Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II) generates a discrete complex not a coordination polymer: Crystal structure of [μ-4,4′-(1,2-ethynediyl)-bis(pyridine-2,6-dicarboxylato)- N, O, O′-μ-N′, O″, O″]-diaqua-bis(imidazole)- dicopper(II)

Article abstract of DOI:10.1007/s10870-007-9179-8

Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II) produces crystals containing a discrete complex of [μ-4,4′-(1,2- ethynediyl)-bis(pyridine-2,6-dicarboxylato)-N, O, O′-μ-N′, O″, O?]-diaqua-bis(imidazole)-dicopper(II) complex (1), [(C ₁₆H₄N₂O₈)(C₃H ₄N₂)₂(H₂O)₂Cu ₂] instead of a linear coordination polymer. The complex crystallizes in the monoclinic space group P2₁/n with a = 5.3763(8) A, b = 18.522(3) A, c = 11.4827(7) A, β = 93.705(3)° and Z = 2. The complex sits on a center of symmetry such that the two halves of the molecule are related by inversion symmetry. The molecules pack in layers in which the molecules are joined by intermolecular N-H...O hydrogen bonds between imidazole and carboxylate groups and O-H...O hydrogen bonds between carboxylate groups and water.

Full text of DOI:10.1007/s10870-007-9179-8

ꢀ
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Journal of Chemical Crystallography, Vol. 37, No. 4, April 2007 ( 2007)
DOI: 10.1007/s10870-007-9179-8
Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne
and imidazole with Cu(II) generates a discrete complex
not a coordination polymer: crystal structure of
[µ-4,4ꢀ-(1,2-ethynediyl)-bis(pyridine-2,6-dicarboxylato)-N,
O, Oꢀ-µ-Nꢀ, Oꢀꢀ, Oꢀꢀꢀ]-diaqua-bis(imidazole)-dicopper(II)
Yu Wang,(1) Christopher G. F. Cooper,(1) Brian S. Luisi,(2) Brian Moulton,(2)
and John C. MacDonald(1)∗
Received August 1, 2006; accepted January 8, 2007
Published Online February 23, 2007
Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II) pro-
duces crystals containing a discrete complex of [µ-4,4ꢁ-(1,2-ethynediyl)-bis(pyridine-2,6-
dicarboxylato)-N, O, Oꢁ-µ-Nꢁ, Oꢁꢁ, Oꢁꢁꢁ]-diaqua-bis(imidazole)-dicopper(II) complex (1),
[(C16H4N2O8)(C3H4N2)2(H2O)2Cu2] instead of a linear coordination polymer. The com-
plex crystallizes in the monoclinic space group P21/n with a = 5.3763(8) Å, b = 18.522(3)
Å, c = 11.4827(7) Å, β = 93.705(3) ◦ and Z = 2. The complex sits on a center of symmetry
such that the two halves of the molecule are related by inversion symmetry. The molecules
–
pack in layers in which the molecules are joined by intermolecular N H· · ·O hydrogen
–
bonds between imidazole and carboxylate groups and O H· · ·O hydrogen bonds between
carboxylate groups and water.
KEY WORDS: 2,6-dicarboxypyridine; 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne; imidazole.
Introduction
We are investigating the design and synthesis
of metal-organic frameworks using ligands that
promote divergent assembly as a means to prepare
porous crystalline solids. Solids that obtain their
porosity from rigid frameworks of molecules
joined by metal ions are receiving considerable
attention because these materials exhibit useful
properties such as permanent porosity, high ther-
mal stability, large void volumes and pores with
adjustable structures.1–26 Accordingly, porous
solids are promising materials for molecular
storage, separation and sensing, production
of nanoparticles, catalysis, and exchange of
molecules and ions.27–47 We have demonstrated
previously that 2,6-dicarboxypyridine is a versa-
tile ligand that coordinates a range of transition
metals and that metal-ligand coordination can
be used to promote assembly of molecules in
solids48–53 and on surfaces.54,55 Based on this
work, we are now investigating the utility of lig-
ands containing multiple 2,6-dicarboxypyridine
groups as building blocks for preparing 1-D, 2-D
(1) Department of Chemistry & Biochemistry, Worcester Polytechnic
Institute, Worcester, MA 01609, USA.
(2) Department of Chemistry, Brown University, Providence, RI
02912, USA.
∗
To whom correspondence should be addressed; e-mail:
jcm@wpi.edu
299
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1074-1542/07/0400-0299/0 2007 Springer Science+Business Media, LLC

300
Wang, Cooper, Luisi, Moulton, and MacDonald
O
O
O
O OH2
H
N
+2
+2
N Cu
N
N
Cu N
N
OH
OH
H
1
H2O O
O
N
N
O
O
O
O
H
H
CuCl2
N
N
H
N
N
+
DMSO
H2O
RT
N
H
N
H
O
N
O
O
O
O
O
O
O
O O
O O
OH
OH
+2
+2
Cu
N
N
Cu N
O O
O O
O
O
n
H
N
H
N
N
N
H
H
Fig. 1. Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with CuCl2 followed by crystallization gave
crystals containing complex 1 (top) rather than a rigid 1-D metal-organic framework consisting of a linear coordination
polymer (bottom).
and 3-D frameworks. Our objective is to develop
a library of poly(2,6-dicarboxypyridin-4-yl)
ligands that form frameworks with a range of
architectures and porosity that can be selected
based on the geometry of the ligand and the
choice of transition or lanthanide metals. Toward
this goal, we are investigating ligands substituted
with two, three or four 2,6-dicarboxypyridine
units attached covalently to a central core by
rigid linkers that consist of alkynyl groups and
benzene rings. Here we report results from
initial efforts to prepare a linear framework
by reacting 1,2-bis(2,6-dicarboxypyridin-4-
yl)ethyne with Cu(II) metals ions in the presence
of imidazole. We chose to study 1,2-bis(2,6-
dicarboxypyridin-4-yl)ethyne as a candidate for
preparing linear frameworks for the following
reasons: (1) 1,2-bis(2,6-dicarboxypyridin-4-
yl)ethyne contains two 2,6-dicarboxypyridine
groups bonded at either end of ethyne, as shown
in Fig. 1; (2) the rigid ethyne group holds the 2,6-
dicarboxypyridine groups in a linear arrangement
with metal-binding sites oriented facing in
opposite directions; (3) this ligand is known to
bind lanthanide metals and form coordination
networks in solution.56 Our previous studies of
bis(imidazolium 2,6-dicarboxypyridine) M(II)
complexes (M = Mn, Co, Ni, Cu, and Zn),50–53
bis(2-methylimidazolium 2,6-dicarboxypyridine)
M(II) complexes (M = Mn, Co, Ni, Cu, and Zn)49
and bis(imidazolium 2,4,6-tricarboxypyridine)
M(II) complexes (M = Co, Ni, Cu, and Zn)48
established that imidazole functioned not as a
ligand, but as a base. In each family of com-
plexes, we found that imidazole deprotonated one
carboxyl group on each 2,6-dicarboxypyridine
ligand when an equimolar amount of imidazole
was present during formation of the metal
complex. The resulting imidazolium cations
played an important structural role by linking
adjacent metal complexes into hydrogen-bonded
–
layers via N H· · ·O interactions with carboxylate

Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II)
301
groups. A consistent feature of the layers was
the presence of large (e.g., 17Å × 21Å) holes.
We reasoned it should be possible to form
solids with open channels by tethering two
2,6-dicarboxypyridine units covalently in order
to align the holes in adjacent layers.
measured by SynPep Corporation. Ionization was
performed using electrospray, with acetonitrile as
the carrier solvent, and nitrogen as a curtain gas.
Preparation of 4-bromo-2,6-bis
(ethoxycarbonyl)pyridine (i)
Shown at the bottom in Fig. 1 is the structure
of a coordination polymer with pendant imida-
zolium cations that we targeted by reacting react-
ing 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and
imidazole with Cu(II) ions. Our attempts to pre-
pare porous solids from these components unex-
pectedly yielded a discrete coordination complex.
In this report, we present the crystal structure of
[µ-4,4ꢁ-(1,2-ethynediyl)-bis(pyridine-2,6-dicarb-
oxylato)-N, O, Oꢁ-µ-Nꢁ, Oꢁꢁ, Oꢁꢁꢁ]-diaqua-bis
(imidazole)-dicopper(II) complex (1).
Chelidamic acid (23.9 g, 100 mmol)
was combined with phosphorous pentabromide
(43.1 g, 100 mmol) under an N2 atmosphere. The
mixture was heated gently at 100◦C for 0.5 h.
The reaction was cooled to RT, and the solid was
dissolved in ether (150 mL). The ethereal solution
was then added dropwise to a stirred solution of
absolute ethanol (100 mL) at 0◦C. Recrystalliza-
tion of the product from 30 % nitric acid yielded
22.0 g (66.1 % yield) of i as white needles.1H
NMR (d6-DMSO) δ (ppm) 1.30 (t, 6 H), 4.29 (q,
4 H), 9.0 (s, 2 H).
Experimental
All reagents and solvents were purchased
from Aldrich or Acros and were used without
further purification. Bis(2,6-dicarboxypyridin-
4-yl)ethyne was synthesized in three steps
(Scheme 1) by adapting procedures reported pre-
viously by Latva et al. as described below.56
Acetylene (30% in acetone) was purchased from
Mattheson Gas, Inc. Phosphorous pentabromide
was prepared freshly via addition of bromine
and phosphorous tribromide. The resulting yel-
low solid was reslurried in tert-butyl methyl ether
several times until the filtrate was no longer yel-
low in color, and was used immediately after.
NMR spectra were obtained with a 400 MHz
Avance Bruker spectrometer. Mass spectra were
Preparation of bis[2,6-bis(ethoxycarbonyl)
pyridin-4-yl]ethyne (ii)
Compound i (22.0 g, 66.1 mmol) was dis-
solved in triethyl amine (200 mL) under an N2
atmosphere, and PdCl2 (1.1 g, 6.6 mmol), CuI
(2.5 g, 13.2 mmol), and triphenyl phosphine
(1.7 g, 6.6 mmol) were added. Acetylene was bub-
bled through the reaction mixture, and the mix-
ture stirred for 18 h at RT. The resultant mixture
was poured into water (400 mL), and extracted
three times with ethyl acetate (900 mL total). The
organic fractions were combined, washed twice
with citric acid, dried over anhydrous sodium sul-
fate, and evaporated under reduced pressure. The
EtO2C
N
CO2Et
HO2C
N
CO2H
acetylene (g)
PdCl2, CuI, PPh3
Et3N
OH
N
Br
1. PBr5, 100 °C
2. EtOH, 0 °C
NaOH
H2O, EtOH
HO2C
CO2H
EtO2C
N
i
CO2Et
EtO2C
N
ii
CO2Et
HO2C
N
iii
CO2H
Scheme 1.

302
Wang, Cooper, Luisi, Moulton, and MacDonald
resulting crude product was purified over silica
gel (40% ethyl acetate:hexane) to give 10.2 g of
ii (33.0% yield) as a white powder. 1H NMR (d6-
DMSO) δ (ppm) 1.25 (t, 12 H), 4.38 (q, 8 H), 8.6
(s, 4 H).
were removed from solution and dried on filter
paper.
Crystal structure determination
A crystal of 1 having approximate dimen-
sions 0.11 mm × 0.10 mm × 0.03 mm was
placed on a glass fiber that was mounted on a
Siemens SMART/CCD diffractometer equipped
with an LT-II low temperature device. Diffrac-
tion data were collected at 90(2) K using
graphite monochromated Mo-Kα radiation with
the omega-scan technique. Diffraction data were
corrected for absorption using the SADABS
program.57 The unit cell and space group were
determined using the SAINT + program.57 The
structure was solved by direct methods and refined
by full matrix least-squares using the SHELXTL
program on a PC platform.57 Refinement was
based on F2 using all reflections. All non-
hydrogen atoms were refined anisotropically. Hy-
drogen atoms on oxygen atoms were located and
refined with isotropic thermal parameters. Hydro-
gen atoms on oxygen atoms on the molecules
of water bonded to Cu were located in the dif-
ference Fourier map, the coordinates constrained
to ride on the coordinates of the bonded oxygen
atom, and refined isotropically with thermal pa-
rameters based upon the corresponding attached
oxygen atoms [U(H) = 1.2 Ueq (O)]. The re-
maining hydrogen atoms on carbon were fixed in
calculated positions and refined isotropically with
thermal parameters based upon the corresponding
attached carbon atoms [U(H) = 1.2 Ueq (C)].
Preparation of bis(2,6-dicarboxypyridin-4-yl)
ethyne (iii)
Compound ii (10.2 g, 21.7 mmol) was dis-
solved in 10% aqueous ethanol (300 mL), and
sodium hydroxide (1.6 g, 65.1 mmol) was added.
The reaction mixture was heated to reflux for 3 h,
with progress being monitored via TLC. The re-
action mixture was cooled to RT, and solvent re-
moved under reduced pressure. The crude product
was dissolved in water (500 mL), and extracted
twice with TBME (500 mL total). The pH of
the aqueous layer was adjusted to 6 with ammo-
nium chloride, and the product was extracted three
times with methylene chloride (250 mL per wash).
The organic fractions were combined, dried over
anhydrous sodium sulfate, and concentrated un-
der reduced pressure to give 6.2 g of iii (80%
1
yield) as a white solid. H NMR (d6-DMSO) δ
(ppm) 9.3 (s, 4 H), 12.6 (bs, ∼ 4 H). ESI-MS,
m/z [M + H]∗ = 357.1, [M + Na]∗ = 379.8.
Preparation of [µ-4,4ꢁ-(1,2-ethynediyl)-
bis(pyridine-2,6-dicarboxylato)-N, O, Oꢁ-µ-Nꢁ,
Oꢁꢁ, Oꢁꢁꢁ]-diaqua-bis(imidazole)-dicopper(II)
complex (1)
A solution containing 10 mg (0.028 mmol) of
compound iii and 3.7 mg (0.054 mmol) imidazole
in 10 mL of 1:1 DMSO/H2O (v/v) was placed in a
glass vial. Ten mL of 1:1 DMSO/H2O (v/v) were
carefully layered on top followed by 20 mL of
H2O. A solution containing 4.6 mg (0.0027 mmol)
of CuCl2·2H2O in 5 mL of H2O was added as a
fourth layer. The vial was covered and allowed to
sit undisturbed until crystals of 1 formed as green
plates in solution after one week. Single crystals
Results and discussion
The molecular structure of 1 with thermal el-
lipsoids and atomic numbering is shown in Fig. 2.
A view of crystal packing perpendicular to the
103 plane is shown in Fig. 3. A summary of the
crystallographic information is given in Table 1.
Selected bond lengths and bond angles are given
in Table 2.

Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II)
303
Fig. 2. Molecular structure of 1, ellipsoids at 50% probability.
2,6-dicarboxylato)-N, O, Oꢁ-µ-Nꢁ, Oꢁꢁ, Oꢁꢁꢁ]-
diaqua-bis(imidazole)-dicopper(II) complex, 1.
As shown in Fig. 2, this complex has a struc-
ture in which both Cu ions coordinate to ligands
in a distorted square pyramidal geometry. The
Cu ions bond to 1,2-bis(2,6-dicarboxypyridin-
Molecular structure of 1
Reaction of 1,2-bis(2,6-dicarboxypyridin-
4-yl)ethyne and imidazole with copper(II)
chloride in DMSO/H2O gave the corre-
sponding [µ-4,4ꢁ-(1,2-ethynediyl)-bis(pyridine-
Fig. 3. A hydrogen-bonded layer in the crystal structure of 1 viewed perpendicular to the 103 plane. Isolated molecules of
water shown within the layer are bonded to metal complexes that have been omitted for clarity. Thin blue lines represent
hydrogen bonds.

304
Wang, Cooper, Luisi, Moulton, and MacDonald
Table 1. Crystallographic Data and Refinement Information for 1
–
of a molecule of water via one Cu O bond
–
[Cu(1) O(5W) = 2.342(3)Å].
Empirical formula
CCDC deposit no.
Formula weight
Crystal system
C22H16N6O10Cu2
CCDC 615638
651.48
Monoclinic
P21/n
To compare the Cu-N and Cu-O bond
distances and coordination in the 2,6-
dicarboxypyridine moiety of 1 to those reported
previously in the crystallographic literature, we
searched the Cambridge Structural Database
(CSD) for complexes that contained Cu bonded to
at least one 2,6-dicarboxypyridine moiety.58 The
search, which allowed for substituents other than
hydrogen on carbons 3, 4 and 5 on the pyridine
ring, yielded 36 crystal structures of complexes
Space group
Crystal description
Unit cell dimensions
a(Å)
b(Å)
c(Å)
green plate
5.3763(8)
18.522(3)
11.4827(17)
93.705(3)
1141.1(3)
2
1.896
90(2)
656
2.09–28.28
1.940
9892
β(deg)
Volume (Å3)
Z
Densitycalc(g/cm3)
Temperature (K)
F(000)
–
–
that featured 52 Cu N bonds and 101 Cu O
bonds that were unique crystallographically.
θ range for data collection (◦)
Absorption coefficient (mm−1
Reflections collected
Independent reflections
Reflections [I > 2σ (I)]
Data/restraints/parameters
Goodness of fit on F2
R [I > 2σ (I)]
–
The Cu N bond distances ranged from 1.844–
)
2.034Å with an average distance of 1.931(48)Å.
–
The Cu O bond distances ranged from 1.987–
2707 [Rint = 0.1240]
2054
2.536Å with an average distance of 2.131(144)Å.
By comparison, the Cu N and Cu O bond lengths
in the structure of 1 fall within one e.s.d. of
2707/0/183
1.001
R1 = 0.0560, wR2 = 0.0945
R1 = 0.0881, wR2 = 0.1042
0.872, – 0.646
–
–
R (all data)
Max. and min. peaks in final
diff. map (eÅ−3
–
–
the average Cu N and Cu O distances, and
thus are average within experimental error.
Twelve of the structures from the CSD had Cu
ions coordinated to two 2,6-dicarboxypyridine
ligands; the other 24 had Cu ions coordinated
to one 2,6-dicarboxypyridine ligand and to
nitrogen and oxygen atoms on other ligands
that included various derivatives of pyridine,
pyrazine, pyrimidine, bipyridine, terpyridine,
phenanthroline, propane-1,3-diamine, imidazole,
imidazole-N-oxide, methanol and water.
)
4-yl)ethyne in a meridonial arrangement and to
one molecule of imidazole and one molecule of
water. The carboxylic acid groups at both ends
of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne lose
protons to form negatively charged carboxylate
groups that balance the 2 + charge of the Cu
metal ions. The least squares planes of the pyri-
dine rings are parallel but separated by 0.3Å
because the central C≡C bond bends out of
the planes of the rings by 2.7◦. The carboxy-
late groups are nearly copla◦nar with the pyri-
It was surprising to find Cu ions coordinated
to molecules of imidazole in addition to
the 2,6-dicarboxypyridine groups. We reported
previously that transition metals coordinate to two
2,6-dicarboxypyridine ligands in bis(imidazolium
2,6-dicarboxypyridine) M(II) dihydrate com-
–
dine ring and rotate by just 1.2 [O(1)-C(1) O(2)]
and 2.1◦ [O(3) C(7) O(4)] out of the plane
plexes,50–53
bis(2-methylimidazolium
2,6-
–
–
of the ring. The Cu ion bonds to 1,2-bis(2,6-
dicarboxypyridine) M(II) dihydrate complexes49
and bis(imidazolium 2,4,6-tricarboxypyridine)
M(II) trihydrate complexes (where M = Mn,
Co, Ni, Cu or Zn).48 The chemical structures
of those complexes are shown in Fig. 4. Those
studies established that 2,6-dicarboxypyridine
is a versatile ligand that reliably coordinates to
a variety of first-row transition metal ions in an
–
dicarboxypyridin-4-yl)ethyne via one Cu N bond
–
[Cu(1) N(1) = 1.910(3)Å] to the pyridine ring
–
and two Cu-O bonds [Cu(1) O(1) = 2.022(3)Å,
–
Cu(1) O(3) = 2.033(3)Å] to the carboxy-
late groups. In addition, the metal ion
–
bonds to imidazole via one Cu N bond
–
[Cu(1) N(2) = 1.926(3)Å] and to the oxygen

Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II)
305
Table 2. Bond Lengths (Å) and Angles (◦) for 1
–
–
–
Cu(1) N(1)
1.909(3)
1.926(3)
2.022(3)
2.034(3)
2.343(3)
1.329(5)
1.343(5)
1.328(5)
1.383(5)
1.332(5)
1.369(5)
1.288(5)
1.223(5)
1.271(5)
1.249(5)
1.513(6)
1.385(6)
1.400(6)
1.403(6)
1.436(6)
1.383(5)
1.527(6)
1.189(8)
1.341(6)
169.56(15)
80.96(13)
97.94(13)
80.31(13)
99.58(13)
160.64(11)
89.77(12)
100.60(13)
O(1) Cu(1) O(5W)
88.84(11)
95.96(11)
123.6(3)
118.9(3)
117.3(3)
105.5(4)
127.7(3)
126.8(3)
107.8(4)
114.7(2)
114.4(2)
126.0(4)
119.9(4)
114.1(3)
119.5(4)
112.4(3)
128.1(4)
118.5(4)
120.2(4)
119.5(4)
120.3(4)
118.2(4)
120.1(4)
110.9(3)
128.8(4)
125.6(4)
119.2(3)
115.2(3)
177.0(7)
110.8(4)
106.5(4)
109.4(4)
–
– –
O(3) Cu(1) O(5W)
– –
C(6) N(1) C(2)
– –
C(6) N(1) Cu(1)
– –
C(2) N(1) Cu(1)
– –
C(9) N(2) C(11)
– –
C(9) N(2) Cu(1)
– –
C(11) N(2) Cu(1)
– –
C(9) N(3) C(10)
– –
C(1) O(1) Cu(1)
– –
C(7) O(3) Cu(1)
– –
O(2) C(1) O(1)
– –
O(2) C(1) C(2)
– –
O(1) C(1) C(2)
– –
N(1) C(2) C(3)
– –
N(1) C(2) C(1)
– –
C(3) C(2) C(1)
– –
C(2) C(3) C(4)
– –
C(3) C(4) C(5)
– –
C(3) C(4) C(8)
– –
C(5) C(4) C(8)
– –
C(6) C(5) C(4)
– –
N(1) C(6) C(5)
– –
N(1) C(6) C(7)
– –
C(5) C(6) C(7)
– –
O(4) C(7) O(3)
– –
O(4) C(7) C(6)
– –
O(3) C(7) C(6)
– –
C(8)#1 C(8) C(4)
– –
N(2) C(9) N(3)
– –
C(11) C(10) N(3)
– –
C(10) C(11) N(2)
Cu(1) N(2)
–
Cu(1) O(1)
–
Cu(1) O(3)
–
Cu(1) O(5W)
–
N(1) C(6)
–
N(1) C(2)
–
N(2) C(9)
–
N(2) C(11)
–
N(3) C(9)
–
N(3) C(10)
–
O(1) C(1)
–
O(2) C(1)
–
O(3) C(7)
–
O(4) C(7)
–
–
–
–
–
–
–
–
C(1) C(2)
C(2) C(3)
C(3) C(4)
C(4) C(5)
C(4) C(8)
C(5) C(6)
C(6) C(7)
C(8) C(8)#1
–
C(10) C(11)
–
–
–
–
–
–
–
–
–
N(1) Cu(1) N(2)
–
N(1) Cu(1) O(1)
–
N(2) Cu(1) O(1)
–
N(1) Cu(1) O(3)
–
N(2) Cu(1) O(3)
–
O(1) Cu(1) O(3)
–
N(1) Cu(1) O(5W)
–
N(2) Cu(1) O(5W)
Note. Symmetry transformations used to generate equivalent atoms: #1 − x + 1, − y, − z.
octahedral arrangement in which the metal ion is
encapsulated by two ligands. Moreover, this work
showed that imidazole and 2-methylimidazole
did not compete with 2,6-dicarboxypyridine or
2,4,6-tricarboxypyridine in coordinating to metal
ions when both ligands were present in solution
CO2H
• 2 H2O
• 3 H2O
O
O
O
O
O
O
O
O
O
O
O
O
H
N
H
N
M
N
H
N
M
N
N
H
N
H
N
H
CH3
N
2+
2+
O-
O-
O-
O-
2+
O-
O-
O-
O-
O-
O-
O-
O-
N
N
N
N
M
N
N
CH3
H
N
H
H
N
H
H
H
N
CO2H
M = Zn, Cu, Ni, Co
M = Zn, Cu, Ni, Co, Mn:Cu (7:3)
M = Zn, Cu, Ni, Co, Mn
Fig. 4. Chemical structures of bis(imidazolium 2,6-dicarboxypyridine) M(II) dihydrate complexes,50–53 bis(2-
methylimidazolium 2,6-dicarboxypyridine) M(II) complexes49 and bis(imidazolium 2,4,6-tricarboxypyridine) M(II) tri-
hydrate complexes.48

306
Wang, Cooper, Luisi, Moulton, and MacDonald
prior to addition of the metal ion. Instead,
imidazole acted as a base by deprotonating
carboxylic acid groups on the pyridine ligands.
The resulting imidazolium cations served
as hydrogen-bonding donors that promoted
formation of 2-D networks of hydrogen bonds
involving carboxylate acceptors. Accordingly,
we expected 1,2-bis(2,6-dicarboxypyridin-4-
yl)ethyne ligands to coordinate Cu preferentially
in a 1:1 ratio, thereby forming linear coordination
polymers with pendant imidazolium cations
hydrogen bonded along the backbone (Fig. 1).
The structure of complex 1, while unexpected,
is not unprecedented. For example, several
structures featuring Cu(II) ions coordinated by
2,6-dicarboxypyridine and substituted imidazole
ligands, [Cu(2,6-dicarboxypyridine)(L)], where
L = 1-methylimidazole, 2-methylimidazole and
4-methylimidazole, have been reported with
coordination similar to that in complex 1.59–61
In one study, Su investigated the structure and
electronic properties of the complex with 1-
methyl imidazole and proposed that monodentate
bonding of imidazoles with the ring coplanar to
the pyridine ligand is necessary to maximize π-
donation of electron density from the imidazole to
the pyridine ring.61 The structure of complex 1 is
consistent with this finding in that the imidazole
coordinates to Cu so that the two rings are nearly
coplanar (the imidazole ring twists 11.4◦ out of
the plane of the pyridine ring). In a recent study,
we found that the monodentate ring nitrogen
of 2,6-dicarboxypyrazine rings in coordination
polymers of catena-[diaqua-(µ-pyrazine-2,6-
mation of discrete complexes with mixed
ligands over coordination polymers. In related
work, we recently reported that reaction and
crystallization of 1,2-bis(2,6-dicarboxypyridin-
4-yl)ethyne with Co(II) ions in the absence of
imidazole produced a discrete complex of 4-
(2,6-dicarboxypyridin-4-yl)ethynylpyridine-2,6-
dicarboxylatotriaqua cobalt(II) monohydrate,
[Co(C16H6N2O8)(H2O)3] · H2O.63 That complex
featured a Co ion coordinated octahedrally to a
single 2,6-dicarboxypyridine group at one end of
the ligand and three molecules of water, leaving
the second 2,6-dicarboxypyridine group uncom-
plexed. We hypothesize that coordination of the
positive Co ion by one 2,6-dicarboxypyridine
group withdraws enough electron density from
the second 2,6-dicarboxypyridine group through
the conjugated acetylenic backbone to inhibit
coordination to a second Co ion. Differences in
the relative ratio of ligand to metal and modes
of coordination between the Co complex and
complex 1 likely are determined by the presence
or absence of imidazole in solution during
reaction of the components. The fact that Cu
ions coordinate to both ends of the 1,2-bis(2,6-
dicarboxypyridin-4-yl)ethyne ligand in complex
1 can be explained by considering that imidazole
behaves as a π-donor that stabilizes mixed ligand
Cu complexes with 2,6-dicarboxypyridine (a
π-acceptor) by donating electron density to the
complex.61 In the absence of imidazole, we expect
1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and Cu
ions to form a discrete complex similar to that of
the Co complex. Accordingly, we predict that for-
mation of linear coordination polymers is unlikely
to occur with 1,2-bis(2,6-dicarboxypyridin-4-
yl)ethyne or related ligands in which two electron-
deficient 2,6-dicarboxypyridine groups are linked
electronically via a conjugated backbone.
dicarboxylato-N,O,Oꢁ-µ-Nꢁ)-copper(II)
also
bond to Cu(II) ions in a coplanar arrangement
with adjacent 2,6-dicarboxypyrazine rings.62 It
is notable that 1,2-bis(2,6-dicarboxypyridin-4-
yl)ethyne, Cu and imidazole were incorporated
into complex 1 in a 1:2:2 molar ratio, re-
spectively, from solutions containing the three
components in a 1:1:2 molar ratio. Considering
that half of the 1,2-bis(2,6-dicarboxypyridin-
4-yl)ethyne molecules remained uncomplexed
in solution, these results indicate there is
a significant driving force for selective for-
The molecule of water coordinated to Cu
sits in an axial position with respect to the 2,6-
dicarboxypyridine and imidazole moieties. Water
also was present in a similar arrangement in four
of the five structures of mixed ligand Cu com-
plexes with 2,6-dicarboxypyridine and substituted
imidazoles present in the CSD.59–61 We did not

Reaction of 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne and imidazole with Cu(II)
307
attempt to prepare crystals of 1 from solvents
gen atoms distal [O(5W) · · · O(4) = 2.881(4)Å,
H(2W) · · · O(1) = 1.95Å, O(5W)-H(2W) · · ·
O(1) = 170.7◦] and proximal [O(5W) · · · O(1)
= 2.880(4)Å, H(2W) · · · O(1) = 2.00Å, O(5W)-
H(2W) · · · O(1) = 164.0◦] to Cu on opposite
sides of the 2,6-dicarboxypyridine groups at both
ends of the molecule that generate 3-D networks
of hydrogen bonds.
other than DMSO/H2O mixtures; therefore it is
not known what effect solvents other than water
might have on the composition of the final prod-
uct. Our earlier investigation of bis(imidazolium
2,6-dicarboxypyridine) M(II) dihydrate comple-
xes50–53 and bis(imidazolium 2,4,6-tricarboxy-
pyridine) M(II) trihydrate complexes48 revealed
that water was included in all structures as
hydrates (i.e., not coordinated to metal ions)
regardless of whether DMSO, water or a mixture
of DMSO and water was used as the solvent
for crystallization. In contrast, we found water
coordinated to the metal ions in the structures of
catena-[diaqua-(µ-pyrazine-2,6-dicarboxylato-
N,O,Oꢁ-µ-Nꢁ)-copper(II) and 4-(2,6-dicarboxy-
pyridin-4-yl)ethynylpyridine-2,6-dicarboxylatot-
riaqua cobalt(II) monohydrate.62,63 It is likely
that coordination of water will occur in other
solvents to form complexes of 1 unless water is
rigorously excluded.
Conclusions
Our attempts to prepare a crystalline solid
containing a 1-D metal-organic framework by re-
acting 1,2-bis(2,6-dicarboxypyridin-4-yl)ethyne
and imidazole with Cu metal ions resulted in
an unexpected metal complex. Instead of form-
ing a linear coordination polymer composed
of alternating Cu metal ions and 1,2-bis(2,6-
dicarboxypyridin-4-yl)ethyne ligands, Cu metal
ions coordinated to molecules of 1,2-bis(2,6-
dicarboxypyridin-4-yl)ethyne, imidazole and wa-
ter, forming discrete complex 1.
Crystal packing and hydrogen bonding in 1
Supplementary Materials CCDC 615638 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via by e-mailing data request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK, Fax: + 44(0)1223−336033.
Complex 1 crystallizes in the monoclinic
space group P21/n with the asymmetric unit con-
sisting of half the complex. The complex sits on
a center of symmetry such that the two halves of
the compound are related by inversion symmetry.
Packing in the solid is dominated by hydrogen
bonding involving imidazole, water and car-
boxylate groups. Each molecule is joined to two
adjacent molecules by four symmetry equivalent
Acknowledgements
Acknowledgement is made to the U. S. Army
Research Office (grant # W911NF-05-0293) and
to the Donors of The Petroleum Research Fund
(grant # 34604-B3), administered by the Ameri-
can Chemical Society, for support of this research
at Worcester Polytechnic Institute.
–
N H · · · O interactions between imidazole and
carboxylate groups [N(3) · · · O(4) = 2.843(4)Å,
–
H(3N3) · · · O(4) = 1.98Å,
N(3) H(3N3) · · ·
O(4) = 177.8◦] that form hydrogen-bonded rings
with graph set R22(16).64,65 These interactions
form on both sides of each molecule, thereby
generating chains of molecules with graph set
C22(16)[R22(16)]. Adjacent chains are joined into
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