Synthesis and physicochemical characterization of two lead(II) complexes with O-, N-donor ligands. Lone pair functionality and crystal structure

Article abstract of DOI:10.1016/j.saa.2014.07.084

A dinuclear [Pb₂(4-CHO-5-MeIm)₆(NO₃)₂](NO₃)₂ (1) and a polynuclear [Pb(2-pzc)₂(H₂O)]_n (2) complexes (where 5(4)-carbaldehyde-4(5)-methylimidazole (5(4)-CHO-4(5)-MeIm) and pyrazine-2-carboxylic acid (2-pzcH)) have been synthesized and characterized by elemental analysis, IR spectroscopy and X-ray crystallography. Structural determination for complex 1 reveals a cationic species [Pb(4-CHO-5-MeIm)₃]²⁺ connected through bridging nitrate(V) ions. There are also an uncoordinated nitrate ions as counterions. Complex 2 is a three-dimensional architecture consisting of Pb₆O₁₂ building units. The pyrazine-2-carboxylato ligand behaves as a chelating agent and a bi-connective bridge. The coordination polyhedra around lead(II) ion could be described as a distorted docecahedron (1) or monocapped trigonal prism (2). The luminescent properties of 1 and 2 investigated in the solid state at room temperature indicate structure-dependent photoluminescent properties. The DFT calculations and the X-ray structural data point on rather hemidirected type of coordination around Pb(II) ions of 1 and 2.

Full text of DOI:10.1016/j.saa.2014.07.084

Accepted Manuscript
Synthesis and physicochemical characterisation of two lead(II) complexes with
O-, N-donor ligands. Lone pair functionality and crystal structure
Joanna Masternak, Barbara Barszcz, Maciej Hodorowicz, Oleksiy V.
Khavryuchenko, Alina Majka
PII:
S1386-1425(14)01165-2
http://dx.doi.org/10.1016/j.saa.2014.07.084
SAA 12504
DOI:
Reference:
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
Received Date:
Revised Date:
Accepted Date:
29 November 2013
25 July 2014
30 July 2014
Please cite this article as: J. Masternak, B. Barszcz, M. Hodorowicz, O.V. Khavryuchenko, A. Majka, Synthesis
and physicochemical characterisation of two lead(II) complexes with O-, N-donor ligands. Lone pair functionality
and crystal structure, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://
dx.doi.org/10.1016/j.saa.2014.07.084
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Synthesis and physicochemical characterisation of two lead(II) complexes with O-, N-
donor ligands. Lone pair functionality and crystal structure.
a∗
a
b
Joanna Masternak , Barbara Barszcz , Maciej Hodorowicz ,
c
d
Oleksiy V. Khavryuchenko , Alina Majka
a
b
c
Institute of Chemistry, Jan Kochanowski University, 15G Świętokrzyska Str., 25-406 Kielce, Poland
Faculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060 Kraków, Poland
Chemistry Department, Taras Shevchenko National University of Kyiv, 64 Volodymyrska Str., Kyiv UA-
1601, Ukraine
0
d
Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka Str., 01-224 Warsaw, Poland
Abstract
A dinuclear [Pb
(H O)] (2) complexes (where 5(4)-carbaldehyde-4(5)-methylimidazole (5(4)-CHO-
(5)-MeIm) and pyrazine-2-carboxylic acid (2-pzcH)) have been synthesized and
characterized by elemental analysis, IR spectroscopy and X-ray crystallography. Structural
2 6 3 2 3 2
(4-CHO-5-MeIm) (NO ) ](NO ) (1) and a polynuclear [Pb(2-
pzc)
2
2
n
4
2
+
determination for complex 1 reveals a cationic species [Pb(4-CHO-5-MeIm)
through bridging nitrate(V) ions. There are also an uncoordinated nitrate ions as counterions.
Complex 2 is a three-dimensional architecture consisting of Pb 12 building units. The
3
]
connected
6
O
pyrazine-2-carboxylato ligand behaves as a chelating agent and a bi-connective bridge. The
coordination polyhedra around lead(II) ion could be described as a distorted docecahedron
(
1) or monocapped trigonal prism (2). The luminescent properties of 1 and 2 investigated in
the solid state at room temperature indicate structure-dependent photoluminescent
properties. The DFT calculations and the X-ray structural data point on rather hemidirected
type of coordination around Pb(II) ions of 1 and 2.
Keywords
Lead(II) complexes, X-ray structure, Lone pair steroactivity, Luminescence properties
Introduction
Of all p-block elements, lead(II) has a particular fascination for coordination
chemists as it can adopt many different geometries in its complex. Extensive structural
studies on this subject give some evidence why Pb(II) as a central ion in complexes displays
such an interesting feature. The literature data mainly pointed on: (i) the effect of so-called
“
inert electron-pair”, which means the size and extent of the lone-pair in the coordination
sphere, (ii) the broad range of coordination numbers from 2 to 12, (iii) the kind of donating
ligands and their flexibility.
Moreover, the interest of coordination chemistry of Pb(II) is also motivated by the
toxicity of lead and its widespread occurrence in the environment due to its numerous
industrial applications [1-5]. The coordination properties of lead over other essential metal
ions is crucial to understand the toxicological properties of this metal, the design of selective
∗
Corresponding author. Tel.: +48 (41) 349 70 39.
E-mail address: joanna.masternak@ujk.edu.pl
1

chelation therapy agents, and the development of efficient chelating agents for the
remediation of polluted water and soil. On the other hand, lead has been used as a heavy
atom derivative to solve a crystal structure of proteins, and inspection of these structures
provides useful insights into the biological chemistry of Pb(II). A landmark in the field of
RNA biochemistry was the crystal structure of the transfer RNA for phenylalanine
(
tRNAPhe) because it constituted the first crystal structure of a transfer RNA molecule and
laid the foundation for the field of catalytic RNA [6,7]. Although considerably larger than
both Ca(II) and Zn(II), Pb(II) can bind to both calcium- and zinc-binding sites in proteins [8-
1
2]. In both cases, lead binds a high coordination number site than that is made up of
carboxylate groups.
In view of the aforementioned, we have successfully synthesized Pb(II) coordination
dimmer and polymer using 5(4)-carbaldehyde-4(5)-methylimidazole (5(4)-CHO-4(5)-
MeIm) and pyrazine-2-carboxylic acid (2-pzcH) as ligands, namely [Pb (4-CHO-5-
MeIm) (NO ](NO (1), [Pb(2-pzc) (H O)] (2). To realize the one of the goal of this
2
6
3
)
2
3
)
2
2
2
n
work we select 5(4)-CHO-4(5)-MeIm and 2-pzcH as ligands because from bioinorganic
point of view, heterocyclic aldehydes and acids are of great importance as compounds
containing N,O-donor atoms which can be adopted to modeling coordination environment of
toxic metal ions in biological systems. Both complexes have been characterised by X-ray
single crystal structure analysis. The spectral (FTIR, PL) and thermogravimetric
characterisation was also performed. Additionally, a possible stereochemical activity of the
lone pair in the complexes obtained has been discussed by us based on the molecular
structure of the complexes provided by DFT calculations. Such investigations are very
important because despite the fact that coordination environment of Pb(II) has been
subjected to detailed analyses it is frequently difficult to establish a proper coordination
number and geometry. According to the literature data, holo- and hemidirected geometry are
two general structural types of lead(II) complexes [13,14]. But in complexes with an
intermediate coordination number (6-8) the prediction is less reliable because both types of
arrangement can be observed.
Experimental
Materials and physical measurements
All of the starting materials were commercially available reagents for analytical
grade and were used without further purification. Lead(II) nitrate and acetate were
purchased from the Merck Chemical Company, 5(4)-CHO-4(5)-MeIm and 2-pzcH from
Sigma-Aldrich. Elemental analyses (C, H and N) were performed on VarioMicro Cube
elemental analyzer. Infrared (FTIR) spectra were measured on Nicolet 380
-
1
spectrophotometer in the spectral range 4000-500 cm
Thermogravimetric analyses were carried out using a TG/SDTA 851 METTLER-TOLEDO
thermobalance with Al crucible. The thermoanalytical curves were obtained using
STAR System METTLER-TOLEDO software. The powder X-ray diffraction (XRD)
patterns of the products of decomposition were recorded by DRON-2 using CuK radiation
λ = 1.54178 Å) over 2θ angle range 8-90°. Fluorescence spectra of solid samples were
using a KBr pellet.
e
2
O
3
e
α
(
recorded with a Spex Fluorolog 3 spectrofluorometer equipped with a Quanta-ϕ integrating
2

sphere. In order to further clarify the origin of photoluminescence, the solid-state emission
spectra of complexes and ligands were measured by varying the excitation wavelengths
under the same metrical conditions.
Single crystal X-ray diffraction studies
Diffraction intensity data for single crystal of lead(II) complexes were collected at room
temperature on a KappaCCD (Nonius) diffractometer with graphite-monochromated MoK
radiation (λ = 0.71073 Å). Corrections for Lorentz, polarization and absorption effects
15,16] were applied. The structure was solved by direct methods using SIR-92 program
α
[
2
package [17] and refined using a full-matrix least square procedure on F using SHELXL-
013 [18]. Anisotropic displacement parameters for all non-hydrogen atoms and isotropic
2
temperature factors for hydrogen atoms were introduced. In the structure, the hydrogen
atoms connected to carbon atoms were included in calculated positions from the geometry of
molecules, whereas hydrogen atoms of water molecules were included from the difference
maps and were refined with isotropic thermal parameters. The figures were made using
DIAMOND software [19].
Quantum-chemical method
Quantum-chemical calculations were performed in the framework of density-functional
theory (DFT). Hybrid three-parameter Becke Lee-Yang-Parr functional B3LYP [20]
together with Ahlrichs’ triple-zeta split-valence basis set augmented by Coulomb fitting
(
def2-TZVPP) [21] using resolution of the identity approximation [22]. Stuttgart-Dresden
effective core potential (ECP) for 60 core electrons of Pb was used [23]. The ORCA ab
initio, DFT and semiempirical SCF-MO package [24] was used for all calculations.
Monomeric form of 1 with two nitrate counter-ions and truncated dinuclear part of 2 with
‘
outer’ coordination positions saturated with hydrogen atoms were used as models for the
DFT calculations. Positions of hydrogen atoms were optimized, while other atoms retained
the same coordinates as determined from single-crystal X-ray analysis. Molecular orbitals
were localized for analysis using Foster-Boys (spatial localization) algorithm [25].
Syntheses
2 6 3 2 3 2
Preparation of [Pb (4-CHO-5-MeIm) (NO ) ](NO ) (1)
3
The complex was prepared by layering method. A water solution (10 cm ) of lead(II) nitrate
3
(
1 mmol) was carefully layered over 20 cm of isopropanol-water (1:3, v/v) solution of 5(4)-
CHO-4(5)-MeIm (3 mmol). Colourless crystals were finally formed within six weeks (at
°C in a refrigerator). The product was collected in 39% yield. Anal. calcd (%) for
Pb (661.56): C, 27.23; H, 2.74; N, 16.94; Found (%): C, 27.60; H, 2.53; N,
4
C
1
15
H
18
N
8
O
9
−1
6.77. FTIR (cm ): 3320 (m), 3120 (w), 3066 (w), 3001 (w), 2933 (m), 2885 (m), 2857
(
(
m), 2811 (m), 1767 (w), 1738 (w), 1651 (s), 1625 (m), 1584 (w), 1517 (w), 1446 (m), 1415
m), 1356 (m), 1303 (s), 1251 (m), 1107 (w), 1093 (w), 1033 (m), 948 (s), 867 (w), 806 (s),
7
21 (m).
Preparation of [Pb(2-pzc)
2
(H
2
O)] (2)
n
3

The formation of the complex was accomplished by neutralization of the pyrazine-2-
carboxylic acid (2-pzcH). Lead(II) nitrate (1 mmol) was dissolved in warm distilled water
3
3
(
(
(
10 cm ) and added to an aqueous solution (15 cm ) of sodium pyrazine-2-carboxylate
generated in situ by reacting (2 mmol) pyrazine-2-carboxylic acid with sodium bicarbonate
2 mmol)) under continuous stirring. The suspension was heated at 40 °C until a clear
solution was obtained. The solution was cooled and left at room temperature. After five
weeks the colourless crystals suitable for X-ray crystallography were obtained. Yield 31%
(
1
based on Pb(II) salt). Anal. calcd (%) for C10
1.88; Found (%): C, 25.41; H, 1.58; N, 11.82. FTIR (cm ): 3369 (m, br), 1592 (s), 1580
H
8
N
4
O Pb (471.40): C, 25.48; H, 1.71; N,
5
−1
(
(
s), 1522 (m), 1459 (w), 1410 (m), 1359 (s), 1319 (m), 1280 (m), 1157 (s), 1047 (m), 1026
s), 819 (m), 789 (s), 718 (s).
Results and discussion
Description of the structures
We describe in detail the structure of [Pb
2 6 3 2 3 2
(4-CHO-5-MeIm) (NO ) ](NO ) (1) and
[
Pb(2-pzc) (H O)] (2) complexes which were determined by X-ray crystallography. Crystal
2
2
n
data (selected bond lengths, valence angles, hydrogen bonds) and structure refinement of
these compounds are given in Tables 1-4.
Table 1
Table 2
Table 3
Table 4
Crystal structure of complex 1
A single crystal X-ray structural analysis shows that complex 1 consists of a neutral
2
+
dimmeric coordination unit which consist of [Pb
2
(4-CHO-5-MeIm)
anions (see Fig. 1). Each lead atom is chelated by the nitrogen and oxygen
atoms of three 4-carbaldehyde-5-methylimidazole ligands with Pb–N distances ranging from
.457(3) to 2.703(3) Å and Pb–O distances of 2.596(3), 2.667(3) and 2.803(3) Å which is in
the normal range and well comparable with such complexes [13 and literature cited there].
6
(NO
3
)
2
]
cation and
-
two free NO
3
2
+
3 3
Two [Pb(4-CHO-5-MeIm) (NO )] are bridged into a centrosymmetric dimmer by two
bidentate bridging nitrate ligand in µ-O, O’ fashion. The presence of a lone pair of the lead
ion, is apparently the reason why the bridging interactions are so long. The interaction of
-
lead(II) with oxygen atoms of nitrate anions (Pb–O(NO
3
) distances 2.911(4) and 3.019(3)
Å) are weaker [26] but not so long as bonds in hemidirected lead(II) complexes presented in
literature [27]. Simultaneously, all Pb-N and Pb-O distances are shorter than the sum of the
ionic radii (3.54 Å and 3.40 Å, respectively) [28-34], therefore these Pb ions have significant
first-order Pb-N and first-, second-order Pb-O interactions. In fact the coordination number
of Pb(II) in this complex is eight (CN=8): six normal bonds with donor atoms of 4-CHO-5-
MeIm and two “weak” Pb···O bonds of nitrite ligands. Additionally, the lone pair impact is
4

plainly evident in distortion of all valence angles (Table 3). Therefore, coordination
polyhedron can be considered a pseudo-dodecahedron. Additionally, the arrangement of
donor atoms suggests gaps or holes in the coordination geometry at the site of stereo-active
lone pair of electrons. The coordination sphere of Pb(II) could be best described as “pseudo-
hemidirected” (i.e., transition between hemidirected and holodirected) [35].
In the crystal structure of 1 there are intramolecular hydrogen bonds of the N-H···O
type formed by the imidazole N atoms as proton donors and nitrate atoms (see Table 2). In
addition, the weak intermolecular interaction (N-H···O and C-H···O type), between the
adjacent complex molecules are responsible for the molecular packing (Fig. 2).
Fig. 1.
Fig. 2.
Crystal structure of complex 2
The X-ray analysis of compound 2, of formulation [Pb(2-pzc)
2
(H
2
O)] , revealed that
n
-
it is composed of coordination polymer (Fig. 3). Each lead atom is chelated by two 2-pzc
ligands through the pyridine N-donor and one carboxylate oxygen atom. The bond distances
associated to the chelating ligands are Pb–N 2.591(4) Å and Pb-O 2.364(3) Å and 2.585(4)
-
Å. It is interesting to note that both 2-pzc ligands are coordinated to the central Pb(II) ion in
-
different bridging modes. One of the 2-pzc anions is coordinated to two different Pb(II) ions
through one of the carboxylate oxygen atoms (O(8B)(-x+1,-y,-z)), which functions as a
2
2
-
monoatomic bridge (µ -η ) (Fig. 4a). The second independent 2-pzc ligand functions as a
2
1
1
bridging bidentate ligand (µ -η :η ) which is linked to two symmetry related Pb(II) ions via
O(8A) and O(9A) oxygen atoms (Fig. 4b). Moreover, lead ion is also coordinated by oxygen
atom from water molecule (Pb-O(1) bond distance 2.596(3) Å). Thus, the Pb(II) ion
coordinates to four carboxylate oxygen atoms and one oxygen from water molecule. The
coordination sphere of Pb(II) ion is completed by two pyridine-like nitrogen atom (N(1A)
2 5
and N(1B) giving {PbN O } chromophore, resulting in coordination polyhedron described
as distorted monocapped trigonal prism. The most interesting feature of the crystal packing
is existence of mesh network which is built of two kinds of cavities: (1) the small one
constructed by two lead(II) ions bridged by the carboxylate groups (Pb···Pb distance 4.163
-
Å), (2) the bigger are built up six Pb(II) ions joined by six bidentate bridging 2-pzc ligands,
leading to the formation of a 2-D infinite network, as illustrated in Fig. 5a building blocks
along [1 0 1] plane.
The crystal polymeric structure is stabilized by typical and trifurcated hydrogen
bonds (Fig. 5b). A scrutiny of the structure reveals that the noncoordinating oxygen atom of
the carboxylate group (O(9B) and the coordinated water as well as nitrogen atom (N(4))
from pyrazine ring are involved in C–H···O and C–H···N interactions. The H-bonding
interactions with H···O(N) distances ranging from 2.54 to 2.62 Å (Table 2) are observed.
These can be classified as intra- and interchain interactions.
Fig. 3.
5

Fig. 4.
Fig. 5.
DFT calculations
Optical activity of Pb(II) compounds is often related to spatial localization of its
valent s-orbital [12,13]. In order to check if the compound synthesized are potentially
interesting for optics we have performed DFT calculations on selected structures. Such
2
+
investigations are also very important for prediction of geometry around Pb ion in
complexes with intermediate coordination number (6-8) in which both holo- and
hemidirected arrangement can be observed. The results are controversial: on the one hand
some orbitals of Pb are significantly asymmetric - more localized in the “empty” hemisphere
of the lead surrounding; on the other hand they do not have essential contributions from s-
orbitals. The most asymmetrically localized molecular orbitals of 1 and 2 are demonstrated
in Fig. 6, while complete data, as well as protocols of calculations can be found in
Supplementary materials (Fig. S1, S2). One can see that there is an obvious asymmetry of
the electrons localization on certain orbitals (although total electronic density around Pb
atom is rather spherical). However, the contributions to those localized orbitals (25% of s-,
2
5% of p-, 49% of d-orbitals for MO#59 of 1; 32% of s-, 25% of p-, 40% of d-orbitals for
MO#81 of 1; 8% of s-, 40% of p-, 51% of d-orbitals for MO#35 of 2) proves that the main
role in localizing of electron density in the space responsible for the potential stereoactivity
of Pb(II) complexes is due to d-orbitals. This seems logical, since s-orbitals are spherical
and, thus, cannot have selected direction with higher localization, while p-orbitals are often
strongly involved into forming covalent bonds with donating atoms of ligands. Such
distribution of electronic density around Pb atom of 1 and 2 point at a hemidirected type of
coordination.
Fig. 6.
Spectral studies
FTIR analysis
Selected vibration bands of free ligands and their Pb(II)complexes, which are useful
for determining the ligand’s mode of coordination, are given in Table 5. In the FTIR spectra
-
1
(
Fig. S3) the ν(C=O) bands of 5(4)-CHO-4(5)-MeIm appears at 1663 cm , typical of an
-
1
aldehyde group [36] and it is slightly red-shifted in complex 1 (ca. 12 cm ). A strong peak
-1
at 1458 cm , assigned to the ν(C=N) of heteroaromatic imidazole ring vibrations is also
shifted to lower energy (1446 cm ). Such shifts in the mentioned bands upon coordination
are consistent with bidentate coordination of the ligand through carbonyl oxygen and
-
1
pyridine-like nitrogen atom of 5(4)-CHO-4(5)-MeIm. In addition, the IR spectra have
-
presented evidence for the coordination mode of NO
3
. The frequency difference of the
-1
vibrations ∆ν = 29 (∆ν = 1767 - 1738 cm ) indicate a bidentate, bridging coordination mode
of nitrates in 1 [37-39]. Moreover, a characteristic band of the NO counterion at about
-
3
6

-1
1
356 cm was observed [26]. These features are consistent with the X-ray diffraction data
for complex 1.
In the FTIR spectrum of 2 (Fig. S3), the strong and broad absorption band in the
-1
-1
range of 3200-3500 cm (max. 3369 cm ) is assigned as the characteristic peak of ν(O-H)
vibration from the coordinated water molecules present in the crystal structure. Moreover,
the spectra of 2 confirm the structural data indicating that the carboxylic group of ligand is
-
1
deprotonated. Strong absorption bands of the free 2-pzcH ligand at 1707 and 1392 cm
which correspond to the asymmetric and symmetric ν(COOH) vibrations after coordination
-
1
to the Pb(II) ions were shifted to lower wavenumbers and were observed at 1592, 1580 cm
-
1
and 1359, 1319 cm , respectively. Band splitting of the stretching vibrations of the
carboxylate group, suggest that 2-pzc exhibited a different coordination mode. It may
-
1
1
2
function as: (a) bridging (µ
2
-η :η ) mode and (b) bridging (µ
2
-η ) mode (see Fig. 4a, b).
The differences between the asymmetric and symmetric stretches of the carboxylate groups
-
1
of complex 2, ∆ν = 233 and 261 cm , give some qualitative information which allows the
correlation of the data of the infrared spectra with the coordination modes of lead(II)
carboxylates (a) and (b) respectively [40-43].
Table 5
Luminescence spectra
Complexes 1, 2 reported in this work exhibit the photoluminescence properties at
room temperature in the solid-state (Fig. 7). The luminescent spectrum of 1 indicated an
emission maximum at 575 nm upon excitation at 350 nm (Fig. 7a). In order to assess the
emission behaviour, comparison with the corresponding spectrum of the 5(4)-CHO-4(5)-
MeIm as ligand was made. The free ligand exhibited emission band with maxima at 554 nm
upon excitation with the same wavelength. The red-shifted band can be assigned to metal-to-
2
+
ligand charge transfer (MLCT) [44] transition with electrons being transferred from Pb
2
(
6s ) lone pair to the unoccupied π* orbitals of 4-CHO-5-MeIm. On the other hand these
bands can be derived from intra atomic transitions from the Pb(II) 6s orbital to the Pb(II) 6p
orbital [45,46].
2
Upon replacement of the 4-CHO-5-MeIm ligand by 2-pzcH a broad emission shifted
to the blue part of the spectra in complex 2 was observed. Polymer display a broad emission
band with maxima at 538 nm upon excitation with wavelength of 350 nm. As shown in Fig.
7
b the free ligand, 2-pzcH shown a broad maximum at 553 nm upon excitation with the
same wavelength. Hypsochromic shift in complex 2 has the same background as was
mentioned above for 1 but the emission band has a significantly stronger luminescence
intensity compare to complex 1 (Fig. 8).
These phenomena indicate that the ligands participates in the emission of complexes
and the Pb(II) monomer in comparison to polymer is a weaker emitter. That may be
attributed to the coordination diversity of the lead(II) centres and the rigidity difference of
these lead(II) complexes [47].
Fig. 7.
7

Fig. 8.
Thermogravimetric analyses
The TG/DTG/SDTA of compounds 1, 2 were performed on crystalline samples
-1
under an air atmosphere from 25 to 800 °C with heating rate 5° min . The TG curve of 1
see Fig. 9) has indicated that it is stable up to 154 °C. The first weight loss in the range
54-235 °C (6.56 %) is in good agreement with the calculated value (6.8%) for the
decomposition of three methyl groups. The second, clear weight loss (16.49 %) between 235
and 330 °C suggests that the N molecules (16.32 %) vaporized. SDTA curve shows
(
1
2
O
5
exothermic peak. Over the range 330-790 °C, the weight loss (30.44 %) should correspond
to the loss of imidazole molecules (calcd 29.51 %). The total mass loss occurring up to 800
°C is in good agreement with the formation of metallic lead and gaseous products as a final
residue (observed 46.51 %, calculated 47.37 %). The final solid product was identified on
the basis of powder XRD studies [48].
For compound 2, there are five obvious weight-loss steps of decomposition. The first
step occurred at ca. 35-90 °C (2.8 %), corresponding to the removal of two water molecules
per formula unit (3.8 % calcd). In the second one, weight loss (4.4 %) happened at 90-150
°C, which indicates that noncoordinated part of carboxylate group (4.4 %) from complex 2
was removed. The third step (31.33 %) with exothermic effect, between 150 and 400 °C
suggests the defragmentation of two pyrazine rings inclusive N-donor atoms. Lastly, steps
four and five, 400-800 °C, are connected with last part of organic ligand including the O-
donor atoms connected with central lead(II). Assuming that the residue at 800 °C
corresponds to metallic lead, the overall observed weight loss (57.85%) is in good
agreement with that calculated for Pb (58.47%). Additionally, the final product was
confirmed from XRD patterns [48] and identified on the basis of ICDD using XRAYAN
package.
Fig. 9.
Conclusion
In summary, the self-assembly of lead(II) salts with 5(4)-carbaldehyde-4(5)-
methylimidazole (5(4)-CHO-4(5)-MeIm) and pyrazine-2-carboxylic acid (2-pzcH) affords
Pb(II) coordination dimmer (1) and polymer (2). Complex 1 consists of a neutral dimmeric
2
+
coordination unit which is constructed by [Pb
2
(4-CHO-5-MeIm)
6
(NO
3
)
2
]
cation and two
-
free NO anions. The polymer features (2) a 3D-like network formed by the linkage of
lead(II) ions and 2-pzc ligands. The both 2-pzc ligands are coordinated in different bridging
modes: monoatomic (µ -η ) or bidentate (µ -η :η ). The most interesting feature of the
crystal packing is existence of mesh network constructed of two kinds of cavities: the small
one (Pb···Pb distance 4.163 Å) and the bigger which is built up six Pb(II) ions joined by six
3
-
-
2
2
2
1
1
-
bidentate bridging 2-pzc ligands. Therefore, it can be consider as promising metal-organic
frameworks (MOFs).
The nonspherical distribution of ligands surrounding the lead(II) ion in 1, 2 (pseudo-
hemidirected geometry, see Fig. 10) results from the presence of the stereochemically active
8

lone electron pair. The DFT computations of the isolated complexes are in good agreement
with structural results obtained by X-ray diffraction.
Based on the literature data, the Pb-O bonds lengths in complexes belongs to the first
or second coordination sphere. According to R.L. Davidovich [13] the secondary bonds are
-
-
mostly realised by a bridging ligands (COO ) or NO
3
(additionally mentioned by Hakimi
[
26] and Marandi [27]). In our case (1) this group of bonds is presented by the interaction of
-
lead(II) with oxygen atoms of bridging nitrate anions (Pb–O(NO ) with distances 2.911(4)
3
and 3.019(3) Å. Consistent with Davidovich suggestion [13] the primary coordination
sphere of the Pb(II) atom can be limited to the Pb-O distance of 2.70 Å. Taking into
consideration complex 2 with pyrazine-2-carboxylate as ligand, Pb(II)-O bonds (2.363-2.585
Å) belongs to the primary coordination sphere of Pb(II) atom but Pb-O(9A) bond (2.862 Å)
of bridging carboxylate group belongs to the secondary sphere of metal atom.
Complexes reported in this work exhibit the photoluminescence properties at room
temperature in the solid-state. The bands at 574 nm (1) and 638 nm (2) can be assigned to
metal-to-ligand charge transfer (MLCT) transition.
The thermogravimetric analysis showed that complex 1 exhibited higher thermal stability in
comparison to hydrated complex 2.
Fig. 10.
Acknowledgement
We would like to thank Dr J. Karpiuk (Polish Academy of Sciences) for cooperation during
data collections.
Appendix A. Supplementary material
CCDC 973041, 973040 contains the supplementary crystallographic data for 1, 2. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can
be found, in the online version, at http…
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1
1

Figure captions:
Fig. 1. Coordination arrangements of the Pb in complex 1.
2
+
Fig. 2. View of (a) inter- and intramolecular hydrogen bonds C-H···O and N-H···O type in
complex 1, (b) the packing arrangement along c axis.
Fig. 3. Asymmetric unit of polymer 2 with atom numbering scheme.
-
Fig. 4. Coordination modes of the 2-pzc ligand and carboxylate group.
Fig. 5. Part of the polymer structure exhibiting the a) macrometallocycle structural motif
along [1 0 1] plane, b) hydrogen interactions between water molecule and neighboring
monomeric units in complex 2.
Fig. 6. Molecular orbitals localized on Pb atoms in 1 (MO#35) and 2 ((a) MO#59; (b)
MO#81), which have the most asymmetric character.
Fig. 7. Solid-state luminescence emission spectra of complexes and free ligands (1 (a), 2 (b),
respectively) at room temperature.
Fig. 8. Comparison of the complexes 1 and 2 showing growth of the emission.
Fig. 9. TG/DTG/SDTA curves of compounds 1 and 2.
2
+
Fig. 10. The coordination polyhedra around Pb ion in the complexes.
1
2

2
+
Fig. 1. Coordination arrangements of the Pb in complex 1.

2
+
Fig. 1. Coordination arrangements of the Pb in complex 1.

a
b
Fig. 2. View of (a) inter- and intramolecular hydrogen bonds C-H···O and N-H···O type in
complex 1, (b) the packing arrangement along c axis.

a
b
Fig. 2. View of (a) inter- and intramolecular hydrogen bonds C-H···O and N-H···O type in
complex 1, (b) the packing arrangement along c axis.

Fig. 3. Asymmetric unit of polymer 2 with atom numbering scheme.

Fig. 3. Asymmetric unit of polymer 2 with atom numbering scheme.

a
b
1
1
2
(
 - : ) mode
( - ) mode
2
2
-
Fig. 4. Coordination modes of the 2-pzc ligand and carboxylate group.

a
b
1
1
2
(
 - : ) mode
( - ) mode
2
2
-
Fig. 4. Coordination modes of the 2-pzc ligand and carboxylate group.

a
b
Fig. 5. Part of the polymer structure exhibiting the a) macrometallocycle structural motif
along [1 0 1] plane, b) hydrogen interactions between water molecule and neighboring
monomeric units in complex 2.

a
b
Fig. 5. Part of the polymer structure exhibiting the a) macrometallocycle structural motif
along [1 0 1] plane, b) hydrogen interactions between water molecule and neighboring
monomeric units in complex 2.

1
a
b
2
Fig. 6. Molecular orbitals localized on Pb atoms in 1 (MO#35) and 2 ((a) MO#59; (b)
MO#81), which have the most asymmetric character.

1
a
b
2
Fig. 6. Molecular orbitals localized on Pb atoms in 1 (MO#35) and 2 ((a) MO#59; (b)
MO#81), which have the most asymmetric character.

553 nm
5
1
(4)-CHO-4(5)-MeIm
554 nm
2-pzcH
2
538 nm
575 nm
400
500
600
700
400
500
600
700
Wavelenght (nm)
Wavelenght (nm)
a
b
Fig. 7. Solid-state luminescence emission spectra of complexes and free ligands (1 (a), 2 (b), respectively) at room temperature.

Figure 7 mono
a
b
Fig. 7. Solid-state luminescence emission spectra of complexes and free ligands (1 (a), 2 (b), respectively) at room temperature.

1
2
400
500
600
700
Wavelenght (nm)
Fig. 8. Comparison of the complexes 1 and 2 showing growth of the emission.

Figure 8 mono
Fig. 8. Comparison of the complexes 1 and 2 showing growth of the emission.

6
.56%
1
6.49%
DTG
30.44%
TG
SDTA
100
200
300
400
500
600
700
800
o
Temperature ( C)
a
7.2%
DTG
31.33%
6.32%
13.00%
SDTA
TG
100
200
300
400
500
600
700
800
o
Temperature ( C)
b
Fig. 9. TG/DTG/SDTA curves of compounds 1 (a) and 2 (b).