Towards rational synthesis of polar solids. Synthesis and X-ray structures of cadmium(n) w<?ta-pyridinecarboxylate coordination polymers f

Article abstract of DOI:10.1039/b003441j

Two cadmium(n) coordination polymers, Cd2L'4(n-H2O) 1 and Cd2L24(u-H2O) 2 [L1 = (￡)-4-(2-thiazolyl)ethenylbenzoate, L2 = (￡)-4-(3-pyridyl)ethenylbenzoate] based on /e/o-pyridinecarboxylate bridging ligands, were synthesized by the reactions of cadmium perchlorate and the corresponding benzonitrile precursor ligands under hydro(solvo)thermal conditions. Although both compounds 1 and 2 are constructed from the Cd2L"4(u-H2O) building block, X-ray single crystal structure determinations show that 1 adopts a 2-D polymeric network structure while 2 exhibits a 3-D framework structure as a result of the slightly different configurations of the bridging pyridinecarboxylate ligands. Owing to the presence of the bridging water molecules, both 1 and 2 crystallize in centrosymmetric space groups. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b003441j

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Towards rational synthesis of polar solids. Synthesis and X-ray
structures of cadmium(II) meta-pyridinecarboxylate coordination
polymers†
Owen R. Evans and Wenbin Lin*
Department of Chemistry, Brandeis University, Waltham, MA 02454, USA.
E-mail: wlin@brandeis.edu
Received 28th April 2000, Accepted 30th June 2000
First published as an Advance Article on the web 25th September 2000
Two cadmium() coordination polymers, Cd2L1 (µ-H2O) 1 and Cd2L2 (µ-H2O) 2 [L1 = (E)-4-(2-thiazolyl)ethenyl-
4
4
benzoate, L2 = (E)-4-(3-pyridyl)ethenylbenzoate] based on meta-pyridinecarboxylate bridging ligands, were
synthesized by the reactions of cadmium perchlorate and the corresponding benzonitrile precursor ligands under
hydro(solvo)thermal conditions. Although both compounds 1 and 2 are constructed from the Cd2Ln (µ-H2O)
4
building block, X-ray single crystal structure determinations show that 1 adopts a 2-D polymeric network structure
while 2 exhibits a 3-D framework structure as a result of the slightly different configurations of the bridging
pyridinecarboxylate ligands. Owing to the presence of the bridging water molecules, both 1 and 2 crystallize in
centrosymmetric space groups.
composed of “basic” trinuclear carboxylates with 3-fold
Introduction
rotational symmetry. By using para-pyridinecarboxylates, we
have successfully designed acentric diamondoid coordination
networks [Fig. 1(a)].6a Interestingly, when “bent” linking groups
such as meta-pyridinecarboxylates were used, acentric coordin-
ation polymers with 2-D grid structures were obtained [Fig.
1(b)].6b We have recently discovered that acentric 2-D coordin-
ation networks containing an unprecedented “basic” tricad-
mium carboxylate structural motif could be prepared under
basic conditions [Fig. 1(c)].6c Because of the 3-fold rotational
symmetry of the “basic” tricadmium carboxylate chromo-
phoric building blocks, these 2-D coordination networks are
octupolar in nature and are expected to have a superior
transparency/non-linearity tradeoff.
Non-centrosymmetric organization of molecular building
blocks is an essential requirement for many solid state proper-
ties including second-order nonlinear optical (NLO) effects.1
The synthesis of such acentric supramolecular assemblies still
presents a significant challenge for synthetic chemists.2 On
the other hand, construction of metal–organic coordination
frameworks via metal coordination-directed self-assembly pro-
cesses has become an area of intense research in recent years.3
Strong metal–ligand coordination bonds and their direction-
ality can be exploited for the rational design of extended solid
materials.4 Moreover, both the network structures and proper-
ties of these coordination networks can be fine-tuned via a sys-
tematic change of the organic linking ligands.5 In our work, we
have become particularly interested in utilizing metal–ligand
coordination to counteract unfavorable centric interactions
(such as dipole–dipole repulsions) and thus developing general
strategies for the construction of acentric polymeric coordin-
ation networks for second-order NLO applications.6
In order to test the generality of the above crystal engineering
approaches to polar solids, we have recently systematically
examined the reactions of a series of pyridinecarboxylic acids
(and their precursors) with Zn() and Cd() salts. We report in
this contribution the synthesis and X-ray structures of two
Cd() coordination polymers, Cd2L1 (µ-H2O) 1 and Cd2L2 -
4
4
Our approach to the rational synthesis of transparent polar
polymeric coordination networks is very straightforward. First,
we focus on d10 metal centers Zn() and Cd() for their trans-
parency and their favorable thermal and oxidative stabilities.
Second, we have chosen pyridinecarboxylate groups as bridging
ligands. We hypothesize that polymeric coordination networks
will result if these pyridinecarboxylate ligands can use both the
pyridyl and carboxylate functionalities to coordinate to Zn()
or Cd() centers. Third, the unsymmetrical structure and con-
jugate nature of pyridinecarboxylate groups will impart elec-
tronic asymmetry to the resulting coordination networks. Such
electronic asymmetry is an essential requirement for second-
order NLO properties. We have thus far identified three differ-
ent structure motifs with which we can rationally construct
polar solids containing pyridinecarboxylate ligands: (a) 3-D
diamondoid networks; (b) 2-D grid structures; (c) 2-D networks
(µ-H2O) 2 [L1 = (E)-4-(2-thiazolyl)ethenylbenzoate, L2 = (E)-4-
(3-pyridyl)ethenylbenzoate] based on meta-pyridinecarboxylate
bridging ligands. Owing to the presence of bridging water
molecules, 1 and 2 do not adopt any of the three structures in
Fig. 1 and are both centrosymmetric. Detailed studies of the
subtle differences among these solids should lead to better
understanding of the above crystal engineering approaches and
ultimately result in superior second-order NLO materials.
Experimental
Materials and methods
2-Bromothiazole and 4-bromopyridine hydrochloride were
purchased from TCI America and all other chemicals were pur-
chased from Aldrich Chemical Company. 4-Ethynylbenzo-
nitrile was synthesized according to the published procedure.7
The IR spectra were recorded as KBr pellets on a Paragon 1000
FT-IR spectrometer. 1H and 13C NMR spectra were taken on a
Varian Unity-Plus 400 spectrometer. Thermogravimetric anal-
yses were performed in air at a scan speed of 10 ЊC minϪ1 on a
Shimadzu TGA-50 TG analyzer.
† Based on the presentation given at Dalton Discussion No. 3, 9–11th
September 2000, University of Bologna, Italy.
Electronic supplementary information (ESI) available: rotatable 3-D
crystal structure diagram in CHIME format. See http://www.rsc.org/
suppdata/dt/b0/b003441j/
DOI: 10.1039/b003441j
J. Chem. Soc., Dalton Trans., 2000, 3949–3954
This journal is © The Royal Society of Chemistry 2000
3949

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was removed and made alkaline with 3 M NaOH, and extracted
with ethyl acetate. The extract was dried over MgSO4 and evap-
orated to dryness to afford 2.426 g of pure product (76% yield);
1
mp 124–126 ЊC. H NMR: δ 8.71 (br s, 1H, Ha), 8.51 (d, 1H,
J = 4.6 Hz, Hb), 7.82 (d, 1H, J = 1.8 Hz, Hd), 7.62 (d, 2H, J = 8.3
Hz, Hg), 7.57 (d, 2H, J = 8.3 Hz, Hh), 7.28 (dd, 1H, Jcb = 4.9,
Jcd = 4.9 Hz, Hc), 7.18 (d, 1H, J = 16.5 Hz), 7.11 (d, 1H, J = 16.5
Hz). 13C{1H} NMR: δ 149.4, 148.8, 141.0, 132.9, 132.5, 131.9,
128.7, 128.5, 127.0, 123.6, 118.8, 111.1.
Synthesis of (E)-4-(2-thiazolyl)ethenylbenzonitrile
A mixture of 4-ethynylbenzonitrile (2.58 g, 15.7 mmol) and
2-bromothiazole (2.02 g, 10 mmol) in 50 mL of diethylamine
was refluxed in the presence of dichlorobis(triphenylphos-
phine)palladium (40 mg, 0.06 mmol) and copper() iodide
(10 mg, 0.05 mmol) for 12 h. Diethylamine was removed under
reduced pressure, and the residue dissolved in 50 mL of ethyl
acetate and washed twice with 50 mL of water. The organic
layer was dried over MgSO4 and evaporated to dryness. Silica
gel column chromatography eluting with a mixture of hexane
and ethyl acetate (4:1 v/v) afforded 2.68 g of pure 4-(2-
1
thiazolyl)ethynylbenzonitrile (81% yield); mp 149–150 ЊC. H
NMR: δ 7.90 (d, 1H, J = 3.4 Hz, Hb), 7.66 (br s, 4H, Hc ϩ Hd),
7.45 (d, 1H, J = 3.4 Hz, Ha). 13C{1H} NMR: δ 147.7, 144.1,
132.4, 132.2, 126.3, 121.7, 118.2, 112.8, 91.5, 85.9.
4-(2-Thiazolyl)ethynylbenzonitrile (1.0 g, 4.8 mmol) in 50 mL
of methylene chloride was hydrogenated in the presence of
Lindlar’s catalyst under 40 psi of H2 for 12 h. Methylene
chloride was removed in vacuo to yield crude (Z)-4-(2-
thiazolyl)ethenylbenzonitrile, which was refluxed in nitro-
benzene (25 mL) containing a trace of iodine (0.3 g) for 3 h. The
mixture was then extracted with dilute hydrochloric acid, and
then made alkaline with 3 M NaOH. The alkaline aqueous
phase was extracted with ethyl acetate, dried over MgSO4,
Fig. 1 Three structural motifs for the rational synthesis of polar
solids. (a) A 3-D diamondoid network; (b) a 2-D grid structure; (c)
a 2-D polymeric network of “basic” trinuclear carboxylates with 3-fold
rotational symmetry.
Synthesis of (E)-4-(3-pyridyl)ethenylbenzonitrile
and evaporated to dryness. Silica gel column chromatography
eluting with a mixture of hexane and ethyl acetate (4:1 v/v)
afforded 0.75 g of pure product (74% yield); mp 134–136 ЊC. 1H
NMR: δ 7.82 (d, 1H, J = 3.0 Hz, Hb), 7.61 (d, 2H, J = 8.6 Hz,
Hf), 7.56 (d, 2H, J = 8.6 Hz, He), 7.38 (d, 1H, J = 16.2 Hz, Hc),
7.34 (d, 1H, J = 16.2 Hz, Hd), 7.30 (d, 1 H, J = 3.0 Hz, Ha).
13C{1H} NMR: δ 143.9, 140.1, 132.5, 131.7, 127.3, 124.4, 119.3,
118.6, 111.7.
A mixture of (4-cyanobenzyl)triphenylphosphonium bromide
(7.786 g, 17 mmol) and 3-pyridinecarbaldehyde (1.66 g, 15.5
mmol) in 30 mL of methylene chloride was stirred vigorously in
a 250 mL Erlenmeyer flask. To this suspension was added 10
mL of 50% aqueous NaOH dropwise over the course of 1 h.
After stirring vigorously for one additional hour, the mixture
was washed with three portions of deionized water (150 mL).
The organic layer was dried over MgSO4 and evaporated to
dryness. The resulting white solid was then transferred to a
100 mL round bottom flask containing iodine crystals (0.1 g,
0.788 mmol) and 40 mL of nitrobenzene, and the mixture
was refluxed for 3 h. Upon cooling, the reaction mixture was
extracted with dilute hydrochloric acid. The aqueous phase
Synthesis of Cd2L1 (ꢀ-H2O) 1
4
A mixture of Cd(ClO4)2ؒ6H2O (0.110 g, 0.25 mmol) and (E)-4-
(2-thiazolyl)ethenylbenzonitrile (0.104 g, 0.5 mmol) was thor-
oughly mixed with methanol (0.07 mL), water (0.3 mL) and
pyridine (0.1 mL) in a heavy-walled Pyrex tube. The tube was
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Table 1 X-Ray diffraction data for 1 and 2a
Table 2 Selected bond distance (Å) and bond angles (Њ) for 1 and 2
Chemical formula
M
Space group
Z
T/ЊC
λ(Mo-Kα)/Å
a/Å
b/Å
Cd2C48H34N4O9S4, 1
1163.89
P21/c (No. 14)
4
Ϫ75(1)
0.71073
15.099(1)
20.340(1)
15.116(1)
97.982(1)
4597.3(4)
1.682
11.7
8106
4721
612
0.061
0.142
1.02
Ϫ0.63, 2.22
CdC28H21N2O4.5, 2
567.85
C2/c (No. 15)
8
Ϫ75(1)
0.71073
11.458(2)
16.087(2)
25.536(4)
91.230(3)
4705.9(2)
1.609
9.69
3996
2238
325
0.066
0.173
0.98
Ϫ1.33, 1.67
1
Cd1–O1
Cd1–O4
Cd1–O5
Cd1–O9
Cd1–N3c
Cd1–N2d
2.350(6)
2.224(6)
2.280(6)
2.314(5)
2.332(7)
2.442(6)
Cd2–O2
Cd2–O3
Cd2–O7
Cd2–O9
Cd2–N1a
Cd2–N4b
2.226(6)
2.352(6)
2.239(6)
2.336(5)
2.440(6)
2.320(6)
c/Å
O1–Cd1–O4
O1–Cd1–O5
O1–Cd1–O9
O1–Cd1–N3c
O1–Cd1–N2d
O4–Cd1–O5
O4–Cd1–O9
O4–Cd1–N3c
O4–Cd1–N2d
O5–Cd1–O9
O5–Cd1–N3c
O5–Cd1–N2d
O9–Cd1–N3c
O9–Cd1–N2d
N2d–Cd1–N3c
105.2(2)
81.1(2)
90.7(2)
85.0(2)
173.6(2)
172.1(2)
87.5(2)
97.8(2)
81.1(2)
87.8(2)
87.4(2)
92.7(2)
173.9(2)
91.0(2)
92.9(2)
O2–Cd2–O3
O2–Cd2–O7
O2–Cd2–O9
O2–Cd2–N1a
O2–Cd2–N4b
O3–Cd2–O7
O3–Cd2–O9
O3–Cd2–N1a
O3–Cd2–N4b
O7–Cd2–O9
O7–Cd2–N1a
O7–Cd2–N4b
O9–Cd2–N1a
O9–Cd2–N4b
N1a–Cd2–N4b
111.6(2)
165.1(2)
86.0(2)
82.6(2)
99.9(2)
80.3(2)
87.9(2)
165.6(2)
84.1(2)
85.6(2)
86.1(2)
89.9(2)
95.8(2)
171.4(2)
91.2(2)
β/Њ
V/Å3
Dc/g cmϪ3
µ(Mo-Kα)/cmϪ1
Nref (total)
Nref (obs.) [I > 2σ(I)]
Npar
R1a
wR2a
Goodness of fit (GOF)
Min., max. residual
density/e ÅϪ3
a R1 = Σ Fo| Ϫ |Fc /Σ|Fo|; wR2 = [Σ[w(Fo2 Ϫ Fc2)2]/Σ[w(Fo )2]]1/2; GOF =
[Σ[w(Fo2 Ϫ Fc2)2]/(Nref(obs.) Ϫ Npar)]1/2 where Nref(obs.) = number of
observed reflections and Npar = number of parameters.
2
2
Cd1–O1
Cd1–O3
Cd1–O5
2.281(6)
2.324(6)
2.318(6)
Cd1–O2e
Cd1–N1f
Cd1–N2g
2.313(6)
2.332(7)
2.354(8)
frozen under liquid nitrogen, sealed, and heated inside an oven
at 140 ЊC. After 72 h of heating, the tube was opened, and
colorless crystals were obtained. Yield: 80 mg (28%). IR (cmϪ1):
1646s, 1595s, 1572m, 1551s, 1474w, 1431w, 1378s, 1176w,
1127w, 1047w, 1027w, 970m, 955m, 799m, 768m, 699m.
O1–Cd1–O3
O1–Cd1–O5
O1–Cd1–O2e
O1–Cd1–N1f
O1–Cd1–N2g
O3–Cd1–O5
O2e–Cd1–O3
O3–Cd1–N1f
170.7(2)
104.06(19)
96.3(2)
87.5(2)
87.3(2)
85.22(19)
83.5(2)
83.2(2)
O3–Cd1–N2g
O2e–Cd1–O5
O5–Cd1–N1f
O5–Cd1–N2g
O2e–Cd1–N1f
O2e–Cd1–N2g
N1f–Cd1–N2g
94.1(2)
86.0(2)
167.7(2)
86.6(2)
88.5(2)
172.4(2)
98.4(3)
Synthesis of Cd2L24(ꢀ-H2O) 2
A mixture of Cd(ClO4)2ؒ6H2O (0.055 g, 0.125 mmol) and (E)-
4-(3-pyridyl)ethenylbenzonitrile (0.053 g, 0.25 mmol) was
thoroughly mixed with methanol (0.07 mL) and water (0.3 mL)
in a heavy-walled Pyrex tube. The tube was frozen under liquid
nitrogen, sealed, and heated inside an oven at 130 ЊC. After
1 week of heating, the tube was opened, and pale yellow crys-
tals were obtained. Yield: 32 mg (22%). IR (cmϪ1): 1595s,
1551s, 1485m, 1382s, 1231w, 1145m, 948m, 797w, 776m, 717m,
670w, 628w.
Symmetry codes: a = Ϫx, Ϫ1/2 ϩ y, 3/2 Ϫ z; b = Ϫx, 1/2 ϩ y, 3/2 Ϫ z;
c = 1 Ϫ x, Ϫ1/2 ϩ y, 3/2 Ϫ z; d = 1 Ϫ x, 1/2 ϩ y, 3/2 Ϫ z; e = 2 Ϫ x, y,
3/2 Ϫ z; f = x, Ϫy, Ϫ1/2 ϩ z; h = Ϫ1/2 ϩ x, 1/2 Ϫ y, 1/2 ϩ z.
using 50% aqueous sodium hydroxide to generate the ylide, as
illustrated in Scheme 1. The reaction gave a mixture of (E)-
and (Z)-products. Refluxing this mixture in nitrobenzene in the
presence of iodine for 3 h afforded the pure (E)-product in 76%
overall yield.11 Pure (E)-4-(3-pyridyl)ethenylbenzonitrile was
characterized by 1H and 13C{1H} NMR spectroscopy.
X-Ray data collections and structure determinations
Data collection for 1 (2) was carried out with a colorless crystal
of dimensions of 0.05 × 0.16 × 0.22 (0.05 × 0.10 × 0.24 mm)
on a Siemens SMART system equipped with a CCD detector
using Mo-Kα radiation. Of the 8106 (3996) unique reflections
measured, 4721 (2238) reflections with I > 2σ(I) were used in
structure solution and refinement for 1 (2). The structures were
solved by direct methods using SHELX-TL8 and refined on F2
by full-matrix least squares using anisotropic displacement
parameters for all non-hydrogen atoms.8 All the hydrogen
atoms in 1 and 2 were located by geometric placing. Final
refinement gave an R1 = 0.061 (0.066), wR2 = 0.142 (0.173) and
goodness of fit = 1.02 (0.98) for 1 (2). Experimental details
for X-ray data collections of 1 and 2 are tabulated in Table 1,
while selected bond distances and angles for 1 and 2 are listed
in Table 2. All the drawings were made using either XP8 or
CAMERON9 programs.
The ligand precursor (E)-4-(2-thiazolyl)ethenylbenzonitrile
was synthesized by a Heck coupling reaction12 between
4-ethynylbenzonitrile and 2-bromothiazole, followed by hydro-
genation in the presence of Lindlar’s catalyst. The hydrogen-
ation afforded the pure (Z)-isomer, which was converted to
the desired (E)-isomer by refluxing in nitrobenzene in the
presence of iodine. Pure (E)-4-(2-thiazolyl)ethenylbenzonitrile
was characterized by 1H and 13C{1H} NMR spectroscopy.
Cd2L1 (µ-H2O) 1, was obtained as large colorless crystals
4
in 28% yield by a hydro(solvo)thermal reaction between
Cd(ClO4)2ؒ6H2O and (E)-4-(2-thiazolyl)ethenylbenzonitrile in
a mixture of methanol, water and pyridine at 140 ЊC (Scheme
2). The IR spectrum of 1 exhibits peaks at 1572 and 1378 cmϪ1
that can be assigned to the antisymmetric and symmetric C᎐O
᎐
stretches, respectively.13 Carboxylate formation in 1 has evi-
dently resulted from the in situ hydrolysis of ligand during the
hydro(solvo)thermal synthesis.14 The presence of bridging
water molecules in 1 is clearly indicated in the TGA results
which exhibited a weight loss of 1.54% in the temperature range
219–259 ЊC [calc. 1.55% for the loss of one water molecule per
CCDC reference number 186/2070.
See http://www.rsc.org/suppdata/dt/b0/b003441j/ for crystal-
lographic files in .cif format.
Results and discussion
Synthesis
Cd2L1 (µ-H2O) unit].
4
Cd2L2 (µ-H2O) 2 was obtained similarly by hydro(solvo)-
4
The ligand precursor (E)-4-(3-pyridyl)ethenylbenzonitrile was
synthesized via a Wittig reaction10 between (4-cyanobenzyl)-
triphenylphosphonium bromide and 4-pyridinecarbaldehyde,
thermal reaction between Cd(ClO4)2ؒ6H2O and (E)-4-(3-
pyridyl)ethenylbenzonitrile (Scheme 2). The structure of 2 can
be inferred from the IR spectrum showing strong peaks at 1551
J. Chem. Soc., Dalton Trans., 2000, 3949–3954
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Scheme 1
Scheme 2
Fig. 3 2-D sheet of 1 as viewed down the c axis. Open circles represent
Cd atoms.
Fig. 2 Coordination environment of 1. The asymmetric unit is shown
with ellipsoids at 30% probability. Hydrogen atoms have been omitted
for clarity.
geometry. The geometry around both Cd() centers can be
described as a highly distorted octahedron. The bond angles
around the central Cd1 atom and the cis ligands lie in the range
81.1–102.5Њ while the bond angles around the central Cd2 atom
and the cis ligands lie in the range 80.3–111.6Њ. All the Cd–O
and Cd–N bond distances are normal. The Cd1–Cd2 distance
in 1 is 3.74 Å. The dihedral angles between the thiazolyl and
phenyl rings for the four L1 ligands are 19.6, 18.1, 14.2 and 6.6Њ.
and 1382 cmϪ1 which are characteristic of the antisymmetric
and symmetric C᎐O stretches. TGA results indicated that 2
᎐
exhibited a weight loss of 1.57% in the temperature range 229–
302 ЊC [calc. 1.58% for the loss of one water molecule per
Cd2L2 (µ-H2O) unit].
The structure of 1 is based upon the Cd2L1 (µ-H2O) building
4
4
block. The Cd2L1 (µ-H2O) unit uses a pair of the eight different
4
Single crystal X-ray structures
L1 ligands to connect to an adjacent Cd2L1 (µ-H2O) unit. Each
4
Compound 1 crystallizes in the monoclinic space group P21/c.
The asymmetric unit of 1 consists of two cadmium atoms, four
4-(2-thiazolyl)ethenylbenzoate (L1) ligands, and one bridging
water molecule (Fig. 2). The thiazolyl nitrogen atoms of all the
four L1 ligands coordinate to Cd centers. Two of these L1
ligands adopt exotridentate bridging mode (with µ,η2-carboxyl-
ato groups), while the other two L1 ligands are exobidentate
(with monodentate carboxylato groups). With a bridging water
molecule (O9), each Cd center in 1 is coordinated to three carb-
oxylate oxygen atoms, to one oxygen atom of the bridging
water molecule, and to two pyridyl nitrogen atoms in a cis
Cd2L1 (µ-H2O) unit is thus connected to four neighboring
4
Cd2L1 (µ-H2O) units in the ab plane to form a 2-D sheet struc-
4
ture (Fig. 3). Adjacent Cd2L1 (µ-H2O) units along the b axis
4
adopt the same orientation and form an infinite chain structure.
These infinite chains are related by a 21 operation along the
b axis. Overall, each 2-D sheet lacks a center of symmetry.
However, the adjacent 2-D sheets are related by an inversion
center and 1 crystallizes in the centrosymmetric space group
P21/c. It is interesting that within each 2-D sheet, Cd centers
are encased between L1 ligands to form a sandwich structure
(Fig. 4); such sandwich structures are presumably favorable for
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The Cd–Cd distance in the Cd2L2 (µ-H2O) unit is 3.72 Å. In
4
contrast to 1, the phenyl and pyridyl rings for the two L2
ligands in 2 are very non-coplanar with dihedral angles of 62.5
and 46.5Њ.
Although compounds 1 and 2 contain similar building
blocks, they have drastically different network topologies. Each
Cd2L2 (µ-H2O) unit in 2 is also connected to four neighboring
4
Cd2L2 (µ-H2O) units via four different pairs of L2 bridging
4
ligands. Unlike 1, these Cd2L2 (µ-H2O) units in 2 are not in
4
the same plane. Three of these Cd2L2 (µ-H2O) units are in the
4
same zigzag chains along the c axis which are linked to the two
Fig. 4 Stacking of 2-D sheets of 1 viewed down the a axis.
Cd2L2 (µ-H2O) units from two other zigzag chains to result in a
4
highly regular 3-D network (Fig. 6). There are inversion centers
relating these zigzag chains. We believe this drastic structure
difference between 1 and 2 is a result of the isomerism of the
bridging ligands. While the coordinated oxygen (O5 and O7)
and nitrogen (N3 and N4) atoms in exobidentate L1 ligands in 1
are cis to each other (Fig. 1), the coordinated oxygen (O3) and
nitrogen (N2) atoms in exobidentate L2 ligands in 2 are trans to
each other (Fig. 2). Such a trans configuration of the L2 ligands
is responsible for the linking of one zigzag Cd2L2 (µ-H2O) chain
4
to two different zigzag Cd2L2 (µ-H2O) chains to result in a 3-D
4
network structure of 2.
Fig. 5 Coordination environment of 2. The asymmetric unit is shown
with ellipsoids at 30% probability. Hydrogen atoms have been omitted
for clarity.
In summary, we have synthesized two Cd() coordination
polymers based on meta-pyridinecarboxylate bridging ligands.
Subtle configurational isomerism between the two meta-
pyridinecarboxylate bridging ligands in 1 and 2 is responsible
for the drastic difference of the resulting coordination poly-
mers. Both 1 and 2 are centrosymmetric presumably as a result
of the presence of bridging water molecules.
Acknowledgements
We acknowledge financial support from the National Science
Foundation (DMR-9875544). We thank Dr Scott R. Wilson
and the Materials Chemistry Laboratory of University of
Illinois for the collection of X-ray diffraction data. W. L. is an
Alfred P. Sloan Fellow, an Arnold and Mabel Beckman Young
Investigator, and a Cottrell Scholar of Research Corporation.
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Compound 2 crystallizes in the monoclinic space group C2/c.
The asymmetric unit of 2 contains one Cd atom, two 4-(3-
pyridyl)ethenylbenzoate (L2) ligands, and one water molecule
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4
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J. Chem. Soc., Dalton Trans., 2000, 3949–3954
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