Aromatic N-oxide bridged copper(II) coordination polymers: Synthesis, characterization and magnetic properties

Article abstract of DOI:10.1016/j.ica.2010.03.050

The complexes [Cu₂(o-NO₂-C₆H ₄COO)₄(PNO)₂] (1), [Cu₂(C ₆H₅COO)₄(2,2′-BPNO)]_n (2), [Cu₂(C₆H₅COO)₄(4,4′-BPNO)] _n (3), [Cu(p-OH-C₆H₄COO)₂(4, 4′-BPNO)₂·H₂O]_n (4), (where PNO = pyridine N-oxide, 2,2′-BPNO = 2,2′-bipyridyl-N,N′-dioxide, 4,4′-BPNO = 4,4′-bipyridyl-N,N′-dioxide) are prepared and characterized and their magnetic properties are studied as a function of temperature. Complex 1 is a discrete dinuclear complex while complexes 2-4 are polymeric of which 2 and 3 have paddle wheel repeating units. Magnetic susceptibility measurements from polycrystalline samples of 1-4 revealed strong antiferromagnetic interactions within the {Cu₂}⁴⁺ paddle wheel units and no discernible interactions between the units. The complex 5, [Cu(NicoNO)₂·2H₂O]_n·4nH ₂O, in which the bridging ligand to the adjacent copper(II) ions is nicotinate N-oxide (NicoNO) the transmitted interaction is very weakly antiferromagnetic.

Full text of DOI:10.1016/j.ica.2010.03.050

Inorganica Chimica Acta 363 (2010) 2279–2286
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Aromatic N-oxide bridged copper(II) coordination polymers: Synthesis,
characterization and magnetic properties
Rupam Sarma a, Athanassios K. Boudalis b, Jubaraj B. Baruah a,
*
a Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
b Institute of Materials Science, NCSR ‘‘Demokritos”, 15310 Aghia Paraskevi Attikis, Greece
a r t i c l e i n f o
a b s t r a c t
The complexes [Cu2(o-NO2–C6H4COO)4(PNO)2] (1), [Cu2(C6H5COO)4(2,20-BPNO)]n (2), [Cu2(C6H5COO)4-
(4,40-BPNO)]n (3), [Cu(p-OH–C6H4COO)2(4,40-BPNO)2ꢀH2O]n (4), (where PNO = pyridine N-oxide, 2,20-
BPNO = 2,20-bipyridyl-N,N0-dioxide, 4,40-BPNO = 4,40-bipyridyl-N,N0-dioxide) are prepared and character-
ized and their magnetic properties are studied as a function of temperature. Complex 1 is a discrete dinu-
clear complex while complexes 2–4 are polymeric of which 2 and 3 have paddle wheel repeating units.
Magnetic susceptibility measurements from polycrystalline samples of 1–4 revealed strong antiferro-
magnetic interactions within the {Cu2}4+ paddle wheel units and no discernible interactions between
the units. The complex 5, [Cu(NicoNO)2ꢀ2H2O]nꢀ4nH2O, in which the bridging ligand to the adjacent cop-
per(II) ions is nicotinate N-oxide (NicoNO) the transmitted interaction is very weakly antiferromagnetic.
Ó 2010 Elsevier B.V. All rights reserved.
Article history:
Received 30 January 2010
Received in revised form 16 March 2010
Accepted 19 March 2010
Available online 28 March 2010
Keywords:
Coordination polymer
Copper(II) complexes
2,20-Bipyridyl-N,N0-dioxide
4,40-Bipyridyl-N,N0-dioxide
Crystal structure
Magnetochemistry
1. Introduction
copper(II) N-oxide complexes for two reasons: (i) copper(II) N-
oxide complexes have been shown to exhibit biological activity
Interest in heterocyclic aromatic N-oxides has flourished be-
cause of their practical impact on biological activity [1]. A number
of N-oxide derivatives are known to show herbicidal activity [2],
inhibitory activity [3] such as against feline coronavirus and hu-
man SARS-CoV, as well as antibiotic properties, as for example 2-
n-heptyl-4-hydroxyquinoline N-oxide [4,5]. This latter N-oxide is
also known for mediating proton transfer across the mitochondrial
membrane as well as for inhibiting electron transport across cell
membranes in complex form [4,6]. Moreover, researchers have re-
ported several aromatic N-oxide derivatives capable of forming
biologically active metal complexes [7,8]. Apart from these, coordi-
nation complexes of aromatic N-oxides are known to have exten-
sive applications as magnetic materials [7,9–12], in catalysis [13–
19] as well as in supramolecular systems [20–31].
The N-oxides can act as monodentate or bridging ligands
(Scheme 1) and we have shown that they can lead to polymeric
complexes even with simple mono carboxylic acids [32–36]. It
has been found that in the case of pyridine N-oxide bridged coor-
dination polymers the metal–metal separations lie in the 3.6–
3.7 Å range and they exhibit antiferromagnetic exchange [36–41].
In this present study we have decided to undertake the study of
[7,8] and (ii) to study the antiferromagnetic properties in paddle
wheel containing copper complexes.
In general copper(II) carboxylates prefers paddle wheel geome-
try and in such complexes axial positions are occupied by solvents
or by ancillary ligands. Thus, by use of bidentate spacer ligands to
connect these paddle wheel units the distance of separation be-
tween the paddle wheel cores may be controlled. As shown in
Scheme 2 such distance will be different for different ligands used
(d1 > d2), which in turn may have significant effects on their mag-
netic exchange properties. However, in the case of 2,20-bipyridyl-
N,N0-dioxide there is other possibilities such as chelation, which
need to be overcome so that polymeric structures are preferred.
With the intention of studying magnetostructural correlations
with some copper(II) complexes of such polymeric complexes we
have synthesized a number of coordination complexes/polymers
of copper(II) aromatic N-oxides having carboxylate as the anionic
ligand and studied their magnetic properties as a function of tem-
perature. We have studied complexes of N-oxide ligands such as
pyridine N-oxide, 2,20-bipyridyl-N,N0-dioxide, 4,40-bipyridyl-N,N0-
dioxide and nicotinic acid N-oxide to compare the structural as
well as magnetic features of the different complexes formed. All
the complexes reported here are characterized by conventional
spectroscopic techniques as well as with single-crystal X-ray
diffraction.
* Corresponding author.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.03.050

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R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
Scheme 1. Some common binding mode of N-oxides.
Scheme 2. The separation of paddle wheel units by N-oxide connectors.
2.3. Synthesis of 4
2. Experimental
To a solution of p-hydroxybenzoic acid (1 mmol, 0.138 g) in
methanol (15 mL) copper(II) acetate monohydrate (0.5 mmol,
0.100 g) was added and stirred for 10 min. To this reaction mixture
4,40-bipyridyl-N,N0-dioxide (0.5 mmol, 0.095 g) was added with
constant stirring at room temperature. A small amount (ꢁ5 mL)
of dimethylformamide was then added to dissolve the precipitate
that appeared after addition of 4,40-bipyridyl-N,N0-dioxide. Diffrac-
tion quality crystals were collected after 12 days. Yield of the pure
crystalline complex was found to be ꢁ40%. IR (KBr, cmꢂ1): 3116
(bm), 1614 (s), 1598 (s), 1574 (s), 1475 (m), 1372 (s), 1225 (s),
1029 (m), 838 (m), 785 (m). UV–Vis kmax (methanol): 703 nm;
The complexes [Cu2(o-NO2–C6H4COO)4(PNO)2] (1), [Cu2(C6H5-
COO)4(2,20-BPNO)]n (2), [Cu2(C6H5COO)4(4,40-BPNO)]n (3), [Cu(p-
OH–C6H4COO)2(4,40-BPNO)2ꢀH2O]n (4), [Cu(NicoNO)2ꢀ2H2O]n ꢀ
4nH2O (5) were prepared through solution phase synthetic
route (where PNO = pyridine N-oxide; 2,20-BPNO = 2,20-bipyri-
dyl-N,N0-dioxide; 4,40-BPNO = 4,40-bipyridyl-N,N0-dioxide; NicoN-
O = nicotinate N-oxide) .
2.1. Synthesis of 1, 2
To a solution of o-nitrobenzoic acid (1 mmol, 0.167 g) in meth-
anol (15 mL) copper(II) actetate monohydrate (0.5 mmol, 0.100 g)
was added and stirred for 10 min. To this reaction mixture pyridine
N-oxide (1 mmol, 0.095 g) was added with constant stirring at
room temperature. A small amount (ꢁ5 mL) of toluene was then
added to dissolve the precipitate that appeared after addition of
pyridine N-oxide. Diffraction quality crystals were collected after
7 days and dried in air. Yield of the pure crystalline complex was
found to be >70%. IR (KBr, cmꢂ1): 3421 (bw), 3084 (w), 1643 (s),
1614 (s), 1575 (m), 1529 (s), 1467 (m), 1410 (s), 1369 (m), 1351
(m), 1227 (m), 839 (m), 702 (m). UV–Vis kmax (methanol):
e
= 117 Mꢂ1 cmꢂ1
.
2.4. Synthesis of 5
To a solution of nicotinic acid N-oxide (1 mmol, 0.139 g) in
methanol (15 mL) methanolic solution of copper(II) acetate mono-
hydrate (0.5 mmol, 0.100 g) (10 mL) was added. After stirring this
reaction mixture for about 30 min precipitation appeared. The pre-
cipitate was dissolved by adding 5–10 mL of water to it. The reac-
tion mixture was then allowed to stir for another 30 min and then
kept for crystallization. Crystals of 5 were obtained after about
7 days. Yield of the crystalline complexes were found to be >80%.
IR (KBr, cmꢂ1): 3403 (bm), 3081 (m), 1657 (m), 1591 (s), 1557
(s), 1477 (w), 1387 (s), 1219 (m), 731 (m). UV–Vis kmax (methanol):
733 nm;
e .
= 108 Mꢂ1 cmꢂ1
Complex 2 was prepared with a similar procedure and crystal-
line pure products with 55% yield were obtained. IR (KBr, cmꢂ1):
3445 (bw), 3085 (w), 1618 (s), 1597 (m), 1572 (s), 1410 (s), 1228
e .
= 249 Mꢂ1 cmꢂ1
(s), 719 (m). UV–Vis kmax (methanol): 715 nm;
e .
= 99 Mꢂ1 cmꢂ1
705 nm;
2.2. Synthesis of 3
2.5. X-ray crystallography
Single-crystals of 3 were grown by layering a solution of 4,40-
bipyridyl-N,N0-dioxide (0.25 mmol, 0.045 g) in water (3 mL) over
a reaction mixture containing copper(II) acetate monohydrate
(0.25 mmol, 0.050 g) and benzoic acid (0.5 mmol, 0.061 g) in tolu-
ene: methanol (3:1, 10 mL). IR (KBr, cmꢂ1): 3421 (bw), 3110 (m),
1633 (s), 1614 (s), 1572 (m), 1462 (m), 1403 (s), 1230 (s), 1175
(w), 840 (m), 716 (m). UV–Vis kmax (methanol): 711 nm;
The X-ray crystallographic data were collected at 296 K with Mo
radiation (k = 0.71073 Å) using a Bruker Nonius SMART CCD dif-
Ka
fractometer equipped with graphite monochromator. The SMART
software was used for data collection and also for indexing the
reflections and determining the unit cell parameters; the collected
data were integrated using SAINT software. The structures were
solved by direct methods and refined by full-matrix least-squares
calculations using SHELXTL software. All the non-H-atoms were
e
= 246 Mꢂ1 cmꢂ1
.

R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
2281
Table 1
Crystallographic parameters of the complexes 1, 2 and 4.
Compound
number
1
2
4
Formulae
Formula weight
Crystal system
Space group
a (Å)
C38H26Cu2N6O18 C19H14CuNO5
C24H20CuN2O9
543.96
monoclinic
Cc
18.9930(7)
15.8109(7)
7.7635(3)
90.00
981.73
399.85
monoclinic
P21/c
monoclinic
C2/c
11.3721(4)
10.1756(3)
19.7779(6)
90.00
20.6947(2)
10.10310(10)
19.1416(2)
90.00
b (Å)
c (Å)
a
(°)
b (°)
117.384(2)
90.00
2032.20(11)
2
118.17(10)
90.00
755.32(13)
8
107.791(4)
90.00
2219.86(15)
4
c
(°)
V (Å3)
Z
Density (Mg mꢂ3
)
1.604
1.506
1.628
Absolute coefficient
(mmꢂ1
F(0 0 0)
1.133
1.267
1.045
)
996
1632
1116
Total number of
reflections
21419
18542
12347
Reflections [I > 2
Maximum h (°)
Ranges (h, k, l)
r
(I)]
3672
25.50
ꢂ13 6 h 6 13,
ꢂ12 6 k 6 10,
ꢂ24 6 l 6 24
3105
25.00
ꢂ23 6 h 6 24,
ꢂ11 6 k 6 12,
ꢂ22 6 l 6 22
99.9
5104
28.93
ꢂ23 6 h 6 25,
ꢂ21 6 k 6 21,
ꢂ10 6 l 6 10
99.1
Fig. 1. Dinuclear paddle wheel unit of 1.
Completeness to 2h (%) 97.0
Data/restraints/
parameters
3672/0/289
3105/0/235
5104/0/332
Goodness-of-fit (GOF)
1.081
1.052
1.082
1467 cmꢂ1 due to the carboxylate stretching, the N-oxo stretching
appears at 1227 cmꢂ1. The single-crystal X-ray analysis reveals
that 1 crystallizes in the monoclinic P21/c space group. The asym-
metric unit of 1 consists of two o-nitrobenzoate ligands and one
pyridine N-oxide ligand coordinated to a copper(II) centre. In the
dinuclear molecule each of the copper centres (Cu1) adopts a
square-pyramidal geometry with the four oxygen atoms O1, O2,
O5 and O9 from four different carboxylate ligands residing at the
corners of the basal plane. The apical position is occupied by the
N-oxo, O8, of the pyridine N-oxide ligand. The basal atoms are al-
most coplanar and the Cu1 is positioned 0.224 Å above the basal
plane. The bond lengths for the basal bonds (Cu–O1, Cu–O2, Cu–
O5, Cu–O9) lie in the range 1.963–1.993 Å, while that of the axial
bond (Cu1–O8) is 2.116 Å and the Cu–Cu separation is 2.686 Å
which is normal for other copper complexes reported.
(F2)
R indices [I > 2
r(I)]
R1 = 0.0349,
wR2 = 0.0861
R1 = 0.0459,
wR2 = 0.0919
R1 = 0.0223,
wR2 = 0.0614
R1 = 0.0258,
wR2 = 0.0636
R1 = 0.0451,
wR2 = 0.1113
R1 = 0.0558,
wR2 = 0.1179
R indices (all data)
refined in the anisotropic approximation against F2 of all reflec-
tions. The H-atoms, except those attached to O were placed at their
calculated positions and refined in the isotropic approximation;
those attached to hetero-atoms (O) were located in the difference
Fourier maps, and refined with isotropic displacement coefficients.
The crystallographic parameters of three complexes are given in
Table 1. The crystallographic parameters for the complex 3 are
not that satisfactory; also the crystal structure of 5 is already re-
ported and hence not included in the Table 1.
With the intention of linking together the dinuclear paddle
wheel units as formed in case of 1 we have reacted copper(II)
0
acetate, benzoic acid with the spacer0 ligand 2,20-bipyridyl-N,N0-
3. Magnetic measurements
dioxide. This ligand being a bidentate ligand resulted in the
formation of the 1D coordination polymer 2 with the composition
[Cu2(C6H5COO)4(2,20-BPNO)]n. The 2,20-bipyridyl-N,N0-dioxide
ligand can act either as a chelating or as bridging ligand. This is
possible because of the free rotation around the C–C single bond
between the two rings of 2,20-bipyridyl-N,N0-dioxide. In other
words it can be said that 2,20-bipyridyl-N,N0-dioxide can coordinate
to metal centres either in a cis or a trans configuration as shown in
Scheme 3.
There are a few reports available on the chelating coordination
mode of 2,20-bipyridyl-N,N0-dioxide [43–45]. However, in case of
the coordination polymer 2, the 2,20-bipyridyl-N,N0-dioxide coordi-
nates in the trans-fashion to bridge two paddle wheel units of cop-
per(II) benzoate. The crystal structure of 2 is shown in Fig. 2. It
crystallizes in the monoclinic space group C2/c and each asymmet-
ric unit of 2 consists of one copper(II) atom, two benzoate ligands
and half a 2,20-bipyridyl-N,N0-dioxide molecule. Like in 1, here also
each of the copper(II) centres adopts a square-pyramidal geometry,
with the N-oxo of 2,20-bipyridyl-N,N0-dioxide apically coordinated.
Four of the carboxylate oxygens O1, O2, O3 and O4 occupy the cor-
ners of the basal plane, with bond lengths spanning the 1.957–
1.983 Å range. The apical oxygen O5 is residing at a distance
Variable temperature magnetic susceptibility measurements
were carried out on polycrystalline samples of 1–3 (5–300 K) and
5 (2–300 K) using a Quantum Design MPMS SQUID susceptometer
under magnetic fields of 0.5 (1–3) and 0.1 (5) T. Diamagnetic
corrections for the complexes were estimated from Pascal0s con-
stants. The magnetic susceptibility of 1–3 has been computed by
exact calculation of the energy levels associated with the spin
Hamiltonian through diagonalization of the full-matrix with a
general program for axial symmetry [42]. Least-squares fits were
accomplished with an adapted version of the function-minimiza-
tion program MINUIT [43]. The error-factor
R is defined as
2
P
v
ð
expꢂvcalc
Þ
R ¼
, where N is the number of experimental points.
N
v2exp
4. Results and discussion
Copper(II) acetate reacts with o-nitrobenzoic acid and pyridine
N-oxide resulting in the formation of the complex 1, [Cu2(o-NO2–
C6H4COO)4(PNO)2]. It has a dinuclear paddle wheel structure with
two terminal pyridine N-oxide coordination (Fig. 1). The complex
exhibits characteristic IR stretching frequencies at 1614 and

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R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
Scheme 3. Two conformations of 2,20-bipyridyl-N,N0-dioxide towards coordination.
Fig. 2. 1D chain of the coordination polymer 2.
Fig. 3. 1D chain of the coordination polymer 3.
2.171 Å from the copper(II) centre and the copper(II) centre is at a
distance of 0.202 Å from the basal plane. The Cu–Cu distance in the
paddle wheel unit is 2.639 Å, while the smallest CuꢀꢀꢀCu distance
between two paddle wheel units is 8.035 Å. Moreover the two
planes of the two aromatic rings of 2,20-bipyridyl-N,N0-dioxide
form an angle of 67.12° which differs significantly from those re-
ported for the cis-coordination geometries.
A similar reaction of copper(II) acetate monohydrate and
benzoic acid with 4,40-bipyridyl-N,N0-dioxide led to an one-dimen-
sional coordination polymer 3 of composition [Cu2(C6H5COO)4-
(4,40-BPNO)]n with the paddle wheel building blocks of copper(II)
benzoate bridged through the spacer N-oxide ligand. We have
ꢀ
evidence that the complex belongs to the triclinic space group P1
and the structure of the coordination polymer is as shown in

R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
2283
Table 2
Selected bond lengths (Å) and bond angles (°) of the complexes 1–4.
Bond distance (Å)
Bond angle (°)
Bond distance (Å)
Bond angle (°)
For 1
For 2
Cu1 O1
Cu1 O2
Cu1 O5
Cu1 O8
Cu1 O9
1.9926(16)
1.9749(17)
1.9625(18)
2.1158(19)
1.9646(18)
O5 Cu1 O9
O5 Cu1 O2
O5 Cu1 O1
O9 Cu1 O1
O2 Cu1 O1
O5 Cu1 O8
O9 Cu1 O8
O2 Cu1 O8
O1 Cu1 O8
166.78(8)
88.50(8)
90.70(7)
89.15(7)
167.12(7)
92.32(8)
100.85(8)
97.22(8)
95.66(8)
Cu1 O1
Cu1 O2
Cu1 O3
Cu1 O4
Cu1 O5
1.9837(12)
1.9568(12)
1.9567(12)
1.9745(12)
2.1715(11)
O3 Cu1 O2
O3 Cu1 O4
O2 Cu1 O4
O3 Cu1 O1
O2 Cu1 O1
O4 Cu1 O1
O3 Cu1 O5
O2 Cu1 O5
O4 Cu1 O5
O1 Cu1 O5
87.82(6)
168.02(5)
89.33(5)
90.65(6)
168.26(5)
89.77(5)
95.27(5)
104.72(5)
96.71(5)
87.01(4)
For 3
For 4
Cu1 O1
Cu1 O2
Cu1 O3
Cu1 O4
Cu1 O5
Cu2 O6
Cu2 O7
Cu2 O8
Cu2 O9
Cu2 O10
1.942(13)
1.971(12)
1.942(12)
2.137(12)
1.962(11)
1.950(11)
1.970(13)
2.119(11)
1.935(12)
1.956(12)
O3 Cu1 O5
O1 Cu1 O5
O3 Cu1 O2
O3 Cu1 O4
O1 Cu1 O4
O5 Cu1 O4
O2 Cu1 O4
O9 Cu2 O10
O6 Cu2 O10
O9 Cu2 O7
O6 Cu2 O7
O9 Cu2 O8
O6 Cu2 O8
O10 Cu2 O8
O7 Cu2 O8
90.6(5)
88.0(5)
90.6(5)
94.6(5)
96.7(5)
100.3(5)
91.5(5)
88.8(5)
90.8(5)
91.1(6)
86.7(6)
98.0(5)
93.1(5)
100.4(5)
92.5(5)
Cu1 O1
Cu1 O3
Cu1 O6
Cu1 O5
Cu1 O7
1.908(3)
1.910(4)
1.966(3)
2.046(3)
2.527
O1 Cu1 O3
O1 Cu1 O6
O3 Cu1 O6
O1 Cu1 O5
O3 Cu1 O5
O6 Cu1 O5
O3 Cu1 O7
O1 Cu1 O7
O5Cu1 O7
O6Cu1 O7
175.96(11)
92.20(13)
90.38(13)
87.28(13)
90.97(14)
165.96(15)
88.74
87.94
99.10
94.91
Fig. 4. (a) Coordination environment around Cu in 4, (b) self assembly of 4 showing various short range interactions.
Fig. 3. As in 2, in the coordination polymer 3 also the copper(II)
adopts the expected tetragonal pyramidal geometry. However,
the difference comes in the orientation of the 4,40-bipyridyl-N,N0-
dioxide ligand which gets distorted may be due to the geometry
forced by the copper(II) ions. The two aromatic rings of the ligand
are inclined at an angle 23.96° to each other, departing from the
reported ones where they are mostly coplanar [10]. The separation
between two nearest paddlewheel units is 12.57 Å. Some of the
important bond lengths and angles are tabulated in Table 2.
The reaction between copper(II) acetate monohydrate,
p-hydroxybenzoic acid and 4,40-bipyridyl-N,N0-dioxide leads to
the formation of another 1D coordination polymer 4 with compo-

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R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
Fig. 5. (a) Hexameric aqua-nets formed by the aqua-groups in 5. (b) The one-dimensional chains of coordination polymer separated by the aqua-nets.
sition [Cu(p-OH–C6H4COO)2(4,40-BPNO)2ꢀH2O]n. It differs signifi-
cantly from the earlier two polymers in that unlike 2 and 3, 4 does
not possess paddle wheel units, rather it forms mononuclear cop-
per(II) units bridged through the spacer ligand. The structure of 4 is
shown in Fig. 4 which shows a square-pyramidal geometry around
the copper centre. Two of the coordination come from two benzo-
ate ligands, two from the 4,40-bipyridyl-N,N0-dioxide and the rest
from the aquo-ligand. The oxygen O7 from 4,40-bipyridyl-N,N0-
dioxide occupies the apical position with a Cu–O bond distance
of 2.527 Å. Here, the distance between two nearest copper(II)
centres bridged by 4,40-bipyridyl-N,N0-dioxide is 12.007 Å. The
one-dimensional chains interact through the hydrogen bonds
O6–H6AꢀꢀꢀO2 and O6–H6BꢀꢀꢀO4 between aquo-groups and the car-
boxylate oxygen and O8–H8ꢀꢀꢀO7, between the hydroxy group on
the aromatic carboxylate ring and one of the N-oxo groups. The
hydrogen bonded self assembly is shown in the Fig. 4b along with
the coordination environment around Cu1 in Fig. 4a.
ing net-like structures of hexameric aqua-units as shown in Fig. 5a.
These aqua-nets surround the one-dimensional chain of the metal
complex separating them from each other as shown in Fig. 5b. This
makes a layered structure comprising of polymeric chains of the
metal complex embedding a layered sheets of infinite chains like
structures of water.
At room temperature the complexes 1–3 are ESR silent; this
could be due to antiferromagnetic coupling between the two cop-
per(II) centres within the paddle wheel units. The ESR spectrum of
4 shows a well resolved four line spectra typical of copper(II)
monomeric units expected for the coupling of the electron with
the nuclear spin. On the other hand the ESR spectrum of complex
5 is axial in character (please refer to Supplementary material).
5. Magnetic properties
A completely different product is found to be formed in the
reaction between copper(II) acetate and nicotinic acid N-oxide
where six coordination rather than five coordination of copper(II)
is observed. The reaction yields the one-dimensional coordination
polymer 5 with the composition [Cu(NicoNO)2ꢀ2H2O]nꢀ4nH2O crys-
tallizing in the monoclinic space group P21/c. The crystal structure
of 5 has already been reported [47] with space group P21 and then
correctly reinterpreted by Clemente [48] and is not discussed here.
However the magnetic measurement for 5 is carried out and dis-
cussed in the next section. It should be noted that the structure
contains as many as four water molecules per copper core. These
water molecules are strongly hydrogen bonded to each other form-
Magnetic susceptibility data for complexes 1–3 are shown in
Fig. 6. For all three complexes, the
vMT product at 300 K is 0.44–
0.45 cm3 molꢂ1 K, significantly lower than the values predicted
for two non-interacting S = 1/2 spins (0.82 cm3 molꢂ1 K, g = 2.1),
suggesting the interplay of antiferromagnetic interactions. This
conclusion is corroborated by the decrease of the
vMT products
of all three complexes upon cooling. These are stabilized below
ꢃ50 K, forming plateaus of different values. This indicates the pres-
ence of paramagnetic impurities, of different fractions for each
complex.
For the analysis of the magnetic data a simple isotropic ex-
change model was employed, according to the Hamiltonian:

R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
2285
Fig. 6. vM vs. T and
vMT vs. T experimental data for complexes 1–3 and calculated curves according to the model described in the text.
Magnetic susceptibility data for complex 5 are shown in Fig. 7.
The
MT product for 5 is 0.42 cm3 molꢂ1 K, the value corresponding
v
to an isolated S = 1/2 spin (g = 2.12). This remains constant upon
cooling down to ꢃ50 K, and then decreases down to a value of
0.36 cm3 molꢂ1 K, indicating weak antiferromagnetic interactions.
Given the 1D structure of 5, the magnetic properties were ana-
lyzed according to the Bonner–Fisher model [49] for an equally
spaced S = 1/2 Heisenberg chain. The Hamiltonian considered was:
X
^
^ ^
SiSiþ1
H ¼ ꢂ2J
i
For the fitting of the data the empirical equation derived by Hall
was employed [50]:
"
#
Ng2b2
kT
A þ Bx þ Cx2
v
¼
1 þ Dx þ Ex2 þ Fx3
with A = 0.25, B = 0.14995, C = 0.30094, D = 1.9862, E = 0.68854,
F = 6.0626 and x = |J|/kT for the above mentioned Hamiltonian
formalism.
Fig. 7.
vMT vs. T experimental data for complex 5 and calculated curve according to
the model described in the text.
Best-fit parameters according to this model are J = ꢂ0.71 cmꢂ1
,
g = 2.12 with R = 8.2 ꢄ 10ꢂ6
.
^ ^
H ¼ ꢂ2JSiSj
ð1Þ
The very weak interaction can be rationalized by (i) the rela-
tively long superexchange pathway provided by the nicotinic N-
oxide ligand and (ii) the fact that magnetic exchange is transmitted
between the O(3) atom of the ligand, coordinated to an axial coor-
dination position [Cu(1)ꢀꢀꢀO(3), 2.530 Å] and the O(1) atom of the
ligand coordinated to an equatorial coordination position
[Cu(1)ꢀꢀꢀO(1), 1.955 Å]. Since the axial coordination positions occu-
py non-magnetic orbitals (of dz2 character), magnetic exchange
transmitted through them is significantly reduced.
In this model we also took into account a fraction
magnetic impurity following the Curie law.
q
of S = 1/2 para-
Fits to these data using this model were of good quality, yield-
ing best-fit parameters J = ꢂ159 cmꢂ1, g = 2.07,
q
= 2.0%, R = 2.0 ꢄ
10ꢂ4 (complex 1), J = ꢂ168 cmꢂ1, g = 2.13,
(complex 2) and J = ꢂ157 cmꢂ1, g = 2.04,
q
q
= 1.2%, R = 1.2 ꢄ 10ꢂ3
= 4.9%, R = 2.2 ꢄ 10ꢂ4
(complex 3). These values fall in the range of values previously
determined for paddlewheel dicopper(II) carboxylate complexes
[44–46].
The quality of the fits does not allow us the incorporation of
additional parameters, namely a mean-field correction for the
determination of eventual interactions between dinuclear units
in 2 and 3, without overparametrizing the problem. Therefore,
we conclude that such interactions, if indeed operative, must be
very weak. This is in agreement with the fact that such interactions
would have to be transmitted through a superexchange pathway
involving the apical coordination position of the square-pyramidal
copper(II) ions; since this position occupies a non-magnetic orbital
(of dz2 character), this interaction is expected to be very weak.
Moreover, the size and shape of the bridging ligands is probably
not favourable for the transmission of significant magnetic
exchange.
6. Thermogravimetric analysis
Thermal stability of the complexes 1–4 is studied. The thermo-
grams show that all the complex/coordination polymers are ther-
mally stable up to ꢃ200 °C (please refer to Supplementary
materials). Thermogram of the complex 1 reveals weight loss in
the range 225–275 °C which corresponds to 67.2% (calc. 67.6%) of
the total weight and is accounted for loss of four o-nitrobenzoic
acid. Then the weight loss due to the two pyridine N-oxide mole-
cules takes place. For the coordination polymer 2, weight loss oc-
curs in two steps: the first step within the range 195–250 °C
corresponds to weight loss of 59.5% (calc. 58.3%) due to loss of
the two benzoic acid molecules and the second step, 350–525 °C,

2286
R. Sarma et al. / Inorganica Chimica Acta 363 (2010) 2279–2286
is due to the loss of half a 2,20-bipyridyl-N,N0-dioxide molecule
(experimental 57.4% from the first step; calc. 59.1%). The coordina-
tion polymer 3 also shows a similar thermal behaviour as 2 and the
coordination polymer 4 shows a continuous thermal degradation
in the range 200–525 °C.
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In conclusion we have synthesized and characterized copper(II)
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binding mode of 2,20-bipyridyl-N,N0-dioxide to form copper(II)
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quently come across. Depending on the aromatic N-oxide ligands
different architectures of the coordination polymers with repeated
mononuclear or dinuclear copper(II) units are observed. The pad-
dle wheel structures of copper(II) separated by different N-oxide
spacers controls their structural features. The nicotinic acid N-
oxide leads to dinuclear repeated units of copper(II) to form the
1D chains with relatively larger copper(II) inter-metal separations.
Interactions within the paddlewheel moieties of 1–3 were found to
be strongly antiferromagnetic, with values typical of copper(II) car-
boxylates. However, inter-dinuclear interactions between these
moieties were too weak to be detected, or non-existent. In the case
of 5, in which the coordination of the bridging ligand to the adja-
cent copper(II) ions is both axial and equatorial, the transmitted
interaction is very weakly antiferromagnetic.
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Acknowledgements
The authors thank Department of Science and Technology, New
Delhi, India for financial support and the author R.S. thanks Council
of Scientific and Industrial Research, New Delhi, India for Junior Re-
search Fellowship.
Appendix A. Supplementary material
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CCDC 664104, 672157, 759711, and 759712 contain the supple-
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tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2010.03.050.
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