Silica-immobilized zinc β-diiminate catalysts for the copolymerization of epoxides and carbon dioxide

Article abstract of DOI:10.1021/om030209w

A synthetic protocol has been developed to prepare silica-supported Zn-β-diiminate catalysts for the copolymerization of cyclohexene oxide (CHO) and CO₂. Multiple strategies have been developed for the immobilization of these β-diiminate zinc complexes onto the surface of model silica materials such as mesoporous SBA-15 and controlled-pore glass (CPG). The β-diiminate ligand has been modified to incorporate a C=C double bond or an alkane spacer with a trimethoxysilyl end group, allowing immobilization via direct reaction of the alkoxysilanes with silanols on the surface or via AIBN-promoted C=C bond coupling with thiol-functionalized silica. The immobilization process has been followed using FT-Raman spectroscopy and thermogravimetric analysis, whereas polymers have been characterized by GPC and NMR. The resulting silica-supported catalysts exhibit good activity in the alternating copolymerization of CHO and CO₂, leading to polymers with varying degrees of carbonate linkages (copolymerization) relative to ether linkages (homopolymerization of epoxide). Immobilizing the complexes on the silica support leads to catalysts that give more polymeric ether linkages than their corresponding homogeneous analogues. Control studies indicate that this is due, at least in part, to starvation of the active site of CO₂, especially at later stages of the polymerization.

Full text of DOI:10.1021/om030209w

Organometallics 2003, 22, 2571-2580
2571
Silica -Im m obilized Zin c â-Diim in a te Ca ta lysts for th e
Cop olym er iza tion of Ep oxid es a n d Ca r bon Dioxid e
Kunquan Yu and Christopher W. J ones*
School of Chemical Engineering, Georgia Institute of Technology, 311 Ferst Drive,
Atlanta, Georgia 30332
Received March 19, 2003
A synthetic protocol has been developed to prepare silica-supported Zn-â-diiminate
catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2. Multiple strategies
have been developed for the immobilization of these â-diiminate zinc complexes onto the
surface of model silica materials such as mesoporous SBA-15 and controlled-pore glass (CPG).
The â-diiminate ligand has been modified to incorporate a CdC double bond or an alkane
spacer with a trimethoxysilyl end group, allowing immobilization via direct reaction of the
alkoxysilanes with silanols on the surface or via AIBN-promoted CdC bond coupling with
thiol-functionalized silica. The immobilization process has been followed using FT-Raman
spectroscopy and thermogravimetric analysis, whereas polymers have been characterized
by GPC and NMR. The resulting silica-supported catalysts exhibit good activity in the
alternating copolymerization of CHO and CO2, leading to polymers with varying degrees of
carbonate linkages (copolymerization) relative to ether linkages (homopolymerization of
epoxide). Immobilizing the complexes on the silica support leads to catalysts that give more
polymeric ether linkages than their corresponding homogeneous analogues. Control studies
indicate that this is due, at least in part, to starvation of the active site of CO2, especially
at later stages of the polymerization.
In tr od u ction
exception of transport of the reactants to the active site
and the products away from the active site. Because the
product of the reaction is a polymer that has a substan-
tially lower diffusion coefficient than typical small
molecules (and because the polymer remains attached
to the catalyst in coordination-insertion catalysts), the
area around the catalytic active site can be a very
dynamic environment due to the inability of the polymer
to effectively move away from the site. If a porous
catalyst is used, at the outset of the reaction, the
reaction will usually be kinetically limited. However,
as time progresses, the pores of the catalyst can fill with
polymer, the process can enter a diffusion-limited
regime, and the overall rate of polymerization can be
significantly retarded. Two key issues arise as a result
of this: (1) the kinetics of the reaction become complex
and more difficult to analyze, and (2) the recovery of
the catalyst after reaction can become impossible if the
sparingly soluble or completely insoluble polymer en-
gulfs the pores of the catalyst. Hence, in the design of
catalysts for polymerization reactions when catalyst
recovery is desired, both the design of the active site
and the design of the catalyst pore structure must be
considered simultaneously. In particular, the following
three factors are critically important: (1) when a single-
site, homogeneous catalyst is immobilized, extreme care
must be taken to develop a synthetic protocol that gives
a solid catalyst with a single type of site, (2) the support
pore structure must be amenable to facile transport of
reactants to the active sites and polymers away from
the sites, and (3) when the reaction is quenched, the
active site must not be altered (quenching the reaction
The immobilization of homogeneous catalysts to fa-
cilitate easy product separation, catalyst recovery, and
recycling constitutes a well-established research area
in small-molecule catalysis. In contrast, it is signifi-
cantly less common in polymerization catalysis.1 Cata-
lyst recovery and recycling in polymerization reactions
is exceedingly rare, yet a technology that would poten-
tially allow recovery could have important benefits, such
as allowing the preparation of metal-free polymers for
biomedical applications. Similarly, such a technology
could make systems that utilize expensive catalysts of
relatively low activity commercially attractive. To this
end, in our laboratories, we are investigating potential
strategies for the preparation of recoverable and recy-
clable polymerization catalysts.
There are several issues that make the design of
recoverable polymerization catalysts different from, and
generally more difficult than, the design of correspond-
ing small-molecule catalysts. In small-molecule reac-
tions (e.g. hydrogenation of simple olefins) on solid
catalysts, the catalytic process involves diffusion of the
reactants to the active site, binding at the active site,
progression through the various steps of the catalytic
cycle, and then desorption from the active site and
diffusion away from the solid catalyst. In polymerization
catalysis, most of these steps are the same, with the
* To whom correspondence should be addressed. E-mail: cjones@
che.gatech.edu. Tel: 404-385-1683.
(1) Homogeneous catalysts such as metallocenes are routinely
immobilized for R-olefin polymerizations, but the catalysts are not
recovered.
10.1021/om030209w CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/22/2003

2572 Organometallics, Vol. 22, No. 13, 2003
Yu and J ones
Sch em e 1
cannot decompose the catalyst) if the catalyst is meant
to be recycled. Hence, the design of supported, recover-
able polymerization catalysts is a very complex process
with demanding requirements.
Inoue first reported the alternating copolymerization
of carbon dioxide and epoxide using ZnEt2 and H2O in
1969.2 Since then, a variety of other zinc complexes have
been investigated for this copolymerization process.3-10
We have identified the work of Coates and co-workers
on well-defined â-diiminate (BDI) zinc complexes for the
alternating copolymerization of cyclohexene oxide (CHO)
and CO2 as potential catalysts for our initial studies of
recoverable polymerization catalysts.9 These well-
defined zinc catalysts are among the first catalysts
reported for the living synthesis of high-molecular-
weight polymer from epoxides and CO2. The catalysts
display high activity under mild conditions and are
single-site in nature. The fact that copolymerizations
using these catalysts can be quenched using simple
alcohols and that the initiating group on the catalyst
can be an alkoxy species gave some hope to the pos-
sibility that such a supported catalyst could be ef-
fectively recycled.
Resu lts a n d Discu ssion
Syn th eses a n d Sp ectr oscop ic Stu d ies. Two po-
tential ligand modifications were envisioned that could
potentially be used to facilitate binding the zinc BDI
complex to a silica surface. Both involved modification
of the BDI ligand at locations that would likely prevent
detrimental effects on the activity and selectivity of the
catalyst. These locations were off the terminal methyl
group on the dione precursor and para to the nitrogen
on the aromatic ligand (Scheme 1). Protocols were
developed to add terminal olefins in these locations.
For functionalization on the dione precursor, the
reaction of allyl bromide with the dianion of 2,4-
pentanedione was utilized to afford the ligand precursor
for (BDI-1)H. This precursor was then reacted with 2
equiv of 2,6-diisopropylaniline in acidic ethanol followed
by neutralization with saturated sodium carbonate to
give the ligand (BDI-1)H (Scheme 2a). In the other
approach, reacting allyl chloride with 2,6-diisopropyl-
aniline followed by an aza-Claisen rearrangement af-
forded the allyl-modified aniline. Further reaction with
0.5 equiv of 2,4-pentanedione gave (BDI-2)H (Scheme
2b). The ZnCl2-catalyzed aza-Claisen rearrangement
always resulted in a mixture of N-allyl- and 4-allyl-2,6-
diisopropylaniline. Although separation can be ac-
complished by careful vacuum distillation, yields were
low, with an overall yield for (BDI-2)H of only 18%. The
final ligand employed, (BDI-1-MPTS)H, was prepared
in a quantitative yield by utilizing an AIBN-initiated
coupling reaction between the CdC double bond and the
-SH group (Scheme 3).12
In this work, we report our initial investigations into
the design of recoverable and potentially recyclable
supported polymerization catalysts. Modified zinc â-di-
iminate complexes can be easily immobilized on silica
supports such as mesoporous SBA-1511 and controlled-
pore glass (CPG). The resulting solid-supported system
exhibits good activity for the copolymerization of CHO
and CO2 and offers a potential way for catalyst separa-
tion and recycling. Here we report the detailed synthesis
and characterization of the silica-supported catalyst,
and the composition of the produced polycarbonate is
related to the synthetic protocol employed.
(2) Inoue, S.; Koinuma, H.; Tsuruta, T. J . Polym. Sci., Part B:
Polym. Lett. 1969, 7, 287-292.
(3) For reviews on epoxide/CO2 copolymerization, see: (a) Darens-
bourg, D. J .; Holtcamp Coord. Chem. Rev. 1996, 153, 155-174. (b)
Super, M. S.; Beckman Trends Polym. Sci. 1997, 5, 236-240. (c)
Rokicki, A.; Kuran, W. J . Macromol. Sci. Rev. Macromol. Chem. Phys.
1981, C21, 135-186.
Diethylzinc reacts with the â-diimine ligands in
quantitative yields to produce (BDI-1)ZnEt, (BDI-2)-
ZnEt, and (BDI-1-MPTS)ZnEt (Scheme 3) and afford the
corresponding methoxide complexes in high yields after
treatment with 1 equiv of methanol.9a However, treat-
ment of the zinc amide complex with 1 equiv of 2-pro-
panol gave zinc isoproxides in poor to moderate yields
(56%) after recrystallization from toluene at -30 °C. In
the case of (BDI-1-MPTS)ZnN(SiMe3)2, 4 equiv of alcohol
was used, since 2-propanol also reacts with the tri-
methoxy group, though it is unlikely that it would
displace all three -OMe groups due to steric constraints
(proton NMR indicates a mono displacement). Whereas
for [(BDI-1)ZnOMe]2 the syn/anti ratio of the complex
is ∼1:3 by proton NMR, the 1H NMR spectra of both
the methyl and isopropyl BDI-1-MPTS-Zn-alkoxy
complexes show only one set of peaks. This presumably
suggests that only the anti arrangement exists in the
solution, due to the steric bulkiness of the ligand. Note
that the coupling of the organosilane to the modified
BDI ligands was carried out using the reaction of the
(4) (a) Darensbourg, D. J .; Wildeson, J . R.; Yarbrough, J . C. Inorg.
Chem. 2002, 41, 973-980. (b) Darensbourg, D. J .; Adams, M. J .;
Yarbrough, J . C. Inorg. Chem. 2001, 40, 6543-6544. (c) Darensbourg,
D. J .; Rainey, P.; Yarbrough, J . Inorg. Chem. 2001, 40, 986-993. (d)
Darensbourg, D. J .; Zimmer, M. S.; Rainey, P.; Larkins, D. L. Inorg.
Chem. 2000, 39, 1578-1585. (e) Darensbourg, D. J .; Wildeson, J . R.;
Yarbrough, J . C.; Reibenspies, J . H. J . Am. Chem. Soc. 2000, 122,
12487-12496. (f) 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, 107-116. (g)
Darensbourg, D. J .; Holtcamp, M. W. Macromolecules 1995, 28, 7577-
7579.
(5) Saˆrbu, T.; Beckman, E. J . Macromolecules 1999, 32, 6904-6912.
(6) (a) Nozaki, K.; Nakano, K.; Hiyama, T. J . Am. Chem. Soc. 1999,
121, 11008-11009. (b) Nakano, K.; Nozaki, K.; Hiyama, T. J . Am.
Chem. Soc., in press.
(7) Dinger, M. B.; Scott, M. J . Inorg. Chem. 2001, 40, 1029-1036.
(8) Chisholm, M. H.; Navarro-Llobet, D.; Zhou, Z. Macromolecules
2002, 35, 6494-6504.
(9) (a) Cheng, M.; Moore, D. R.; Reczek, J . J .; Chamberlain, B. M.;
Lobkovsky, E. B.; Coates, G. W. J . Am. Chem. Soc. 2001, 123, 8738-
8749. (b) Cheng, M.; Lobkovsky, B. M.; Coates, G. W. J . Am. Chem.
Soc. 1998, 120, 11018-11019. (c) Cheng, M.; Darling, N. A.; Lobkovsky,
E. B.; Coates, G. W. Chem. Commun. 2000, 2007-2008.
(10) (a) Chisholm, M. H.; Gallucci, J .; Phomphrai, K. Inorg. Chem.
2002, 41, 2785-2794. (b) Chisholm, M. H.; Huffman, J . C.; Phomphrai,
K. J . Chem. Soc., Dalton Trans. 2001, 222-224.
(11) (a) Zhao, D.; Feng, J .; Hou, Q.; Melosh, N.; Fredrickson, G. H.;
Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (b) Zhao, D.;
Hou, Q.; Feng, J .; Chmelka, B. F.; Stucky, G. D. J . Am. Chem. Soc.
1998, 120, 6024-6036.
(12) Burger, H.; Sawodny, W.; Wannagat, U. J . Organomet. Chem.
1965, 3, 113-120.

Silica-Immobilized Zinc â-Diiminate Catalysts
Organometallics, Vol. 22, No. 13, 2003 2573
Sch em e 2
Sch em e 3
terminal olefin with a thiol, because more traditional
approaches such as hydrosilylation of the terminal
olefins gave either no product or very low yields,
presumably due to binding of the platinum hydrosi-
lylation catalyst by the uncomplexed BDI ligands.
Im m obiliza tion on th e Silica Su r fa ce. Two dif-
ferent silica materials with different pore sizes and pore
connectivities were used as hosts for the zinc catalysts.
SBA-15 is a well-defined hexagonal mesoporous silica
material with straight mesopores that are connected
through small micropores.11 The SBA-15 used in this
study has an average mesopore size of 105 Å and a
surface area of 830 m2/g on the basis of low-temperature
N2 physisorption measurements. Hexagonal mesoporous
silicas of this type are useful model supports because
they have well-defined, tailorable mesopore systems
with little polydispersity. This well-defined pore system
will facillitate quantitative kinetic analysis of the
transition between kinetic and diffusive regimes in
these catalysts, a potential topic of future investigation.
However, for industrial application and maximization
of reaction rates, these supports are not optimal due to
the unidimensional nature of the pores.
material that was employed is controlled-pore glass
(CPG), a lower surface area (∼80 m2/g), mesoporous
solid with an average pore size of 246 Å and a narrow
pore size distribution. This material has pores that are
interconnected randomly. Because of the sensitivity of
the zinc complexes toward water, the calcined silica
materials SBA-15 and CPG-246 were fully dried under
high vacuum, first at room temperature overnight and
then at 120 °C for 3 h. The surface silanol content13 for
SBA-15 determined by TGA is approximately 4.0 mmol/g
of silica, whereas for CPG, it is only about 1.0 mmol/g
of silica.
Three different methods were explored for the im-
mobilization of the zinc complexes onto the silica
surface.
(a ) Meth od 1 (Sch em e 4). This method was based
on a direct reaction of the trimethoxysilyl group of
metalated BDI-1-MPTS with surface silanols to form a
Si-O-Si bond with concomitant release of methanol.
(13) Based on the condensation reaction of two Si-OH groups in
forming a Si-O-Si bond with concomitant release of a H2O molecule.
This approach assumes that all silanol groups are surface groups that
all are accessible for functionalization. A more quantitative method
to determine the silanol content was described in: (a) Fujdala, K. L.;
Tilley, T. D. J . Am. Chem. Soc. 2001, 123, 10133-10134. (b) Rice, G.
L.; Scott, S. L. Langmuir 1997, 13, 1545.
For this reason, a different silica material with larger,
interconnected pores was also studied. This second silica

2574 Organometallics, Vol. 22, No. 13, 2003
Yu and J ones
Sch em e 4. Meth od 1
Sch em e 5. Meth od 2
F igu r e 1. FT-Raman spectra: (A) SBA-15-SH-Capped; (B)
[(BDI-1)ZnOMe]2; (C) species after [(BDI-1)ZnOMe]2 was
immobilized on the thiol-functionalized SBA-15. Raman
samples were prepared in a drybox with ∼20 mg of the
solid sample placed into a J . Young Raman tube for
analysis under dry N2.
Sch em e 6. Meth od 3
This method is the most straightforward one, and the
only real concern is whether the surface silanols will
interact with the zinc metal center.
trum for thiol-functionalized SBA-15 shows a peak
around 2577 cm-1, characteristic of the S-H stretching.
FT-Raman spectra of [(BDI-1)ZnOMe]2 showed two
peaks at 1662 and 1641 cm-1 for the cis and trans
configurations, respectively, of the CdC bond in the
ligand.14 The disappearance of the thiol and double-bond
stretching after the coupling reaction was apparent in
Figure 1C. The existence of the aromatic C-H stretch-
ing at 3058 cm-1 further supports that the â-diiminate
ligand is incorporated in the solid catalysts. The de-
creased intensity was due to the low concentration of
the â-diiminate ligand on the silica surface. Due to
sample fluorescence, we were not able to observe the
Zn-O and Zn-N stretching.
(c) Meth od 3 (Sch em e 6). In this method, the silica
surface is first functionalized with the ligand and,
subsequently, the immobilized ligand is metalated. For
example, the (BDI-1-MPTS)H ligand can be immobilized
on the silica surface to afford the surface-silanol-free
â-diiminate-functionalized silica after capping with
(b) Meth od 2 (Sch em e 5). Method 2 was based on
a coupling reaction between the double bond and a thiol
group, the same reaction used to prepare (BDI-1-
MPTS)H (Scheme 3). However, in this case, the thiol
group was first immobilized on silica and then the
olefin-terminated metal complex was reacted with the
functionalized silica. Thiol-functionalized SBA-15 was
prepared by refluxing with excess (3-mercaptopropyl)-
trimethoxysilane (MTPS) in a toluene solution for 24 h
and recovering by filtration with toluene washing in a
drybox. Capping the remaining surface silanols was
achieved by further reacting at room temperature with
hexamethyldisilazane in toluene. Following this capping
treatment, the thiol-functionalized SBA-15 was dried
under vacuum overnight at room temperature. Using
this method, potential interactions between zinc and the
silanols on the silica surface are strongly reduced, due
to the coverage of the silica surface with thiol and
-SiMe3 groups. However, interactions between the
acidic thiol species and the zinc metal center are
possible. This issue will be addressed in a subsequent
section.
(14) The two BDI-1 ligands in [(BDI-1)Zn(µ-OMe)]2 adopted cis and
trans configurations; for CdC stretching frequencies in Raman see:
Bowen, R. D.; Edwards, H. G. M.; Farwell, D. W.; Rusike, I.; Saunders,
D. M. J . Chem. Res.,Synop. 1998, 426-427. J . Chem. Res., Miniprint
1998, 1901-1918.
The immobilization through method 2 was character-
ized by FT-Raman spectroscopy (Figure 1). The spec-

Silica-Immobilized Zinc â-Diiminate Catalysts
Organometallics, Vol. 22, No. 13, 2003 2575
Ta ble 1. Ca ta lytic Resu lts for th e
Cop olym er iza tion of CHO a n d CO2
[(BDI)ZnOR]2 revealed that subtle ligand modifications
(electronic, steric) led to dramatic differences in catalytic
activity. Generally, a more electron-deficient zinc center
would increase the reaction rate due to more efficient
CHO coordination, and on the other hand, a decrease
of the catalytic activity is expected with a more electron-
rich zinc metal center. Coates and co-workers16 have
demonstrated that the addition of an electron-with-
drawing cyano group to the â-diimine ligand results in
superior activities, giving an 8-fold increase in the
copolymerization of CHO/CO2. Similar trends are ob-
served here, with the addition of an electron-donating
allyl or bulky alkane group to the â-diiminate ligand
decreasing the catalytic activity. As was shown in Table
1, all the modified zinc complexes had lower activities
than the simple BDI zinc complex. In the case of BDI-
1-MPTS-based zinc complexes, steric effects also may
play a role in the activity decrease and the broader PDI.
Nonetheless, all the modified zinc â-diiminate complexes
still exhibited good activities in the copolymerization,
with excellent carbonate content in the polymer and
narrow PDIs.
a
carbonate Mn
linkages (kg/ Mw/
en-
try
c
complex
TON TOF
(%)b
mol) Mn
1
2
3
4
5
6
7
8
9
[(BDI)ZnOMe]2
370 185
220 110
98
96
91
95
98
93
95
98
92
15.2 1.04
11.1 1.03
18.2 1.10
13.4 1.06
9.7 1.04
14.6 1.09
12.1 1.07
8.7 1.28
[(BDI-1)ZnOMe]2
(BDI-1)ZnN(SiMe3)2
[(BDI-1)ZnOiPr]2
[(BDI-2)ZnOMe]2
(BDI-2)ZnN(SiMe3)2
[(BDI-2)ZnOiPr]2
[(BDI-1-MPTS)ZnOMe]2 120
(BDI-1-MPTS)ZnN-
(SiMe3)2
160
80
216 108
140
125
132
70
62
66
60
65
130
13.3 1.09
a
All reactions were performed in neat CHO with a monomer-
to-zinc ratio of 1000:1 at 50 °C for 2 h with a constant CO2 pressure
of 100 psi. The turnover number (TON) is given in moles of CHO
consumed per mole of zinc, assuming all zinc sites are active; the
turnover frequency (TOF) is given in moles of CHO consumed per
mole of zinc per hour (thus, this is a lower bound on the TOF, as
the catalyst may not be active for the full 2 h and perhaps not all
b
zinc sites participate in the reactions). Calculated by integration
of methine resonances in 1H NMR of the polymer; estimated error
is (0.5%. c The polymer was characterized with GPC using
polystyrene standards in THF.
Silica-supported solid catalysts prepared from each
of the three methods were investigated for their per-
formance in the copolymerization of CHO and CO2. The
polymerizations were performed under conditions iden-
tical with those for for the homogeneous systems, in
neat CHO at 50 °C and with a CO2 pressure of 100 psi.
In several cases, the copolymerization was performed
at a higher temperature of 80 °C, but only a slight
activity increase was observed. Table 2 summarizes the
results from the catalysts supported on SBA-15. A
decrease in polymerization activity was observed in
these solid catalysts, to about one-sixth the activity of
their homogeneous counterparts. Furthermore, there
was a reduction in the number of carbonate linkages
relative to ether linkages in the resulting polymers
compared to the homogeneous systems, with the highest
carbonate content being 73% for I-OMe prepared using
method 1 and the lowest being 33% for the heteroge-
neous metalation of III-OMe prepared using method
3. This gradient in copolymer content depending on
synthetic procedure may track with the propensity for
side reactions to occur during synthesis. Side reactions
that cause the partial or complete decomposition of the
Zn-BDI complexes would be expected to give zinc
species that are either inactive for polymerization or
active for the homopolymerization of the epoxide.
The catalytic results also indicate that the methoxy
group is a more effective initiating group in the sup-
ported catalysts, perhaps because it is less likely to
undergo side reactions with the silanols (method 1) or
the thiols on the silica surface (method 2) than the
amide group. This also could be due to the formation of
a different type of immobilized active site upon im-
mobilization of the dimeric methoxy-bridged catalysts
compared to the monomeric amide- and ethyl-containing
catalysts. Immobilization of the dimeric complexes could
lead to two complexes immobilized next to each other
on the surface. If the catalytic process required two zinc
centers, this could lead to a more active and selective
site (vide infra). Alternately, if the presumed anti
hexamethyldisilazane. Following this step, solid-sup-
ported catalysts can be made by heterogeneous meta-
lation with a zinc source. This process, however, likely
gives the least defined catalyst geometry among the
three methods. The quantitative preparation of the
homogeneous complex (BDI-1-MPTS)ZnEt indicates
that diethylzinc will not react with the Si-OMe bond.
Furthermore, it has been reported that dimethylzinc
caused insignificant or no damage to the framework of
dry silica and chlorinated silica as well as trimethylsilyl-
capped silica.15 Thus, it is unlikely that that diethylzinc
will interact with the framework of the trimethylsilyl-
capped â-diiminate-functionalized silica and metalation
of the surface-bound ligand should proceed. However,
the ethyl form of the complex needs to be converted to
an alkoxy form for the complex to initiate the polym-
erization. It is not clear if there was a complete
conversion of Zn-Et to Zn-OMe upon treatment with
methanol, since methanol can also react with the silica
surface. Elemental analysis of the nitrogen and zinc
content of the as-prepared III-ZnOMe showed a N:Zn
molar ratio of 1.67, very close to the theoretical value
of 2, but it is unclear if all or most of the zinc is still
complexed with the BDI ligand, as excess methanol can
demetalate the ligand. Unfortunately, neither NMR nor
FT-Raman spectroscopic methods could shed light on
Zn coordination. It is believed that incomplete conver-
sion to the metal alkoxide may well account for the low
activity (vide infra) of the catalyst prepared from this
method.
Cop olym er iza tion of Cycloh exen e Oxid e a n d
Ca r bon Dioxid e. A variety of modified zinc â-diiminate
complexes were investigated for the copolymerization
of CHO and CO2 under the same conditions used by
Coates,9a and the polymerization results are sum-
marized in Table 1. Our catalytic reaction protocol was
checked using Coates’ original [(BDI)ZnOMe]2, and our
observations were very consistent with those reported
by Coates.9a Previous mechanistic studies9 involving
(16) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew.
Chem., Int. Ed. 2002, 41, 2599-2602.
(15) Tao, T.; Maciel, G. E. J . Am. Chem. Soc. 2000, 122, 3118-3126.

2576 Organometallics, Vol. 22, No. 13, 2003
Ta ble 2. SBA-15-Su p p or ted Ca ta lysts for th e Cop olym er iza tion of CHO a n d CO2
Yu and J ones
a
Zn loadingc
(mmol/g of silica)
[monomer]/
[Cat.]
carbonate
Mn
(kg/mol)
entry
catalystb
I-OMe
TON
TOF
linkages (%)d
Mw/Mn
1
2
3
4
5
0.15
0.14
0.14
0.12
0.11
1457
2170
1467
1680
1546
72
60
70
60
40
18
12
14
10
5
73
65
61
43
33
15.5
21.4
14.7
20.3
25.2
1.31
1.40
1.48
1.65
1.89
I-N(SiMe3)2
II-OMe
II- N(SiMe3)2
III-OMe
a
b
All reactions were performed in neat CHO at 50 °C with a constant CO2 pressure of 100 psi. I-III stand for catalysts prepared by
methods 1-3 with appropriate initiating groups, respectively. c Zinc complex loading was determined with TGA and was verified by
elemental analysis. Calculated by integration of methine resonances in 1H NMR of the polymer; estimated error is (1.0%. All catalysts
contain the BDI-1 ligand.
d
a
Ta ble 3. CP G-Su p p or ted Ca ta lysts for th e Cop olym er iza tion of CHO a n d CO2
Zn loadingc
(mmol/g of silica)
[monomer]/
[Cat.]
carbonate
Mn
(kg/mol)
entry
catalystb
I-OMe
TON
TOF
linkages (%)d
Mw/Mn
1
2
3
4
5
0.08
0.09
0.06
0.07
0.05
2000
1855
2800
2486
3400
126
78
102
54
21
13
17
9
78
72
69
52
41
16.6
19.7
15.9
17.4
23.4
1.29
1.36
1.40
1.55
1.62
I-N(SiMe3)2
II-OMe
II- N(SiMe3)2
III-OMe
48
8
a
b
All reactions were performed in neat CHO at 50 °C with a constant CO2 pressure of 100 psi. I-III stand for catalysts prepared with
methods 1-3 with appropriate initiating groups, respectively. c Zinc complex loading was determined with TGA and was verified by
elemental analysis. Calculated by integration of methine resonances in 1H NMR of the polymer; estimated error is (1.0%. All catalysts
contain the BDI-1 ligand.
d
arrangement of the complexes hinders immobilization
of both ligands to the surface, it could lead to the
covalent immobilization of one complex and the leaching
of the other, nonbound complex after activation with
CO2. Method 3 is clearly the least useful method, as the
catalyst prepared using this method gave very low rates
and poor CO2 incorporation in the polymer. As men-
tioned earlier, diethylzinc is unlikely to interact with
the silica framework of the silanol-free â-diiminate
functionalized SBA-15 or CPG. As (BDI)ZnEt showed
no activity in the copolymerization,9a the poor perfor-
mance from III-ZnOMe is more likely due to the
difficulty in generating the corresponding zinc methox-
ide catalysts in situ on the solid support. This trans-
formation is limited by the reaction of methanol with
the silica surface, and using a large excess of methanol
is not an option, since it leads to demetalation of the
ligand.
It is hypothesized that polymer filling of the catalyst
pores may limit the kinetics of this system, leading to
lower activities and broader PDIs. To probe this, solid
catalysts supported on CPG were also evaluated. Due
to its larger pore size and interconnected three-
dimensional mesoporous structure, CPG should have
more efficient mass transport and diffusion. Using CPG,
an increase in catalytic activity and a higher percentage
of carbonate linkages were observed. However, a major
drawback of using CPG as a support is its low surface
area, only about 1/10 of that of SBA-15, resulting in low
zinc catalyst loading and decreased activity per gram
of catalyst. Table 3 lists the copolymerization results
from the CPG-supported systems. In all cases, the CPG-
supported catalysts showed comparable activities but
improved carbonate content in the polymers.
center that contains two zinc species, as previous reports
indicate that copolymerization is second order with
respect to the zinc complex.16 The fact that supported
catalysts based on Zn-amide initiating groups promote
the copolymerization with activities and copolymer
contents only slightly lower than those of supported
catalysts derived from dimeric Zn-OMe catalysts may
indicate that a site that is monomolecular in zinc
complex may also be able to promote the copolymeri-
zation, as it is highly unlikely that the complexes are
immobilized in such a manner that allows easy interac-
tion of adjacent surface-bound sites. Certainly, the
determination of the nature of the active site both in
solution and on supports warrants future investigation.
Another possible cause for lower incorporation of CO2
in polymers prepared using the supported catalysts may
be related to active site accessibility. We hypothesized
that the lower carbonate content could be due to
starvation of the active site for CO2, especially during
the latter stages of the polymerization when the pores
have begun to fill with the polymer. To probe this, the
copolymer content was studied both at the early stages
of the polymerization and at later points in the process.
Using the I-N(SiMe3)2 catalyst supported on SBA-15,
it was found that the carbonate content in the polymer
was substantially higher at an early stage (76% after
40 min) than at a later stage of the process (65% after
5 h). This implies that, as the pores fill with polymer,
the limiting reactant, CO2, may be less accessible to the
active site, leading to lower carbonate contents. Ad-
ditional studies were undertaken with the overall pres-
sure of CO2 modulated to further probe the effect of CO2
pressure on copolymer content. With the catalyst
I-N(SiMe3)2 supported on SBA-15, no polymerization
occurred at a reduced pressure of 50 psi. A pressure of
100 psi appears to be near the lower bound of where
the copolymerization will occur. At higher pressures, it
was found that the copolymer content increased. For
instance, at 600 psi, polymerization with I-N(SiMe3)2
resulted in a TON of 56, a TOF of 14, and a carbonate
content of 74% (60, 12, and 65%, respectively, with a
A number of control reactions were undertaken to try
and elucidate the cause for the lower CO2 incorporation
in the polymers prepared using supported catalysts. Of
course, one possible cause is the potential presence of
multiple types of zinc sites on the surface resulting from
side reactions during the synthesis. Another possibility
is that the copolymerization may require an active

Silica-Immobilized Zinc â-Diiminate Catalysts
Organometallics, Vol. 22, No. 13, 2003 2577
CO2 pressure at 100 psi). These results indicate that
poor diffusion of CO2 into the pores of the supported
catalyst is one contributing cause for lower carbonate
contents in the polymers made via this method. How-
ever, the contribution of multiple types of zinc sites
caused by side reactions during the supporting process
cannot be ruled out, due to the inability to probe the
state of the zinc center directly via spectroscopic tech-
niques.
In comparison to other immobilization methods, po-
lymerization studies of the catalysts I-OMe and
-N(SiMe3)2 derived from method 1 indicate that this
method is the most likely to give good catalysts with
relatively well-defined zinc sites. Useful catalysts were
prepared despite the possibility of a side reaction
occurring between the weakly Brønsted acidic silanol
groups on the surface and the Zn-OMe or Zn-
N(SiMe3)2 groups. The lack of this side reaction occur-
ring in appreciable amounts was hypothesized to be due
to steric constraints on the silica surface within the
mesopores. To probe whether surface silanols could have
detrimental effects on Zn-BDI catalysts, Coates’ origi-
nal [(BDI)ZnOMe]2 was stirred with dry SBA-15 in a
toluene suspension at room temperature overnight and
the solid was recovered by filtration and washed with
hexanes and toluene in a drybox. The filtrate was
collected, the solvent was removed, and upon addition
of CHO and CO2, it was determined that the homoge-
neous catalyst still gave comparable activity (TON )
312, TOF ) 156) and the resulting copolymer had good
carbonate content (93%). This indicates that extensive
contact of the catalyst with mesoporous silica does not
significantly deactivate the zinc complex. In addition,
the recovered solid SBA-15 was tested for its activity
in the copolymerization and no polymer was produced,
indicating that essentially no zinc catalyst was phys-
isorbed onto the solid. However, different behavior was
observed if a nonporous fumed silica was used as a
support. In another experiment, [(BDI)ZnOMe]2 was
dissolved in C6D6 in an NMR tube and a small amount
(∼20 mg) of soluble, nonporous, fumed silica (Cab-O-
Sil EH-5) was added. 1H NMR analysis showed that the
silica caused complete decomposition of the catalyst,
with no methoxy protons visible, only those of the
protonated ligand. Furthermore, all attempts to im-
mobilize the BDI-1-based Zn catalysts on the fumed
silica support resulted in catalysts that had a very low
activity and only traces of homopolymer were formed.
These results indicate that the structure of the support
is crucial. In SBA-15, the steric constraints at the silica
surface imposed by the pore size and shape seem to
prevent facile interaction of the zinc center with the
surface (concave surface). In contrast, on the nonporous
solid with a different, more open radius of curvature
on the surface (flat or convex surface), the zinc center
can interact with the surface OH groups and decompose
the catalyst (see Figure 2). Referring back to the cause
of the lower copolymer content in the supported cata-
lysts I-OMe and I-N(SiMe3)2 compared to their homo-
geneous analogues, the above results imply that side
reactions involving the zinc center and the mesoporous
silica surface may not be the primary cause of the lower
carbonate content in the polymer. Instead, the primary
cause of the lower carbonate content in the polymer
F igu r e 2. Schematic illustration of the Zn-BDI complex
interacting with a silica surface: (a) in mesoporous SBA-
15 with unidimensional pores of 105 Å in diameter, where
the curvature of the surface could prevent facile interaction
of the zinc atom with the silanols of the surface; (b) on a
nonporous support such as fumed, colloidal silica, where
the relatively flat surface leaves open the possibility of
interaction between the silica surface and the zinc center
and subsequent decomposition of the Zn-BDI complex.
relative to the homogeneous system could be a lack of
CO2 getting to the active site as mentioned above. In
the absence of CO2, these catalysts do not promote the
homopolymerization of the epoxide, either at atmo-
spheric pressure under argon or at 100 psi of argon
pressure. Hence, it is unlikely that the reduced carbon-
ate content in the polymers from the heterogeneous
catalysts prepared via method 1 are due to homopoly-
mer specific sites; rather, it is more likely that a
copolymer is initiated and this growing chain begins to
incorporate more ether linkages as the concentration
of CO2 around the active species drops with time.
It is noteworthy that, in both the homogeneous and
immobilized catalyst systems, we did not observe cyclic
cyclohexene carbonate. This may be attributed to the
steric encumbrance provided by the cyclohexene carbon-
ate linkages and the â-diiminate ligand, which inhibit
the back-biting mechanism that leads to the formation
of cyclic carbonates seen in the reaction between pro-
pylene oxide (PPO) and CO2.17 Coates18 recently re-
ported the copolymerization of PPO/CO2 using unsym-
metrical BDI-Zn catalysts with high activity, and we
are currently investigating whether immobilized BDI-
Zn catalysts will behave differently.
Finally, mechanistic studies by Coates and co-workers
suggested that a monomeric zinc â-diiminate species
is responsible for the ring-opening polymerization of
lactide19 and â-butyrolactone and â-valerolactone.20
(17) (a) Kuran, W.; Listos, T. Macromol. Chem. Phys. 1994, 195,
997-984. (b) Listos, T.; Kuran, W.; Siwiec, R. J . Macromol. Sci.-Pure
Appl. Chem. 1995, A32, 393-403. (c) Koinuma, H.; Hirai, H. Makromol.
Chem. 1977, 178, 1283.
(18) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J .
Am. Chem. Soc. 2002, 124, 14284-14285.
(19) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J . Am. Chem. Soc. 2001, 123, 3229-
3238.

2578 Organometallics, Vol. 22, No. 13, 2003
Yu and J ones
Aldrich), diethylzinc (Alfa-Aaser), and the surfactant Pluronic
P123 (Mav ) 5800, EO20PO70EO20, Aldrich) were used as
received. Cyclohexene oxide (CHO) was distilled over calcium
hydride under vacuum twice and stored in a drybox after three
freeze-pump-thaw cycles. Carbon dioxide (Matheson Re-
search Purity Grade, 99.995%) was used as received.
(B) Syn th esis of SBA-15 a n d Th iol-F u n ction a lized
SBA-15. SBA-15 was synthesized by literature methods.11b The
as-prepared material was calcined using the following tem-
perature program: (1) increasing the temperature (1.2 °C/min)
to 200 °C, (2) heating at 200 °C for 1 h, (3) increasing the
temperature (1.2 °C/min) to 550 °C, and (4) holding at 550 °C
for 6 h. The pore size and surface area were measured by low-
temperature N2 physisorption. Prior to functionalization, the
SBA-15 was dried under vacuum at room temperature over-
night and then at 120 °C for 3 h and stored in a drybox.
Thiol-functionalized SBA-1521 was synthesized by refluxing
However, in the copolymerization of CHO and CO2, it
is less clear whether the active species is of a dimeric
or a monomeric form.9a,16 As mentioned above, by
immobilizing monomeric zinc amide catalysts on a solid
support, we can significantly reduce the likelihood of
dimer formation. Because the silica-supported zinc
amide catalysts are effective for promoting polycarbon-
ate formation, it is suggested that a monomeric species
is capable of promoting CHO and CO2 copolymerization
as well. This conclusion cannot be drawn from simple
homogeneous studies, because dimers could always form
in situ in solution.
Con clu sion s
Several new â-diiminate zinc complexes have been
prepared and immobilized on silica supports using a
variety of synthetic protocols. The reactivity of these
mesoporous silica-supported catalysts is influenced by
a number of factors, including catalyst preparation
methodology, structure of the silica support, and CO2
pressure. Immobilized catalysts gave moderate polym-
erization activities compared to their homogeneous
analogues, potentially due to pore diffusion effects. The
relative compositions of carbonate linkages compared
to ether linkages in the produced copolymers have been
found to vary with the catalyst preparation procedure,
the initiating group, and the CO2 pressure. The struc-
ture of the support was illustrated to have a strong
influence on the ability to support the zinc complexes,
with mesoporous materials such as SBA-15 and CPG-
246 effectively serving as useful supports and nonporous
solids such as fumed silica causing complete complex
decomposition. The catalysts are currently being evalu-
ated in the ring-opening polymerization of lactide.
Future studies concerning polycarbonate synthesis may
include a more detailed kinetic study of the initial stages
of the process.
a
toluene (40 mL) suspension of SBA-15 (1 g) and (3-
mercaptopropyl)trimethoxysilane (1 g) for 24 h. The solid was
filtered and washed with anhydrous toluene in a drybox and
then dried under vacuum at room temperature overnight. TGA
showed that 0.372 mmol/g of SiO2 of the thiol was immobilized
on SBA-15. Capping the remaining surface silanols was
achieved by stirring the as-synthesized materials with a large
excess of hexamethyldisilazane (1.0 g) in toluene (30 mL) at
room temperature overnight. The solid was then filtered and
washed with toluene in a drybox and dried under vacuum at
room temperature overnight.
(C) Syn th esis of Liga n d P r ecu r sor s. 1-Allyl-2,4-p en -
ta n ed ion e. A modified literature procedure was used.22 To a
suspension of NaH (2.08 g, ca. 46 mmol) in THF (100 mL) was
slowly added 2,4-pentanedione (4.30 mL, 42.1 mmol) at 0 °C.
After the mixture was stirred for 10 min, n-BuLi (1.6 M in
hexane, 26.4 mL, 42.2 mmol) was added over a period of 15
min to afford a deep yellow solution of the pentadione dianion.
When the mixture was stirred for another 20 min at 0 °C, allyl
bromide (3.84 mL, 44.4 mmol) in 5 mL of THF was added
dropwise and the yellow color faded. The reaction mixture was
then warmed to room temperature and was kept stirring for
another 20 min. It was then quenched with concentrated HCl
(8 mL in 10 mL H2O and 30 mL of Et2O). The organic layer
was separated, washed with saturated sodium chloride, and
dried over anhydrous MgSO4. Vacuum distillation (20 mTorr,
50 °C) afforded the pure product as a colorless liquid (yield
4.0 g, 72%). 1H NMR (CDCl3; exists mainly in the hydroxyl
form): δ 5.80 (1H, m, CHdCH2), 5.02 (2H, t, J ) 14.9 Hz, CHd
CH2), 5.49 (1H, s, â-CH), 2.37 (4H, m, -C2H4-), 2.05 (3H, s,
Me) ppm. Mass (FAB): M+ m/z 140.
Exp er im en ta l Section
(A) Ma ter ia ls a n d In str u m en ta tion . All reactions with
air- and moisture-sensitive compounds were carried out under
a dry nitrogen/argon atmosphere using an MBraun UniLab
2000 drybox and/or standard Schlenk line techniques. Gel
permeation chromatography (GPC) analyses were carried out
using a Waters instrument equipped with a Waters 410
differential refractometer. The GPC columns (American Poly-
mer Standards) were eluted with THF at a rate of 1 mL/min
4-Allyl-2,6-d iisop r op yla n ilin e. This synthesis was based
on
a
literature preparation of 4-allyl-2,6-xylidine.23 Allyl
chloride (2.90 g, 37.89 mmol) and 2,6-diisopropylaniline (8.20
g, 42.55 mmol) were mixed in xylene (20 mL) and heated to
120 °C overnight. After it was cooled to room temperature,
the mixture was stirred with 20% NaOH (10 mL), washed with
saturated NaCl (3 × 12 mL), and then dried by distilling off
the xylene. Fractional distillation (70 °C, 20 mTorr) afforded
the pure N-allyl-2,6-diisopropylaniline as a colorless liquid. 1H
NMR (CDCl3): δ 7.05 (2H, d, J ) 7.5 Hz, Ar H), 6.82 (1H, t,
J ) 7.7 Hz, Ar H), 6.05 (1H, m, CHdCH2), 5.33 (1H, m, J )
17.1 Hz, CHdCHH′), 5.17 (1H, m, J ) 9.9 Hz, CHdCHH′),
3.75 (1H, b, NH), 3.52 (2H, m, J ) 5.2 Hz, CH2), 2.94 (2H,
hept, J ) 6.9 Hz, CHMe2), 1.25 (12H, d, J ) 7.2 Hz, CHMe2)
ppm.
1
and were calibrated using 11 polystyrene standards. H (300
MHz) and 13C (75 MHz) NMR spectra were recorded on a
Mercury VX instrument. Nitrogen physisorption measure-
ments were conducted on a Micromeritics ASAP 2010 system.
Samples were pretreated by heating under vacuum at 150 °C
for 8 h. A Netzsch Thermoanalyzer STA 409 was used for
simultaneous thermal analysis combining thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
with a heating rate of 10 °C/min in air. FT-Raman spectra
were obtained on a Bruker FRA-106 instrument. At least 128
scans were collected for each spectrum, with a resolution of
N-Allyl-2,6-diisopropylaniline (7.02 g, 32.31 mmol) and
ZnCl2 (4.86 g, 35.74 mmol) were mixed in 20 mL of xylene and
heated to reflux for 4 h to effect an aza-Claisen rearrangement.
2-4 cm-1
.
CPG with 246 Å pores was purchased (CPG, Inc.). (3-
Mercaptopropyl)trimethoxysilane (95%, Gelest), 1,1,1,3,3,3-
hexamethyldisilazane (99.9%, Aldrich), 2,6-diisopropylaniline
(92%, Acros Organics), zinc bis(bis(trimethylsilyl)amide) (97%,
(21) Yiu, H. H. P.; Wright, P. A.; Botting, N. P. J . Mol. Catal. B
2001, 15, 81-92.
(22) Stuart, N. H.; Weiler, L. J . Am. Chem. Soc. 1974, 96, 1082-
(20) Rieth, L. R.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J .
Am. Chem. Soc. 2002, 124, 15239-15248.
1087.
(23) Elliott, M.; J anes, N. F. J . Chem. Soc. C 1967, 1780-1782.

Silica-Immobilized Zinc â-Diiminate Catalysts
Organometallics, Vol. 22, No. 13, 2003 2579
Next, the mixture was cooled to room temperature and then
stirred with NaOH (5.4 g in 20 mL of H2O). The organic layer
was extracted with ether, washed with saturated NaCl, and
dried over anhydrous MgSO4. The pure 4-allyl-2,6-diisopro-
pylaniline was separated from the unreacted N-allyl complex
through fractional distillation (80 °C, 20 mTorr) as a colorless
liquid. Yield: 3.30 g (47%). 1H NMR (CDCl3): δ 6.84 (2H, s,
Ar H), 5.96 (1H, m, CHdCH2), 5.03 (2H, t, J ) 15.45 Hz, CHd
CH2), 3.62 (2H, b, NH), 3.30 (2H, d, J ) 7.2 Hz, CH2), 2.90
(2H, septet, J ) 6.9 Hz, CHMe2), 1.25 (12H, d, J ) 7.2 Hz,
CHMe2) ppm.
32.97, 32.03, 30.03, 28.54, 27.05, 25.02, 24.56, 23.72, 23.32,
21.50, 8.99 ppm.
(E) Zin c Com p lex Syn th eses. (BDI-1)Zn Et. To a solution
of diethylzinc (0.760 g, 6.18 mmol) in toluene (10 mL) was
slowly added (BDI-1)H (0.52 g, 1.14 mmol) in toluene (5 mL)
at 0 °C. After being stirred overnight at 80 °C, the clear yellow
solution was cooled to room temperature and the solvent was
removed under vacuum, giving the product as a colorless solid
in a quantitative yield. 1H NMR (C6D6): δ 7.10 (6H, m, Ar H),
5.51 (1H, m, CHdCH2), 5.08 (1H, s, â-CH), 4.83 (2H, m, J )
13.0 Hz, CHdCH2), 3.20 (4H, septet, J ) 6.0 Hz, CHMe2), 2.25
(4H, m, CH2), 1.75 (3H, s, R-Me), 1.28 (6H, d, J ) 5.7 Hz, Me),
1.26 (6H, d, J ) 5.7 Hz, Me), 1.20 (6H, d, J ) 6.6 Hz, Me),
1.15 (6H, d, J ) 6.6 Hz, Me), 0.90 (3H, t, J ) 8.1 Hz, CH2CH3),
0.26 (2H, q, J ) 7.8 Hz, CH2CH3) ppm.
(D) Liga n d Syn th esis. (BDI-1)H and (BDI-2)H were
synthesized using a reported procedure with some modifica-
tion.24
(BDI-1)H. 2,6-Diisopropylaniline (9.0 g, 46.71 mmol) and
1-allyl-2,4-pentanedione (3.0 g, 21.43 mmol) were dissolved in
absolute ethanol (50 mL). Anhydrous sodium sulfate (1.0 g)
was used as water adsorbent, and concentrated hydrochloric
acid (1.8 mL, 22.3 mmol) was added via syringe. The mixture
was refluxed for 3 days. The solvent was then evaporated to
afford a light brown solid. The solid was washed with hexanes
to leave a cream-colored solid, which was suspended in CH2-
Cl2 and neutralized with saturated sodium carbonate (80 mL).
The organic collection was passed through a short silica gel
column, and the solvent was removed under vacuo. Excess 2,6-
diisopropylaniline was distilled off under vacuum (90 °C, 20
mTorr), leaving the pure (BDI-1)H as a viscous light yellow
[(BDI-1)Zn (µ-OMe)]2. To a solution of (BDI-1)ZnEt (0.628
g, 1.14 mmol) in toluene (10 mL) was added methanol (40 mg,
1.25 mmol) at room temperature. After the mixture was stirred
for 2 h, the solvent was removed to afford the product as a
pale yellow oily solid in quantitative yield. 1H NMR (C6D6): δ
7.12 (6H, m, Ar H), 5.40 (1H, m, CHdCH2), 4.86 (1H, s, â-CH),
4.79 (2H, m, J ) 15.3 Hz, CHdCH2), 3.42 (3H, s, OMe), 2.95
(4H, m, CHMe2), 2.16 (2H, m, CH2), 2.07 (2H, m, CH2), 1.56
(3H, s, R-Me), 1.19 (24H, m, CHMe2) ppm. Anal. Calcd for
C
33H48N2OZn: C, 71.53; H, 8.73; N, 5.06. Found: C, 71.44; H,
8.82; N, 4.95.
(BDI-1)Zn N(SiMe3)2. To a solution of (BDI-1)H (0. 60 g,
1.31 mmol) in toluene (10 mL) was added Zn[N(SiMe3)2]2 (0.505
g, 1.31 mmol), and the solution was stirred for 6 days at 80
°C. After removal of solvent under vacuum and recrystalliza-
tion from toluene at -30 °C, a yellow oily solid was recovered.
1
liquid. Yield: 4.5 g (46%). H NMR (CDCl3): δ 12.10 (1H, b,
NH), 7.10 (6H, m, Ar H), 5.71(1H, m, CHdCH2), 4.96 (1H, m,
CHdCHH′), 4.90 (1H, m, CHdCHH′), 4.87 (1H, s, â-CH), 3.07
(4H, septet, J ) 6.4 Hz, CHMe2), 2.22 (2H, m, CH2CH2), 2.05
(2H, m, CH2CH2), 1.71 (3H, s, R-Me), 1.19 (12H, dd, J ) 6.6
Hz, J ) 3.7 Hz, CHMeMe′), 1.09 (12H, dd, J ) 7.2 Hz, J ) 3.5
Hz, CHMeMe′) ppm. 13C{1H} NMR (CDCl3): δ 163.28, 162.25,
146.46, 143.09, 142.09, 139.54, 137.09, 125.49, 124.97, 123.73,
123.18, 115.11, 91.77, 41.50, 32.67, 28.48, 25.02, 24.60, 23.72,
23.35, 21.53 ppm. Anal. Calcd for C32H46N2: C, 83.79; H, 10.11;
N, 6.11. Found: C, 83.84; H, 10.15; N, 5.95.
1
Yield: 0.565 g (63%). H NMR (C6D6): δ 7.11 (6H, m, Ar H),
5.48 (1H, m, CHdCH2), 4.94 (1H, s, â-CH), 4.80 (2H, m, J )
10.7 Hz, CHdCH2), 3.27 (4H, septet, J ) 7.1 Hz, CHMe2), 2.15
(4H, m, CH2), 1.68 (3H, s, Me), 1.21 (6H, d, J ) 7.2 Hz,
CHMeMe′), 1.17 (6H, d, J ) 6.9 Hz, CHMeMe′), 1.14 (6H, d, J
) 2.1 Hz, CHMe′′Me′′′), 1.11 (6H, d, J ) 2.7 Hz, CHMe′′Me′′′),
0.19 (18H, s, SiMe) ppm. Anal. Calcd for C38H63N3Si2Zn: C,
66.78; H, 9.29; N, 6.15. Found: C, 66.67; H, 9.23; N, 6.07.
[(BDI-1)Zn (µ-OiP r )]2. To a solution of (BDI-1)ZnN(SiMe3)2
(0.44 g, 0.64 mmol) in toluene (10 mL) was added 2-propanol
(40 mg, 0.66 mmol), and the mixture was stirred overnight at
room temperature. Removal of solvent and recrystallization
from toluene at -30 °C afforded the product as a light yellow
(BDI-2)H. This ligand was prepared in a manner analogous
to that described for (BDI-1)H, by simply using 4-allyl-2,6-
diisopropylaniline (1.70 g, 7.83 mmol) and 2,4-pentanedione
1
(0.39 g, 3.90 mmol). Yield: 0.74 g (38%). H NMR (CDCl3): δ
12.07 (1H, b, NH), 6.90 (4H, m, Ar H), 5.97 (2H, m, CHdCH2),
5.05 (4H, m, CHdCH2), 4.82 (1H, s, â-CH), 3.34 (4H, d, J )
6.6 Hz, -CH2-), 3.05 (4H, septet, J ) 6.9 Hz, CHMe2), 1.69
(6H, s, R-Me), 1.17 (12H, d, J ) 7.2 Hz, CHMeMe′), 1.08 (12H,
1
oily solid. Yield: 0.217 g (58%). H NMR (C6D6): δ 7.15 (6H,
m, Ar H), 5.54 (1H, m, CHdCH2), 4.96 (1H, s, â-CH), 4.83 (2H,
m, CHdCH2), 3.63 (1H, m, J ) 4.2 Hz, OCHMe2), 3.31 (4H,
septet, J ) 7.2 Hz, CHMe2), 2.18 (4H, m, CH2), 1.71 (3H, s,
R-Me), 1.24 (6H, d, J ) 6.6 Hz, CHMeMe′), 1.20 (6H, d, J )
6.6 Hz, CHMeMe′), 1.17 (6H, d, J ) 2.7 Hz, CHMe′′Me′′′), 1.14
(6H, d, J ) 2.4 Hz, CHMe′′Me′′′), 0.93 (6H, d, J ) 6.3 Hz,
OCHMe2) ppm.
d, J ) 7.2 Hz, CHMeMe′) ppm. 13C{1H} NMR (CDCl3):
δ
161.43, 142.43, 138.87, 138.11, 136.51, 123.36, 115.62, 93.31,
40.65, 28.57, 24.60, 23.62, 21.13 ppm. Anal. Calcd for
C
35H50N2: C, 84.28; H, 10.10; N, 5.62. Found: C, 84.24; H,
10.23; N, 5.59.
(BDI-1-MP TS)H. (BDI-1)H (1.0 g, 2.18 mmol), (3-mercap-
topropyl)trimethoxysilane (MPTS; 1.3 g, 6.57 mmol), and AIBN
(40 mg) were dissolved in chloroform (30 mL), and the solution
was refluxed overnight. After the mixture was cooled to room
temperature, chloroform was removed and excess MPTS was
distilled off under vacuum (90 °C, 20 mTorr) to afford pure
product as a light yellow viscous liquid. Yield: 1.36 g (100%).
1H NMR (CDCl3): δ 12.09 (1H, b, NH), 7.09 (6H, m, Ar H),
4.86 (1H, s, â-CH), 3.54 (9H, s, OMe), 3.07 (4H, septet, J )
4.5 Hz, CHMe2), 2.44 (4H, m, J ) 6.9 Hz, CH2SCH2), 1.97 (2H,
t, J ) 8.7 Hz, CH2), 1.71 (3H, s, R-Me), 1.65 (2H, m, CH2),
1.56 (2H, m, CH2), 1.47 (2H, m, CH2), 1.19 (12H, dd, J ) 6.6
Hz, J ) 3.9 Hz, CHMeMe′), 1.08 (12H, dd, J ) 6.6 Hz, J ) 4.5
Hz, CHMeMe′), 0.72 (2H, t, J ) 8.3 Hz, CH2Si) ppm. 13C{1H}
NMR (CDCl3): δ 163.58, 162.25, 143.09, 142.06, 139.54,
125.46, 124.91, 123.70, 123.15, 91.77, 50.85, 42.02, 35.31,
Similar procedures were followed to synthesize the corre-
sponding BDI-2-based and BDI-1-MPTS-based complexes.
(BDI-2)Zn Et. 1H NMR (C6D6): δ 7.14 (2H, s, Ar H), 6.96
(2H, s, Ar H), 6.03 (2H, m, CHdCH2), 5.04 (4H, m, CHdCH2),
5.02 (1H, s, â-CH), 3.37 (4H, d, J ) 6.6 Hz, CH2), 3.00 (4H,
septet, J ) 7.5 Hz, CHMe2), 1.75 (6H, s, R-Me), 1.22 (12H, m,
J ) 3.7 Hz, CHMeMe′), 1.11 (12H, m, J ) 3.7 Hz, CHMeMe′),
0.42 (3H, t, J ) 8.4 Hz, CH2CH3), -0.29 (2H, q, J ) 8.1 Hz,
CH2CH3) ppm.
[(BDI-2)Zn (µ-OMe)]2. 1H NMR (C6D6): δ 7.09 (2H, s, Ar
H), 6.92 (2H, s, Ar H′), 6.00 (1H, m, CHdCH2), 5.32 (1H, s,
â-CH), 5.06 (2H, t, J ) 14.9 Hz, CHdCH2), 3.67 (3H, s, OMe),
3.34 (4H, d, J ) 6.6 Hz, CH2), 2.84 (4H, septet, J ) 6.9 Hz,
CHMe2), 1.79 (6H, s, R-CH3), 1.11 (24H, m, CHMe2) ppm. Anal.
Calcd for C36H52N2OZn: C, 72.77; H, 8.82; N, 4.71. Found: C,
72.71; H, 8.77; N, 4.65.
[(BDI-2)Zn (µ-OiP r )]2. 1H NMR (C6D6): δ 7.07 (4H, s, Ar
H), 5.98 (2H, m, CHdCH2), 5.04 (4H, m, J ) 16.8 Hz, CHd
CH2), 4.86 (1H, s, â-CH), 3.44 (1H, m, OCHMe2), 3.30 (4H, m,
(24) Feldman, J .; McLain, S. J .; Parthasaranthy, A.; Marshall, W.
J .; Calabrese, J . C.; Arthur, S. D. Organometallics 1997, 16, 1514-
1516.

2580 Organometallics, Vol. 22, No. 13, 2003
Yu and J ones
CHMe2), 2.07 (4H, s, CH2), 1.66 (6H, s, R-Me), 1.20 (12H, d,
J ) 7.2 Hz, CHMeMe′), 1.15 (12H, d, J ) 6.6 Hz, CHMeMe′),
0.85 (6H, m, J ) 6.5 Hz, OCHMe2) ppm.
CPG-240 (1.0 g) and was stirred for 24 h at room temperature.
It was then filtered and washed with toluene to afford the
â-diimine ligand functionalized SBA-15 or CPG. Metalation
was carried out by adding a 5 equiv excess of ZnEt2 or 1 equiv
of Zn(N(SiMe3)2)2 based on the loading of the BDI-MPTS ligand
on the solid, typically in 40 mL of toluene. The metalation for
diethylzinc was achieved at 80 °C for 24 h, with a reaction
time of 1 week for the zinc amide. The resulting solid catalysts
were collected by filtration and washed with toluene in a
drybox. The former was then stirred in toluene with 5 equiv
of methanol in toluene for 24 h and then filtered and dried.
(G) Gen er a l P olym er iza tion P r oced u r e. In a drybox, the
desired zinc complex (3.0 × 10-5 mol) and cyclohexene oxide
(3.0 g, 30.6 mmol) were placed in a 60 mL Fischer-Porter bottle
with a magnetic stir bar. The vessel was pressurized to 100
psi with CO2, and the mixture was stirred at the desired
temperature for the necessary reaction time. After reaction, a
small sample of the crude mixture was removed for NMR
analysis, the product was dissolved in minimal methylene
chloride, and the polymer was precipitated with methanol,
which was centrifuged and dried under vacuum to constant
weight.
In a similar preparation, the solid catalysts (120∼200 mg)
and cyclohexene oxide (3.0 g, 30.6 mmol) were placed into a
Fischer-Porter bottle and pressurized with 100 psi of CO2. A
small sample of the crude mixture was removed after the
necessary reaction time at a given temperature, and the
product was precipitated with 20 mL of methanol and centri-
fuged. The crude sample was dissolved in deuterated chloro-
form and passed through a small alumina plug before NMR
analysis. The rest of the polymer was separated from the silica
support by passing through a short alumina column and eluted
with large amount of methylene chloride, and the solution was
concentrated and precipitated with methanol again. The
polymer was then obtained via centrifugation and dried under
vacuum to constant weight.
(BDI-2)Zn N(SiMe3)2. 1H NMR (C6D6): δ 7.12 (2H, s, Ar
H), 7.10 (2H, s, Ar H′), 6.00 (2H, m, CHdCH2), 5.06 (4H, m,
J ) 11.7 Hz, CHdCH2), 4.89 (1H, s, â-CH), 3.31 (8H, m,
CHMe2 +CH2), 1.69 (6H, s, R-Me), 1.20 (24H, m, CHMe2), 0.09
(9H, s, SiMe3), 0.01 (9H, s, SiMe′3) ppm.
(BDI-1-MP TS)Zn Et. 1H NMR (CD2Cl2): δ 7.14 (6H, m, Ar
H), 5.03 (1H, s, â-CH), 3.52 (9H, s, OMe), 3.01 (4H, septet,
J ) 6.6 Hz, CHMe2), 2.42 (4H, pentet, J ) 6.9 Hz, CH2SCH2),
2.05 (2H, t, J ) 8.0 Hz, CH2), 1.77 (3H, s, R-Me), 1.61 (4H, m,
CH2), 1.47 (2H, m, CH2), 1.22 (12H, d, J ) 6.6 Hz, CHMeMe′),
1.11 (12H, dd, J ) 7.2, 2.7 Hz, CHMeMe′), 0.69 (2H, t, J ) 8.3
Hz, CH2Si), 0.37 (3H, t, J ) 8.3 Hz, CH2Me), -0.32 (2H, q,
J ) 8.1 Hz, CH2Me) ppm.
[(BDI-1-MP TS)Zn (µ-OMe)]2. 1H NMR (C6D6): δ 7.08 (6H,
m, Ar H), 4.98 (1H, s, â-CH), 3.45 (3H, s, OMe), 3.38 (9H, s,
OMe), 3.00 (4H, m, J ) 6.6 Hz, CHMe2), 2.73 (2H, m, J ) 7.2
Hz, CH2), 2.43 (2H, t, J ) 7.4 Hz, CH2), 2.30 (2H, t, J ) 7.2
Hz, SCH2), 2.17 (2H, t, J ) 6.9 Hz, CH2S), 2.06 (2H, t, J ) 8.6
Hz, CH2), 1.73 (2H, t, J ) 9.5 Hz, CH2), 1.58 (3H, s, Me), 1.22-
1.13 (24H, m, CHMe2), 0.72 (2H, t, J ) 8.3 Hz, CH2Si) ppm.
Anal. Calcd for C39H64N2O4SSiZn: C, 62.42; H, 8.60; N, 3.73;
S, 4.27. Found: C, 62.34; H, 8.53; N, 3.69; S, 4.18.
1
(BDI-1-MP TS)Zn N(SiMe3)2. H NMR (C6D6): δ 7.13 (6H,
m, Ar H), 4.99 (1H, s, â-CH), 3.38 (9H, s, OMe), 3.29 (4H, m,
J ) 6.6 Hz, CHMe2), 2.44 (2H, t, J ) 7.5 Hz, CH2), 2.29 (2H,
t, J ) 7.2 Hz, CH2), (4H, m, CH2SCH2), 2.06 (2H, m, CH2),
1.81 (2H, m, J ) 7.6 Hz, CH2), 1.32 (3H, s, Me), 1.26-1.11
(24H, m, CHMe2), 0.77 (2H, t, J ) 8.9 Hz, CH2Si), 0.19 (18H,
s, SiMe3) ppm. Anal. Calcd for C44H79N3O3SSi3Zn: C, 60.07;
H, 9.05; N, 4.78; S, 3.64. Found: C, 59.91; H, 8.93; N, 4.62; S,
3.47.
(F ) Im m obiliza tion of Zin c Ca ta lysts on
a Silica
Su r fa ce. With each method, the resulting solid catalysts were
dried under vacuum at room temperature overnight and
characterized by FT-Raman spectroscopy. The catalyst loading
was determined by TGA and elemental analysis.
Meth od 1. In a typical preparation, [(BDI-1-MPTS)ZnOMe]2
(100 mg, 0.268 mmol) was added into a toluene suspension
(30 mL) of dry SBA-15 (400 mg) or CPG-246 (500 mg), and
the mixture was stirred for 24 h at room temperature. The
solid catalyst was collected by filtration and washed with
anhydrous toluene and dry hexanes in a drybox.
Meth od 2. (BDI-1)ZnN(SiMe3)2 (150 mg, 0.170 mmol), AIBN
(40 mg), and thiol-functionalized SBA-15 (400 mg) or CPG-
(246) (700 mg) were mixed in chloroform (30 mL). The mixture
was refluxed overnight. After it was cooled to room tempera-
ture, it was filtered and washed with hexanes and toluene in
a drybox.
For polymerizations at pressures greater than 100 psi, a
25 mL stainless steel stirred Parr 4590 micro autoclave was
utilized. The reactor was loaded with catalyst and CHO in a
drybox as described above, removed from the box, and con-
nected to a CO2 cyclinder via a “quick-connect” Swagelock
fitting. The reactor was heated to 50 °C using the associated
Parr 4843 controller.
Ack n ow led gm en t. The National Science Founda-
tion is gratefully acknowledged for partial support of
this work (Grant No. CTS-0210460). C.W.J . thanks the
Shell Oil Company Foundation and Oak Ridge Associ-
ated Universities for partial support of this work
through a Faculty Career Initiation Award and a Ralph
Powe J unior Faculty Award, respectively.
Meth od 3. (BDI-1-MPTS)H (346 mg, 0.755 mmol) was
added to a toluene (30 mL) suspension of SBA-15 (600 mg) or
OM030209W