Magnetic coupling in EE and EO azido-bridged binuclear copper complexes: Synthesis, structure and magnetic studies

Article abstract of DOI:10.1016/j.poly.2009.12.023

Four azide bridged dinuclear copper(II) complexes, [Cu₂(L^X)₂(N₃)₂](ClO₄)₂, with L^X = substituted N,N-bis[(3,5-dimethylpyrazole-1-yl)-methyl]benzylamine, [X = H (1), OM

Full text of DOI:10.1016/j.poly.2009.12.023

Polyhedron 29 (2010) 1201–1208
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Polyhedron
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Magnetic coupling in EE and EO azido-bridged binuclear copper
complexes: Synthesis, structure and magnetic studies
Ishita Banerjee a, Jaromir Marek b, Radovan Herchel c, Mahammad Ali a,
*
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Laboratory of Functional Genomics and Proteomics, Institute of Experimental Biology, Faculty of Science, Masaryk University, Kamenice 5/A2, CZ-625 00 Brno, Czech Republic
c Department of Inorganic Chemistry, Faculty of Science, Palacky University, Tr. 17, Listopadu 12, CZ-771 46 Olomouc, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Four azide bridged dinuclear copper(II) complexes, [Cu2(LX)2(N3)2](ClO4)2, with LX = substituted N,N-
bis[(3,5-dimethylpyrazole-1-yl)-methyl]benzylamine, [X = H (1), OMe (2), Me (3) and Cl (4)] have been
synthesized, out of which complexes 1 and 2 have been characterized structurally. In Complex 1 the
two bridging azide ligands have connected the two metal centers in an end-on (EO) fashion with aSP
(asymmetric Square Pyramidal) geometry and showed an weak antiferromagnetic interaction
(J = ꢀ3.34 cmꢀ1). On the contrary, in complex 2, the two metal centers have been connected in end-to-
end (EE) fashion exhibiting moderately strong ferromagnetic interaction (J = +19.7 cmꢀ1). Cyclic voltam-
metric studies performed on all the four complexes show a reasonably good correlations when E1/2 for
Article history:
Received 4 August 2009
Accepted 15 December 2009
Available online 14 January 2010
Keywords:
EE and EO bridging
Synthesis
Crystal structures
Ferro and antiferromagnetic coupling
CuIICuII ? CuIICuIII and CuIICuIII ? CuIIICuIII oxidations are plotted against
r
(substituent constants) with
q
= ꢀ0.182 (R2 = 0.92) and ꢀ0.684 (R2 = 0.99) respectively.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
mode of cooperative magnetic interactions between the metal
centers. From the magnetic point of view, an unusual range of
magnetic behavior can be obtained as a function of coordination
mode of the azide bridging group. In case of di-bridged com-
plexes with one or more symmetric EO azido bridging, the inter-
action between the metal ions is strongly antiferromagnetic
[32,11], but when it bridges asymmetrically, the interaction is
weak to moderately strong ferromagnetic [33]. In contrast, with
symmetric EE bridging the interaction is dominated to be ferro-
magnetic, while for asymmetric EE bridge with short and long
Cu–Nazide bonds, the interaction is either negligible or very
weakly antiferromagnetic depending on the geometry which
tends to be trigonal bipyramidal (TBP) or Square Pyramidal
(SP), respectively [31]. The azido-bridged binuclear complexes
provide a stringent test for any theoretical analysis of the ex-
change coupling [34].
Copper(II) complexes of multidentate heterocyclic amine li-
gands coordinated to metal centers have found extensive use to
generate biological models that mimic proteins such as hemocya-
nin and tyrosinase [35], a Type-3 copper proteins with strong anti-
ferromagnetically coupled copper(II) centers and therefore EPR
silent in the oxidized state [36]. Although there are few reports
on the copper complexes of pyrazole-based ligands, which are
mostly mononuclear in nature [37–47], no report on the azido-
bridged copper(II) complexes of such ligands are available in the
literature. Here, we report four azide bridged dinuclear copper(II)
complexes of the ligands LX (X = Cl, H, Me and OMe), two of them
have been characterized structurally and magnetically.
The development of the chemistry of binuclear metal com-
plexes has been stimulated by a desire to synthesize model sys-
tems that may mimic the active sites of metallo-biomolecules
[1–6], bind and activate small molecules [7–10] and be used to
investigate the mutual influence of two metal centers in terms
of cooperative effect on the electronic, magnetic and redox prop-
erties. One of the most appealing properties of binuclear transi-
tion metal complexes is the possible appearance of exchange
interactions between the metal centers. A synthetic strategy that
is generally applied to build such molecular architectures is to
use pseudohalides as ligands [11–14], because they provide ver-
satile coordination behavior and generates dimeric and multidi-
mensional polymeric magnetic materials that facilitate ferro-
and antiferromagnetic interactions among the metal centers
[15–30]. Depending upon the steric and electronic demand of
coligands, azide can serve as end-on (
l-1,1 or EO) or an end-
to-end ( -1,3 or EE) bridging ligand Chart 1. Such azido-bridged
l
copper(II) complexes are of great interest for biologists and bio-
inorganic chemists to investigate the structural and functional
role of active sites in copper proteins [11,31], and also for phys-
ical inorganic chemists seeking to design new magnetic materi-
als. Chart 1 displays different modes of bridging of azide ligand
along with the geometry around the metal center, and also the
* Corresponding author. Tel.: +91 9433249716 (m); fax: +91 33 2414 6223.
E-mail addresses: m_ali2062@yahoo.com, mali@chemistry.jdvu.ac.in (M. Ali).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.12.023

1202
I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
N
N
N
N
N
N
N
N
N
Cu
N
N
N
Cu
Cu
Cu
Cu
Cu
Cu
Cu
N
N
N
N
N
N
N
N
N
N
N
N
(a)
(b)
(c)
(d)
(a) : S (F), (b): S (AF), (c) AS (F), (d) AS (weak AF)
S = symmetric, AS = antisymmetric, F= Ferromagnetic and AF = antiferromagnetic
Chart 1. Different modes of bridging of azide ion to metal center and the nature of magnetic interactions.
in a stoppered vessel at room temperature for 24 h. The water pro-
duced in the reaction was removed by drying over anhydrous
Na2SO4. It was filtered and evaporated under reduced pressure.
Oily substance thereby appeared was dissolved in diethylether
and kept in refrigerator whereupon white crystalline product ap-
peared was filtered and air dried in air.
N
X
N
N
N
N
LCl: Yield 55%. Calculated for Molecular formula: C19H24N5Cl
(357.5): C, 63.8; H, 6.71; N, 19.6; Found: C, 64.04; H, 6.49; N,
19.7%.
Cl
H
OMe
Me
X = H(L ), OMe(L ), Me(L ) and Cl(L )
LH: Yield 60%. Calculated for Molecular formula: C19H25N5
(323.0): C, 70.5; H, 7.73; N, 21.67; Found: C, 70.62; H 7.76; N
21.5%.
2. Experimental
LMe
: Yield 60%. Calculated value for Molecular formula:
C20H27N5 (337.0): C, 71.21; H, 8.01; N, 20.77; Found: C, 71.15;
2.1. Materials and reagents
H 7.89; N 20.80.
LOMe
: Yield 65%. Calculated value for Molecular formula:
2.1.1. Synthesis of tripodal ligands
C20H27N5O (353.0): C, 67.90; H, 7.64; N, 19.80; Found: C,
68.06; H 7.53;N 20.06.
Preparation of 3,5-dimethyl pyrazole and (1-hydroxy methyl)-3,5-
dimethyl pyrazole: 3,5-Dimethyl pyrazole and corresponding (1-hy-
droxy methyl)-3,5-dimethyl pyrazole have been prepared by fol-
lowing the literature procedures [48].
Results of 1H and 13C NMR studies on all the four ligands are
listed in Table 1.
2.1.2. Preparation of ligands (LX with X = Cl, H, Me and OMe)
All the ligands (LX) were synthesized by following the literature
procedure [48]. In a typical attempt, a solution of 12 mmol of
respective substituted benzyl amines and 24 mmol of (1-hydroxy
methyl)-3,5-dimethyl pyrazole in 50 ml acetonitrile was stirred
2.2. Preparation of azide bridged complexes
To a methanolic solution of 0.25 mmol of Cu(ClO4)2ꢁ6H2O
respective ligands were added. Immediately the colour of the solu-
Table 1
1H and 13C NMR of ligand LX.
6
5
4
1
8
3
N
14
12
15
11
7
N2
N
16
X
N
10
13
9
N
Ligand
1H NMR (CDCl3, d, ppm)
13C NMR (CDCl3, d, ppm):
LOMe (X = OMe)
1.98 (6H,s); 2.19 (6H,s); 3.62 (2H,s); 3.76 (2H,s); 4.88 (4H,s);
5.77 (2H,s); 6.78–7.09 (4H,d)
147.52 (C3); 139.91(C5); 129.9 (C11,15); 127.9 (C10); 113.69 (C12,14);
105.64(C4); 64.74 (C8); 55.24(C16); 51.98(C9); 13.50 (C7); 10.67 (C6);
55.24 (C16, OMe)
147.52 (C3); 139.91 (C5); 136.78 (C13); 134.96 (C10); 128.98 (C11, 12, 14,
15); 105.63 (C4); 64.74 (C8); 52.22 (C9); 13.50 (C7); 10.67 (C6); 21.08 (C16,
Me)
149.4 (C3); 140.9 (C5); 105.4 (C4); 127.3 (C13); 135.6 (C10); 128.5 (C11, 12,
14, 15); 61.4 (C8); 55.7 (C6); 18.0 (C7); 11.2 (C6)
148.32 (C3); 139.92 (C5); 132.8 (C13); 129.9 (C11, 15); 128.02 (C12, C14);
106.04 (C4); 64.84 (C8); 55.24 (C6); 13.50 (C7); 11.2 (C6)
LMe (X = Me)
1.99 (6H,s); 2.18 (6H,s); 3.63 (2H,s); 4.90 (4H,s); 5.77 (2H,s);
7.06 (4H,q)
LH (X = H)
LCl (X = Cl)
1.99 (6H,s); 2.17 (6H,s); 3.69 (2H,s); 4.93 (4H,s); 5.42 (2H,s);
7.1–7.3 (5H,m)
2.32 (6H,s); 3.62 (1H,s); 4.80 (4H,s); 5.82 (2H,s), 7.78 (4H,s)

I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
1203
Table 2
tion turned to blue, which on subsequent addition of 0.75 mmol of
Crystal data and details of the structure determination.
NaN3 changed to deep green. It was stirred for further six hours
whereupon the solution turned to brown. It was filtered and the fil-
trate was kept in refrigerator. Green crystals were formed. Com-
plexes 1 and 2 yield single crystals on slow evaporation within
couple of weeks. We failed to get suitable single crystals of com-
plexes 3 and 4 even after several trials with different solvents.
Formula
(1)
(2)
C38H50Cu2N16
2(ClO4)
,
4(C40H54Cu2N16O2),
8(ClO4), O
Formula weight
Crystal system
Space group
a, b, c (Å)
1056.94
Triclinic
P1 (No. 2)
8.9889(6),
11.1947(6),
12.2791(7)
74.740(5),
4483.97
Monoclinic
C2/c (No. 15)
14.0929(4),
15.6767(6),
21.6474(10)
90, 91.966(3), 90
ꢀ
[{Cu(LH)(N3)}]2(ClO4)2 (1)
Yield 60%, Anal. Calc.: C 43.18, H 4.73, N 21.21%; Found: C
42.9, H 4.77, N 19.82%. IR [m
(N3), cmꢀ1] = 2054.
a
, b, c (°)
[{Cu(LOMe)(N3)}]2(ClO4)2 (2) = 2054
89.188(5), 70.984(5)
1123.53(12)
1
1.562
1.136
V (Å3)
4779.8(3)
1
1.558
1.076
2320
Yield 70%, Anal. Calc.: C 43.0, H 4.83, N 20.07%; Found: C
Z
D (calc.) (g cmꢀ3
)
42.83, H 4.80, N 19.98%. IR [m(N3)] = 2055.
[{Cu(LMe)N3}]2(ClO4)2 (3)
l
(Mo K
a
) (mmꢀ1
)
F(0 0 0)
546
Yield 64%, Anal. Calc.: C 44.3, H 4.98, N 20.66%; Exptal. Calc.
Crystal size (mm)
Temperature (K)
0.20 ꢃ 0.20 ꢃ 0.40
0.40 ꢃ 0.40 ꢃ 0.40
value C 45.4, H 4.87, N 20.2%. IR [m(N3)] = 2038.
120
120
[{Cu(LCl)N3}]2(ClO4)2 (4)
Radiation (Å) Mo K
a
0.71073
0.71073
3.2, 25.0
Yield 64%, Anal. Calc.: C 40.46, H 4.43, N 19.87%; Exptal. Calc.
h Minimum–maximum (°)
3.3, 25.0
Dataset
ꢀ10:10; ꢀ13:11;
ꢀ16:16; ꢀ18:15;
value C 40.6, H 4.80, N 19.9%. IR [m(N3)] = 2073.
ꢀ12:14
ꢀ25:23
Total unique data, Rint
Observed data [I > 2
7849, 3954, 0.013
3344
16,202, 4227, 0.012
3943
2.3. Physical measurements
r(I)]
Nref, Npar
3954, 302
0.0238, 0.0664, 1.09
4227, 351
0.0310, 0.0743, 1.04
0.00, 0.00
R, wR2, S
Elemental analyses were carried out using a Perkin–Elmer 240
elemental analyzer. Infrared spectra (400–4000 cmꢀ1) were re-
corded from KBr pellets on a Nickolet Magna IR 750 series-II FT-
IR spectrophotometers. 1H and 13C NMR were recorded in CDCl3
on a Bruker 300 MHz NMR Spectrophotometer using tetramethyl-
silane (d = 0) as an internal standard. Electronic spectra were re-
corded on Agilent 8453 diode array UV–vis spectrophotometer.
Electrochemical measurements were carried out using a com-
puter-controlled AUTOLAB (model 263A VERSASTAT) electrochem-
ical instrument with platinum tip as working electrode.
Maximum and average shift/ 0.00, 0.00
error
Minimum and maximum
ꢀ0.27, 0.35
ꢀ0.48, 0.55
resd. densities (e Å3)
2
w ¼ 1=½r2ðFo2Þ þ ð0:0700PÞ þ 1:5000Pꢂ where P ¼ ðFo2 þ 2F2c Þ=3.
Table 3
Selected bond distances (Å) and bond angles (°) for 1a and 2.b
1
2
2.3.1. Crystal structure determination and structural refinement
Intensity data for complexes 1 and 2 were collected on a four-
circle diffractometer Kuma KM-4 equipped with a CCD detector
Cu1–N1
Cu1–N9
Cu1–N11
Cu1–N18
Cu1–N18_a
1.9798(17)
2.0802(16)
1.9715(17)
2.4272(16)
1.9619(17)
Cu1–N1
Cu1–N9
Cu1–N11
Cu1–N17
Cu1–N19_a
1.9656(17)
2.0755(17)
1.9743(19)
1.9593(19)
2.403(2)
and an Oxford Cryostream Cooler (Oxford Cryosystems). The
scan technique with different and / offsets for covering the
whole independent part of reflections in the 2 < 2h < 25° for mono-
chromated (monochromator Enhance, Oxford Diffraction) Mo K
x-
j
N1–Cu1–N9
N1–Cu1–N11
N1–Cu1–N18
N1–Cu1–N18_a
N9–Cu1–N11
81.04(6)
163.01(7)
91.05(6)
97.57(7)
82.17(6)
95.45(6)
178.30(7)
93.09(6)
99.27(7)
83.59(6)
126.87(13)
136.52(13)
N1–Cu1–N9
N1–Cu1–N11
N1–Cu1–N17
N1–Cu1–N19_a
N9–Cu1–N11
82.13(7)
161.20(7)
98.11(8)
94.77(7)
79.89(7)
174.60(8)
89.86(7)
99.28(8)
90.63(7)
95.50(7)
119.26(15)
120.32(16)
a
radiation (k = 0.71073 Å) was performed. The data reductions were
carried out using the CrysAlis RED (Oxford Diffraction) program.
The cell parameters were refined from all strong reflections. We
applied an empirical absorption correction as implemented in
SCALE3 ABSPACK scaling algorithm (CrysAlis RED) on both data-
N9–Cu1–N18
N9–Cu1–N17
N9–Cu1–N18_a
N11–Cu1–N18
N11–Cu1–N18_a
N18–Cu1–N18_a
Cu1_a–N18–N19
Cu1–N18–N19
N9–Cu1–N19_a
N11–Cu1–N17
N11–Cu1–N19_a
N17–Cu1–N19_a
Cu1_a–N19–N18
Cu1–N17–N18
sets. The structures were determined by direct methods (SHELXS
-
97) and refined anisotropically on F2 using full matrix least-squares
procedure by SHELXL-97 [49]. Hydrogen atoms were included in the
riding model approximation with C–H = 0.95, C–H2 = 0.99, C–
H3 = 0.98 Å. The final refinement details are given in Table 1.
Cu1–N18–Cu1_a
96.41(7)
a
Symmetry x + 1, ꢀy + 1, ꢀz + 1.
Symmetry ꢀx + 1, y, ꢀz + 3/2.
b
3. Results and discussion
The IR peaks corresponding to m((l-1,1 or EO) or an end-to-end
for 1 are given in Table 3. The ORTEP diagram of cationic part of 1 is
2þ
given in Fig. 1. There are one dimeric unit, ½CuLHð
l-N3Þꢂ2 with two
(l
-1,3 or EE) appear in the range 2038–2073 cmꢀ1 for all the com-
plexes. This indicates that the Nꢀ3 group bound to both the cop-
per(II) center through its nitrogen atom, exhibiting an increase in
its stretching frequency.
equivalent (symmetry x + 1, ꢀy + 1, ꢀz + 1) copper atoms in the
unit cell (Fig. 2) and two perchlorate anions just in trans disposi-
tion. The perchlorate ions are found to be normal though they
are not coordinated to the metal centers. The center of one dimeric
cation is located about a crystallographic center of inversion. With-
in a dimeric unit, the copper(II) centers are centro-symmetrically
related and are bridged by azide ions in an asymmetric EO fashion.
The copper(II) ion is coordinated to five nitrogens; three nitrogen
atoms from the pyrazolyl-N and amin-N of LH (X = H) and a nitro-
3.1. Structural description of [{Cu(LH)((
l-1,1-N3}]2(ClO4) (1) (EO)
Complex 1 crystallizes from methanol as dark green solids. The
details of structural determination for the compounds reported
here are given in Table 2. Selected bond distances and bond angles

1204
I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
Fig. 1. The ORTEP diagram of complex 2 plotted at 30% probability.
Fig. 2. The ORTEP diagram of complex 1 plotted at 30% probability.
gen atom of one of the bridging azide, Cu1–N18_a = 1.9619(17) oc-
cupy the equatorial plane. The axial position is occupied by the sec-
ond bridging azide group [Cu1–N18 = 2.4272(16) Å] at a rather
longer distance. The two azide ions are related by crystallographic

I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
1205
center of inversion. The Cu1–N1 = 1.980(2) Å, Cu1–N9 = 2.080
(16) Å, Cu1–N11 = 1.972(17) Å, and Cu1–N18_a = 1.962(17) Å dis-
tances are similar to those observed for related complexes of li-
gands with pyrazole and pyridine donor groups [50,51]. N–Cu–N
bond angles vary from 81.04(6)° to 178.30(7)°. The two coordi-
bonds, the interaction is either negligible or very weakly antiferro-
magnetic, depending on the geometry which tends to be trigonal
bipyramidal or Square Pyramidal, respectively [31].
3.3.1. Magnetic susceptibility of complex 1 (EO)
nated pyrazolyl nitrogens make
a biting angle of N1–Cu1–
The two copper atoms are symmetrically equivalent and posses
aSP (asymmetric Square Pyramidal) geometry. Complexes with
asymmetric EO azido bridge are rare, and the interaction between
metal centers is weak to moderately strong ferromagnetic [33].
Thus based on the earlier observations it is expected that complex
1 would provide moderate ferromagnetic interaction between two
copper(II) paramagnetic centers.
N11 = 163.01(7)° with central metal ion. Similarly, the amino
nitrogen (N9) and azide nitrogen (N18) make an angle of
178.30(7)°. Thus, the geometry around copper ion can be best de-
scribed as a distorted square pyramid (aSP). The Cuꢁ ꢁ ꢁCu distance
is 3.287 Å. The linear azide ions related by the molecular inversion
center are bridging the two copper centers through one nitrogen
atom (EO mode). Each bridging nitrogen atom simultaneously
occupies an equatorial position on Cu1 and an axial position on
the other Cu1.
Variable-temperature magnetic susceptibility measurements
were performed on a polycrystalline sample in the range 2–
300 K, and the results are shown in Fig. 3 in the form of leff
and Mmol/NAlB leff versus T plots. At room temperature, the eff va-
lue is 2.14 B, which is close to the spin-only value of 2.45 lB ex-
/lB
l
3.2. Structural description of [{Cu(LOMe) (
l-1,3-N3}]2(ClO4) (2) (EE)
l
pected for two uncoupled S = 1/2 spin systems. As the
temperature is lowered, the leff value continuously increases and
reaches a value of 3.32 lB at 5.00 K.
The magnetic data were successfully characterized with the
spin Hamiltonian for a coupled dimer is (Eq. (1))
Compound 2 crystallizes from methanol as dark green solids.
The details of crystal structural determination for the compound
2 reported here are given in Table 2. Selected bond distances and
angles for compound 2 are given in Table 3. The ORTEP diagram
of cationic part of 2 is given in Fig. 2. There are four dimeric units,
^
2þ
H ¼ ꢀJðSA ꢁ SBÞ þ lBB ꢁ g ꢁ ðSA þ SBÞ
ð1Þ
½CuLð
l-1; 3-N3Þꢂ2 in the unit cell, and two are isolated from other
two by six perchlorate anions. The perchlorate ions are found to be
normal though they are not coordinated to the metal ions. Within a
dimeric unit, the copper(II) centers are symmetrically related
(symmetry ꢀx + 1, y, ꢀz + 3/2) and are bridged by azide ions in
an asymmetric EE fashion. As like 1 here also Cu(II) ion is coordi-
nated to five nitrogens, three nitrogen atoms from two pyrazolyl
and one amino nitrogen of LOMe and a nitrogen atom of one of
the bridging azide (Cu1–N17 = 1.9593(2) Å) lying in the equatorial
plane. The axial position is occupied by the second bridging azide
group at a rather longer distance (Cu1–N19_a = 2.403(2) Å). The
Cu–NPyrazole (Cu1–N1 = 1.9656(2) Å and Cu1–N11 = 1.974(2) Å)
and Cu–Namine (Cu1–N9 = 2.0755(2) Å) distances are similar to
those observed for related complexes of ligands with pyrazole
and pyridine donor groups [52]. N–Cu–N bond angles vary from
82.13(7)° to 174.6(8)°. The two coordinated pyrazolyl nitrogens
make a biting angle of 161.2(7)° with central metal ion. Similarly,
the amino nitrogen (N9) and azide nitrogen N17 and N19 make an
angle of 174.6(8)° and 89.86(7)° respectively. Thus the geometry
around copper ion can be best described as a distorted square pyr-
amid. The Cuꢁ ꢁ ꢁCu distance is 5.063 Å and comparable in the case
of a similar compound [50,51]. It is noteworthy that in an
analogous asymmetrically bridged aliphatic triamine ligand com-
where |J| parameter characterizes the energy gap between the sin-
glet (S = 0) and triplet state (S = 1) resulting from the coupling of
two local spins SA = SB = 1/2. For the molar magnetization simple
formula exists which is expressed as in Eq. (2):
Mmol
¼
lBgNA½expððJ þ xÞ=kTÞ ꢀ expððJ ꢀ xÞ=kTÞꢂ
=½1 þ expððJ þ xÞ=kTÞ þ expðJ=kTÞ þ expððJ ꢀ xÞ=kTÞꢂ
ð2Þ
where x = BgB [54,55]. The experimental data (both temperature
l
and field dependent magnetization) were corrected to the underly-
ing diamagnetism and fitted simultaneously by varying J and g
parameters with fixed values of the temperature-independent para-
magnetism term for two copper(II) centers as
v
TIP = +1.5 m3 molꢀ1
(in SI units). The maximum on the susceptibility should be observed
for antiferromagnetically coupled dimer and it holds Tmax = (J/k)/
1.599. By applying the best-fitted value, we are left with
Cal
T
= 3.0 K, J = ꢀ3.34 cmꢀ1, g = 2.14. The TmCaalx is in good agreement
max
obs
max
with the experimental value T
= 2.9 K [54,55].
3.3.2. Magnetic susceptibility of complex 2 (EE)
The magnetic data were successfully characterized with the
spin Hamiltonian for coupled dimer as in Eq. (1) where |J| param-
eter characterize the energy gap between the singlet (S = 0) and
triplet state (S = 1) resulting from the coupling of two local spins
SA = SB = 1/2. For the molar magnetization Eq. (2) exists.
plex Cu2(l-N3)2(Me5dien)2(BPh4)2 the Cu–Cu separation is only
(5.2276(7) Å) [53]. The EE bridging azide is quasi-linear; the
N17–N18–N19 bond angle is 177.71°. The Cu1_a–N19–N18 and
Cu1–N17–N18 angles are 119.26(15)° and 120.32(16)° respec-
tively, and the Cu1–(N3)2–Cu1_a unit forms a chair configuration.
The N–N bond lengths for the bridging azide ligand are different
from one another (N(6)–N(7) 1.181(6) Å and N(7)–N(8)
1.161(6) Å). The amine and pyrazole rings can be considered as
practically planar.
The experimental data (both temperature and field dependent
magnetization) were corrected to the underlying diamagnetism
and fitted simultaneously by varying J and g parameters with fixed
values of the temperature-independent paramagnetism term for
two Cu(II) centers as
v
TIP = +1.5 m3 molꢀ1 (in SI units) (Fig. 4).
Fig. 5 shows alternative fit with the molecular-field correction with
spin Hamiltonian by Eq. (3):
3.3. Magnetic properties
^
H ¼ ꢀJðSA ꢁ SBÞ þ lBB ꢁ g ꢁ ðSA þ SBÞ ꢀ zjhSziSz
ð3Þ
As mentioned in the introduction, an unusual range of magnetic
behavior (from ferro- to antiferro-) can be observed based on the
nature of coordination of the azide bridging groups. In di-bridged
complexes with one or more symmetric EO bridges, the interaction
between the metal ions is strongly ferromagnetic [33]. On the
other hand, with one or more symmetric EE azido bridges, the
interaction is strongly antiferromagnetic [32,11]. In complexes
with asymmetric EE azido bridge with short and long Cu–Nazide
where zj is the common molecular-field parameter which is due to
small intermolecular interactions and Sz is a thermal average of the
spin [56]. There is no visible change in parameters (J = +19.7 cmꢀ1
,
g = 2.17, Eq. (2)) on considering the small intermolecular interac-
tions (J = +19.9 cmꢀ1, g = 2.16 and zj = +0.025 cmꢀ1, Eq. (3)) indicat-
ing negligible intermolecular interactions and this is supported by
the wide separation (8.279 Å) between the two nearest copper(II)
centers of two separate dimmers.

1206
I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
Fig. 3. Left: temperature dependence of the effective magnetic moment of complex 1 (EO) (calculated from magnetization at B = 1.0 T) with the low-temperature region
expanded in the inset; right: field-dependence of magnetization at T = 2.0 and 4.6 K. circles – experimental points, lines – calculated using the best-fit parameters:
J = ꢀ3.34 cmꢀ1, g = 2.14.
Fig. 4. Left: temperature dependence of the effective magnetic moment of complex 2 (EE) (calculated from magnetization at B = 1.0 T) with the low-temperature region
expanded in the inset; right: field-dependence of magnetization at T = 2 and 4.6 K. circles – experimental points, lines – calculated using the best-fit parameters:
J = +19.7 cmꢀ1, g = 2.17.
3.4. Electronic spectra
tammetric studies on the metal centers showed quasi-reversible
waves (Fig. 7) and represented in Eq. (4)
The UV spectra of the complexes 1–4 in MeCN are very similar
(Fig. 6). The electronic spectra show bands in the range of wave-
length 393–414 nm with molar extinction coefficients values,
1eꢀ
ꢀ1eꢀ
CuIICuꢀII ꢀ CuIICuIII ꢀ CuIIICuIII
ð4Þ
e
= 2692, 4860, 4880 and 2516 dm3 molꢀ1 cmꢀ1 for X = Cl, H, Me
Details of E1/2 values are shown in Table 4. An attempt was taken to
correlate the E1/2 values for the two processes with substituent con-
and OMe, respectively and arise mainly due to the pyrazole nitro-
gen to metal charge transfer transition (LMCT).
stants (
r).
The plots of E1/2 of the dinuclear copper(II) complexes versus
r
of pyrazole-based tripodal ligands bearing different substituents
on the para-position of benzene ring seem to be interesting to
understand the effect of substituents on E1/2 and to elucidate the
reduction mechanism. Many electrochemical processes have been
shown to correlate quite well with Hammett substituent constants
[57]. It is not surprising that the electrochemical processes which
involve addition or removal of electrons from an organic frame-
work should correlate with the ability of substituents to withdraw
or supply electron density to that framework.
3.5. Cyclic voltammetric studies
Electrochemical measurements were carried out using a com-
puter-controlled AUTOLAB (model 263A VERSASTAT) electrochem-
ical instrument with Pt-tip as working electrode. Cyclic
voltammograms were recorded at 25 °C versus Ag/AgCl electrode
in MeCN under pure N2 atmosphere with. 0.1 M tetrabutylammo-
nium perchlorate (TBAPC) as supporting electrolyte. The cyclic vol-

I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
1207
Fig. 5. Left: temperature dependence of the effective magnetic moment of complex 2 (EE) (calculated from magnetization at B = 1 T) with the low-temperature region
expanded in the inset; right: field-dependence of magnetization at T = 2 and 4.6 K. circles – experimental points, lines – calculated using the best-fit parameters:
J = +19.9 cmꢀ1, g = 2.16 and zJ = +0.025 cmꢀ1
.
2.0
Table 4
Summary of electrochemical studies of complexes 1–4 in MeCN under nitrogen
3
1
atmosphere with [C] = 1.0 ꢃ 10ꢀ3 M, TBAP = 0.10 M as supporting electrolyte.
Compounds
Epa
Epc
E1/2
Epa
Epc
0.046
E1/2
0.140
1.5
1.0
0.5
0.0
1 (LH)
ꢀ0.198
ꢀ0.107
ꢀ0.180
ꢀ0.170
ꢀ0.436
ꢀ0.399
ꢀ0.435
ꢀ0.535
ꢀ0.317
ꢀ0.253
ꢀ0.308
ꢀ0.353
0.233
0.486
0.261
0.200
2 (LOMe
)
0.125
0.060
ꢀ0.254
0.306
0.161
ꢀ0.027
3 (LMe
4 (LCl
)
4
)
2
0.45
y = -0.6843x + 0.1358
R
2 = 0.9925
0.3
0.15
0
300
400
Wavelength (nm)
500
600
Fig. 6. Electronic spectra of complexes 1 (H), 2 (OMe), 3 (Me) and 4 (Cl) in MeCN,
[C] = 5.0 ꢃ 10ꢀ4 M.
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
2.0x10-5
1.5x10-5
1.0x10-5
-0.15
Fig. 8. Hammette plot of E1/2 vs. (for the CuIICuIII–CuIIICuIII couple. Conditions are
same as mentioned in Fig. 7.
5.0x10-6
0.0
A reasonable linear relation was observed when we plot E1/2 (for
CuIICuII–CuIICuIII and CuIICuIII–CuIIICuIII oxidation) versus
r and q
-5.0x10-6
-1.0x10-5
-1.5x10-5
-2.0x10-5
values obtained from the slopes are ꢀ0.182 (R2 = 0.92) and
ꢀ0.684 (R2 = 0.99) (Fig. 8) respectively. The strong dependence of
E1/2 on the substituent constants for the latter reaction compared
to the former is due to the fact that electron withdrawing substit-
uents destabilize the higher oxidation states more prominently
than lower one. This also explains the observation of negative
values for these processes.
q
-1.42
-0.95
-0.47
0.00
0.47
0.95
1.42
E/V vs. SCE
Fig. 7. Cyclic voltammograms of 1–4 in MeCN under N2 atmosphere in MeCN using
4. Conclusion
a
Pt-tip as working electrode and TBAP (0.10 M) as supporting electrolyte at
50 mV sꢀ1 scan rate vs. Ag/AgCl. [{Cu(LH)N3}]2(ClO4)2 (1) (colour), [{Cu(LOMe)N3}]2-
(ClO4)2 (2) (colour), [{Cu(LMe)N3}]2(ClO4)2 (3) (colour), [{Cu(LCl)N3}]2(ClO4)2 (4)
(colour).
Four azido-bridged dinuclear copper(II) complexes of N-methyl
pyrazole-based tripodal ligands, LX, [X = H (1), OMe (2), Me (3) and

1208
I. Banerjee et al. / Polyhedron 29 (2010) 1201–1208
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acterized structurally and cryo-magnetic studies were performed
on them. In Complex 1 the two bridging azide ligands connect
the two metal centers by EO fashion with aSP geometry and
showed very weak antiferromagnetic interactions (J = ꢀ3.34 cmꢀ1).
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exhibits moderately strong ferromagnetic interaction (J = +19.7
cmꢀ1). Cyclic voltammetric studies were performed on all the four
complexes and E1/2 for CuIICuII ? CuIICuIII and CuIICuIII ? CuIIICuIII
oxidations were plotted against
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CCDC 716781 and 716782 contain the supplementary crystal-
lographic data for complexes 1 and 2. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
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Financial supports from DRDO (Ref. ERIP/ER/0503510/M/904)
and DST (Ref. SR/S1/IC-35/2006), New Delhi, India are gratefully
acknowledged. JM (MSM0021622415) and RH (MSM619
8959218) acknowledge the grants from the Ministry of Education,
Youth and Sports of the Czech Republic.
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