Metallacycles of cadmium, mercury dicarboxylates

Article abstract of DOI:10.1039/c2dt30554b

A series of linear coordination polymers, metallacycles of cadmium(ii) and mercury(ii) of flexible carboxylic acid ligands, RCH{3-CH₃-,5-CH ₃-,6-(-OCH₂CO₂H)C₆H ₂}₂, (when R = C₆H₅, (H ₂L¹); 2-NO₂C₆H₄- (H ₂L²) and 3-NO₂C₆H₄- (H₂L³)) are synthesized and characterized. [CdL ¹ (py)₃]_n·3nH₂O (py = pyridine) is a linear coordination polymer, whereas [CdL²(py)(CH ₃OH)]₂·CH₃OH is a dinuclear complex of cadmium with a Cd₂O₂ type of core. The latter is obtained from reaction of cadmium(ii) acetate with H₂L² in methanol followed by reaction with pyridine. A similar reaction of cadmium(ii) acetate with H₂L² in dimethylformamide results in the formation of a cadmium metallacycle, namely [CdL² (py)₂(H ₂O)]₂·H₂O. The H₂L ³ reacted with cadmium(ii) acetate in the presence of pyridine to form a metallacycle [CdL³(py)₂(H₂O)] ₂·3H₂O. The ligand H₂L² form mercury(ii) metallacycle [HgL²(4-mepy)₂]₂ in the presence of 4-methylpyridine (4-mepy) and the ligand H₂L ³ forms metallacycle [HgL³(3-mepy)₂] ₂·DMF in the presence of 3-methylpyridine (3-mepy). The potassium salts of H₂L¹ and H₂L² were found to be coordination polymers and these potassium coordination polymers were structurally characterized.

Full text of DOI:10.1039/c2dt30554b

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PAPER
Metallacycles of cadmium, mercury dicarboxylates†
Bhaskar Nath and Jubaraj B. Baruah*
Received 8th March 2012, Accepted 30th March 2012
DOI: 10.1039/c2dt30554b
A series of linear coordination polymers, metallacycles of cadmium(II) and mercury(II) of flexible
carboxylic acid ligands, RCH{3-CH3–,5-CH3–,6-(–OCH2CO2H)C6H2}2, (when R = C6H5, (H2L1);
2-NO2C6H4– (H2L2) and 3-NO2C6H4– (H2L3)) are synthesized and characterized. [CdL1 (py)3]n·3nH2O
(py = pyridine) is a linear coordination polymer, whereas [CdL2(py)(CH3OH)]2·CH3OH is a dinuclear
complex of cadmium with a Cd2O2 type of core. The latter is obtained from reaction of cadmium(II)
acetate with H2L2 in methanol followed by reaction with pyridine. A similar reaction of cadmium(II)
acetate with H2L2 in dimethylformamide results in the formation of a cadmium metallacycle, namely
[CdL2 (py)2(H2O)]2·H2O. The H2L3 reacted with cadmium(II) acetate in the presence of pyridine to form
a metallacycle [CdL3(py)2(H2O)]2·3H2O. The ligand H2L2 form mercury(II) metallacycle [HgL2(4-
mepy)2]2 in the presence of 4-methylpyridine (4-mepy) and the ligand H2L3 forms metallacycle
[HgL3(3-mepy)2]2·DMF in the presence of 3-methylpyridine (3-mepy). The potassium salts of H2L1 and
H2L2 were found to be coordination polymers and these potassium coordination polymers were
structurally characterized.
with a large spacer.7 The use of a flexible or less-rigid spacer for
Introduction
the construction of a coordination complex has an added advan-
tage as they have many degrees of freedom and a few confor-
mational restraints, which can give various topologies. These
types of conformational flexible ligands give unique opportu-
nities to construct novel structures with desirable characteristics.
Complicacy in predicting the architecture of the coordination
complexes derived from these types of flexible ligands arises due
to different coordination modes of carboxylate ligands, which
are further complicated by factors such as the variation of crys-
tallization conditions. However, one may simplify this by
restricting to one, or two binding modes of carboxylates8 leading
to a coordination polymer, metallacycle and dinuclear complex,
as shown in Scheme 1. Flexibility associated with CH2COOH
parts and the shape and rigidity associated with the bis-phenol
part makes this class of ligands unique, as cyclic structures may
be anticipated when these ligands are attached to appropriate
connections. Further to this interest in d10 metal ions-containing
dicarboxylate coordination polymers, those with cadmium9 or
mercury10 ions are especially of great importance. The cadmium
dicarboxylate polymers show interesting optical properties9 and
mercury dicarboxylates have medical applications,11 such as
their role in the treatment of renal failure. In addition to these,
dicarboxylic acids in ionic liquids bind mercury ions and have
the property to absorb gases.12 The mercury can have relatively
stable variable valences13 and can be easily functionalised to
organometallics. Wide variations in the structure of cadmium
carboxylate coordination polymers are possible by changing the
reaction conditions.9 Further to this, they have the possibility for
variations of coordination numbers9 and they are diamagnetic in
nature; so these metal ions can be studied in solution by
Multidentate ligands have often been observed to coordinate
with more than one metal centre to form infinitely extended
structures, such as coordination polymers, polynuclear com-
plexes or metallacycles.1 Enormous efforts are being made for
the construction of metal–organic frameworks and coordination
polymers, as they show interesting material properties.2 Avariety
of organic ligands with different backbones and functional
groups have been used to form coordination motifs with various
dimensions.3 The structural features of the outcome polymers are
controlled by various factors, such as coordination modes, shape
and size of ligands and the solvent used. Based on these, rigid
bridging ligands with nitrogen or oxygen as donor atoms, such
as 4,4′-bipyridine,4 terephthalic acid5 etc., are commonly used to
generate polymeric networks with beautiful topologies. There is
a growing trend in using flexible ligands for the construction of
coordination polymers with intriguing network structures.6 V-
shaped molecules with carboxylic acid functionality at the two
ends are most appealing for the construction of various metal–
organic frameworks because of two different metal binding sites
Department of Chemistry, Indian Institute of Technology Guwahati,
North Guwahati, 781039, India. E-mail: juba@iitg.ernet.in
†Electronic supplementary information (ESI) available: The powder
X-ray diffraction patterns of the cadmium complexes (4–7), IR spectra
of the ligands and complexes, ESI mass of the ligands H2L2 and H2L3,
1
1HNMR spectra of complex 7 and 8, H HOMOCOSY spectra of com-
plexes 5 and 6, thermogravimetry of complexes 4–8 and 12, 13 are avail-
able. CCDC 840829, 806671, 837719, 844623, 837720, 837718,
851819, 851820, 859335–859337. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt30554b
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7115–7126 | 7115

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common spectroscopic tools, like NMR spectroscopy. Thus, we
identified cadmium and mercury as the metal ions for the
synthesis of coordination polymers with dicarboxylic acid
ligands with a flexible part anchored to a rigid V-shape
geometry.
In the literature it is found that the presence of nitro substitu-
ent at o-, m- and p-positions of benzoates have a striking effect
on the structure and dimensionality of the resulting coordination
complexes.14 In our earlier studies, we have also found that nitro
substituent on aromatic carboxylic acids decides the formation of
mononuclear or dinuclear complexes.15 In addition, we have also
used bis-phenol-based flexible V-shaped dicarboxylic acids
(H2L1) to bind zinc to form penta-coordinated mononuclear
complexes.16 In this study, we tried to explore whether an appar-
ently innocent nitro substitution on this V-shaped ligand (H2L1)
can bring about any change in the molecular architecture of
cadmium and mercury complexes. Furthermore, we also
attempted to explore the structural changes in these complexes
by varying the position of the nitro group in these ligands
(Scheme 2a).
Herein, we report the synthesis and characterisation of some
cadmium and mercury complexes, which were synthesised under
identical ambient conditions in methanol or dimethyl forma-
mide. An interesting structural variation from the one dimen-
sional coordination polymer (4) to metallacycle (7) is observed
Scheme 1 The formation of metallacycle (A) (red arrows show the
possible path for a metallacycle converted to a dinuclear complex);
dinuclear complex (B); linear coordination polymer (C) from flexible V-
shaped ligand.
Scheme 2 (a) Ligands H2L1, H2L2 and H2L3. (b) The formation of one dimensional coordination polymer (4), dinuclear complex (5) and metalla-
cycles of cadmium (6 and 7).
7116 | Dalton Trans., 2012, 41, 7115–7126
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To prepare 3a, 3-nitrobenzaldehyde was used instead of 2-nitro-
benzaldehyde. Overall yield: 70%. IR (KBr, cm−1): 3433 (bs),
2079 (w), 1636 (s), 1523 (s), 1476 (m), 1352 (m), 1297 (w),
1234 (w), 1214 (m), 1142 (m), 1067 (m), 867 (w), 827 (w), 703
1
(m). HNMR (DMSO-d6, 400 MHz): 2.13 (s, 6H), 2.16 (s, 6H),
3.78 (s, 2H), 3.81 (s, 2H), 6.48 (s, 2H), 6.89 (s, 2H), 7.01 (s,
1H), 7. 51 (d, J = 7.6, 1H), 7.57 (t, J = 7.6 Hz, 1H), 7.79 (s,
1H), 8.05 (d, J = 7.6 Hz, 1H). LCMS [M + Na+]: 516.1430.
Scheme 3 The synthesis of ligands 1–3.
Synthesis of cadmium and mercury complexes
on the introduction of the nitro substituent on the ligand. Again,
it is also observed that on changing the solvent from methanol to
dimethyl formamide, the dinuclear complex (5) is converted to a
metallacycle (6) (Scheme 2b).
Complex [CdL1(py)3]n·3nH2O (4). To a well-stirred solution
of H2L1 (0.224 g, 0.5 mmol) and sodium hydroxide (0.02 g) dis-
solved in methanol (10 ml), cadmium(II) acetate monohydrate
(0.133 g, 0.5 mmol) was added. The white precipitate obtained
was dissolved by the addition of the minimum amount of pyri-
dine. The reaction mixture was filtered and the transparent liquid
was kept for crystallization. After one week colorless needle-like
crystals were obtained. Isolated yield: 50%. IR (KBr, cm−1):
3426 (bs), 2924 (s), 2855 (w), 1600 (s), 1447 (s), 1415 (m),
1327 (w), 1245 (w), 1208 (m), 1141 (m), 1036 (s), 760 (w), 702
(s). 1HNMR (DMSO-d6): 2.11 (s, 6H), 2.14 (s, 6H), 4.01 (s,
4H), 6.37 (s, 1H), 6.43 (s, 2H), 6.85 (s, 2H), 6.98 (d, 6.2 Hz,
2H), 7.18 (t, 7.0 Hz, 1H), 7.21 (5.3 Hz, t, 3H), 7.79 (7.6 Hz, t,
6H), 8.58 (s, 6H).
Experimental
The ligands RCH{3-CH3–,5-CH3–,6-(–OCH2CO2H)C6H2}2,
(when R = C6H5, (H2L1); 2-NO2C6H4– (H2L2) and 3-
NO2C6H4– (H2L3)) used in this study were synthesized by func-
tionalization of their respective parent bis-phenol. The ligand
H2L1 (1) was prepared by reported procedure.16 The procedure
was extended to ligand H2L2 (2) and H2L3 (3).
Synthesis of the ligand H2L2 (2)
Complex [CdL2(py)(CH3OH)]2·CH3OH (5). To a well-stirred
solution of H2L2 (0.246 g, 0.5 mmol) and sodium hydroxide
(0.02 g) dissolved in methanol (10 ml), cadmium(II) acetate
monohydrate (0.133 g, 0.5 mmol) was added. Awhite precipitate
was obtained, which was dissolved by the addition of the
minimum amount of pyridine. The reaction mixture was filtered
and the transparent liquid was kept for crystallization. After one
week colorless needle-like crystals were obtained. Isolated yield:
60%. IR (KBr, cm−1): 3427 (bs), 1630 (s), 1526 (m), 1475 (w),
1443 (w), 1419 (w), 1351 (w), 1324 (w), 1238 (w), 1212 (m),
The bis-phenol (as in Scheme 3) (2a) (1.89 g, 5 mmol) was dis-
solved in dry acetone (30 ml). To this solution anhydrous potass-
ium carbonate (1.38 g, 10 mmol) was added and stirred for
20 min. Then, methyl bromoacetate (0.95 ml, 10 mmol) was
added and the reaction mixture was refluxed at 70 °C for 22 h.
Progress of the reaction was monitored at regular intervals using
TLC. After completion of the reaction, the reaction mixture was
filtered. The solvent from the filtrate was removed under reduced
pressure to obtain the crude product, which was further purified
by thin layer chromatography (silica gel; hexane–ethyl acetate).
Isolated yield: 78%. The ester (1.04 g, 2 mmol) was taken with
sodium hydroxide (0.16 g, 4 mmol) in a mixed solvent of metha-
nol–water (4 : 1, 20 ml) and refluxed for 1 h at 70 °C. After com-
pletion of the reaction, the solvent was removed under reduced
pressure, then 10 ml of water was added to it and the solution
was acidified with dilute hydrochloric acid (20 ml, 10%) solution
to precipitate out the dicarboxylic acid. The solid was filtered
and washed with water until it was free from acid. The product
was isolated as a white solid and was further purified by recrys-
tallising it from methanol. Isolated yield: 59%. IR (KBr, cm−1):
3468 (bs), 2067 (w), 1748 (s), 1638 (s), 1526 (s), 1474 (m),
1441 (m), 1354 (m), 1300 (w), 1254 (w), 1235 (w), 1209 (m),
1143 (s), 1067 (m), 968 (w), 869 (w), 714 (m). 1HNMR
(DMSO-d6, 400 MHz): 2.11 (s, 6H), 2.17 (s, 6H), 4.08 (s, 1H),
4.18 (s, 4H), 6.28 (s, 2H), 6.53 (s, 1H), 6.94 (s, 2H), 6.98 (d, J =
8 Hz, 1H), 7.51 (t, J = 8 Hz, 1H), 7.63 (t, J = 8 Hz, 1H), 7.99
(d, J = 8 Hz, 1H). Mass (ESI) [M + Na+]: 516.681.
1
1143 (m), 1051 (w), 1035 (w), 697 (m). HNMR (DMSO-d6):
2.10 (s, 1H), 2.15 (s, 6H), 4.10 (s, 4H), 6.25 (s, 2H), 6.65 (s,
1H), 6.88 (d, 8 Hz, 1H), 6.90 (s, 2H), 7.39 (t, 5.6 Hz, 2H), 7.49
(t, 7.6 Hz, 1H), 7.60 (t, 7.6 Hz, 1H), 7.79 (t, 6.4, 1H), 7.93 (d, 8
Hz, 1H), 8.58 (d, 6.2 Hz, 2H).
Complex [CdL2(py)2(H2O)]2·H2O (6). This was prepared by
exactly the same procedure as for 5, but instead of methanol
dimethylformamide was used as the solvent. After one week
light yellow crystals were obtained. Isolated yield: 80%. IR
(KBr, cm−1): 3435 (bs), 1602 (s), 1536 (m), 1473 (w), 1447 (w),
1415 (w), 1362 (w), 1321 (w), 1239 (w), 1214 (m), 1142 (m),
1
1031 (s), 933 (w), 860 (m), 691 (w). HNMR (DMSO-d6): 2.10
(s, 6H), 2.16 (s, 6H), 4.09 (s, 4H), 6.23 (s, 2H), 6.66 (s, 1H),
6.87 (d, 8.0 Hz, 1H), 6.90 (d, 6.4 Hz, 2H), 7.39 (t, 1.6 Hz, 4H),
7.51 (t, 7.6 Hz, 1H), 7.59 (t, 6 Hz, 1H), 7.79 (t, 1.6 Hz, 2H),
7.93 (t, 5.6 Hz, 1H), 8.58 (d, 4.0 Hz, 4H).
Alternatively, complex 6 can be prepared by dissolving
complex 5 in a DMF–pyridine solvent and slow evaporation of
the solvent at room temperature for several days.
Synthesis of the ligand H2L3 (3)
Complex [CdL3(py)2(H2O)]2·3H2O (7). To a well-stirred sol-
ution of H2L3 (0.25 g, 0.5 mmol) and sodium hydroxide
(0.02 g) dissolved in methanol (10 ml), cadmium(II) acetate
Ligand 3 was synthesized by a similar procedure adopted for
ligand H2L2 (2) and it was prepared starting from bis-phenol 3a.
This journal is © The Royal Society of Chemistry 2012
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X-ray crystallography
monohydrate (0.13 g, 0.5 mmol) was added. A white precipitate
was obtained, which was dissolved by the addition of the
minimum amount of pyridine. The reaction mixture was filtered
and the transparent liquid was kept for crystallization. After one
week colorless needle-like crystals were obtained. Isolated yield:
55%. IR (KBr, cm−1): 3433 (bs), 2923 (m), 1633 (s), 1525 (w),
1446 (w), 1348 (w), 1213 (m), 1143 (m), 1055 (w), 864 (w),
Diffraction data for compounds were collected using a Bruker
Nonius SMART Apex CCD diffractometer equipped with graph-
ite monochromator and Apex CCD camera. The SMART soft-
ware was used for data collection and also for indexing the
reflections and determining the unit cell parameters. Data
reduction and cell refinement were performed using SAINT soft-
ware and the space groups of these crystals were determined
from systematic absences by XPREP and further justified by the
refinement results. The structures were solved by direct methods
and refined by full-matrix least-square calculations using
SHELXTL software. All of the non-H atoms were refined in the
anisotropic approximation against F2 of all reflections. The H-
atoms attached to heteroatoms in these crystals were located in
the difference Fourier synthesis maps and refined with isotropic
displacement coefficients. The locations of acidic protons were
justified by a difference Fourier synthesis map and in the refine-
ment these were allowed for as riding atoms. The crystallo-
graphic parameters are listed in Table 1. Some of the labile
hydrogen atoms in the complexes 4–7 could not be located.
1
698 (m). HNMR (DMSO-d6): 2.12 (s, 6H), 2.16 (s, 6H), 4.02
(d, 14.4 Hz, 4H), 4.08 (d, 14.4 Hz, 4H), 6.39 (s, 2H), 6.74 (s,
1H), 6.91 (s, 2H), 7.37 (t, 5.6 Hz, 2H), 7.50 (t, 6.8 Hz, 1H), 7.74
(s, 1H), 7.77 (t, 7.6 Hz, 2H), 8.02 (d, 6.8 Hz, 1H), 8.58 (s, 2H).
Complex [HgL2(py)2]2·DMF (8). To a well-stirred solution of
H2L2 (0.25 g, 0.5 mmol) and sodium hydroxide (0.02 g) dis-
solved in dimethylformamide (10 ml), mercury(II) acetate
(0.16 g, 0.5 mmol) was added. A white precipitate was obtained,
which was dissolved in the minimum amount of pyridine. Any
residue at this stage was filtered and the transparent liquid was
kept for crystallization. After one week crystals were obtained.
Isolated yield: 55%. IR (KBr, cm−1): 3432 (bs), 1609 (s), 1537
(m), 1474 (w), 1415 (m), 1363 (w), 1321 (w), 1299 (w), 1239
1
(w), 1214 (w), 1142 (m), 1031 (m), 860 (w), 713 (m). HNMR
Results and discussion
(DMSO-d6): 2.11 (s, 6H), 2.17 (s, 6H), 4.24 (d, 15.2 Hz, 2H),
4.45 (d, 14 Hz, 2H), 6.26 (s, 2H), 6.75 (s, 1H), 6.87 (d, 7.6 Hz,
1H), 6.92 (s, 2H), 7.52 (t, 7.6 Hz, 4H), 7.60 (t, 7.2 Hz, 1H), 7.89
(t, 1.6 Hz, 1H), 7.94 (s, 2H), 7.97 (s, 1H), 8.69 (d, 4.4 Hz, 4H).
Alternatively, in the syntheses of complexes 4–11, the use of
potassium hydroxide instead of sodium hydroxide does not
effect the formation of these products. Complex [{CdL1(4-
mepy)2}·H2O]n (9) was prepared in a similar procedure to that
for complex 4, but 4-mepy was used instead of pyridine.
Mercury complexes [HgL2(4-mepy)2]2 (10) and [HgL3(3-
mepy)2]2·DMF (11) were prepared in a similar procedure as for
8, but the two different isomeric methyl-pyridines were used
instead of pyridine.
Ligands synthesis and characterisation
The dicarboxylic acid ligands H2L1 (1), H2L2 (2) and H2L3 (3)
were synthesized by multi-step synthetic procedures starting
from bis-phenols (1a–3a). The bis-phenols (1a–3a) were reacted
with methyl bromoacetate to obtain the corresponding ester (1b–
3b). The esters (1b–3b) were further hydrolyzed to the dicar-
boxylic acids. The reaction steps are illustrated in Scheme 2.
These esters and acids are characterized from their spectroscopic
properties, such as 1HNMR, IR, mass spectra, and also by deter-
mining some of their structures.
We have already reported the X-ray crystal structure of
H2L1.16 The structure determined by single crystal X-ray of the
ligand H2L2 shows a propeller-like geometry (Fig. 1). The
ligand has two carboxylic group containing arms that are
attached to a central carbon to provide a V-shape geometry in
two dimensions. The propeller-like geometry originates from the
projection of the two aromatic rings bearing a central methine
carbon (C11). The arms bearing flexible carboxylic acid groups
are attached to the two aromatic rings through oxygen atoms.
Thus, the flexible arms are suitably oriented to adopt a favorable
conformation for coordination to metal ions. We could not
obtain suitable crystals of ligand H2L3 for its X-ray structure
determination, however, we have established its structure by
other spectroscopic techniques.
Synthesis of potassium complexes
Complex [K4(L1)2(μ-H2O)2(H2O)2](H2O)n (12). To a well-
stirred solution of ligand H2L1 (0.9 g, 2 mmol) in dimethylfor-
mamide, potassium acetate (4 mmol) was added. The reaction
mixture was stirred for another half an hour to dissolve it and
then filtered to remove any solid impurity. The transparent liquid
was kept for crystallization. After one week colorless block crys-
tals of 12 were obtained. Isolated yield 65%. IR (KBr, cm−1):
3400 (s), 2921 (m), 1591 (s), 1467 (m), 1448 (m), 1416 (s),
1327 (m), 1297 (w), 1239 (w), 1212 (m), 1140 (m), 1033 (s),
933 (w), 861 (w), 779 (w), 946 (w), 706 (m).
Cadmium and mercury complexes of the ligands
The cadmium complex of the ligand H2L1, [CdL1(py)3]n·3nH2O
(4) is a coordination polymer (H2L1 = (C6H5)CH{3-CH3–,5-
CH3–, 6-(–OCH2CO2H)C6H2}2 and py = pyridine). The ligand
H2L2 (2) either formed dinuclear complex [CdL2(py)
(CH3OH)]2·CH3OH (5) or metallacycle [CdL2(py)2(H2O)]2·H2O
(6) (where H2L2 = (2-NO2-C6H4)CH{3-CH3-,5-CH3-,6-(–OCH2
CO2H)C6H2}2) depending on the solvent used; whereas the
Complex [K2L2(H2O)]n (13). The potassium salt of H2L2 (13)
was also synthesized in a similar way to 12, but it was syn-
thesised from ligand H2L2. Isolated yield: 60%. IR (KBr, cm−1):
3389 (s), 2923 (m), 1599 (s), 1537 (s), 1473 (m), 1413 (s), 1364
(w), 1320 (m), 1298 (m), 1239 (m), 1214 (m), 1141 (m), 1030
(s), 932 (w), 860 (m), 822 (w), 786 (w), 713 (m), 582 (w).
7118 | Dalton Trans., 2012, 41, 7115–7126
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Table 1 Crystallographic parameters of ligand and complexes
Compound no.
2
4
5
6
Formulae
CCDC no.
Mol. wt.
Crystal system
Space group
Temperature (K)
Wavelength (Å)
a (Å)
C27H27NO8
840829
493.50
monoclinic
P21/n
C42H41CdN3O9
806671
844.18
monoclinic
P21/c
C68H74Cd2N4O22
837719
1524.11
monoclinic
P21/c
C74H74Cd2N6O20
844623
1592.19
monoclinic
P21/c
296(2)
296(2)
296(2)
296(2)
0.71073
16.297(3)
9.222(2)
16.374(4)
90.00
0.71073
16.3357(5)
14.1196(5)
18.3475(6)
90.00
0.71073
17.4571(11)
21.0538(12)
9.2040(5)
90.00
0.71073
12.5560(10)
25.231(2)
12.2248(10)
90.00
b (Å)
c (Å)
α (°)
β (°)
94.019(15)
90.00
2454.8(9)
4
1.335
0.099
104.841(2)
90.00
4090.7(2)
4
1.371
0.591
93.921(4)
90.00
3374.9(3)
2
1.500
0.710
101.953(4)
90.00
3788.8(5)
2
1.396
0.635
γ (°)
V (Å3)
Z
Density (Mg m−3
)
Abs. coeff./mm−1
Abs. correction
F(000)
none
1040
none
1736
none
1564
empirical
1632
Total reflection
Reflections, I > 2σ(I)
Max. θ/°
4427
2439
25.50
−16 ≤ h ≤ 19
−11 ≤ k ≤11
−19 ≤ l ≤19
97.00
4427/0/331
1.062
7341
4341
25.25
−19 ≤ h ≤19
−16 ≤ k ≤16
−22 ≤ l ≤22
99.1
7341/0/500
0.820
5843
3876
25.50
−19 ≤ h ≤20
−24 ≤ k ≤25
−10 ≤ l ≤10
93.0
5843/0/440
1.259
9492
6870
28.48
−16 ≤ h ≤ 16
−32 ≤ k ≤33
−15 ≤ l ≤16
99.0
9492/0/460
1.294
Ranges (h, k, l)
Complete to 2θ (%)
Data/restrain/parameter
Goof (F2)
R indices [I > 2σ(I)]
R indices (all data)
Residual electron density
0.0658
0.1216
0.20
0.0366
0.0715
0.44
0.0789
0.1242
1.81
0.0867
0.1088
0.97
Compound no.
7
8
12
13
Formulae
CCDC no.
Mol. wt.
Crystal system
Space group
Temperature (K)
Wavelength (Å)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
74H70Cd2N6O26
C86H96Hg2N10O20
837718
1990.91
C27H30K2O11
851819
608.71
monoclinic
P21/c
C27H27K2NO11
851820
619.70
orthorhombic
Pbca
837720
1684.16
triclinic
P1
296(2)
0.71073
11.7715(8)
12.4713(9)
15.3045(11)
80.416(3)
80.098(3)
67.749(3)
2035.8(2)
1
triclinic
ˉ
ˉ
P1
296(2)
296(2)
296(2)
0.71073
9.7843(3)
12.7257(4)
19.1614(6)
90.829(2)
97.845(2)
111.982(2)
2186.08(12)
1
1.512
3.582
empirical
1002
0.71073
17.4531(15)
14.4853(15)
11.8689(10)
90.00
78.186(4)
90.00
2937.1(4)
4
1.377
0.71073
13.8855(4)
12.4867(3)
33.5892(10)
90.00
90.00
90.00
5823.8(3)
8
1.414
0.385
none
γ (°)
V (Å3)
Z
Density (Mg m−3
)
1.374
0.600
none
860
Abs. coeff. (mm−1
Abs. correction
F(000)
)
0.380
none
1272
2576
Total reflection
Reflections, I > 2σ(I)
Max. θ/°
7252
6287
25.50
7501
5716
25.50
7250
5475
28.29
7251
5245
28.34
Ranges (h, k, l)
−14 ≤ h ≤ 14
−15 ≤ k ≤ 15
−18 ≤ l ≤ 18
95.6
7252/0/491
1.079
−11 ≤ h ≤ 11
−15 ≤ k ≤ 15
−23 ≤ l ≤ 23
92.1
7501/0/539
0.995
−23 ≤ h ≤ 23
−16 ≤ k ≤ 19
−15 ≤ l ≤ 15
99.5
7250//4/381
1.027
−18 ≤ h ≤ 16
−16 ≤ k ≤ 16
−43 ≤ l ≤ 44
99.6
7251/2/382
1.098
Complete to 2θ (%)
Data/restrain/parameter
Goof (F2)
R indices [I > 2σ(I)]
R indices (all data)
Residual electron density
0.0384
0.0449
0.63
0.0341
0.0503
0.54
0.0957
0.1154
0.66
0.0646
0.0882
0.52
ligand H2L3 (3) formed a metallacycle [CdL3(py)2(H2O)]2·3H2O
(7) (where H2L3
(3-NO2–C6H4)CH{3-CH3–, 5-CH3–,
ions are linked by carboxylate functional groups present at two
ends of the ligands. A similar reaction of H2L2 with mercury(II)
acetate in DMF gave metallacycle [HgL2(py)2]2·DMF (8). The
=
6-(–OCH2CO2H)C6H2}2). In all of the complexes, the metal
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synthesis of cadmium metallo-organic frameworks of different
dimensions,7,9,10 but the formation of metallacycles from those
ligands were not observed.
1
The HNMR spectra of coordination polymer 4 is shown in
Fig. 3. There are three singlets at 2.11 ppm, 2.14 ppm and
4.01 ppm; the former two signals are for two different types of
methyl groups on the aromatic rings and the latter is for –CH2O–
group. Again, the singlet peaks at 6.37 ppm, 6.43 ppm and
6.85 ppm appear for the methine proton and aromatic protons of
methyl group-containing ring, respectively. The peaks for the
coordinated pyridine molecule appear at 7.39 (triplet), 7.79
(doublet) and 8.58 ppm (singlet), as illustrated in Fig. 3.
The reaction of cadmium(II) acetate with H2L2 in MeOH–pyri-
dine forms a dinuclear complex [CdL2(py)(CH3OH)]2·CH3OH
(5), where both of the cadmium ions are in identical environ-
ments (Fig. 4a). Each cadmium ion has one pyridine and one
methanol molecule in its coordination sphere and the methanol
ligands are independently attached to two cadmium ions. They
project away from each other at two ends of the dimeric struc-
ture. The presence of methanol molecules as ligands at the term-
inal ends inhibits the formation of a polymeric structure. It helps
in providing an adequate geometry to the carboxylate ligands to
form a bridged structure. The nitro group present at the ortho-
position of the aromatic ring also plays a role by providing the
adequate twist to the carboxylate group to form the Cd2O2 type
core. Such effects were revealed in 2-nitrobenzoate complexes in
our earlier studies with metal ions, such as zinc, manganese and
cadmium.15 Alternatively, the 2-nitro benzoate provides the
appropriate orientation to two carboxylate groups attached two
metal ions to form the Cd2O2 core.15c Complex 5 has one pyri-
dine ligand per cadmium ion and these two metal ions in close
proximity form the seven coordinate dinuclear complex and the
metal–metal distance in this dinuclear complex is found to be
3.97 Å. Generally, to form Cd2O2 core with η1 and η3-carboxy-
late binding mode the cadmium–cadmium separations has to be
magnitude15a of 3.8 to 4.15 Å. The selected bond angles and
bond distances are listed in Table 3.
Fig. 1 The crystal structure of ligand H2L2 (H-atoms are omitted).
Fig. 2 Zig–zag one dimensional coordination polymer 4 formed by
ligand L1.
crystal structures of all of these complexes were determined.
Despite the facts that all of the skeletal parameters, other than
the substituent on the aromatic ring of the ligands, are similar
and the syntheses of these complexes were carried out at
ambient conditions, the number of pyridine molecules per
cadmium ion in these complexes varies. This suggests that the
substituent present at the remote site of the aromatic ring guides
the incoming pyridine molecules to the coordination sphere of
cadmium ions.
The coordination polymer 4 has seven coordinated cadmium
ions, each having a pentagonal bipyramidal geometry. These
geometries are constructed by three monodentate pyridine
ligands and two chelating carboxylate groups from two indepen-
dent ligands. The coordination polymer has a spiral structure
(Fig. 2). Each pentagonal bipyramid has two pyridine ligands at
its axial positions. The equatorial positions are occupied by four
oxygen atoms of carboxylate ligands and a nitrogen atom of a
pyridine ligand. The metal–ligand bond parameters are listed in
Table 2. A similar reaction of H2L1 with cadmium(II) acetate in
the presence of 4-methylpyridine (4-mepy) yielded linear coordi-
nation polymer [{CdL1(4-mepy)2}·H2O]n (9). It has a similar
structure to that of 4, but due to poor data quality the structure is
not discussed, but is available as ESI.† The V-shaped carboxylic
acid ligands along with less flexible ligands were used in the
The 1HNMR spectra of dinuclear complex 5 is shown in
Fig. 5a. From the spectra, it is seen that here also there are three
singlets at 2.10 ppm, 2.15 ppm and 4.10 ppm for two types of
methyl protons and –CH2O–, respectively. Again, the singlets at
6.23 ppm, 6.90 ppm and 6.65 ppm are due to the aromatic
protons of the methyl group-containing aromatic ring and
methine proton of the parent bis-phenol molecule, respectively.
The signals for the protons of the coordinated pyridine mol-
ecules appear at 7.39 ppm (triplet), 7.79 ppm (triplet) and
8.58 ppm (doublet), these assignments are confirmed by record-
ing HOMO-COSY spectra (ESI†).
Ligand 2 reacted with Cd(O2CCH3)2 in dimethylformamide
(DMF) and pyridine to form metallacycle [CdL2(py)2-
(H2O)]2·H2O (6). This metallacycle may be attributed to opening
of a Cd2O2 core in a dinuclear complex that could have been an
intermediate compound. Each cadmium ion has a coordination
number of seven, comprised of two chelating carboxylates and a
water molecule along with two pyridine ligands. Although the
DMF was used as the solvent, we did not observe DMF mol-
ecules in its crystal lattice as the solvent of crystallization, nor it
was observed as a ligand. However, DMF can easily interact
with different functional groups (in this case either nitro or
7120 | Dalton Trans., 2012, 41, 7115–7126
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Table 2 The bond lengths and angles in coordination polymer 4
M–L
dM–L (Å)
∠L–M–L
Angle (°)
∠L–M–L
Angle (°)
∠L–M–L
Angle (°)
Cd1–N3
Cd1–N2
Cd1–O1
Cd1–N1
Cd1–O5
Cd1–O6
Cd1–O2
2.333(3)
2.344(3)
2.360(2)
2.375(3)
2.474(3)
2.478(3)
2.548(3)
N3–Cd1–N2
N3–Cd1–O1
N2–Cd1–O1
N3–Cd1–N1
N2–Cd1–N1
O1–Cd1–N1
N3–Cd1–O5
95.27(11)
84.24(10)
136.16(10)
170.04(10)
88.85(11)
86.51(10)
83.78(11)
N2–Cd1–O5
O1–Cd1–O5
N1–Cd1–O5
N3–Cd1–O6
N2–Cd1–O6
O1–Cd1–O6
N1–Cd1–O6
129.82(10)
93.83(9)
100.54(11)
107.21(11)
82.30(10)
139.73(10)
82.30(11)
O5–Cd1–O6
N3–Cd1–O2
N2–Cd1–O2
O1–Cd1–O2
N1–Cd1–O2
O5–Cd1–O2
O6–Cd1–O2
51.12(9)
82.01(10)
82.98(9)
53.44(8)
89.52(10)
145.35(9)
163.28(10)
Fig. 3 The 1HNMR spectrum of the coordination polymer 4 in DMSO-d6.
Fig. 4 (a) The structure of dinuclear complex 5 and (b) the structure of metallacycle 6. In both cases the uncoordinated solvent molecules are
omitted for clarity.
carboxylic acid) during the course of the reaction, to guide the
carboxylate to form a metallacycle. The structure of the complex
determined by X-ray single crystal diffraction is shown in
Fig. 4b. The very weak bonds Cd1–O7 and Cd1–O2 are
2.501(6) and 2.555(5) Å, whereas the Cd1–O8 and Cd1–O1 are
2.327(5) and 2.308(5) Å, respectively, supporting that the carbox-
ylates are ligated as bidentate chelate. The metal–ligand bond
distances and bond angles are listed in Table 3. The cadmium ions
have pentagonal bipyramid structures, in which one pyridine and
two carboxylate groups occupy the equatorial positions; a water
molecule and a pyridine molecule occupy the axial positions.
From the HNMR spectra of the metallacycle 6 (Fig. 5b) it is
seen that there are three singlets at 2.09 ppm, 2.15 ppm and
4.12 ppm for two types of methyl protons of aromatic rings and
1
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Table 3 The metal–ligand bond parameters of dinuclear complex 5 and metallacycle 6
M–L
dM–L (Å)
∠L–M–L
Angle (°)
∠L–M–L
For 5
Angle (°)
∠L–M–L
Angle (°)
Cd1–N3
Cd1–N2
Cd1–O5
Cd1–O2
Cd–O6
Cd1–O1
Cd1–N1
2.333(3)
2.344(3)
2.474(3)
2.548(3)
2.478(3)
2.360(2)
2.375(3)
O9–Cd1–O7
O2–Cd1–O8
N2–Cd1–O8
O9 Cd1–O8
N2–Cd1–O2
O9–Cd1–O2
O9–Cd1–N2
87.85(18)
112.39(17)
142.6(2)
98.20(18)
91.20(19)
123.32(19)
91.54(19)
N2–Cd1–O7
O2–Cd1–O7
O8–Cd1–O7
O9–Cd1–O1
N2–Cd1–O1
O2–Cd1–O1
O8–Cd1–O1
89.14(18)
148.80(18)
55.56(16)
82.1(2)
128.9(2)
53.94(18)
88.27(18)
O1–Cd1–O2
O7–Cd1–O2
O8–Cd1–O2
O2–Cd1–O2
N2–Cd1–O2
O9–Cd1–O2
O7–Cd1–O1
113.83(18)
78.47(16)
81.05(16)
70.9(2)
80.01(18)
163.95(18)
140.63(17
For 6
Cd1–O1
Cd1–N3
Cd1–O9
Cd1–O8
Cd1–N2
Cd1–O7
Cd1–O2
2.308(5)
2.312(6)
2.322(5)
2.327(5)
2.334(6)
2.501(6)
2.555(5)
O1–Cd1–N3
O1–Cd1–O9
N3–Cd1–O9
O1–Cd1–O8
O1–Cd1–N2
O1–Cd1–O7
N3–Cd1–O7
136.38(19)
82.17(19)
87.7(2)
89.98(17)
97.0(2)
141.39(17)
80.0(2)
N3–Cd1–N2
O9–Cd1–N2
O8–Cd1–N2
N3–Cd1–O8
O9–Cd1–O7
O8–Cd1–O7
N2–Cd1–O7
91.7(3)
178.0(2)
89.6(3)
132.9(2)
88.2(2)
52.96(19)
93.5(2)
O1–Cd1–O2
N3–Cd1–O2
O9–Cd1–O2
O8–Cd1–O2
N2–Cd1–O2
O7–Cd1–O2
53.46(16)
84.6(2)
90.7(2)
142.51(18)
87.3(2)
164.53(18)
Fig. 5 The 1HNMR spectra of (a) dinuclear complex 5 and (b) metallacycle 6 in DMSO-d6.
–CH2O–, respectively. Again, the singlets at 6.24 ppm,
6.90 ppm and 6.65 ppm are due to the aromatic protons of
methyl groups attached to aromatic rings and methine proton of
the parent bis-phenol molecule, respectively. The signals for the
protons of the coordinated pyridine molecules appear at
7.39 ppm (triplet), 7.81 ppm (triplet) and 8.59 ppm (doublet).
7122 | Dalton Trans., 2012, 41, 7115–7126
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Fig. 7 The structure of metallacycle 7 (solvent molecules are omitted
for clarity).
Fig. 6 The FT-IR (KBr, cm−1) of the dinuclear complex 5 (top) and
the metallacycle 6 (bottom).
From the 1HNMR spectra of 5 and 6, it is seen that although the
position of the peaks are almost similar, the splitting pattern and
coupling constant values are different, suggesting that in solution
they retain their identity. Furthermore, the coupling scheme inter-
1
preted from HNMR is confirmed by recording HOMO-COSY
spectra (ESI†).
The formation of different types of structures in different sol-
vents shows that the solvent guides the coordination of pyridine
into the coordination spheres of the metal ions in these com-
plexes. Conversely, the numbers of pyridine molecules also
decide the formation of a dinuclear metal complex or metalla-
cycle. This result demonstrates a process that has close analogy
to the off and on of a metallacycle through the coordination
effect. This important observation has relevance and analogy
with tunable on–off responses for select guest molecules17,18 by
metal–organic frameworks, in which molecules may come close
to self-assemble or get dis-assembled by solvent molecules or by
an external ligands. Furthermore, there are examples of cyclic
molecules adopting different polymorphic structures to controls
pores in transport processes.19
The PXRD patterns of complex 5 and 6 are distinguishable
(ESI†). The experimental PXRD pattern of complex 6 is in
excellent agreement with the theoretical pattern, however, the
PXRD pattern of complex 5 is in agreement with the simulated
Miller indices, but with a slight shifting of the peaks in θ-values.
We do not have a proper explanation for the shift. Thermogravi-
metry shows that the compound easily loses methanol on heating
at 25–70 °C. The IR spectra of complexes 5 and 6 are dis-
tinguishable, as the carbonyl stretching of complex 5 appears at
1630 cm−1, whereas the carbonyl stretching of complex 6
appears at 1602 cm−1, which is shown in the Fig. 6.
Fig. 8 The powder X-ray diffraction pattern of complex 7 (the upper
one is experimental, whereas the lower one is simulated).
monodentate and another bidentate with distorted chelate struc-
ture, where Cd1–O2, Cd1–O7 and Cd1–O9 bond distances are
2.346 Å, 2.263 Å and 2.341 Å, respectively. These distances are
within the limit of generally observed Cd–O bond distances in
related compounds. Whereas the Cd1–O1 separation at 2.524 Å
and Cd1–O8 separations at 2.677 Å are long for the formation of
a bond, but the Cd1–O1 can be suggested as weakly interac-
ting.2b,15c This makes the two slightly different coordination
modes of the two carboxylate attached to a cadmium ion. The
Cd1–N2 and Cd1–N3 distances in the complex are 2.361 Å and
2.332 Å, respectively. Thus, the molecule adopts distorted penta-
gonal bipyramid geometry, in which one pyridine and two car-
boxylates connected to Cd1 make the five-member geometry and
the axial bonds are Cd–N2 and Cd1–O9 bonds. The bond angles
∠N3–Cd1–O2, ∠N3–Cd–O7 and ∠O2–Cd1–O7 are 134.32°,
139.46° and 85.07°, respectively. If the two weak contacts are
not taken into consideration, then each of the cadmium ions in
the complex can be described as a distorted trigonal bipyramid.
Nonetheless, the overall structure of the metallacycle is highly
symmetric and contains a mirror plane that bisects it into two
halves. Each equivalent half contains one cadmium ion with one
carboxylate, two pyridine and one water molecules. The simu-
lated and the experimental powder X-ray diffraction patterns of 7
are shown in Fig. 8. All of the principal peaks for different
Metallacycle [CdL3(py)2(H2O)]2·3H2O (7) was obtained from
the reaction of cadmium(II) acetate with H2L3 (3) in methanol
and pyridine, where both of the cadmium ions are in identical
environments (Fig. 7). The other coordination sites of the metal
atoms are occupied by two pyridine molecules and a water mol-
ecule. It is evident from the Cd–O bond distances that the car-
boxylate groups in this complex are a combination of a
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these atoms. This shows that the carboxylates are coordinated in
a monodentate fashion to the mercury(II). From the 1HNMR
spectrum of the mercury-containing metallacycle, it is seen that
signals from the hydrogen atoms of coordinated pyridine mol-
ecules appear at 7.52 ppm, 7.91 ppm and 8.69 ppm (ESI†).
The packing pattern of the mercury complex suggests that the
solvent molecules are held between the interstitial spaces of the
metallacycles. The coordinating carboxylate groups on the cyclic
part have no interactions with the DMF molecules of the lattice.
The carbonyl oxygen of DMF and a C–H bond of the aromatic
ring of a ligand provide C15–H⋯O9 interactions (dD–H
,
0.929 Å; dD–A, 3.295 Å; ∠D–H⋯A, 140.6°) to hold the DMF in
the interstices, as shown in Fig. 9b.
We have also obtained macrocyclic compounds of mercury
with composition [HgL2(4-mepy)2]2 (10) and [HgL3(3-
mepy)2]2·DMF (11) from ligands H2L2 and H2L3 in the pres-
ence of 4-methylpyridine (4-mepy) or 3-methylpyridine (3-
mepy) under similar reaction conditions to those used for the
preparation of 8. Since their data quality are poor, they are not
discussed to elucidate their geometry, but the crystallographic
information files are provided as ESI.†
Potassium complexes of the ligands
Since the presence of a nitro group enabled us to isolate two
different structures of cadmium complexes, we have examined
the structure of the potassium salts as these may be used as pre-
cursors for the preparation of the cadmium complexes through
cation exchange reactions. The reaction of H2L1 or H2L2 with
Fig. 9 (a) The X-ray single crystal structure of metallacycle 8. The
hydrogen atoms and solvents are omitted for clarity. (b) Weak inter-
actions of DMF molecules in the crystal lattice of 8.
potassium
hydroxide
in
DMF
resulted
in
Table 4 The metal–ligand bond lengths and bond angles of complex 8
[K4(L1)2(μ-H2O)2(H2O)2](H2O)n (12) and [K2L2(H2O)]n (13),
respectively. Potassium complex 12 is a 2-D coordination
polymer. The polymer is comprised of potassium ions that have
a coordination geometry of seven with a pentagonal bipyramid
structure, as well as with six coordination potassium ions with
distorted octahedral geometry (Fig. 10a). It has two different
types of water ligands; one set is bridging and the other is
monodentate.
M–L
dM–L (Å)
∠L–M–L
Angle (°)
Hg1–N3
Hg1–O1
Hg1–O8
Hg1–N2
2.227(3)
2.233(3)
2.291(3)
2.381(4)
N3–Hg1–O1
N3–Hg1–O8
O1–Hg1–O8
N3–Hg1–N2
O1–Hg1–N2
O8–Hg1–N2
119.95(13)
142.46(13)
87.03(13)
103.76(13)
101.88(13)
94.40(14)
In coordination polymer 13, the interesting feature is the
coordination of the nitro group present at the ortho-position of
the aromatic ring (Fig. 10b). The coordination polymer has two
different coordination environments for potassium; these are
seven and eight coordination geometry. The nitro group also acts
as a bridging ligand to hold two potassium ions. Between the
two ethereal oxygen atoms of the ligand L2, one oxygen atom
coordinates, while the other remains uncoordinated. In alkali
metal carboxylate polymers ethereal oxygen plays an important
role in providing the final geometry.20 But due to the involve-
ment of oxygen atoms of the nitro group in coordination, it hin-
dered the coordination of one of the ethereal oxygens. From this
observation, it is clear that the presence of a nitro group at the
ortho-position plays a role by coordination to the alkali metal
cation in the synthesis of complexes 5 and 6 when potassium
hydroxide or sodium hydroxide is used to generate the dicarbox-
ylate anions.
Miller indices are observed, confirming the phase purity and
homogeneity of the samples.
Since both metallacycles as well as a binuclear complex of
cadmium with the H2L2 are observed, we have examined the
effect of size of the metal ions in deciding their formation. Thus,
mercury metallacycle [HgL2(py)2]2·DMF (8) was prepared by
reacting H2L2 with mercury(II) acetate in DMF, followed by
treatment with pyridine (Fig. 9a). In this case, we were not suc-
cessful in obtaining crystals from the reaction carried out in
methanol. The size of mercury being bigger than cadmium, it
accommodated two pyridine ligands and retained a cyclic struc-
ture, where each mercuric ion has a distorted tetrahedral geome-
try. The metal–ligand bond parameters are listed in Table 4. The
Hg1–O2 and Hg1–O7 separations are 2.91 Å and 2.68 Å,
respectively, suggesting that there is no Hg–O bond between
7124 | Dalton Trans., 2012, 41, 7115–7126
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heating loses two molecules of H2L2. Potassium salts 12 and 13
lose five water and three water molecules in the temperature
ranges 50–150 °C and 55–130 °C, respectively, on heating.
In conclusion, we have shown the formation of metallacycle
and coordination polymers of cadmium and mercury from three
flexible dicarboxylate ligands. As an exceptional case, we found
the formation of a dinuclear complex, as well as the metallacycle
of cadmium of ligand H2L2 by changing the solvent from
methanol to dimethylformamide. It is suggested that the solvent
used in these reactions guides the number of incoming ancillary
ligands, such as pyridine, that are anchored to cadmium. Again,
this series of complexes also clearly demonstrates the role of the
nitro group on the ligand; it is shown that even when it doesn’t
coordinate to the cadmium or mercury ions, its presence has a
striking effect on the structure of cadmium and mercury com-
plexes. These types of substituent effect in controlling and
tuning the molecular architecture of the coordination polymers
would be a potentially effective approach. Strikingly, the ability
of the nitro group on the ligand attached to potassium suggests
the role of the alkali metal ion in guiding these types of coordi-
nation polymer frames. Although there have been substantial
contributions to cadmium coordination chemistry, the approach
to obtain metallacycles have not been pursued. We have demon-
strated that a flexible tether (–CH2CO2) with rigid directing func-
tionality (in this case the bis-phenol part) makes it possible to
obtain metallacycles in ambient conditions.
Acknowledgements
BN thanks the Council of Scientific and Industrial Research,
New Delhi, India for a senior research fellowship. The authors
thank the Department of Science and Technology, New-Delhi,
India for financial support for the X-ray facility.
Fig. 10 The repeat units of the coordination polymer of potassium salt
(a) 12 (b) 13 (hydrogen atoms are omitted for clarity).
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Thermogravimetry
For coordination polymer 4, in the temperature range 50–180 °C,
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molecules of the ligand (H2L3) at above 165 °C. Metallacycle 8
loses four coordinated pyridine molecules and two uncoordi-
nated DMF molecules when heated to 215 °C, which on further
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7126 | Dalton Trans., 2012, 41, 7115–7126
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