Transformation of mixed anionic coordination polymers of lead

Article abstract of DOI:10.1016/j.ica.2008.07.018

The coordination polymerization from lead(II) nitrate on reaction with 4-nitrobenzoic acid and pyridine N-oxide at room temperature passes through stepwise ligand substitution reaction. An intermediate polymer [Pb(NB)(PyO)₂(NO₃)]

Full text of DOI:10.1016/j.ica.2008.07.018

Inorganica Chimica Acta 362 (2009) 1681–1686
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Transformation of mixed anionic coordination polymers of lead
*
Rupam Sarma, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039 Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2008
Received in revised form 12 July 2008
Accepted 12 July 2008
Available online 18 July 2008
The coordination polymerization from lead(II) nitrate on reaction with 4-nitrobenzoic acid and pyridine
N-oxide at room temperature passes through stepwise ligand substitution reaction. An intermediate
polymer [Pb(NB)(PyO)2(NO3)]n (where NB = 4-nitrobenzoate, PyO = Pyridine N-oxide) is formed to give
the final polymer [Pb(NB)2(PyO)]n. A hydrated mononuclear complex [Pb(NB)2(PyO)(H2O)] is also formed
if rigorous anhydrous condition is not maintained. The reaction is extended to 4,40-bipyridyl N-oxide
(BPNO), which initially gives a coordination polymer [Pb2(NO3)(NB)3(BPNO)2]n which gets converted to
another coordination polymer [Pb(NB)2(BPNO)2]n. All these complexes are structurally characterized.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Coordination polymers
Ligand substitution
Lead
N-Oxides
4-Nitrobenzoic acid
1. Introduction
ical reactions. Reactions carried out in solid state, followed by crys-
tallization can also be useful in describing inter conversion of
The field of hybrid inorganic–organic framework materials spe-
cially coordination polymers has emerged as a subject of intensive
research not only for their fascinating topologies but also for the
promise they have offered as storage materials, catalyst, and so
on [1–10]. General strategy during syntheses of such hybrid mate-
rials involves reaction of a ligand with appropriate coordination
modes, and a metal ion that can act as a multidimensional hub
to get a thermodynamically stable polymer [1–13]. A good number
of reports regarding coordination polymers of lead with carboxyl-
ate ligand are available in the literature [14–17]. Recently, it has
been shown that both hemi and hollow directed coordination
spheres can be generated in lead carboxylates [18–19]. Presence
of underlying s orbital of lead results in interesting coordination
behavior owing to the inert pair effect [18–21]. Numbers of coordi-
nation polymers are prepared through solvothermal reactions and
their transformations to another coordination polymer are re-
ported [22–27]. Most of these transformations are driven by ther-
mal or photochemical means and are basically solid state
transformations [22–27]. Unfortunately, intermediate species
formed during such reactions are often not isolated. Alternatively,
solution chemistry can still be very useful for syntheses of coordi-
nation polymers, since, transient complexes formed during the
reaction can sometimes be isolated [28–29]. Moreover, there is
need for systematic study, especially in synthesizing mixed anionic
coordination polymers and in transformation of a coordination
back-bone of one coordination polymer to another through chem-
different supramolecular isomers or polymorphs which may even-
tually show path for controlled and selective syntheses [30]. With
the objective of understanding possible transformation among
polymeric species, we have studied the transformation of coordi-
nation polymers of lead derived from 4-nitrobenzoate and aro-
matic N-oxides in solution. Here we report the transformation of
mixed anionic polymeric species containing pyridine N-oxide
either to supramolecular self assembly or to homo anionic coordi-
nation polymer. In the case of 4,40-bipyridyl N-oxide (BPNO) com-
plex, inter conversion of one mixed anionic 2-D polymeric species
to a homo anionic 2-D polymer is reported.
2. Experimental
Synthesis of complexes [Pb(NB)(PyO)(NO3)]n (1), [Pb(NB)2–
(PyO)(H2O)] (2) and [Pb(NB)2(PyO)]n (3) [Pb2(NO3)(NB)3(BPNO)2]n
(4), [Pb(NB)2(BPNO)2]n (5):
To a solution of 4-nitrobenzoic acid (2 mmol, 0.334 g) in meth-
anol (20 ml) methanolic solution of NaOH (2 mmol, 0.080 g, 5 ml)
was added. After stirring this reaction mixture for about 30 min
Pb(NO3)2 (1 mmol, 0.331 g) was added and stirred for another
30 min and then PyO (1 mmol, 0.095 g) was added. A small amount
of toluene was added. The clear solution was then kept for crystal-
lization and good quality colorless crystals of 1 were obtained after
about 24 h. IR (KBr, cmꢀ1): 1573 (m), 1555 (m), 1470 (s), 1385 (s),
1350 (s), 1216 (m), 832 (m). 1H NMR (DMSO-d6, ppm): 8.29 (d, 2H,
J = 8.8 Hz), 8.21 (d, 2H, J = 7.2 Hz), 8.11 (d, 2H, J = 8.8 Hz), 7.41 (m,
2H,), 7.34 (t, 1H, J = 7.6 Hz). Elemental Anal. Calc. for
C17H14N4O9Pb: C, 32.61; H, 2.24. Found: C, 32.65; H, 2.26%.
* Corresponding author. Tel.: +91 361 2582082; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.07.018

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R. Sarma, J.B. Baruah / Inorganica Chimica Acta 362 (2009) 1681–1686
The complex 1 along with the mother liquor was kept and from
fined in the anisotropic approximation. The crystallographic
parameters of the compounds studied are given in Table 1.
The elemental analyses of the complexes were carried out with
pure crystals and satisfactory results are obtained, however, from
bulk materials we could not get satisfactory data.
this crystals of 2 were obtained after 2 days. Isolated IR (KBr,
cmꢀ1): 1615 (m), 1578 (s), 1509 (m), 1384 (s), 1351 (s), 1220
(m), 831 (m). 1H NMR (DMSO-d6, ppm): 8.22 (m, 4H), 8.09 (d,
2H, J = 8.4 Hz), 7.41 (m, 2H), 7.33 (t, 1H, J = 7.6 Hz). Elemental Anal.
Calc. for C19H15N3O10Pb: C, 35.05, H, 2.31. Found: C, 35.12, H,
2.02%.
3. Results and discussion
Crystals of the complex 3 were obtained when 2 was dissolved
in methanol and crystallized. Isolated yield: 93%. IR (KBr, cmꢀ1):
1621 (m), 1583 (s), 1510 (m), 1474 (m), 1384 (s), 1350 (s), 1227
(m), 832 (m). 1H NMR (DMSO-d6, ppm): 8.22 (m, 4H), 8.11 (d,
2H, J = 8.4 Hz), 7.41 (m, 2H), 7.33 (t, 1H, J = 7.6 Hz). Elemental Anal.
Calc. for C19H13N3O9Pb: C, 35.93, H, 2.05. Found: C, 35.91; H, 2.11%.
For the synthesis of 4; 4-nitrobenzoic acid (2 mmol, 0.334 g)
and sodium hydroxide (2 mmol, 0.080 g, 5 ml) was dissolved in
methanol (20 ml). After stirring this reaction mixture for about
30 min lead nitrate (1 mmol, 0.331 g) and 4,40-bipyridine N-oxide
(0.188 g, 1 mmol) was added and stirred for another 30 min. Yel-
low colored precipitate appeared which was dissolved in DMSO.
The clear solution was then kept for crystallization and good qual-
ity colorless crystals of 4 were obtained after about 8 h. Isolated
yield 48%. IR (KBr, cmꢀ1): 1619 (m), 1563 (s), 1519 (m), 1470
(m), 1384 (s), 1342 (s), 1214 (m), 833 (m). Elemental Anal. Calc.
for C31H20N6O17Pb2: C, 32.01 H, 1.73. Found: C, 32.15; H, 2.06%.
The complex 4 along with the mother liquor was kept and from
this crystals of 5 were obtained after 24 h. Isolated yield: 65%. IR
(KBr, cmꢀ1): 1619 (m), 1580 (s), 1473 (m), 1389 (m), 1348 (s),
1231 (m), 840 (m). Elemental Anal. Calc. for C24H16N4O10Pb: C,
39.61 H, 2.24. Found: C, 39.45; H, 2.31%.
We have observed that the reaction of lead(II) nitrate with 4-
nitrobenzoate and pyridine N-oxide in methanol initially gives a
mixed anionic coordination polymer
1 having composition
[Pb(NB)(PyO)2(NO3)]n (where NB = 4-nitrobenzoate, PyO = Pyridine
N-oxide). This compound on keeping dissolved in the mother li-
quor for 24 h gets converted to a mononuclear complex 2 having
composition [Pb(NB)2(PyO)(H2O)]. Finally, the complex 2 slowly
gets converted to a new coordination polymer 3 [Pb(NB)2(PyO)]n
on stirring in methanol solution. The reaction is presented in
Scheme 1.
The complex 1 has its characteristic IR absorptions at 1555
cmꢀ1 due to the metal bound chelating nitrate group and at
1216 cmꢀ1 due to pyridine N-oxide, the carbonyl frequency
appears at 1573 cmꢀ1. In the 1H NMR spectra, the complex has
the A2B2 pattern for the 4-nitrobenzoate group at 8.1 ppm and
8.3 ppm, in addition to the aromatic peaks of pyridine N-oxide.
Fig. 1b shows the structure of the coordination polymer 1. Each
of the metal centers in 1 is octa coordinated in which 4-nitro-
benzoate ligands satisfy three of the coordination sites and are in
bidentate bridging mode. The nitrate groups in the polymer are
chelating and each lead is associated with three different N-oxide
oxygen two bridging and the other mono-dentate. The Pb–O dis-
tance for the bridging N-oxide is 2.746 Å whereas that for the
mono-dentate N-oxide is 2.44 Å. The Pb–O distances for the nitrate
ligand are Pb1–O1, 2.82 Å; Pb1–O2, 2.57 Å. A careful look on the
coordination polymer shows that it comprises mononuclear build-
ing blocks, which are of hemi directed geometry (Fig. 1a), and on
self-assembling such hemi directed units result in the coordination
polymer 1 (Fig. 1b) with hollow directed lead centers.
The X-ray diffraction data were collected at 296 K with Mo K
a
radiation (k = 0.71073 Å) using a Bruker Nonius SMART CCD dif-
fractometer equipped with a graphite monochromator. SMART soft-
ware was used for data collection and also for indexing the
reflections and determining the unit cell parameters; the collected
data were integrated using SAINT software. The structures were
solved by direct methods and refined by full-matrix least-squares
calculations using SHELXTL software. All the non-H atoms were re-
Table 1
Crystallographic parameters for complexes 1–5
Compound number
1
2
3
4
5
Formulae
Formula weight
Crystal system
Space group
a (Å)
C17H14N4O9Pb
625.51
monoclinic
P2(1)/n
12.1032(5)
7.5460(3)
21.6511(8)
90.00
94.402(2)
90.00
1971.58(13)
4
2.107
8.617
1192
C19H15N3O10Pb
652.53
C19H13N3O9Pb
634.51
monoclinic
P2(1)/c
10.62940(10)
7.71560(10)
24.6416(3)
90.00
98.6340(10)
90.00
1998.01(4)
4
2.109
8.504
1208
C31H20N6O17Pb2
1162.91
monoclinic
P2(1)/c
17.6692(13)
8.1098(6)
24.3357(17)
90.00
100.471(4)
90.00
3429.1(4)
4
C24H16N4O10Pb
727.60
monoclinic
P2(1)/c
13.061(2)
7.4870(11)
26.345(4)
90.00
108.057(10)
90.00
2449.3(6)
4
1.973
6.955
1400
triclinic
ꢀ
P1
6.8980(3)
11.7165(5)
13.6190(7)
72.189(3)
83.549(3)
84.304(3)
1038.85(8)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Density (Mg mꢀ3
)
2.080
8.184
620
2.253
9.896
2192
Absorption coefficient (mmꢀ1
F(000)
)
Total number of reflections
19740
9628
26282
31952
25184
Reflections, I > 2
r(I)
4855
3757
4903
8458
5996
Maximum 2h (°)
Ranges (h, k, l)
28.30
25.50
28.37
28.33
28.45
ꢀ16 P h P 16
ꢀ9 P k P 10
ꢀ28 P l P 28
99.2
ꢀ5 P h P 8
ꢀ14 P k P 14
ꢀ16 P l P 16
96.9
ꢀ13 P h P 13
ꢀ10 P k P 10
ꢀ31 P l P 32
97.9
ꢀ21 P h P 23
ꢀ10 P k P 10
ꢀ31 P l P 32
99.0
ꢀ17 P h P 17
ꢀ9 P k P 9
ꢀ30 P l P 35
97.0
Complete to 2h (%)
Data/restraints/parameters
4855/0/280
1.039
3757/0/298
1.056
4903/0/289
1.036
8458/0/505
1.070
5996/0/352
0.990
Goodness-of-fit (F2)
R indices [I > 2
r(I)]
0.0190
0.0508
0.0197
0.0260
0.0463
R indices (all data)
0.0275
0.0732
0.0275
0.0426
0.0826

R. Sarma, J.B. Baruah / Inorganica Chimica Acta 362 (2009) 1681–1686
1683
2.77 Å) are much stronger than those in 1 (Pb1–O1, 2.46 Å; Pb1–
O2, 2.72 Å). This may be one of the various factors responsible
for the degradation of the coordination polymer as it leads to for-
mation of much stronger bonds. Also the Pb1–O9 distance for the
N-oxide ligand here is 2.44 Å. The complex self assembles through
weak O2ꢁꢁꢁH–O10 (dD–H 0.818 Å; dDꢁꢁꢁA, 2.781 Å; \D–HꢁꢁꢁA,
161.58°), O6ꢁꢁꢁH–O10 (dD–H 0.833 Å; dDꢁꢁꢁA, 2.792.Å; \D–HꢁꢁꢁA,
MeOH/RT
8hrs
[Pb (NB)(PyO)2(NO3)]n
Pb(NO3)2
+
NB + PyO
(1)
Mother
liquor
MeOH/RT
24hrs
60hrs
[Pb (NB)2(PyO)(H2O)]
MeOH/RT
Where
PyO =
(2)
141.42°) interactions and
p p interactions between the aromatic
ꢁꢁꢁ
O
N
rings of 4-nitrobenzoate ligand (Fig. 2). The Pb1ꢁꢁꢁO10 distance is
2.612 Å. The aromatic rings of the 4-nitro benzoate groups are
stacked parallel with the nitro-groups slightly off-set with a dis-
tance of separation between C2 and C9 of two rings is 3.392 Å
(Fig. 2) which is well within the limit for having center to center
MeOH/RT
48hrs
O
O
O2N
[Pb (NB)2(PyO)]n
NB =
(3)
p p stacking interactions.
ꢁꢁꢁ
Scheme 1.
It is interesting to note that the lead coordination polymers are
used for reduction of nitrate as well as nitration of aromatic sub-
strates [31]. However, in our case we could not obtain any nitrated
product from the reaction of resorcinol with the complex 1; what
we could obtain is the complex 2 and the solution showed the test
for presence of nitrate anion in solution. The coordination polymer
1 also reacts with sodium salt of 4-nitrobenzoic acid to give 3. The
coordination polymer 3 is also formed on reaction of lead(II) ni-
trate with sodium salt of 4-nitrobenzoic acid and pyridine N-oxide
for a longer reaction time. We have observed that the complex 2 is
unstable and slowly gets converted to the coordination polymer 3
by losing the water molecule in the presence or absence of sodium
4-nitrobenzoate.This suggests both ligand disproportionation as
well as ligand exchange process to be possible for interconversion
of 2 to 3. However, we have not been successful in characterizing
any other product formed from ligand disproportionation of 3.
The complex 2 is thus a side product in this reaction and appar-
ently can be considered to be an isolable reaction intermediate.
The complex 3 has IR absorptions at 1227 cmꢀ1 due to pyridine
N-oxide and 1621 cmꢀ1, 1583 cmꢀ1 due to the two carboxylate li-
gands. The complex 3 has a seven coordinated geometry around
lead ions and remains as one dimensional coordination polymer.
Six of these coordinations come from the 4-nitrobenzoate groups
Fig. 1. (a) Smallest building block of the coordination polymer [Pb(NB)(PyO)2-
(NO3)]n (1) (b) the coordination polymer.
The coordination polymer 1 degrades under solution state and
gets converted to the mononuclear complex 2. The degradation
process involves the removal of the nitrate group and incorpora-
tion of one 4-nitrobenzoate group. The complex 2 has hexa coordi-
nated hemi directed lead center with two bidentate carboxylates,
one PyO and one aqua ligand co-ordinated to the metal center.
The aqua ligand in the complex is derived from the residual water
in methanol. When dry methanol was used instead of ordinary
methanol, complex 2 could not be isolated and complex 3 was
found to be formed directly. From the crystal structure of the com-
plex it is seen that the Pb–O bonds to the 4-nitrobenzoate groups
in 2 (Pb1–O1, 2.78 Å; Pb1–O2, 2.37 Å; Pb1–O5, 2.38 Å; Pb1–O6,
Fig. 2. The dimeric assembly of [Pb(NB)2(PyO)(H2O)] showing intermolecular
H-bonding interaction and
pꢁꢁꢁp stacking interactions in (2).

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R. Sarma, J.B. Baruah / Inorganica Chimica Acta 362 (2009) 1681–1686
while the seventh coordination is from a monodentate PyO ligand.
It is the tendency towards expansion of coordination number of
the metal that acts as the driving force towards the elimination
of the aqua ligand and results in the polymerization of 2 to give
3. The bond lengths of the Pb–O bonds in 3 are in the range of
2.40–2.83 Å. The structure of the complex with selected bond dis-
tances is shown in Fig. 3. The 1H NMR spectra of the three com-
plexes recorded in DMSO-d6 clearly indicates the presence of
pyridine N-oxides and 4-nitrobenzoate groups (please refer to Sup-
plementary material). The powder X-ray diffraction of the three
complexes 1–3 are recorded (please refer to Supplementary mate-
rial) and compared with the simulated pattern. It is found that in
addition to desired peaks there are additional peaks in each case.
Careful look at the powder pattern of 2 and 3, shows that there
is some amount of each of them in both the XRD patterns. This
happens due to incomplete transformation of 2 to 3.
Further support to these results comes from the fact that similar
reactions of lead nitrate, 4-nitrobenzoate with 4,40-bipyridyl N-
oxide initially gives a mixed anionic two dimensional coordination
polymer 4 which gets converted to another 2-D coordination poly-
mer 5 in the reaction mixture with time. This conversion is shown
in Scheme 2.
Both the complexes are characterized by single crystal XRD and
also by other conventional spectroscopic techniques. The complex
4 has the composition [Pb2(NO3)(NB)3(BPNO)2]n and is featured by
two crystallographically distinct Pb centers. One of these lead cen-
ters (Pb1) is octa co-ordinated (three from carboxylate oxygen,
three from nitrate ligands and rest are from BPNO ligands) with
a hollow directed geometry and contains tridentate nitrate ligands
MeOH, Toluene
DMSO/RT
[Pb2(NO3)(NB)3(BPNO)2]n
Pb(NO3)2
+ NB + BPNO
8hrs
4
RT
Mother
liquor 24hrs
MeOH, Tol, DMSO
72 hrs/RT
[Pb(NB)2(BPNO)2]n
5
Where,
O
O
O2N
NB =
O
N
N O
BPNO =
Scheme 2.
gives a new two dimensional coordination polymer 5 having com-
position [Pb(NB)2(BPNO)2]n with the removal of the weakly bound
nitrate ligands. This time the complex lacks any IR absorptions
around 1520 cmꢀ1 which was present in 4 due to the nitrate
ligand. The peaks at 1619 cmꢀ1 and 1580 cmꢀ1 are due to the
carboxylate ligand and at 1231 cmꢀ1 due to the BPNO ligand. Each
of the lead centers in 5 has a hemi directed hepta coordinated
geometry, four of which are from two chelating carboxylate ligands
and rest are from bridging as well as monodentate BPNO ligands.
Here the Pb–O bond lengths lie in the range 2.48–2.77 Å. In this
case also the formation of comparatively stronger Pb–O (carboxyl-
ate) forces the dissociation of the Pb–O (nitrate) bonds thereby
forming the homo anionic coordination polymer from the mixed
anionic one. The dimensionality of this 2-D coordination polymer
extends along the b and c crystallographic axes as shown in Fig.
5. The 1H NMR spectra of both these complexes are recorded in
DMSO-d6 and shows the presence of 4-nitrobenzoate group as well
as the BPNO ligand (please refer to Supplementary material).
In both these reaction schemes it is observed that mixed anionic
coordination polymer containing 4-nitrobenzoate as well as nitrate
is formed initially with the excess 4-nitrobenzoate remaining in
reaction mixture. These mixed anionic complexes are unstable in
the mother liquor and gets converted to polymeric species contain-
ing only 4-nitrobenzoate as anionic ligand through substitution of
the nitrate ligand with a 4-nitrobenzoate. There are examples of
lead complexes derived from N-oxide of nictonic acid and they also
lead to coordination polymers of lead and in that case the ligand
itself has N-oxide and carboxylic acids, however, our system is dif-
ferent in a sense that we are being able to manipulate the stoichi-
ometry of the reactant to control the reactions which are not
possible when two different types of coordinating sites are bound
to rigid system [14].
along with carboxylate and
l-oxo containing BPNO ligands. The
other lead center (Pb2) has hexa coordinated hemi directed geom-
etry. Four of this hexa coordination come from carboxylate oxy-
gens and rest are from BPNO ligands. Pb1 is connected to
another Pb1 through the tridentate nitrate groups while it is con-
nected to Pb2 via N-oxide oxygens as well as carboxylate oxygen.
It is interesting to note here that among all the Pb–O bonds in 4
the ones to the nitrate ligand viz. Pb1–O29 (2.91 Å), Pb1–O30
(2.87 Å), Pb1–O31(2.82 Å) are the weaker. Bond lengths of all other
bonds lies in the range 2.47–2.65 Å. The complex 4 has IR absorp-
tions at 1519 cmꢀ1 characteristic of the nitrate ligand. The peaks at
1619 cmꢀ1 and 1563 cmꢀ1 are due to the carboxylate ligand and at
1214 cmꢀ1 is due to the N–O stretching of BPNO ligand. It has a two
dimensional network structure growing along b and c crystallo-
graphic axes as shown in Fig. 4.
It has been observed that the complex 4 is unstable in the reac-
tion mixture and on leaving dissolved in the mother liquor for 24 h
Fig. 3. (a) The building block of coordination polymer [Pb(NB)2(PyO)]n (3) (b) the polymeric structure.

R. Sarma, J.B. Baruah / Inorganica Chimica Acta 362 (2009) 1681–1686
1685
Fig. 5. (a) The building block of coordination polymer [Pb(NB)2(BPNO)2]n (b) 2-D
polymeric structure.
Acknowledgements
The authors thank Department of Science and Technology, New
Delhi, India for financial support and one of the author R.S. thanks
Council of Scientific and Industrial Research, New Delhi, India for
Junior Research Fellowship.
Fig. 4. (a) The building block of coordination polymer [Pb2(NO3)(NB)2(BPNO)2]n
(b) 2-D polymeric structure.
Appendix A. Supplementary material
In conclusion we have isolated and characterized mixed anionic
coordination polymers in the reaction of lead(II) nitrate with ben-
zoate and N-oxide ligands that are intermediates formed during
the reactions. In addition to this, the conversion of a mixed anionic
coordination polymer to homo anionic coordination polymer and
also formation of a self-assembled mono-nuclear coordination
complex from mixed anionic coordination polymer through hydra-
tion is demonstrated.
CCDC 673855, 673856, 673857, 684603 and 684604 contain the
supplementary crystallographic data for 1, 2, 3, 4 and 5. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
The CIF files for the five complexes and the 1H NMR spectra of
the complexes 1–5 and the powder X-ray diffraction pattern of
1–3 are available. Supplementary data associated with this article

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