1. SHORT COMMUNICATION
DOI:10.1002/ejic.201301536
Formation of a Magnetically Coupled Neutral [4؋4]
Square Grid from a 2,6-Pyridinedicarbaldehyde
Bis(hydrazone) Ligand
Amit Adhikary,[a]
Soumyabrata Goswami,[a]
Javeed Ahmad Sheikh,[a]
and Sanjit Konar*[a]
Dedicated to Professor V. Chandrasekhar on the occasion of his 55th birthday
Keywords: Self-assembly / Magnetic properties / Magnetostructural correlation / Copper / Square grids
The synthesis, characterization, magnetic properties, and
magnetostructural correlation of a square-grid complex
having the molecular formula [Cu16L1
8](dmf)3 (1) {H4L1
= 2,6-
bis[(3-methoxysalicylidene)hydrazinecarbonyl]pyridine} is
described. Reaction of the H4L1
ligand and the Cu2+
salt gen-
erates complex 1, which exhibits a rare grid structure with a
[4ϫ4] metallic core of Cu2+
. Structural studies revealed a
Introduction
Ligand-directed self-assembly is a powerful synthetic
tool for molecular fabrication. Predetermined polymetallic
architectures can be developed if specifically designed li-
gands are coordinated to metal ions according to their co-
ordination requirements.[1]
Although significant contri-
butions to polymetallic cages with polypyridyl ligands have
already been documented,[2–5]
logical approaches for the in-
clusion of large numbers of metal ions in a small, single
molecular entity with interesting functional properties[6–9]
is
a synthetic challenge and is of current research interest.
Often the topology of the assembly can be predicted in ad-
vance if relatively rigid organic ligands are combined with
transition-metal ions that have a definite coordination pref-
erence. With the appropriate balance between the coordi-
nating features of the ligand and the coordination require-
ments of the metal, several [2ϫ2] and [3ϫ3] grid complexes
have been reported by using ditopic and tritopic ligands,
respectively. As the number of coordination sites and
thereby the number of coordinating metal centers increases
the self-assembly strategies, the premeditated polynuclear
structural arrays become exponentially more complicated
because of chemical and thermodynamic reasons. As a re-
[a] Department of Chemistry, IISER Bhopal,
Bhopal 462066, MP, India
E-mail: skonar@iiserb.ac.in
http://www.iiserb.ac.in/index.php
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301536.
Eur. J. Inorg. Chem. 2014, 963–967 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim963
mixed coordination geometry around the Cu2+
core. The mo-
lecule has a fourfold rotational axis and one inversion center.
Magnetic investigations showed the coexistence of both
ferro- and antiferromagnetic spin exchanges, though antifer-
romagnetic exchange dominates over ferromagnetic cou-
pling in the low-temperature region.
sult, polymetallic grid complexes larger than [3ϫ3] are very
rare.[1]
In this paper, we report the synthesis, structural
analysis, and magnetic properties of a novel [4ϫ4] grid com-
plex having the molecular formula [Cu16L8](dmf)3 (1). The
ligand, H4L = {2,6-bis[(3-methoxysalicylidene)hydrazino-
carbonyl]pyridine}, consists of a central pyridine ring and
two o-vanillin moieties on each side connected by a car-
bohydrazide linkage.
Results and Discussion
Complex 1 was derived from H4L and Cu(ClO4)2·6H2O
in methanol at room temperature through self-assembly. To
date, all of the reported [4ϫ4] grids were formed with li-
gands having a central dinucleating fragment.[1,2e]
So, we
expected preferably a [5ϫ5] or a [3ϫ3] grid, but a [4ϫ4]
grid was obtained with unsymmetrical coordination modes
of the H4L ligand (Scheme 1). Bond valence sum (BVS) cal-
culations[10]
suggest that all of the copper ions are in the
+2 oxidation state (Table S1, Supporting Information). The
ORTEP diagram of the complex is given in Figure S1. The
X-ray crystal data and structure refinement parameters are
summarized in Table S2, and relevant bond lengths are
listed in Table S3. The title compound consists of 16 Cu2+
ions, 8 L4–
, and 3 DMF molecules as the solvent of
crystallization (Figure 1). The structure of 1 involves a
novel [4ϫ4] grid of 16 Cu2+
centers coordinated by 2 groups
of 4 roughly parallel ligands arranged above and below the
metal pseudoplane; the metals are bridged by μ2-hydrazone

2. www.eurjic.org SHORT COMMUNICATION
oxygen atoms. The ligands are parallel but not eclipsed to
each other to avoid steric interactions of the o-vanillin
groups. The destabilization caused by steric hindrance of
the o-vanillin groups may be stronger than the stability ex-
pected from π···π interactions between aromatic rings.
Scheme 1. Schematic view of the H4L ligand showing compart-
ments for metal binding.
Figure 1. Molecular structure of 1. Color code, Cu: cyan, C: gray,
O: red, N: blue.
A central pyridine moiety with two 2,6-carbonyl func-
tionalities should ideally act as a tridentate or pentadentate
ligand through a symmetrical coordination mode. However,
one terminal methoxy group of all the ligands does not co-
ordinate with any metal center. This unsymmetrical coordi-
nation mode results in tetradentate behavior of the ligand.
The 16 Cu2+
ions of the molecules can be distributed into
2 groups: 8 Cu2+
ions with distorted square-pyramidal ge-
ometry with a N1O4 coordination environment (τ, distor-
tion from ideal square-pyramidal geometry: 0.06–0.13) and
the remaining 8 Cu2+
ions have a distorted octahedral ge-
ometry with a N2O4 coordination environment (Figure 2
and Figure S2). The N1O4 coordination environment of the
metal centers is satisfied by one oxygen atom of the meth-
oxy group from one o-vanillin moiety, two oxygen atoms
from the hydroxy groups of two o-vanillin moieties, and one
nitrogen atom and one oxygen atom from the hydrazine
moiety. In contrast, the N2O4 coordination environment of
the metal centers is fulfilled by one nitrogen atom from the
pyridine ring, one nitrogen atom from the hydrazone moi-
ety, and four oxygen atoms of the two hydrazone moieties
through μ2 bridging. The bent shape of the ligand induces
a pronounced bending in the overall arrangement of the
Eur. J. Inorg. Chem. 2014, 963–967 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim964
metal centers (Figure S3). The structure as a whole can be
best described as if it comprises two components: the cen-
tral Cu4N8O16 core, made of four corner-sharing CuN2O4
Oh units, in which the oxygen atoms lie alternatively above
and below the metal pseudoplane and the peripheral ring,
which itself is made of alternate arrangements of one Oh
and two distorted square-pyramidal CuN1O4 units (Fig-
ure 2). The central core and peripheral units are connected
through eight corner-sharing oxygen atoms. The [4ϫ4] grid
can also be described as if it is composed of nine [2ϫ2]
square segments, in which the central segment (“B” in Fig-
ure 2) is unique. All of the Cu···Cu distances in the central
segment are approximately 4.1 Å. The four corner segments
(represented by “A”) are identical and are related to the in-
ternal C2 axis. The remaining four segments (C) lie in be-
tween the corner segments and also have the same geometry
and symmetry. The molecule contains a center of inversion,
and the dimension of the metallic core is approximately
10.9ϫ10.7 Å2
. The Cu···Cu distances in the B and C seg-
ments are in the range from 3.4–4.3 Å. Detailed examina-
tion of the Cu–O bond lengths reveal Jahn–Teller elong-
ation for all of the Cu2+
centers. Additionally, two types of
Cu–O–Cu bridging angles are present in the core of the
grid: one is in the range from 113 to 115° and another is in
the range from 139 to 145°.
Figure 2. Polyhedral view of the core of 1 showing the mixed coor-
dination environment around the Cu2+
center. Six-coordinate: light
gray, five-coordinate: dark gray. “A”, “B”, and “C” represent the
different segments in the grid.
The electron paramagnetic resonance (EPR) spectra re-
corded for a polycrystalline sample of complex 1 at 300 and
77 K are displayed in Figure 3; there is no splitting at lower
temperatures. However, a ͗g͘ value of 2.08 with a line width
of 33 mT for complex 1 is obtained. This “g” value was
used to fit the magnetic data.
The bulk-phase powder X-ray diffraction pattern of com-
plex 1 is in good agreement with the simulated one obtained
on the basis of the single-crystal structure data (Figure S4),
and this indicates the purity of the as-synthesized product.
Variable-temperature magnetic susceptibility data for 1 is
shown in the form of χmT (χm = molar magnetic suscep-
tibility) in Figure 4. A room-temperature χmT value of

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Figure 3. EPR spectra of complex 1 at 300 K with a microwave
frequency of 9848.5 MHz (top) and at 77 K with a microwave fre-
quency of 9126.3 MHz (bottom).
6.43 cm3
mol–1
K was obtained, which is slightly less than
the expected value of 6.48 cm3
mol–1
K for 16 isolated Cu2+
ions (g = 2.08) owing to antiferromagnetic coupling.
Figure 4. Temperature dependence of the direct current suscep-
tibilities χMT collected for complex 1 in an applied field of 0.1 T.
The red line represents the best-fit obtained.
Upon lowering the temperature, the value of χmT grad-
ually decreased down to 1.80 cm3
mol–1
K at 20 K. With any
further decrease in the temperature, the value of χmT de-
creased rapidly to a final value of 1.01 cm3
mol–1
K at 1.8 K.
The susceptibility data were fitted according to the simpli-
fied spin-exchange model given in Figure 5 and as defined
by the Hamiltonian in Equation (1). Considering the com-
plexity of magnetic exchange, a nice fitting of the χmT ver-
sus T plot was obtained from the model to give J1 =
2.01 cm–1
, J2 = –55.4 cm–1
, and TIP = 320ϫ10–6
(102
R =
1.2) (TIP = temperature-independent paramagnetism) with
a g value of 2.08 (obtained from EPR), in which the ex-
change pathways were proposed to be based on the Cu–O–
Cu bridging angle.[11]
J1 and J2 represent coupling con-
Eur. J. Inorg. Chem. 2014, 963–967 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim965
stants for the smaller (113–115°) and larger (139–145°) Cu–
O–Cu angles, respectively (Figure 5). |J2|Ͼ|J1| explains the
overall antiferromagnetic exchange in the system.
HCu16 = –J1(S1S2 + S2S6 + S4S8 + S7S8 + S9S10 + S9S13 + S11S15
+ S15S16) – J1(S2S3 + S3S4 + S1S5 + S3S7 + S5S6 + S6S7 + S5S9 +
S6S10 + S7S11 + S8S12 + S10S11 + S11S12 + S13S14 + S14S15 + S10S14
+ S12S16) – gμBH ͚
16
1
Si (1)
Figure 5. Exchange model used for the data fitting of 1. Balls repre-
sent metal centers and lines signify connectivity between two metal
centers. Arrows signify Jahn–Teller axes around the metal centers.
In general, a Cu–O–Cu angle larger than 97° shows anti-
ferromagnetic interaction between the metal ions, and if this
angle is increased, the coupling parameter (–J) increases.[12]
According to the model, a large Cu–O–Cu angle in the 139–
145° range corresponds to strong antiferromagnetic cou-
pling. In the proposed model, ferromagnetic coupling was
observed through the angle in the 113–115° range, which is
higher than 97°. This can be explained by considering the
coordination environment around the Cu2+
center.
Because of Jahn–Teller elongation (Figure 5), the effec-
tive magnetic orbital becomes dx2
–y2 for distorted octahedral
and distorted square-pyramidal Cu2+
centers. As magnetic
planes of the adjacent Cu2+
centers are canted at an angle
of 72.6° (Figure S5), the magnetic couplings do not occur
in the parallel plane. Consequently, symmetry mismatch
leads to ferromagnetic spin exchange (J1)[13]
despite the fact
that the bond angle is higher than ideal.
The isothermal magnetization plot (M/NμB vs. H) shows
that complex 1 does not saturate even at the highest field
measured, 7 T, and the highest magnetization value of
3.03 μB, obtained at 2 K and 7 T, is inconsistent with the
theoretical value[1a]
of 16 μB, (for g = 2.0, 16 isolated Cu2+
;
Figure 6). This suggests the presence of a dominant antifer-
romagnetic exchange within the grid. The M/NμB versus H/
T plots show the presence of anisotropy in the low-tempera-
ture region (Figure S6). However, the magnetization dy-
namics for 1 investigated by alternating current suscep-
tibility measurements as a function of temperature (1.8–
10 K) and frequency, in zero and higher direct current

4. www.eurjic.org SHORT COMMUNICATION
fields, do not show any frequency dependence, which indi-
cates a lack of single molecular magnet behavior in the
complex.
Figure 6. Field-dependencies of the isothermal-normalized magne-
tizations of 1.
Conclusions
This report described the synthesis, characterization,
magnetic properties, and magnetostructural correlation of a
relatively rare [4ϫ4] square grid complex. Structural studies
revealed that the molecule has a fourfold rotational axis and
one inversion center. Magnetic investigations showed the
coexistence of both ferro- and antiferromagnetic spin ex-
change, although the latter dominates in the low-tempera-
ture region. This report revealed that pyridine-based li-
gands might help in the formation of polymetallic cages
with preferred and even more complex geometrical shapes.
Experimental Section
The H4L1
ligand was synthesized by following a previously re-
ported procedure[14]
(see the Supporting Information for details).
Synthesis of [Cu16L1
8](dmf)3 (1): The H4L1
ligand (46.3 mg,
0.1 mmol) and Cu(ClO4)2·6H2O (74.1 mg, 0.2 mmol) were taken
up methanol (10 mL) in a round-bottomed flask, and the reaction
mixture was stirred for 4 h. A brown precipitate was obtained,
which was collected by filtration, washed with Et2O, and air dried.
The dried precipitate was dissolved in DMF and Et2O was layered
carefully on top of the DMF solution in a narrow glass tube. After
a few weeks, dark brown colored single crystals suitable for X-ray
diffraction were separated out by filtration from the junction of the
layers. The crystals were washed with Et2O and air dried, yield ca.
49% (based on Cu). C193H157Cu16N43O51 (4911.36): calcd. C 47.20,
N 12.26, H 3.22; found C 47.31, N 12.21, H 3.07. IR (KBr pellet,
4000–600 cm–1
): ν˜ = 3389(w), 3063(w), 2927(m), 2359(m), 2128(s),
1651(s), 1596(s), 1496(s), 1453(m), 1371(s), 1167(s), 1086(s), 814(s),
748(s), 665(s) cm–1
.
X-ray Crystallography: Data collection of complex 1 was performed
at 120 K with a Bruker D8 venture CCD diffractometer by using
graphite monochromated Cu-Kα (λ = 1.54718 Å) radiation under
a cold nitrogen stream. Data reduction and cell refinements were
performed with the SAINT[15]
program, and the absorption correc-
Eur. J. Inorg. Chem. 2014, 963–967 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim966
tion program SADABS[16]
was employed to correct the data for
absorption effects. Crystal structures were solved by direct methods
and refined with full-matrix least squares (SHELXTL-97[17]
) with
atomic coordinates and anisotropic thermal parameters for all non-
hydrogen atoms.
CCDC-939879 (for 1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Crystal Data for 1: C193H157Cu16N43O51, triclinic, P1¯, a =
21.571(1) Å, b = 21.695(3) Å, c = 22.184(9) Å, α = 80.09(7)°, β =
86.77(7)°, γ = 70.95(7)°, V = 9667.9(6) Å, M = 4911.30, Dcalcd. =
1.678 gcm–3
, Z = 2, R1 = 0.1042 for 43968 reflections.
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis of ligand, BVS calculations, crystallographic bond
lengths, ORTEP diagram, PXRD, magnetic plot, and MALDI-
TOF mass spectral analysis.
Acknowledgments
A. A. acknowledges the Council of Scientific and Industrial Re-
search (CSIR), New Delhi for his SRF fellowship. S. G. thanks the
Indian Institute of Science Education and Research (IISER), Bho-
pal for a PhD fellowship. We are also thankful to Dr. H. S. Jena
for helpful scientific discussions. S. K. thanks the Government of
India, Department of Science and Technology (DST) (project
number SR/FT/CS-016/2010) and IISER, Bhopal for generous fin-
ancial and infrastructural support.
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Received: December 7, 2013
Published Online: January 28, 2014