Exploring supramolecular interactions between inorganic tetrachlorometallate and organic pyridinium dication: Synthesis, characterization and structural investigations

Article abstract of DOI:10.1016/j.molstruc.2012.01.023

The crystal structures of three isostructural salts 2, 3 and 4 consisting of dication (α,α′-p-xylylene-N,N′-bis(3- hydroxymethylpyridinium) (L²⁺)), and tetrachlorometallates, [MCl ₄]^2- (M = Co(II), Zn(II) and Cu(II) in 2,

Full text of DOI:10.1016/j.molstruc.2012.01.023

Journal of Molecular Structure 1013 (2012) 102–110
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Exploring supramolecular interactions between inorganic tetrachlorometallate
and organic pyridinium dication: Synthesis, characterization and
structural investigations
⇑
Kamal Kumar Bisht, Amal Cherian Kathalikkattil, Suresh Eringathodi
Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), GB Marg, Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
The crystal structures of three isostructural salts 2, 3 and 4 consisting of dication (a,
a0-p-xylylene-N,N0-
Article history:
bis(3-hydroxymethylpyridinium) (L2+)), and tetrachlorometallates, [MCl4]2ꢀ (M = Co(II), Zn(II) and Cu(II)
in 2, 3 and 4 respectively) have been reported. Synthesized salts are well characterized by various phys-
ico-chemical techniques such as CHN analysis, IR, TGA, DSC, NMR, PXRD and single crystal X-ray diffrac-
tion. The crystal lattice of complex 2 is constructed from two types of interactions viz., C–Hꢁ ꢁ ꢁCl and
O–Hꢁ ꢁ ꢁCl H-bonding between the pyridinium dication and the complex anions [CoCl4]2ꢀ, whereas only
C–Hꢁ ꢁ ꢁCl interaction was found to play major role in stabilizing the structures of organic inorganic hybrid
complexes 3 and 4. The hierarchy of intermolecular interactions present in solid state, factors controlling
the adopted structures of these compounds and their thermal stability are discussed in detail by using
various analytical techniques including single crystal X-ray.
Received 2 November 2011
Received in revised form 14 January 2012
Accepted 17 January 2012
Available online 25 January 2012
Keywords:
Supramolecular chemistry
Tetrachlorometallate
Hydrogen bonding
Pyridinium dication
X-ray crystallography
Organic–inorganic hybrid
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
tetrahalometallates have gained enormous attention due to the
ease of designer synthesis, and applications [14–20]. In the case
Crystal engineering is a rapidly growing area of research, which
mainly try to establish the command over the preparation of crys-
talline solid materials with detailed understanding of intermolecu-
lar attractions in the solid state, and the use of this understanding
in the design of new solids with specific structural topologies and
applications. Wide range of possible molecular and supramolecular
interactions has been utilized for the construction of multidimen-
sional networks [1]. Hydrogen bonding, van der Waals forces and
of N-alkylpyridinium tetrahalometallates, most of the studies
focused on their liquid crystal or ionic liquid behavior [14–16].
However, synthesis and crystallographic studies on dicationic
pyridinium salts involving tetrahalometallate are relatively few
in the literature [21–23]. Interestingly, several reports including
crystallographic studies in which tetrahalometallate (II) dianion
combined with two organic cations possessing unit formal positive
charge, is available in literature [24–29]. Self-assembly process of
such hybrid materials in solid state arises due to the variety
of interactions which include, ionic interactions, hydrogen bonding
within the organic moiety, hydrogen bonding between organic and
inorganic components, halogen bonding and van der Waals inter-
actions [17]. Some pioneering work in this field by utilizing supra-
molecular synthon approach and exploiting the directional
properties of hydrogen bonding interactions among perhalometal-
lates and various organic cations has been carried out by Orpen
and Brammer groups. In fact, Orpen, Brammer and their co-
workers have utilized supramolecular synthons based on N–Hꢁ ꢁ ꢁCl
hydrogen-bonds in the construction of organic–inorganic hybrid
solids comprising organic cations with linear connectivity
[18–20]. The effect of functional groups such as pendent hydroxyl
groups on the physical properties of organic salts has been re-
ported by several groups; Armstrong et al., have demonstrated
the effect of symmetry on the melting points and other physical
p p stacking etc. weak molecular interactions chiefly direct the
ꢁ ꢁ ꢁ
design of organic solid state [2–5] whereas coordinative covalent
bonds are equally important in the design and construction of
coordination polymers [6–9]. Salts consisting of organic cations
frequently exhibit ionic liquid behavior, and the properties associ-
ated with them such as magnetic, optical, electrical and catalytic
activity can be tuned with subtle structural changes on cationic
and/or anionic moieties [10,11]. In order to design a salt with pre-
ferred properties it is needful to precisely develop structure–prop-
erty relationships for such compound [12,13]. This is one of the
major reasons for the popularity of organic salts among crystal
engineers and solid state researchers. Salts of organic cations with
⇑
Corresponding author.
E-mail addresses: esuresh@csmcri.org, sureshe123@rediffmail.com (S. Erin-
gathodi).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.01.023

K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
103
properties of dicationic ionic liquids [30]. On the other hand,
there are very few reports demonstrating the role of O–Hꢁ ꢁ ꢁCl–M
interactions in structural assemblies of organic tetrachlorometal-
late salts [31–33]. We have initiated some work in the area of
self-assembled hybrid organic/inorganic materials as either multi-
dimensional coordination polymers or layered structures via coor-
dinate covalent bond or in conjunction with weak supramolecular
hydrogen bonding interaction [9,34–38].
solvents such as methanol, ethanol, dimethylformamide and di-
methyl sulfoxide. Similar solubility trend was observed for 2–4.
Melting point: 182–184 °C; Elemental Anal. Calc. for 1.2H2O: C,
56.08; H, 5.88; N, 6.54; O, 14.94%. Found: C, 55.97; H, 5.92; N, 6.51;
O, 15.03%. m/z: 357.20 ([L2+Clꢀ], calculated 357.13), 321.19 ([L2+
–
H+], calculated 321.16). IR (KBr): 3405s, 3190s, 3023s, 2940m,
1625s, 1500s, 1467m, 1443m, 1430s, 1371w, 1224s, 1191m,
1140s, 1057s, 994m, 928w, 863w, 816w, 768s, 745m. 1H NMR
(MeOD) d: 9.07 (2H, s), 9.01 (2H, d, J = 5.94 Hz), 8.56 (2H, d,
J = 7.99 Hz), 8.09 (2H, dd, J = 5.94 Hz, J = 7.99 Hz), 7.64 (4H, s),
5.92 (4H, s), 4.83 (4H, s); 13 C NMR (MeOD) d: 143.9, 143.1,
142.6, 141.7, 134.6, 129.3, 127.3, 63.1, 59.2.
Herein we report the structural characterization of three tetra-
chlorometallate salts of organic pyridinium dication
ene-N,N0-bis(3-hydroxymethylpyridinium (L2+
and discuss
a,
a0-p-xylyl-
)
various intermolecular interaction particularly the O–Hꢁ ꢁ ꢁCl–M,
C–Hꢁ ꢁ ꢁCl interactions in the absence of charge assisted N+–Hꢁ ꢁ ꢁCl–
M or N–Hꢁ ꢁ ꢁCl–M interactions. In addition to the hydrogen bonding
between the metal halide with the organic dication, conformational
flexibility around the methylene carbon bridging the central phenyl
and pyridine rings in the symmetrically disposed dication and the
2.4. Synthesis of a,
a0-p-xylylene-N,N0-bis(3-
hydroxymethylpyridinium) tetrachlorocobaltate (II) monohydrate:
L2+ꢁ[CoCl4]2ꢀꢁH2O (2)
p
-stacking between the six membered ring as well as C–Hꢁ ꢁ ꢁ
p
A mixture of the 1 (42 mg, 0.1 mmol) and CoCl2ꢁ6H2O (24 mg,
0.1 mmol), dissolved in 14 mL water–methanol (1:1) mixture was
heated at 85 °C with continuous stirring for 2 h. The pink colored
solution obtained was filtered and filtrate was dried using a rota-
tory evaporator to obtain Complex 2 as a blue crystalline powder.
Blue colored single crystals suitable for crystallographic studies
were grown by recrystallization from methanol in a period of
2 weeks. Yield 50 mg (92%). Melting point: 110–112 °C; Elemental
Anal. Calc. for 2: C, 44.39; H, 4.47; N, 5.18; O, 8.87%. Found: C,
44.36; H, 4.54; N, 5.16; O, 8.93%. IR (KBr): 3393br, 3065m, 2940w,
2361m, 1626s, 1500s, 1472s, 1444m, 1293w, 1226m, 1143m,
1051s, 913w, 746m, 601w, 480w. 1H NMR (MeOD) d: 9.03 (2H, s),
8.96 (2H, d, J = 5.63 Hz), 8.54 (2H, d, J = 7.58 Hz), 8.07 (2H, dd,
J = 5.63 Hz, J = 7.58 Hz), 7.61 (4H, s), 5.88 (4H, s), 4.84 (4H, s).
interaction also partake in the supramolecular arrangement be-
tween the pyridinium salts of perchlorometallates in the present
investigation. The effect of employed metal and geometry of tetra-
chlorometallate on the thermal stability is also accounted.
2. Experimental
2.1. Materials
Pyridine-3-methanol and
a,
a0-dichloro-p-xylene were pur-
chased from Sigma–Aldrich. Metal chloride salts and organic sol-
vents were obtained from SD Fine Chemicals, India. Double
distilled water and methanol were used as solvents for the synthe-
ses of the complexes. All the reagents and solvents were used as
received without further purification.
2.5. Synthesis of a,
a0-p-xylylene-N,N0-bis(3-
hydroxymethylpyridinium) tetrachlorozincate (II) monohydrate
L2+ꢁ[ZnCl4]2ꢀꢁH2O (3)
2.2. Physical measurements
A mixture of the 1 (42 mg, 0.1 mmol) and ZnCl2ꢁ6H2O (25 mg,
0.1 mmol), dissolved in 14 mL water–methanol (1:1) mixture was
heated at 85 °C with continuous stirring for 2 h. The pink colored
solution obtained was filtered and filtrate was dried using a rota-
tory evaporator to obtain Complex 3 as a white crystalline powder.
Transparent single crystals suitable for crystallographic studies
were grown by recrystallization from methanol in a period of
2 weeks. Yield 52 mg (95%). Melting point: 107–109 °C; Elemental
Anal. Calc. for 3: C, 43.87; H, 4.42; N, 5.12; O, 8.76%. Found: C,
43.82; H, 4.49; N, 5.08; O, 8.83%. IR (KBr): 3395br, 3069m, 2942w,
2362m, 1626s, 1500s, 1473s, 1446m, 1292w, 1228m, 1144m,
1053s, 912w, 745m, 602w, 485w. 1H NMR (MeOD) d: 9.03 (2H, s),
8.96 (2H, d, J = 5.15 Hz), 8.55 (2H, d, J = 7.74 Hz), 8.08 (2H, dd,
J = 5.15 Hz, J = 7.74 Hz), 7.62 (4H, s), 5.89 (4H, s), 4.85 (4H, s).
CHNS analyses were done using a Perkin–Elmer 2400 CHNS/O
analyzer. IR spectra were recorded using KBr pellets on a Perkin–
Elmer GX FTIR spectrometer. For each IR spectra 10 scans were
recorded at 4 cmꢀ1 resolution. 1H and 13C NMR spectra for the
compounds were recorded on Bruker AX 500 spectrometer
(500 MHz) at temperature 25 °C. NMR Spectra was calibrated with
respect to internal reference TMS. Melting points of finely pow-
dered samples were obtained using a Mettler Toledo FP62 instru-
ment and are uncorrected. TGA analyses were carried out using
Mettler Toledo Star SW 8.10. DSC analyses were carried out using
Mettler Toledo DSC822e. Mass spectra were recorded on a Q-TOF
Micro™ LC–MS instrument. Solution phase and solid state UV–
Vis spectra were recorded using a Shimadzu UV-3101PC spectro-
photometer. X-ray powder diffraction data were collected using a
Philips X-Pert MPD system with Cu K
structures were determined using a BRUKER SMART APEX (CCD)
diffractometer.
a radiation. Single crystal
2.6. Synthesis of a,
a0-p-xylylene-N,N0-bis(3-
hydroxymethylpyridinium) tetrachlorocuprate (II) monohydrate
L2+ꢁ[CuCl4]2ꢀꢁH2O (4)
2.3. Synthesis of
a
,
a0-p-xylylene-N,N0-bis(3-
A mixture of the 1 (42 mg, 0.1 mmol) and CuCl2.6H2O (24 mg,
0.1 mmol), dissolved in 14 mL water–methanol (1:1) mixture was
heated at 85 °C with continuous stirring for 2 h. The pink colored
solution obtained was filtered and filtrate was dried using a rota-
tory evaporator to obtain Complex 4 as an orange–yellow colored
crystalline powder. Bright yellow colored single crystals suitable
for crystallographic studies were grown by recrystallization from
methanol in a period of 3 weeks. Yield 50 mg (91%). Melting point:
95–97 °C; Elemental Anal. Calc. for 4: C, 44.01; H, 4.43; N, 5.13; O,
8.79%. Found: C, 43.87; H, 4.52; N, 5.07; O, 8.84%. IR (KBr): 3400br,
3060m, 2948w, 2361m, 1624s, 1499s, 1470s, 1423m, 1291w,
hydroxymethylpyridinium) dichloride: L2+ꢁ2Clꢀ (1)
L2+ꢁ2Clꢀ has been synthesized by following a procedure reported
by us recently [38]. To a solution of a,
a0-dichloro-p-xylene (175 mg,
1 mmol) in 20 mL acetonitrile, pyridine-3-methanol (240 mg,
2.2 mmol) dissolved in 15 mL acetonitrile was added slowly. The
reaction mixture was set to reflux for 6 h when the product formed
as a white precipitate. The reaction mass was filtered, washed with
5 mL acetonitrile and dried in the air. The pure L2+ꢁ2Clꢀ (1) obtained
in good yield (294 mg, 74%) was soluble in water and organic

104
K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
Table 1
hydrogen atoms could not be located from the difference Fourier
map for lattice water molecules.
Crystal data and refinement parameters for Compound 2, 3 and 4.
Identification
code
Compound 2
Compound 3
Compound 4
3. Results and discussion
Chemical
formula
Formula
weight
C20H22Cl4Co1N2O3 C20H22Cl4Zn1N2O3 C20H22Cl4Cu1N2O2
3.1. IR, LCMS and PXRD analysis
539.13
545.57
527.74
Crystal color
Crystal Size
(mm)
Temperature
(K)
Crystal
system
Space group
a (Å)
b (Å)
Blue
Colorless
Yellow
The IR spectra of pyridinium dicationic salt 1 consists of a weak
band around 912 cmꢀ1 which can be attributed to the C–N stretch-
ing band of quarternary pyridinium entity, suggesting the formation
0.37 ꢂ 0.33 ꢂ 0.11 0.47 ꢂ 0.28 ꢂ 0.14 0.21 ꢂ 0.15 ꢂ 0.05
110 (2)
110 (2)
110 (2)
of adduct between
a,
a0-dichloro-p-xylene and pyridine-3-metha-
Triclinic
Triclinic
Triclinic
nol. However, the formation of symmetric dicationic salt is con-
firmed by the LCMS and NMR techniques. Methanol solution 1
was subjected to time of flight mass spectrometry (TOF-MS). The
mass spectra clearly shows m/z peaks corresponding to [L2+ꢁClꢀ] at
P-1
P-1
P-1
8.1498(8)
8.5690(8)
16.9722(16)
97.460(2)
93.936(2)
101.587(2)
2
8.1684(11)
8.6173(12)
17.170(2)
97.796(2)
93.831(2)
102.295(2)
2
8.0770(14)
8.6476(15)
17.209(3)
99.131(3)
101.853(3)
98.832(3)
2
c (Å)
357.20 (calculated value 357.13), and low intensity peak for [L2+
–
H+] at 321.19 (calculated value 321.16). The base peak at m/z
161.09 and second most intense peak at m/z 230.10 represent two
unsymmetrical but complementary molecular fragments of 1 re-
sulted by cleaving of one of the arene methylene bond and equal
distribution of two chloride ions. The IR spectra of 1–4 encompass
two broad bands around 3550 and 3190 cmꢀ1 suggesting the pres-
ence of lattice water molecules in these compounds in addition to
the alcohol functionality. O–H stretching bands of the alcoholic
group for 1–4 is observed at 3190, 3393, 3395, 3400 cmꢀ1 respec-
tively. The aromatic C–H stretching frequencies for tetrahalometal-
lates 2–4 appeared at 3065, 3069 and 3060 cmꢀ1 respectively. The
quantitative differences among these three values suggest
the involvement of hydrogen atoms attached to aromatic rings in
hydrogen bonding interactions. The aromatic C–H deformation
bands for 1–4 appeared at 1467, 1472, 1473, 1470 cmꢀ1 respec-
tively. Medium intensity signals around 1625, 1500 and
1446 cmꢀ1 for all the compounds 1–4 are attributable to the aro-
matic unsaturated C–C bonds. Medium intensity signals at 1430,
1444, 1426 and 1423 cmꢀ1 can be attributed to the deformation
mode of hydrogen bonded methylene groups in the case of 1–4.
The presence of sharp peaks around 770 cmꢀ1 in all the compounds
1–4 can be attributed to the methylene rock mode. Weak bands
around 480 and 420 cmꢀ1 in infrared profiles of 2–4 can be attrib-
uted to the metal chloride linkage of tetrachlorometallate anions.
In an attempt to confirm the homogeneity of the synthesized mate-
rials, we have analyzed the XRPD pattern of compounds 2–4 and
correlated the results with the powder pattern obtained from the
single crystal XRD data simulation. The experimental XRPD patterns
corroborate well with the corresponding powder patterns simu-
lated from single crystal data, indicating the phase purity of bulk
products. Phase purity was further confirmed by comparing the unit
cell parameters obtained from single crystal XRD data and the same
calculated from XRPD data. Details of unit cell parameter determi-
nation, XRPD profiles and IR spectra for 2–4 and LCMS for 1 are given
in Supplementary data information.
a
(°)
b (°)
c
(°)
Z
V (Å3)
1145.79(19)
1.563
1164.2(3)
1.556
1139.7(3)
1.538
Density
(Mg mꢀ3
)
Absorption
coefficient
(mmꢀ1
F(000)
1.240
1.538
1.446
)
550
6921
556
8195
538
7495
Reflections
collected
Independent
reflections
R(int)
Number of
264
S (Goodness
of fit) on F2
Final R1/wR2
5061
4048
3557
0.0190
parameters
0.0372
273
0.0379
273
1.099
1.040
1.188
0.0671/0.1609
0.0723/0.2034
0.0844/0.2146
0.0967/0.2111
0.1154/0.2207
(I > 2r(I)
Weighted R1/ 0.0735/0.1652
wR2(all
data)
CCDC number 831,072
831,073
831,074
1225m, 1142m, 1054s, 912w, 745m, 601w, 486w. 1H NMR (MeOD)
d: 9.09 (2H), 9.02 (2H), 8.58 (2H), 8.12 (2H), 7.68 (4H), 5.94 (4H),
4.89 (4H).
2.7. X-ray crystallography
Summary of crystallographic data and details of data collection
for all the three compounds are given in Table 1. In case of all the
three compounds, single crystals of suitable size were mounted on
the tip of a glass fiber and the intensity data were collected using
MoKa (k = 0.71073 Å) radiation on a Bruker SMART APEX diffrac-
tometer equipped with CCD area detector at 100 K.
The data integration and reduction were carried out with SAINT
[39] software. An empirical absorption correction was applied to
the collected reflections with SADABS [40]. The structures were
solved by direct methods using SHELXTL [41] and were refined
on F2 by the full-matrix least-squares technique using the SHEL-
XL-97 [42] program package. The lattice water molecule present
in the case of Complex 4 was highly disordered and their contribu-
tion to the diffraction pattern has been removed using the
SQUEEZE subroutine as implemented in PLATON [43]. Refinement
of 4 using the modified reflection data set apparently improved the
R-factor. All non-hydrogen atoms were refined anisotropically till
convergence is reached. Hydrogen atoms attached to the dication
moieties are stereochemically fixed in all the complexes. However,
3.2. NMR analysis
13C NMR spectra of 1 comprise of nine well distinguished reso-
nance signals in the chemical shift range of 143.9–59.2 ppm. These
signals are attributable to the centrosymmetric structure of
the dication which is further evident by 1H NMR spectroscopy.
The 13C NMR spectra for 1 and 1H NMR spectra for 1–4 are given
in Supplementary information with the structural assignment of
observed signals. The proton shifts for tetrachlorometallate salts
2–4 were consistent with that of 1 indicating same dication in all
of the reported compounds 1–4. However, the observed differences
in the chemical shifts and coupling constants can be attributed to
the different metal ions, different anionic geometry and difference

K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
105
Fig. 1. A segment of room temperature 1H NMR spectra exhibiting the chemical
shifts and coupling patterns for pyridinium protons in salts 1–4 in MeOD-d solvent.
Lowercase alphabets in parentheses represent the magnetically non-equivalent
pyridinium protons of dication as represented in inset.
in hydrogen bonding contacts between the cation and anion in
solution phase. Fig. 1 presents a comparison of chemical shifts
for pyridinium protons in 1–4 and reveals slight shielding for the
same in 2–4 with respect to 1. The NMR spectra of paramagnetic
substances contain valuable information, but the scattered chemi-
cal shifts and fast nuclear relaxation (broad lines) renders them dif-
ficult to interpret [44–46]. In an attempt to deal with the
paramagnetic Co2+ (d7 system) system we have recorded solution
state proton NMR spectra for 2 in different deuterated solvents
viz., D2O, methanol-d, DMSO-d6 and DMF-d7. Except in DMF-d7,
the occurrence of clean NMR spectra for 2 can be attributed to
the highly symmetric Td geometry for [CoCl4]2ꢀ ion [14c]. Dimethyl
formamide being highly polar and good coordinating solvent is ex-
pected to interact with [CoCl4]2ꢀ persuading in strong distortions
from Td symmetry, resulting the clumsiness of proton NMR spectra
in DMF-d7. The effect of paramagnetic nature of Co2+ (d7) and Cu2+
(d9) systems can be observed in the coupling behavior of protons as
depicted in Fig. 1. The signals for pyridinium protons b, c and d
Fig. 2. TGA and DSC plots for pyridinium salts 2–4 in temperature range 50–450 °C.
The endothermic signals in DSC near melting points of 2–4 are surrounded by a
rectangle. (All TGA and DSC plots were obtained at a scan rate of 10 °C minꢀ1; (a)
and (b) share the identical x-axis.)
reside in the range 106.5–108.5 and 103–105 °C respectively.
TGA profiles suggest excellent thermal stability of these com-
pounds as higher as 100° above their respective melting points. A
two step decomposition is observed in the temperature range
220–340 °C for both 2 and 3 with 35% weight loss of their initial
weight followed by the slow decomposition of the residual materi-
als with increase in temperature. Close resemblance of DSC Plots
for 2 and 3 indicate three unknown phase changes immediate after
the expulsion of lattice water molecules around 110, 120.5 and
140 °C for 2 and 108.5, 122 and 144.5 °C for 3 respectively. Tetra-
chlorocuprate salt 4 exhibits weight loss process in the tempera-
ture ranges 85–145 and 195–260 °C. A weight loss of 3.1% near
about 95 °C can be attributed to loss of lattice water molecule (cal-
culated weight loss 3.32%). The absence of intense heat flow signal
near 95 °C in the DSC plot for 4 suggests weak interactions be-
tween the guest water molecules and the organic salt. The second
decay process recorded on TGA comprises of 31.48% mass loss at
235 °C indicating the commencement of decomposition of the
compound which follows the same trend as observed in the case
of 2 and 3. The determined melting point for 4 is 140–142 °C.
The decomposition of compound 4 maps out intense endothermic
minima at 204.5 °C whereas preceding shoulder nearby 140 °C rep-
resents melting phenomenon. The correlations among melting
points, thermograms and DSC plots for all three compounds 2–4
suggest that the tetrachlorometallates 2–4 are fairly stable salts
and their thermal decomposition realize after melting. Moreover,
complexes 2 and 3 exhibited excellent thermal stability even up
to the temperatures as high as 100° above their melting points.
become considerably unsymmetrical for 2 (Co2+ d7 system) and
,
interestingly, peak broadening for signals resulted in single bands
in the case of 4 (Cu2+, d9 system) may be due to the highly distorted
tetrahedral geometry adopted by the [CuCl4]2ꢀ as observed in
X-ray data.
3.3. Thermal analyses
All the tetrachlorometallate salts are air-stable and retain their
crystalline integrity under ambient conditions. Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) experi-
ments in nitrogen atmosphere were carried out in order to deter-
mine the thermal stability, dehydration and decomposition of
compounds 2–4 and the results are presented in Fig. 2.
The thermograms for 2 and 3 apparently suggest the higher
thermal stability of tetrachlorocobaltate and tetrachlorozincate
salts. Both 2 and 3 loss lattice water (observed (calculated) weight
loss for 2 and 3 4.21 (3.33) and 4.1(3.29)%) for 2 and 3 respectively)
in 55–105 °C temperature range with exclusion points nearing 79
and 78 °C respectively. The determined melting points of 2 and 3
3.4. UV–VIS absorption study
Uv–vis absorption experiments were performed at room tem-
perature for 1–4 in 10ꢀ4 M methanolic solutions as well as in solid
state. The solution phase absorption spectra presented in Fig. 3a

106
K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
Fig. 3. (a) Absorption spectra for 1–4 in 10ꢀ4 M methanol solutions recorded at
room temperature; d–d band observed for 10ꢀ3 M methanolic solution of cobalt(II)
compound 2 is presented in inset. (b) Solid state uv–vis absorption spectra of 1–4
recorded at room temperature.
shows that the compound 1 absorbs strongly at 212 nm and
267 nm, however, methanolic solutions of 2–4 follow similar trend.
Compounds 2, 3 and 4 exhibited a weak bathochromic shift as
compared to 212 nm signal of 1, and absorb at 213, 217 and
214 nm respectively. The d–d transition for 2 appears at 530 nm
whereas the same transition for 4 could not be recorded at studied
dilution. Solid state absorption spectra are recorded with unpolar-
ized source and are presented in Fig. 3b. Salt 1 intensely absorbs in
the uv region 272 nm at room temperature. The tetrachlorometal-
late salts 2–4 also absorb in the same region at 274, 274 and
279 nm respectively with considerable decrease in absorbance
intensity. Salt 2, that contain [CoCl4]2ꢀ showed absorption bands
at 449 and 535 nm (22,271 and 18,691 cmꢀ1) for ligand field tran-
sitions and a broad band around 685 nm (14,598 cmꢀ1) that may
envelop d–d (4A2 ? 4T2, 4A2 ? 4T1(F), 4A2 ? 4T1(P)) transitions.
Compound 3 exhibited feebly distinct ligand field transitions in
345–455 nm range. This moderate intensity band appears as a
shoulder to the 274 nm band observed for cation L2+. Compound
4, which contains the D2d distorted tetrahedral [CuCl4]2ꢀ complex
exhibited LMCT band around 410 nm (24,390 cmꢀ1). The broad
overlapping bands in lower energy region that extends to the
near-IR region envelop d–d (2B2 ? 2A1, 2B2 ? 2B1, 2B2 ? 2E) transi-
tions. The spectral features of 4 with average trans angle ꢃ135° are
in accordance with previously reported tetrachlorocuprate salts
[47–49]. A comparative overview of Fig. 3 reflects the considerable
differences between solution and solid state spectra for each of 1–4
and points out that choice of metal center and coordination driven
slight variations in crystal packing may considerably affect the
absorption properties of isostructural salts.
Fig. 4. ORTEP diagram of tetrachlorometallate salts 2–4 (40% probability factor for
the thermal ellipsoids; lattice water molecules are omitted for clarity).
Fig. 4 and Table 1 respectively. It is worthy to mention that the
three crystal structures possess almost the same unit cell parame-
ters. All the three complexes are isostructural and crystallized in
triclinic space group P-1 with slight variation in the crystallo-
graphic parameters viz., cell dimensions and cell volume. The
variation in crystal parameters may be attributed to the existence
of different metal ions, flexibility of the dication and difference in
mode of intermolecular interactions in these complexes. The
asymmetric unit of all the three complexes consists of one [MCl4]2ꢀ
anion occupying the general position and two centrosymmetric
half cations with the center of inversion located at the middle of
the phenyl ring along with one water molecule as solvent of
crystallization.
The crystal structures of all the three complexes consist of iso-
lated pyridinium dications and distorted tetrahedral [MCl4]2ꢀ
(where M = Co(II), Zn(II) and Cu(II)) anions along with one water
molecule as solvent of crystallization. The anion of complex 2
and 3 i.e. [CoCl4]2ꢀ and [ZnCl4]2ꢀ reveal a slightly distorted tetrahe-
dral geometry with Cl–M–Cl angles in the range 105.26(5)–
114.81(5)° and 104.75(8)–113.81(7)° respectively. In the case of
4, [CuCl4]2ꢀ exhibits a distinctive, highly distorted tetrahedral
geometry in which the Cl–Cu–Cl is in the range of 98.93(15)–
138.11(13)°. The average value of the Cl–Cu–Cl trans angles is
135.64°, indicating the high distortion in the tetrahedral geometry
that can be attributed to the Jahn–Tellar effect of the Cu(II) and the
packing arrangement imposed by the dication. In the case of Co
3.5. Crystal and molecular structures of L2+ꢁ[CoCl4]2ꢀꢁH2O; 2,
L2+ꢁ[ZnCl4]2ꢀꢁH2O; 3 and L2+ꢁ[CuCl4]2ꢀꢁH2O; 4
ORTEP diagram ([MCl4]2ꢀ with one pyridinium dication) and
crystallographic data for all the three complexes are given in

K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
107
Fig. 5. Packing diagram of compound 2 viewed down a-axis showing stacking, C–Hꢁ ꢁ ꢁ
p
interaction between the layered dications and the orientation of the [MCl4]2ꢀ anion
and water molecules between the adjacent dicationic layers.
and Zn complexes the Cl–M–Cl are much closer to the ideal tetra-
hedral value (111.29° and 112.55° respectively). M–Cl distance
central phenyl ring and the pyridine rings shows slight differences.
The tilt angle between the phenyl ring and pyridyl rings in the cen-
trosymmetric dication for 2–4 are 75.77, 73.18°; 75.78, 72.23°;
76.78, 74.60° respectively.
In an attempt to understand the molecular interactions be-
tween the [MCl4]2ꢀ anion and pyridinium dication we have ana-
lyzed the packing of the molecules and hydrogen bonding
interactions in detail. Since the anions carry four potential hydro-
gen bond acceptors and range of apparent hydrogen bond donor
sites is available with the organic dication, numerous ways in
which the cations and anions might be arranged to give satisfac-
tory hydrogen bonds are feasible. Furthermore flexibility of the
range in compounds 2,
3 and 4 are 2.2570(13)–2.2946(13),
2.2534(18)–2.282(2) and 2.226(3)–2.247(3) respectively. These
bond distances as well as the bond angles cited for the tetrachloro-
metallate complexes are well within the range of the earlier re-
ports [18–29]. Tetrahedral geometry is less distorted as expected
for the d7 and d10 tetrahedral ions in the cases 2 and 3 compared
to d9 system in the case of Cu complex 4. In all the three salts,
the dication adopts anti conformation. The pyridinium rings of
symmetrically disposed dication in 2–4 are essentially planar and
are parallel to each other. However, the tilt angle between the
Fig. 6. Close up view of the hydrogen bonding interactions between [MCl4]2ꢀ and L2+ in compounds 2–4.

108
K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
organic cation and stacking interactions among organic moieties
also promote the weak molecular interaction in the formation of
supramolecular network.
C–Hꢁ ꢁ ꢁ parameters (H3ꢁ ꢁ ꢁCg = 2.53 Å for 1; 2.63 Å for 2 and 2.78Å
p
for 3; C3ꢁ ꢁ ꢁCg = 3.402(5) Å for 1; 3.481(6) Å for 2; 3.625(10) Å for 3;
C3–H3ꢁ ꢁ ꢁCg = 157° for 1; 152° for 2 and 151° for 3 for the first
dicationic layer and H13ꢁ ꢁ ꢁCg = 2.61 Å for 1; 2.70 Å for 2 and
2.57 Å for 3; C13ꢁ ꢁ ꢁCg = 3.402(5) Å for 1; 3.515(7) Å for 2 and
3.448(9) Å for 3; C13–H13...Cg = 157° for 1; 147° for 2 and 158°
for 3 for the second dicationic layer).
Packing diagram of the Co complex (2) as a representative of the
isostructural organic salts 2–4 is viewed down a-axis is shown in
Fig. 5. It depicts two different types of superimposed rows of pyrid-
inium dications in 2. There are two crystallographically distinct L2+
cations aligned along the b-axis in these isostructural compounds.
Each centrosymmetric cation is associated with the same cation via
intermolecular stacking resulting in the columns of alternate cat-
ionic layers (in plane and perpendicular) in their orientation (in
bc-plane) with the stacking axis down a-axis (Fig. 5). Single row
of [MCl4]2ꢀ anions and the lattice water molecules are positioned
between these two different dicationic layers and are held together
through secondary C–Hꢁ ꢁ ꢁCl/O–Hꢁ ꢁ ꢁCl hydrogen bonding interac-
tions. Stacking interactions between pyridinium rings can be seen
along the a-direction and b-direction defining two different types
Fig. 5, clearly depicts that alternating rows of
pꢁ ꢁ ꢁp stacked
pyridinium rings are separated by rows of [MCl4]2ꢀ anions. Even
though complex 2–4 are isostructural, crystal structure analysis
showed that hydrogen bonding interaction between the organic
dication and [MCl4]2ꢀ anions are quite different. Close up view of
the hydrogen bonding interaction in all the three organic salts are
depicted in Fig. 6. In the case of Co salt (2), six dicationic moieties
are involved in two O–Hꢁ ꢁ ꢁCl and five C–Hꢁ ꢁ ꢁCl contacts surround-
ing the [MCl4]2ꢀ anion as depicted in Fig 6a. Thus, the H1, H2 alco-
holic hydrogens from different dications are involved in O–Hꢁ ꢁ ꢁCl
contact with Cl3 and Cl2 of the [CoCl4]2ꢀ (H1ꢁ ꢁ ꢁCl3 = 3.425(7) Å;
H2ꢁ ꢁ ꢁCl1 = 3.158(4) Å with O–Hꢁ ꢁ ꢁCl = 152°and 169° respectively).
Further, Cl2 and Cl3 acts as acceptors and involved in C–Hꢁ ꢁ ꢁCl
contacts with methylene hydrogens H16A, H16B and the cationic
pyridine hydrogen H15 respectively. Cl1 and Cl4 are involved in
only one C–Hꢁ ꢁ ꢁCl contact with H12 and H1A of the pyridine
moiety. Thus, each [CoCl4]2ꢀ anion is held in place between the
alternate dicationic layers by five C–Hꢁ ꢁ ꢁCl and two O–Hꢁ ꢁ ꢁCl total-
ing seven secondary hydrogen bonding interactions. Hydroxyl
groups (O1 and O2) of the dicationic moiety and lattice water mol-
ecule O3 acts as hydrogen bonding acceptors and involve in C–
Hꢁ ꢁ ꢁO interactions with hydrogen atoms of the pyridinium ring in
stabilizing the organic salt in the crystal lattice. In the case of Com-
plex 3 only four units of dication are surrounded by the [ZnCl4]2ꢀ
involving in only C–Hꢁ ꢁ ꢁCl hydrogen bonding interactions. Thus,
Cl1 is involved in two CHꢁ ꢁ ꢁCl contacts with the methylene hydro-
gens H16A and H16B of different dicationic moiety while Cl2, Cl3
and Cl4 are involved in one C–Hꢁ ꢁ ꢁCl contact each with H12, H15
and H1A from the pyridine moiety of the L2+ in clasping the
[ZnCl4]2ꢀ. Unlike 1, complex 2 is occupied by only five C–Hꢁ ꢁ ꢁCl con-
tacts involving all four Cl of the anion, hence hydroxyl group has no
role in holding the anionic moiety. However, Hydroxyl group
involving O1 and the lattice water molecule O3 acts as acceptors
of superimposed rows of pyridinium rings. Both the
pꢁ ꢁ ꢁp interac-
tions are reached through almost face-to-face alignment with
inter-planar distances 3.56, 3.61 Å (and the centriodꢁ ꢁ ꢁcentroid dis-
tance 3.76, 3.85 Å), and horizontal slippage of 1.23 and 1.35 Å for
the Co complex. Similar trend is observed in the isostructural com-
plexes 3 and 4, with the face-to-face interplanar stacking distance
of 3.58, 3.71 Å (centroid...centroid distance 3.84, 3.92 Å) and hori-
zontal slippage of 1.41and 1.28 Å for Zn salt and 3.73, 3.58 Å
(centroid...centroid distance 4.07, 3.78 Å) with horizontal slippage
of 1.63 and 1.22 Å for the copper complex generating layers of
the dications. In addition to the above stacking interactions be-
tween the terminal pyridinium rings originated from the exclusive
stacking of the symmetrically disposed dication present in the
asymmetric unit from either side, alternate layers of pyridinium
dications within the cationic layer is further stabilized by intermo-
lecular C–Hꢁ ꢁ ꢁ
the H3 from the terminal pyridinium ring of the L2+ is involved in
C–Hꢁ ꢁ ꢁ with the central phenyl ring of the adjacent molecule
p interaction as depicted in Fig. 5 along b-axis. Thus,
p
within the self assembled first dicationic layer and H13 of the ter-
minal pyridinium ring of the L2+ is involved in C–Hꢁ ꢁ ꢁ
p with the
central phenyl ring with the adjacent molecule within the
p
stacked second dicationic layer. Same trend of C–Hꢁ ꢁ ꢁ
p interaction
is observed in all the three complexes with slight variations in the
Table 2
Hydrogen bonding interactions in Compound 2, 3 and 4.
Compound
D–Hꢁ ꢁ ꢁA
d(Hꢁ ꢁ ꢁA) (Å)
d(Dꢁ ꢁ ꢁA) (Å)
D–Hꢁ ꢁ ꢁA (°)
Compound 2
O(1)–H(1)ꢁ ꢁ ꢁCl(3)1
2.68
2.73
2.81
2.66
2.35
2.57
2.72
2.32
3.425(7)
3.594(5)
3.482(5)
3.563(5)
3.158(4)
3.489(4)
3.568(5)
3.216(8)
152
156
127
155
169
170
151
161
C(15)–H(15)ꢁ ꢁ ꢁCl(3)1
C(16)–H(16A)ꢁ ꢁ ꢁCl(2)2
C(16)–H(16B)ꢁ ꢁ ꢁCl(2)1
O(2)–H(2)ꢁ ꢁ ꢁCl(2)3
C(1)–H(1A)ꢁ ꢁ ꢁCl(1)4
C(12)–H(12)ꢁ ꢁ ꢁCl(4)5
C(11)–H(11)ꢁ ꢁ ꢁO(3)5
Symmetry code: (1) x, y, z; (2) 1 ꢀ x,ꢀy,1 ꢀ z; (3) x,1 + y, z; (4) 1 + x,1 + y, z; (5) 1 ꢀ x,1 ꢀ y,1 ꢀ z
Compound 3
C(16)–H(16A)ꢁ ꢁ ꢁCl(1)1
C(16)–H(16B)ꢁ ꢁ ꢁCl(1)2
C(12)–H(12)ꢁ ꢁ ꢁCl(2)3
C(15)–H(15)ꢁ ꢁ ꢁCl(3)1
C(1)–H(1A)ꢁ ꢁ ꢁCl(4)4
C(2)–H(2A)ꢁ ꢁ ꢁO(1)5
C(11)–H(11)ꢁ ꢁ ꢁO(3)6
2.69
2.82
2.78
2.66
2.59
2.59
2.29
3.574(8)
3.556(8)
3.632(7)
3.507(6)
3.518(6)
3.500(9)
3.205(9)
152
133
153
152
172
168
167
Symmetry code: (1) x, y, z; (2) 1 ꢀ x, 2 ꢀ y, ꢀz; (3) 1 ꢀ x, 1 ꢀ y, ꢀz; (4) 2 ꢀ x, 2 ꢀ y, 1 ꢀ z; (5) 1 + x, y, z; (6) x, y, ꢀ1 + z
Compound 4
C(6)–H(6B)ꢁ ꢁ ꢁCl(1)1
C(11)–H(11)ꢁ ꢁ ꢁCl(4)2
C(15) –H(15)ꢁ ꢁ ꢁCl(4)1
C(16)–H(16B)ꢁ ꢁ ꢁCl(4)1
O(2) –H(2)ꢁ ꢁ ꢁO(2)3
C(12)–H(12)ꢁ ꢁ ꢁO(2)2
2.67
2.61
2.75
2.82
2.56
2.56
3.612(11)
3.534(9)
3.624(8)
3.695(9)
3.323(9)
3.440(10)
163
172
156
151
155
158
Symmetry code: (1) x, y, z; (2) 1 + x, y, z; (3) 1 ꢀ x, ꢀy, ꢀz

K.K. Bisht et al. / Journal of Molecular Structure 1013 (2012) 102–110
109
via inter/intramolecular C–Hꢁ ꢁ ꢁO interaction with the pyridinium
hydrogens H2A, H5 and H11 in stabilizing the solid state structure.
For complex 4, only three dicationic moieties are drawn in hydro-
gen bonding with [CuCl4]2ꢀ via C–Hꢁ ꢁ ꢁCl interaction. Interestingly,
for 1 and 2, all the four chloride ions of [MCl4]2ꢀ moiety are impli-
cated as good hydrogen bond acceptors in C–Hꢁ ꢁ ꢁCl/O–Hꢁ ꢁ ꢁCl
contacts; while in the case of Cu complex 4, only two chlorides
Cl1 and Cl4 are making contacts with three different dicationic moi-
eties. Thus, Cl1 as an acceptor is making C–Hꢁ ꢁ ꢁCl H bonding with
the methylene hydrogen H6B, while Cl4 is making three C–Hꢁ ꢁ ꢁCl
hydrogen bonds from the other two dicationic moieties chipping
in methylene hydrogen H16B and pyridinium hydrogen H15 of
the same cation and pyridinium hydrogen H11 of another cationic
moiety. Hydroxyl group involving O2 is making intermolecular
O–Hꢁ ꢁ ꢁO H-bonding with the same dication and acts as an acceptor
in C–Hꢁ ꢁ ꢁO interaction with H12 in the generation of supramolecu-
lar hydrogen bonded nets. Details of all these pertinent hydrogen
bonding interaction mentioned above for all the three tetrachloro-
metallates and their symmetry codes are given in Table. 2.
(DST), New Delhi, India (Grant No. SR/S1/IC-37/2006) for financial
support. K.K.B. and A.C.K. acknowledge CSIR (India) and DST for JRF
and SRF, respectively. The authors are grateful to Dr. Pragnya Bhatt
for PXRD data Mr. Vinod Kumar Agrawal for IR data, Mr. Hitesh
Bhatt for NMR data, Mrs. Sheetal N. Patel for TGA data, Mr. Viral
Vakani for microanalysis, Mr. Arun Das for LCMS and Dr. P. Paul
for all-round analytical support.
Appendix A. Supplementary material
CCDC 831072, 831073 and 831074 contain the supplementary
crystallographic data for complex 2-4 respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/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. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.molstruc.2012.01.023.
Even though all the three complexes crystallized in the same
space group with almost identical cell dimensions and crystal
packing, hydrogen bonding interactions between [MCl4]2ꢀ moiety
and dications is quite different. This can be attributed to the subtle
changes in the tetrahedral geometry of [MCl4]2ꢀ due to the varia-
tion in the central metal, flexibility of the dication around the
methylene groups which all concomitantly control the closer
approach of the organic moiety surrounding the tetrachlorometal-
late in making effective hydrogen bonding interactions.
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