Self assembled tetranuclear Cu4(ii), Ni4(ii) [2 × 2] square grids and a dicopper(ii) complex of heterocycle based polytopic ligands - Magnetic studies

Article abstract of DOI:10.1039/c1dt10813a

The ditopic ligand PyPzOAP (N-[(Z)-amino(pyridin-2-yl)methylidene]-5- methyl-1-(pyridin-2-yl)-1H-pyrazole-3-carbohydrazonic acid) and the polytopic ligand 2-PzCAP (N′³,N′⁵-bis[(1E)-1-(pyridin-2- yl)ethylidene]-1H-pyrazole-3,5-dicarbo

Full text of DOI:10.1039/c1dt10813a

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PAPER
Self assembled tetranuclear Cu4(II), Ni4(II) [2 ¥ 2] square grids and a
dicopper(II) complex of heterocycle based polytopic ligands - Magnetic
studies†‡
Tarak Nath Mandal,a Somnath Roy,a Saugata Konar,a Atanu Jana,a Sangita Ray,a Kinsuk Das,a Rajat Saha,b
Mohamed Salah El Fallah,*c Ray J. Butcher,d Sudipta Chatterjeee and Susanta Kumar Kar*a
Received 2nd May 2011, Accepted 23rd August 2011
DOI: 10.1039/c1dt10813a
The ditopic ligand PyPzOAP (N-[(Z)-amino(pyridin-2-yl)methylidene]-5-methyl-1-(pyridin-2-yl)-1H-
3
5
pyrazole-3-carbohydrazonic acid) and the polytopic ligand 2-PzCAP (N¢ ,N¢ -bis[(1E)-1-(pyridin-2-yl)-
ethylidene]-1H-pyrazole-3,5-dicarbohydrazide) were synthesized in situ by condensation of methyl
imino picolinate with 5-methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide and 2-acetyl pyridine with
pyrazole-3,5-dicarbohydrazide respectively. The ligands PyPzOAP and PzOAP (reported earlier, Dalton
Trans., 2007, 1229) self-assemble to form homoleptic [2 ¥ 2] tetranuclear M4 (M = Cu(II) and Ni(II))
square grids structures [Cu4(PyPzOAP)4](NO3)4 (1), [Cu4(PzOAP)4](ClO4)4 (2) and
[Ni4(PyPzOAP)4](NO3)4·8H2O·2CH3CN (3). While the ligand 2-PzCAP forms a dicopper(II) complex
˚
[Cu2(2-PzCAP)(OH)(NO3)(H2O)](NO3)·2H2O (4). The complex 1 is a perfect square grid (a = 4.201 A),
whereas, 2 and 3 are almost square grids. All these compounds have been characterized by X-ray
structural analyses and variable temperature magnetic susceptibility measurements. EPR studies have
also been carried out for complexes 1, 2 and 4. In the Cu4 grid (1), all the Cu(II) centers are in a
distorted octahedral environment with N4O2 chromophore, while, in complex 2, all four Cu(II) centers
have a square pyramidal environment with N3O2 chromophore. In complex 3, all four Ni(II) centers
have distorted octahedral geometry with N4O2 chromophore. In compound 4, the Cu(II) centers are in
square pyramidal environment with N3O2 chromophore. The magnetic properties of compounds 1 and
2 show the presence of intramolecular ferromagnetic exchange interaction (J = 5.88 cm-1 for 1 and 4.78
cm-1 for 2). The complex 3 shows weak intramolecular antiferromagnetic interaction (J = -4.02 cm-1).
While, complex 4, shows strong antiferromagnetic behavior (J = -443 cm-1).
ligands represent a successful paradigm for the single step
Introduction
synthesis of new clusters of well-defined aesthetical architectures
with electronic, redox, magnetic, photo-physical and catalytic
properties.4–7 The major challenging goal in this area is the
design and synthesis of new polytopic ligands which can be
used for synthesis of molecular metal grids. Hence, with the
proper choice of paramagnetic metal ions which interact with
these polytopic ligands, magnetic coupling can be fine-tuned. Spin
exchange interactions occur between the paramagnetic metal ions
through the direct bridging (e.g. m-O, m-N–N) connections of the
ligands.8 Moreover, depending on the packing, such assemblages
have the unique potential to exhibit either long-range order or
single molecule magnetic behavior. Transition metal ion templated
cyclizations have successfully produced many tetranuclear [2 ¥ 2]
Mn4, Ni4, Cu4 and Zn4 grid complexes,9–15 in which the bridging
arrays among the metal ions involve oxygen donor groups. Most
of the so far reported alkoxo-bridged polynuclear grids [Mn(II),
Co(II) and Ni(II)]11–15 show anti-ferromagnetic coupling, while the
ferromagnetic interaction prevails only in Cu(II) grids.8b–d,16 The
origin of this ferromagnetic exchange is due to the orthogonal
The binding of transition metal ions to properly designed poly-
topic ligands has attracted continuous scientific interest since the
seminal reports on the metal-mediated self-assembly processes.1–3
Self-assembly of transition metals with multifunctional organic
aDepartment of Chemistry, University College of Science, University of
Calcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: skkar_cu@
yahoo.co.in
bDepartment of Physics, Jadavpur University, Kolkata, 700 032, India
cDepartament de Qu´ımica Inorga`nica, and Institut de Nanocie`ncia i Nanotec-
nologia, Universitat de Barcelona, Mart´ı i Franque`s 1-11, 08028, Barcelona,
Spain. E-mail: salah.elfallah@qi.ub.es
dDepartment of Chemistry, Howard University, 2400 Sixth Street, N.W.,
Washington, DC, 200 59, USA
eDepartment of Chemistry, Serampore College, Serampore, Hooghly, Pin -
712 201, India
† Electronic supplementary information (ESI) available. CCDC reference
numbers 796246–796249 for 1–4. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1dt10813a
‡ Dedicated to the late Dr Golam Mostafa.
11866 | Dalton Trans., 2011, 40, 11866–11875
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2
2
orientation of the magnetic orbitals (dx ). Thus, the major
CHN analyzer 2400. Mass spectra were done with a JEOLJMS-
AX 500 mass spectrometer. The electronic spectra ofthe complexes
were recorded on a Hitachi model U-3501 spectrophotometer.
IR spectra (KBr pellet, 400–4000 cm-1) were recorded on a
Perkin-Elmer model 883 infrared spectrophotometer. Magnetic
susceptibility measurements for the compounds were carried out
on polycrystalline samples, at the Servei de Magnetoqu´ımica of
the Universitat de Barcelona, with a Quantum Design SQUID
MPMS-XL susceptometer apparatus working in the range 2–
300 K under a magnetic field of approximately 500 G (2–30 K)
and 1000 G (35–300 K). Diamagnetic corrections were estimated
from Pascal Tables. The EPR spectra have been recorded on an
X-band Bruker spectrometer (ESR 300E) working with an Oxford
helium liquid cryostat for variable temperature measurements.
–y
direction in the syntheses of new magnetically coupled metal
grids and clusters is the exploration of the various super-exchange
bridges embedded inside the polytopic ligands which will lead to
the development of materials with the desired magnetic properties.
In this work, we have continued our pursuit of studying the
coordination properties of N-heterocycle-based polytopic ligands.
As a part of this effort, we herein report two such new ligands,
PyPzOAP and 2-PzCAP (Scheme 1). The ligand PyPzOAP formed
a ferromagnetically coupled Cu4(II) [2 ¥ 2] square grid (1) and a
weak antiferromagnetically coupled Ni4(II) [2 ¥ 2] square grid (3).
While, PzOAP formed a ferromagnetically coupled Cu4(II) [2 ¥ 2]
square grid (2). A polytopic ligand 2-PzCAP (1 : 2 condensation
product of pyrazole-3,5-dicarbohydrazide and 2-acetyl pyridine)
formed only a dicopper(II) complex (4) instead of a molecular grid.
All the complexes are structurally and magnetically characterized.
EPR studies have also been carried out for complexes 1, 2 and 4.
Syntheses of the ligands
5-methyl-1-(2-pyridyl)pyrazole-3-carbohydrazide was prepared
following our previously reported method.18 The ligand
PzOAP was synthesized as reported previously.8b,c Pyrazole-
3,5-dicarbohydrazide was prepared according to the literature
procedure.19
Synthesis of PyPzOAP
The methyl ester of iminopicolinic acid was prepared in situ
by reaction of 2-cyanopyridine (5.2 g, 0.05 mol) with sodium
methoxide solution, produced by dissolving sodium metal (2.5 g,
0.108 mol) in dry methanol (25 mL). 5-methyl-1-(2-pyridyl)
pyrazole-3-carbohydrazide (10.85 g, 0.05 mol) in 25–30 mL dry
methanol, was added to the above solution with stirring for 1 h.
The mixture was refluxed for 5 h and cooled to room temperature.
Excess methanol was removed in a rotary evaporator to leave a
yellow coloured oily sticky liquid. It was then mixed with water (50
mL) and neutralized with AcOH (pH~5) which afforded a shining
yellow powder. The solid was filtered off, washed thoroughly with
water and methanol and dried in vacuo over fused CaCl2. Yield
(10.43 g, 65%). Mp 220 ◦C (decomp.). MS (m/z) 321 (M+, 100%).
IR/cm-1: nNH 3360; nCO/CN 1655(s), 1540(s); nN–Npz 1060(s); npy
1006(s). Anal. calc. for C16H15N7O: C, 59.81; H, 4.67; N, 30.52.
Found: C, 59.40; H, 4.62; N, 30.60%.
Scheme 1
Synthesis of 2-PzCAP
Experimental
To a dry methanolic solution (30 mL) of 2-acetyl pyridine (0.242 g,
2 mmol), solid pyrazole-3,5-dicarbohydrazide (0.184 g, 1 mmol)
was added with constant stirring. The suspension was stirred for
30 min and refluxed for 5 h at water bath temperature. After reflux,
the suspension turned into a transparent solution. The volume of
the mixture was reduced to one-third of its original volume and
kept for slow evaporation. After two days, a white microcrystalline
solid product was afforded. The solid was filtered off, washed
several times with cold methanol and dried in vacuo over fused
CaCl2. Yield: 0.341 g, 80%. M.P. 197–200 ◦C (decomp.). MS (m/z)
390 (M+, 100%). IR/cm-1: nNH 3348; nCO/CN 1673(s), 1536(s); nN–Npz
1123(s); npy 1005(s). Anal. calc. for C19H18N8O2: C, 58.46; H, 4.61;
N, 28.72. Found: C, 58.55; H, 4.65; N, 28.69%.
Materials
2-Cyano pyridine, 2-cyanopyrazine and 2-acetyl pyridine were
purchased from Aldrich. Other commercially available chemicals
and solvents were used and purified by standard procedures.17
Caution! Although we have not encountered any problem, it
should be kept in mind that only a small amounts of perchlorate
compound should be prepared and it should be handled with care.
Physical measurement
Elemental analyses (carbon, hydrogen and nitrogen) of the ligand
and the metal complexes were determined with a Perkin-Elmer
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Table 1 Crystallographic data of 1–4
Compound
1
2
3
4
Empirical formula
Formula weight
T/K
C64H56Cu4N32O16
1783.61
293 (2)
C44H44Cl4Cu4N24O20
1625.04
150(2)
C68H75Ni4N34O24
1987.29
293(2)
C19H24Cu2N10O12
711.56
200(2)
˚
Wavelength (A)
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions
Monoclinic
C2/c
Monoclinic
P21/c
Tetragonal
I41/a
Triclinic
P1
¯
˚
˚
˚
˚
˚
˚
˚
˚
a = 19.921(7) A
a = 14.745(1) A
a = 14.105(9) A
a = 8.8162(4) A
˚
˚
˚
b = 38.224(16) A
b = 21.597(1) A
b = 14.105(9) A
b = 10.2990(5) A
˚
c = 14.084(5) A
c = 26.180(2) A
c = 38.379(4) A
c = 16.3062(5) A
a = g = 90◦
a = g = 90◦,
a = b = g = 90◦
a = 74.779(3)◦
b = 88.629(3)◦
g = 65.135(4)◦
1289.87(9)
b = 134.890(5)
b = 121.57(1)◦
3
˚
Volume (A )
7598(5)
7103.1(11)
7636(7)
Z
4
2
4
2
Density (calc) (Mg m-3)
Absorption Coefficient (mm-1)
F(000)
1.559
1.192
3632
1.07 to 20.41
-19 £ h £ 19
-37 £ k £ 37
-13 £ l £ 13
1.190
1.520
1.413
3280
4.69 to 32.48
-21 £ h £ 21
-31 £ k £ 30
-38 £ l £ 37
1.043
1.719
1.076
4056
1.54 to 24.07
-16 £ h £ 16
-16 £ k £ 16
-44 £ l £ 44
1.086
1.832
1.733
724
4.55 to 34.78
-13 £ h £ 14
-15 £ k £ 16
-21 £ l £ 25
0.965
q Range (◦) for data collection
Index ranges
Goodness-of-fit on F2
Completeness to q
99.6%(= 20.00◦)
3741 [0.0305]
Full-matrix least squares
on F2
98% (= 32.48◦)
25170 [0.0572]
Full-matrix least squares
on F2
99.7% (24.07◦)
3019 [0.0382]
Full-matrix least squares
on F2
99.2% (34.78◦)
10328 [0.0224]
Full-matrix least squares
on F2
Independent reflections [Rint
]
Refinement method
Data/restraints/parameters
Reflections collected
3741/0/523
22805
R1 = 0.0619, wR2 = 0.1122
R1 = 0.0776, wR2 = 0.1178
0.255 and -0.213
25170/0/869
58450
R1 = 0.0992, wR2 = 0.2698
R1 = 0.1792, wR2 = 0.3486
2.289 and -0.905
3019/0/285
41211
R1 = 0.0535, wR2 = 0.1256
R1 = 0.0572, wR2 = 0.1284
1.351and -0.488
10328/46/454
22018
R1 = 0.0345, wR2 = 0.0832
R1 = 0.0591, wR2 = 0.0875
1.103 and -0.852
Final R indices [I > 2s(I)]
R indices (all data)
Largest difference peak and
-3)
˚
hole (e A
Syntheses of the complexes
X-ray quality crystals of 3 were obtained by slow evaporation of
acetonitrile solution of the complex. Yield 0.245 g, 40%. IR/cm-1:
nNH 3365; nCO/CN 1660(s), 1532(s); nN–Npz 1056(s); npy 1030(s).
UV-vis (lmax): 392, 630, 990 nm. mRT 6.11 B.M. Anal. calc. for
C68H75Ni4N34O24: C, 41.06; H, 3.77; N, 23.95. Found: C, 41.04; H,
3.74; N, 23.91%.
[Cu4(PyPzOAP)4](NO3)4 (1). The ligand PyPzOAP (0.321 g,
1 mmol) was added to a hot solution of Cu(NO3)2·6H2O (0.148 g,
1 mmol) in H2O/CH3OH (30 mL) [60 : 40 v/v]. The resulting
suspension was stirred with heating (~60 ◦C) until complete
dissolution of the ligand occurred. The resulting deep green
solution was filtered to remove any undissolved ligand and left
at room temperature. Dark green crystals suitable for X-ray
diffraction were isolated from the filtrate after standing for two
weeks. Yield 0.211 g, 45%. IR/cm-1: nNH 3372; nCO/CN 1665(s),
1535(s); nN–Npz 1060(s); npy 1020(s). UV-vis (lmax): 670 nm. mRT 3.67
B.M. Anal. calc. for C64H56Cu4N32O16: C, 43.05; H, 3.13; N, 25.11.
Found: C, 43.10; H, 3.18; N, 25.15%.
[Cu2(2-PzCAP)(OH)(NO3)(H2O)](NO3)·2H2O (4). The lig-
and 2-PzCAP (0.195 g, 0.5 mmol) was added to a solution of
Cu(NO3)2·6H2O (0.295 g, 1 mmol) in CH3CN-H2O (30 mL) [10 : 1
v/v]. The suspension was stirred for 1 h with constant stirring
at ~60 ◦C until complete dissolution of the ligand occurred.
The resulting deep green solution was filtered and left at room
temperature. Rectangular dark green crystals suitable for X-ray
diffraction were isolated after standing for several days. Yield:
0.270 g, 55%. IR/cm-1: nNH/H2O 3372, 3136; nCO/CN 1669(s), 1533(s);
nN–Npz 1119(s); npy 997(s). UV-vis (lmax): 385, 590(br) nm. mRT 1.85
B.M. Anal. calc. for C19H24Cu2N10O12: C, 32.06; H, 3.37; N, 19.67.
Found: C, 32.10; H, 3.39; N, 19.65%.
[Cu4(PzOAP)4](ClO4)4 (2). Complex 2 was prepared following
the same procedure as that of complex 1 using Cu(ClO4)2·6H2O
as metal salt. Yield 0.429 g, 70%. IR/cm-1: nNH 3350, 3134; nCO/CN
1645(s), 1525(s); nN–Npz 1051(s); npy 1021(s). UV-vis (lmax): 368,
694(br)nm. mRT 3.54 B.M. Anal. calc. for C44H44Cl4Cu4N24O20: C,
32.49; H, 2.70; N, 20.68. Found: C, 32.53; H, 2.72; N, 20.55%.
Single crystal X-ray crystallography
[Ni4(PyPzOAP)4](NO3)4·8H2O·2CH3CN (3). The ligand
PyPzOAP (0.321 g, 1 mmol) was added to a hot solution of
Ni(NO3)2·6H2O (0.29 g, 1 mmol) taken in H2O/CH3CN (20 mL)
[60 : 40 v/v]. Then pH of the solution was adjusted to ca. 6 by
adding few drops of NEt3. A red coloured compound precipitated
and the stirring was continued for ca. 45 min. This was filtered
under suction, washed with methanol and dried over fused CaCl2.
Selected crystal data for 1–4 are given in Table 1 and selected
metrical parameters of these complexes are given in Table 2. For 1
and 3 the data collections were made using Bruker SMART APEX
II CCD area detector equipped with graphite monochromated
˚
Mo-Ka radiation (l = 0.71073 A) source in w scan mode. For
2 and 4, data collections were made using Oxford Diffraction
11868 | Dalton Trans., 2011, 40, 11866–11875
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◦
˚
Table 2 Selected bond distances (A) and angles ( ) in 1–4
Table 2 (Contd.)
Selected angles (◦)
Selected bonds
Selected angles (◦)
˚
˚
Value (A)
Selected bonds
Value (A)
Complex 1
Cu1–O1
Cu1–N4
Cu1–N6
Cu1–O2
Cu1–N1A
Cu1–N2A
Cu2–N4A
Cu2– N2
Cu2– O1
Cu2– N1
Cu2– N6A
Cu2– O2
N4B–Cu2–N2B
N1B–Cu2–N2B
Cu2–O–Cu1
Cu2–O–H1O
Cu1–O–H1O
89.71(5)
81.79(5)
118.43(6)
113.3(16)
111.7(16)
2.376(5)
1.895(7)
2.329(8)
2.064(5)
2.086(7)
1.983(7)
1.913(7)
1.998(8)
2.070(5)
2.070(7)
2.348(8)
2.388(5)
N4–Cu1–N2A
N4–Cu1–O2
N2A–Cu1–O2
N4–Cu1–N1A
N2A–Cu1–N1A
O2–Cu1–N1A
N4–Cu1– N6
N2A–Cu1–N6
O2–Cu1–N6
N1A–Cu1–N6
N4–Cu1–O1
O2–Cu1–O1
N4A–Cu2–N2
N4A–Cu2–O1
N2–Cu2–O1
O1–Cu2–N1
O1–Cu2–N6A
N2–Cu2–O2
O1–Cu2–O2
175.7(3)
99.2(2)
77.2(3)
105.7(3)
78.0(4)
154.7(3)
72.0(4)
105.5(3)
92.2(2)
99.2(2)
74.0(3)
91.83(18)
175.9(3)
99.1(2)
77.5(3)
155.3(3)
90.9(2)
108.3(2)
92.01(18)
Complex 3:a Symmetry elements = 5/4 -y,-3/4+x,1/4 -z, b Symmetry
elements = 3/4+y,5/4 -x,1/4 -z.
Gemini and Oxford Diffraction Gemini R equipped with graphite
monochromated Mo-Ka radiation (l = 0.71073 A) source j–
˚
w scan mode. For 1 and 3 the structures were solved using
the Patterson method by using the SHELXS-97.20 Subsequent
difference Fourier synthesis and least-square refinement revealed
the positions of the remaining non hydrogen atoms. Non-hydrogen
atoms were refined with independent anisotropic displacement
parameters. Hydrogen atoms were placed in idealized positions
and their displacement parameters were fixed to be 1.2 times
larger than those of the attached non-hydrogen atom. Successful
convergence was indicated by the maximum shift/error of 0.001
for the last cycle of the least squares refinement. All calculations
were carried out using SHELXS-97,20 SHELXL-97,20 PLATON-
99,21 ORTEP-3222 and WinGX system Ver-1.64.23 For 2 and
4, cell parameter refinement and data reduction were carried
out using the CRYSALIS RED, Oxford Diffraction Ltd., Ver-
sion 1.171.31.8 (release 12-01-2007 CRYSALIS171. NET) and
Version 1.171.32.15 (release 10-01-2008 CRYSALIS171. NET)
respectively. All the structures were solved by conventional direct
methods and refined by full-matrix least square methods using
F2 data. SHELXS-97 and SHELXL-97 programs20 were used
for structure solution and refinement respectively. For 3, H-
atoms were not included in acetonitrile and water molecules.
For complex 1, due to the wrong data collection strategy the
data are complete only to q = 20◦ resulting in a low data :
parameter ratio and, in consequence, in a low-bond precision.
Thus it would not be appropriate to use these bond parameters in
fine comparison with similar entities in future. Since we failed to
generate another suitable quality single crystal of this compound
it thus restricts the possibility to recollect the data to rectify this
problem. But it suffices to establish the identity, connectivity of
the molecule and gross feature of the system. We have critically
examined the data and found that R2 value is higher for the
‘Tetragonal’ system than that in the ‘Monoclinic’ system. In
the Tetragonal system the R2 value is 0.218 whereas in the
Monoclinic system the R2 value is 0.031. The structure can be
solved both in Tetragonal as well as Monoclinic systems and the
structural model is the same. The reported structure model exhibits
a partial pseudosymmetry. Thus we decided to solve it in the
‘Monoclinic’ crystal system. Compound 2 appears to be crystalline
but, in general, was not suitable for X-ray study. Fortunately,
after several trials X-ray quality crystals of [Cu4(PzOAP)4](ClO4)4
could be formed. Crystals of this compound were found to be
weak scatterers and thus the quality of the structure is not
very high. However, the structural analysis of the reference com-
pound suffices to establish the identity and gross features of the
system.
Complex 2
Cu1–O1
Cu1–O4
Cu1–N1
Cu1–N3
Cu1–N23
Cu2–O1
Cu2–O2
Cu2–N5
Cu2–N7
Cu2–N9
Cu2–O2
Cu3–O3
Cu3–N11
Cu3–N13
Cu3–N15
Cu3–O3
Cu4–O4
Cu4–N17
Cu4–N19
Cu4–N21
Complex 3
Ni1–O1
1.958(5)
2.333(4)
2.050(7)
1.903(5)
1.961(5)
2.324(6)
1.967(4)
1.952(5)
1.994(6)
1.915(7)
2.396(6)
1.985(5)
1.969(5)
2.030(7)
1.887(5)
2.302(4)
1.958(5)
1.983(5)
2.020(7)
1.926(6)
Cu1–O4–Cu4
Cu1–O1–Cu2
Cu2–O2–Cu3
Cu3–O3–Cu4
O1–Cu1–N3
N3–Cu1–N23
O1–Cu1–N1
O1–Cu2–N5
N5–Cu2–N9
O1–Cu2–N5
N11–Cu3–N15
O3–Cu3–N13
N13–Cu3–O2
N17–Cu4–N21
O4–Cu4–N19
O4–Cu4–N17
N17–Cu4–N19
N19–Cu4–N21
136.4(4)
139.0(2)
138.2(2)
138.3(2)
79.7(2)
167.7(2)
159.6(2)
76.2(2)
168.2(3)
76.2(2)
176.1(3)
159.5(2)
95.1(3)
173.36(19)
159.8(3)
97.2(2)
102.4(3)
81.0(3)
2.079(3)
2.115(3)
1.958(4)
2.280(3)
1.936(3)
2.177(3)
O1–Ni1–N1
O1–Ni1–N2
O1–Ni1–O1b
O1–Ni1–N4b
O1–Ni1–N6b
N1–Ni1–N2
O1b–Ni1–N1
N1–Ni1–N4b
N1–Ni1–N6b
O1b–Ni1–N2
N2–Ni1–N4b
N2–Ni1–N6b
O1b–Ni1–N4b
O1b–Ni1–N6b
N4b–Ni1–N6b
Ni1–O1–Ni1a
154.78(12)
77.46(11)
92.31(10)
105.68(11)
94.08(11)
77.38(13)
91.46(10)
99.23(13)
96.33(11)
102.85(11)
174.17(12)
110.34(13)
72.33(11)
146.80(12)
74.58(13)
137.83(13)
Ni1–N1
Ni1–N2
Ni1–O1b
Ni1–N4b
Ni1–N6b
Complex 4
Cu1–O
1.9197(11)
1.9289(13)
1.9590(14
2.0251(13)
2.264(7)
O–Cu1–N4A
O–Cu1–N1A
N4A–Cu1–N1A
O–Cu1–N2A
N4A–Cu1–N2A
N1A–Cu1–N2A
O–Cu1–O11A
O–Cu2–N4B
88.91(5)
99.14(5)
169.25(6)
169.48(6)
89.32(5)
81.47(6)
90.59(12)
89.32(5)
98.88(5)
169.02(6)
177.46(5)
Cu1–N4A
Cu1–N1A
Cu1–N2A
Cu1–O11A
Cu1–O11B
Cu2–O
Cu2–N4B
Cu2–N1B
Cu2–N2B
2.38(3)
1.9142(11)
1.9201(13)
1.9622(13)
2.0154(13)
O–Cu2–N1B
N4B–Cu2–N1B
O–Cu2–N2B
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Fig. 1 (a) Structural representation of cationic complex 1. (b) Structural representation with atom numbering scheme of the tetranuclear core in 1.
Fig. 2 (a) Structural representation of cationic complex 2. (b) Structural representation with atom numbering scheme of the tetranuclear core in 2.
all the copper(II) centers are arranged in a perfect square (a =
4.199 A), with alkoxide oxygen atoms acting as connector between
Results and discussion
˚
the two adjacent metal atoms. The alkoxide bridge angles are
in the range 141.0(3)–141.6(3)◦. For this complex, the ligands
PyPzOAP are aligned in two parallel pairs, above and below
the tetra copper square arrangements. The two-fold rotational
symmetry leads to two equivalent pairs of copper centers with
Structural description of 1, 2 and 3
The structures of the tetranuclear cationic part in 1–3 are illus-
trated in Fig. 1(a), 2(a) and 3(a) respectively. The core structures
are shown in Fig. 1(b), 2(b) and 3(b) respectively. The complex 1
is a homoleptic Cu4(II) [2 ¥ 2] square grid, where all four Cu(II)
centers are bridged by alkoxide oxygen atoms. It consists of four
six-coordinate distorted octahedra with N4O2 chromophore where
˚
an alternating arrangement of axial [Cu1–O1 2.376(5) A, Cu2–
˚
O2 2.388(5) A] and equatorial Cu–O bonds [Cu1–O2 2.064(5)
˚
˚
A, Cu2–O1 2.070(5) A] around the square. The Jahn–Teller axis
11870 | Dalton Trans., 2011, 40, 11866–11875
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Fig. 3 (a) Structural representation of cationic complex 3. (b) Structural representation with atom numbering scheme of the tetranuclear core in 3.
of Cu1 and Cu2 are defined as O1–Cu1–N6 (Cu1–O1 2.376(5)
central Ni(II) atom is slightly pulled toward the N6 atom (mean
˚
˚
˚
˚
A, Cu1–N6 2.329(8) A) and O2–Cu2–N6A (Cu2–O2 2.388(5) A,
plane deviation, 0.068 A) from the basal plane formed by the
˚
Cu2–N6A 2.348(8) A), with the equatorial planes O2–N2A–N1A–
equatorial atoms. From the analysis of all nickel–nitrogen/oxygen
bond distances it is clear that each nickel environment undergoes
tetragonal elongation. The pyridyl-pyrazole part of the ligand
PyPzOAP is planar (dihedral angle 1.44◦) as that of complex 1.
All the three complexes are stabilized through H-bond networks.
In the case of 1, there is moderately strong H-bonding involving
the hydrogen atoms of dangling -NH2 groups with the oxygen
atoms of ionic nitrate. In 2 there are weak H-bonding interactions
involving hydrogen atoms of pyrazole-N and dangling -NH2 group
with the oxygen atoms of ionic perchlorate. Relevant H-bond
parameters are listed in the ESI (Table-S1†). In 3 weak H-bonding
exist between H2W1 of water molecule and N3 atom of ligand
moiety, H7A and H7B of free –NH2 group and O1 W of water and
O101 of nitrate molecule respectively. The nitrate anions present
in this molecule is responsible for the formation honeycomb like
superstructure (Fig. 4). This is a unique example of a honeycomb
network consisting of a self-assembly of Ni4(II) grid.
N4 and O1–N2–N1–N4A respectively.
For complex 2, all the four copper(II) centers are arranged
in a squarish manner. The ligands (PzOAP) are arranged in
two parallel pairs, above and below the tetra copper square
arrangements. The Cu–Cu separations are closed to 4 A (Cu1–Cu2
4.014 A, Cu2–Cu3 4.079 A, Cu3–Cu4 4.008 A and Cu4–Cu1 3.987
˚
˚
˚
˚
˚
A) with O–Cu–O bridge angles falling in the range 94.97(17)–
96.37(16)◦. Here all the square pyramidal Cu1, Cu2, Cu3 and Cu4
centers have the CuN3O2 chromophore. The structure of 2 involves
the use of the four five-coordinate Cu(II) centers for formation of
the homoleptic Cu4 [2 ¥ 2] square grid.
Complex 3 consists of a [2 ¥ 2] square grid involving four
nickel(II) centers bridged by four alkoxide oxygen atoms. Here
the PyPzOAP ligand behaves as a uninegative-pentadentate N4O
donor, where the oxygen atom acts as a bridging atom between
the two adjacent nickel(II) centers. Each nickel atom has distorted
octahedral geometry and is bound to two pyridine nitrogen atoms,
one pyrazole nitrogen atom, one imine nitrogen atom, and two
alkoxide oxygen atoms belonging to two orthogonal ligands. All
˚
the Ni–Ni bond lengths are exactly 4.067 A forming a perfect
Ni4 [2 ¥ 2] square grid. All the Ni–O bonds have alternating long
˚
and short distances (2.280(3) and 2.079(3) A respectively) around
the Ni4O4 arrangement. All the Ni–O–Ni angles are 137.83(13)◦.
Two of the oxygen atoms are located above the mean Ni4 plane
˚
˚
(0.765 A), while the other two are located below it (0.765 A)
resulting a boatlike arrangement. Each of the Ni(II) atom have
˚
four short in-plane bonds (1.936(3)–2.115(3) A), formed by the
three nitrogen atoms (N1, N2 and N4) and one oxygen atom of
ligand moiety in the equatorial plane. While the apical positions
are occupied by one nitrogen atom (N6) and one oxygen atom with
˚
˚
longer distance (Ni1–O1 2.280(3) A and Ni1–N6 2.177(3) A). The
shorter Ni–N6 bond distance than that of Ni–O1 suggests that the
Fig. 4 Honeycomb like superstructure of 3 (Solvent molecules were
omitted for clarity).
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Fig. 5 Structural representation and atom numbering scheme of the cationic complex 4.
Structural description of 4
The structural representation of 4 is shown in Fig. 5. The structure
consists of a dinuclear cation, with two copper centers bridged
by the central pyrazole moiety and an adventitious hydroxide
group. The ligand adopts an unusual cis conformation, in which
the hydrazone oxygen and diazine N–N groups are not involved
in bridging and the ligand end pieces form a tridentate N3
coordinating pocket, with a mixture of five- and six-membered
chelate rings. The net result is a near perfect fit of the metal ions
in the pockets forming an almost planar dinuclear entity. The Cu–
◦
˚
Cu separation is 3.294 A and the Cu–O–Cu angle is 118.43(6) .
The two copper ions are almost square pyramidal (t values are
0.004 for Cu1 and 0.14 for Cu2), with an axial nitrate bound
to Cu1 and an axial water molecule bound to Cu2 forming an
unusual trans arrangements between them. Hence, the absence
of any intramolecular hydrogen bonding interaction indicates the
inherent stability of the dinuclear unit and the apparently preferred
dinucleating ligand bonding mode. In this complex, there is a weak
H-bonding interaction involving H3AB and H3BB with O3W and
O2W respectively of lattice water molecules. Details of H-bond
parameters are provided in the ESI (Table-S1†).
Fig. 6 Plots of the cMT ns T and M N-1b ns H at T = 2 K (inset) for 1
and 2. Solid line corresponds to the best fit (see text). The experimental
magnetization curves of the complexes (1) and (2) were fitted to the
Brillouin equation at T = 2.0 and S = 2.0 giving the following values
of g = 2.08 for 1 and g = 2.02 for 2.
The field dependence of magnetization (0–5 T) measured at 2 K
of 1 and 2 shown in Fig. 6, inset in the form of M N-1b (per Cu4
entity) ns H suggests that the magnetization tends to 4 Nb. This
feature agrees with the global weak ferromagnetic coupling within
the four copper(II) atoms.
Magnetic studies
The variable temperature magnetic susceptibility data for the
compounds 1 and 2 in form cMT ns T plots is shown in Fig. 6
indicating ferromagnetic behaviors of the two compounds. cMT
values are 1.672 cm3 mol-1 K (for 1) and 1.556 cm3mol-1 K (for
2) at 300 K being close to that expected for four uncoupled S =
1/2 spins (with g = 2.1 for 1 and 2.00 for 2). cMT increases up
slightly when temperature was lowered and reached a maximum
value of 3.162 cm3 mol-1 K and 2.878 cm3 mol-1 K at 2 K for 1 and
2 respectively. These maxima are consistent with that expected for
a ferromagnetically coupled copper(II) ions.
The structures of 1 and 2 consist of copper ions linked between
them by the ligands PyPzOAP and PzOAP giving tetranuclear
compounds respectively. Taking into account the compound
topology (slight distortion from square array, see crystallographic
data), coupling parameters J, (Fig. 7) can be considered to
interpret the magnetic interactions in the complex. For this
square disposition, the En value can be obtained by using Kambe
method24 from the Hamiltonian:
H = -J1(S1·S2+ S2·S3+S3·S4+S1·S4)
(1)
11872 | Dalton Trans., 2011, 40, 11866–11875
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isolated Ni(II) ion (8 with g = 2.0 Nb). Confrontation of the overall
shape of the plot of the compound with the Brillouin one (dashed
plot) for four isolated Ni(II) with S = 1 and g = 2 at T = 2 K
indicates slower magnetisation which is consistent with a weak
antiferromagnetic interaction.
As in 1 and 2 the structure of 3 consists of nickel ions linked
between them by the ligand PyPzOAP giving a tetranuclear com-
pound with the same topology, therefore one coupling parameter
J, (Fig. 7) can be considered to interpret the magnetic interactions
in the complex using the previous assumption and Hamiltonian
of the eqn (1). The analysis of the experimental susceptibility data
have been performed by the use of expression (3):
Fig. 7 Structural representation of the [M4O4] core and exchange
coupling constant in 1, 2 and 3. (M = Cu in 1 and 2, and M = Ni
in 3).
2exp(J KT ) +14exp(3J KT ) + 22exp(5J KT ) +52exp(6J KT )
N b2g2 +10exp(7J KT ) +56exp(8J KT ) +60exp(10J KT )
kT 1+3exp(J KT ) +11exp(3J KT ) + exp(4J KT ) +13exp(5J KT )
cM
=
+24exp(6J KT ) +5exp(7J KT ) +14exp(8J KT ) + 9exp(10J KT )
The analysis of the experimental susceptibility data have been
performed by the use of expression:
(3)
The best fit parameters found are: J = -4.02 cm-1 and g = 2.17
Ng2b2 6exp(J KT ) +12exp(2J KT ) + 30exp(3J KT )
3K 1+3exp(J KT ) + 7exp(2J KT ) +5exp(3J KT )
(2)
4
with an error R = R[(cMT)obs-(cMT)calc]2/R[(cMT)obs]2 = 6.1 ¥ 10- .
cM
=
The magnetic behavior of 4 in the forms of cMT ns T and cM ns T
(inset) plots is shown in Fig. 9. At 300 K, the cMT value per dimer
is 0.3203 cm3 mol-1 K, this value is very much smaller than that
expected for two uncoupled S = 1/2 spins (0.75 cm3 mol-1 K). cMT
decreases rapidly with decreasing temperature to reach a plateau
at ca. 70 K giving a value of 0.00541 cm3 mol-1 K, which indicates
the existence of strong antiferromagnetic coupling. The plateau
observed at low temperature is may be due to the non coupled
Cu(II) ions in the polycrystalline powder sample. The cM shows a
maximum of 1.07 ¥ 10-3 cm3 mol-1 at 300 K and then decreases,
reaching a minimum (7.73 ¥ 10-5 at ca. 70 K). The next increase in
the cM values is indicative of the presence of a small quantity of
paramagnetic impurities. The magnetic susceptibility data were
quantitatively analyzed by simple treating it as an interacting
dimer [eqn (4)].
The parameters N, b and K in eqn (2) have their usual meanings,
J = Singlet–triplet splitting.
The best fit parameters found are: J = +5.88 and +4.78 cm-1 and
g = 2.09 and 2.05 with an error R = 8.6 ¥ 10-5 and 1.4 ¥ 10-4 for 1
and 2 respectively, where R = R[(cmT)obs - (cmT)calc]2/R[(cmT)obs]2.
The global feature of the cMT vs. T curve of complex 3 is
characteristic of very weak antiferromagnetic interaction in a
tetranuclear nickel(II) complex and/or intermolecular interaction
(Fig. 8). The value of cMT at 300 K is 4.6212 cm3 K mol-1 which is
slightly high to the expected for four uncoupled nickel(II) ions (4
¥ 0.99 cm3 K mol-1 with g = 2.0). The cMT values are more or less
constant at high temperature and then decrease suddenly in the
low-temperature region reaching a value of 0.7455 cm3 K mol-1 at
2 K.
Ng2b2 2exp(J KT )
r
2
⎛
⎞
⎟
⎟
⎟
⎟
⎠
⎜
⎜
⎜
(4)
cM
=
1− r +
( )
⎜
KT 1+3exp(J KT )
⎝
Fig. 8 Plots of the cMT ns T and M N-1b ns H at T = 2 K (inset) for
3. Solid and dashed lines correspond to the best fit and the simulation M
N-1b ns H at T = 2 K with g = 2 respectively (see text).
Fig. 9 Plots of cMT vs. T and cM vs. T (inset) for 4. Solid lines correspond
to the best fit (see text).
The reduced molar magnetisation (M N-1b) at 2 K for 3 reaches
2.24 (Fig. 8, inset). This value is less than the expected for four
Here, we take into account a proportion of a monomeric
impurity r, for which the susceptibility is assumed to follow the
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Curie law c = (NmB g2/kT). Least-square fitting of all experimental
(118.4◦) obviously falling in the antiferromagnetic regime but
not enough to reach a value of J = -443.0 cm-1. The reasons
may found in terms of the complementary effect of the HOMO
of the second bridging ligand different to the hydroxide.29,30
Indeed, to transmit the antiferromagnetic coupling, the most
useful molecular orbitals of the hydroxide and pyrazolato bridges
are the highest filled MOs (HOMOs) which interacts more strongly
2
data leads to the following parameters: J = -443.0 cm-1, g =
ꢀ
2.2, and r = 0.008 with the agreement factor R =
[(cM)obs -
ꢀ
(cM)calc]2/ [(cM)obs2] = 1.2 ¥ 10-6.
EPR spectra of 1
The X-band EPR spectra of 1 at room temperature (Fig. 10) shows
an isotropic band centered at g = 2.11 (3311 G for n = 9.7920 GHz)
which corresponds to the transition DMS = 1.
2
2
with the same combination of the atomic dx
orbitals allowing
–y
more separation between the symmetric js and the antisymmetric
ja combinations, they add their effect, thus increasing the energy
gap D = E(js) - E(ja) and consequently increasing the JAF values
as was demonstrated by the theoretical calculations reported by
Escriva et al.30 in (m- hydroxo)(m-pyrazolato) dicopper(II) model.
In view of this, the coupling constant of J = -443.0 cm-1 calculated
for 4 is in accordance with expectation.
Conclusion
The mixed pyrazole-pyridine based ditopic ligands PyPzOAP and
PzOAP self-assemble in the presence of Cu(II) and Ni(II) salts to
form a series of [2 ¥ 2] molecular grids. The ligand PyPzOAP forms
a Cu4(II) grid 1 and a Ni4(II) grid 3 whereas, the ligand PzOAP
forms the Cu4(II) grid 2. One polytopic ligand 2-PzCAP having
a linear arrangement of a number of coordination pockets forms
solely a dicopper(II) complex 4. The ligand negates our expectation
for grid formation. This is probably due to the six-membered (not
five) chelate rings in the tridentate N3 coordinating pockets of
2-PzCAP ligand. In 1 and 3 all the metal centers have distorted
octahedral geometry, whereas in 2, all the four Cu(II) centers have
a square pyramidal geometry. In the dicopper(II) complex 4, both
the metal centers have square pyramidal geometry. The square
copper clusters (1 and 2) exhibit intramolecular ferromagnetic spin
exchange associated with orthogonal alkoxide bridging arrange-
ment and close proximity of the Cu(II) centers. The square Ni(II)
complex (3) exhibits intramolecular antiferromagnetic exchange
in which the four octahedral Ni(II) centers are bridged by alkoxide
groups with a Ni–O–Ni bridge angle 137.83 (13)◦. There is a
fundamental difference between the magnetic behaviours of Cu(II)
and Ni(II) grids of the same ligand which is in agreement with
previous reports.14
Fig. 10 EPR spectra of 1 at room temperature.
Magnetic interpretation
The values of the superexchange parameters J = +5.88 and +4.78
cm-1 for compounds 1 and 2 respectively, are in agreement with the
observed ones in the [2 ¥ 2] grid complexes previously reported.8b,d
These complexes are formed by similar four ligands giving a
self-assembled, ferromagnetically coupled tetranuclear CuII with
only one interaction J. The origin of the ferromagnetic coupling
can be associated to the quasi orthogonality between copper
atoms established recently by theoretical DFT calculations.8b,d The
pathways interaction, J, is among an apical and equatorial (a–e)
position of the adjacent coppers atoms. Generally, for this kind
of interaction one can expect a slight ferro- or anti-ferromagnetic
interaction depending on the relative disposition of the planes that
contain the magnetic orbitals.25
For the antiferromagnetic nickel compound 3 the pathways
interaction, J, is also among an apical and equatorial (a–e)
position of the adjacent nickel(II) atoms. In this case, two adjacent
Acknowledgements
Financial support [Sanc. No. SR/S1/IC-27/2008, dated
13.03.2009] from Department of Science & Technology (DST),
New Delhi, India, is gratefully acknowledged. T.N.M. thanks the
University Grants Commission, New Delhi, India, for financial
assistance. The Spanish (CTQ2009-07264/BQU) and Catalan
(2009 SGR-1454) financial support are acknowledged.
2
2
2
orbitals dx
and dz are located in the same plane causing
–y
2
2
2
the propagation of the magnetic exchange via the dz and dx
–y
orbitals which interact with the s orbitals of the oxygen atoms of
the bridging ligand resulting, thus, in a global antiferromagnetic
contribution.
For the compound 4, the significant antiferromagnetic coupling
observed is due to the large s in-plane overlap between the
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