Pyridine- and Imidazoledicarboxylates of Zinc: Hydrothermal Synthesis,
Structure, and Properties
Partha Mahata
[a]
and Srinivasan Natarajan*
[a]
Keywords: Luminescence / N ligands / Pi interactions / Zinc
The reaction of hetrocyclic dicarboxylic acids, such as pyri-
dine-2,5-dicarboxylic acid and imidazole-4,5-dicarboxylic
acid, under hydrothermal conditions in the presence of an
appropriate zinc salt yields three new zinc coordination poly-
mers
0
⬁
[{Zn
2
(H
2
O)
4
}{C
5
H
3
N(COO)
2
}
2
](1),
1
⬁
[{Zn(C
12
H
8
N
2
)}-
{C
5
H
3
N(COO)
2
}·0.5H
2
O] (2), and
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
3
HN
2
-
(COO)
2
}] (3). While 1 forms with a zero-dimensional molecu-
lar rectangular box structure, 2 and 3 have zig-zag one-di-
mensional chain structures. The Zn
2+
ions are coordinated by
both the carboxylate oxygen atoms and also by the nitrogen
Introduction
Research in the area of metal-organic frameworks
(MOFs) exhibiting open structures continues to be interest-
ing due to their many applications, both actual as well as
potential.
[1]
Of the many MOF compounds investigated,
those containing benzenecarboxylates constitute an impor-
tant family
[1–10]
as they combine the principles of supramo-
lecular chemistry along with favorable π···π interactions
that give rise to fascinating crystal structures. The role of
hydrogen bonding in metal-coordinated network structures
is beginning to gain importance as it results in a large
number of coordination polymers. Recently, the scope of
the investigations on benzenecarboxylates has been en-
hanced by the use of heterocyclic carboxylic acids such as
pyridine-, pyrazole-, and imidazolecarboxylic acids. These
acids can act both as a multiple proton donor and acceptor
and can use their carboxylate oxygen and nitrogen atoms,
which are highly accessible to metal ions, to form interest-
ing network structures. Thus, Lin and co-workers have em-
ployed pyridinecarboxylic acid to prepare a series of Zn
2+
and Cd
2+
coordination polymers using a molecular build-
ing-block approach.
[11–14]
Pyrazoledicarboxylates have also
given rise to interesting network structures of varying di-
mensionality.
[15–17]
Although many polymeric complexes
have been prepared employing heterocyclic carboxylates,
previously uncharacterized compounds with novel crystal
[a] Framework Solids Laboratory, Solid State and Structural
Chemistry Laboratory, Indian Institute of Science
Bangalore 560012, India
E-mail: snatarajan@sscu.iisc.ernet.in
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
atoms of the heterocycles. The 1,10-phenanthroline mole-
cules in 2 and 3 act as a secondary ligands and occupy the
inter-chain spaces. The moderate hydrogen-bond interaction
energy in 1 and the π···π interactions in 2 and 3 appear to
play an important role for the structural stability. The struc-
tures of 2 and 3 appear to be related, even though they are
formed with different carboxylic acids. All three compounds
exhibit photoluminescence at room temperature.
structures often emerge during hydrothermal synthetic con-
ditions. We have combined the advantages of the hydrother-
mal method of synthesis and multifunctional carboxylic ac-
ids in the presence of 1,10-phenanthroline to form a large
number of new inorganic coordination polymers.
[18,19]
In
continuation of this theme, we have now synthesized three
new Zn coordination polymers, namely
0
⬁
[{Zn
2
(H
2
O)
4
}-
{C
5
H
3
N(COO)
2
}
2
](1),
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
5
H
3
N(COO)
2
}·
0.5H
2
O] (2), and
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
3
HN
2
(COO)
2
}] (3),
by employing pyridine- and imidazoledicarboxylic acids.
Compounds 1 and 2 were prepared from pyridine-2,5-di-
carboxylic acid, whereas 3 was prepared from imidazole-
4,5-dicarboxylic acid. While 1 possesses a zero-dimensional
structure with a rectangular molecular-box arrangement, 2
and 3 have one-dimensional structures and all the structures
are stabilized by hydrogen-bonding and π···π interactions.
In this paper we describe the synthesis, structure, and prop-
erties of these compounds.
Results and Discussion
The asymmetric unit of 1 consists of 30 non-hydrogen
atoms, of which two zinc atoms are crystallographically in-
dependent (Figure 1). Both the Zn
2+
ions have a distorted
square-pyramidal geometry formed by two carboxylate
oxygen atoms, two bonded water molecules and a nitrogen
atom of the pyridine ring. An average distance of 2.0395
and 2.092 Å for the Zn–O and Zn–N bonds, respectively,
results from this connectivity. The O/N–Zn–O/N bond
angles are in the range 78.01(2)–156.2(2)°. There are two
different pyridine-2,5-dicarboxylate anions present in the
structure and all the carboxylate groups have only mono-
dentate connectivity with the Zn
2+
cations. The bond
lengths and angles associated with the pyridine-2,5-dicar-
boxylate anions are in the range expected for this type of
bonding. The terminal Zn–O bonds formed by the oxygen
atoms [O(1) and O(4) for Zn(1) and O(6)and O(8) for Zn(2)]
are formally water molecules. The two proton positions ob-
served in the difference Fourier map for each of these oxy-
gen atoms also confirm this. Selected bond lengths and
angles are listed in Table 1. The connectivity between Zn
2+
and the pyridine-2,5-dicarboxylate anions gives rise to a
zero-dimensional unique molecular box. Each molecular
box consists of four Zn
2+
cations and four carboxylate
anions, as shown in Figure 2a. The terminal water mole-
cules and the presence of terminal C–O bonds in 1 gives
rise to a large number of significant O–H···O hydrogen
bonds. The hydrogen-bond interactions between the mol-
ecular box units form an extended two-dimensional struc-
ture (Figure 2b and c). Both intra- and intermolecular box
hydrogen bonds are observed in 1; a complete list of these
interactions is given in Table 2.
Figure 1. ORTEP drawing of
0
⬁
[{Zn
2
(H
2
O)
4
}{C
5
H
3
N(COO)
2
}
2
](1)
showing the asymmetric unit. Thermal ellipsoids are given at 50%
probability.
Table 1. Selected bond lengths [Å] and angles [°] in
0
⬁
[{Zn
2
-
(H
2
O)
4
}{C
5
H
3
N(COO)
2
}
2
](1).
Zn(1)–O(1) 2.012(5) Zn(2)–O(5) 2.103(5)
Zn(1)–O(2) 2.113(4) Zn(2)–O(6) 1.983(6)
Zn(1)–O(3) 2.022(5) Zn(2)–O(7) 2.036(5)
Zn(1)–O(4) 2.039(5) Zn(2)–O(8) 2.008(5)
Zn(1)–N(2) 2.113(5) Zn(2)–N(12) 2.070(6)
O(1)–Zn(1)–O(3) 100.1(2) O(6)–Zn(2)–O(8) 113.4(2)
O(1)–Zn(1)–O(4) 102.5(2) O(6)–Zn(2)–O(7) 99.6(2)
O(3)–Zn(1)–O(4) 91.1(2) O(8)–Zn(2)–O(7) 89.6(2)
O(1)–Zn(1)–O(2) 105.7(2) O(6)–Zn(2)–N(12) 111.0(2)
O(3)–Zn(1)–O(2) 83.05(2) O(8)–Zn(2)–N(12) 135.5(2)
O(4)–Zn(1)–O(2) 151.7(2) O(7)–Zn(2)–N(12) 86.8(2)
O(1)–Zn(1)–N(2) 99.7(2) O(6)–Zn(2)–O(5) 102.8(2)
O(3)–Zn(1)–N(2) 155.9(2) O(8)–Zn(2)–O(5) 88.6(2)
O(4)–Zn(1)–N(2) 98.0(2) O(7)–Zn(2)–O(5) 156.2(2)
O(2)–Zn(1)–N(2) 78.44(2) N(12)–Zn(2)–O(5) 78.01(2)
The asymmetric unit of 2 consists of 28 non-hydrogen
atoms, of which only one Zn atom is crystallographically
independent (Figure 3). The Zn
2+
ions have a distorted oc-
tahedral geometry formed by three carboxylate oxygen
atoms and three nitrogen atoms, two of which belong to the
1,10-phenanthroline ligand. The Zn–O and the Zn–N
bonds have average values of 2.162 and 2.145 Å, respec-
tively. The O/N–Zn–O/N bond angles are in the range
58.98(2)–164.70(2)°, thereby indicating the heavy distortion
of the Zn
2+
octahedra (ideal octahedral values are 90° and
180°). The two carboxylate units of the pyridine-2,5-dicar-
boxylate show differences in their connectivity with respect
to the Zn
2+
ions − one has a monodentate connectivity and
the other a bis(didentate) connectivity. The bond lengths
and angles associated with the carboxylate group have typi-
cal values. Selected bond lengths and angles are listed in
Table 3. In 2, the connectivity between Zn
2+
and the pyri-
dine-2,5-carboxylate units gives rise to one-dimensional zig-
zag chains (Figure 4). The 1,10-phenanthroline molecules
act as a ligand to the Zn
2+
ions and occupy the inter-chain
spaces, along with the lattice water, and contribute to the
stability of 2 by forming favorable π···π interactions (Fig-
ure 4). The presence of these interactions gives rise to a
channel-like structure in 2. Unlike 1, no significant hydro-
gen-bond interactions are observed in 2.
The asymmetric unit of 3 consists of 26 non-hydrogen
atoms, of which only one Zn atom is crystallographically
independent (Figure 5). The Zn
2+
ions have a distorted oc-
tahedral geometry formed by two carboxylate oxygen atoms
and four nitrogen atoms, two of which belong to the 1,10-
phenanthroline ligand and the other two to the imidazole
ring. The Zn–O and Zn–N bonds have average values of
2.225 Å and 2.151 Å, respectively. The O/N–Zn–O/N bond
angles are in the range 74.63(8)–171.54(8)°, thereby indicat-
ing distortion of the Zn
2+
octahedra. Both the carboxylate
groups of the imidazole-4,5-dicarboxylate have only mono-
dentate connectivity with Zn
2+
ions. The bond lengths and
angles associated with the imidazole-4,5-dicarboxylate have
typical values. Selected bond lengths and angles are given
in Table 3. The connectivity between the Zn
2+
and the imid-
azole-4,5-carboxylate units gives rise to one-dimensional
zig-zag chains (Figure 6). The 1,10-phenanthroline mole-
cules act as a ligand to the Zn
2+
ions and occupy the inter-
chain spaces (Figure 6).
Room-temperature solid-state photoluminescence studies
performed on powdered samples are presented in Figure 7.
Photoluminescence studies of coordination polymers of the
type discussed here have been investigated in great detail
during the past few years.
[19–28]
In our present study, we
found that coordination polymers 1–3 all exhibit photolu-
minescence. Compound 1 exhibits a single broad emission
band at 430 nm, whilst compounds 2 and 3 exhibit two
peaks at about 375 nm and at 390 nm when excited at
337 nm. The emission peak at 430 nm for 1 and at about
375 nm for 2 and 3 can be assigned to the intraligand fluo-
rescent emission, since the acid displays a rather weak emis-
sion (λ
max
= 410 nm). The lifetime, τ, for the emissions are
0.016 and 0.018 ns for 2 and 3, respectively. This indicates
that the luminescence should be assigned to fluorescence.
The peak at 390 nm observed for 2 and 3 can be assigned
to intraligand emission from the 1,10-phenanthroline li-
gand.
[28]
Similar ligand-to-metal charge transfer (LMCT)
Figure 2. (a) A figure showing the connectivity between the Zn
2+
ions and the pyridine-2,5-dicarboxylate anions. Note that the connectiv-
ity forms a molecular box. Dotted lines represent hydrogen-bond interactions. (b) The connectivity between the molecular box in the bc
plane. Dotted lines represent hydrogen-bond interactions. (c) The connectivity between the molecular box in the ac plane.
Table 2. Selected hydrogen-bond interactions in
0
⬁
[{Zn
2
(H
2
O)
4
}-
{C
5
H
3
N(COO)
2
}
2
](1).
D–H···A
[a]
D–H H···AD···AD–H···A
[Å] [Å] [Å] [°]
O(1)–H(10)···O(2)
#1
0.85 2.09 2.882(8) 156
O(1)–H(11)···O(12)
#2
0.85 1.87 2.676(7) 156
(intra)
O(4)–H(20)···O(2)
(#3
0.85 2.13 2.943(8) 159
O(4)–H(21)···O(9)
#4
0.85 1.79 2.618(8) 166
O(6)–H(30)···O(10)
#1
0.85 1.89 2.726(9) 173
O(6)–H(31)···O(11)
#5
0.85 1.81 2.653(8) 171
O(8)–H(40)···O(5)
#6
0.85 1.85 2.674(8) 162
O(8)–H(41)···O(10)
#3
0.85 2.45 3.202(9) 147
[a] #1: 1 – x,–y,1–z;#2:2–x,1–y,1–z;#3:1–x,1–y,1–
z;#4:x,–1+y, z;#5:1–x,–y,–z;#6:x,–1+y,–1+z.
transitions have been observed in many metal-organic coor-
dination polymers.
[19–28]
It is known that free 1,10-phenan-
throline exhibits weak emission peaks at 425 and 445 nm in
the solid state at room temperature (see Supporting Infor-
mation). The enhancement and the blue shift of the lumi-
nescence of the 1,10-phenanthroline ligand compared to
that of free 1,10-phenanthroline may, therefore, be attrib-
uted to the chelating effect of the 1,10-phenanthroline li-
gand to the Zn
2+
ion. This effectively enhances the rigidity
of the ligand and reduces the loss of energy by radiationless
decay of the intraligand emission of the excited state. Sim-
ilar blue-shifts involving rigid ligand molecules and coordi-
nation polymers have been observed before.
[28–30]
In ad-
dition, fluorescent emission of carboxylate ligands resulting
Figure 3. ORTEP drawing of
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
5
H
3
N(COO)
2
}·
0.5H
2
O] (2) showing the asymmetric unit. Thermal ellipsoids are
given at 50% probability.
Table 3. Selected bond lengths [Å] and angles [°] in
1
⬁
[{Zn(C
12
H
8
-
N
2
)}{C
5
H
3
N(COO)
2
}·0.5H
2
O] (2)and
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
3
HN
2
-
(COO)
2
}] (3).
23
Zn(1)–O(1) 2.057(4) Zn(1)–O(1) 2.2271(19)
Zn(1)–O(2) 2.312(5) Zn(1)–O(4) 2.222(2)
Zn(1)–O(3) 2.118(4) Zn(1)–N(1) 2.143(2)
Zn(1)–N(1) 2.151(4) Zn(1)–N(2) 2.286(2)
Zn(1)–N(2) 2.163(4) Zn(1)–N(3) 2.073(2)
Zn(1)–N(3) 2.121(4) Zn(1)–N(4) 2.100(2)
O(1)–Zn(1)–O(3) 103.08(17) N(3)–Zn(1)–N(4) 103.30(8)
O(1)–Zn(1)–N(3) 79.07(14) N(3)–Zn(1)–N(1) 158.67(8)
O(3)–Zn(1)–N(3) 98.19(14) N(4)–Zn(1)–N(1) 96.94(8)
O(1)–Zn(1)–N(1) 90.69(14) N(3)–Zn(1)–O(4) 98.01(8)
O(3)–Zn(1)–N(1) 95.17(14) N(4)–Zn(1)–O(4) 78.54(8)
N(3)–Zn(1)–N(1) 164.70(14) N(1)–Zn(1)–O(4) 92.48(8)
O(1)–Zn(1)–N(2) 104.39(15) N(3)–Zn(1)–O(1) 79.28(7)
O(3)–Zn(1)–N(2) 151.60(16) N(4)–Zn(1)–O(1) 92.60(8)
N(3)–Zn(1)–N(2) 93.93(14) N(1)–Zn(1)–O(1) 93.29(8)
N(1)–Zn(1)–N(2) 77.50(14) O(4)–Zn(1)–O(1) 169.95(7)
O(1)–Zn(1)–O(2) 158.01(15) N(3)–Zn(1)–N(2) 84.99(8)
O(3)–Zn(1)–O(2) 58.98(16) N(4)–Zn(1)–N(2) 171.54(8)
N(3)–Zn(1)–O(2) 90.45(15) N(1)–Zn(1)–N(2) 74.63(8)
N(1)–Zn(1)–O(2) 102.86(15) O(4)–Zn(1)–N(2) 102.24(7)
N(2)–Zn(1)–O(2) 95.50(14) O(1)–Zn(1)–N(2) 87.23(7)
from the π* 씮 n transition is very weak compared with
that of the π* 씮 π transition of the 1,10-phenanthroline
ligand. The strongly electron-withdrawing carboxylate
group results in a fluorescence quenching, so the carboxyl-
ate ligands hardly contribute to the fluorescent emission of
the as-synthesized polymers. As can be seen, the main emis-
sion bands of 2 and 3 are located almost at the same posi-
tion, but with differences in the band shapes, which has
been attributed to the π* 씮 π transition of the coordinated
1,10-phenanthroline ligand.
[28–30]
The differences in the
band shape might also be due to the minor differences in
the structural topologies of the two structures.
The three new compounds were obtained by employing
hydrothermal methods. The compounds have zero- and
one-dimensional structures. While 1 forms with a zero-di-
Figure 4. One-dimensional zig-zag chains observed in 2 in the ab
plane. Note that the 1,10-phenanthroline ligands occupy inter-
chain spaces and interact through π···π interactions (see text).
Figure 5. ORTEP drawing of
1
⬁
[{Zn(C
12
H
8
N
2
)}{C
3
HN
2
(COO)
2
}]
(3) showing the asymmetric unit. Thermal ellipsoids are given at
50% probability.
mensional molecular box structure, 2 and 3 are formed with
zig-zag one-dimensional chain structures. However, all three
compounds are related in a subtle way. While compound 2
is formed by adding 1,10-phenanthroline to the synthesis
mixture of 1, 3 is obtained by replacing pyridine-2,5-dicarb-
oxylic acid with imidazole-4,5-dicarboxylic acid in the syn-
thesis mixture of 2. The secondary ligand, 1,10-phenan-
throline, replaces the terminal water molecules in 1 and,
during this process, the coordination environment of the
Zn
2+
ions also changes from distorted square-pyramidal
(five-coordinate) to an octahedral one (six-coordinate). In
addition, the coordination mode of the pyridine-2,5-dicar-
boxylate also changes from being a simple monodentate co-
ordination in 1 to a combination of mono- and bis(dident-
ate) in 2.In3, however, the imidazole-4,5-dicarboxylate has
Figure 6. One-dimensional chains observed in 3 in the ab plane.
Note that the 1,10-phenanthroline ligands occupy the inter-chain
spaces and interact through π···π interactions (see text).
Figure 7. Emission spectra in the solid state at room temperature
for (a) 1,(b)2, and (c) 3.
a simple monodentate connectivity with Zn
2+
ions. The
striking similarities between 2 and 3 can best be seen when
viewing the structures down the chain axis. The 1,10-phen-
anthroline ligands occupy similar positions in both 2 and
3, thus indicating that the favorable π···π interactions be-
tween the ligand molecules play an important role in the
formation and stability of these compounds.
The use of heterocyclic carboxylic acids provides both
the proton donor as well as the acceptor through the car-
boxylate oxygen and the ring nitrogen atoms. Both these
centers are highly accessible to the participating metal ions
during the synthesis for the formation of both monodentate
and/or multidentate M–O and M–N bonds. The structural
motifs thus formed can then readily participate in hydrogen
bonding to give rise to a variety of networks. In the present
system of compounds, we observe differences in the connec-
tivity of the carboxylate oxygen atoms but the nitrogen
atom, in all cases, participates in bonding with the Zn
2+
ions. Similar behavior has been observed before.
[17]
In ad-
dition, moderate O–H···O-type hydrogen bonding is ob-
served in 1 that gives rise to extended networks. It is likely
that the presence of the rather bulky 1,10-phenanthroline
as the secondary ligand in 2 and 3 prevents the formation
of any hydrogen-bond interactions and only π···π interac-
tions are observed (Figure 8).
Figure 8. Arrangement of the one-dimensional chains in the ac
plane for (a) 2 and (b) 3. Note the similarity between the two struc-
tures and the arrangement of the 1,10-phenanthroline ligands.
The role of π···π interactions in the stability of lower di-
mensional structures in metal-organic coordination poly-
mers has been a topic of much interest.
[31,32]
In the present
compounds π···π interactions involving the 1,10-phenan-
throline ligands are observed, especially in 2 and 3. The
centroid–centroid distance (d) between the 1,10-phenan-
throline rings and their interplanar angles (θ)for2 and 3
are shown in Figure 9. Favorable π···π interactions between
these rings, with d = 3.66 Å and 3.45 Å and θ = 0.8° and
1.93° for 2 and 3, respectively, are observed. From the in-
terplanar angles (θ), it is clear that the two 1,10-phenan-
throline rings are arranged one over the other, but are
stacked anti-parallel to each other. This type of anti-parallel
arrangement of aromatic rings is commonly observed in
systems exhibiting dipolar properties. To understand the
role of π···π interactions, we have performed preliminary
calculations using the AM1-parameterized Hamiltonian
available in the Gaussian program suite.
[33,34]
AM1 meth-
ods, together with a semi-classical dipolar description, have
been employed recently to establish the relationship be-
tween the stability and geometries of organic molecules.
[35]
![Table 1. Selected bond lengths [Å] and angles [°] in 0 [{Zn2(H2O)4}{C5H3N(COO)2}2] (1).](/figures/table-1-selected-bond-lengths-a-and-angles-deg-in-0-zn2-h2o-32t97hcj.webp)
![Figure 1. ORTEP drawing of 0 [{Zn2(H2O)4}{C5H3N(COO)2}2] (1) showing the asymmetric unit. Thermal ellipsoids are given at 50% probability.](/figures/figure-1-ortep-drawing-of-0-zn2-h2o-4-c5h3n-coo-2-2-1-1lvh8yyp.webp)
![Table 4. Crystal data and structure-refinement parameters for 0 [{Zn2(H2O)4}{C5H3N(COO)2}2] (1), 1 [{Zn(C12H8N2)}{C5H3N(COO)2}·0.5H2O] (2), and 1 [{Zn(C12H8N2)}{C3HN2(COO)2}] (3).](/figures/table-4-crystal-data-and-structure-refinement-parameters-for-2n7abhcd.webp)
![Table 2. Selected hydrogen-bond interactions in 0 [{Zn2(H2O)4}{C5H3N(COO)2}2] (1).](/figures/table-2-selected-hydrogen-bond-interactions-in-0-zn2-h2o-4-h8oczgha.webp)