Copper(II) azide complexes of 1-alkyl-2-(arylazo)imidazoles: Structure and magnetism

Article abstract of DOI:10.1016/j.poly.2006.04.007

Tetranuclear Cu(II)-azido complexes of the composition [Cu(RaaiR′)(N₃)₂]₄ have been synthesized by reacting 1-alkyl-2-(arylazo)imidazole (RaaiR′), Cu(OAc)₂ · H₂O and NaN₃ in 1:1:2 mole proportion [RaaiR′; R = H (3), Me (4); R′ = Me (a), Et (b)]. The structure of [Cu(HaaiMe)(N₃)₂]₄ (3a) has been confirmed by an X-ray diffraction study. The tetranuclear copper core is generated by dimerisation via μ_1,1-N₃ of two dimer units. In each dimer one copper centre is five-coordinated (CuN₅) and the other one is six-coordinated (CuN₆). Variable temperature magnetic measurements of 3a show the occurrence of ferromagnetic (J₁ = +25.3 cm^-1) and antiferromagnetic (J₂ = -4.7 cm^-1) interactions through the double and single end-on azido bridges, respectively. Upon addition of excess ligand, a 2-methoxy ethanol solution of the tetranuclear complex fragmented into a mononuclear copper(II) complex of the formula [Cu(RaaiR′)₂(N₃)₂], and in one case, [Cu(HaaiEt)₂(N₃)₂], the structure was established by X-ray diffraction.

Full text of DOI:10.1016/j.poly.2006.04.007

Polyhedron 25 (2006) 2764–2772
www.elsevier.com/locate/poly
Copper(II) azide complexes of 1-alkyl-2-(arylazo)imidazoles:
Structure and magnetism
a
a
b
c
Umasankar Ray , Kamal Krishna Sarker , Golam Mostafa , Tian-Huey Lu ,
d,
a,
*
M. Salah El Fallah *, Chittaranjan Sinha
a
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
b
Department of Physics, Jadavpur University, Kolkata 700032, India
Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
` `
´
Department de Quı´mica Inorganica, Universitat de Barcelona, Martı i Franques, 1-11, 08028 Barcelona, Spain
c
d
Received 15 February 2006; accepted 2 April 2006
Available online 27 April 2006
Abstract
Tetranuclear Cu(II)-azido complexes of the composition [Cu(RaaiR0)(N3)2]4 have been synthesized by reacting 1-alkyl-2-
(arylazo)imidazole (RaaiR0), Cu(OAc)2 Æ H2O and NaN3 in 1:1:2 mole proportion [RaaiR0; R = H (3), Me (4); R0 = Me (a), Et (b)].
The structure of [Cu(HaaiMe)(N3)2]4 (3a) has been confirmed by an X-ray diffraction study. The tetranuclear copper core is generated
by dimerisation via l1,1-N3 of two dimer units. In each dimer one copper centre is five-coordinated (CuN5) and the other one is six-coor-
dinated (CuN6). Variable temperature magnetic measurements of 3a show the occurrence of ferromagnetic (J1 = +25.3 cmꢀ1) and anti-
ferromagnetic (J2 = ꢀ4.7 cmꢀ1) interactions through the double and single end-on azido bridges, respectively. Upon addition of excess
ligand, a 2-methoxy ethanol solution of the tetranuclear complex fragmented into a mononuclear copper(II) complex of the formula
[Cu(RaaiR0)2(N3)2], and in one case, [Cu(HaaiEt)2(N3)2], the structure was established by X-ray diffraction.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Copper(II)-azide; 1-Alkyl-2-(arylazo)imidazoles; Tetranuclear; Magnetic properties
1. Introduction
blockers [12,13] of varied denticities have been used to pre-
pare such molecular architectures. Multidentate Schiff base
ligands [12] and polyamines [13] have generally been used
to stabilize the core unit. Use of the azo function in this
direction has rarely been explored [14].
We have been engaged for some time in exploring the
chemistry of 1-alkyl-2-(arylazo)imidazole with transition
and non-transition metal ions [14–21]. Our understanding
of copper complexes with arylazoimidazole has mainly
focused on the structural, spectroscopic and electrochemi-
cal properties of copper(I) complexes. Attempts to synthe-
size copper(II)–RaaiR0 have failed eveꢀn under aerobic
conditions in the presence of Clꢀ or ClO4 as counter ions,
and [Cu(RaaiR0)2]+ꢀ has been crystallized [15]. However,
the presence of N3 or NCSꢀ in the reaction mixture has
enabled Cu(II) complexes to be stabilized [16]. Towards
the preparation of multinuclear copper(II) complexes via
Polynuclear metal complexes are of contemporary inter-
est [1–7]. A careful selection of ligand, metal centre and
reaction conditions can confer control over the topology
of the resulting framework [8–10]. Pseudohalides, espe-
cially azide, are versatile polyatomic bridging units that
can bind metal ions via end-on (EO, l1,1) and end-to-end
(EE, l1,3) motifs [11] with novel magnetic properties. This
leads to extreme structural and electronic versatility allow-
ing ferromagnetic (F) or antiferromagnetic (AF) coupling
via EO and EE bridges, respectively, and tunes magnetic
behaviors via modification of the bond parameters, in
particular in the bridging region [12]. Different organic
*
Corresponding author. Fax: +91 33 2414 6584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.04.007

U. Ray et al. / Polyhedron 25 (2006) 2764–2772
2765
self-assembly of metal, bidentate azoimine blocker and
pseudohalide, we have been successful in isolating and
characterizing tetranuclear complexes with the formula
[Cu(RaaiR0)(N3)2]4 [RaaiR0; R = H (3), Me (4); R0 = Me
(a), Et (b)]. The structure has been confirmed by a single
crystal X-ray diffraction study of [Cu(HaaiMe)(N3)2]4
(3a). Variable temperature magnetic measurements of 3a
binding of N3 in the complexes 3 and 4 [9,11,22]. Moder-
ately intense stretchings at 1583–1598 and 1438–
1448 cmꢀ1 are due to m(C@N) and m(N@N), respectively
[14].
Because of the insolubility of the complexes in common
organic solvents, the electronic spectra of the complexes
have been recorded in N,N-dimethyl formamide (DMF)
solution in the UV–Vis region (200–900 nm). The com-
plexes (3, 4) exhibit four weak transitions around 715,
585, 475 and 450 nm. The position and intensity of the
bands may not only be of d–d origin rather an admixture
of d–d and MLCT (dp(Cu) ! p*(ligand)) transitions. The
bands 6400 nm are presumably due to intra-ligand p–p*
and n–p* transitions.
show the occurrence of ferromagnetic (J1 = +25.3 cmꢀ1
)
and antiferromagnetic (J2 = ꢀ4.7 cmꢀ1
)
interactions
through the double and single end-on azido bridges,
respectively. Addition of excess of RaaiR0 to the respective
tetranuclear copper(II) complex in 2-methoxy ethanol has
enabled the cluster to be split into a mononuclear cop-
per(II) complex of the composition [Cu(RaaiR0)2(N3)2].
The structure and magnetism are detailed below.
2.3. Crystal structures
2. Results and discussion
2.3.1. [Cu(HaaiMe)(N3)2]4 (3a)
2.1. The complexes
The X-ray structure (Fig. 1) reveals that complex 3a is a
neutral tetranuclear copper(II) derivative, comprising a
dimer-of-dinuclear copper(II) subunits. Selected bond
lengths and angles are given in Table 1. There are two crys-
tallographically unique copper(II) environments; one of
them is a five-coordinated square-based pyramid with a
CuN5 donor set and other is a six-coordinated distorted
octahedral CuN6 type. The dinuclear core unit is generated
by a two l1,1-N3 bridging scheme. Dimerisation of the
dinuclear motif is carrying out by l1,1-N3-group bridging.
Thus, an arrangement of four copper atoms is formed.
Each copper centre has been chelated with a HaaiMe
ligand in which the ligand acts as an unsymmetric bidentate
N,N0-donor type where N(imidazole) and N(azo) refer to
N and N0, respectively.
The complexes are synthesized by the reaction of 1-
alkyl-2-(arylazo)imidazole (RaaiR0), Cu(OAc)2 Æ H2O and
NaN3 in MeOH in a 1:1:2 mole proportion at 298 K.
The ligand is an unsymmetric, N,N0-bidentate chelator
where N and N0 refer to N(imidazole) and N(azo) donor
centres, respectively. The azide ðN3ꢀÞ is a bridging ligand
showing its versatile coordination motifs. The composition,
[Cu(RaaiR0)(N3)2]4 (R = H (3), Me (4); R0 = Me (a), (b))
has been confirmed by X-ray crystal structure determina-
tion of one of the compounds, [Cu(HaaiMe)(N3)2]4 (3a).
The tetranuclear cluster is dissociated into a mono-copper
complex, [Cu(RaaiR0)2(N3)2] (5, 6) upon addition of excess
of RaaiR0 to a 2-methoxy ethanol solution of the complex.
The structure of the mononuclear copper(II) compound
has also been established by X-ray structure study in one
case, [Cu(HaaiEt)2(N3)2] (6a), and by comparing with pre-
viously reported results [16].
The atomic groups Cu(1), N(5), C(13), N(7), N(8) and
Cu(2), N(1), C(3), N(3), N(4) constitute two chelate planes
˚
of mean deviation 60.02 A. The pendant phenyl ring
makes a dihedral angle of about 8ꢁ with the chelating
N
N
R'
N
N
R
RaaiR/
R = H (a), Me (b);
R’ = Me (1), Et (2)
2.2. Spectral studies
Infrared spectral data were assigned on comparing
with the free ligand data and on comparing with
[Cu(RaaiR0)2(N3)2] [16]. The most significant observation
is the appearance of strong sharp stretches at 2069–2082,
2045–2050 and 2026–2028 cmꢀ1 for 3 and 4. These corre-
spond to masym(N3). This supports a bridging end-on type
Fig. 1. ORTEP view of 3a showing the metal environment and the atom
labelling.

2766
U. Ray et al. / Polyhedron 25 (2006) 2764–2772
Table 1
parameters around the CuN6 coordination environment
shows a distorted octahedral arrangement. In the TBP
CuN5, the triangular base is constituted by N(8), N(13),
N(15), Cu(1) and apical positions are occupied by N(9)
and N(5). The azido bridge angles in the dinuclear cop-
per(II) unit (Cu(1)–{l1,1-N3}2–Cu(2)) are 100.60(11)ꢁ and
104.72(12)ꢁ and lie in the range of reported values
[9,11,27]. The bridge angle has a major effect on the spin-
exchange phenomena (evaluated from the exchange param-
eter J). Planarity of the Cu2N2 unit, chelate angle of the
capping agent, dihedral angle between two coordination
planes, steric and electronic factors of the ligand signifi-
cantly control the magnetic properties of the complexes.
The two dinuclear copper(II) cores (Cu(1)ꢂ ꢂ ꢂCu(2) unit)
are bridged by a l1,1-N3 group in an inverted fashion
(Cu(1) of core-1 is bridged by l1,1-N3 to Cu(2a)* (* symme-
try: 2 ꢀ x, 1 ꢀ y, 1 ꢀ z) of core-2 and vice versa) to consti-
tute an eight-member ring. The Cu(1)ꢂ ꢂ ꢂCu(2) distance is
˚
Selected bond distances (A) and angles (ꢁ) for [Cu(HaaiMe)(N3)2]4 (3a)
Cu(1)–N(5)
Cu(1)–N(8)
Cu(1)–N(9)
Cu(1)–N(12)
Cu(1)–N(15)
Cu(2)–N(1)
Cu(2)–N(4)
Cu(2)–N(9)
Cu(2)–N(12)
Cu(2)–N(18)
N(3)–N(4)
2.002(2)
2.639(1)
2.017(2)
1.973(2)
1.961(3)
1.973(2)
2.632(3)
2.051(2)
1.980(2)
1.967(3)
1.260(3)
1.325(4)
1.387(4)
1.243(4)
1.332(5)
1.372(5)
1.213(4)
1.168(5)
1.195(4)
1.126(5)
1.191(4)
1.143(8)
1.182(4)
1.152(5)
3.130
N(5)–Cu(1)–N(9)
N(5)–Cu(1)–N(12)
N(5)–Cu(1)–N(15)
N(9)–Cu(1)–N(12)
N(9)–Cu(1)–N(15)
N(12)–Cu(1)–N(15)
N(8)–N(7)–C(13)
Cu(1)–N(8)–N(7)
Cu(1)–N(5)–C(13)
Cu(1)–N(15)–N(16)
N(1)–Cu(2)–N(9)
N(1)–Cu(2)–N(12)
N(1)–Cu(2)–N(18)
N(9)–Cu(2)–N(12)
N(9)–Cu(2)–N(18)
N(12)–Cu(2)–N(18)
Cu(2)–N(1)–C(3)
N(1)–C(3)–N(3)
168.06(10)
92.35(10)
97.47(10)
77.77(10)
93.52(10)
165.59(12)
113.2(3)
107.24
120.7(2)
122.6(2)
95.87(10)
169.23(11)
94.00(10)
76.82(10)
170.03(10)
93.21(11)
121.4(2)
128.1(3)
178.0(3)
112.0(3)
100.59(12)
124.5(2)
N(1)–C(3)
N(3)–C(3)
N(7)–N(8)
N(5)–C(13)
N(7)–C(13)
N(9)–N(10)
N(10)–N(11)
N(12)–N(13)
N(13)–N(14)
N(15)–N(16)
N(16)–N(17)
N(18)–N(19)
N(19)–N(20)
Cu(1)ꢂ ꢂ ꢂCu(2)
Cu(1)ꢂ ꢂ ꢂCu(2_a)#
N(15)–N(16)–N(17)
N(4)–N(3)–C(3)
˚
3.13 A and is similar to reported azido bridged (end-on)
Cu(1)–N(15)–Cu(2_a)*
Cu(1)–N(9)–N(10)
Cu(2)–N(9)–N(10)
N(9)–N(10)–N(11)
Cu(1)–N(12)–Cu(2)
Cu(1)–N(12)–N(13)
Cu(2)–N(12)–N(13)
N(12)–N(13)–N(14)
N(8)–Cu(1)–N(9)
N(1)–Cu(2)–N(4)
N(5)–Cu(1)–N(8)
dicopper complexes [13] but shorter than oxobridged
(Cu–O–Cu) phenolato compounds [7a]. In the single azido
bridged (Cu(1)–{l1,1-N3}–Cu(2_a)*) (* symmetry: 2 ꢀ x,
1 ꢀ y, 1 ꢀ z) dinuclear copper(II) unit the Cuꢂ ꢂ ꢂCu separa-
127.2(2)
177.9(4)
104.71(12)
131.3(2)
123.8(2)
177.8(3)
107.01(10)
69.54(9)
3.592
˚
tion is 3.59(2) A and is longer than the double azido
bridged dicopper unit.
There are three types of (azido) N3-groups present in
the tetranuclear copper(II) unit: two N3-units bridge two
Cu(II)-centres in the dimeric core; one N3-function serves
to bridge two dimer cores and the last N3-unit coordinates
as a monodentate donor with one of the two copper cen-
tres in the dimer. Other N-donor centres are N(imidazole)
and N(azo) from the chelated HaaiMe. Out of them, the
N(azo) requires considerably more room than the other
N-donors [23]. There are at least two steric reasons for this.
First, the N(azo) belongs to an exocyclic N@N along with
a pendant aryl group; thus the van der Waals repulsion
will be greater than N(imidazole). Second, the N(azido)
has larger s-character compared to N(azo) and thus
N(azo) is more space demanding. The Cu–N(azo) dis-
tances are the longest in the series and are Cu(1)–N(8),
68.75(10)
#
Symmetry code: 2 ꢀ x, 1 ꢀ y, 1 ꢀ z.
Cu
N
N
C
N
plane. The chelate angles of
are N(1)–Cu(2)–N(4), 69.54(9)ꢁ, N(5)–Cu(1)–N(8),
68.75(10)ꢁ. From the respective basal plane, the Cu(1)
and Cu(2) centres are moved in same direction by
˚
ꢀ0.168(1) and ꢀ0.123(1) A. These two planes are inclined
at a dihedral angle of 87.22(10)ꢁ. In the dinuclear motif
the bridging unit Cu2N2 is a four armed distorted plane
˚
of mean deviation ꢁ0.02 A. The Cu–N distances in the
Cu2N2 unit reveal the distortion of the tetraatomic cycle:
Cu(1)–N(1), 2.017(2); Cu(2)–N(1), 2.051(2); Cu(1)–N(12),
˚
2.639(1) and Cu(2)–N(4), 2.632(3) A. However, the Cu–
˚
1.973(2); Cu(2)–N(12), 1.980(2) A. The bridge angles are
N(azo) distances are less than the sum of the van der
Waals radii of Cu(II) and N(azo) [24]. This implies a cova-
lent interaction of N(azo) and Cu(II). Cu(II) has some
preference to bind to N(imidazole) [25] compared to
100.60(11)ꢁ and 104.71(12)ꢁ. The other two angles (N(9)–
Cu(1/2)–N(12)) are 77.77(10)ꢁ and 76.82(10)ꢁ. The chelate
and bridge planes are inclined at an angle of 76.30(13)ꢁ.
Double bridged azide groups are inclined at an acute angle
with the bridge plane [N(9)–N(10)–N(11) makes 55ꢁ and
N(12)–N(13)–N(14) makes 15ꢁ angle with Cu2N2]. The sin-
gle-bridged azido group [N(15)–N(16)–N(17)] lies at an
angle of 61ꢁ with the Cu2N2 motif. Dimerisation of the
dicopper core generates the Cu2N bridge unit (Cu(2_a)*–
N(15)–Cu(1)) (* symmetry: 2 ꢀ x, 1 ꢀ y, 1 ꢀ z) of angle
106.59(12)ꢁ. The geometrical arrangement reflects the dis-
torted orientation of the groups around the bridge. In the
CuN6 arrangement the donor centres are as follows: chelat-
ing HaaiMe gives two N-donor centres; three end-on azido
bridged-N centres and a terminal azido-N donor. The bond
˚
N(azo) and is shortened by 0.6 A. The azido bond lengths
˚
lie in the range 1.12–1.21 A. The azo N@N distances in the
two chelated HaaiMe of the dimer are N(3)–N(4), 1.260(3)
˚
and N(7)–N(8), 1.243(4) A, which are near to the free
˚
ligand value (1.250(1) A) [26].
2.4. Magnetic study
The variable temperature magnetic susceptibility data
for complex 3a was recorded between 300 and 2 K. A plot
of vMT versus T for the four copper(II) ions is shown in
Fig. 2. At 300 K, the vMT value is 1.64 cm3 molꢀ1 K, which

U. Ray et al. / Polyhedron 25 (2006) 2764–2772
2767
vM ¼ Ng2b2=KT½A=Bꢃ;
ð1Þ
is close to that expected for four uncoupled spins, with
g = 2.09. The value increases slightly on lowering the tem-
perature and reaches a maximum (1.83 cm3 molꢀ1 K) at
22 K. Below this temperature vMT decreases to a value of
1.38 cm3 molꢀ1 K at 2 K. The shape of this curve indicates
dominant ferromagnetic coupling until 22 K and below this
temperature vMT decreases from 1.8 to 1.4 cm3 molꢀ1 K,
which is the signature of antiferromagnetic exchange and/
or the presence of the ZFS of the ground state. To analyze
the magnetic pathways it is necessary to schematize them as
is shown in Fig. 3. This scheme matches with the structure
that, as indicated above, can be considered as a dimer of
dimers. The J1 corresponds to the double end-on azido
bridges with an equatorial–equatorial (short–short) coordi-
nation mode, whereas J2 corresponds to the single end-on
azido bridge, but in equatorial–axial (short–long) coordi-
nation mode. Thus, considering these two different J val-
ues, the analysis of the experimental susceptibility values
has been performed using the following expression:
where A = [30exp(ꢀE1/kT) + 6exp(ꢀE2/kT) + 6exp(ꢀE3/
kT) + 6exp(ꢀE4/kT)], B = 5[exp(ꢀE1/kT) + 3exp(ꢀE2/kT) +
3exp(ꢀE3/kT) + 3exp(ꢀE4/kT) + exp(ꢀE5/kT) + exp(ꢀE6/
kT)].
En values can be obtained using the Kambe method
from the Hamiltonian:
H ¼ ꢀJ1ðS1 ꢂ S2 þ S3 ꢂ S4Þ ꢀ J2ðS1 ꢂ S3 þ S2 ꢂ S4Þ
E1 ¼ ðꢀJ1 ꢀ J2Þ=2; E2 ¼ ðꢀJ1 þ J2Þ=2;
E3 ¼ ðJ1 þ J2Þ=2; E4 ¼ ðJ1 ꢀ J2Þ=2;
1=2
E5 ¼ ðJ1 þ J2Þ=2 þ ðJ21 þ J22 ꢀ J1J2Þ
;
1=2
E6 ¼ ðJ1 þ J2Þ=2 ꢀ ðJ21 þ J22 ꢀ J1J2Þ
where Ji parameters correspond to those given in Fig. 4.
The best fit parameters found are: J1 = +25.3 cmꢀ1
,
J2 = ꢀ4.7 cmꢀ1
,
g = 2.07 and R = 1.6 · 10ꢀ4
,
where
P
P
2
2
R ¼ ½ðvMTÞexp ꢀ ðvMTÞcalꢃ = ðvMTÞexp. Attempts to fit
the experimental data introducing D, zJ0 or h parameters
in the expression (1) did not improve the R factor.
The values of the superexchange parameters are
J1 = +25.3 cmꢀ1 and J2 = ꢀ4.7 cmꢀ1 which are typical
for such Cu(II) complexes with these structural features.
The bonding pattern of Cu–(N3)–Cu is unique in this
example, having one single end-on and double end-on
azide bridging, and is different from so far reported end-
on azido bridged Cu(II) complexes (Table 2). Indeed, for
the equatorial–equatorial coordination mode (such as in
the J1 pathway), the magnetic behavior of copper-azide
bridged complexes in end-on mode is well-known [28–35]
and the general trends of their magnetic properties are
well-correlated to the structural parameters of the bridge
(mainly Cu–Nazide–Cu angle). Currently, the end-on or
(l1,1) double bridges are usually associated with ferromag-
netic behavior [5], although for angles out of the range
96–104ꢁ the interaction can be antiferromagnetic [27,36].
However, for azido anions bridging in an end-on fashion
at the axial–equatorial position (such in the J2 case,
Fig. 3), the reported J values are always small and they
can be ferromagnetic or antiferromagnetic [37–42], depend-
ing on the Cu–N distance and the s parameter (s = 0 for
square pyramidal or elongated octahedral geometries and
1.9
1.8
1.7
1.6
1.5
T
M
χ
1.4
1.3
1.2
0
50
100
150
200
250
300
T / K
Fig. 2. Plot of the vMT vs. T data for complex 3a. Solid line represents the
best fit (see text).
Fig. 3. Magnetic interaction pathways J1 and J2 between the different
Fig. 4. ORTEP view of 6a Æ C3H8O2 showing the metal environment and
copper(II) centres.
the atom labelling.

2768
U. Ray et al. / Polyhedron 25 (2006) 2764–2772
Table 2
Selected structural and magnetic data of some single- and double end-on azido bridged Cu(II) complexes
˚
Complexa
Cu–N (A)
Cu–N–Cu (ꢁ)
J (cmꢀ1
)
Ref.
Double end-on azido bridged Cu(II) complexes
[Cu2(OBISDIEN)(l2-N3)2(N3)2]
[Cu2(t-Bupy)4(l2-N3)2](ClO4)2
[Cu2(tbz)2(l2-N3)2(N3)2] Æ 2CH3OH
[Cu2(DMPTD)(l2-N3)2(N3)2]
[Cu2(l1,1N3)2(4-Etpy)4(l-NO3)2]
[Cu2(l1,1N3)2(3-ampy)4(l-NO3)2] Æ C2H5OH
[Cu3(l1,1N3)4(Meinic)2(N3)2(DMF)2]
[Cu(L1)(N3)]2
1.990, 2.043
1.99
105.5, 101.6
100.5
104.7
98.3, 101.9
98.3
97.5
100.8, 99.2
90.93
101.4
102.3
103.0
70
105
23
[28]
[29]
[31]
[36]
[11b]
2.059, 1.97
1.995, 2.043
1.996, 2.004
2.000, 2.006
1.998, 2.012
2.215, 2.174
2.002, 1.998
2.023, 2.010
1.970, 1.979
2.012, 1.996
85
230
223
47.3
1.03
11.7
[33]
[34]
[Cu4(bpm)2(N3)8]n
[Cu(4-Etpy)(N3)2]
[Cu4(L2)(l2-N3)4](PF6)4
weak ferro
94
[11c]
[35]
98.1, 96.6
Single end-on azido bridged Cu(II) complexes
[Cu2(terpy)2-l-(N3)4Cu2-l-N3)2(N3)2]
Cu3(SE)2(N3)4(DMF)2
[Cu2(l-oxen)(l1,1-N3)2(Br)]n
[Cu4(l-oxen)2(l1,1-N3)2(l-1,3-N3)]n(ClO4)n
1.943, 2.374
2.001, 2.042
1.98, 1.95
1.98, 1.98
2.41, 1.95
2.28, 2.01
2.34, 1.99
2.30, 1.97
2.42, 1.96
2.60, 1.95
107.11
102.33
112.5
111.8
97.3
130.0
116.6
110.9
113.6
107.1
ꢀ100
32.6
ꢀ85.7
[42a]
[42b]
[42c]
[4]
[Cu2(hppz)2(N3)2(l1,1-N3)(l1,3-N3)]n
[Cu2(L3)2(N3)2(l1,1-N3)(l1,3-N3)]n
[Cu2(Me-L4)(l1,1-N3)4]n
ꢀ11.5
[42d]
[42e]
14.1
[Cu(L5)(N3)(l1,1-N3)]n
ꢀ2.2
ꢀ3.7
[42f]
[Cu(L6)(N3)(l1,1-N3)]n
Single- and double end-on azido bridged, together, in compound 1
Compound 3a
1.973, 2.017
2.051, 1.961
104.7, 100.6
106.59
25.3
ꢀ4.7
this work
a
OBISDIEN = 1,13-dioxa-4,7,10,16,19,23-hexaazacyclotetracosane; t-bupy = 4-tert-butylpyridine; (tbz = bis(2-benzimidazolyl)propane; DMPDT =
2,5-bis-((pyridylmethy1)thio)thiadiazole; 4-Etpy = 4-ethylpyridine; 3-ampy = 3-aminopyridine; Meinic = methylisonicotinate; HL1 = 1-(2-hydroxyben-
zyl)-1,5-diazacicloctane; bpm = bis(pyrazol-1-yl)methane; L2 = 1,2,4,5-tetrakis-(1,4,7-triazacyclonon-1-ylmethyl)benzene, terpy = 2,20:6,20 0-terpyridine;
SE = result from the condensation of N,N-dimethylethylenediamine with salicylaldehyde; H2oxen = N,N0-bis(2-aminoethyl)oxamide; hppz = homopip-
erazine; L3 = 2-(pyrazol-1-yl-methyl)pyridine; Me-L4 = 2-(3-methylpyrazol-1-ylmethyl)pyridine. L5 and L6 are obtained from the condensation of
2-pyridylaldehyde with aniline and p-chloroaniline, respectively.
s = 1 for trigonal bipyramidal geometry). Thus, the J2
value of ꢀ4.7 cmꢀ1 (small for copper(II) ions), can be
unequivocally assigned to the monobridge azido ligand
(equatorial–axial). The two J values found in 3a match
with those shown in Table 2.
tural confirmation has been carried out in one case by a
single crystal X-ray structure study and is given below in
detail. Room temperature (298 K) magnetic data of mono-
nuclear complexes 5 and 6 show the typical values of vMT
for one unpaired electron (d9), around 0.4 cm3 molꢀ1 K.
2.4.1. Reaction of excess ligand with [Cu(RaaiR0)(N3)2]4
To a 2-methoxy-ethanol solution of the tetranuclear
complex, an excess of the ligand (the same ligand as is pres-
ent in the tetranuclear copper(II) complex) was added and
stirred for a few minutes. Upon addition of water to this
solution a brown product was separated. It was then puri-
fied by recrystallisation from the mixture of methanol and
2-methoxy ethanol (1/1 v/v). Infrared spectra of the com-
plex show a sharp single band at an average of
2045 cmꢀ1. This supports the presence of terminal N3-
bonding. Moderately intense stretchings at 1585–1595
and 1440–1450 cmꢀ1 are referred to m(C@N) and
m(N@N), respectively. These complexes are non-conducting
and sufficiently soluble in common organic solvents like
chloroform, dichloromethane and acetonitrile. The UV–
Vis spectra are recorded in acetonitrile solution and show
weak absorptions at 706–730 and 570–580 nm. The struc-
2.4.2. Cu(HaaiEt)2(N3)2 Æ C3H8O2 (6a Æ C3H8O2)
Crystals are obtained on slow evaporation of a mixture
of 2-methoxy ethanol and a methanol solution of [Cu(Haa-
iEt)2(N3)2]4 and HaaiEt (in excess). An ORTEP view of
Cu(HaaiEt)2(N3)2 Æ C3H8O2 is shown in Fig. 4. Selected
bond lengths and angles are given in Table 3. In the dis-
crete molecule, copper(II) is surrounded by two ligands
and two azido groups. The ligand serves as an unsymmetric
N,N0-chelating agent (N refers to imidazole-N and N0
refers to azo-N donor centres) and two ligands chelate in
a manner to give cis-N,N and trans-N0,N0 arrangements.
ꢀ
Two N3 coordinate in a cis-fashion around Cu(II). Thus,
the coordination sequence of azides, imidazole-N(N) and
azo-N(N0) describe a cis–cis–trans stereochemical orienta-
tion. The geometry of the copper ion is an axially elongated
octahedron; the equatorial plane is formed by two imidaz-
ole-N (each from an individual ligand) and two azido-N

U. Ray et al. / Polyhedron 25 (2006) 2764–2772
2769
Table 3
N(13), 152.30(10)ꢁ]. The large deviation (ꢁ35ꢁ) from the
trans angle (180ꢁ) is undoubtedly from the combined con-
tribution of short chelate angle, lifting of the inherent
degeneracy of the d9 configuration and the steric require-
ment of azo-N centres. The azido bond lengths and angles
are within the range for reported data [9–13]. The azo,
˚
Bond distances (A) and bond angles (ꢁ) of Cu(HaaiEt)2(N3)2 Æ C3H8O2
(6a Æ C3H8O2)
Cu(1)–N(1)
Cu(1)–N(5)
Cu(1)–N(12)
Cu(1)–N(13)
Cu(1)–N(2)
Cu(1)–N(7)
N(1)–C(17)
N(11)–C(17)
N(11)–N(12)
N(13)–N(14)
N(2)–N(3)
2.003(3)
2.018(3)
2.541(3)
2.569(3)
1.985(4)
1.994(3)
1.319(5)
1.396(4)
1.264(4)
1.269(5)
1.201(6)
1.153(6)
1.164(6)
1.159(6)
1.326(5)
1.383(5)
N(1)–Cu(1)–N(12)
N(2)–Cu(1)–N(13)
N(5)–Cu(1)–N(7)
N(5)–Cu(1)–N(8)
N(5)–Cu(1)–N(12)
N(5)–Cu(1)–N(13)
N(7)–Cu(1)–N(12)
N(7)–Cu(1)–N(13)
N(12)–Cu(1)–N(13)
N(1)–Cu(1)–N(12)
N(1)–Cu(1)–N(13)
Cu(1)–N(1)–C(17)
Cu(1)–N(2)–N(3)
N(2)–N(3)–N(4)
Cu(1)–N(5)–C(7)
N(1)–Cu1–N2
70.28(10)
105.40(13)
90.13(14)
70.84 (12)
88.04 (12)
69.60 (12)
106.21(12)
90.48(13)
152.30(10)
70.28(10)
93.35(11)
119.5(2)
120.0(3)
177.9(5)
119.0(3)
91.00(13)
176.16(14)
174.63(15)
87.90(15)
111.6(2)
˚
N@N, distances lie between 1.26–1.27 A which is longer
˚
than the free ligand value (1.250(1) A) [26].
3. Experimental
3.1. Materials
N(3)–N(4)
N(7)–N(8)
N(8)–N(9)
N(5)–C(7)
Cu(OAc)2 Æ H2O, NaN3 were purchased from Sisco
Research Lab, India, and 1-alkyl-2-arylazoimdazoles
(RaaiR0) were prepared following the literature procedures
[20]. All other chemicals and solvents were analytical
reagent grade. Solvents were used after drying. For spectral
studies solvents of spectroscopic grade were purchased
from Lancaster, UK. The reaction conditions have been
set up following the procedure of synthesis of [Mn-
(MeaaiEt)(N3)2]n [14].
N(14)–C(7)
N1–Cu(1)–N(7)
N(2)–Cu(1)–N(5)
N(2)–Cu(1)–N(7)
Cu(1)–N(12)–N(11)
donor centres. The two axial positions are occupied by two
azo-N donor centres of two HaaiEt. The N–Cu–N bond
angles subtended by the chelating ligands are N(1)–Cu–
N(12), 70.28(10)ꢁ and N(5)–Cu–N(13), 69.60(12)ꢁ which
are close to the precursor tetranuclear copper(II)
compound. The arylazoimidazole moiety is almost planar
Caution! Azido complexes of transition metal ions con-
taining organic ligands are potentially explosive. Only a
small amount of material should be prepared, and it should
be handled with care.
3.2. Physical measurements
˚
with a deviation from the least square plane <0.04 A.
The chelate rings constitute good planes (mean deviation
Microanalyses (C, H, N) were performed using a
Perkin–Elmer 2400 CHNO/S elemental analyzer. Copper
analyses were carried out by iodometric titration [43].
Spectroscopic measurements were carried out using the fol-
lowing instruments: UV–Vis spectra, reflectance spectra,
JASCO UV–Vis/NIR model V-570; IR spectra (KBr disk,
4000–200 cmꢀ1), JASCO FT-IR model 420. The room tem-
perature magnetic moment was measured using a vibrating
sample PAR-155 magnetometer at 298 K. Molar conduc-
tances (KM) were measured in a Systronics conductivity
meter 304 model using ca. 10ꢀ3 M solutions in MeOH.
The magnetic susceptibility of a powdered sample was
measured using a SQUID Magnetometer in the tempera-
ture range 300–2 K at an applied field of 0.1 T. The contri-
bution of the sample holder was determined separately in
the same temperature range and field. Diamagnetic correc-
tions were estimated from Pascal’s constants. The magnetic
susceptibility was fitted by least square techniques.
˚
ꢁ0.03 A). The pendant aryl ring makes small a dihedral
angle with the chelate plane and lie in the range 7–8ꢁ.
The proposed molecular planes in the octahedral geometry
are found to be heavily distorted in this structure. The dis-
tortion in CuN6 coordination is supported from the bond
parameters of the structures (Table 3).
There are three types of N-donor centres: N(imidazole),
N(azo) and N(azido). Usually Cu–N(azo) distances are
longer than the other distances and lie in the range of
˚
2.6–2.7 A. The reasons of more space of N(azo) have been
accounted for earlier (see above) [23–25]. The structural
distortion is not only of electronic origin (Jahn–Teller dis-
tortion for d9, Cu(II)) but also arises from the steric effect
of N-donor centres. The lowering in overall symmetry is
manifested by the elongation of axial bonds and the two
N(azo) donor centres occupy two axial positions. There is
˚
a small difference (ꢁ0.01 A) between two Cu–N(imidazole)
˚
distances [Cu(1)–N(1), 2.003(3); Cu(1)–N(5), 2.018(3) A].
The sum of the equatorial bond angles (plane is constituted
by two imidazole-N and two azido-N donor centres) at Cu
is 360.31ꢁ. It shows that the metal ion is essentially in the
mean plane. A significant deviation from the trans angle
is observed for the two axial N-donors, more than that
of the equatorial N-donors in the chelated fragment
Cu(HaaiEt). In the equatorial plane the trans angles are
>170ꢁ [N(1)–Cu(1)–N(7), 176.16(14)ꢁ; N(2)–Cu(1)–N(5),
174.63(15)ꢁ]. The axial \N–Cu–N is >150ꢁ [N(12)–Cu(1)–
3.3. Syntheses
3.3.1. [Cu(HaaiMe)(N3)2]4 (3a)
A
solution
of
1-methyl-2-(phenylazo)imidazole
(HaaiMe) (0.045 g, 0.24 mmol) in methanol (10 ml) was
added dropwise to a stirred solution of Cu(OAc)2 Æ H2O
(0.05 g, 0.25 mmol per copper) in MeOH (10 ml) at
298 K. The greenish brown solution was stirred for
15 min. NaN3 (0.033 g, 0.51 mmol) in MeOH (10 ml) was

2770
U. Ray et al. / Polyhedron 25 (2006) 2764–2772
Table 4
added to this solution. The colour changed from greenish
brown to red brown. The solution was filtered and left
undisturbed for a few days. The dark brown crystalline
compounds were collected, washed with aqueous methanol
(1:1, v/v) and finally with ether. It was dried in vacuo. Yield
was 0.063 g (74%). The microanalytical data of the two
complexes is as follows, [Cu(HaaiMe)(N3)2]4 (3a): Anal.
Calc. for C40H40N40Cu4: C, 35.97; H, 2.99; N, 41.97; Cu,
19.05. Found: C, 35.86; H, 3.05; N, 41.84; Cu, 18.91%.
[Cu(MeaaiMe)(N3)2]4 (3b) (yield, 65%). Anal. Calc. for
C44H48N40Cu4: C, 37.98; H, 3.45; N, 40.28; Cu, 18.28.
Found: C, 37.89; H, 3.39; N, 40.37; Cu, 18.14%. [Cu(Haa-
iEt)(N3)2]4 (4a) (yield, 67%). Anal. Calc. for C44H48N40Cu4:
C, 37.98; H, 3.45; N, 40.28; Cu, 18.28. Found: C, 37.92; H,
3.51; N, 40.33; Cu, 18.17%. [Cu(MeaaiEt)(N3)2]4 (4b)
(yield, 72%). Anal. Calc. for C48H56N40Cu4: C, 39.83; H,
3.87; N, 38.73; Cu, 17.57. Found: C, 39.89; H, 3.81; N,
38.68; Cu, 17.45%.
Summarized crystallographic data for 3a and 6a Æ C3H8O2
Compound
3a
6a Æ C3H8O2
Empirical formula
Formula weight
Temperature (K)
Crystal system
C40H40N40Cu4
1335.32
293
orthorhombic
Pbca
C25H32N14O2Cu
624.19
294
orthorhombic
Pbca
Space group
Crystal size (mm)3
0.10 · 0.08 · 0.05
19.2947(14)
14.9915(11)
19.2947(14)
90
5581.1(7)
8
0.71073
1.576
1.589
379
31861
6529
0.15 · 0.20 · 0.20
12.5670(7)
15.8577(9)
31.2045(18)
90
6218.5(6)
8
0.71073
0.750
1.134
383
35918
7195
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A)
Z
˚
k (A)
l (Mo Ka) (mmꢀ1
)
Dcalc (Mg mꢀ3
)
Refined parameters
Reflection number total
Unique reflections
a
R1 [I > 2r(I)]
wR2
0.0444
0.0970
0.0652
0.1322
b
3.3.2. Reaction of HaaiMe with [Cu(HaaiMe)(N3)2]4
To a well-stirred suspension of [Cu(HaaiMe)(N3)2]4 (3a)
(0.1 g, 0.30 mmol) in 2-methoxy ethanol (5 ml), a small
excess to one equivalent (with reference to per copper
atom) of HaaiMe (0.60 g, 0.32 mmol) in methanol (10 ml)
was added and stirred continuously for 30 min. By this
time the solution colour changed to brown violet. It was
then filtered through a G-4 crucible, to reject any residue,
and left undisturbed for slow evaporation. Dark coloured
crystals appeared after a week. These were then filtered,
washed with cold methanol and dried in vacuo over CaCl2.
Yield was 0.09 g (58%). [Cu(HaaiMe)2(N3)2] (5a): Anal.
Calc. for C20H20N14Cu: C, 35.97; H, 2.99; N, 41.97; Cu,
19.05. Found: C, 35.86; H, 3.05; N, 41.84; Cu, 18.91%.
[Cu(MeaaiMe)2(N3)2] (5b) (yield, 55%). Anal. Calc. for
C22H24N14Cu: C, 37.98; H, 3.45; N, 40.28; Cu, 18.28.
Found: C, 37.89; H, 3.39; N, 40.37; Cu, 18.14%. [Cu(Haa-
iEt)2(N3)2] (6a) (yield, 60%). Anal. Calc. for C22H24N14Cu:
C, 37.98; H, 3.45; N, 40.28; Cu, 18.28. Found: C, 37.92; H,
3.51; N, 40.33; Cu, 18.17%. [Cu(MeaaiEt)2(N3)2] (6b)
(yield, 58%). Anal. Calc. for C24H28N14Cu: C, 39.83; H,
3.87; N, 38.73; Cu, 17.57. Found: C, 39.89; H, 3.81; N,
38.68; Cu, 17.45%.
Goodness-of-fit
0.92
0.83
P
P
a
R = (jFo ꢀ Fcj)/ jFoj.
P
P
2 1=2
b
wR ¼ ½ wðF 2o ꢀ F c2Þ= wðF oÞ ꢃ , w = 1/[r2(Fo)2 + (0.0462P)2] for
3a and w = 1/[r2(Fo)2 + (0.0659P)2] for 6a, where P = ((Fo)2 + 2(Fc)2)/3.
A summary of the crystallographic data and structure
refinement parameters is given in Table 4. Reflection data
were recorded using the x scan technique. Data were cor-
rected for Lorentz polarization effects and for linear decay
using the SAINT program. Semi-empirical absorption cor-
rections based on w-scans were applied. The structure
was solved by direct methods using SHELXS-97 [44,45] and
successive difference Fourier syntheses. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms
were fixed geometrically and refined using the riding model.
In the final difference Fourier map the residual maxima and
minima were 0.396 and ꢀ0.206 for 3a and 0.313 and
ꢀ0.230 for 6a.
4. Conclusion
In this research we demonstrate the synthesis of tetranu-
clear [Cu(RaaiR0)(N3)2]4 complexes from the reaction of
Cu(OAc)2 Æ H2O, RaaiR0 and NaN3 in a 1:1:2 mole ratio.
Addition of RaaR0 (a slight excess of one equivalent) to a
2-methoxy ethanol solution of the tetranuclear compound
has degraded it to a mono-copper compound of composi-
tion, [Cu(RaaiR0)2(N3)2]. In each case, the structural confir-
mation has been done by X-ray diffraction studies. The
tetranuclear arrangement has been achieved by end-on
sharing of N3-groups in the dimers in which six-coordinated
CuN6 and five-coordinated CuN5 units are associated via
two end-on-N3 bridging groups. IR spectra also distinguish
the bonding pattern. Variable temperature magnetic sus-
ceptibility measurement (300–2 K) shows both anti- and
ferromagnetic interactions for [Cu(HaaiMe)(N3)2]4.
3.4. Crystallographic data collection and refinement of the
structure
Good quality and stable to X-ray crystals for both the
complexes were collected from slow evaporation of syn-
thetic mixtures in methanol and 2-methoxy ethanol. Data
were collected with a Siemens SMART CCD diffractome-
ter using fine focus sealed graphite-monochromatized Mo
˚
Ka radiation (k = 0.71073 A). Unit cell parameters were
determined from least-squares refinement of setting angles
of 6529 reflections at 293 K for [Cu(HaaiMe)(N3)2]4 (3a)
and 7195 reflections for [Cu(HaaiEt)2(N3)2] Æ C3H8O2 (6a)
at 294 K. Data were collected with 2h in the range
4.04 6 2h 6 56.54ꢁ (for 3a) and 2.6 6 2h 6 56.6ꢁ (for 6a).

U. Ray et al. / Polyhedron 25 (2006) 2764–2772
2771
[10] (a) F. Averseng, P.G. Lacroix, I. Malfant, N. Perisse, C. Lepetit, K.
Nakatani, Inorg. Chem. 40 (2001) 3797;
Acknowledgements
(b) H. Oshio, Y. Saito, T. Ito, Angew. Chem., Int. Ed. 36 (1997) 2673.
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(b) A. Escuer, M.A.S. Goher, F.A. Mautner, R. Vicente, Inorg.
Chem. 39 (2000) 2107;
Financial support from the Council of Scientific and
Industrial Research, and Department of Science and
Technology, New Delhi are gratefully acknowledged.
C.S. also acknowledges UGC for providing him a Visiting
Associateship to I.I.Sc., Bangalore. M.S.F. is also grateful
for the grant given by the Spanish Ministerio de Ciencia y
Tecnologia (Programa Ramon y Cajal). Our sincere
thanks are due to Prof. J. Ribas, Universitat de Barce-
lona, Spain for his helpful discussion on magnetism of
the complexes.
(c) M.A.S. Goher, A. Escuer, F.A. Mautner, N. Al-Salem, Polyhe-
dron 20 (2001) 2971;
(d) R. Costa, M.K. Urtiaga, L. Lezama, J. Ignacio, R. Larramendi,
M.I. Arriortua, T. Rojo, J. Chem. Soc., Dalton Trans. (1993) 3685;
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Appendix A. Supplementary material
[14] U.S. Ray, Sk. Jasimuddin, B.K. Ghosh, M. Monfort, J. Ribas, G.
Mostafa, T.-H. Lu, C. Sinha, Eur. J. Inorg. Chem. (2004) 250.
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(b) P.K. Santra, T.K. Misra, C. Sinha, Indian J. Chem. 38a (1999) 82;
(c) J. Dinda, U.S. Ray, G. Mostafa, T.-H. Lu, A. Usman, I.A. Razak,
S. Chantrapromma, H.-K. Fun, C. Sinha, Polyhedron 22 (2003) 247.
[16] U.S. Ray, D. Banerjee, G. Mostafa, T.-H. Lu, C. Sinha, New J.
Chem. 28 (2004) 1432.
[17] U.S. Ray, B.G. Chand, G. Mostafa, J. Chang, T.-H. Lu, C. Sinha,
Polyhedron 22 (2003) 2587.
[18] (a) T.K. Misra, D. Das, C. Sinha, P. Ghosh, C.K. Pal, Inorg. Chem.
37 (1998) 1672;
UV–Vis and infrared spectral data are submitted in Sup-
plementary Table. Crystallographic data for the structures
have been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 249036 for Cu(HaaiEt)2(N3)2 Æ
C3H8O2 and 249037 for [Cu(HaaiMe)(N3)2]4. Copies of
this information may be obtained free of charge from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.
uk). Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.2006.
04.007.
(b) S. Pal, T.K. Misra, C. Sinha, A.M.Z. Slawin, J.D. Woollins,
Polyhedron 19 (2000) 1925;
(c) P. Byabartta, S. Pal, T.K. Misra, C. Sinha, F.-L. Liao, K.
Panneerselvam, T.-H. Lu, J. Coord. Chem. 55 (2002) 479.
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Organomet. Chem. 629 (2001) 28;
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