Heavy metal complexes of 4-chlorodipicolinic acid - structural, spectral and thermal correlations

Article abstract of DOI:10.1080/00958972.2019.1675873

The ligand, 4-chloro-2,6-dipicolinic acid (H₂PDA-Cl), and its two heavy metal complexes, [Ag(HDPA-Cl)(H₂DPA-Cl)·2H₂O] (1) and {[Cd(μ₂-H₂O)(H₂O)(PDA-Cl)]}_n (2), were prepared and then characterized by single-crystal X-ray diffraction. In addition, spectral and thermal correlations with structural results complete their solid-state description and facilitate complex 3 ([Pb(DPA-Cl)]) composition determinations. Complex 1 crystallizes in a monoclinic space group C2/c and each DPA-Cl ligand is tridentate to Ag(I) through the pyridine N and two monodentate carboxyl O atoms. The carboxy group with carbonyl C1 is semideprotonated and forms a symmetric hydrogen bond with a carboxy group of a neighboring complex. Complex 2 crystallizes in a triclinic lattice with space group P (Formula presented.) The Cd(II) ions are seven-coordinate and the coordination polyhedra can be described as a distorted pentagonal bipyramid. IR data are consistent with monodentate coordination of the carboxylate to Ag(I), Cd(II), and Pb(II) and observed wavenumber shifts confirm PDA-Cl ligand coordination to Pb(II) in 3. Thermal stability of anhydrous complexes indicates the metal–ligand interactions. The thermal stability of prepared compounds is reflected by the strength of interaction between metal–ligand and hydrogen bonds.

Full text of DOI:10.1080/00958972.2019.1675873

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Heavy metal complexes of 4-chlorodipicolinic acid
-
structural, spectral and thermal correlations
Z. Vargová, M. Almáši, R. Gyepes & R. Vetráková
To cite this article: Z. Vargová, M. Almáši, R. Gyepes & R. Vetráková (2019): Heavy metal
complexes of 4-chlorodipicolinic acid - structural, spectral and thermal correlations, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2019.1675873
To link to this article: https://doi.org/10.1080/00958972.2019.1675873
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1675873
Heavy metal complexes of 4-chlorodipicolinic acid -
structural, spectral and thermal correlations
a
a
b
a
Z. Vargov ꢀa , M. Alm ꢀa ꢁs i , R. Gyepes and R. Vetr ꢀa kov ꢀa
a
ꢁ
Department of Inorganic Chemistry, Faculty of Science, P. J. Saf ꢀa rik University, Ko ꢁs ice, Slovak
b
Republic; Department of Inorganic Chemistry, Faculty of Science, Charles University, Praha,
Czech Republic
ABSTRACT
ARTICLE HISTORY
The ligand, 4-chloro-2,6-dipicolinic acid (H
2
PDA-Cl), and its two
Received 14 February 2019
Accepted 9 September 2019
heavy metal complexes, [Ag(HDPA-Cl)(H DPA-Cl)ꢀ2H O] (1) and
2
2
2 2 2 n
f[Cd(l -H O)(H O)(PDA-Cl)]g (2), were prepared and then charac-
KEYWORDS
terized by single-crystal X-ray diffraction. In addition, spectral and
thermal correlations with structural results complete their solid-
state description and facilitate complex 3 ([Pb(DPA-Cl)]) compos-
ition determinations. Complex 1 crystallizes in a monoclinic space
group C2/c and each DPA-Cl ligand is tridentate to Ag(I) through
the pyridine N and two monodentate carboxyl O atoms. The car-
boxy group with carbonyl C1 is semideprotonated and forms a
symmetric hydrogen bond with a carboxy group of a neighboring
complex. Complex 2 crystallizes in a triclinic lattice with space
group P: The Cd(II) ions are seven-coordinate and the coordin-
ation polyhedra can be described as a distorted pentagonal bipyr-
amid. IR data are consistent with monodentate coordination of
the carboxylate to Ag(I), Cd(II), and Pb(II) and observed wavenum-
ber shifts confirm PDA-Cl ligand coordination to Pb(II) in 3.
Thermal stability of anhydrous complexes indicates the metal–li-
gand interactions. The thermal stability of prepared compounds is
reflected by the strength of interaction between metal–ligand
and hydrogen bonds.
Lead; cadmium; silver;
chelating agent;
coordination
CONTACT Z. Vargovꢀa
zuzana.vargova@upjs.sk
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2019.1675873.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

ꢀ
2
Z. VARGOVA ET AL.
1. Introduction
Environmental pollution by heavy metals is found increasingly in military, industrial
and agricultural zones. Fuel and power industries generate 2.4 million tons of As, Cd,
Cr, Cu, Hg, Ni, Pb, Se, V, and Zn. Metals, discharged or transported into the environ-
ment, may undergo transformations and can have large environmental, public health
and economic impact [1]. Many different methods are available for removal of toxic
metal ions. Among them, the most commonly used are ion exchange, adsorption,
reduction, and precipitation. In many cases, the environmentally most compatible and
cost-effective solutions include combination of two or more of these processes.
Chelating agents are offered as one solution and are widely used in many indus-
trial, domestic and agriculture applications due to their ability to complex metals.
They have been used in applications, such as scale and corrosion inhibitors, pulp,
paper and textile production, cleaning and laundry operations, prevention/inhibition
of the growth of microorganisms, soil remediation, waste and effluent treatment, agri-
culture, metal electroplating and other surface treatments, tanning processes, cement
admixtures, photography, food products, pharmaceuticals, and cosmetics. Among the
common chelating agents, organophosphonates and aminopolycarboxylates (APCs)
stand out as they are good metal chelators with a good quality/price ratio [2].
The chelating agent pyridine-2,6-dicarboxylic acid (H PDA), also known as dipicolinic
2
acid, contains a pyridine ring and carboxylic functions with N- and O-donors. Due to its
structure, it tends to chelate metals in a proportion of 2:1 (ligand/metal), improving the
stability of the complex [2]. Moreover, PDA is a low toxic and ready-biodegradable com-
pound according to the OECD guidelines [3]. Due to its ability to complex with metal

JOURNAL OF COORDINATION CHEMISTRY
3
ions, it has been studied for remediation of soils, used in detergents and cleaning
applications.
To test the chelating ability of 4-chloro-2,6-dipicolinic acid (4-chloro-2,6-pyridinedicarboxylic
acid, H PDA-Cl), we prepared the chlorine-functionalized variant that can be a prospect-
2
ive ligand in many areas from the structural points of view. Previous results revealed that
chlorine function is not connected directly to metals and the N- and O-atoms remain as
donors. However, this function contributes significantly to intermolecular interactions,
frequently through hydrogen bonding with secondary entities such as solvent mole-
cules [4–11].
In 1999, Shaw et al. demonstrated promising separations involving HPCIC (high
performance chelation ion chromatography) using polystyrene-divinyl (PS-DVB) resin
dynamically modified with H PDA-Cl. The retention order found for transition and heavy
2
metal ions was U(VI) > Fe(III) > lanthanides > Cd(II) > Pb(II) > Cu(II) > Zn(II) > Ni(II) >
ꢂ4
Co(II) > Mn(II) using an eluent consisting of 1 M KNO and 5 ꢁ 10 M H PDA-Cl at pH 1.2
3
2
(
HNO ). They concluded that PS-DVB dynamically modified with H PDA-Cl has promising
3
2
results giving both, good peak efficiency and unusual metal selectivity [12].
Moreover, H PDA-Cl and other 4-substituted analogues, like chelators, were used
2
for enhancement of terbium(III) and europium(III) complexes luminescence. Because
of the ease which functional groups can be placed at the 4 position of dipicolinic
acid, and the efficiency with which dipicolinic acid sensitizes lanthanide ion
emission in aqueous solution, compounds of the type 4-X-dipicolinic acid represent
attractive intermediates for formation of lanthanide energy transfer complexes with
long emission lifetimes. In the case of Tb(III) complexes, maximum emission inten-
sities were in the order: X ¼ NH > NHAc > OH > H > Cl > Br and measurements of
2
emission decay kinetics revealed lifetimes of 1.0–2.0 ms [13]. However, the opposite
trend was observed for Eu(III) complexes luminescence sensitization finding:
X ¼ Cl > H > OH [14] and the Eu(III) luminescence lifetime varied from 1.16
to 2.90 ms.
Further H PDA-Cl complexes testing was observed for insulin-enhancing activity in
2
streptozotocin (STZ) induced diabetic Wister rats. The effect of the chloro-substitution
on lowering diabetic hyperglycaemia was evaluated and differences were found
depending on the oxidation state [5]. There were observed significant differences in
the abilities of H[V(PDA-Cl) ]ꢀ5.5H Oꢀ0.5NH and [VO(PDA-Cl)(H O) ] to lower diabetic
2
2
3
2
2
hyperglycaemia when compared to the blood glucose levels in normal and diabetic
rats. Moreover, in biological activity evaluation, a mixed-ligand Ni(II) complex ([Ni(PDA-
þ
Cl)(APYM)(H O) ], APYM ¼ 2-aminopyrimidine) was tested against three G and three
2
2
ꢂ
G
bacteria (the highest ATB activity was observed toward S. epidermis) and its fluor-
escence properties were studied in solvents with different dipole moments (fluores-
cence intensity increased in the order CH OH > H O > DMSO) [10].
3
2
To study toxic Cd(II), Pb(II), and Ag(I) metal ions–ligand H PDA-Cl interactions (and
2
to compare H PDA-Cl coordination ability with H PDA) in aqueous solution and in
2
2
solid state, we observed the fast crystals elimination from mother liquor at very low
species concentrations. Therefore we decided to determine and characterize the metal
complexes in the solid state and compare their structural, spectral, and thermal fea-
tures. Unfortunately, all attempts to find solution experimental conditions failed

ꢀ
Z. VARGOVA ET AL.
4
(metal/ligand concentrations, metal/ligand ratio). In all cases, at the beginning of the
experiment, precipitates were produced.
2. Experimental
2.1. Materials and measurements
Cd(NO ) ꢀ4H O, Pb(NO ) and AgNO were obtained from Lachema (Czech Republic).
3
2
2
3 2
3
The ligand 4-chloropyridine-2,6-dicarboxylic acid was prepared by the reaction proced-
ure described in Section 2.2.1. Chemicals used in the synthesis were obtained from
Sigma-Aldrich or Acros Organics companies. All chemicals were analytically pure and
used without purification. Demineralized water was used for all solution preparations.
2.2. Preparation of the complexes
2
.2.1. Synthesis of 4-chloropyridine-2,6-dicarboxylic acid (H DPA-Cl)
2
4
4
-Chloropyridine-2,6-dicarboxylic acid was prepared by two-step organic reactions from
-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid) as a precursor and phenylphos-
phonic dichloride as a standard chlorinating agent. In the first step 2 g (10.92 mmol) of
3
chelidamic acid and 10 cm (70.51 mmol) of phenylphosphonic dichloride were heated
ꢃ
and stirred at 130 C for 3 h in reflux under nitrogen. After selected reaction time, the
mixture was cooled to laboratory temperature. During the reaction 4-chloropyridine-2,6-
dicarbonyl dichloride was formed. To prepare the desired product, hydrolysis of acyl
3
chloride was performed by addition of 50 cm water dropwise to the reaction mixture
(be careful, strong exothermic reaction). The reaction residue was then suspended in dis-
tilled water, filtered off and slightly washed with ethanol, chloroform, diethyl ether and
dried in a stream of air. Final product was isolated in the form of white powder with yield
ꢂ1
1
.43 g (65% based on chelidamic acid). Elem. analysis for C H Cl N O (201.56 g.mol
)
7
4
1
1 1 4
Calc. (Found) %: C, 41.71 (41.96); H, 2.00 (1.96); N, 6.95 (7.01). H NMR (300 MHz, DMSO-
13
d ): d ¼ 8.24 (s, 2 H) ppm; C NMR (100 MHz, DMSO-d ): d ¼ 164.8, 150.1, 145.1, 127.2.
6
6
H DPA-Cl in the form of single crystals suitable for single crystal X-ray analysis were pre-
2
pared by recrystallization from mixture of solvents EtOH/H O (v:v, 2:1).
2
2
.2.2. Synthesis of [Ag(H PDA-Cl)(HPDA-Cl)]ꢀ2H O (1)
2
2
3
A solution of silver(I) nitrate (62.4 mg, 0.25 mmol) in ethanol/water (10 cm , 2:1 molar
ratio) was slowly added to a solution of H PDA-Cl (50 mg, 0.25 mmol) in ethanol/water
2
3
(
40 cm , 2:1 molar ratio). After 24 h the yellow precipitate of 1 was filtered and the
clear solution was kept in the dark for evaporation. After a month, beige needles suit-
able for X-ray analysis were obtained. Elem. analysis for C H Ag Cl N O
2 2 10
1
4
11
1
ꢂ1
(
546.02 g.mol ) Calc. (Found) %: C, 30.79 (31.03); H, 2.03 (1.98); N, 5.13 (5.19), yield:
23 mg corresponding to 17%.
2
.2.3. Synthesis of f[Cd(l -H O)(H O)(PDA-Cl)]g (2)
2
2
2
n
A solution of cadmium(II) nitrate tetrahydrate (95 mg, 0.25 mmol) in ethanol/water
3
(10 cm , 2:1 molar ratio) was slowly added to a solution of H PDA-Cl (50 mg,
2
3
0.25 mmol) in ethanol/water (40 cm , 2:1 molar ratio). After 24 h the yellow precipitate

JOURNAL OF COORDINATION CHEMISTRY
5
of 2 was filtered and the clear solution was kept for slow evaporation. After a month,
beige crystals suitable for X-ray analysis were afforded. Elem. analysis for
ꢂ1
C H Cd Cl N O (347.98 g mol ) Calc. (Found) %: C, 24.16 (24.05); H, 1.73 (1.70); N,
7
6
1
1 1 6
4.03 (3.92), yield: 31 mg corresponding to 36%.
2
.2.4. Synthesis of [Pb(PDA-Cl)] (3)
3
A solution of lead(II) nitrate (66 mg, 0.20 mmol) in ethanol/water (30 cm , 2:1 molar ratio)
was slowly added to an aqueous solution of PDA-Cl (40 mg, 0.20 mmol, 40 cm ). After
3
24 h the precipitate of 3 was filtered and the clear solution was kept for slow evaporation.
The result of slow evaporation was polycrystalline material not suitable for single crystal
ꢂ1
X-ray analysis. Elem. analysis for C H Pb Cl N O (406.76 g mol ) Calc. (Found) %: C,
7
2
1
1 1 4
20.67 (20.53); H, 0.49 (0.25); N, 3.44 (3.34), yield: 46 mg corresponding to 57%.
2.3. Elemental analysis and infrared spectroscopy
The elemental analyses were performed on a CHNOS Elemental Analyzer Vario MICRO
(
Elemental Analyzer system GmbH). Infrared spectra were recorded on a Nicolet Avatar
ꢂ1
FT-IR 6700 spectrometer from 4000 to 400 cm using KBr pellets. KBr pellets were
prepared by grinding sample with potassium bromide in mass ratio 2:200.
2.4. NMR spectroscopy
1
13
H and C NMR spectra were recorded with a Varian Mercury Plus 400 spectrometer
1
13
at frequency of magnetic field 400 ( H NMR) and 100 ( C NMR) MHz, respectively,
using TMS as the internal reference.
2.5. X-ray diffraction
Single-crystal diffraction data of the free ligand (CCDC number 1940563) and 1 (CCDC
number 194056) and 2 (CCDC number 1940562) were collected on a Nonius Kappa
four-circle CCD diffractometer equipped with an Apex II (Bruker) detector and a
Cryostream Cooler (Oxford Cryosystems). In all cases, MoKa radiation was used
(
k ¼ 0.71073 Å) and reduction of collected data was carried out by diffractometer soft-
ware. The phase problem was solved by intrinsic phasing and structure models were
refined by full matrix least squares on F using the Shelx program suite [15]. Non-
2
hydrogen atoms were refined anisotropically. Hydrogens located on aromatic rings
were included in their ideal positions with their isotropic thermal parameters fixed to
1
.2 U . The figures of final crystal structures were drawn using DIAMOND [16] and
eq
Mercury [17] software. Detailed crystallographic data are given in Table 1.
2.6. Thermal analysis
Stability and thermal behavior of the prepared compounds were studied by thermog-
ravimetry (TG) using TA instrument TGA Q-500 in air. Samples were heated in dynamic
3
ꢂ1
atmosphere of air (argon was used as a protecting gas) with air flow rate 60 cm ꢀmin

ꢀ
Z. VARGOVA ET AL.
6
Table 1. Crystal data and structure refinement summary for H DPA-ClꢀH O, 1 and 2.
2
2
H
2
DPA-ClꢀH
Cl
219.58
2
O
1
2
Formula
7
C H
6
1
N
1
O
5
Ag
C
1 14
H11Cl
1
N
2
O
10
1 7 6 1 1 6
Cd C H Cl N O
Formula weight
Color
Temperature (K)
Crystal system
Space group
a (Å)
546.02
Yellow
150(2)
Monoclinic
C2/c
6.5673(3)
11.6494(5)
24.1220(12)
90
347.98
Yellow
150(2)
Triclinic
P-1
5.3994(2)
8.8389(4)
10.5007(5)
101.207(2)
95.930(2)
93.338(2)
0.71073
487.38(4)
2
Colorless
150(2)
Monoclinic
P2
1
5.1861(3)
9.4760(6)
8.5224(5)
90
94.147(2)
90
b (Å)
c (Å)
ꢃ
a ( )
ꢃ
b ( )
93.017(2)
90
ꢃ
c ( )
k (MoKa) (Å)
V (Å )
Z
0.71073
417.72(4)
2
1.746
0.452
0.71073
1842.90(15)
4
1.968
1.443
3
–
3
D (calcd) (g.cm )
2.371
2.527
–
1
m (mm )
F [000]
Crystal size (mm)
h min, h max ( )
Reflections collected/unique
Data/restraints/parameters
224
1080
336
0.299 ꢁ 0.240 ꢁ 0.142
2.1114, 28.8561
5630/1924
0.451 ꢁ 0.298 ꢁ 0.276
1.691, 27.561
8721/2138
0.584 ꢁ 0.278 ꢁ 0.114
1.991, 27.499
9084/2199
ꢃ
1924/1/136
1.056
2138/1/146
1.251
2199/4/151
1.054
2
Goodness-of-fit on F
Final R indices [I > 2r(I)]
R indices (all data)
R
wR
R
wR
1
¼ 0.0311,
R
wR
R
wR
1
¼ 0.0417,
R
wR
R
1
wR
1
¼ 0.0241,
2
2
¼ 0.0739
¼ 0.0343,
2
¼ 0.0902
¼ 0.0458,
¼ 0.0605
¼ 0.0267,
1
1
2
¼ 0.0754
2
¼ 0.0918
2
¼ 0.0622
ꢃ
ꢂ1
ꢃ
and heating rate 10 Cꢀmin from 25 to 800 C. Before thermal measurements, gentle
grinding of the samples and careful packing into the platinum crucibles were performed.
The mass of samples used in the analyses was 10–15 mg. Thermoanalytical curves
obtained were analyzed using the Origin 6.1 computational program [18].
3. Results and discussion
3.1. Single crystal X-ray studies
3
.1.1. Crystal structure of H DPA-ClꢀH O
2
2
The chloro derivative of dipicolinic acid was synthesized by the procedure described in
Section 2.2.1. After slow evaporation of the solvents 4-chloropyridine-2,6-dicarboxylic
acid crystallizes as a monohydrate, H DPA-ClꢀH O (Figure 1a,b). Ligand preparation was
2
2
confirmed by elemental analysis whose experimental and theoretical values are in agree-
1
ment as well as by NMR spectroscopy. H NMR spectra contain only one singlet at
13
8
1
.24 ppm, corresponding to hydrogen in the para position and four signals in C NMR at
64.8, 150.1, 145.1, and 127.2 ppm for carbons. The crystal structure of prepared ligand
has been re-determined by the single crystal X-ray diffraction with aim to use its struc-
tural data for 3 composition determination (using IR spectroscopy, Section 3.2). The com-
pound crystallizes in a monoclinic space group P2 , with cell dimensions a ¼ 5.1861(3) Å,
1
ꢃ
b ¼ 9.4760(6) Å, c ¼ 8.5224(5) Å, and b ¼ 94.147(2) . Racemic twinning was present and
its batch scale factor was refined to 0.317. Nieger et al. [19] noted similar twinning.
Detailed crystallographic data and refinement details for H DPA-ClꢀH O are listed in
2
2
Table 1 and Supplementary Information Table S1. Protonation does not occur on the

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. (a) A view of the crystal structure of 4-chloropyridine-2,6-dicarboxylic acid monohydrate.
b) Content of unit cell. (c) 1D supramolecular chains generated between H DPA-Cl and crystalliza-
(
2
tion water molecules propagating along the b crystallographic axis.
Figure 2. (a) The coordination environment of Ag(I) ion and coordination fashion of 4-chloropyri-
ud ni ni et - c2 e, 6l l -. dicarboxylate ligand in the crystal structure of 1 (i ¼ ꢂx þ 2, y, ꢂz þ 1/2). (b) Content of
nitrogen of the pyridine ring and both carboxyl groups consist of carbonyl and hydroxyl
groups. Each H DPA-Cl molecule generated O-HꢀꢀꢀO and O-HꢀꢀꢀN hydrogen bonds with
2
crystallization water molecule to form endless chains propagated along the b crystallo-
graphic axis. Formation of 1D supramolecular polymeric chain is depicted in Figure 1c
and hydrogen bond lengths and angles are summarized in Supplementary Information
Table S2. Bond distances and angles are similar to the values presented previously [20]
and they change in the range 1.374(4)–1.507(4) Å for C-C, 1.203(4)–1.217(4) Å for C-O,
ꢃ
ꢃ
ꢃ
1
17.0(3)–119.6(3) for C-C-C and 110.8(3) , 123.2(3) for O-C-C.
3.1.2. Crystal structure of 1
The crystal structure of [Ag(HDPA-Cl)(H DPA-Cl)ꢀ2H O] is depicted in Figure 2a,b and
2
2
selected bond distances and angles are summarized in Supplementary Information
Table S3. The prepared complex crystallized in a monoclinic lattice with space group
C2/c, with cell dimensions a ¼ 6.5673(3) Å, b ¼ 11.6494(5) Å, c ¼ 24.1220(12) Å,
ꢃ
b ¼ 93.017(2) and with four formula units in the unit cell. The asymmetric unit is
formed of one silver(I), one ligand molecule and one crystal water molecule. Each
DPA-Cl ligand is tridentate to Ag(I) through the pyridine N and two monodentate
carboxyl O atoms. The silver is six-coordinate and the coordination polyhedron can be
described as a distorted tetragonal bipyramid. The coordination environment is
formed by four oxygens and two nitrogens of two crystallographically equivalent lig-
and molecules with a [AgN O ] donor set. The interaction between Ag1-N1 (2.324(3)
2
4
Å) is stronger than Ag1-O1 and Ag1-O3. The Ag1-O1/O3 distances are 2.558(3) Å and
.617(6) Å, suggesting weak interactions. Although the Ag-O distances in 1 are longer,
2

ꢀ
Z. VARGOVA ET AL.
8
Figure 3. (a) Hydrogen bonds generated between carboxylic/carboxylate groups of ligand mole-
cules and crystallization water molecule for 1 (i ¼ ꢂx þ 2, y, ꢂz þ 1/2; ii ¼ x ꢂ 1, y, z; iii ¼ x þ 1/
2, y þ 1/2, z). (b) Formation of final 2D supramolecular sheets.
they are comparable with bond distances of other silver complexes containing coordi-
nated protonated carboxylic ligands [21–25].
The interesting feature of 1 is that carboxy group with carbonyl C1 is semideproto-
nated and forming a symmetric hydrogen bond with a carboxy group of a neighbor-
ing complex (Figure 3a). H1 lies on a center of symmetry and is involved in a strong
i
O1-H1ꢀꢀꢀO1 hydrogen bond (i ¼ ꢂx þ 2, y, ꢂz þ 1/2; d(DꢀꢀꢀA) ¼ 2.451(5) Å) and the O-
H bond distance is 1.226(4) Å. Medium strength hydrogen bonds are formed between
ii
crystallization water molecules and carboxylic groups. O5 is an acceptor in O5ꢀꢀꢀH4 -
ii
iii
O4 (ii ¼ xꢂ1, y, z; d(DꢀꢀꢀA) ¼ 2.532(4) Å) hydrogen bond formation and donor for O2
i
(
iii ¼ x þ 1/2, y þ 1/2, z; d(DꢀꢀꢀA) ¼ 2.686(4) Å) and O3 (d(DꢀꢀꢀA) ¼ 2.933(4) Å).
Hydrogen bonds are depicted in Figure 3a and corresponding bond lengths and
angles are summarized in Supplementary Information Table S4.
The crystal structure shows 1 is a monomer, however, via formation of strong
hydrogen bonds between adjacent complex units and medium strength hydrogen
bonds formed through the crystallization water molecules, the final crystal packing of
1
is two-dimensional supramolecular sheets (Figure 3b).
For comparison, a similar complex, [Ag(HDPA)(H DPA)ꢀ2H O] [26, 27], was prepared
2
2
using dipicolinate ligand (DPA). The coordination environment of Ag(I) ions and coord-
ination fashion of DPA ligand is similar to 1. The difference between our compound
and [Ag(HDPA)(H DPA)ꢀ2H O] is in ligands and interactions of silver with donors.
2
2
However, the bond distance between Ag-N in dipicolinate complex is similar (2.326(4)
Å); the presence of chloro group as electron-withdrawing group on pyridine ring in
DPA-Cl ligand had an impact on Ag-O bond lengths. Corresponding atom distances in
1
are longer (see text above) compared to [Ag(HDPA)(H DPA)ꢀ2H O] in which Ag-O
2
2
bond lengths are 2.505(2) Å and 2.507(3) Å, indicating stronger interaction between
coordinated carboxylic group and silver(I) ion.
3.1.3. Crystal structure of 2
The cadmium(II) compound containing 4-chloropyridine-2,6-dicarboxylate was pre-
pared as single crystals and the solid-state structure is depicted in Figure 4a–c. Results

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 4. (a) Atomic labelling diagram of f[Cd(l -H O)(H O)(PDA-Cl)]g (i ¼ 1ꢂx, 1ꢂy, ꢂz; ii ¼
2
2
2
n
ꢂx, 1ꢂy, ꢂz). (b) Final 1D polymeric chains propagated along the c crystallographic axis. (c)
Content of unit cell.
of X-ray diffraction studies show that the compound crystallizes in a triclinic lattice
with space group P, having cell parameters a ¼ 5.3994(2) Å, b ¼ 8.8389(4) Å,
ꢃ
ꢃ
ꢃ
c ¼ 10.5007(5) Å, a ¼ 101.207(2) , b ¼ 95.930(2) , and c ¼ 93.338(2) . The asymmetric
unit is formed from one cadmium(II), one PDA-Cl ligand and two water molecules
coordinated in different modes. Cd(II) ions are seven-coordinate and the coordination
polyhedron could be described as a distorted pentagonal bipyramid. The equatorial
i
plane is formed from three oxygens (O1, O1 and O3; i ¼ 1ꢂx, 1ꢂy, ꢂz), one nitrogen
(
N1) of two DPA-Cl molecules and a coordinated water (O5). The axial positions are
ii
occupied by two oxygens (O6 and O5 , ii ¼ ꢂx, 1ꢂy, ꢂz) of two different water mole-
cules as depicted in Figure 4a. Both carboxylic groups of H PDA-Cl acid are deproto-
2
nated and coordinated to cadmium in different coordination modes. Carboxylate with
C4 is monodentate mode and carboxylate (C1) makes a bridge between two
cadmium(II) ions via O1. The distances between cadmium(II) ions and carboxylate oxy-
gens are 2.3469(18)–2.4286(18) Å and are typical for similar cadmium carboxylate com-
pounds [28, 29, 30, 31]. Pyridine nitrogen of PDA-Cl ligand is coordinated to cadmium
with shorter bond length 2.299(2) Å, indicating stronger interaction with Cd(II). As
mentioned above, there are two water molecules coordinated in different coordination
fashion in the crystal structure of 2. Water molecule (O6) is terminally coordinated to
Cd(II) with bond length 2.2944(19) Å and the other water molecule (O5) generates a
bridge between two cadmium ions; the Cd-O5 bond is therefore longer (2.3145(19) Å,
Supplementary Information Table S5). The bond angles around cadmium(II) indicate
distortion from ideal pentagonal–bipyramidal geometry, which is reflected especially
ii
ꢃ
ꢃ
in the axial angle O6-Cd-O5 of 155.588(61) and in the equatorial bite angles
ꢃ
i
ꢃ
i
O3–Cd–N1 (69.560(67) ), N1–Cd–O1 (68.306(66) ), O1–Cd–O1 (73.626(68) ), O1 –Cd–O5
ꢃ
ꢃ
(
74.614(62) ) or O5–Cd–O3 (76.248(66) ).

ꢀ
Z. VARGOVA ET AL.
1
0
Figure 5. (a) Intramolecular and intermolecular hydrogen bonds in f[Cd(l -H O)(H O)(PDA-Cl)]g
2
2
2
n
(
i ¼ 1ꢂx, 1ꢂy, ꢂz; iii ¼ x ꢂ 1, y, z; iv ¼ ꢂx, ꢂy, ꢂz). (b) View of the final supramolecular 2D poly-
meric sheets propagated along ac crystallographic plane.
Because of one bridging carboxylate O and two bridging water molecules, the adja-
cent cadmium(II) ions are joined to the formation of 1D polymeric chains propagating
along the a crystallographic axis (Figure 4b). Cadmium ions are arranged in zig-zag
ii
i
chains with CdꢀꢀꢀCd separation of 3.7164(3) Å for Cd-Cd and 3.8235(3) Å for Cd-Cd .
Due to the presence of water molecules in the crystal structure of 2, there is a rich net-
work of hydrogen bonds (Figure 5). Intramolecular hydrogen bonds are formed between
bridging water (O5) acting as donor, while the acceptor is O6 of the terminal water molecule
iii
having both the O5-H5AꢀꢀꢀO6 (iii ¼ xꢂ1, y, z) distance 2.819(3) Å and the carboxylate
i
oxygen O2 with O5-H5BꢀꢀꢀO2 distance 2.578(3) Å. The water O6 creates an intermolecular
hydrogen bond as donor with carboxylate O4 from an adjacent polymeric chain with O6-
iv
H6BꢀꢀꢀO4 (iv ¼ ꢂx, ꢂy, ꢂz) distance of 2.608(3) Å. The polymeric chains are stabilized in the
solid state by hydrogen bonds with formation of supramolecular 2D polymeric sheets propa-
gating along the ac crystallographic plane (Figure 5b). Corresponding hydrogen bond
lengths and angles are summarized in Supplementary Information Table S6.
Similar coordination of ligands to cadmium(II) ions was observed in f[Cd (DPA-
2
OH) (H O) ]ꢀ4H Og [32] and f[Cd (DPA-OH) (H O) ]ꢀ0.5MeOHꢀ3H Og [33] (DPA-OH ¼
2
2
2
2
n
2
2
2
2
2
n
4-hydroxypyridine-2,6-dicarboxylate, chelidamate). The compounds are also seven-
coordinate binuclear polymeric complexes with distorted pentagonal bipyramidal
geometry around Cd(II). In their binuclear monomeric units, cadmiums are joined by
the O atoms of two bridging tridentate DPA-OH ligands and the polymer propagates
via two bridging water molecules that link each Cd(II) of one monomer to the neigh-
bor. Their Cd–O and Cd–N distances are in the same range as 2. The difference
between 2 and Cd-chelidamate complexes is in the number and type of crystallization
solvent molecules, which separate 1D chains from each other and are able to form dif-
ferent types of hydrogen bonds.
3.2. Infrared spectroscopy
Comparing IR spectra of H PDA and H PDA-Cl (Supplementary Information Figure S1), the
2
2
absorptions slightly change their positions, attributed to the chlorine in the ortho position.

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 6. Infrared spectra of prepared compounds.
Table 2. The IR spectral data assignments for H DPA, H DPA-Cl and 1-3 (band positions are
2
2
ꢂ1
in cm ).
–
1
Wavenumbers (cm ) and intensities
Assignments
2
H PDA
H
2
PDACl
1
2
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(OH)
(CH)
3479(w)
3092(w)
1726(s)
3490(w)
3089(w)
1707(m)
1660(w)
1573(m)
1468(m)
1412(m)
3330(vw)
3060(w)
3065(w)
1692(s)
3074(w)
(C ¼ O)
–
as(COO )
1645(w)
1562(m)
1428(m)
1385(m)
1590(s)
1569(s)
1430(s)
1371(s)
(C ¼ C)
1573(m)
1454(m)
1411(m)
1575(m)
1477(w)
1439(w)
(C ¼ N)
–
s
(COO )
(C–O)þd(OH) 1295(m), 1263(m) 1311(s), 1211(m) 1295(m), 1250(m) 1330(m), 1266(m) 1311(m), 1263(m)
d(CCH)
c(CCH)
d(COO )
D(ꢀas – ꢀ
D(ꢀas – ꢀ
1161(m), 1079(m) 1170(s), 1050(w)
1003(w)
783(m)
702(m)
248
1021(w)
765(w), 723(m)
640(m)
1014(m)
764(s), 730(s)
600(m)
783(s), 748(s)
692(s), 646(s)
783(m), 729(m)
677(s)
ꢂ
s
)
)
exp
calc
260
234
219
s
246
s, strong; m, medium; w, weak.
Comparing to IR data of complexes, the observed shifts and splitting of corresponding
bands reveal ligand coordination to Ag(I), Cd(II) even the Pb(II). The water molecule and
hydrogen bonds in the structure are associated with stretching vibrational mode ꢀ(O-H) at
ꢂ1
3
479 (for H PDA-Cl), 3490 (for 1) and 3330cm (for 2), similar to the terbium(III) com-
2
plexes with 4-substituted dipicolinic acid analogues [13] and silver dipicolinate complex
[
26, 27]. Contrary, IR spectra indicate anhydrous complex 3, because of absence of ꢀ(O-H)
ꢂ1
(Figure 6). Furthermore, significant band at 1707cm confirms the carbonyl stretching
vibration in 1 that is in accordance with structural data. The characteristic free carboxylic
acid vibrations, ꢀ(C ¼ O) and ꢀ(C-O), are converted to asymmetric, ꢀ (COO), and symmet-
as
ric, ꢀ (COO) stretching vibrations of carboxylate as expected after its coordination to cen-
s
tral atom in 1, 2, and 3 that also confirm its bond distances and angles change in free
ligand and in complexes (Section 3.1). The wavenumber difference, D(ꢀ
ꢀs), is consistent
as ꢂ
with monodentate coordination of the carboxylate to Ag(I), Cd(II), and Pb(II) (Table 2), in
agreement with structural results (carboxylate monodentate coordination mode). The
same monodentate coordination mode was assigned to dipicolinate for Cd(II) and Pb(II)

ꢀ
Z. VARGOVA ET AL.
1
2
Figure 7. Scheme of in-plane bending vibrations with Wilso nꢂs notation and comparison of
ꢂ1
cadmium(II), lead(II) compound and free H PDA-Cl ligand in wavenumber region 1200–800 cm
.
2
ꢂ1
complexes, D(ꢀ
ꢀ ) ¼ 212 and 220cm , respectively [28, 34]. However, ꢀ (COO) at
as ꢂ
s
as
ꢂ1
ꢂ1
ꢂ1
lower wavenumber (1587, 1700 cm [28, 34]) than in 2 (1645 cm ) and in 3 (1590 cm ).
Similar differences were observed for symmetric stretching vibrations in the case of Cd(II)
ꢂ1
ꢂ1
ꢂ1
complexes (1375 cm for a Cd(II) complex [28], 1385cm for 2). However, symmetric
stretch comparison (in the case of Pb(II) complexes), 1480 cm for Pb(II) complex [34] and
371cm for 3, respectively. This points to the fact that carboxylates are involved in differ-
ꢂ1
1
ent systems of hydrogen bonds.
Both deformation vibrations were observed in all cases (ligands and 1, 2, and 3),
ꢂ1
the in-plane bending modes d(CCH) from 1180 to 800 cm and out-of-plane bending
ꢂ1
modes c(CCH) from 790 to 600 cm (Table 2).
Additionally, to confirm PDA-Cl coordination to Pb(II) we correlated pyridine ring
in-plane bending vibrations of free ligand and 2 (crystalline products with deproto-
nated ligand in the structure) and 3 (precipitate product). The pyridine ring vibra-
ꢂ1
tions are 1200 to 850 cm
[35]. The typical ring vibrations in accordance with
Wilson ꢂs notation [36] at 964, 995(1), 1007, 1045(12), 1044, 1085, (18b), 1083, 1093
ꢂ1
(9b), 1250, 1254(15), 1292, 1292(3) cm
were noted for PIC and AgPIC [37].
Significant changes were observed in the case of PDA-Cl ligand and its Cd(II) and
Pb(II) complexes comparing mentioned bands range. The free ligand pyridine ring
vibrations were noted at 846(1), 899(12), 997(18b), 1109(9b), 1171(3, 15). Spectra
indicate 3 and 15 vibrations overlap. In-plane bending vibrations 1, 12, 18b and 15
(and 3) change their positions to higher wavenumbers and 9b to lower values in 2
and 3 (Figure 7). Observed wavenumber shifts confirm PDA-Cl ligand coordination
to Pb(II) in 3.

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 8. Thermoanalytical curves of prepared complexes (a) [Ag(H PDA-Cl)(HPDA-Cl)]ꢀ2H O, (b)
2
2
f[Cd(l -H O)(H O)(PDA-Cl)]g and (c) [Pb(PDA-Cl)] measured in air atmosphere. Inset shows a
2
2
2
n
ꢃ
detailed view on the dehydration process of compounds in temperature range 30–200 C.
3.3. Thermal analysis
To estimate the thermal stability of our compounds, thermogravimetric analysis was
carried out under air (TG curves shown in Figure 8). [Ag(H PDA-Cl)(HPDA-Cl)]ꢀ2H O is
2
2
ꢃ
thermally stable after heating at 70 C (inset of Figure 8). Above this temperature,
ꢃ
7
0–155 C, dehydration of 1 takes place in two decomposition steps. In mentioned
temperature range total mass loss of 6.41% was observed and corresponds to release
of two crystallization water molecules (Anal Calcd mass loss 6.60%). Observed decom-
position step could be divided into two separate processes in which step-by-step
ꢃ
dehydration of water molecules takes place, from 70–110 C (mass loss observed
ꢃ
3
.18%, calculated 3.30%) and 110–155 C (mass loss observed 3.23%, calculated
3
.30%). At higher temperature, decomposition of anhydrous complex occurs. From 155
ꢃ
to 270 C mass loss of 38.36% was observed (Anal calcd mass loss 36.91%) followed
with two steps on TG curve and this process was assigned to loss of H PDA-Cl mol-
2
ecule. This molecule is fully protonated and bonded with less energy compared to
deprotonated form of the ligand, which forms stronger interactions. Therefore, this
ꢂ
molecule is evolved before HPDA-Cl ligand. Based on literature data it is known that
ꢃ
H PDA-Cl melts at 210 C and evaporates [38]. Similar thermal behavior was also
2
observed in fAg(INA) (HINA)g (INA – isonicotinate), where protonated and deproto-
3
n
nated forms of isonicotinate are present. In the first stage of thermal decomposition
ꢂ
HINA was evolved and then release of INA ligands occurred [39]. The thermal decom-
ꢃ
position of 1 ends at 780 C and final decomposition product was elemental silver
(obs. residual mass 21.28%, Anal Calcd residual mass 19.76%).
As can be seen from the TG curve for f[Cd(l -H O)(H O)(PDA-Cl)]g (Figure 8), 2 is stable
2
2
2
n
ꢃ
in air to 85 C. When heating above this temperature, release of coordinated water takes
ꢃ
place. Dehydration occurs in two decomposition steps from 85 to 185 C, first step at
ꢃ
ꢃ
8
5–125 C with mass loss of 7.42% and the second step at 125–185 C with mass loss of
3.57%. Observed mass changes were attributed to the gradual release of 1.5 moles (Anal.
calcd mass loss 7.76%) and 0.5 mole (Anal. calcd mass loss 2.59%) of water molecules (inset

ꢀ
Z. VARGOVA ET AL.
1
4
Figure 9. Proposed molecular diagram of 3 derived from spectral and thermal studies.
of Figure 8). The total observed mass loss of the dehydration process is 10.35% (Anal. Calcd
mass loss 10.99%) and corresponds to the release of two water molecules, in agreement
with the crystal structure and the chemical composition of 2. The dehydrated form of 2 is
ꢃ
thermally stable from 185 to 380 C as is seen from the plateau on the TG curve (Figure 8).
The decomposition of f[Cd(PDA-Cl)]g takes place in four overlapping decomposition
n
2ꢂ
steps. In the first stage loss of chlorine from PDA-Cl occurs, which was reflected by the
mass change 10.60% on TG from 380 to 420 C (Anal. Calcd mass loss 10.19%). In further
ꢃ
decomposition steps thermal decomposition of residual organic moiety (probably dipicoli-
2
ꢂ
ꢃ
nate, PDA ) occurs and decomposition ends at 750 C. Final product of thermal decom-
position was CdO (obs. residual mass 39.94%, Anal. calcd residual mass 38.67%). Similar
polymeric complex containing the same building blocks (water, Cd(II) ions) was prepared
with dipicolinic acid with composition f[Cd(H O) (PDA)]g . After dehydration
2
1.5
n
ꢃ
ꢃ
(
150–220 C) this complex is stable to 330 C [28]. When we compare thermal stability of
anhydrous forms for 2 and Cd(II)-dipicolinate complex, it is obvious that f[Cd(PDA-Cl)]g
n
framework is more stable. This observation could be explained by positive mesomeric
effect of chloro substituent in molecular structure of 4-chloropyridine-2,6-dicarboxylate lig-
and. It is apparent that an electron-releasing group enhances the binding strength
between the carboxylate group and the central atom, which is reflected in higher ther-
mal stability.
As predicted by IR spectroscopy, [Pb(PDA-Cl)] was prepared in anhydrous state (see
Section 3.2) and TG analysis also confirmed this presumption. As can be seen from the
ꢃ
TG curve (Figure 8), 3 is thermally stable from 30 to 370 C without significant mass
change. The decomposition of organic part occurs in four stages as in 2. In the first
ꢃ
stage chlorine is evolved from 370 to 400 C with observed mass loss 8.29%, which is
in agreement with calculated value 8.72%. The decomposition of dipicolinic acid takes

JOURNAL OF COORDINATION CHEMISTRY
15
ꢃ
place in the next three overlapping stages from 400 to 710 C (obs. mass loss 37.36%,
Anal. calcd mass loss 36.41%). The final product of thermal decomposition of 3 was
PbO (obs. residual mass 55.45%, Anal. calcd residual mass 54.87%).
Based on the obtained results, the thermal stability of the prepared compounds
can be arranged in the following order:
ꢃ
ꢃ
½
AgðH PDA-ClÞðHPDA-ClÞꢄ ꢀ 2H O ð70 CÞ < f½Cdðl -H OÞðH OÞðPDA-ClÞꢄg ð85 CÞ
2
2
2
2
2
n
ꢃ
<
½PbðPDA-ClÞꢄð370 CÞ,
and for anhydrous forms in following order:
ꢃ
ꢃ
ꢃ
½
AgðH2PDA-ClÞðHPDA-ClÞꢄð155 CÞ < ½PbðPDA-ClÞꢄð370 CÞ < f½CdðPDA-ClÞꢄg ð380 CÞ:
n
The conclusion of thermal behavior for anhydrous compounds can be summarized
as follows: lower thermal stability of the silver(I) complex could be explained due to
the presence of the protonated (H PDA-Cl), which forms weak interactions with the
2
central atom and therefore is released at a lower temperature. In the case of the
cadmium(II) and lead(II) complexes, their thermal stability is comparable, because of
2
ꢂ
deprotonated ligand in both compounds (PDA-Cl ), which could form strong ion–ion
interaction with central atoms. The increased thermal stability of the cadmium com-
plex can be explained by the polymeric nature of the complex.
4. Conclusion
A new series of 4-chloro-dipicolinate metal complexes has been prepared and charac-
terized. The single crystal structure reveal the asymmetric unit formed from one
silver(I), one ligand molecule and one crystallization water molecule in the case of 1.
Each PDA-Cl ligand acts as a tridentate ligand of Ag(I), through the pyridine N and
two monodentate carboxyl O atoms. In the case of 2 the asymmetric unit is formed
from one cadmium(II) ion, one PDA-Cl ligand and two water molecules coordinated in
different modes. Cd(II) ions are seven-coordinate in a distorted pentagonal bipyramid.
The spectral, thermal and elemental analysis comparison confirms the [Pb(PDA-Cl)]
empirical formula for 3. The wavenumber differences, D(ꢀ
ꢀs), are consistent with
as ꢂ
monodentate coordination of the carboxylate to Ag(I), Cd(II), and Pb(II), in agreement
with structural results (carboxylate monodentate coordination mode). Moreover, PDA-
Cl, 2 and 3 in-plane bending vibrations (by Wilson ꢂs notation) confirm PDA-Cl ligand
coordination to Pb(II).
The thermal stability of the anhydrous compounds can be arranged in the following
ꢃ
ꢃ
order: [Ag(H PDA-Cl)(HPDA-Cl)] (155 C) < [Pb(PDA-Cl)] (370 C) < f[Cd(PDA-Cl)]g
2
n
ꢃ
(
380 C) and can be explained by the presence of the protonated ligand molecule
(H2PDA-Cl) in the silver(I) complex and deprotonated ligand molecule (PDA-Cl) in
cadmium(II) and lead(II) complexes.
From infrared spectroscopy and thermal analysis we proposed the molecular dia-
gram of 3 (Figure 9).
Disclosure statement
No potential conflict of interest was reported by the authors.

ꢀ
Z. VARGOVA ET AL.
1
6
Funding
This work was supported by the Ministry of the Education, Science, Research and Sport of the
Slovak Republic [grant number KEGA 008UPJS-4/2018]. Authors Z.V. and M.A. thank for the
financial support of the TRIANGEL team in the frame of the scheme “Top Research Teams in
Slovakia”. R.Gy. is grateful to OP VVV “Excellent Research Teams” (Project No. CZ.02.1.01/0.0/0.0/
15_003/0000417 – CUCAM).
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