Catalytic reactions involving C1 feedstocks: New high-activity Zn(II)- based catalysts for the alternating copolymerization of carbon dioxide and epoxides [22]

Full text of DOI:10.1021/ja982601k

11018
J. Am. Chem. Soc. 1998, 120, 11018-11019
Scheme 1
Catalytic Reactions Involving C1 Feedstocks: New
High-Activity Zn(II)-Based Catalysts for the
Alternating Copolymerization of Carbon Dioxide and
Epoxides
Ming Cheng, Emil B. Lobkovsky, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology
Baker Laboratory, Cornell UniVersity
Ithaca, New York 14853
ReceiVed July 23, 1998
Scheme 2
Carbon dioxide is an ideal synthetic feedstock since it is
abundant, inexpensive, nontoxic, and nonflammable. Although
it is estimated that Nature uses CO2 to make over 200 billion
tons of glucose by photosynthesis each year,1 synthetic chemists
have had little success in developing efficient catalytic processes
that exploit this attractive raw material.2 There has been consider-
able recent interest in the development of catalysts for the
alternating copolymerization of carbon dioxide with epoxides to
produce aliphatic polycarbonates.3 Because of the low cost and
accessibility of the monomers and the attractive properties of
polycarbonates, the development of new, efficient initiators for
this polymerization process is a significant scientific goal.4-10 We
report here a new class of well-defined, high-activity Zn(II)
catalysts that copolymerize carbon dioxide and epoxides under
exceptionally mild conditions.
The proposed mechanism of the copolymerization reaction is
depicted in Scheme 1.11 Repetition of the sequence in which CO2
inserts into a metal alkoxide, followed by epoxide ring-opening
with the metal carbonate forms the alternating copolymer. Two
catalytic systems have been recently discovered that, prior to the
present work, were the most active reported zinc-based systems
for CO2/epoxide polymerization. Darensbourg and Holtcamp have
reported novel Zn(II) bis(2,6-diphenylphenoxide) initiators for the
copolymerization of CO2 and epoxides.12 Immediately after
initiation, it is probable that the remaining bulky phenoxide ligand
of the complex prevents aggregation yet allows monomer
coordination. Beckman et al. have developed a highly active Zn-
based compound for cyclohexene oxide (CHO) copolymerization
in supercritical CO2.13,14 The key to success in this system was a
fluorinated carboxylate ligand that provided catalyst solubility in
supercritical CO2. In each of these systems, it is vital that the
monomer does not displace the ligand from the active site during
the polymerization reaction. On the basis of this lead, we
investigated the synthesis and application of Zn(II) complexes
incorporating chelating, bulky â-diimine ligands15 as catalysts for
epoxide/CO2 copolymerization.
Deprotonation of the â-diimine ligand (BDI-H, derived from
2,6-diisopropylaniline)16 with n-butyllithium and subsequent
reaction with zinc acetate yields [(BDI)ZnOAc] (1) (Scheme 2).17
Reaction of BDI-H with ZnEt2 gives [(BDI)ZnEt], which produces
[(BDI)ZnOMe] (2) upon reaction with methanol. The X-ray
crystal structure of 118 is shown in Figure 1; the structure reveals
a tetrahedral zinc center and aryl groups that are approximately
perpendicular to the plane of the N-Zn-N chelate. Complex 1
exists as the acetate-bridged dimer in the solid-state. The 1H NMR
spectrum of 1 in benzene-d6 (1 × 10-2 M) exhibits two sets of
shifts whose intensities vary with concentration; the set that
becomes more intense as concentration decreases was assigned
(1) Behr, A. Carbon Dioxide ActiVation by Metal Complexes; VCH:
Weinheim, 1988.
(2) For some leading references, see: (a) Ayers, W. N. Catalytic ActiVation
of Carbon Dioxide; ACS Symposium Series 363; American Chemical
Society: Washington DC, 1988. (b) Jessop, P. G.; Ikariya, T.; Noyori, R.
Chem. ReV. 1995, 95, 259-272. (c) Leitner, W. Angew. Chem., Int. Ed. Engl.
1995, 34, 2207-2221. (d) Leitner, W. Coord. Chem. ReV. 1996, 153, 257-
284. (e) Kro¨cher, O.; Ko¨ppel, R. A.; Baiker, A. Chem. Commun. 1997, 453-
454.
(3) For reviews, see: (a) Darensbourg, D. J.; Holtcamp, N. W. Coord.
Chem. ReV. 1996, 153, 155-174. (b) Super, M. S.; Beckman, E. J. Trends
Polym. Sci. 1997, 5, 236-240. (c) Rokicki, A.; Kuran, W. J. Macromol. Sci.,
ReV. Macromol. Chem. 1981, C21, 135-186.
(4) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Polym. Lett. 1969,
B7, 287-292.
(5) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1983, 105, 1304-1309.
(6) Chen, X.; Shen, Z.; Zhang, Y. Macromolecules 1991, 24, 5305-
5308.
(7) Darensbourg, D. J.; Stafford, N. W.; Katsurao, T. J. Mol. Catal. A 1995,
104, L1-L4.
(15) For nickel and aluminum complexes containing this â-diimine ligand,
see: (a) Feldman, J.; McLain, S. J.; Parthasaranthy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516. (b)
Qian, B.; Ward, D. L.; Smith, M. R. Organometallics 1998, 17, 3070-3076.
(16) The 2,6-diisopropylphenyl group has been used extensively as a
sterically bulky ligand substituent in the development of highly active
polymerization catalysts: (a) Oskam, J. H.; Fox, H. H.; Yap, K. B.;
McConville, D. H.; O’Dell, R.; Lichtenstein, B. J.; Schrock, R. R. J.
Organomet. Chem. 1993, 459, 185-198. (b) Johnson, L.; Killian, C.;
Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414-6415. (c) Scollard, J.;
McConville, D.; Payne, N.; Vittal, J. Macromolecules 1996, 29, 5241-5243.
(d) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998,
120, 4049-4050.
(8) Kruper, W. J.; Dellar, D. V. J. Org. Chem. 1995, 60, 725-727.
(9) Tan, C.-S.; Hsu, T.-J. Macromolecules 1997, 30, 3147-3150.
(10) Kuran, W.; Listos, T.; Abramczyk, M.; Dawidek, A. J. Macromol.
Sci. Pure Appl. Chem. 1998, A35, 427-437.
(11) Darensbourg, D. J.; Niezgoda, S. A.; Draper, J. D.; Reibenspies, J. H.
J. Am. Chem. Soc. 1998, 120, 4690-4698 and references therein.
(12) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995, 28,
7577-7579.
(13) Super, M. S.; Berluche, E.; Costello, C.; Beckman, E. J. Macromol-
ecules 1997, 30, 368-372.
(14) Super, M. S.; Beckman, E. J. Macromol. Symp. 1998, 127, 89-108.
10.1021/ja982601k CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/09/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 42, 1998 11019
Table 1. Results of Cyclohexene Oxide/CO2 Polymerization by Zinc Compounds 1-4a
temp
(°C)
pressure
(psig)
reaction
length (h)
% carbonate
linkages
Mn (×10-3
)
Mw/Mn
(GPC)
TOFc
catalyst
(GPC)
TONb
(h-1
)
1
1
1
20
50
80
50
80
100
100
100
100
800
2
2
2
2
69
24
95
96
95
95
91
93
21.3
31.0
25.7
19.1
38.0
17.0
1.07
1.11
1.17
1.07
4.5
270
494
412
449
173
216
135
247
206
224
2.5
9.0
2
3d
4e
100
2000
6.4
a All of the reactions were performed in neat CHO. b Moles of CHO consumed per mole of zinc. c Moles of CHO consumed per mole of zinc
per hour. d Data from ref 12 for (2,6-Ph2C6H3O)2Zn(Et2O)2 (3). e Data from ref 13 for HO2CCHdCHCO2(CH2)2C6F13/ZnO (4).
Compounds 1 and 2 are virtually inactive for the homopolym-
erization of CHO (50 °C, 2h).19 However the addition of low
pressures of CO2 gas to neat solutions of 1 or 2 in the epoxide
monomer rapidly produces the polycarbonate. Data for the
copolymerization of CHO and CO2 with 1 and 2 are summarized
in Table 1 (data for initiators [(2,6-Ph2C6H3O)2Zn(Et2O)2] (3)12
and HO2CCHdCHCO2(CH2)2C6F13/ZnO (4)13 are included for
comparison). All four catalysts form polymers containing trans-
rings and predominately carbonate linkages.20 However complexes
1 and 2 operate under substantially milder conditions; even at
moderate reaction temperatures (20-80 °C) and low CO2 pres-
sures (100 psig), the compounds are the most active reported
catalysts for CHO/CO2 copolymerization. Furthermore, the poly-
mers produced are of high molecular weight and exhibit very
narrow polydispersities, consistent with a living polymerization
process. Compounds 1 and 2 under the same reaction conditions
(50 °C, 100 psig) exhibit nearly identical activities, consistent
with the proposal that alkoxide and carbonate intermediates are
produced during the reaction.
In summary, we have discovered a new class of well-defined,
highly active catalysts for the synthesis of highly alternated, high
molecular weight, and monodispersed carbon dioxide/epoxide
copolymers. Future work will explore the mechanism and
modification of the current system to develop new generations
of catalysts that exhibit higher activities or stereochemical control
during the polymerization reaction.
Figure 1. X-ray crystal structure of the dimer of 1. Selected bond
distances (Å) and angles (deg): Zn(1)-N(1), 1.970(2); Zn(1)-N(2),
1.969(2); Zn(1)-O(1), 1.946(2); Zn(1)-O(2), 1.965(2); C(1)-O(1),
1.239(3); C(1)-O(2A), 1.227(3); N(1)-Zn(1)-N(2), 98.48(8); O(1)-
Zn(1)-O(2), 117.27(9); O(1)-Zn(1)-N(1), 112.33(8); O(2)-Zn(1)-
N(2), 107.92(9); Zn(1)-O(1)-C(1), 138.3(2); O(1)-C(1)-O(2A),
128.4(3).
Acknowledgment. We thank the Cornell University Center for
Biotechnology, New York State Center for Advanced Technology
supported by the New York State Science and Technology Foundation
and industrial partners for partial support of this research, as well as
Amoco, Bayer, and Eastman Chemical for unrestricted research grants.
This work made use of the Cornell Center for Materials Research facilities
(supported by the NSF; DMR-9632275), and the Cornell Chemistry
Department X-ray Facility (supported by the NSF; CHE-9700441).
G.W.C. gratefully acknowledges a Camille and Henry Dreyfus New
Faculty Award.
to the monomeric species. Thus, Keq for the 2 • 1 h [1]2
equilibrium is 34 M-1 (C6D6 at 20 °C). In tetrahydrofuran-d8
solution (1 × 10-2 M) only one set of shifts is present, which we
have tentatively assigned to the monomeric species. Complexes
1 and 2 provide excellent model compounds for the putative metal
carbonate and alkoxide intermediates of the polymerization
reaction (Scheme 1).
Supporting Information Available: Syntheses of 1 and 2, polym-
erization procedures, and crystal structure data for 1 (tables of crystal
data, structure solution and refinement, atomic coordinates, bond lengths
and angles, and anisotropic thermal parameters (10 pages, print/PDF).
See any current masthead page for ordering information and Web access
instructions.
(17) (BDI)ZnOAc (1). To a solution of BDI-H (0.535 g, 1.28 mmol) in
THF (10 mL) was slowly added n-BuLi (1.6 M in hexane, 0.88 mL, 1.41
mmol) at 0 °C. After stirring for 5 min at 0 °C, the solution was cannulated
to a solution of zinc acetate (0.240 g, 1.31 mmol) in THF (15 mL). After
stirring overnight at RT, the suspension was filtered over a frit, and the clear
solution was dried in vacuo. The light yellow solid was recrystallized from a
minimum amount of methylene chloride at -20 °C (0.436 g, 63% yield). 1H
NMR (25 mg complex in 0.55 mL C6D6, 300 MHz, M denotes monomer, D
denotes dimer) δ 7.11 (18H, m, ArH, M + D), 4.93 (1H, s, â-CH, M), 4.64
(2H, s, â-CH, D), 3.29 (12H, m, CHMe2, M + D), 1.73 (6H, s, OC(O)Me,
D), 1.67 (6H, s, R-Me, M), 1.55 (12H, s, R-Me, D), 1.41 (12H, d, J ) 6.4
Hz, CHMeMe′, M), 1.31 (3H, s, OC(O)Me, M), 1.19 (24H, d, J ) 7.0 Hz,
CHMeMe′, D), 1.14 (24H, d, J ) 7.0 Hz, CHMeMe′, D), 0.87 (12H, d, J )
6.4 Hz, CHMeMe′, M).
JA982601K
(19) Compound 1 is, however, active for the homopolymerization of
lactones such as ꢀ-caprolactone. Cheng, M.; Coates, G. W., unpublished results.
(20) Polymers formed using 1 and 2 were subjected to basic hydrolysis.
Analysis of the 1,2-cyclohexenediol degradation product by GC revealed that
the diol was greater than 99% trans in configuration, consistent with inversion
of stereochemistry during epoxide ring-opening. Consecutive epoxide insertion
produces polymers that contain ether linkages.
(18) Crystal data of 1: monoclinic, P21/n, colorless; a ) 13.0558(3) Å, b
) 15.1725(4) Å, c ) 15.9712(2) Å; â ) 106.142(1); V ) 3038.99(11); Z )
4; R ) 0.0535; GOF ) 1.121.