A new luminous end-to-end thiocyanato bridged heptacoordinated coordination polymer of lead(II) containing a tetradentate Schiff base

Article abstract of DOI:10.1016/j.inoche.2011.11.019

A new end-to-end thiocyanato bridged coordination polymer [Pb(enbzpy)(μ_1,3-NCS)(NCS)]_n (1) has been synthesized using a one-pot reaction of Pb(OAc)₂.3H₂O, tetradentate Schiff base N,N-(bis(pyridine-2-yl)benzylidene)ethane-1,2-diamine (enbzpy) and NH₄NCS in MeOH solution at room temperature. Single crystal X-ray diffraction measurement reveals that each lead(II) adopts a pentagonal bipyramidal geometry with a PbN₆S chromophore coordinated through four N atoms of enbzpy, two μ_1,3-bridged N and S atoms and one terminal N atom of thiocyanates. In crystalline state, 1 forms a 2D honeycomb like (6³) topology through S...S interaction and the layers stack through face-to-face π...π interaction giving rise to a 3D network. At room temperature DMF solution of 1 displays a high-energy intraligand ¹(π-π*) fluorescence.

Full text of DOI:10.1016/j.inoche.2011.11.019

Inorganic Chemistry Communications 16 (2012) 21–24
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A new luminous end-to-end thiocyanato bridged heptacoordinated coordination
polymer of lead(II) containing a tetradentate Schiff base
Soumi Chattopadhyay a, Kishalay Bhar a, Somnath Choubey a, Sumitava Khan a,
Partha Mitra b, Barindra Kumar Ghosh a,
⁎
a
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2011
Accepted 10 November 2011
Available online 18 November 2011
A new end-to-end thiocyanato bridged coordination polymer [Pb(enbzpy)(μ1,3-NCS)(NCS)]n (1) has been
synthesized using a one-pot reaction of Pb(OAc)2.3H2O, tetradentate Schiff base N,N -(bis(pyridine-2-yl)ben-
zylidene)ethane-1,2-diamine (enbzpy) and NH4NCS in MeOH solution at room temperature. Single crystal X-
ray diffraction measurement reveals that each lead(II) adopts a pentagonal bipyramidal geometry with a
PbN6S chromophore coordinated through four N atoms of enbzpy, two μ1,3-bridged N and S atoms and one
terminal N atom of thiocyanates. In crystalline state, 1 forms a 2D honeycomb like (63) topology through
S∙∙∙S interaction and the layers stack through face-to-face π∙∙∙π interaction giving rise to a 3D network. At
room temperature DMF solution of 1 displays a high-energy intraligand 1(π-π*) fluorescence.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Heptacoordinated lead(II) polymer
Schiff base
Thiocyanate bridge
S∙∙∙S interaction
Luminescence
A vast majority of current work centers around the use of molec-
ular building components to isolate inorganic–organic hybrid [1,2]
functional materials [3–7] of different shapes and sizes [8] through
control and manipulation of metal ligand covalent bonds [9,10] with
malleable coordination spheres and multiple lateral non-covalent
forces [11–15] like hydrogen bondings and C-H∙∙∙π, π∙∙∙π, S···S,
Se···Se, etc. interactions. One-pot synthesis [16] using metal ion
templates, organic spacers and inorganic/organic terminals/bridges
in varied molar ratios may lead to different molecular and crystalline
architectures with tunable target properties. Recently, we are inter-
ested [17–19] in the construction of different coordination polymers
and polymer based supramolecular entities through variation of li-
gand backbones and metal ion coordination environments; in this re-
gard, Schiff base spacers and pseudohalide terminals/bridges have
been remarkable. Schiff bases [20] are useful ligands because of
their ease of preparation, structural variety, varied denticities and
subtle steric and/or electronic effects. Ambidentate thiocyanate [21]
may act as terminal and bridging ligands, and in terminal mode
often participates in hydrogen bondings [22], S∙∙∙S interactions
[23–25] that influence crystalline architectures. The present work
stems from our interest to build new molecular and crystalline aggre-
gates of lead(II), a heavy p-block metal ion that has intrinsic features
owing to the presence of stereochemically active 6 s2 electrons
[26–29]. In the present work we have examined the coordination be-
haviour of a tetradentate Schiff base, N,N -(bis(pyridin-2-yl)benzyli-
dene)ethane-1,2-diamine (enbzpy) towards lead(II) in combination
with thiocyanate, that remains unexplored to date. Successfully, we
have synthesized and X-ray crystallographically characterized one lu-
minous end-to-end thiocyanate bridged heptacoordinated neutral 1D
coordination polymer of the type [Pb(enbzpy)(μ1,3-NCS)(NCS)]n (1)
through a one-pot reaction of lead(II) acetate trihydrates, enbzpy and
NH4NCS in a 1:1:2 molar ratio in methanolic solution [30]. Compound
1 assembles into a 2D honeycomb like (63) topology through S···S in-
teraction and the successive 2D sheets are stacked via π∙∙∙π interaction
affording a 3D supramolecular architecture. This luminous compound
is an example of a unique heptacoordinated thiocyanato bridged coor-
dination polymer of lead(II) which has very scarce literature [31,32]
Scheme 1.
The air- and moisture-insensitive compound is insoluble in metha-
nol, ethanol, dichloromethane, acetonitrile but soluble in dimethylfor-
mamide and dimethylsulphoxide. In IR spectrum of 1 three strong
peaks at 2073, 2047, 2024 cm-1 are observed assignable to νas(CN)
stretching vibrations of the thiocyanate ligands [33]. Three bands corre-
sponding to ν(CS) stretching frequency appear at 766, 749, 701 cm-1
.
The position and number of signals strongly suggests mutual cis align-
ment and terminal as well as N- and S-coordinated bridging behaviour
of the bound thiocyanates [33]. X-ray study [34] corroborates this hy-
pothesis and defines the molecular structure and crystalline architec-
ture. The stretching vibrations ν(C=N) plus ν(C=C) of the Schiff
base are observed at 1630 and 1580 cm-1, and 1621 and 1584 cm-1 for
free and in metal bound states, respectively.
⁎
Corresponding author. Tel.: +91 342 2533913; fax: +91 342 2530452.
E-mail address: barin_1@yahoo.co.uk (B.K. Ghosh).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.11.019

22
S. Chattopadhyay et al. / Inorganic Chemistry Communications 16 (2012) 21–24
towards S2. Five equatorial nitrogens, N1, N2, N3, N4 and N5 also
show considerable deviation (N1 0.096 Å, N2 0.093 Å, N3 0.064 Å, N4
0.025 Å and N5 0.036 Å) from the mean plane. The ethylenic part
(N2-C26-C25-N4) of the Schiff base is to some extent puckered. The
intrachain Pb∙∙∙Pb separation across the single μ1,3-NCS bridge is 7.414 Å.
The 1D covalent chains in 1 are associated into a 2D honeycomb like
(63) topology (Fig. 2) arising from S···S [S(4)···S(4)** separation:
3.473(6) Å, symmetry code: **2-x, 2-y, -z; which is lower [35] than the
sum of the van der Waals radii (4.06 Å) of S atoms] interaction between
two S [S(4)] atoms of terminal thiocyanates along a-axis. The successive
2D sheets pack alongside each other to give a 3D network (Fig. 3)
through face-to-face π∙∙∙π interaction [Ring(1)-Ring(1): Cg(1)-Cg(1)***
separation: 3.864(5) Å, vertical displacement of Cg: 3.578 Å; dihedral
angle 0.0°; symmetry code: ***1-x, 2-y, -z; Cg(1)=N(1)-C(1)-C(2)-C
(3)-C(4)-C(5)] propagating along c-axis due to the availability of pyridyl
π interaction sites which align optimally to facilitate the π···π stacking
between the sheets.
Scheme 1. (Np, Ni, Ni, Np) donor set in enbzpy.
Single crystal X-ray diffraction study of 1 reveals that the crystal lat-
tice consists of [Pb(enbzpy)(μ1,3-NCS)(NCS)]n unit. An ORTEP diagram
with atom numbering scheme is shown in Fig. 1. The coordination poly-
hedra around each lead(II) center is best described as a distorted pen-
tagonal bipyramid (pbp) with a PbN6S chromophore. The distortion
from ideal pbp geometry is due to the asymmetric nature of the
bound enbzpy and the deviation of the refine angles formed at each
metal center. The coordination includes tetradentate Schiff base,
enbzpy, ligated by two Np atoms (N1, N5), two Ni atoms (N2, N4),
two end-to-end (μ1,3-) bridged nitrogen and sulphur atoms (N3*, S2;
symmetry code: *1-x, 1/2+y, 1/2-z) and one terminal N (N6) of thiocy-
anates. The equatorial plane of the pentagon consists of one bridging
thiocyanate nitrogen (N3*), and N1, N2, N4, N5 atoms of enbzpy, the
axial positions are occupied by terminal N6 and bridging S2 of two thio-
cyanates. The degrees of distortion from an ideal pbp geometry are
reflected in the equatorial [60.58(17)-85.3(2)°] and the axial 154.0(2)°
angles. Each lead(II) center deviates 0.434 Å from the mean plane
To examine thermal stability, thermogravimetric and differential
thermal analyses (TG-DTA) were made between 40 and 900 °C in
the static atmosphere of nitrogen. Compound 1 is stable up to
229 °C, and the TG curve (Fig. S1) indicates that the weight loss
(obs.: 42.66%; calcd.: 54.73%) between 229 and 585 °C corresponds
to the departure of the Schiff base (enbzpy) with an exothermic effect
at 266 °C and two thiocyanate groups (obs.: 21.79%, calcd.: 16.24%) in
the range 585–674 °C followed by another exothermic peak with a
maximum at 626 °C.
Faint yellow DMF solutions of the free ligand (enbzpy) and the
compound 1 show strong absorption band at 273 nm presumably
due to ligand based transition [36,37]. Excitation at 273 nm, the free
Schiff base containing the pyridine moieties exhibits emission at
378 nm with lower intensity value, whereas the compound 1 shows
greater intensity at ~378 nm presumably due to the increase in con-
formational rigidity of the ligand upon coordination [38]. In each
Fig. 1. An ORTEP view of a segmented 1D zig-zag chain structure in 1 with atom labeling scheme and 30% thermal ellipsoid probability for all non-hydrogen atoms. Selected bond
distances (Å) Pb(1)-S(2) 3.192(3), Pb(1)-N(1) 2.642(6), Pb(1)-N(2) 2.527(6), Pb(1)-N(4) 2.642(6), Pb(1)-N(5) 2.754(6), Pb(1)-N(6) 2.416(8), Pb(1)-N(3)* 2.835(8), and bond
angles (˚) N(1)-Pb(1)-N(2) 62.97(18), N(2)-Pb(1)-N(4) 65.68(18), N(4)-Pb(1)-N(5) 60.58(17), N(5)-Pb(1)-N(3)* 80.15(19), N(1)-Pb(1)-N(3)* 85.3(2), S(2)-Pb(1)-N(6) 154.0
(2); *1-x, 1/2+y, 1/2-z.

S. Chattopadhyay et al. / Inorganic Chemistry Communications 16 (2012) 21–24
23
Fig. 2. A view of 2D honeycomb like (63) topology in 1 formed through S···S interactions along a-axis.
case the luminescence may be attributed to the intraligand 1(π-π*)
transition (Fig. 4a). The lead(II) salt and ammonium thiocyanate are
not luminescent. To observe the formation of the polynuclear com-
pound, fluorescence behaviour of the Schiff base was studied as a
function of lead(II) using a solution concentration of enbzpy and
NH4NCS in the 1:2 molar ratio. The plot of fluorescence intensity as
a function of concentration of the metal ion (Table S1) shows that
the intensity increases linearly with the concentration of lead(II)
[(0.1-0.2)×10-6 M] up to a mole ratio of 1:1 ligand mixture (enbzpy
polynuclear formula gives their almost same intensity value in sepa-
rate experiments. This shows the polynuclear compound formation
is very facile in reaction condition as expected from labile non-
transition lead(II) ion.
In summary, we report an interesting crystal structure and lumi-
nous behaviour of a heptacoordinated coordination polymer of lead
(II) containing thiocyanate and a tetradentate Schiff base in which a
2D honeycomb like (63) topology through S∙∙∙S interaction has been
accomplished through deliberate choice of building units. In addition,
weak π∙∙∙π interaction among two closest pyridine rings of enbzpy
plays an important role for stacking these 2D sheets into a 3D
network structure. This indicates that the complex 1 can be effec-
tively used as a potential scaffold for the design of supramolecular
0.197×10-6
M
plus NH4NCS 0.421×10-6 M) and lead(II) ion
(0.23784×10-6 M); after that it remains constant (Fig. 4b). Use of
similar concentration of solid compound 1 and the concentration
of lead(II), ammonium thiocyanate and enbzpy according to
π…π interaction
Fig. 3. 3D network structure in 1 formed through π…π stacking of 2D honeycombs along c-axis.

24
S. Chattopadhyay et al. / Inorganic Chemistry Communications 16 (2012) 21–24
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C
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ν(C=C) 1621, 1584. UV–vis (λ, nm): 273.
+
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fully acknowledged. S. Chattopadhyay is grateful to the UGC and K.
Bhar, S. Choubey, S. Khan to the CSIR, New Delhi, India for fellowships.
The authors also acknowledge the use of DST-funded National Single
Crystal X-ray Diffraction Facility at the Department of Inorganic
Chemistry, IACS, Kolkata, India for crystallographic study.
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Appendix A. Supplementary data
Crystallographic data for the structural analysis (excluding struc-
ture factors) has been deposited with the Cambridge Crystallographic
Data Center (CCDC No. 832925 for 1). Copy of this information can be
obtained free of charge from The Director, CCDC, 12 Union Road, Cam-
bridge, CB2 1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk). Supplementary materials
related to this article can be found online at doi:10.1016/j.inoche.
2011.11.019.
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Appendix A. Supplementary data
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