synthesized terminal oxoꢀtitanium compound is the first example of structurally
characterized dimeric terminal oxoꢀtitanium compound having no Ti=O→Ti bonds. Two titanium
complexes were used as effective catalysts for the cycloaddition of CO2 to propylene oxide in the
15 presence of various kinds of cocatalysts such as nꢀBu4PBr, nꢀBu4NI, nꢀBu4NBr, nꢀBu4NCl, PPNCl, and
DMAP.
the literature.20ꢀ32 Interestingly, only titanium compounds
chelated by sterically encumbering ligands such as tetradentate
porphyrinꢀtype ligands,20ꢀ23 two bidentate ketiminate ligands,24
two Cp* ligands,25ꢀ27 tetradentate salenꢀtype ligand,28 tridentate
50 Me3tacn ligands (where Me3tacn is 1,4,7ꢀtrimethylꢀ1,4,7ꢀ
triazacyclononae),29,30 two bidentate benzamidinate ligands,31 and
tridentate PNP scaffold ligand32 could generate terminal oxo
product. Their major coordination geometries around titanium are
square pyramidal.20ꢀ24 Others are trigonal planar,25 tetrahedral,26,27
55 trigonal bipyramidal,28 and octahedral.29ꢀ32 Interestingly,
structurally characterized terminal oxoꢀ titanium complexes are
all monomeric though some dimeric Ti compounds with
Ti=O→Ti bonds are reported in the literature.33ꢀ36 In those cases,
TiꢀO bond distances are about 1.80 Å, which is much longer than
60 pure Ti=O bond distances of 1.65 Å.20ꢀ32 To our best knowledge,
real dimeric terminal oxotitanium compound without Ti=O→Ti
bonds has never been reported in the literature. In this regard, we
focused on the development of new dimeric or polymeric
terminal oxoꢀTi compounds having no Ti=O→Ti bonds.
Introduction
The efficient capture and utilization of carbon dioxide, a
valuable C1 building block, have been an important theme
20 environmentally and chemically for a long time.1 Thus, the
development of chemical methods to activate and convert CO2
into methanol, formic acid, urea, cyclic carbonate, and
polycarbonates has drawn a significant attention in recent years.
Among them, the cyclic carbonates from CO2 and epoxide have
25 been produced since 1950s and find their use as aprotic solvents,2
electrolytes for secondary batteries,3 and monomers for polymers4
and pharmaceutical intermediate.5
A variety of metalꢀbased catalytic systems, including those
with Cr, Fe, Co, Ni, Cu, Al, and so on, have been developed to
30 couple CO2 and epoxide.6ꢀ10 Titanium, the seventh most abundant
metal in earth’s crust (e.g., Al, Fe, Ca, Na, K, Mg, Ti in the
order), could be an attractive metal candidate due to its low
toxicity and high Lewis acidity.10 Although Tiꢀbased complexes
have been widely used as catalysts for various organic
35 transformation and polymer synthesis,11ꢀ13 a few examples of Tiꢀ
based catalytic systems for the cycloaddition of CO2 to epoxide
have been reported in the literature.14ꢀ20
65
Whereas a lot of catalytic systems for CO2 conversion into
cyclic carbonates have been reported in the literature, the research
of Tiꢀbased complexes is an unexplored field.14ꢀ17 Previous work
in our group explored the importance of electronic effect at Ti
center on catalytic activity for cycloaddition reaction.16 During
Due to high oxophilic property of titanium metal, many
titanium compounds could easily react with oxygenꢀdonating
40 sources such as H2O, O2, and CO to form oxotitanium complexes.
In general, oxotitanium compounds can be classified as two types
of complexes depending on functional connectivity with: (1)
quite common and bridging [TiꢀOꢀTi]6+ unit and (2) very rare and
terminal [Ti=O]2+ moiety. Several structurally characterized
45 titanium complexes with terminal Ti=O group were reported in
70 our continuous search for new Tiꢀbased catalysts, we report here
the synthesis and characterization of diisopropoxytitanium
complex 1 containing the novel 2ꢀ tridentate aminopyridineꢀbased
ligand LH2 and its facile transformation to terminal oxo complex
2, along with their catalytic behavior in the cycloaddition reaction
75 of CO2 to epoxides.
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6.8 and 1.2 Hz, 1H, Ar–H), 7.13(t, J = 7.9 Hz, 1H, Ar–H), 6.56(d,
J = 9.5 Hz, 1H, Ar–H), 5.92(dt, J = 6.7 and 1.2 Hz, 1H, Ar–H),
45 4.04 (s, 2H, Ar=NCH2–), 3.03 (s, 2H, –NCH2–), 1.25 (s, 6H, –
CMe2OH), 1.19 (s, 6H, –CMe2OH). 13C NMR (CDCl3, 100.63
MHz):
δ 156.7, 139.8, 136.3, 113.2, 102.5 (Ar), 71.30
(Ar=NCH2–), 70.56(–NCH2–), 64.00 (–CMe2OH), 58.80 (–
CMe2OH), 27.99 (–CMe2OH), 27.61 (–CMe2OH). 13C NMR
50 (CD3OD, 100.63 MHz): δ 157.9, 141.8, 138.0, 113.6, 103.9(Ar),
72.64 (Ar=NCH2–), 72.36(–NCH2–), 64.76 (–CMe2OH), 59.92 (–
CMe2OH), 27.95 (–CMe2OH), 27.86 (–CMe2OH). HRMS m/z
calcd for C13H22N2O2 239.1760, found 239.1754.
Synthesis of LTi(O-i-Pr)2 (1)
55
Scheme 1 Synthetic routes for complexes 1 and 2.
Experimental
Methods and Materials
60
All manipulations were carried out under an atmosphere of
dinitrogen by using standard Schlenkꢀtype glassware on a dual
manifold Schlenk line in
To a stirred colorless solution of LH2 (0.596 g, 2.50 mmol) in 30
mL of toluene was added Ti(O–i–Pr)4 (0.711 g, 2.50 mmol) at 0
oC. The reaction mixture was allowed to warm to room
temperature and stirred for overnight. The residue, obtained by
65 removing the solvent under vacuum, was recrystallized in toluene.
The desired product 1 was isolated as yellow crystals after the
solution remained at ꢀ15 oC in a refrigerator for a few days
(94.7%, 0.953 g). 1H NMR (CDCl3, 400.13 MHz): δ 7.18 (m, 1H,
Ar–H), 7.15 (dt, J = 1.2 and 5.6 Hz, 1H, Ar–H), 6.61 (d, J = 9 Hz,
70 1H, Ar–H), 6.10 (td, J = 6.6 and 1.3 Hz, 1H, Ar–H), 4.63 (m, 2H,
–OCHMe2), 4.04 (br, 2H, Ar=NCH2–), 3.28 (br, 2H, –NCH2–),
1.29 (s, 6H, ꢀCMe2O–), 1.23 (s, 6H, –CMe2O–), 1.15 (d, J = 6.1
Hz, 6H, –OCHMe2), 1.11 (d, J = 6.1 Hz, 6H, –OCHMe2). 1H
NMR (CD3OD, 400.15 MHz): δ 7.58 (s, 1H, Ar–H), 7.49 (d, J
75 =4.9 Hz, 1H, Ar–H), 6.90 (d, J = 5.4 Hz, 1H, Ar–H), 6.35 (d, J =
3.8 Hz, 1H, Ar–H), 4.15 (s, 2H, Ar=NCH2–), 3.91 (m, 2H, –
OCHMe2), 3.18 (s, 2H, –NCH2–), 1.27 (d, J = 7.4 Hz, 12H, –
CMe2O–), 1.15 (d, J = 6.1 Hz, 12H, –CMe2O–) 13C NMR (CDCl3,
100.63 MHz): δ 159.2, 140.2, 136.7, 115.1, 105.2 (Ar), 79.97
80 (Ar=NCH2–), 77.13 (–NCH2–), 75.90 (–OCMe2), 66.01 (–
OCMe2), 64.06 (–OCMe2), 28.81(–OCMe2), 26.06 (–OCMe2),
25.95 (–OCMe2). 13C NMR (CD3OD, 100.63 MHz): δ 157.4,
142.5, 140.4, 113.8, 107.8 (Ar), 72.55 (Ar=NCH2–), 71.82 (–
NCH2–), 65.34 (–OCMe2), 64.74 (–OCMe2), 57.94 (–OCMe2),
85 27.67 (–OCMe2), 27.65 (–OCMe2), 25.25 (–OCMe2). Anal.
Calcd. for C19H34N2O4Ti: C, 56.72; H, 8.52; N, 6.96. Found: C,
56.84; H, 8.28; N, 7.12.
5
a
glove box.37 Dinitrogen was
deoxygenated using activated Cu catalyst and dried with calcium
sulfate.38 All chemicals were purchased from Aldrich and used as
supplied unless otherwise indicated. All solvents such as toluene,
10 diethyl ether, and nꢀhexane, were dried by distillation from
sodium diphenylketyl under dinitrogen and were stored over 3Å
activated molecular sieves. CDCl3 and CD3OD were dried over
4Å activated molecular sieves and used after vacuum transfer to a
Schlenk tube equipped with a J. Young valve.
15
1H and 13C NMR spectra were recorded at ambient temperature
on a 400 MHz NMR spectrometer using standard parameters. All
chemical shifts are reported in δ units with reference to the peaks
of residual CDCl3 (δ 7.24, 1H NMR; δ 77.0, 13CNMR) or CD3OD
(δ 3.30, 1H NMR; δ 49.0, 13C NMR). Elemental analyses and
20 mass data measurements were performed with EA 1110ꢀFISONS
analyser and VG Auto Spec., respectively.
Synthesis of HOCMe2CH2NC5H4=NCH2CMe2OH (LH2)
25
30
Synthesis of [LTi(=O)]2 (2)
Isobutylene oxide (9.01 g, 125 mmol) and 2ꢀaminopyridine (4.71
g, 50.0 mmol) were added to a 20 mL screw cap vial containing
stirring bar. The vial was tightly sealed by Teflon tape and
paraffin film. The mixture was maintained at room temperature
35 for overnight and was then heated for 3 days at 75 °C. The
removed of volatiles at reduced pressure gave the desired product
LH2 (11.8 g, 99.0 %) as a yellow powder. 1H NMR (CDCl3,
400.15 MHz): δ 7.76 (br, 1H, OH), 7.03 (t, J = 7.9Hz 1H, Ar–H),
6.97(d, 1H, J = 6.8Hz, Ar–H), 6.45(d, 1H, J = 9.4 Hz, Ar–H),
40 5.81(t, 1H, J = 6.6 Hz, Ar–H), 3.97 (s, 2H, Ar=NCH2–), 2.99 (s,
2H, –NCH2–), 2.44 (br, 1H, OH), 1.27 (s, 6H, –CMe2OH), 1.23(s,
6H, –CMe2OH). 1H NMR (CD3OD, 400.15 MHz): δ 7.28(dd, J =
90
95 (Route 1) To a stirred colorless solution of LH2 (0.596 g, 2.50
mmol) in 30 mL of toluene was added Ti(OꢀiꢀPr)4 (0.711 g, 2.50
mmol) at 0 oC. The reaction mixture was allowed to warm to
room temperature and stirred for overnight. After 12 h, an air
using air pump was bubbled into the reaction vessel for more than
2
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Table 1 Crystallographic data for LH2, 1 and 2
6 h. The residue, obtained by removing the solvent under vacuum,
was recrystallized in toluene. The desired product 2 was isolated
as yellow crystals after the solution remained at ꢀ15 oC in a
refrigerator for a few days (94.5%, 1.42 g).
(Route 2) To a solution of 1 (1.00 g, 2.50 mmol) in 30 mL
toluene was bubbled an air using air pump for more than 4 h. The
removal of volatiles at reduced pressure gave the desired product
2 (91.8%, 1.38 g) as a yellow powder.
Sample
LH2
1
2
Chemical formula
Formula mass
Crystal system
Space group
a (Å)
C13H22N2O2
238.33
Monoclinic
P21/c
12.9760(3)
9.0107(2)
12.2487(3)
90
C19H34N2O4Ti C28H42Cl6N4O6Ti2
402.38
Monoclinic
P21/n
10.145(2)
14.8386(3)
14.7511(3)
90
839.15
Monoclinic
P21/n
9.5970(3)
19.6755(6)
9.9008(3)
90
5
b (Å)
c (Å)
(Route 3) To a solution of 1 (1.00 g, 2.50 mmol) in 30 mL
α
β
γ
(o)
(o)
(o)
Z
10 toluene was added H2O (0.045 g, 2.5 mmol), which was degassed
prior to adding with dinitrogen for 5 min. After 4 h, the removal
of volatiles at reduced pressure gave the desired product 2 (94.0%,
1.41 g) as a yellow powder. 1H NMR (CDCl3, 400.13 MHz): δ
7.02 (t, J = 7.8 Hz, 1H, Ar–H), 6.97 (d, J = 6.8 Hz, 1H, Ar–H),
15 6.44 (d, J = 9.4 Hz, 1H, Ar–H), 5.80 (t, J = 6.6 Hz, 1H, Ar–H),
3.96 (s, 2H, Ar=NCH2–), 2.98 (s, 2H, –NCH2–), 1.26 (s, 6H, –
CMe2O–), 1.23 (s, 6H, –CMe2O–). 1H NMR (CD3OD, 400.15
MHz): δ 7.36 (d, J = 7.8Hz, 1H, Ar–H), 7.23 (m, 1H, Ar–H), 6.64
(d, J = 9.4Hz, 1H, Ar–H), 6.01 (t, J = 6.7Hz, 1H, Ar–H), 4.07 (s,
20 2H, Ar=NCH2–), 3.07 (s, 2H, –NCH2–), 1.25 (s, 6H, –CMe2O–),
1.21 (s, 6H, – CMe2O–). 13C NMR (CDCl3, 100.63 MHz): δ
156.8, 139.9, 136.3, 113.3, 102.6 (Ar), 71.37 (Ar=NCH2–), 70.63
(–NCH2–), 64.07 (–OCMe2), 58.90 (–OCMe2), 28.07 (–OCMe2),
27.69 (–OCMe2). 13C NMR (CD3OD, 100.63 MHz): δ 158.2,
25 142.0 138.7, 113.5, 105.2 (Ar), 72.63 (Ar=NCH2–), 72.24 (–
NCH2–), 68.86 (–OCMe2), 59.47 (–OCMe2), 27.87 (–OCMe2),
26.49 (–OCMe2). Anal. Calcd. for C26H41N4O6Ti2: C, 51.93; H,
6.87; N, 9.32. Found: C, 52.19; H, 7.04; N, 9.11.
102.498(2)
90
103.9490(10)
90
105.588(2)
90
4
4
2
V (Å3)
Temperature (K)
Density (g cmꢀ3)
ꢀ (mmꢀ1)
1398.22(6)
100(2)
1.132
0.077
520
2146.51(8)
100(2)
1.245
0.423
864
1800.76(10)
100(2)
1.548
0.934
864
F(000)
θ range (o)
2.77 – 25.00 1.977 – 30.689 2.070 – 26.573
ꢀ15 ≤ h ≤ 15
ꢀ10 ≤ k ≤ 10
ꢀ14 ≤ l ≤ 13
ꢀ10 ≤ h ≤ 14
ꢀ21 ≤ k ≤ 16
ꢀ21 ≤ l ≤ 19
ꢀ12 ≤ h ≤ 12
ꢀ24 ≤ k ≤ 24
ꢀ12 ≤ l ≤ 12
Index ranges
Reflections
collected
# of Independent
reflections
# of Parameters
GOF(I > 2σ(I))
R1(I > 2σ(I)) a
wR2(I > 2σ(I)) a
7785
25010
31380
2464
6620
3738
161
272
212
1.009
0.0373
0.0872
1.021
0.0322
0.0816
1.030
0.0396
0.0955
55 a R1 = Σ||F |–|Fc||/Σ|Fo| and wR2 = {Σ[w(Fo –Fc2)2]/Σ[w(Fo )2]}1/2
2
2
o
Representative procedures for the cycloaddition reaction of
epoxide and CO2
(Trial to synthesis of 2 using dry air bubbling method) To a
30 solution of 1 (1.00 g, 2.50 mmol) in 30 mL toluene was bubbled
an air, which was passed through the drierite column, for more
than 4 h. The removal of volatiles at reduced pressure gave only
unreacted 1.
The cycloaddition reaction of CO2 to epoxide was carried out by
charging a stirring bar, epoxide (10 mmol), catalyst (20 ꢁmol),
60 and cocatalyst (20 ꢁmol), into a stainless steel pressure reactor
(10 mL inner volume). Then, CO2 was charged in the reactor and
the pressure was adjusted to desired pressure at appropriate
temperature. The reactor was maintained for the desired time.
After then, the pressure reactor was cooled to ambient
65 temperature, and the excess CO2 was vented. A small sample of
mixture was taken for 1H NMR analysis.
35 Single crystal X-ray diffraction
Single crystals suitable for Xꢀray diffraction analysis were
obtained from NMR cells containing LH2, 1, and 2 dissolved in
CDCl3. The crystallographic measurements were performed at
296(2) K for all compounds LH2, 1, and 2 using a Bruker APEX
40 II diffractometer with Mo Kα (λ= 0.71073 Å) radiation.
Specimens of suitable quality and size were selected, coated with
Paratone® oil, mounted onto a glass capillary, and centred in the
Xꢀray beam by using a video camera. The structures were solved
by the direct method and refined by full matrix leastꢀsquares
45 methods using the SHELXTL39 program package with
anisotropic thermal parameters for all nonꢀhydrogen atoms. Final
refinement based on the reflections (I > 2σ (I)) converged at R1 =
0.0373, wR2 = 0.0872, and GOF = 1.009 for LH2, and at R1 =
Results and Discussion
The tridentate ligand HOCMe2CH2NC5H4=NCH2CMe2OH
(LH2) was synthesized by the reaction of 2ꢀaminopyridine with 2
70 equivalents of isobutylene oxide in almost quantitative yield. As
depicted in Scheme 2, L2H2 was an initial expected product;
however, the isolated product from this reaction was LH2, whose
the resonance in 6ꢀmembered pyridine ring was collapsed. This
structure was confirmed by single crystal Xꢀray analysis (vide
75 infra).
0.0322, wR2 = 0.0876, and GOF = 1.021 for 1, and at R1
=
50 0.0396, wR2 = 0.1022, and GOF = 1.030 for 2. Further details are
listed in Table 1. Xꢀray crystal structures were drawn by the
Diamond Program ver. 2.1e.40 Details for crystallographic data
and parameters are listed in Table 1.
Scheme 2 The result of the reaction between 2ꢀaminopyridine and 2
equiv of isobutylene oxide
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As outlined in Scheme 1,
a
new monomeric
diisopropoxytitanium complex was prepared via simple
1
exchange reaction by adding a solution of Ti(OꢀiꢀPr)4 dropwise to
an equimolar solution of LH2 in toluene. Analytically pure
samples were obtained as yellow crystalline solids after the
removal of the volatiles. Terminal oxo titanium complex 2 can be
made by three different synthetic routes as shown in Scheme 1.
For Route 1, compound 2 was prepared in situ by treating mixed
solution of Ti(OꢀiꢀPr)4 and LH2 with wet air bubbling (Figures 11
5
10 and 12). Wet air bubbling of compound 1 (Figure S16) and the
direct addition of water into compound 1 (Figure S17) were
applied for Route 2 and Route 3, respectively. Interestingly, dry
air bubbling of compound 1 did not work for the synthesis of
compound 2 (Figure S18). Thus, oxygen of Ti=O in compound 2
15 definitely comes from water. Route 1 is better than Routes 2 and
3 in regards to yield and time efficiency. Unlike compound 1,
compound 2 was stable in air. All compounds LH2, 1, and 2 were
characterized by 1H and 13C NMR spectroscopies, HR mass
spectrometry for LH2 and elemental analysis for 1 and 2 and by
20 single crystal Xꢀray crystallographic method.
Fig. 2 Xꢀray structure of 1 with thermal ellipsoids drawn at the 50%
probability level. All hydrogens are omitted for clarity. Selected bond
lengths (Å): Ti1–N2 2.2685(9); Ti1–O1 1.8119(8); Ti1–O2 1.8663(8);
Ti1–O3 1.8074(9); Ti1–O4 1.8586(8); N1–C1 1.3719(14); C1–C2
1.3520(17); C2–C3 1.4076(18); C3–C4 1.3582(16); C4–C5 1.4358(15);
C5–N1 1.3871(14); C5–N2 1.3215(13); C6–N1 1.4685(14); N2–C10
1.4701(14). Selected angles (deg): O1–Ti1–O2 140.83(4); O1–Ti1–O3
106.71(4); O1–Ti1–O4 93.98(4); O2–Ti1–O3 109.73(4); O2–Ti1–O4
89.70(4); O3–Ti1–O4 105.73(4); N2–Ti1–O1 87.29(4); N2–Ti1–O2
76.01(3); N2–Ti1–O3 94.63(4); N2–Ti1–O4 158.25(4); Ti1–O1–C7
152.41(8); Ti1–O2–C11 125.90(7); Ti1–O3–C14 135.37(10); Ti1–O4–
C17 130.52(7).
45
50
1
The H and 13C NMR spectra were in accord with the suggested
structures, and all chemical shifts of the protons and carbons for
1
LH2, 1, and 2 were in the expected range. H NMR spectra of 1
and 2 show the absence of signals for alcoholic protons,
25 corroborating successful formation of Ti complexes chelated by
L2- ligand. Their molecular structures were determined by singleꢀ
crystal Xꢀray diffraction analysis. Xꢀray quality crystals were
o
obtained by fractional recrystallization at ꢀ15 C in a refrigerator
for a few days. Figures 1 – 3 display the molecular structures of
30 LH2, 1, and 2, respectively.
Fig. 3 Xꢀray structure of 2 with thermal ellipsoids drawn at the 50%
probability level. All hydrogens and CDCl3 are omitted for clarity.
Selected bond lengths (Å): Ti1–N2 2.153(2); Ti1–O2 2.0549(16); Ti1–O2’
2.0147(16); Ti1–O1 1.8459(17); Ti1–O3 1.6638(17); N1–C1 1.375(3);
C1–C2 1.351(4); C2–C3 1.399(4); C3–C4 1.360(4); C4–C5 1.430(3);
C5–N1 1.385(3); C5–N2 1.329(3); N1–C6 1.478(3); N2–C10 1.470(3).
55
60 Selected angles (deg): O2–Ti1–O2’ 73.50(7); O2–Ti1–O1 139.32(7); O2–
Ti1–O3 111.88(8); O2’–Ti1–O1 99.43(7); O2’–Ti1–O3 108.45(8); O1–
Ti1–O3 108.35(8); N2–Ti1–O2 77.71(7); N2–Ti1–O2’ 144.99(7); N2–
Ti1–O1 89.54(7); N2–Ti1–O3 100.45(8); Ti1–O1–C7 133.55(15).
Fig. 1 Xꢀray structure of LH2 with thermal ellipsoids drawn at the 50%
probability level. All hydrogens except for alcoholic protons are omitted
for clarity. Selected bond lengths (Å): N1–C1 1.366(2); C1–C2 1.352(2);
C2–C3 1.411(2); C3–C4 1.351(2); C4–C5 1.442(2); C5–N1 1.402(2);
C5–N2 1.296(2); N1–C6 1.474(2); N2–C10 1.455(2). Selected angles
(deg): N1–C5–N2 117.10(12); C5–N1–C1 122.00(12); C5–N1–C6
118.57(11); N1–C1–C2 122.17(14); C1–C2–C3 118.33(14); C2–C3–C4
120.57(15); C3–C4–C5 121.79(14); C4–C5–N1 115.03(12); C4–C5–N2
127.87(13); C5–N2–C10 117.56(12).
35
40
There are only four πꢀelectrons, not six, in sixꢀmembered
65 pyridine ring for tridentate ligand LH2, 1, and 2. As shown in
Figure 4, two different types of carbonꢀcarbon bond lengths
certainly exist. A single crystal Xꢀray diffraction analysis of 1
and 2 confirmed the compounds to be monomeric and dimeric,
respectively (Figures 2 and 3). Complexes 1 and 2 have fiveꢀ
70 coordinate titanium centers. The titanium atom in 1 is ligated to
two OꢀiꢀPr and one tridentate L2-. In addition to anionic oxygens
and neutral nitrogen in tridentate L2-, each titanium metal in
dimeric 2 has one =O ligand and a bridging oxygen from another
4
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tridentate L2-, which is the less encumbered tridentate ligand than
other tridentate alkoxy/phenoxy pincer ligands bridging two Ti 60 always use an oxygen atom of Ti=O as a bridge.33ꢀ36 Thus,
addition, dimeric or polymeric terminal oxoꢀtitanium complexes
centers.20,42 Two Ti–Obridge bond distances in 2 [2.0549(16) and
2.0147(16) Å] are within the range found in previously reported
dimeric Ti complexes having Ti–Obridge bond at 2.0122(16) –
2.2332(16) Å.20,42 Interestingly, Ti–O1(terminal) bond length
[1.8459(17) Å] is extremely shorter than the two other Ti–Obridge
bonds [2.0549(16) and 2.0147(16) Å] in 2. In addition to the short
Ti–O1(terminal) bond length, a wide Ti1–O1–C7 bond angle of
compound 2 is the first example of dimeric terminal oxoꢀtitanium
compound having no Ti=O→Ti bonds.
5
The cycloaddition of CO2 to propylene oxide was performed
without solvent using the titanium compounds 1 and 2 as
65 catalysts in the presence of cocatalyst. The cycloaddition results
o
are summarized in Table 2. We carried out the reaction at 75 C
under the condition of a fixed [Ti]/[cocatalyst] ratio of 1.
Propylene oxide was easily converted into the cyclic carbonate
with high selectivity (99%) without any polymerized products.
70 To check the cocatalyst effect on the catalytic activity, we used
six different kinds of cocatalysts, such as nꢀBu4PBr, nꢀBu4NI, nꢀ
Bu4NBr, nꢀBu4NCl, bis(triphenylphosphoranylidene)ammonium
chloride (PPNCl), and 4ꢀdimethylaminopyridine (DMAP). The
role of cocatalyst has been demonstrated for several epoxide/CO2
75 coupling reactions.14ꢀ20 It is known that Lewis basic cocatalysts
can either 1) be reversibly coordinate to the metal center, increase
the electron density on the metal, and labilize the trans ligand to it,
acting in turn as nucleophile for the ring opening of epoxide, or 2)
the counteranion Xꢀ in cocatalyst act as a nucleophile and it
80 proceeds directly to a nucleophilic attack to the coordinated Tiꢀ
epoxide. Anionic cocatalysts (nꢀBu4NX, nꢀBu4PBr, and PPNCl)
and nonꢀsalt type cocatalyst (DMAP) showed the different
catalytic behavior.
10 133.55(15)o indicates the existence of strong Ti–O πꢀdonation.
Thus, O1 atom of L2- chelated to dimeric 2 is somewhat sp2
hybridized and 2 has two weak Ti–Obridge bonds and a strong Ti–
O1(terminal) bond. This provides some idea on how the
encumbered five coordinate complex 2 could potentially free an
15 open coordination site allowing consequently the epoxide and/or
cocatalyst coordination. Complex 2 has an inversion center (i) at
the centroid of the Ti2O2 ring plane. There is no direct Ti–Ti
interaction for 2.
20
25
30
85
Table 2 Cocatalyst screening for the cycloaddition of CO2 to epoxides
using complexes 1 and 2.
Fig. 4 Comparison of carbonꢀcarbon bond lengths in 6ꢀmembered
pyridine ring for LH2, 1, and 2.
90
Entry
1
2
3
4
5
6
7
8
Cat.
Cocatalyst Conv.(%)
TONb Selectivity(%)c
35 The coordination geometry around metal center in the 5ꢀ
coordinate systems (e.g., trigonal bipyramidal or square
pyramidal) could be determined by the trigonality parameter τ (τ
= [α – β]/60, where α and β are the largest and nextꢀlargest
interligand bond angles).43,44 The largest and nextꢀlargest
40 interligand bond angles for 1 are ∠O4–Ti1–N2 (158.25(5)o) and
∠O1–Ti1–O2 (140.83(5)o), respectively. Those values for 2 are
∠N2–Ti1–O2’ (145.00(7)o) and ∠O1–Ti1–O2 (139.31(7)o),
respectively. Thus, the τ values are 0.29 for 1 and 0.09 for 2. The
trigonality parameter τ for the regular trigonal bipyramidal (tbp)
45 complexes is 1.0 and τ for square pyramidal (sqp) complexes is
zero. The titanium center in 2 is almost regular square pyramidal
geometry; however, 1 could be reasonably defined as having less
square pyramidal geometry than 2.
nꢀBu4PBr
nꢀBu4NI
nꢀBu4NBr
nꢀBu4NCl
PPNCl
68
95
78
83
66
21
69
51
86
49
72
26
20
17
11
13
12
10
13
5
338
470
392
416
328
108
343
255
432
245
362
132
98
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
1
DMAP
nꢀBu4PBr
nꢀBu4NI
nꢀBu4NBr
nꢀBu4NCl
PPNCl
9
2
10
11
12
13
14
15
16
17
18
19
20
DMAP
1
2
none
83
54
64
59
nꢀBu4PBr
nꢀBu4NI
nꢀBu4NBr
nꢀBu4NCl
PPNCl
none
49
64
24
As shown in Figure 3, compound 2 possesses a square
50 pyramidal geometry where three oxygens and one nitrogen from
two L2- ligands occupy the base and oxygen of Ti=O takes the
apical position. The titanium metal is displaced by 0.59 Å toward
the oxygen atom of Ti=O from the base, which is quite planar, the
deviations from planarity ranging from ꢀ0.396 to 0.347 Å. The
55 Ti=O bond length in 2 is 1.664(2) Å is within the normal range of
structurally characterized terminal oxoꢀtitanium complexes.20ꢀ32
Until now, structurally characterized terminal oxoꢀtitanium
complexes reported in the literature are all monomeric. In
DMAP
a Cycloaddition conditions: [propylene oxide] = 10 mmol, [1] = 0.02
mmol, [2] = 0.01 mmol, [cocatalyst] = 0.02 mmol, [propylene oxide]/[Ti]
= 500/1, CO2 = 10 bar, Temperature = 75 oC, time = 24 h, b Turnover
number (TON) = (mol of propylene oxide consumed)/(mol of catalyst), c
95 Calculated by 1H NMR spectral integration.
As expected, cocatalyst nꢀBu4NI for catalyst 1 is superior
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compared with the five others (entries 1–6) because the
nucleophilicity for halide ions decreases on going up from Iꢀ to
Clꢀ. However, no similar trend for catalyst 2 was observed and
the highest catalytic activity for 2 was obtained in the presence of
nꢀBu4NBr (entry 9). Interestingly, amineꢀbased cocatalyst such as
nꢀBu4NBr showed the better activity than phosphineꢀbased
cocatalyst, nꢀBu4PBr in both 1 and 2 (entries 1 and 3 for 1 and
entries 7 and 9 for 2). The reason may be quaternary ammonium
compounds could stabilize the carbonate intermediate.
5
6
24
46
330
406
>99
>99
5
7
54
74
>99
10 Compounds 1 and 2 showed marginal difference in catalytic
activity (entries 1–12). Both 1 and 2 with nonꢀsaltꢀtype cocatalyst
a Cycloaddition conditions: [epoxide] = 10 mmol, [1] = 0.02 mmol, [nꢀ
Bu4NI] = 0.02 mmol, [epoxide]/[1] = 500/1, CO2 = 10 bar, Temperature =
75 oC. b Turnover number (TON) = (mol of epoxide consumed)/(mol of
45 catalyst), c Calculated by 1H NMR spectral integration.
DMAP showed very low activities (entries
6 and 12).
Interestingly, compounds 1 and 2 without any cocatalyst showed
the conversion of 20% and 17%, respectively, under the same
15 cycloaddition condition (entries 13 and 14). In addition, six
cocatalysts without 1 and 2 showed low conversion in the range
of 5 – 13% (entries 15–20). Thus the coupling of catalyst and
cocatalyst showed the synergistic activity for this cycloaddition
reaction.
We next moved to investigate the scope of substrates (Table 3)
using 1 as a catalyst and nꢀBu4NI as a cocatalyst. Substrates
include various kinds of epoxides such as propylene oxide (Table
50 2, entry 1), 1,2ꢀepoxybutane (entry 2), 1,2ꢀepoxyhexane (entry 3),
epichlorohydrin (entry 4), 1,2ꢀepoxyꢀ3ꢀmethoxypropane (entry
5), tertꢀbutyl glycidyl ether (entry 6), and 1,2ꢀepoxyꢀ2ꢀ
methylpropane (entry 7). Epoxides with different alkyl
substituents (R = Me, Et, and nꢀBu) showed high dependence on
55 chain length (entries 1–3). While propylene oxide gave an
excellent conversion (entry 1), a quick loss of reactivity was
observed from ethyl to butyl side chains (entries 2 and 3).
Interestingly, when heteroatom was introduced at 3ꢀposition,
epichlorohydrin (entry 4) showed the better activity than epoxides
60 with –CH2OMe (entry 5) and –CH2OBut (entry 6) substituents.
However, substrate with multiꢀsubstituents such as 1,2ꢀepoxyꢀ2ꢀ
methylpropane (entry 7) exhibited low conversions. The observed
dependency of the nature of the epoxide and the observed TONs
may be related to the polarity of the reaction mixture and thus the
65 solubility of the catalyst species could have been influenced.
20 Until now, four papers on Tiꢀbased systems for the
cycloaddition of CO2 and epoxides to make cyclic carbonates
were reported in the literature.14ꢀ17 Harsh condition such as high
catalyst loading of 1 mol%,14,15,17 high temperature such as
100oC17 and 150oC14,15 and high CO2 pressure such as 15 bar17
25 and 22 bar16 for the reported systems in needed. However, our
system was applied under the condition of 0.2 mol% catalyst
loading, 75oC, and 10 bar CO2 pressure. Even though all systems
conducted in different reaction condition, our systems are one of
the mildest Tiꢀbased systems. Interestingly, compounds 1 and 2
30 (0.2 mol%) showed complete conversion at 125oC within 1 hr. In
addition, catalyst/cocatalyst ratio of 2 and CO2 pressure change
did not show any significant differences. Interestingly, in case of
the synthesis of polymers via coupling reaction of propylene
oxide and styrene oxide with CO2 catalyzed by homogeneous Tiꢀ
35 based complexes under mild or similar conditions (1 – 35 bar of
CO2 pressure and 25 – 80oC).18ꢀ20
Conclusions
The ligand LH2 from the reaction between 2ꢀaminopyridine and 2
equiv of isobutylene oxide was synthesized and used to make
monomeric LTi(OꢀiꢀPr)2 (1) and dimeric [LTi(=O)]2 (2).
70 Compound 2 has terminal oxo group and was bridged by the
oxygen atom of L2ꢀ ligand. Compound 2 is the first example of
terminal oxoꢀtitanium compound having no Ti=O→Ti bonds.
New titanium complexes 1 and 2 could act as catalysts for the
synthesis of cyclic carbonate via the cycloaddition of CO2 to
75 epoxides under somewhat mild condition of 0.2 mol% catalyst
loading, 75oC, and CO2 pressure of 10 bar compared with other
reported Tiꢀbased catalysts.
Table 3 The scope of epoxides on the cycloaddition reaction of epoxide
and CO2 using catalyst 1 a
40
Entry
1
Product
Time (h)
24
TON b
470
Selectivity(%)c
>99
2
48
344
>99
Acknowledgements
Y. K. thanks the National Research Foundation of Korea
80 (NRF), the Korean Ministry of Education (MOE) through the
Creative Human Resource Training Project for Regional
Innovation (grant number 2014H1C1A1066874) and Basic
3
4
48
14
250
403
>99
>99
Science
Research
Program
(grant
number
2015R1D1A1A01061043) for financial support. J. L.
85 acknowledges financial supports by the BRL Program
6
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A Graphic Contents Entry
Dimeric terminal oxoꢀtitanium complex 1 having no Ti=O→Ti bonds could be easily
synthesized. The titanium complexes 1 and 2 could act as catalysts for cycloaddition
between CO2 and epoxides in the presence of cocatalysts.