Synthesis, characterization and single crystal X-ray structure determination of three cadmium(II) complexes derived from picric acid and p-nitrobenzoic acid in the presence and absence of nitrogen donor ligand N-(hydroxyethyl)ethylenediamine

Article abstract of DOI:10.1016/j.poly.2017.09.053

Cadmium, an extremely toxic heavy metal, is of a major public health concern and its efficient removal from the environment is a real challenge. This work aims for a deep understanding of stable cadmium(II) chelation with a tridentate ligand to form complex salts being stabilized by cation–anion hydrogen bonding non-covalent interactions in addition to electrostatic interactions. Three new Cd(II) complexes, [Cd(H₂O)₆](pic)₂·2H₂O 1, [Cd(N-hyden)₂](pic)₂ 2, [Cd(N-hyden)₂](pnb)₂ 3, (where N-hyden = N-(hydroxyethyl)ethylenediamine, Hpic = picric acid and Hpnb = p-nitrobenzoic acid) using the appropriate acids in the absence and presence of the tridentate N-hyden ligand have been synthesized and fully characterized by elemental analyses, thermogravimetric analyses and spectroscopic (FT-IR, NMR) techniques. The exact ionic structures of complexes 1–3 have been unambiguously determined using single crystal X-ray diffraction studies. They comprise octahedrally surrounded cationic Cd(II) centers, [Cd(H₂O)₆]²⁺ in 1 and [Cd(N-hyden)₂]²⁺ in 2 and 3, together with the counter anions pic/pnb, which are stabilized by a mutual interplay of various hydrogen bonding interactions.

Full text of DOI:10.1016/j.poly.2017.09.053

Polyhedron 139 (2018) 178–188
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and single crystal X-ray structure
determination of three cadmium(II) complexes derived from picric acid
and p-nitrobenzoic acid in the presence and absence of nitrogen donor
ligand N-(hydroxyethyl)ethylenediamine
a
b,
Santosh Kumar a, Raj Pal Sharma a, , Paloth Venugopalan , Thammarat Aree
⇑
⇑
a Department of Chemistry, Panjab University, Chandigarh 160014, India
b Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 August 2017
Accepted 23 September 2017
Cadmium, an extremely toxic heavy metal, is of a major public health concern and its efficient removal
from the environment is a real challenge. This work aims for a deep understanding of stable cadmium(II)
chelation with a tridentate ligand to form complex salts being stabilized by cation–anion hydrogen bond-
ing non-covalent interactions in addition to electrostatic interactions. Three new Cd(II) complexes,
[Cd(H2O)6](pic)2ꢀ2H2O 1, [Cd(N-hyden)2](pic)2 2, [Cd(N-hyden)2](pnb)2 3, (where N-hyden = N-(hydrox-
yethyl)ethylenediamine, Hpic = picric acid and Hpnb = p-nitrobenzoic acid) using the appropriate acids
in the absence and presence of the tridentate N-hyden ligand have been synthesized and fully character-
ized by elemental analyses, thermogravimetric analyses and spectroscopic (FT-IR, NMR) techniques. The
exact ionic structures of complexes 1–3 have been unambiguously determined using single crystal X-ray
diffraction studies. They comprise octahedrally surrounded cationic Cd(II) centers, [Cd(H2O)6]2+ in 1 and
[Cd(N-hyden)2]2+ in 2 and 3, together with the counter anions pic/pnb, which are stabilized by a mutual
interplay of various hydrogen bonding interactions.
Keywords:
Cadmium
N-(hydroxyethyl)ethylenediamine
Picrate
p-Nitrobenzoate
Hydrogen bonding interactions
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
biological species, e.g., Zn-Cu superoxide dismutase, carboxypepti-
dase, etc. On the other hand, Cd/Hg salts/complexes are known to
The design and synthesis of novel complexes with toxic metal
ions with an appended framework are the fundamental requisites
for the discovery of various functional supramolecular devices or
technologically useful materials [1]. Recently, a great deal of atten-
tion has been paid to the design and construction of organic–inor-
ganic hybrid frameworks from various synthons or building blocks
owing to their interesting structural diversity and enormous
potential application in chemistry, biology and material science
[2,3]. Heavy metals are environmental pollutants due to their
non-degradable nature. Although Zn, Cd and Hg belong to the same
metal group of the Periodic Table (group 12) with the similar elec-
tronic configuration nd10ns2, their chemical characteristics and
potential applications are different. For example, zinc salts/
complexes have been extensively studied over the years because
a number of zinc containing enzymes are known to exist in
be highly toxic and can accumulate in the human body leading to
renal dysfunction and various other diseases, including cancer, as
Ca2+ and Cd2+ ions have similar ionic radii. Moreover, cadmium
and mercury, having a large size with a half-life of many years in
humans, are known to be toxic [7] and show direct or indirect
interactions with DNA, hence leading to cancer [8,9].
The cadmium(II) ion exhibits different coordination numbers
from 4 to 8 in its coordination complexes with various ligands
[4–6]. Despite the harmful effects of cadmium, an outstanding
example is the discovery of a cadmium-containing carbonic-anhy-
drase enzyme from Thalassiosira weissflogii that specifically uses
cadmium to achieve its biological function, suggesting the impact
of cadmium on the global carbon cycle [9]. In order to reduce the
toxicity of heavy metals, chelation therapy is an important aspect
as metals are removed in the form of stable chelate complexes.
So, the design of drugs to overcome the toxicosis requires an in-
depth insight into the properties of the ligands employed and the
geometry of resultant complexes [9]. Para-nitrobenzoic acid (Hpnb)
and picric acid (Hpic) are used due to their wide applications in
⇑
Corresponding authors.
E-mail addresses: rpsharmapu@yahoo.co.in (R.P. Sharma), thammarat.aree@
gmail.com (T. Aree).
https://doi.org/10.1016/j.poly.2017.09.053
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

S. Kumar et al. / Polyhedron 139 (2018) 178–188
179
Fig. 1. Schematic representation of second-sphere coordination complexes via hydrogen bonding between the complex cation [Cd(N-hyden)2]2+ and pic/pnb anions shown as
X in the second coordination sphere.
Table 1
nuclear chemistry, biology and the pharmaceutical industry
Crystal data and refinement parameters of complexes 1–3.
[10,11]. Moreover, arylcarboxylates (pnb here) can be employed
as building blocks in crystal engineering because of the versatility
of their coordination modes (monodentate, symmetric/asymmetric
chelating and bidentate/monodentate bridging) and ability to form
strong directional hydrogen bonds with predictable/desirable
properties [10–12]. On the other hand, picric acid easily forms salts
with metal ions due to its strong acidity. Its lead salt is widely used
as a primary explosive [13–15]. Picric acid, with three nitro groups,
can form crystalline complexes with various organic molecules
Complex 1
Complex 2
Complex 3
Abbreviated formula
[Cd(H2O)6]
[Cd(N-hyden)2]
(pic)2
[Cd(N-hyden)2]
(pnb)2
(pic)2ꢀ2H2O
Empirical formula
Crystal habit
C12H18CdN6O21 C20H28CdN10O16 C22H32CdN6O10
rod, yellow
rod, yellow
needle, light
yellow
Crystal size (mm)
0.24 ꢂ 0.24 ꢂ
0.30
694.40
orthorhombic
Pccn (No. 56)
0.14 ꢂ 0.22 ꢂ
0.30
776.93
orthorhombic
C2221 (No. 20)
0.15 ꢂ 0.15 ꢂ
0.40
Mw
652.95
triclinic
Crystal system
Space group
with the help of ionic, non-covalent hydrogen bonding and
p–p
P1 (No. 2)
interactions, and has been widely applied as a supramolecular
heterosynthon in the design of new complexes [16–18].
Unit cell dimensions
a (Å)
b (Å)
c (Å)
25.3723(9)
7.2691(3)
13.2050(4)
90
90
90
2435.5(2)
4
1.944
1.012
1432
9.8836(2)
12.9192(3)
46.4409(10)
90
90
90
5930.0(2)
8
1.740
0.829
3152
7.1058(2)
7.5543(2)
13.0469(4)
102.255(1)
104.976(1)
98.515(1)
645.69(3)
1
1.679
0.913
334
296
It is envisaged that complexation of the cadmium(II) ion with
the N-donor ligand N-hyden, which possesses two N–H groups
and one O–H group, could form stable chelated cations,
[Cd(N-hyden)2]2+or [Cd(N-hyden)2(H2O)2]2+ (Fig. 1), depending on
whether a tridentate or bidentate coordination mode of the ligand
is adopted. This doubly-positively charged cation possesses eight
(six N–H and two O–H) hydrogen bond donors and has a stable
structural framework onto which anionic components can be
assembled [19–21] by various hydrogen bonding interactions.
Our research group has successfully isolated new anionic cad-
mium(II) complex salts with the help of large cationic cobalt(III)
species: [Cd2Cl7]3ꢁ and [Cd2Br7]3ꢁ by using the [Co(phen)3]3+
cation [22(a)]; [CdBr4(C7H5O2)]3ꢁ by using the [Co(en)3]3+ cation
[22(b)]; [Cd3Br10(H2O)2]4ꢁ by using the [Co(NH3)6]3+ cation [22
(c)]; [trans-CdBr4Cl2]4ꢁ and [CdBr6]4ꢁ by using the [Co(en)3]3+
cation [22(d)]; and [CdI4]2ꢁ by using the [Co(en)2Cl2]+ cation [22
(e)]. In continuation of our interest in the structural chemistry of
metal-arylcarboxylates [23,24] with nitrogen donor ligands, the
present work reports the synthesis, characterization and single
crystal X-ray structure determination of the cadmium(II) com-
plexes [Cd(H2O)6](pic)2ꢀ2H2O 1 (structure redetermination), [Cd
(N-hyden)2](pic)2 2 and [Cd(N-hyden)2](p-nitrobenzoate)2 3.
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dc (Mg mꢁ3
)
l
(mmꢁ1
)
F(000)
T (K)
Radiation (Å)
296
296
Mo K
a;
Mo Ka; 0.71073 Mo Ka;
0.71073
24708/4918/
3192
0.0344
4918/188
1.020
0.0343, 0.0847 0.0394, 0.0994
0.0609, 0.0980 0.0427, 0.1012
ꢁ0.64, 0.48
0.71073
17664/3975/
3718
0.0325
3975/182
1.044
0.0257, 0.0569
0.0296, 0.0584
ꢁ0.29, 0.53
Reflections collected/
19643/9056/
8474
0.0214
9056/424
1.094
unique/>2r(I)
Rint
Data/parameters
Goodness on fit
R1, wR2 [I > 2
R1, wR2 [all data]
(e Åꢁ3
r(I)]
D
q
)
ꢁ1.14, 0.63
CAUTION! Although we have experienced no problems in handling
picrate complexes, these should be handled with great caution,
because of their explosive nature.
2.2. Sample preparation
2. Experimental
2.2.1. Synthesis of [Cd(H2O)6](pic)2ꢀ2H2O, 1
0.34 g (2 mmol) of cadmium(II) carbonate was suspended in 20
mL of water. To the stirred CdCO3 suspension at 50–60 °C, 0.91 g
(4 mmol) of picric acid was added slowly. The mixture was stirred
for 20 min in the temperature range 50–60 °C until the efferves-
cence ceased completely, indicating formation of cadmium picrate.
The resultant reaction mixture was filtered and put aside for slow
solvent evaporation at room temperature. Yellow block shaped
crystals were harvested after a few days. Complex 1 decomposes
at 236 °C. Complex 1 is sparingly soluble in water and insoluble
in organic solvents like methanol, acetone, chloroform, etc. Anal.
Calc. (%): C, 20.73; H, 2.59; N, 12.09; Cd, 16.18. Found (%): C,
20.81; H, 2.51; N, 12.25; Cd, 16.24.
2.1. Materials and physical measurements
Analytical grade reagents were used without any further purifi-
cation. Carbon, hydrogen and nitrogen contents were measured
micro-analytically with an automatic Perkin Elmer 2400 CHN ele-
mental analyzer and the cadmium content was determined gravi-
metrically [25]. FT-IR spectra were recorded using a PERKIN ELMER
SPECTRUM RXFT-IR system. Conductance measurements were per-
formed with a Pico Conductivity Meter (Model CNO4091201, Lab
India) in aqueous medium at 25 °C, using double distilled water.
Multinuclear NMR spectra were recorded on BRUKER AVANCE II
400 MHz spectrophotometer. Thermo-gravimetric (TG) analyses
of complexes 1–3 were carried out by a Simultaneous Thermal
Analyzer (STA), manufactured and supplied by METTLER TOLEDO,
Model Mettler Toledo 851e. The samples were subjected to heating
from 25 to 1000 °C at a heating rate 10 °C/min under a nitrogen
atmosphere.
2.2.2. Synthesis of [Cd(N-hyden)2](pic)2, 2
Method 1: 0.5 g (2 mmol) of CdCl2 was dissolved in 20 mL of
ethanol. To the stirred CdCl2 solution, N-(hydroxyethyl)ethylenedi-
amine (4 mmol) was added dropwise. The mixture was stirred for

180
S. Kumar et al. / Polyhedron 139 (2018) 178–188
Table 2
solution. The precipitates were filtered and the filtrate was put
Structural parameters [Å, °] of complexes 1–3.
aside to obtain orange yellow crystals after seven days at room
temperature. Complex 2 decomposes at 220 °C. Complex 2 is sol-
uble in water and insoluble in organic solvents like methanol, ace-
tone, chloroform, etc.
Complex 1
Cd1–O1W
Cd1–O2W
Cd1–O3W
Complex 2
Cd1–N1
2.249(2)
2.297(1)
2.225(2)
O1W–Cd1–O2W
O1W–Cd1–O3W
O2W–Cd1–O3W
84.63(6)
90.76(7)
86.16(6)
Method 2: 0.34 g (2 mmol) of cadmium(II) carbonate was sus-
pended in 20 mL of water. To the stirred CdCO3, in the temperature
range 50–60 °C, 0.91 g (4 mmol) of picric acid was added slowly.
The mixture was stirred for 20 min at 50–60 °C until the efferves-
cence ceased completely, indicating the formation of cadmium
picrate. To the resultant solution, still in the temperature range
50–60 °C, N-hyden (4 mmol) was added slowly. The solution was
stirred at 60 °C for 30 min. The resultant reaction mixture was fil-
tered and the filtrate was put aside to obtain orange yellow block
shaped crystals after seven days of evaporation at room tempera-
ture. Complex 2 decomposes at 220 °C. Complex 2 is also sparingly
soluble in water and insoluble in organic solvents like methanol,
acetone, chloroform, etc. Anal. Calc. (%): C, 30.84; H, 3.59; N,
17.99; Cd, 14.39. Found (%): C, 30.97; H, 3.32; N, 18.10; Cd, 14.51.
2.303(4)
2.326(4)
2.509(4)
N1–Cd1–O1
N2–Cd1–O1
N1–Cd1–N2
N1–Cd1–O1i
N3–Cd2–O2
N4–Cd2–O2
N3–Cd2–N4
N3–Cd2–O2ii
146.49(1)
71.55(1)
75.93(1)
72.89(1)
145.59(1)
68.62(1)
77.13(1)
89.48(1)
Cd1–N2
Cd1–O1
Cd2–N3
Cd2–N4
Cd2–O2
2.279(4)
2.298(4)
2.642(4)
Equivalent positions: (i) ꢁx + 1, y, ꢁz + 1.5; (ii) x, ꢁy, ꢁz + 2.
Complex 3
Cd1–N1
Cd1–N2
Cd1–O1
2.298(1)
2.320(1)
2.417(1)
N1–Cd1–N2
N1–Cd1–O1
N2–Cd1–O1
77.72(5)
89.97(4)
75.08(4)
45 min. The mixture was filtered in a filtration unit and dried in
vacuum to avoid moisture. 0.5 g of the above dried product, [Cd
(N-hyden)2]Cl2, was dissolved in a methanol–water solvent mix-
ture (25 mL), then the sodium salt of picric acid was added to
the solution in a 1:2 stoichiometry. Precipitation appeared in the
2.2.3. Synthesis of [Cd(N-hyden)2](pnb)2, 3
Complex 3 was synthesized similarly to method 2 employed for
the synthesis of complex 2. 0.34 g (2 mmol) of cadmium(II)
Table 3
Hydrogen bond parameters [Å, o] of complexes 1–3.
DꢁHꢀ ꢀ ꢀA
d(DꢁH)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\DHA
Complex 1
O1W–H1ꢀ ꢀ ꢀO4W
O1W–H2ꢀ ꢀ ꢀO5
O2W–H1ꢀ ꢀ ꢀO1i
O2W–H2ꢀ ꢀ ꢀO7ii
O3W–H1ꢀ ꢀ ꢀO4Wiii
O3W–H2ꢀ ꢀ ꢀO1iv
O4W–H1ꢀ ꢀ ꢀO1ii
O4W–H1ꢀ ꢀ ꢀO7ii
O4W–H2ꢀ ꢀ ꢀO2Wv
C3–H3ꢀ ꢀ ꢀO3iv
0.96
0.96
0.96
0.96
0.96
0.96
0.96
0.96
0.96
0.93
0.93
0.93
1.81
1.92
1.92
2.03
1.79
1.97
2.25
2.04
2.12
2.60
2.57
2.64
2.755(2)
2.875(2)
2.874(2)
2.915(2)
2.754(2)
2.892(2)
2.929(2)
2.892(2)
2.992(2)
3.418(2)
3.189(2)
3.261(2)
166.4
170.3
176.7
151.9
178.1
160.6
126.6
146.5
149.8
147.2
124.3
124.6
C3–H3ꢀ ꢀ ꢀO6vi
C5–H5ꢀ ꢀ ꢀO3vii
Equivalent positions: (i) x ꢁ 0.5, y + 0.5, ꢁz + 1; (ii) x ꢁ 0.5, ꢁy + 1, ꢁz + 1.5; (iii) x, ꢁy + 1.5, z ꢁ 0.5; (iv) ꢁx + 1.5, ꢁy + 1.5, z; (v) ꢁx + 1, y ꢁ 0.5, ꢁz + 1.5; (vi) ꢁx + 1.5, y, z
ꢁ 0.5; (vii) ꢁx + 1.5, y, z + 0.5.
Complex 2
N1–H9ꢀ ꢀ ꢀO3ii
0.89
0.89
0.89
0.98
0.98
0.930(1)
0.89
0.89
0.89
0.89
0.98
0.98
0.98
0.930(1)
0.97
0.97
0.97
0.97
2.31
2.37
2.31
2.39
2.49
1.81(1)
2.24
2.51
2.62
2.51
2.29
2.51
2.61
1.86(1)
2.58
2.63
2.37
2.65
2.41
3.051(6)
3.154(8)
3.071(18)
3.166(6)
3.320(6)
2.710(5)
3.004(6)
3.270(7)
3.418(12)
3.254(10)
3.218(6)
3.093(7)
3.257(14)
2.772(6)
3.33(3)
140.8
146.4
143.5
135.7
142.1
163.4(8)
144.2
143.1
150.0
141.0
158.3
117.8
123.9
166.7(9)
134.6
141.7
120.5
120.0
139.2
N1–H9ꢀ ꢀ ꢀO9Aii
N1–H9ꢀ ꢀ ꢀO9Bii
N2–H11ꢀ ꢀ ꢀO5iii
N2–H11ꢀ ꢀ ꢀO13iii
O1–H12ꢀ ꢀ ꢀO3
N3–H21ꢀ ꢀ ꢀO10v
N3–H21ꢀ ꢀ ꢀO12Av
N3–H21ꢀ ꢀ ꢀO12Bv
N3–H22ꢀ ꢀ ꢀO15Avi
N4–H23ꢀ ꢀ ꢀO6iv
N4–H23ꢀ ꢀ ꢀO16Aiv
N4–H23ꢀ ꢀ ꢀO16Biv
O2–H24ꢀ ꢀ ꢀO10i
C4–H7ꢀ ꢀ ꢀO9Bi
C4–H7ꢀ ꢀ ꢀO14
3.442(9)
2.978(9)
3.247(7)
3.205(7)
C5–H13ꢀ ꢀ ꢀO16Aiv
C6–H16ꢀ ꢀ ꢀO7iv
C6–H16ꢀ ꢀ ꢀO15Aiii
0.97
Equivalent positions: (i) x + 0.5, y + 0.5, z; (ii) ꢁx + 1, y, ꢁz + 0.5; (iii) x + 0.5, y ꢁ 0.5, z; (iv) x + 1, y, z; (v) x + 0.5, ꢁy + 1.5, ꢁz; (vi) x + 1, ꢁy + 2, ꢁz.
Complex 3
N1–H9ꢀ ꢀ ꢀO2ii
N1–H10ꢀ ꢀ ꢀO2iii
N2–H11ꢀ ꢀ ꢀO2iv
O1–H12ꢀ ꢀ ꢀO3
C1–H2ꢀ ꢀ ꢀO4i
0.89
0.89
0.98
0.78(2)
0.97
2.24
2.27
2.30
1.83(2)
2.60
3.088(2)
3.027(2)
3.087(2)
2.598(2)
3.508(2)
3.359(2)
160.3
142.6
136.9
171(2)
155.1
141.9
C7–H14ꢀ ꢀ ꢀO3v
0.93
2.58
Equivalent positions: (i) x + 1, y, z ꢁ 1; (ii) ꢁx + 2, ꢁy + 1, ꢁz + 1; (iii) x + 1, y, z; (iv) ꢁx + 1, ꢁy + 1, ꢁz + 1; (v) x ꢁ 1, y, z

S. Kumar et al. / Polyhedron 139 (2018) 178–188
181
suite [26], including integration, reduction together with multi-
scan absorption corrections, and merging with SAINT
,
SADABS and
XPREP, respectively. The structures were solved by the intrinsic
phasing method with SHELXTL XT [27] and were refined anisotropi-
cally by full matrix least-squares on F2 with SHELXTL XLMP [28]. All
H atom positions were calculated geometrically and treated with
a riding model. Exceptions are the H-atoms of water molecules
in 1 and the –OH group of the N-hyden groups in 2 and 3 which
were initially located from the difference Fourier electron density
maps and then restrained with ‘AFIX 3’ for the water H-atoms.
For 2, a two-fold disorder of the nitro oxygen atoms of the pic
anions was found from the difference Fourier maps. The 1,3-dis-
tances between the two oxygen atoms were restrained to 1.12 Å,
similar to the adjacent fully ordered NO2 groups, and the displace-
ment parameters of both sites A and B were set to be equal. Note,
the present data quality of 1 is comparable to that of Natarajan
et al. [29], however, the preparation of 1 from both sources are dif-
ferent, see Section 3.1. Further details of the crystal data and
refinement statistics, structural parameters and H-bond parame-
ters of complexes 1–3 are given in Tables 1–3 respectively.
Scheme 1. Schematic representation of the syntheses of complexes 1–3.
carbonate was suspended in 20 mL of water. To the stirred solution
4 mmol of p-nitrobenzoic acid was added slowly. The mixture was
stirred for 20 min in the temperature range 50–60 °C until the
effervescence completely ceased, indicating the formation of
cadmium p-nitrobenzoate. To the resultant solution at 50–60 °C,
N-hyden (4mmol) was added slowly. The solution was stirred at
50–60 °C for 30 min. The resultant reaction mixture was filtered
and the filtrate was put aside to obtain light yellow needle shaped
crystals after 12 days of evaporation at room temperature. Com-
plex 3 decomposes at 193 °C. Complex 3 is sparingly soluble in
water and insoluble in organic solvents like methanol, acetone,
chloroform, etc. Anal. Calc. (%): C, 40.40; H, 4.89; N, 12.86; Cd,
17.20. Found (%): C, 40.32; H, 5.01; N, 12.72; Cd, 17.32.
3. Results and discussion
3.1. Synthesis
Complexes 1–3 were synthesized using the appropriate reac-
tants, as shown in Scheme 1. Complex 1 was isolated in the form
of yellow block shaped crystals when an aqueous reaction mixture
of cadmium(II) carbonate and picric acid was allowed to stir at
50–60 °C for 30 min and put aside for evaporation at room temper-
ature. It is worth noting that in the previous structure determina-
tion of complex 1, colorless rod-shaped crystals were obtained
strangely from a saturated aqueous solution of 1:1 CdCl2-picric
acid [29]. The formation of same complex 2 by employing different
methods, 1 and 2, was indicated by melting point, elemental anal-
yses and spectroscopic techniques (FT-IR and NMR). Complex 3
was isolated in the form of light yellow needle shaped crystals
when the reaction mixture was allowed to evaporate at room tem-
perature after 12 days. The structures of all the newly synthesized
complexes 1–3 have been unambiguously determined by single
crystal X-ray diffraction.
2.3. X-ray crystallography
X-ray diffraction experiments were carried out on a Bruker X8
APEXII Kappa CCD area-detector diffractometer (Mo-Ka, k =
0.71073 Å), with the help of the APEX2 GUI program suite [26].
For the respective complexes 1–3, a total number of 24708,
19643 and 17664 data were collected at 296(2) K and processed
using the standard software programs implemented in the APEX2
Fig. 2. FT-IR spectra of complexes 1–3.

182
S. Kumar et al. / Polyhedron 139 (2018) 178–188
Fig. 3. 1H NMR spectra of complexes 1–3.
Fig. 4. 13C NMR spectra of complexes 1–3.
3.2. Spectroscopic characterization
3293 cmꢁ1 may be attributed to the O–H group of coordinated
N-hyden in complexes 2 and 3, respectively which is less than
the free O–H stretching frequency. The absorption peaks in the
region 3110–3210 cmꢁ1 corresponds to –NH2 and –NH stretching
vibrations of the coordinated ligand. The sharp bands varying in
intensity from weak to medium in the region 3000–2750 cmꢁ1
3.2.1. Infrared spectroscopy
The infrared spectra of complexes 1–3 (neat) were recorded in
the region 4000–400 cmꢁ1 and tentative assignments were made
on the basis of earlier reports in the literature [30–32] (Fig. 2).
The broad band in the region 3500–3300 cmꢁ1 in the spectrum
of complex 1 indicated the presence of water molecule, while the
absence of a broad absorption peak in the region 3500–3100
cmꢁ1 for the O–H bond stretching of water in complexes 2 and 3
showed their anhydrous nature. The strong bands at 3289 and
were assigned to –C–H(mas), –C–H(ms) of the ligand and
–C@C–H(s, aromatic) of the aromatic anionic group. The sharp
absorption bands in the regions 1580–1550 and 1390–1360 cmꢁ1
were due to mas(COO) and
m
s(COO) of the aromatic carboxylate anions
in complex 3, with
D
m
= 171–165 cmꢁ1, corresponding to ionic

S. Kumar et al. / Polyhedron 139 (2018) 178–188
183
spectral peaks in the regions 535–520 and 490–475 cmꢁ1 indicated
the presence of Cd-O and Cd-N stretching frequencies. These types
of stretching vibration showed that the N-donor ligand N-hyden
acts as a chelating agent towards the metal center.
3.2.2. Nuclear magnetic resonance spectroscopy
The 1H and 13C NMR spectra of the complexes were recorded in
DMSO-d6 (Figs. 3 and 4). The peak assignments for various protons
and carbon atoms of complexes 1–3 were assigned on the basis of
literature reports [33].
3.2.2.1. 1H NMR spectra. In the proton NMR spectrum of complex 1,
the peaks observed at d 8.57 (4H, s) ppm were assigned to four aro-
matic protons of two pic anions. In proton NMR spectrum of com-
plex 2, one sharp peak observed at d 8.6014 (4H, s) ppm might be
assigned to four aromatic protons of two pic anions. The remaining
observed peaks in the proton NMR spectrum correspond to protons
present in the counter cation [Cd(N-hyden)2]2+. The methylene pro-
tons adjacent to the hydroxyl group of the ligand appeared more
deshielded, appeared at d 3.566 ppm as a triplet, due to the elec-
tron withdrawing nature of the adjacent oxygen atom. The eight
methylene protons adjacent to the middle N–H group of two N-
hyden ligands were observed at d 2.7009 ppm. The remaining four
methylene protons adjacent to the –NH2 group of the two N-hyden
ligands were observed at d 2.512 ppm. The remaining six protons
of one/two –NH and two –NH2 groups of two moieties of N-hyden
Fig. 5. Molar conductance of complexes 1–3.
mode of carboxylate coordination [31]. In complexes 1 and 2, the
peaks for C–O coordination in the picrate moiety appeared in the
range 1090–1050 cmꢁ1 and the peaks for the nitro group of the
picrate anion appeared in the region 1595–1520 cmꢁ1 [31]. IR
Fig. 6. TGA-DTA curves of complexes 1–3.

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S. Kumar et al. / Polyhedron 139 (2018) 178–188
Fig. 7. ORTEP diagram of the ionic complex 1, [Cd(H2O)6](pic)2ꢀ2H2O (40% probability level). Hydrogen atoms of the picrate anions are omitted for clarity.
Fig. 8. ORTEP diagram of the ionic complex 2, [Cd(N-hyden)2](pic)2 (40% probability level). Hydrogen atoms of the picrate anions are omitted for clarity.
Fig. 9. ORTEP diagram of the ionic complex 3, [Cd(N-hyden)2](pnb)2 (40% probability level). Hydrogen atoms of the picrate anions are omitted for clarity.

S. Kumar et al. / Polyhedron 139 (2018) 178–188
185
Fig. 10. Packing diagram of complex 1, showing a strong network of OꢁHꢀ ꢀ ꢀO hydrogen bonding interactions between the cationic [Cd(H2O)6]2+ and anionic pic units.
Fig. 11. Crystal lattice of complex 2, showing an alternate arrangement of cationic and anionic layers.
appeared at d 3.044 ppm as a broad peak. The two –OH protons of
similar ligands appeared at d 5.573 ppm as a broad peak. Similar to
complex 2, the 1H NMR spectrum of complex 3 showed eight aro-
matic protons for the two pnb anions, which appeared at d 8.18
ppm as a doublet. The observed peaks for the cationic region of
complex 3 were observed at similar d values as those observed
for the complex cation in complex 2 (Fig. 3).
corresponded to aliphatic carbon atoms in the ligand N-hyden. In
complex 3, the peak at d 169.04 ppm corresponded to the carboxy-
late carbon atom. The other peaks at d 148.61, 145.029, 130.74,
123.264 ppm might be assigned to different aromatic carbons of
the pnb anion. The peaks of the ligand N-hyden in complex 3 were
similar to those in complex 2 (Fig. 4).
3.3. Molar conductance measurements
3.2.2.2. 13C NMR spectra. The three complexes contain aromatic
carbon atoms, i.e., the pic moiety in complexes 1 and 2, and the
pnb moiety in 3. In complex 1, the d values at 161.10, 142.03,
125.51, 124.55 ppm were assigned to the aromatic carbon atoms
of the pic moiety. For complex 2, the d 160.83, 141.85, 125.21,
124.24 ppm peaks might be assigned to different carbon atoms of
the pic anion. The peak at d 160.83 ppm corresponds to the carbon
atom which is coordinated to the phenoxo oxygen atom. The peaks
with less intensity in the aromatic region (d = 120–170 ppm) corre-
spond to the quaternary carbon atoms, to which the nitro group is
Conductivity measurements of complexes 1–3 were carried out
in aqueous medium at 25 °C. The limiting molar conductance at
infinite dilution was calculated by plotting
K (molar conductance)
versus C1/2 (square root of concentration). When the
concentrations were extrapolated to zero, they gave Ko values of
261, 243 and 250 S cm2 molꢁ1, respectively for complexes 1–3
(Fig. 5). These values fall into the range observed for 1:2
electrolytes [34]. Therefore, the conductance measurements
revealed that complexes 1–3 behave as 1:2 electrolytes in aqueous
medium, supporting the ionic formulation of the complexes.
attached. The peaks observed at
d 58.7, 49.60, 47.52 ppm

186
S. Kumar et al. / Polyhedron 139 (2018) 178–188
Fig. 12. Hydrogen bonding network in complex 2.
3.4. Thermogravimetric analyses
In order to study thermal stability of synthesized complexes,
thermogravimetric analyses were carried out for all complexes
1–3 under a nitrogen atmosphere. The first weight loss in complex
1 corresponded to the loss of seven water molecules, as revealed
from the DTA peak (endothermic loss) in the temperature range
130–150 °C (Fig. 6). The second weight loss for complex 1, in the
temperature range 170–320 °C, was simultaneous to the first
weight loss and corresponds to the exothermic loss of picrate
counter anions. For complexes 2 and 3, the first weight loss in
the temperature range 170–320 °C corresponded to loss of counter
anions, i.e., pic and pnb, respectively. After 320 °C, the observed
continuous weight loss indicated the formation of some malleable
impurities at high temperatures in all the complexes 1–3 (Fig. 6).
Fig. 13. Crystal lattice of complex 3, showing an alternate arrangement of cationic
and anionic layers.
3.5. X-ray crystallography
angles observed for the N-hyden ligand closely resemble with those
observed in related N-hyden complexes [19–21]. The structural
parameters of the pic anion are similar to those observed in salts
containing pic ions. Complex 3 crystallizes in the triclinic crystal
3.5.1. Coordination geometry of [Cd(H2O)6]2+, [Cd(N-hyden)2]2+ and
pic/pnb
Complex 1 crystallizes in the orthorhombic crystal system with
the space group Pccn. The asymmetric unit comprises half a com-
plex cation, [Cd(H2O)6]2+, with the cadmium (Cd1) ion sitting on
a special position, one anionic pic moiety and one lattice water
molecule. Hence the complete structure and formula of complex
1 is [Cd(H2O)6](pic)2ꢀ2H2O (Fig. 7). The central Cd(II) ion is octahe-
drally surrounded by oxygen atoms of coordinated water mole-
cules. The corresponding CdꢁOw distances are in the range 2.225
(2)–2.297(1) Å (Table 2). The ORTEP diagram of complex 1 from
this work agrees with the previous structure determination [29].
However, the crystal packing of 1 was not described at all previ-
ously [29], and hence is analyzed and compared in detail in this
report with the crystal packing of complexes 2 and 3, see
Section 3.5.2.
The presence of the tridentate N-hyden ligand makes different
scenarios in complexes 2 and 3. Complex 2 crystallizes in the
orthorhombic crystal system with the space group C2221. The
asymmetric unit consists of two complex [Cd(N-hyden)2]2+ cations,
each of which sits on a special position (with two-fold rotation
symmetry), and two pic anionic moieties (Fig. 8). The central Cd
(II) ion adopts a distorted trans-octahedral geometry which is com-
posed of two tridentate ligand N-hyden coordinated through two
nitrogen atoms and one oxygen atom. The bond lengths and bond
system with the space group P1. The asymmetric unit comprises
one trans-octahedral complex cation, [Cd(N-hyden)2]2+, and two
pnb anionic moieties (Fig. 9). The bond lengths and bond angles
of the N-hyden ligand and the pic and pnb anions in complexes
1–3 are very similar to those reported previously [35–46].
3.5.2. 3D arrangements of complexes 1–3
The crystal lattice of complex 1 showed a layered arrangement
of cationic [Cd(H2O)6]2+ and anionic pic moieties (Fig. 10). The six
water molecules coordinated to the Cd(II) ion act as hydrogen bond
donors to the oxygen atoms of the nitro and phenoxo groups of the
pic anions. The adjacent anionic layers are arranged in an opposite
fashion in order to reduce steric repulsion between the functional
groups. The pic moieties act as hydrogen bond acceptors, giving
rise to O–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO extensive hydrogen bonding network
(Table 3).
The crystal lattices of complexes 2 and 3, both having a com-
mon trans-octahedral [Cd(N-hyden)2]2+ complex cation and pic
and pnb anionic counterparts respectively, being stabilized by
second-sphere non-covalent interactions. Complex 2 showed a
layered arrangement of cations and anions, in which layers of pic

S. Kumar et al. / Polyhedron 139 (2018) 178–188
187
Fig. 14. Network of C–Hꢀ ꢀ ꢀO, N–Hꢀ ꢀ ꢀO and O–Hꢀ ꢀ ꢀO hydrogen bonds in complex 3.
as C–Hꢀ ꢀ ꢀO, N–Hꢀ ꢀ ꢀO and O–Hꢀ ꢀ ꢀO, etc. The present structural study
suggests that the tridentate N-hyden ligand may find application to
chelate the toxic cadmium(II) ion for its removal from water and
living organisms.
Acknowledgments
The authors RPS and SK acknowledge the financial support from
UGC, New Delhi (India) as a UGC Emeritus and BSR Meritorious Fel-
lowship respectively. PV thanks the DST PURSE program of Panjab
University, Chandigarh. Dr. P.K. Soni, TBRL, Chandigarh, India is
also acknowledged for his help in the thermogravimetric analyses.
TA is grateful to the Ratchadaphiseksomphot Endowment Fund,
Chulalongkorn University (CU-58-021-FW).
Fig. 15. Influence of anions or co-ligands on the chelation mode of the tridentate N-
hyden ligand to the Cd(II) metal center: (a) saccharinates induce
chelation; (b) pnb and pic induce a tridentate chelation.
a bidentate
anions are sandwiched between layers formed by [Cd(N-hyden)2]2+
moieties (Fig. 11). The C–H groups of the N-hyden ligands act as
hydrogen bond donors to the pic oxygen atoms. The N–H group
of the ligand showed hydrogen bonding interactions with the nitro
oxygen atom of the pic anions (Table 3). The extensive hydrogen
bonding network in complex 2 has been depicted in Fig. 12.
Similarly, complex 3 also showed a layered arrangement of
cations and anions (Fig. 13). A network of C–Hꢀ ꢀ ꢀO, N–Hꢀ ꢀ ꢀO and
Appendix A. Supplementary data
CCDC nos. 1562435, 1562436 and 1562437 contain the supple-
mentary crystallographic data for complex 1, 2 and 3. These data
can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
O–Hꢀ ꢀ ꢀO hydrogen bonds and
p
–
p
interactions have been dis-
nor C–Hꢀ ꢀ ꢀ interactions
played in Fig. 14. Note that neither
p–p
p
have been observed in either complex 1 or 2 because of the steric
crowding from the pic nitro groups, which are not parallel stacked,
but are disoriented.
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