Tetravalent metal complexes as a new family of catalysts for copolymerization of epoxides with carbon dioxide

Article abstract of DOI:10.1021/ja203382q

New tetravalent metal complexes with a trianionic [ONNO]-tetradentate ligand and an ancillary chloride ligand were synthesized as catalysts for the copolymerization of epoxides with carbon dioxide (CO₂). All of the titanium, zirconium, germaniu

Full text of DOI:10.1021/ja203382q

COMMUNICATION
pubs.acs.org/JACS
Tetravalent Metal Complexes as a New Family of Catalysts for
Copolymerization of Epoxides with Carbon Dioxide
Koji Nakano, Kazuki Kobayashi, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
S Supporting Information
b
ABSTRACT: New tetravalent metal complexes with a
trianionic [ONNO]-tetradentate ligand and an ancillary
chloride ligand were synthesized as catalysts for the
copolymerization of epoxides with carbon dioxide (CO2).
All of the titanium, zirconium, germanium, and tin
complexes were found to copolymerize epoxides with
CO2. In particular, the copolymerization of propylene
oxide with CO2 gave the almost-completely alternating
copolymers by using titanium or germanium complexes.
These results are the first example of the copolymeriza-
tion using tetravalent metal complexes as a main com-
ponent of catalysts.
Figure 1. Design strategy for the tetravalent metal complexes.
Scheme 1. Synthesis of (BOXDIPY)MCl Complexes
(M = Ti, Zr, Ge, and Sn)
he alternating copolymerization of epoxides with carbon
T
dioxide (CO2) is one of the most promising processes for
CO2 utilization.1 Since the first report by Inoue and co-workers
with Et2Zn/H2O as a catalyst in the late 1960s,2 a variety of
combinations of metal centers and ligands have been investigated
to improve the catalyst performance. The previously reported
active catalysts are based on either divalent (Zn3) or trivalent
(Al,4 Sc,5 Cr,6 Mn,7 Fe,8 Co,9 Y,5,10 Dy,5,10 Lu5,10) metal com-
plexes: the representatives are Zn(OAr)2,3a,c (BDI)ZnX,3b,d,e and
(salen)MX complexes [Ar = aryl; basic skeleton of BDI-H =
2-amino-4-iminopent-2-ene; basic skeleton of salen-H2 = N,
N0-disalicylidene-1,2-ethylenediamine; X = monoanionic ancillary
ligand; M = Al(III),4d Co(III),9aꢀh,j,l and Cr(III)6bꢀe]. There has
been no literature ever reported on the use of a tetravalent metal
complex as an active catalyst component.11 Here we report the first
achievement in the copolymerization by using tetravalent metal
complexes.
coordination geometry of metal-BOXDIPY complexes depends
on the nature of the central metal ions and ancillary ligands, the
BOXDIPY ligand was found to occupy four equatorial sites in the
octahedral manganese(III) complex,13 similar to the typical salen
ligands.
The synthesis of metal(IV) complexes 1 with a BOXDIPY
ligand 2 was described in Scheme 1. Methyl-protected ligand
precursor 3 was synthesized by reference to the reported
procedures.12b,15 Demethylation was carried out by using so-
dium thiolate, giving the metal-free ligand precursor, 1,9-bis(3,
5-di-tert-butyl-2-hydroxyphenyl)-5-phenyldipyrrin (2-H3). Depro-
tonation of 2-H3 with NaH and the subsequent reaction with the
corresponding metal chlorides afforded titanium, zirconium,
germanium, and tin complexes in high yields (>93%). These
Our design strategy for the use of tetravalent metal complexes
is drawn in Figure 1. The (salen)MX complexes, which have been
known as a catalyst with the highest performance, are composed
of a trivalent metal ion (M), a dianionic [ONNO]-tetradentate
salen ligand, and a monoanionic ancillary ligand (X). We con-
ceived of an idea that the combination of a tetravalent metal ion,
a trianionic [ONNO]-tetradentate ligand, and a monoanionic
ancillary ligand would accurately mimic (salen)MX complexes.
Based on this strategy, we designed the tetravalent metal com-
plexes with a 1,9-bis(2-oxidophenyl)dipyrrinate (BOXDIPY) li-
gand (Figure 1). BOXDIPY ligands are promising trianionic
analogues of salen ligands because of their rigid structure and
[ONNO]-tetradentate coordination mode. Recently, boron-
(III),12 aluminum(III),12b manganese(III, V),13 and silicon(IV)14
complexes with a BOXDIPY ligand were reported. Although the
Received: April 12, 2011
Published: June 21, 2011
r
2011 American Chemical Society
10720
dx.doi.org/10.1021/ja203382q J. Am. Chem. Soc. 2011, 133, 10720–10723
|

Journal of the American Chemical Society
COMMUNICATION
solution state,14 or the formation of a CS-symmetrical octahedral
geometry via THF-d8 coordination.
The copolymerization of propylene oxide (PO) with CO2
([PO]/1 = 2000; CO2 = 2.0 MPa) was carried out at 60 °C for 12
h by using the complex 1a as a catalyst. When the complex was
used without any additive, the copolymerization did not proceed
to give only propylene carbonate (PC). In contrast, the addition
of ammonium salts [Ph3Pd NdPPh3]Cl ([PPN]Cl) resulted in
the successful production of poly(propylene carbonate) (PPC)
with a TOF of 29 and moderate PPC selectivity (PPC/PC = 71/29)
(Table 1, entry 1). The obtained PPC had a highly alternat-
1
ing structure (carbonate linkage: 98%) based on its H NMR
spectrum. Higher polymerization temperatures of 80 and 100 °C
gave a higher conversion of PO, while the selectivity for PPC
significantly decreased to yield a larger amount of PC (entries 2
and 3). The PPC selectivity was also susceptible to the molar
ratio of 1a/[PPN]Cl (entries 4 and 5): a significant decrease in
PPC selectivity was observed with a larger excess amount of
[PPN]Cl (entry 5). CO2 pressure did not show a significant
effect on the polymerization behavior (entries 6 and 7). The
nucleophilic anion of the ammonium salt was reported to affect
the catalytic activity and the PPC/PC selectivity when using
metalꢀsalen complexes.6e,9b,9c,9f In the present system, the
copolymerization with [PPN]N3 or [PPN][OCOC6F5] resulted
in lower PPC/PC selectivity (entries 8 and 9).
The germanium complex was also found to copolymerize PO
with CO2 under the typical conditions ([PO]/[1] = 2000; CO2 =
2.0 MPa, 60 °C, 12 h) to give PPC with high carbonate-linkage
content (entry 11), while the zirconium and tin complexes pro-
duced only PC (entry 10) and the copolymer with high ether-
linkage content (entry 20), respectively. The copolymerization with
germanium complex 1c under higher temperature (entries 12, 13)
or with an excess amount of [PPN]Cl (entry 15) led to a decrease in
PPC selectivity, similar to that with titanium complex 1a. It is
noteworthy that the use of a lower amount of [PPN]Cl (0.75 equiv
to 1c) resulted in a higher PPC selectivity of >90% (entry 14).
Furthermore, the higher catalytic activity and carbonate-linkage
content were achieved under high CO2 pressure (entry 17).
Accordingly, the copolymerization with germanium complex 1c
was found to give almost-completely alternating copolymer selec-
tively. The use of other ammonium salts resulted in a slightly lower
PPC selectivity (entries 18 and 19).
The copolymerization of cyclohexene oxide (CHO) with CO2
was also investigated by using the tetravalent metal complexes 1.
The results are summarized in Table 2. The titanium and
germanium complexes gave almost-completely alternating poly-
(cyclohexene carbonate) (PCHC) selectively without a conco-
mitant amount of cyclohexene carbonate (CHC) (entries 1 and 3).
Zirconium complex 1b, which was not applicable to PO/CO2
copolymerization, was found to produce PCHC, while the activity
was quite low and ether-linkage content was high (entry 2). The
copolymerization with tin complex 1d gave the copolymer with high
ether-linkage content (entry 4), similar to the case in PO/CO2
copolymerization.
Figure 2. ORTEP drawingsof(a) 1aand (b) 1c (50% thermalellipsoids;
solvent (CH2Cl2 for 1a) and all hydrogen atoms are omitted for clarity).
are the first examples of group IV metal, germanium, and tin
1
complexes with a BOXDIPY ligand. H and 13C NMR spectra
showed that each complex forms a CS-symmetry structure in
solution, indicating the planar tetradentate coordination of the
trianionic ligand. In addition, one tetrahydrofuran (THF) mo-
lecule was indicated to coordinate to the metal center in titanium
1
(1a), zirconium (1b), and tin (1d) complexes by H NMR
spectra (vide infra), while no signal assigned to THF protons was
detected for the germanium (1c) complex.
The solid-state structure of each complex was revealed by
single-crystal X-ray structural analysis. As shown in Figure 2a,
titanium complex 1a forms an octahedral geometry where four
equatorial coordination sites are occupied by the BOXDIPY
ligand 2. One of the two apical coordination sites is occupied by a
monoanionic chloride ligand, and the other site is ligated by a
neutral THF molecule as indicated by NMR spectroscopy. The
equatorial plane, which is defined by O1, N1, N2, O2, was nearly
planar with an average atomic displacement of 0.0233(13) Å.
The titanium center deviated by 0.1502(14) Å toward the apical
chloride ligand. Zirconium and tin complexes 1b and 1d also
form a similar structure (see Supporting Information). In con-
trast, germanium complex 1c forms a distorted trigonal-bipyr-
amidal geometry (Figure 2b). The atoms N1 and O2 are located
in the two apical coordination sites with a N1ꢀGe1ꢀO2 angle of
176.98(14)°. The equatorial plane is shared by N2, O1, and Cl1
with N2ꢀGe1ꢀO1, O1ꢀGe1ꢀCl1, and N2ꢀGe1ꢀCl1 angles
of 128.21(14)°, 116.56(10)°, and 115.22(11)°, respectively. The
germanium center lies almost in the equatorial plane. Such a C1-
symmetry structure of complex 1c in the solid state is contra-
dictory to a CS-symmetry structure expected by NMR spectros-
copy, indicating the rapid interconversion between isomers in a
In summary, we have developed a variety of tetravalent metal
complexes with a trianionic tetradentate BOXDIPY ligand as a
new family of epoxide/CO2 copolymerization catalysts. Espe-
cially, the titanium and germanium complexes were found to give
the alternating copolymers. Although the catalytic activity is still
lower than those of the previously reported catalysts such as
metalꢀsalen complexes, the results described here are the
first example of the copolymerization using tetravalent metal
10721
dx.doi.org/10.1021/ja203382q |J. Am. Chem. Soc. 2011, 133, 10720–10723

Journal of the American Chemical Society
COMMUNICATION
Table 1. Copolymerization of Propylene Oxide with CO2 by Using Tetravalent Complexes 1a
carbonate
[PPN]Cl
CO2
temp
yield of
PPC + PC (%)b
TOF
for PPCc
linkage
[%]b
Mn
[g molꢀ1 d
entry
1
(equiv to 1)
[MPa]
[°C]
PPC/PC b
]
Mw/Mn
d
3
1
2
3
4
5
6
7
8
9
1a
1.0
1.0
1.0
0.5
1.5
1.0
1.0
1.0e
1.0f
2.0
2.0
2.0
2.0
2.0
1.0
4.0
1.0
1.0
60
80
25
55
98
21
83
27
24
25
33
71/29
33/67
2/98
29
30
3
98
96
77
96
67
98
98
97
99
8100
9200
ꢀ
1.19
1.35
ꢀ
100
60
82/18
1/99
29
1
14 000
ꢀ
1.28
ꢀ
60
60
73/27
67/33
51/49
37/63
33
27
22
20
14 000
9100
6300
7100
1.36
1.13
1.14
1.15
60
60
60
10
1b
1c
1.0
2.0
60
21
0/100
0
ꢀ
ꢀ
ꢀ
11
12
13
14
15
16
17
18
19
1.0
2.0
2.0
2.0
2.0
2.0
1.0
4.0
4.0
4.0
60
80
15
26
82
13
69
6
71/29
40/60
0/100
93/7
10
17
ꢀ
95
94
ꢀ
91
ꢀ
88
97
98
98
7200
6500
ꢀ
6600
ꢀ
5300
18 000
9400
13 000
1.14
1.16
ꢀ
1.15
ꢀ
1.21
1.22
1.54
1.32
1.0
1.0
100
60
0.75
2.0
19
ꢀ
60
0/100
80/20
93/7
0.75
0.75
0.75e
0.75f
60
8
60
17
14
19
26
21
26
60
89/11
83/17
60
20
1d
1.0
2.0
60
22
32/68
12
87
4300
1.22
a PO (1.0 mL, 14.3 mmol), complex 1 (7.1ꢀ7.2 ꢁ 10ꢀ3 mmol) in a 50-mL autoclave for 12 h. b Determined on the basis of 1H NMR spectroscopy of the
crude product by using phenanthrene as an internal standard. c (mol of repeating unit) (mol of 1)ꢀ1 hꢀ1 d Determined by size-exclusion
.
3
3
chromatography analysis using a polystyrene standard. e [PPN]N3 was used instead of [PPN]Cl. f [PPN][OCOC6F5] was used instead of [PPN]Cl.
Table 2. Copolymerization of Cyclohexene Oxide with CO2
by Using Tetravalent Complexes 1a
less toxic metals such as titanium and germanium here reported
should provide attractive alternatives to metal-salen complexes.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, charac-
b
terization data for new compounds, crystallographic information files
(CIF) for complexes 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
yield
(%)b
TOF for
PCHCc
carbonate
linkage [%]b
Mn
[g molꢀ1 d
Mw/
d
entry
1
]
Mn
3
’ AUTHOR INFORMATION
1
2
3
4
1a
1b
1c
1d
45
5
76
8
99
54
99
75
13 000
1100
1.27
1.25
1.12
1.73
Corresponding Author
nozaki@chembio.t.u-tokyo.ac.jp
36
60
14 000
10
17
2500
a CHO (2.0 mL, 20 mmol), complex 1 (10 ꢁ 10ꢀ3 mmol), [PPN]Cl (1.0
equiv to 1) in a 50-mL autoclave at 60 °C for 12 h. b Determined on the basis
of 1H NMR spectroscopy of the crude product by using phenanthrene as an
’ ACKNOWLEDGMENT
This work was supported by a Funding Program for World-
Leading Innovative R&D on Science and Technology from the
Japan Society for the Promotion of Science (JSPS) and by a
Grant for Practical Application of University R&D Results under
the Matching Fund Method from the New Energy and Industrial
Technology Development Organization (NEDO).
internal standard. c (mol of repeating unit) (mol of 1)ꢀ1
h
ꢀ1. d Determined
3
3
by size-exclusion chromatography analysis using a polystyrene standard.
complexes as a main component of catalysts. Considering the
toxicity of most of the active metals such as cobalt and chromium,16
10722
dx.doi.org/10.1021/ja203382q |J. Am. Chem. Soc. 2011, 133, 10720–10723

Journal of the American Chemical Society
COMMUNICATION
’ REFERENCES
(14) Sakamoto, N.; Ikeda, C.; Yamamura, M.; Nabeshima, T. J. Am.
Chem. Soc. 2011, 133, 4726.
(15) Rieth, R. D.; Mankad, N. P.; Calimano, E.; Sadighi, J. P. Org.
Lett. 2004, 6, 3981.
(16) Manahan, S. E. Toxicological chemistry and biochemistry, 3rd ed.;
Lewis Pub.: Boca Raton, FL, 2003.
(1) (a) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004,
43, 6618. (b) Sugimoto, H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem.
2004, 42, 5561. (c) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388.
(d) Luinstra, G. A. Polym. Rev. 2008, 48, 192.
(2) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Part B: Polym.
Lett. 1969, 7, 287.
(3) (a) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995,
28, 7577. (b) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem.
Soc. 1998, 120, 11018. (c) Darensbourg, D. J.; Holtcamp, M. W.; Struck,
G. E.; Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.;
Draper, J. D.; Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107.
(d) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738.
(e) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599.
(4) (a) Aida, T.; Ishikawa, M.; Inoue, S. Macromolecules 1986, 19, 8.
(b) Sarbu, T.; Bekman, E. J. Macromolecules 1999, 32, 6904. (c) Sarbu,
T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165. (d) Sugimoto,
H.; Ohtsuka, H.; Inoue, S. J. Polym. Sci., Part A: Polym. Chem. 2005,
43, 4172.
(5) Cui, D. M.; Nishiura, M.; Hou, Z. M. Macromolecules 2005,
38, 4089.
(6) (a) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.;
Holmes, A. B. Macromolecules 2000, 33, 303. (b) Darensbourg, D. J.;
Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124, 6335. (c) Eberhardt, R.;
Allmendinger, M.; Rieger, B. Macromol. Rapid Commun. 2003, 24, 194.
(d) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Fang, C. C.;
Billodeaux, D. R.; Reibenspies, J. H. Inorg. Chem. 2004, 43, 6024.
(e) Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005,
127, 14026. (f) Darensbourg, D. J.; Fitch, S. B. Inorg. Chem. 2007,
46, 5474. (g) Li, B.; Wu, G. P.; Ren, W. M.; Wang, Y. M.; Rao, D. Y.; Lu,
X. B. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6102. (h) Nakano, K.;
Nakamura, M.; Nozaki, K. Macromolecules 2009, 42, 6972.
(7) Sugimoto, H.; Ohshima, H.; Inoue, S. J. Polym. Sci., Part A: Polym.
Chem. 2003, 41, 3549.
(8) Buchard, A.; Kember, M. R.; Sandeman, K. G.; Williams, C. K.
Chem. Commun. 2011, 47, 212.
(9) (a) Qin, Z. Q.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew.
Chem., Int. Ed. 2003, 42, 5484. (b) Lu, X. B.; Wang, Y. Angew. Chem., Int.
Ed. 2004, 43, 3574. (c) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am.
Chem. Soc. 2005, 127, 10869. (d) Paddock, R. L.; Nguyen, S. T.
Macromolecules 2005, 38, 6251. (e) Nakano, K.; Kamada, T.; Nozaki,
K. Angew. Chem., Int. Ed. 2006, 45, 7274. (f) Lu, X. B.; Shi, L.; Wang,
Y. M.; Zhang, R.; Zhang, Y. J.; Peng, X. J.; Zhang, Z. C.; Li, B. J. Am.
Chem. Soc. 2006, 128, 1664. (g) Noh, E. K.; Na, S. J.; Sujith, S.; Kim,
S. W.; Lee, B. Y. J. Am. Chem. Soc. 2007, 129, 8082. (h) Sujith, S.; Min,
J. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int. Ed. 2008,
47, 7306. (i) Sugimoto, H.; Kuroda, K. Macromolecules 2008, 41, 312.
(j) Ren, W. M.; Liu, Z. W.; Wen, Y. Q.; Zhang, R.; Lu, X. B. J. Am. Chem.
Soc. 2009, 131, 11509. (k) Okada, A.; Kikuchi, S.; Nakano, K.; Nishioka,
K.; Nozaki, K.; Yamada, T. Chem. Lett. 2010, 39, 1066. (l) Nakano, K.;
Hashimoto, S.; Nozaki, K. Chem. Sci. 2010, 1, 369.
(10) Cui, D. M.; Nishiura, M.; Tardif, O.; Hou, Z. M. Organometallics
2008, 27, 2428.
(11) Kobayashi et al. demonstrated that Sc(OiPr)3 copolymerized
epoxides with CO2 and that the addition of Ti(OiPr)4 improved its
catalytic activity. However, Ti(OiPr)4 itself was found to show no
activity in the copolymerization. See: Kobayashi, S.; Akiyama, R.; Suzuki,
K.; Fujimoto, K. (The University of Tokyo, Japan; Nippon Steel
Corporation; Nippon Steel Engineering Co., Ltd.; Nippon Steel Chemi-
cal Co., Ltd.) Jpn. Kokai Tokkyo Koho JP 2009242794, 2009.
(12) (a) Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.;
Burgess, K. Chem. Commun. 1999, 1889. (b) Ikeda, C.; Ueda, S.;
Nabeshima, T. Chem. Commun. 2009, 2544.
(13) Rausaria, S.; Kamadulski, A.; Rath, N. P.; Bryant, L.; Chen,
Z. M.; Salvemini, D.; Neumann, W. L. J. Am. Chem. Soc. 2011, 133,
4200.
10723
dx.doi.org/10.1021/ja203382q |J. Am. Chem. Soc. 2011, 133, 10720–10723