Solid-state structures of zinc(II) benzoate complexes. Catalyst precursors for the coupling of carbon dioxide and epoxides

Article abstract of DOI:10.1021/ic0107983

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized from Zn^II(bis-trimethylsilyl amide)₂ and the corresponding carboxylic acid (2,6-X₂C₆H₃COOH, where X = F, Cl, or OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes from THF or CH₃CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a combination of 10 μ-benzoates and μ₄-oxo ligands, that is, [(2,6-difluorobenzoate)₁₀O₂Zn₇] (solvent)₂, solvent = THF (1) and CH₃CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a dimer, [(2,6-dichlorobenzoate)₄Zn₂](THF)₃ (2), where the two zinc centers are bridged by three benzoate ligand. One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate, and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages, that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO₂/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar to those previously reported analogous complexes which have not been structurally characterized. Hence, it is suggested here that all of these zinc carboxylates provide similar catalytic sites for CO₂/epoxide coupling processes.

Full text of DOI:10.1021/ic0107983

Inorg. Chem. 2002, 41, 973−980
Solid-State Structures of Zinc(II) Benzoate Complexes. Catalyst
Precursors for the Coupling of Carbon Dioxide and Epoxides
Donald J. Darensbourg,* Jacob R. Wildeson, and Jason C. Yarbrough
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842
Received July 27, 2001
Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized
II
from Zn (bis-trimethylsilyl amide)2 and the corresponding carboxylic acid (2,6-X C H COOH, where X ) F, Cl, or
2
6 3
OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes
from THF or CH CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a
3
combination of 10 µ-benzoates and µ4-oxo ligands, that is, [(2,6-difluorobenzoate)10O Zn7](solvent)2, solvent )
2
THF (1) and CH CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a
3
dimer, [(2,6-dichlorobenzoate)4Zn2](THF)3 (2), where the two zinc centers are bridged by three benzoate ligand.
One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate,
and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding
of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of
these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine
ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the
copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages,
that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO /
2
propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization
of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar
to those previously reported analogous complexes which have not been structurally characterized. Hence, it is
suggested here that all of these zinc carboxylates provide similar catalytic sites for CO /epoxide coupling processes.
2
Introduction
solubility of the catalyst in carbon dioxide. In addition, we
have reported a comparably active catalyst in the form of a
soluble zinc crotonate precursor.5 In all of these instances,
the catalyst structures were not specifically defined because
of the inability to isolate single crystals of these zinc
carboxylates.
To better identify reaction pathways in this rather complex
catalytic process, it is essential to have well-defined structures
of the initial metal species employed as catalysts. This is of
particular consequence because formation of the generally
undesirable cyclic carbonates is the dominant pathway in
these soluble catalytic systems when employing aliphatic
epoxides as monomers. This is in contrast to what is observed
in the heterogeneous systems of Inoue1 and Soga.2 Formation
of cyclic carbonates is attributed to a back biting mechanism
During the 1970s, many heterogeneous multicentered zinc
catalysts were reported for the coupling of epoxides and
carbon dioxide. Prominent among these catalysts were zinc
derivatives of mono- and dicarboxylic acids.1,2 More recently,
a methylene chloride soluble zinc complex derived from the
monoester of maleic acid has been developed by ARCO for
the copolymerization of carbon dioxide and epoxides.3 The
catalytic activity of this system was further enhanced upon
fluorination of the ester group by Super et al.4 which provided
* Author to whom correspondence should be addressed. E-mail:
DJDarens@mail.chem.tamu.edu.
(1) Inoue, S.; Kobayashi, M.; Tsuruta, T. J. Polym. Sci. 1973, 11, 2383.
(2) Soga, K.; Imai, E.; Hattori, I. Polymer 1981, 13 (4), 407.
(3) Sun, H.-N. (Arco Chemical Co.) U.S. Patent 4,783, 445, Nov 8, 1988.
(4) Super, M.; Berluche, E.; Costello, C.; Beckman, E. Macromolecules
1997, 30, 368.
(5) Darensbourg, D. J.; Zimmer, M. S. Macromolecules 1999, 32, 2137.
Inorganic Chemistry, Vol. 41, No. 4, 2002 973
10.1021/ic0107983 CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/01/2002

Darensbourg et al.
Scheme 1
Chemical Co. and were sublimed and stored in a glovebox prior to
use. Zn[N(SiMe3)2]2 was prepared according to published litera-
ture,13 stored in the glovebox, and used immediately after removal
from the box. Infrared spectra were recorded on a Mattson 6081
spectrometer with DTGS and mercury cadmium telluride (MCT)
detectors. All isotopically labeled solvents for NMR experiments
were purchased from Cambridge Isotope Laboratories. 1H and 13
C
in which a propagating polymer chain coordinates itself to
an open zinc center (see Scheme 1). Kuran has suggested
the activity of Inoue’s catalysts for the copolymerization of
propylene oxide is due to an aggregated multicentered zinc
system in which a neighboring zinc prevents a propagating
polymer chain from back biting on itself.6 In general, this
back biting process responsible for cyclic carbonate produc-
tion is not an important side reaction in the copolymerization
of alicyclic epoxides, for example, cyclohexene oxide, with
carbon dioxide. Indeed, several homogeneous catalysts have
been quite effective for the copolymerization of cyclohexene
oxide and carbon dioxide to produce high molecular weight
polycarbonates with little to no cyclic carbonate product.7-11
Although the design of these latter catalyst systems has
focused on providing steric environments around the zinc
center blocking polymer chain “back biting” processes, none
have been reported to be very successful for the copolym-
erization of aliphatic epoxides, such as propylene oxide, with
CO2.
Herein, we describe the synthesis and characterization of
soluble multicentered zinc benzoate clusters utilizing halo-
genated ligands, which coordinate to the zinc centers in a
unidentate, chelating, or bridging fashion. These catalysts
display the ability to copolymerize cyclohexene oxide and
CO2 and terpolymerize cyclohexene oxide and propylene
oxide with CO2. In addition, these catalysts do not lose their
activity when allowed to stand in air similar to previous zinc
phenoxide catalyst systems with halogenated substituents.12
NMR spectra were recorded on Varian XL-200E, Unity +300 MHz,
and VXR 300 MHz superconducting high-resolution spectrometers.
19F data were acquired on a Unity +300 MHz superconducting
NMR spectrometer operating at 282 MHz and referenced to 10%
CFCl3 and 1% CClH2CClF2 in d6-acetone. Elemental analyses were
carried out by Galbraith Laboratories Inc.
Synthesis of [(2,6-Difluorobenzoate)10O2Zn7](THF)2 (1). A
5-mL THF solution of 2,6-difluorobenzoic acid (0.164 g, 1.04
mmol) was added to a 5-mL THF solution of Zn[N(SiMe3)2]2 (0.20
g, 0.52 mmol), leading to a clear, colorless solution which was
stirred at room temperature for 2 h. The solution was then
concentrated to 5 mL and stored at -20 °C. Colorless block crystals
formed after several days. The supernatant was cannulated off, and
crystals were dried under vacuum and collected to yield 0.130 g
of product (77%). Anal. Calcd. for C78H46O24F20Zn7 according
to the crystal structure: C, 42.49; H, 2.11. Found: C, 43.07; H,
1.91. IR(νCO ): (KBr) 1620(s), 1604(s) 1561(s), 1416(br) cm-1
;
2
(THF) 1623(s), 1592(m), 1411(s) cm-1. H NMR (CD3CN): δ
1.78 [m, 8H {THF}], 3.65 [m, 8H {THF}], 6.99 [m, 10H {4-H}],
7.42 [t, 20H {3,5-H}]. 13C{H} NMR (CD3CN): δ 26.62 {THF},
68.73 {THF}, 112.92-113.43 [m, {3,5-C6H3} {4-C6H3}], 132.83
[t, JC-F ) 10.1 Hz {ipso-C6H3}], 161.11 [dd, JC-Fl ) 250.79 Hz,
JC-F2 ) 8.05 Hz {2,6-C6H3}], 169.97 [s, {-CO2}]. 19F{H} NMR
(CD3CN): δ -112.34.
1
Synthesis of [(2,6-Dichlorobenzoate)4Zn2](THF)3 (2). A 10-
mL THF solution of 2,6-dichlorobenzoic acid (0.200 g, 1.04 mmol)
was added to a 5-mL THF solution of Zn[N(SiMe3)2]2 (0.20 g, 0.52
mmol), leading to a clear, colorless solution which was stirred at
room temperature for 2 h. The solution was then concentrated to 5
mL and stored at -20 °C. Colorless block crystals formed after
several days. The supernatant was cannulated off, and crystals were
dried under vacuum and collected to yield 0.161 g of product (56%).
Anal. Calcd. for C40H36O11Cl8Zn2 according to the crystal struc-
ture: C, 43.39; H, 3.28. Anal. Calcd. for C28H12O8Cl8Zn2 without
bound THF molecules: C, 37.75; H, 1.36. Found: C, 38.16; H,
1.86. A better match between the calculated values for 2 which
contained no bound THF molecules was found. This is due to the
lability of THF molecules. 1H NMR also supports this conclusion
by exhibiting only trace amounts of THF once the solvent is
Experimental Section
Methods and Materials. Unless otherwise specified, all syn-
theses and manipulations were carried out on a double manifold
Schlenk vacuum line under an atmosphere of argon or in an argon
filled glovebox. Glassware was flame dried thoroughly prior to use.
Solvents were freshly distilled from sodium benzophenone before
use. Cyclohexene oxide and propylene oxide were purchased from
Aldrich Chemical Co. and purified by distillation over calcium
hydride. Bone dry carbon dioxide was purchased from Scott
Specialty Gases, Inc. 2,6-Difluorobenzoic acid, 2,6-dichlorobenzoic
acid, and 2,6-dimethoxybenzoic acid were purchased from Lancaster
removed under vacuum. IR(νCO ): (KBr) 1610(m), 1591(s), 1556(s),
2
1
1404(s) cm-1; (THF) 1626(s), 1589(m), 1399(m) cm-1. H NMR
(CD3CN): δ 1.78 [m, 4H {THF}], 3.74 [m, 4H {THF}], 6.45
[m, 4H], 6.90 [t, 8H, {3,5-H}]. 13C{H} NMR (CD3CN): δ 26.62
{THF}, 68.73 {THF}, 129.28, 129.59 [s, {4-C6H3}]; 131.09, 131.51
(6) Rokicki, A.; Kuran, W. J. Macromol. Sci., ReV, Macromol. Chem.
Phys. 1981, C21, 135.
(7) (a) Cheng, M.; Lobkovsky, E.; Coates, G. J. Am. Chem. Soc. 1998,
120, 11018. (b) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain,
B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001,
123, 8738.
(8) (a) Darensbourg, D. J.; Zimmer, M.; Rainey, P.; Larkins, D. L. Inorg.
Chem. 2000, 39, 1578. (b) Darensbourg, D. J.; Zimmer, M.; Rainey,
P.; Larkins, D. L. Inorg. Chem. 1998, 37, 2852.
(13) Burger, H.; Sawodny, W.; Wannaget, V. J. Organomet. Chem. 1965,
3, 113.
(14) SMART 1000 CCD; Bruker Analytical X-ray Systems: Madison, WI,
1999.
(15) SAINT-Plus, version 6.02; Bruker Analytical X-ray Systems: Madison,
WI, 1999.
(9) Darensbourg, D. J.; Rainey, P.; Yarbrough, J. C. Inorg. Chem. 2001,
40, 986.
(10) Cheng, M.; Darling, N. A.; Lobkovsky, E. B.; Coates, G. W. Chem.
Commun. 2000, 2007.
(16) Sheldrick, G. SHELXS-86 Program for Crystal Structure Solution;
Institut fur Anorganische Chemie der Universitat: Gottingen, Germany,
1986.
(17) Sheldrick, G. SHELXL-97 Program for Crystal Structure Refinement;
(11) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.; Holmes, A.
B. Macromolecules 2000, 33, 303.
(12) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C.; Reibenspies,
J. H. J. Am. Chem. Soc. 2000, 122, 12487.
Institut fur Anorganische Chemie der Universitat: Gottingen, Germany,
1997.
(18) SHELXTL, version 5.0; Bruker Analytical X-ray Systems: Madison,
WI, 1999.
974 Inorganic Chemistry, Vol. 41, No. 4, 2002

Structures of Zinc(II) Benzoate Complexes
Table 1. Crystallographic Data for Complexes 1a, 1b, 2a, and 2b
1 - 4THF
1a - 4CH3CN
1b - pyridine
2
2a
empirical formula
fw
cryst syst
space group
V, Å3
C86H62F20O28Zn7
2380.94
monoclinic
P2(1)/n
4524(4)
2
C82H48F20O22N6Zn7
2307.02
triclinic
P1h
2368.0(6)
1
C29H21F4O4N3Zn‚pyr
696.02
monoclinic
P2(1)
1556.00(17)
2
C44H44Cl8O12Zn2
1179.24
triclinic
P1h
4921.4(9)
4
C29H21Cl4O4N3Zn
682.72
monoclinic
P2(1)/c
2927(6)
4
Z
a, Å
b, Å
c, Å
15.145(7)
14.852(7)
20.147(9)
12.618(2)
14.324(2)
14.933(2)
62.979(3)
84.892(3)
80.076(3)
110(2)
1.442
1.852
6.47
21.17
11.6569(7)
10.9364(7)
12.8998(8)
10.8158(12)
12.6457(14)
36.794(4)
88.179(2)
83.573(2)
79.803(2)
110(2)
1.602
1.468
9.44
23.81
14.688(17)
9.213(10)
22.59(3)
R, deg
â, deg
93.234(9)
108.8850(10)
106.76(2)
γ, deg
T, K
110(2)
1.748
1.945
6.42
110(2)
1.537
0.862
2.56
110(2)
1.549
1.244
7.71
d(calcd), g/cm3
abs coeff, mm-1
R,a % [I > 2σ(I)]
Rw,a %
13.47
6.59
14.28
a R ) ∑||Fo| - |Fc||/∑Fo. Rw ) {[∑w(Fo - Fc )2/[∑w(Fo )2]}1/2
.
2
2
2
[s, {3,5-C6H3}]; 131.81, 132.59 [s, {ipso-C6H3}], 136.32, 139.31
[s, {2,6-C6H3}], 165.64, 172.46 [s, {-CO2}].
The following software was used for the title compound: data
reduction, SAINTPLUS (Bruker15); program(s) used to solve the
structure, SHELXS-96 (Sheldrick16); program(s) used to refine the
structure, SHELXL-97 (Sheldrick17); program(s) used for molecular
graphics, SHELXTL version 5.0 (Bruker18); software used to
prepare material for publication, SHELXTL version 5.0 (Bruker18).
High-Pressure Copolymerization of CO2 with Cyclohexene
Oxide. A sample of the active catalyst (0.100 g) was dissolved in
20.0 mL of cyclohexene oxide. The solution was loaded via an
injection port into a 300 mL stainless steel Parr autoclave, which
had previously been dried overnight under vacuum at 80 °C. The
reactor was pressurized to 600 psi with CO2 and heated to 80 °C,
which increased the pressure to 750-800 psi. After 24-48 h of
reaction time, the reactor was cooled and opened and the viscous/
solid mixture isolated by dissolution in CH2Cl2 and precipitated
Synthesis of [(2,6-Dimethoxybenzoate)2xZnx](THF)y (3). A 10-
mL THF solution of 2,6-dimethoxybenzoic acid (0.189 g, 1.04
mmol) was added to 5-mL THF solution of Zn[N(SiMe3)2]2 (0.20
g, 0.52 mmol), upon which a white solid, assumed to be an
aggregate of bis-2,6-dimethoxybenzoate zinc, formed immediately.
This precipitate was washed several times with hexanes and dried
under vacuum to yield 0.218 (94%). IR(νCO ): (KBr) 1643(m),
2
1597(s), 1547(s), 1425(s) cm-1 1H NMR (d6-DMSO): δ 1.75
.
[m, 4H {THF}], 3.60 [m, 4H {THF}], 3.71 [s, 12H {-OCH3}],
6.60 [d, 4H {3,5-H}], 7.16 [t, 2H {4-H}]. 13C{H} NMR (d6-
DMSO): δ 25.13 {THF}, 55.48 {-OCH3}, 67.04 {THF}, 104.18
[s, {4-C6H3}]; 119.24 [s, {3,5-C6H3}]; 128.02 [s, {ipso-C6H3}],
155.58 [s, {2,6-C6H3}], 170.39 [s, {-CO2}].
X-ray Crystallography. A Bausch and Lomb 10× microscope
was used to identify suitable colorless crystals of 1, 1a, 1b, 2, and
2a from a representative sample of crystals of the same habit. The
representative crystal was coated in a cryogenic protectant (i.e.,
mineral oil, paratone, or apezeon grease) and was then fixed to a
glass fiber, which in turn was fashioned to a copper mounting pin.
The mounted crystals were then placed in a cold nitrogen stream
(Oxford) maintained at 110 K on a Bruker SMART 1000 three
circle goniometer.
1
out in MeOH. The polymer was analyzed by H and 13C NMR
spectroscopy.
High-Pressure Copolymerization of CO2 with Propylene
Oxide. A 0.100 g amount of active catalyst was dissolved in 20.0
mL of propylene oxide. The resulting solution was added through
the injection port to a predried 300 mL autoclave, and the reactor
was pressurized to 600 psi with CO2. The reactor was heated at 55
°C, raising the pressure to 650-700 psi, for 48 h. After this period
of time, the reaction mixture was diluted with CH2Cl2 (1:10) and
analyzed by infrared spectroscopy in the ν(CO) region.
Crystal data and details of data collection for the complexes are
provided in Table 1. The X-ray data were collected on a Bruker
CCD diffractometer and covered more than a hemisphere of
reciprocal space by a combination of three sets of exposures; each
exposure set had a different æ angle for the crystal orientation,
and each exposure covered 0.3° in ω. The crystal-to-detector
distance was 4.9 cm. Crystal decay was monitored by repeating
the data collection for 50 initial frames at the end of the data set
and analyzing the duplicate reflections; crystal decay was negligible.
The space group was determined on the basis of systematic absences
and intensity statistics.14
High-Pressure Terpolymerization of CO2 with Propylene
Oxide and Cyclohexene Oxide. A 0.100 g amount of active
catalyst was dissolved in a solution composed of 10.0 mL (∼50
mol %) of cyclohexene oxide and 7.0 mL (∼50 mol %) of
propylene oxide. The resulting solution was added through the
injection port of a 300 mL predried autoclave, and the reactor
was pressurized to 600 psi with CO2. The reactor was then heated
to 55 °C, raising the pressure to 650-700 psi, for between 24
and 48 h. After this period of time, the reactor was opened, and
the viscous/solid mixture was dissolved in CH2Cl2 and precipitated
1
out in MeOH. The polymer was analyzed by H and 13C NMR
The structures were solved by direct methods. Full-matrix least-
squares anisotropic refinement for all non-hydrogen atoms yielded
R(F) and wR(F2) values as indicated in Table 1 at convergence.
Hydrogen atoms were place in idealized positions with isotropic
thermal parameters fixed at 1.2 or 1.5 times the value of the attached
atom. Neutral atom scattering factors and anomalous scattering
factors were taken from the International Tables for X-ray Crystal-
lography Vol. C.
spectroscopy.
Results and Discussion
Synthesis and X-ray Structural Characterization of
Zinc Benzoate Complexes. Several multicentered zinc
benzoate derivatives have been synthesized and isolated in
greater than 50% purified yields from the reaction of 2 equiv
Inorganic Chemistry, Vol. 41, No. 4, 2002 975

Darensbourg et al.
Figure 1. Thermal ellipsoid representation of the asymmetric unit of [(2,6-
difluorobenzoate)10O2Zn7](THF)2, 1, with the other half of the molecule
shown as a stick drawing. Only the oxygen atom of the THF molecule was
located and anisotropically refined.
Figure 2. Thermal ellipsoid representation of the asymmetric unit of
[(2,6-difluorobenzoate)10O2Zn7](CH3CN)2, 1a, with the other half of the
molecule shown as a stick drawing.
of the respective benzoic acid and Zn[N(SiMe3)2]2. For
example, the reaction of 2,6-difluorobenzoic acid with Zn-
[N(SiMe3)2] in THF solution afforded complex 1, [(2,6-
difluorobenzoate)10O2Zn7](THF)2, in 77% yield. Crystals of
1 were obtained from a concentrated THF solution of the
complex maintained at -20 °C. We have not specifically
identified the source of oxide ligand. Presumably, it comes
from water, for although the benzoic acid derivative was
sublimed and stored under an inert atmosphere prior to its
use, it is difficult to remove all traces of water from
carboxylic acids. Upon vacuum-drying of complex 1, the
THF ligands are easily removed as indicated by C/H analysis
1
and H NMR spectroscopy. Recrystallization of complex 1
from acetonitrile results in X-ray quality crystals of the
acetonitrile analogue of 1, [(2,6-difluorobenzoate)10O2Zn7]-
(CH3CN)2, 1a. However, dissolution of 1 in the stronger base
pyridine led to aggregate disruption and formation of the
mononuclear pyridine adduct, Zn(2,6-difluorobenzoate)2(py)3,
1b.
Figure 3. Abbreviated representation of the zinc-oxo core of complex 1.
are 1.945(8) and 1.946(8) Å in 1 and 1.947(6) and 1.957(6)
Å in 1a, as compared to the significantly longer octahedrally
coordinated Zn(3)-O distances of 2.207(8) and 2.164(8) Å
in 1 and 2.184(6) and 2.167(6) Å in 1a. The remaining three
benzoate bridges are fairly symmetrically bonded to the zinc
centers. The Zn-Ooxo bond lengths cover a small range
(1.919(7)-1.984(7) Å in 1 and 1.917(6)-1.987(5) Å in 1a)
with an average distance of 1.944 Å in 1 and 1.945 Å in 1a.
The bridging modes for all of the benzoate ligands were
found to be in a syn-syn configuration. The tetrahedral
arrangement of the bridged Zn(1)/Zn(2)/Zn(3)/Zn(4) atoms
is similar to a four zinc centered complex previously reported
by Straughan, as described in Figure 4.19 However, instead
of a benzoate bridge linking Zn(3) and Zn(4) together, thus
satisfying their tetrahedral environs, a THF (1) or acetonitrile
(1a) molecule is bound to Zn(4), leaving a trigonally
coordinated Zn(3) foundation open to symmetrically incor-
porate Zn(1A)/Zn(2A)/Zn(4A) along with their benzoate and
oxo bridging groups. The Zn(4)-OTHF bond distance in
complex 1 was found to be 2.035(10) Å, and the analogous
Figures 1 and 2 display thermal ellipsoid drawings of 1
and 1a, along with partial atom labeling schemes. Table 2
contains a compilation of selected bond distances and bond
angles. The structures of complexes 1 and 1a are nearly
identical with the exception of the identity of the solvate
molecules. These complexes are interesting seven zinc center
aggregates where the metal atoms are interconnected by a
combination of 10 bridging benzoate ligands and 2 µ4-oxo
ligands. The arrangement of the metal clusters is symmetrical
through an inversion center at the central, octahedrally
coordinated Zn(3) centers. The remaining three zinc atoms
(Zn(1), Zn(2), and Zn(4)) are of tetrahedral geometry, along
with their symmetry generated counterparts, and each share
an oxo bridge to the central Zn(3) atom. This is best seen in
the abbreviated representation of the Zn3-µ4O-Zn-µ4OZn3
core with bound THF molecules depicted in Figure 3. Two
benzoate ligands are unsymmetrically bridged between the
tetrahedrally coordinated Zn(1) and Zn(2) centers to Zn(3);
that is, the corresponding Zn(1)-O and Zn(2)-O distances
(19) Clegg, W.; Harbron, D.; Holman, C.; Hunt, P.; Little, I.; Straughan,
B. Inorg. Chim. Acta 1991, 186, 51.
976 Inorganic Chemistry, Vol. 41, No. 4, 2002

Structures of Zinc(II) Benzoate Complexes
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for Complexes 1, 1a, 1b, 2, and 2aa
1
1a
1
1a
Zn(1)-O(1D)
1.979(8)
1.945(8)
1.977(8)
1.919(7)
1.946(8)
1.995(8)
1.941(8)
1.927(7)
2.164(8)
2.207(8)
1.984(7)
1.966(7)
2.053(9)
1.946(8)
2.035(10)
1.965(6)
1.947(6)
1.973(7)
1.917(6)
1.957(6)
1.993(7)
1.945(6)
1.932(6)
2.167(6)
2.184(6)
1.987(5)
1.996(6)
2.071(6)
1.944(6)
O(1D)-Zn(1)-O(2C)
O(2C)-Zn(1)-O(1)
O(1)-Zn(2)-O(1C)
O(1)-Zn(2)-O(1A)
O(1)-Zn(2)-O(1E)
O(1E)-Zn(2)-O(1A)
O(1E)-Zn(2)-O(1C)
O(1A)-Zn(2)-O(1C)
O(1)-Zn(3)-O(2A)
O(1)-Zn(3)-O(1B)
O(1B)-Zn(3)-O(2A)
O(2D)-Zn(4)-O(2)
O(2E)-Zn(4)-O(2)
O(1)-Zn(4)-O(2)
102.6(3)
112.6(3)
108.3(3)
115.1(3)
119.9(3)
108.2(3)
102.8(3)
99.9(3)
98.1(3)
97.3(3)
93.7(3)
105.9(4)
91.9(4)
99.2(3)
114.1(3)
109.7(3)
113.2(2)
117.0(3)
115.7(3)
99.6(3)
98.7(3)
100.4(2)
97.9(2)
Zn(1)-O(2B)
Zn(1)-O(2C)
Zn(1)-O(1)
Zn(2)-O(1A)
Zn(2)-O(1C)
Zn(2)-O(1E)
Zn(2)-O(1)
Zn(3)-O(2A)
Zn(3)-O(1B)
Zn(3)-O(1)
90.6(2)
Zn(4)-O(2D)
Zn(4)-O(2E)
Zn(4)-O(1)
139.8(4)
Zn(4)-O(2)
O(2D)-Zn(4)-N(1)
O(2E)-Zn(4)-N(1)
O(1)-Zn(4)-N(1)
104.0(3)
89.2(3)
140.1(3)
98.4(3)
100.6(2)
112.5(2)
125.3(9)
128.2(8)
127.4(9)
125.4(8)
126.0(9)
Zn(4)-N(1)
2.028(8)
107.6(3)
Zn(1)-O(1)-Zn(2)
Zn(1)-O(1)-Zn(3)
Zn(1)-O(1)-Zn(4)
Zn(2)-O(1)-Zn(3)
Zn(2)-O(1)-Zn(4)
Zn(3)-O(1)-Zn(4)
O(1D)-Zn(1)-O(2B)
O(2B)-Zn(1)-O(1)
O(1D)-Zn(1)-O(1)
O(2B)-Zn(1)-O(2C)
107.7(4)
115.2(4)
109.7(3)
111.7(3)
108.5(4)
103.9(3)
106.6(3)
118.2(3)
112.8(3)
102.4(3)
115.3(3)
110.3(3)
109.9(3)
110.6(3)
103.1(3)
105.8(3)
117.9(3)
109.8(3)
108.1(3)
O(2D)-Zn(4)-O(2E)
O(1)-Zn(4)-O(2E)
O(1)-Zn(4)-O(2D)
O(2E)-C(1E)-O(1E)
O(2D)-C(1D)-O(1D)
O(2B)-C(1B)-O(1B)
O(2A)-C(1A)-O(1A)
O(2C)-C(1C)-O(1C)
99.9(3)
102.8(3)
108.0(3)
126.6(12)
127.3(11)
125.8(12)
124.5(11)
124.9(12)
Complex 1b
Complex 2
Zn(1)-O(3)
Zn(1)-O(1)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
2.024(2)
2.036(2)
2.177(3)
2.1135(18)
2.195(3)
O(1)-Zn(1)-O(3)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(2)
O(1)-Zn(1)-N(3)
N(1)-Zn(1)-N(3)
128.46(8)
88.64(10)
134.85(10)
89.44(10)
177.98(10)
Zn(1)-O(2A)
Zn(1)-O(3A)
Zn(1)-O(5A)
Zn(1)-O(7A)
Zn(2)-O(4A)
Zn(2)-O(6A)
Zn(2)-O(8A)
Zn(2)-O(9A)
Zn(2)-O(10A)
Zn(2)-O(11A)
1.941(5)
O(2A)-Zn(1)-O(7A)
O(3A)-Zn(1)-O(5A)
O(3A)-Zn(1)-O(7A)
O(5A)-Zn(1)-O(7A)
O(4A)-Zn(1)-O(6A)
O(4A)-Zn(1)-O(8A)
O(4A)-Zn(1)-O(9A)
O(4A)-Zn(1)-O(10A)
O(4A)-Zn(1)-O(11A)
O(3A)-C(8A)-O(4A)
O(5A)-C(15A)-O(6A)
O(7A)-C(22A)-O(8A)
108.9(2)
109.8(2)
119.6(2)
109.6(2)
98.1(2)
103.7(2)
166.2(2)
84.6(2)
1.957(5)
1.942(6)
1.950(6)
2.007(5)
2.072(5)
2.040(5)
2.095(6)
2.169(5)
2.165(5)
93.1(2)
84.0(2)
127.1(6)
127.6(7)
126.6(7)
O(2A)-Zn(1)-O(3A)
O(2A)-Zn(1)-O(5A)
114.9(2)
Complex 2a
Zn(1)-O(1)
Zn(1)-O(3)
Zn(1)-O(4)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
O(1)-Zn(1)-O(3)
1.977(11)
2.183(11)
2.348(10)
2.108(12)
2.087(11)
2.100(13)
176.3(4)
O(1)-Zn(1)-O(4)
O(3)-Zn(1)-N(1)
O(3)-Zn(1)-N(2)
O(3)-Zn(1)-N(3)
N(1)-Zn(1)-N(3)
O(1)-Zn(1)-N(2)
N(2)-Zn(1)-N(1)
121.0(4)
91.6(5)
91.7(4)
89.4(5)
157.8(5)
90.1(5)
95.3(5)
a Estimated deviations are given in parentheses.
Zn(4)-Nacetonitrile bond distance in 1a was determined to be
2.028(8) Å. In addition to the two THF or acetonitrile
molecules bound to Zn(4) and Zn(4A) in complexes 1 and
1a, four additional molecules of each respective solvent were
found in the crystal lattices.
where the Zn-O(1) and Zn-O(3) bond distances are
2.036(2) and 2.024(2) Å, with the distal oxygens (O(2) and
O(4)) being 2.796 and 2.953 Å from the zinc center,
respectively. The two axial Zn-N bond lengths were found
to be slightly longer than the Zn-N equatorial bond distance,
A thermal ellipsoid rendering of the pyridine complex (1b)
derived from the dissolution of complex 1 in pyridine can
be seen in Figure 5, with selected bond distances and bond
angles listed in Table 2. The structure of 1b consists of a
distorted trigonal bipyrimidal arrangement of three pyridine
and two benzoate ligands about the zinc center. The benzoate
ligands are bound essentially in an unidentate fashion to zinc,
with the average Zn-Nax ) 2.186[3] Å and the Zn-Neq )
2.1135(18) Å. The two axial pyridine ligands form a
N(1)-Zn-N(3) bond angle of 177.98(10)° and an average
bond angle of 91.33° between the ligands in the trigonal
plane. The remaining pyridine ligand in the metal’s coor-
dination sphere forms a trigonal plane with the benzoate
groups affording an average N(2)-Zn-O bond angle of
Inorganic Chemistry, Vol. 41, No. 4, 2002 977

Darensbourg et al.
Figure 6. Thermal ellipsoid representation of [(2,6-dichlorobenzoate)4Zn2]-
(THF)3, 2.
Figure 4. (a) Thermal ellipsoid representation of the independently defined
portion of complex 1a. (b) [(Benzoate)6OZn] complex reported by Straughan
in ref 19.
Figure 7. Thermal ellipsoid representation of [(2,6-dichlorobenzoate)2Zn-
(NC5H5)3], 2a.
distorted tetrahedral ligand environment, where the fourth
ligand is a unidentate benzoate group (distal O(1A)‚‚‚Zn(1)
separation ) 3.225 Å). The second zinc center has an
octahedral arrangement of ligands which is accomplished by
the additional binding of three THF molecules. The average
Zn-O bond distance for the tetrahedral zinc was found to
be 1.948[5] Å, with an average O-Zn-O bond angle of
109.36°. For the octahedral environment, the average Zn-O
bond distance was determined to be 2.040[5] Å for the
bridging benzoates and 2.143[5] Å for the THF linkage.
Analogous to the behavior of complex 1 upon dissolution
in pyridine, complex 2 afforded X-ray quality crystals of
Zn(2,6-dichlorobenzoate)2(py)3 (2a), which results from
disruption of the zinc dimer by the strongly coordinating
pyridine ligand. A thermal ellipsoid representation of com-
plex 2a is shown in Figure 7, and selected bond distances
and bond angles may be found in Table 2. The structure of
2a closely resembles that of its difluorobenzoate analogue,
complex 1b. That is, complex 2a consists of a distorted
trigonal bipyrimidal arrangement of three pyridine and two
benzoate ligands around a common zinc center. However,
in this instance, possibly because of the stronger binding
ability of the chlorobenzoate ligand relative to its fluoro
counterpart, one of the benzoate ligands is unsymmetrically
bound in a bidentate fashion with the Zn-O(3) bond distance
at 2.183(11) Å being significantly shorter than the Zn-O(4)
bond length of 2.348(10) Å. This change in binding mode
distorts the axial N(1)-Zn-N(3) bond angle to 157.8(5)°
Figure 5. Thermal ellipsoid representation of [(2,6-difluorobenzoate)2Zn-
(NC5H5)3], 1b.
115.77[10]°. A fourth pyridine molecule was found in the
crystal lattice of 1b. The O(1)-Zn-O(3) bond angle of
128.46(8)° exists between the benzoate groups.
By way of contrast to the process described previously
leading to formation of complex 1, the reaction of Zn-
[N(SiMe3)2]2 with 2,6-dichlorobenzoate in THF has led to a
more anticipated product devoid of µ-oxo ligands. That is,
the reaction affords the dimeric complex, [(2,6-dichloro-
benzoate)4Zn2](THF)3 (2). A thermal ellipsoid representation
of complex 2 is shown in Figure 6 for one of the independ-
ently generated molecules in the unit cell. Selected bond
distances and bond angles may be found in Table 2. The
two zinc centers are unsymmetrically bridged by three
benzoate ligands. Interestingly, one zinc center possesses a
978 Inorganic Chemistry, Vol. 41, No. 4, 2002

Structures of Zinc(II) Benzoate Complexes
Table 3. Catalytic Activity for the Copolymerization of Carbon
systems a similar zinc species is responsible for the catalysis
of the process depicted in eq 1. Whether the metal species
is a zinc aggregate complex or some species produced by
degradation of a zinc aggregate is not definitively known.
Although complex 1 can be recovered from a refluxing THF
solution maintaining its original solid-state structure, there
may be significant rearrangement or disruption of all or part
of this structure during the copolymerization process. Similar
to recently developed zinc phenoxide catalysts containing
fluorine substituents,10 complexes 1-3 are stable in moist
air and do not lose activity when allowed to stand in a moist
oxygen atmosphere for prolonged periods of time. In
addition, it is particularly notable that the polymers produced
have essentially 100% carbonate linkages, which is charac-
teristic of polymer produced by zinc catalysts with one site
for epoxide binding. The percentage of carbonate linkages
is assessed from the integration of the methine protons of
the carbonate linkages (δ ) 4.60 ppm) and the ether linkages
(δ ) 3.45 ppm). A short while ago, Scott and co-workers
developed a series of similar zinc cluster catalysts by reacting
diethyl zinc with tris(3,5-dialkyl-2-hydroxyphenyl) meth-
ane.22 Although these complexes are structurally interesting,
they displayed reduced activities (1.33-2.54 g poly/g Zn‚
hr) and inadequate control over CO2 incorporation (60-80%
carbonate linkages).
Dioxide and Cyclohexene Oxidea
turnover
number
turnover
frequency
catalyst
(g‚poly/g‚Zn) (g‚poly/g‚Zn/hr)
[Zn7(O2C-2,6-F2C6H3)10O2](THF)2, 1
[Zn2(O2C-2,6-Cl2C6H3)4](THF)3, 2
[Zn(O2C-2,6-(OMe)2C6H2)2]n(THF)m, 3
494
874
329
10.3
16.7
7.2
a Catalyst loading (0.100 g), 20.0 mL of cyclohexene oxide, CO2 at
ambient temperature 41.3 bar. Reaction conditions: 80 °C at a total pressure
of 55 bar.
as compared to the nearly linear angle (177.98(10)°) seen
in 1b. Concomitantly, the O(1)-Zn-O(3) bond angle of
176.3(4)° is more obtuse than its corresponding value of
128.46(8)° in complex 1b. Hence, the geometry of complex
2a might be better described as a highly distorted octahedral
structure.
Note. Average esd’s are provided in [ ], whereas ( ) are
used for a single esd value.
Reactivity Studies of Zinc Benzoate Derivatives for the
Copolymerization of Carbon Dioxide and Epoxides. The
infrared spectra of the zinc benzoate complexes 1 and 2 in
the ν(CO2) region recorded in both the solid state (KBr) and
solution (THF) were shown to be quite similar (see Experi-
mental Section). These observations are suggestive of the
solution structures of 1 and 2 at ambient temperature strongly
resembling their well-defined solid-state structures. Hence,
it is highly likely that upon dissolution of these derivatives
in epoxides a similar solution structure is present, with
epoxide ligands occupying the zinc sites previously occupied
by THF molecules. Because various nonstructurally char-
acterized zinc carboxylates have been demonstrated to be
quite effective as catalysts for the copolymerization of CO2
and epoxides, it was naturally of interest to examine the
activity of these structurally characterized species. That is,
the soluble, nonstructurally characterized zinc monoesters
of maleic acid3,4 and zinc crotonate catalyst systems5 have
shown high activity for the copolymerization of CO2 and
cyclohexene oxide to afford alternating copolymer with little
to no polyether linkages (eq 1). Of course, the heterogeneous
zinc glutarate catalyst has received much academic and
industrial attention for this process.2,20,21
The terpolymerization of propylene oxide (50 mol %),
cyclohexene oxide (50 mol %), and CO2, employing catalyst
2, the most active catalyst for the copolymerization of
cyclohexene oxide and CO2, exhibited an average turnover
number of 523 g poly/g Zn and a turnover frequency of 10.88
g poly/g Zn/hr. The polymer produced has 88.15 mol %
cyclohexene oxide carbonate linkages, 6.44 mol % propylene
carbonate linkages, and 5.41 mol % propylene ether linkages.
1
Propylene carbonate and ether linkages show up in the H
NMR spectrum at 5.01 and 3.59 ppm, respectively. Interest-
ingly, as with the dimeric zinc phenoxide systems, there are
no cyclohexene oxide ether linkages in the terpolymer. On
the other hand, the copolymerization of propylene oxide and
CO2, using 2 as catalyst, produced only a thin film of
polymer. A comparison of cyclic carbonates to polycarbon-
ates produced was determined by infrared spectroscopy, with
propylene polycarbonate showing a ν(CO2) absorption at
1750 cm-1 and cyclic propylene carbonate exhibiting a
ν(CO2) mode at 1800 cm-1. Under similar reaction conditions
to those used in cyclohexene oxide/CO2 copolymerization,
propylene cyclic carbonate is the major product produced.
Upon lowering the reaction temperature to 40 °C, propylene
polycarbonate dominates over the cyclic carbonate as previ-
ously noted.23
Complexes 1-3 have been tested for their ability to
catalyze the copolymerization of cyclohexene oxide and
carbon dioxide to provide high molecular weight poly-
(cyclohexenylene carbonate). These catalysts exhibit activi-
ties which are compiled in Table 3 which are very close to
those seen in the related soluble zinc carboxylates previously
reported in the literature.3-5 It is likely that in all of these
Summary
Herein, we have reported on the synthesis and structural
characterization of several zinc carboxylates, derivatives
which are closely related to nonstructurally defined hetero-
(20) Ree, M.; Bae, J. Y.; Jung, J. H.; Shin, T. J. J. Polym. Sci., Part A:
Polym. Chem. 1999, 37, 1863.
(21) (a) Rokicki, A. U.S. Patent No. 4, 943, 677, July 24, 1990. (b) PAC
Polymers Inc., Newark, DE.
(22) Dinger, M.; Scott, M. Inorg. Chem. 2001, 40, 1029.
(23) 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.
Inorganic Chemistry, Vol. 41, No. 4, 2002 979

Darensbourg et al.
geneous and homogeneous zinc carboxylates which serve
as catalyst precursors for the copolymerization of CO2 and
epoxide to provide polycarbonates. Although these com-
plexes exist as monomeric species when dissolved in strong
bases such as pyridine, they are dimeric or multinuclear
derivatives in the presence of less coordinating bases such
as THF or acetonitrile. In particular, the acetonitrile and THF
adducts of 1 exist as seven zinc membered clusters, where
the metal centers are linked together by syn-syn µ-benzoate
and µ4-oxo bridges. Solution and solid-state infrared spec-
surface, this observation would appear to discredit the Kuran
hypothesis that multinuclear zinc complexes should lead to
reduced cyclic carbonate formation with concomitant poly-
carbonate production.6 However, even if the assumption that
these zinc aggregates remain intact during polymerization
is true, the systems studied herein and elsewhere do not
possess readily accessible coordination sites on adjacent zinc
centers, a requirement which is essential for inhibiting the
back biting reaction.
Finally, the quite similar catalytic behavior and activity
for the copolymerization of CO2 and cyclohexene oxide
exhibited by these structurally defined zinc carboxylates as
compared to the less well-defined homogeneous catalysts
currently in the literature suggest a close relationship between
the nature of the active zinc species in all of these catalytic
systems.
troscopy in the νCO region demonstrates that these derivatives
2
remain intact upon dissolution in THF and suggests a similar
behavior when dissolved in epoxides. The perception that
the catalytically active zinc species remains an aggregate or
dimer is reinforced by the presence of greater than 99%
carbonate linkages in the resulting polymer. That is, if com-
plexes 1-3 were rendered monomeric in the epoxide solution
during the polymerization process, this degree of CO2
incorporation would be highly unlikely because of multiple
open binding sites for the epoxide coordination and conse-
quent formation of ether linkages.10 Nevertheless, it should
be reiterated here that there may be significant rearrangement
of the solid-state structures during the copolymerization
process. These complexes, although found to be active for
the copolymerization of cyclohexene oxide and CO2 along
with the terpolymerization of cyclohexene oxide/propylene
oxide/CO2, did not fulfill our goal to develop effective
catalysts for propylene oxide/CO2 copolymerization. On the
Acknowledgment. Financial support from the National
Science Foundation (CHE-99-10342 and CHE 98-07975 for
the purchase of X-ray equipment) and the Robert A. Welch
Foundation is greatly appreciated.
Supporting Information Available: Complete details (CIF
format) of the X-ray diffraction studies on 1, 1a, 1b, 2, and 2a.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0107983
980 Inorganic Chemistry, Vol. 41, No. 4, 2002