Hetero-metallic trinuclear nickel(II)-cadmium(II) complexes of a salicylaldimine ligand with thiocyanate, cyanate and azide ions: Isolation of a pair of polymorphs with thiocyanate ion

Article abstract of DOI:10.1016/j.ica.2012.07.007

Four new trinuclear hetero-metallic nickel(II)-cadmium(II) complexes [(NiL)₂Cd(NCS)₂] (1A and 1B), [(NiL)₂Cd(NCO) ₂] (2) and [(NiL)₂Cd(N₃)₂] (3) have been synthesized using [NiL] as a so-called "ligand complex" (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine) and structurally characterized. Crystal structure analyses reveal that all four complexes contain a trinuclear moiety in which two square planar [NiL] units are bonded to a central cadmium(II) ion through double phenoxido bridges. The Cd(II) is in a six-coordinate distorted octahedral environment being bonded additionally to two mutually cis nitrogen atoms of terminal thiocyanate (in 1A and 1B), cyanate (in 2) and azide (in 3). Complexes 1A and 1B have the same molecular formula but crystallize in very different monoclinic unit cells and can be considered as polymorphs. On the other hand, the two isoelectronic complexes 2 and 3 are indeed isomorphous and crystallize only in one form. Their conformation is similar to that observed in 1A.

Full text of DOI:10.1016/j.ica.2012.07.007

Inorganica Chimica Acta 394 (2013) 247–254
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Hetero-metallic trinuclear nickel(II)–cadmium(II) complexes of a salicylaldimine
ligand with thiocyanate, cyanate and azide ions: Isolation of a pair
of polymorphs with thiocyanate ion
Lakshmi Kanta Das a, Michael G.B. Drew b, Ashutosh Ghosh a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2012
Received in revised form 10 July 2012
Accepted 11 July 2012
Available online 26 July 2012
Four new trinuclear hetero-metallic nickel(II)–cadmium(II) complexes [(NiL)2Cd(NCS)2] (1A and 1B),
[(NiL)2Cd(NCO)2] (2) and [(NiL)2Cd(N3)2] (3) have been synthesized using [NiL] as a so-called ‘‘ligand
complex’’ (where H2L = N,N0-bis(salicylidene)-1,3-propanediamine) and structurally characterized.
Crystal structure analyses reveal that all four complexes contain a trinuclear moiety in which two square
planar [NiL] units are bonded to a central cadmium(II) ion through double phenoxido bridges. The Cd(II)
is in a six-coordinate distorted octahedral environment being bonded additionally to two mutually cis
nitrogen atoms of terminal thiocyanate (in 1A and 1B), cyanate (in 2) and azide (in 3). Complexes 1A
and 1B have the same molecular formula but crystallize in very different monoclinic unit cells and can
be considered as polymorphs. On the other hand, the two isoelectronic complexes 2 and 3 are indeed
isomorphous and crystallize only in one form. Their conformation is similar to that observed in 1A.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Nickel(II)
Cadmium(II)
Hetero-metallic complexes
Crystal structure
Polymorphism
Isomorphism
1. Introduction
of molecules in the crystal lattice is stabilized through various non-
covalent forces like H-bonds, p–p, and C–H/p interactions [11–15].
The isolation, identification and characterization of different
crystal forms having similar chemical composition represents one
of the most active areas of modern solid state chemistry [1–3]. This
type of solid state phenomenon may be related to isomerism or
polymorphism. Isomers are substances that have the same number
and kinds of atoms arranged differently in solid state, gas phase or
in solution. On the other hand, polymorphs are the compound with
identical chemical composition and the same molecular connectiv-
ity resulting from the possibility of at least two different arrange-
ments of the molecules of that compound in the solid state but
lead to identical liquid and vapor states with the condition that
no bonds are broken or created [4,5]. Polymorphism is an intriguing
phenomenon which has a solid-state property of outstanding fun-
damental and practical importance in view of both academic and
industrial purpose [6–8]. Polymorphism provides a useful tool for
investigating the structure–property relationship as the different
intermolecular interactions leading to significantly different crystal
packing that serve as tools in crystal engineering approach associ-
ated with the pharmaceutical industry [9,10]. Different polymorphs
appear because different arrangements and different conformation
The different solid forms show different solid-state properties such
as stability, crystal shape, compressibility and density which leads
to differences in handling and processing properties of the com-
pound [9,16].
In some cases the differentiation between polymorphism and
isomerism in organic and metal organic networks is not very clear.
This is due to the fact that since polymorphs can be rationalized on
the basis of supramolecular interactions, polymorphism can be
regarded as a type of supramolecular isomerism. Therefore, all sets
of polymorphs can be regarded as being supramolecular isomers of
one another but the reverse is not necessarily the case [17]. Again,
different conformations of the molecule with different bond angles
and internuclear distances are stabilized in the solid state which is
commonly called distortion isomerism or bond-stretch isomerism
[18]. Polymorphism is somehow different from distortion isomer-
ism because distortion isomerism is not only a solid-state phenom-
enon [19].
Recently, we reported some hetero-metallic polynuclear
complexes based on neutral copper(II) complexes of salen type
di-Schiff base ligands with transition and nontransition-metal ions
[20–26]. Now, we focused our attention on the synthesis of hetero-
metallic polynuclear complexes based on such neutral nickel(II)
complexes. In the present paper, we use [NiL] [where H2L = N,N0-
bis(salicylidene)-1,3-propanediamine (Scheme 1)] as a ‘‘ligand
⇑
Corresponding author.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.07.007

248
L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
yellow X-ray quality single crystals of 1A on slow evaporation at
room temperature. The compounds were washed with metha-
nol–water mixture and dried in a desiccator containing anhydrous
CaCl2.
Complex 1A: Yield: 0.639 g (70%). Anal. Calc. for C36H32CdNi2-
N6O4S2: C, 47.69; H, 3.56; N, 9.27. Found: C, 47.63; H, 3.52; N,
9.17%. UV–Vis: kmax(CH3OH) = 590, 403 and 344 nm, kmax (solid,
reflectance) = 619, 501 and 387. IR (KBr):
m ,
(C@N) 1613 cmꢀ1
m
(SCN) 2068 and 2033 cmꢀ1
.
2.5. Synthesis of the complex [(NiL)2Cd(NCS)2] (1B)
The resulting complex [NiL] (0.642 g, 2 mmol) was dissolved in
methanol (20 mL) and a water solution (0.5 mL) of Cd(ClO4)2
(0.311 g, 1 mmol) followed by an aqueous solution (0.5 mL) of so-
dium thiocyanate (0.162 g, 2 mmol) were added with stirring. A
small amount of orange precipitate separated immediately. The
stirring was continued for 1 h at room temperature when the
amount of the orange solid increases. The solid was filtered and
the filtrate was allowed to stand overnight at open atmosphere
when a rectangular shaped orange (1B) X-ray quality single crys-
tals appeared at the bottom of the vessel. The compound was
washed with methanol–water mixture and dried in a desiccator
containing anhydrous CaCl2.
Scheme 1. Formation of the complexes (1A, 1B, 2 and 3).
complex’’ to form four hetero-metallic coordination compounds
(1A, 1B, 2 and 3) with Cd(II) along with the pseudohalides, NCSꢀ,
NCOꢀ and N3 as anionic coligands. All the resulting complexes
ꢀ
are angular trinuclear species having general formula [(NiL)2Cd
(X)2] (X = NCSꢀ, NCOꢀ or N3ꢀ). Interestingly, a pair of polymorphs
(1A and 1B) with distinctly different geometries is isolated by
using thiocyanate ion as anionic coligands.
Complex 1B: Yield: 0.630 g (69%). Anal. Calc. for C36H32CdNi2-
N6O4S2: C, 47.69; H, 3.56; N, 9.27. Found: C, 47.58; H, 3.47; N,
9.18%. UV–Vis: kmax (CH3OH) = 592, 404 and 344 nm, kmax (solid,
reflectance) = 613, 494 and 378. IR (KBr):
m ,
(C@N) 1611 cmꢀ1
m
(SCN), 2051 cmꢀ1
.
2. Experimental
2.6. Synthesis of the complexes [(NiL)2Cd(NCO)2] (2) and
[(NiL)2Cd(N3)2] (3)
2.1. Starting materials
The salicylaldehyde and 1,3-propanediamine were purchased
from Lancaster and were of reagent grade. They were used without
further purification.
Caution: Perchlorate and azide salts of metal complexes with or-
ganic ligands are potentially explosive. Only a small amount of
material should be prepared and it should be handled with care.
The ‘‘ligand complex’’ [NiL] (0.642 g, 2 mmol) was dissolved in
methanol (20 mL) and then aqueous solution (0.5 mL) of Cd(ClO4)2
(0.311 g, 1 mmol) followed by the aqueous solution (0.5 mL) of so-
dium cyanate (0.130 g, 2 mmol) or sodium azide (0.130 g, 2 mmol)
for complexes 2 and 3, respectively were added to this solution
with stirring. A small amount of reddish precipitates separated
immediately. The stirring was continued for 1 h at room tempera-
ture when the amount of the reddish product increases. The fil-
trates were allowed to stand overnight at room temperature
when rectangular shaped red X-ray quality single crystals ap-
peared in both cases. The crystalline compounds were collected
by filtration, washed with methanol and dried in vacuum desicca-
tors containing anhydrous CaCl2.
2.2. Synthesis of the Schiff base ligand N,N0-bis(salicylidene)-1,3-
propanediamine (H2L)
The Schiff base ligand was synthesized by standard methods.
Briefly, 5 mmol of 1,3-propanediamine (0.42 mL) was mixed with
10 mmol of the salicylaldehyde (1.04 mL) in methanol. The result-
ing solution was refluxed for ca. 2 h, and allowed to cool. The yel-
low methanolic solution was used directly for complex formation.
Complex 2: Yield: 0.580 g (66%). Anal. Calc. for C36H32CdNi2N6O6:
C, 49.45; H, 3.69; N, 9.61. Found: C, 49.53; H, 3.62; N, 9.51%. UV–Vis:
kmax (CH3OH) = 588, 405 and 343 nm, kmax (solid, reflectance) = 627,
515 and 390. IR (KBr): m , m .
(C@N) 1610 cmꢀ1 (OCN) 2190 cmꢀ1
2.3. Synthesis of the ‘‘ligand complex’’ [NiL]
Complex 3: Yield: 0.530 g (60%). Anal. Calc. for C34H32CdNi2N10O4:
C, 46.70; H, 3.69; N, 16.02. Found: C, 46.69; H, 3.65; N, 16.08%. UV–
Vis: kmax (CH3OH) = 592, 404 and 344, kmax (solid, reflectance) = 632,
An aqueous solution (20 mL) of Ni(ClO4)2ꢁ6H2O (1.825 g,
5 mmol) and 10 mL ammonia solution (20%) were added to a
methanolic solution of H2L (10 mL, 5 mmol) to prepare the ‘‘ligand
complex’’, [NiL] as reported earlier [27].
500 and 384. IR (KBr):
m , m .
(C@N) 1608 cmꢀ1 (N3), 2044 cmꢀ1
2.7. Physical measurements
2.4. Synthesis of the complex [(NiL)2Cd(NCS)2] (1A)
Elemental analyses (C, H and N) were performed using a Perki-
n-Elmer 2400 series II CHN analyzer. IR spectra in KBr pellets
(4000–500 cmꢀ1) were recorded using a Perkin-Elmer RXI FT-IR
spectrophotometer. Electronic spectra in methanol and the solid
state were recorded in a Hitachi U-3501 spectrophotometer. Pow-
der X-ray diffraction patterns are recorded on a Bruker D-8 Ad-
vance diffractometer operated at 40 kV voltage and 40 mA
current and calibrated with a standard silicon sample, using Ni-fil-
The resulting complex [NiL] (0.642 g, 2 mmol) was dissolved in
methanol (20 mL) and then a water solution (0.5 mL) of Cd(ClO4)2
(0.311 g, 1 mmol) followed by an aqueous solution (0.5 mL) of so-
dium thiocyanate (0.162 g, 2 mmol) were added. A small amount
of orange precipitation separated immediately. The resulting mix-
ture was put under reflux for about 10 h. The amount of solid prod-
uct increases and the entire solid turned into a reddish yellow. The
solid was isolated by filtration. The filtrate gives rhombic shaped
tered Cu Ka (a = 0.15406 nm) radiation.

L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
249
2.8. Crystallographic data collection and refinement
pure 1A. On the other hand, complex 1B separated as an orange so-
lid when the solution was stirred at room temperature. The prod-
uct is pure 1B as is evident from the powder XRD pattern. The
diffraction quality rectangular shaped orange single crystals of 1B
were obtained on keeping the filtrate overnight at open atmo-
sphere. Again, if the mixing of the components is done at low tem-
perature (ca. 0 °C) the separated orange product is found to be 1B
as in the case of room temperature. The filtrate on keeping in a
refrigerator (ꢂ 5 °C) also produced the crystals of 1B. Therefore,
it may be concluded that 1B is formed when the constituents are
mixed at room temperature or at low temperature but at higher
temperature (when the mixture is put under reflux) 1B transforms
to 1A. Complexes [(NiL)2Cd(NCO)2] (2) and [(NiL)2Cd(N3)2] (3)
were synthesized by following a similar procedure to that of 1,
by using sodium cyanate and sodium azide respectively instead
of sodium thiocyanate. However, in both cases the reddish micro-
crystalline products which were isolated after stirring are the sin-
gle species of complexes 2 and 3 as their powder XRD patterns are
identical to those of the simulated ones (Fig. 1). Moreover, unlike 1
on refluxing the mixture or by mixing the components at low tem-
perature no change in the powder XRD pattern of this reddish
product is observed. On examining the crystals of 2 and 3 under
the microscope, that appeared from the filtrate it was readily
established from the crystalline forms that both samples were
homogeneous.The determination of the cell dimensions for several
crystals by diffractometer as well as the powder XRD patterns of
these crystals confirm that only one product is formed for 2 and 3.
9564, 5095 independent reflection data for 1A and 1B were col-
lected with Mo Ka and Cu Ka radiation, respectively at 150 K using
the Oxford Diffraction X-Calibur CCD System. The crystals were
positioned at 50 mm from the CCD. 321, 1564 frames were mea-
sured with counting times of 10, 1 s, respectively. Data analyses
were carried out with the CRYSALIS program [28]. 6371, 7694 inde-
pendent reflection data for 2 and 3 were collected with Mo Ka radi-
ation at 293 K using the Bruker-AXS SMART APEX II diffractometer.
The crystals were positioned at 60 mm from the CCD. 360 frames
were measured with a counting time of 5 s. All four structures
were solved using direct methods with the SHELXS97 program
[29]. The non-hydrogen atoms were refined with anisotropic ther-
mal parameters. The hydrogen atoms bonded to carbon were in-
cluded in geometric positions and given thermal parameters
equivalent to 1.2 times those of the atom to which they were at-
tached. Some disorder was apparent with the central carbon of
the propylene link in one ligand of 3 and two positions were re-
fined with occupation factors of x and 1 ꢀ x, x refining to 0.65(1).
The terminal atoms of the monodentate ligands, thus O in 2 and
N in 3, showed high thermal motion but no appropriate disordered
model was found. Absorption corrections were carried out using
the ABSPACK program [30] for 1A and 1B and SADABS program [31]
for 2 and 3. The four structures were refined on F2 to R1 0.0565,
0.0900, 0.0408, 0.0481; wR2 0.1308, 0.2526, 0.1146, 0.1606, for
6211, 4535, 5160, 5060 data respectively with I > 2r(I).
3.2. IR and UV–Vis spectra of the complexes
3. Results and discussion
Besides elemental analysis, all the complexes were initially
characterized by IR spectra. Like the precursor ‘‘ligand complex’’
3.1. Syntheses of the complexes
[NiL], a strong and sharp band due to the azomethine m(C@N) group
The Schiff-base ligand (H2L) and its Ni(II) complex, [NiL] were
synthesized using the reported procedure [27]. The Ni(II) complex
on reaction with cadmium perchlorate and sodium thiocyanate in
MeOH–H2O medium (20:1, v/v) resulted in a pair of polymorphs
[(NiL)2Cd(NCS)2] (1A) and [(NiL)2Cd(NCS)2] (1B) depending upon
the temperature. Complex 1A separated as a reddish yellow solid
when the solution was refluxed for about 10 h after mixing the
components. The diffraction quality rhombic shaped yellow single
crystals of 1A were obtained on keeping the filtrate overnight at
open atmosphere. A comparison of the powder XRD patterns of
the reddish yellow compound (Fig. 1) with that of the simulated
powder XRD pattern of the single crystal clearly shows that it is
of the Schiff base appears at 1614, 1611, 1610 and 1608 cmꢀ1 for
complexes1A, 1B, 2 and 3, respectively. The precursor ‘‘ligand com-
plex’’ [NiL] is neutral and obviously is not associated with any coun-
ter anion. Therefore, the characteristics peaks in the region of
(2200–2000 cmꢀ1) indicate the presence of counter anions that
indicate the formation of the complexes. Although, both complexes
1A and 1B contain the same counter anion (SCNꢀ), they were distin-
guished by showing different splitting patterns of the
m
(SCN) peak
in IR spectra. In the case of 1A (Fig. S1) splitting of the
m(SCN) band
(2068 and 2033 cmꢀ1) is obvious, while there was only a single
peak at 2050 cmꢀ1 in the spectrum of 1B (Fig. S2). The splitting of
the thiocyanate peak may be attributed to the presence of two
Fig. 1. Left: (a) experimental pattern of 1A obtained after refluxing the mixture about 10 h (b) simulated pattern of 1A (c) experimental pattern of 1B obtained after stirring
the mixture at room temperature (d) experimental pattern of 1B obtained after stirring the mixture at low temperature (e) simulated pattern of 1B. Right: (a) experimental
pattern of 2 obtained after stirring the mixture at room temperature (b) simulated pattern of 2 (c) experimental pattern of 3 obtained after stirring the mixture at room
temperature (d) simulated pattern of 3.

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L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
different types of SCNꢀ anion in 1A. The characteristic intense sin-
gle peaks for cyanate at 2190 cmꢀ1 and for azide 2044 cmꢀ1 were
clearly detected in the IR spectra of 2 and 3, respectively.
The electronic spectra were measured in CH3OH as well as in
solid state. All the complexes show a sharp single absorption band
near 344, 344, 343 and 344 nm along with a less intense shoulder
at 403, 404, 405 and 404 nm in methanol and 387, 378, 390, and
384 nm along with a less intense shoulder at 501, 494, 515, and
500 nm in solid state for 1A, 1B, 2 and 3, respectively (Figs. S3
and S4), attributed to ligand-to-metal charge transfer transitions.
Besides this band, a broad absorption band is observed in the vis-
ible region at 590, 592, 588 and 592 nm in methanol and 619, 613,
627 and 632 in solid state for 1A, 1B, 2 and 3, respectively (Figs. S3
and S4). This band is typical of d–d transitions of Ni(II) ions with a
square planar environment. The two polymorphs show closely
similar electronic spectra indicating almost identical environments
around Ni(II).
confirming a slightly distorted square planar geometry around
both nickel centers. A notable difference is however found in the
conformations of the two ligands. Taking the angle between the
two aromatic rings as a measure, the angle between planes in L
around Ni(2) is 9.7(2)°, while that around Ni(3) is 58.1(1)°.
The cadmium atom has an octahedral environment being
bonded to the four oxygen atoms from the two ligands which there-
fore bridge to the nickel atoms and two terminal –NCS groups. The
geometry is very distorted primarily by the small angles subtended
by the two ends of individual ligands, thus O(11)–Cd(1)–O(31) is
59.94(10)° and O(41)–Cd(1)–O(61) is 63.57(10)°. The bond lengths
are somewhat irregular with Cd(1)–N(CS) 2.198(4), 2.247(4) Å and
2.328(3), 2.406(3), 2.263(3), 2.375(3) Å to the four oxygen atoms.
The Niꢁ ꢁ ꢁCd distances are 3.333(1), 3.336(1) Å.
The structure of 1B, has equivalent stoichiometry to 1A, namely
[(NiL)2Cd(NCS)2] but is not isomorphous and shows significant
variations in structure (Fig. 3 and Table 1). Dimensions are com-
pared to those in 1A in Table 2. Bond lengths are similar to those
in 1A. Thus Ni–O distances range from 1.873(7)–1.883(7) Å, Ni–N
from 1.869(8)–1.902(8) Å, Cd–O 2.306(7)–2.392(7) Å and Cd–N
2.239(9) to 2.244(10) Å. The distortion in the geometry of the cad-
mium coordination sphere is also found. Again there is a small tet-
rahedral distortion in the equatorial planes around nickel with
r.m.s. deviations of coordinating atoms 0.036, 0.161 Å respectively
around Ni(2) and Ni(3); the metal atoms 0.027(4), 0.026(4) Å from
the planes. The s4 value are 0.148 and 0.169 for Ni(2) and Ni(3),
respectively. The Cdꢁ ꢁ ꢁNi distances are 3.282(2), 3.343(2) Å.
Complexes 1A and 1B both crystallize in the monoclinic system,
space group No. 14, but have very different cell dimensions. The
two complexes can be considered as polymorphs. The different de-
gree of distortion particularly in the bond angles around the cad-
mium atom is also observed between the two polymorphs. A
close examination of the two crystal structures reveals that the
two cis angles [O(11)–Cd(1)–O(41) and O(31)–Cd(1)–O(61)] and
the trans angle [O(11)–Cd(1)–O(61)] around the cadmium atom,
which are originated from the O atoms of two different ‘‘ligand
complex’’ are considerably different in two structures. In the case
3.3. Description of the structures
The four structures all consist of trinuclear neutral molecules of
formula [(NiL)2Cd(X)2], whereꢀX is a monodentate anion, NCSꢀ in
1A and 1B, NCOꢀ in 2 and N3 in 3. The structures are all similar
although 1B is somewhat different from the others.
The structure of 1A, [(NiL)2Cd(NCS)2] is shown in Fig. 2 together
with the atomic numbering scheme in the coordination spheres.
The structure contains a six-coordinate cadmium in a distorted
octahedral environment together with two four-coordinate square
planar nickel atoms with equivalent geometries. The two nickel
atoms have square planar environments, being bonded to two
nitrogen atoms and two oxygen atoms from the tetradentate ‘‘li-
gand complex’’. Bond lengths to oxygen range from 1.867(3)–
1.878(3) Å and to nitrogen from 1.884(4)–1.922(3) Å. There are
slight tetrahedral distortions in the two square planes with r.m.s
deviations of 0.040, 0.176 Å for the donor atoms. The nickel atoms
are 0.019(2), 0.026(2) Å from these planes. The distortion of the
square planer environment is also confirmed by the so-called
dex that measures the distortion between a perfect tetrahedron
4 = 1) and a perfect square planar geometry ( 4 = 0) with the for-
mula: + b)]/141°, with and b (in °) being the two
s4 in-
(s
s
s
4 = [360° ꢀ (
a
a
largest angles around the central metal in the complex. The s4
values are 0.138 and 0.187 for Ni(2) and Ni(3), respectively,
Fig. 2. The structure of 1A with ellipsoids at 30% probability.
Fig. 3. The structure of 1B with ellipsoids at 30% probability.

L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
251
Table 1
Crystal data and structure refinement of complexes 1A, 1B, 2 and 3.
Complex
1A
1B
2a
3
Formula
M
Crystal system
Space group
a (Å)
C
36H32CdNi2N6O4S2
C36H32CdNi2N6O4S2
906.61
monoclinic
P21/n
11.0391(3)
16.4365(4)
19.2764(5)
(90)
90.517(2)
(90)
C36H32CdNi2N6O6
874.47
C34H32CdNi2N10O4
874.49
906.61
Monoclinic
P21/c
13.3489(11)
11.8017(10)
21.7504(19)
(90)
92.174(8)
(90)
3424.1(5)
4
triclinic
triclinic
ꢀ
ꢀ
P1
P1
9.985(5)
11.982(5)
13.895(5)
90.774(5)
100.150(5)
90.465(5)
1636.1(12)
2
9.8995(4)
12.0305(6)
14.0228(6)
89.238(2)
79.420(2)
89.455(2)
1641.48(13)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
3497.45(16)
4
1.722
Z
Dcalc (g cmꢀ3
)
1.759
1.877
1.775
1.842
1.769
1.835
l
(mmꢀ1
)
7.602
F(000)
1832
1832
884
884
R(int)
0.051
0.137
0.022
0.057
Total reflections
Unique reflections
17242
9564
30128
5095
12492
6371
19638
7694
I > 2r(I)
6211
4535
5160
5060
R1, wR2
T (K)
0.0565, 0.1308
150
0.0900, 0.2526
150
0.0408, 0.1146
293
0.0481, 0.1606
293
a
2 is isostructural with 3. However the required transformation of the data for 2 to show this directly is not ꢀh, ꢀk, ꢀl but rather ꢀh, ꢀk, l which gives cell dimensions of
9.985, 11.982, 13.895 Å, 89.23°, 79.85°, 90.47°. As this is not an allowed set of angles, the transformation cannot be made.
Table 2
of complex 1A both these cis angles [100.14(10)° and 107.67(10)°]
Bond distances (Å) and angles (°) for complexes 1A, 1B, 2 and 3.
are significantly greater than 90° whereas the trans angle
[162.47(10)°] is less by 17.53° from the ideal value. On the other
hand, in complex 1B, one cis angle [O(11)–Cd(1)–O(41) 86.4(2)°]
is less than 90° while the other one [O(31)–Cd(1)–O(61) 91.5(2)°]
is greater. The corresponding trans angle [143.6(2)°] is smaller by
36.4° from 180°. The relative disposition of the two ‘‘ligand com-
plexes’’ around the Cd atom is thus different: they slide away from
each other more in 1A than in 1B resulting a greater Niꢁ ꢁ ꢁNi sepa-
ration in 1A. Another major difference between the two com-
pounds is in the folding of the ligands as the angles between the
two phenyl rings are 7.1(4) and 34.8(4)o in 1B compared to
9.7(2)o and 58.1(1)o in 1A.
1A
1B
2
3
Cd(1)–O(11)
Cd(1)–O(31)
Cd(1)–O(41)
Cd(1)–O(61)
Cd(1)–N(1)
Cd(1)–N(2)
Ni(2)–O(11)
Ni(2)–O(31)
Ni(2)–N(19)
Ni(2)–N(23)
Ni(3)–O(41)
Ni(3)–O(61)
Ni(3)–N(49)
Ni(3)–N(53)
2.328(3)
2.406(3)
2.263(3)
2.375(3)
2.198(4)
2.247(4)
1.867(3)
1.874(3)
1.922(3)
1.892(4)
1.878(3)
1.874(3)
1.884(4)
1.909(4)
2.306(7)
2.392(7)
2.324(6)
2.312(7)
2.244(10)
2.239(9)
1.883(7)
1.880(7)
1.897(8)
1.902(8)
1.873(7)
1.875(7)
1.899(8)
1.869(8)
2.353(3)
2.426(3)
2.357(3)
2.388(3)
2.238(5)
2.194(4)
1.868(3)
1.874(3)
1.914(4)
1.897(4)
1.853(3
1.861(3)
1.890(4)
1.889(4)
2.350(3)
2.389(3)
2.356(3)
2.369(3)
2.261(7)
2.209(6)
1.862(3)
1.880(3)
1.915(5)
1.899(5)
1.856(3)
1.862(3)
1.886(5)
1.890(5)
It is difficult to rationalize the different folds in the two ligands
in these two structures. One reason can be found in the packing of
the molecules. It is noteworthy that in each structure there is a close
contact between Ni(2), which is bonded to the more planar of the
two ligands, and a second Ni(2) across a center of symmetry. In each
case this second Ni(2) occupies a position close to an axial site. This
packing is found in these two structures with Ni(2)ꢁ ꢁ ꢁNi(2) dis-
tances of 3.346(1) and 3.461(3) Å. While these distances do not rep-
resent a metal–metal bond they do represent various non-covalent
interactions between the ligands of two trinuclear units. A close
examination reveals that the aliphatic chain that connects the
nitrogen atoms of the ligand plays a crucial role. In case of 1A,
two hydrogen atoms (H20A and H22B) of each terminal carbon
atom of aliphatic chain of one unit establish two hydrogen bonds
with two phenolic oxygen atoms (O31 and O11) of the other unit
and vice versa, giving rise to four C–Hꢁ ꢁ ꢁO hydrogen bonds
(O31ꢁ ꢁ ꢁH20A 2.90 Å and O11ꢁ ꢁ ꢁH22B 2.61 Å) (Fig. 4). This hydrogen
bonding network is also observed in case of 1B at distances
O31ꢁ ꢁ ꢁH20B 2.84 Å and O11ꢁ ꢁ ꢁH22A 2.67 Å. Besides these interac-
tions an additional interaction that differentiates the polymorphs
O(11)–Cd(1)–O(31)
O(11)–Cd(1)–O(41)
O(11)–Cd(1)–O(61)
O(11)–Cd(1)–N(1)
O(11)–Cd(1)–N(2)
O(31)–Cd(1)–O(41)
O(31)–Cd(1)–O(61)
O(31)–Cd(1)–N(1)
O(31)–Cd(1)–N(2)
O(41)–Cd(1)–O(61)
O(41)–Cd(1)–N(1)
O(41)–Cd(1)–N(2)
O(61)–Cd(1)–N(1)
O(61)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
O(11)–Ni(2)–O(31)
O(11)–Ni(2)–N(19)
O(11)–Ni(2)–N(23)
O(31)–Ni(2)–N(19)
O(31)–Ni(2)–N(23)
N(19)–Ni(2)–N(23)
O(41)–Ni(3)–O(61)
O(41)–Ni(3)–N(49)
O(41)–Ni(3)–N(53)
O(61)–Ni(3)–N(49)
O(61)–Ni(3)–N(53)
N(49)–Ni(3)–N(53)
Cd(1)–N(1)–C(1)
59.94(10)
100.14(10)
162.47(10)
98.65(13)
92.18(12)
77.30(11)
107.67(10)
92.53(13)
149.31(12)
63.57(10)
150.16(13)
97.15(13)
94.05(13)
96.12(12)
105.09(15)
78.42(12)
91.82(14)
170.39(14)
170.11(14)
92.73(14)
97.12(15)
81.35(12)
92.12(14)
165.59(15)
168.07(14)
92.16(14)
96.35(16)
132.4(3)
60.9(2)
86.4(2)
143.6(2)
95.8(3)
102.8(3)
77.5(2)
91.5(2)
90.9(3)
162.8(3)
63.3(2)
165.3(3)
97.1(3)
100.7(3)
100.7(3)
96.5(3)
78.6(3)
91.4(3)
170.2(4)
169.9(3)
92.5(3)
97.6(4)
81.0(3)
92.1(3)
167.5(4)
168.7(3)
95.0(3)
93.5(4)
147.0(9)
156.2(8)
59.27(10)
97.85(10)
157.08(10)
102.00(14)
93.93(14)
74.27(10)
103.11(10)
96.62(14)
148.99(14)
61.27(9)
149.73(15)
96.44(15)
94.16(14)
97.60(14)
104.68(18)
78.35(12)
92.43(15)
169.06(15)
170.13(15)
92.68(15)
96.86(18)
81.25(12)
92.08(16)
168.20(17)
169.26(15)
91.71(16)
96.20(18)
150.0(5)
59.81(12)
97.96(11)
157.22(12)
101.3(2)
91.67(16)
75.41(12)
103.10(12)
99.02(19)
148.85(16)
61.28(11)
153.7(2)
98.54(18)
96.1(2)
100.22(17)
98.7(2)
78.31(14)
92.91(17)
169.01(18)
170.75(18)
92.40(18)
96.6(2)
80.73(15)
92.60(18)
168.64(19)
169.60(17)
92.12(19)
95.6(2)
involves the SCNꢀ ligand. 1A establishes a weak C–H/
p interaction
through the hydrogen atom (H21A) of the central carbon atom of
the aliphatic chain with the electron cloud of SCNꢀ ligand at
3.49 Å where as in 1B the hydrogen atom (H21B) forms a C–Hꢁ ꢁ ꢁS
hydrogen bond (2.83 Å) (Fig. 4). This difference seems to arise be-
cause of the different disposition of the thiocyanate ions. In 1B
the Cd–N–C angle is small (147.0°) which facilitates the formation
Cd(1)–N(2)–C(2)
177.5(4)
154.7(4)

252
L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
0
Fig. 4. Non-covalent interactions observed between two trinuclear units of 1A and 1B and its location in the crystal packing. Distances in ÅA.
Fig. 6. The structure of 3 with ellipsoids at 30% probability.
Fig. 5. The structure of 2 with ellipsoids at 30% probability.
Complexes 2 and 3 are isostructural. The structure of 2,
of the C–Hꢁ ꢁ ꢁS hydrogen bond with the hydrogen atom (H21B) of
the central carbon atom of the aliphatic chain whereas due to the
large Cd–N–C angle (177.5°) in 1A, the hydrogen atom (H21A) is
far from the sulfur atom S(2) of SCNꢀ and consequently cannot form
the C–Hꢁ ꢁ ꢁS hydrogen bond.
[(NiL)2Cd(NCO)2] is shown in Fig. 5 together with the atomic num-
bering scheme in the coordination spheres. Dimensions are com-
pared to those in 1A and 1B in Table 2 and show only minor
differences. Thus Ni–O range from 1.853(3) –1.874(3) Å, Ni–N from
1.889(4)–1.914(4) Å, Cd–O 2.353(3)–2.426(3) Å and Cd–N

L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
253
0
Fig. 7. Non-covalent interactions observed between two trinuclear units of 2 and 3 and its location in the crystal packing. Distances in ÅA.
2.194(4)–2.238(5) Å. Again there is a small tetrahedral distortion
in the equatorial planes around nickel with r.m.s. deviations of
the coordinating atoms of 0.074, 0.137 Å, respectively around
Ni(2) and Ni(3). The metal atoms are 0.024(2), 0.009(2) Å from
the planes. The s4 values (0.147 and 0.160 for Ni(2) and Ni(3),
respectively) also confirm a slightly distorted square planar geom-
etry around the metal centers. Here the angles between the two
aromatic rings are 14.3(2)° and 51.6(1)°, thus showing a similar
difference to that found in both 1A and 1B. The Cdꢁ ꢁ ꢁNi distances
are 3.338(1), 3.363(1) Å.
The structure of 3, [(NiL)2Cd(N3)2] is shown in Fig. 6. Dimen-
sions are compared to those in 2 as well as those of 1A and 1B in
Table 2 and show only minor differences. Thus Ni–O range from
1.856(3)–1.880(3) Å, Ni–N from 1.886(5)–1.915(5) Å, Cd–O
2.350(3)–2.389(3) Å and Cd–N 2.209(6)–2.261(7) Å. The r.m.s.
deviations of the coordinating atoms around Ni(2) and Ni(3) are
0.067 and 0.129 Å, respectively with the metal atoms 0.026(2),
0.007(2) Å from the planes. The s4 values are 0.147 and 0.154 for
Ni(2) and Ni(3), respectively. The angles between the two aromatic
rings in each ligand are 13.8(3)° and 50.6(1)°. The Cdꢁ ꢁ ꢁNi distances
are 3.310(1), 3.347(1) Å.
O(61)) are considerably less than 180° for both complexes
(157.08(10)° for 2 and 157.22(12)° for 3); all these data along with
the Niꢁ ꢁ ꢁNi separation (4.860 Å for 2 and 4.812 Å for 3) clearly
show that structures of 2 and 3 are close to 1A.The non-covalent
interactions present between the two trinuclear units in complexes
2 and 3 are also similar to those in 1A. In case of complexes 2 and 3,
the central carbon atom of the aliphatic chain that connꢀects the
nitrogen atoms of the ligand is pointing to the OCNꢀ/N3 ligand
(Fig. 7), in the opposite side with respect to 1A (see Fig. 4). This
change causes a different hydrogen bonding network. In 1A two
hydrogen atoms belonging to two different carbon atoms interact
with two phenolic oxygen atoms of the other molecule. However,
in 2 and 3 one hydrogen atom (H21A for 2 and H21B for 3) of
the central carbon atom forms a bifurcated C–Hꢁ ꢁ ꢁO2 hydrogen
bond (O31ꢁ ꢁ ꢁH21A 2.84 Å and O11ꢁ ꢁ ꢁH21A 2.62 Å for 2 and
O31ꢁ ꢁ ꢁH21B 2.83 Å and O11ꢁ ꢁ ꢁH21B 2.61 Å for 3) with two pheno-
lic oxygen atoms (O31 and O11). The other hydrogen atom (H21B
for 2 and H21A for 3) establishes a C–H/p interaction (3.04 and
3.10 Å for complexes 2 and 3, respectively) with the triatomic li-
gand (OCNꢀ/N3ꢀ) presumably due to the large distances from the
oxygen atom (O1) in 2 and nitrogen atom (N12) in 3 which prevent
the formation of H-bond.
Thus the two isoelectronic complexes 2 and 3 resemble each
other in shape, possess similar chemical formulation and are iso-
morphous. Both have structural similarities more with complex
1A than with 1B. For example the two cis angles (O(11)–Cd(1)–
O(41) and O(31)–Cd(1)–O(61)) around the cadmium atom in 2
(97.85(10)° and 103.11(10)°) and 3 (97.96(11) and 103.10(12)°)
are greater than 90° whereas the trans angles (O(11)–Cd(1)–
4. Conclusions
Four hetero-metallic NiII2CdII complexes have been character-
ized by X-ray crystallography. Among these four complexes, two

254
L.K. Das et al. / Inorganica Chimica Acta 394 (2013) 247–254
are polymorphs (1A and 1B) while the other two are isomorphous
(2 and 3). From X-ray crystal structures, it is clear that the cad-
mium atoms in all four complexes show significant amounts of dis-
tortion from an ideal octahedral environment. Moreover, the
distortions around the nickel atoms are similar and in particular
it is noteworthy that the ligand L is folded more around one nickel
atom than the other. The different degrees of distortions between
the polymorphs (1A and 1B) can be related to variations in the dis-
torted octahedral environment of the cadmium atoms. Complexes
2 and 3 have one form and their degree of distortion is similar to
1A. Both polymorphs are stabilized in the solid state through var-
ious kinds of non-covalent weak interactions. In the present case,
due to the variations in the orientation of SCNꢀ ligand in 1A and
1B, different non-covalent weak interactions arise between the
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2012.07.007.
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Acknowledgements
L.K. Das is thankful to CSIR, India for awarding Junior Research
Fellowship (Sanction No. 09/028 (0805)/2010-EMR-I, dated
29.01.2011). We thank EPSRC and the University of Reading for
funds for the X-Calibur system. We also thank the DST-FIST, India
for funds for the Single Crystal Diffractometer Facility at the
Department of Chemistry, University of Calcutta.
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Appendix A. Supplementary material
CCDC 876920–876923 contains the supplementary crystallo-
graphic data for 1A, 1B, 2 and 3, respectively. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary