A rare case of solution and solid state inter-conversion of two copper(II) dimers and a copper(II) chain

Article abstract of DOI:10.1016/j.ica.2011.07.027

Three Cu(II)-azido complexes of formula [Cu₂L₂(N ₃)₂] (1), [Cu₂L₂(N₃) ₂]·H₂O (2) and [CuL(N₃)]_n (3) have been synthesized using the sa

Full text of DOI:10.1016/j.ica.2011.07.027

Inorganica Chimica Acta 377 (2011) 26–33
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A rare case of solution and solid state inter-conversion of two copper(II) dimers
and a copper(II) chain
a,
Subrata Naiya a,b, Saptarshi Biswas a, Michael G.B. Drew c, Carlos J. Gómez-García d, , Ashutosh Ghosh
⇑
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, APC Road, Kolkata 700 009, India
b Susil Kar College, Ghoshpur, Champahati, Baruipur, 24 Parganas(S), West Bengal 743 330, India
c School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG 66AD, UK
d Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia, 46980 Paterna, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2011
Received in revised form 9 July 2011
Accepted 19 July 2011
Available online 4 August 2011
Three Cu(II)-azido complexes of formula [Cu2L2(N3)2] (1), [Cu2L2(N3)2]ꢀH2O (2) and [CuL(N3)]n (3) have
been synthesized using the same tridentate Schiff base ligand HL (2-[(3-methylaminopropylimino)-
methyl]-phenol), the condensation product of N-methyl-1,3-propanediamine and salicyldehyde). Com-
pounds 1 and 2 are basal-apical
l
-1,1 double azido bridged dimers. The dimeric structure of 1 is centro-
-1,1 single azido bridged 1D chain. The
symmetric but that of 2 is non-centrommetric. Compound 3 is a
l
three complexes interconvert in solution and can be obtained in pure form by carefully controlling the
synthetic conditions. Compound 2 undergoes an irreversible transformation to 1 upon dehydration in
the solid state. The magnetic properties of compounds 1 and 2 show the presence of weak antiferromag-
netic exchange interactions mediated by the double 1,1-N3 azido bridges (J = ꢁ2.59(4) and
Keywords:
Copper(II)
Schiff base
Antiferromagnetic
Crystal transformation
1
ꢁ0.10(1) cmꢁ , respectively). The single 1,1-N3 bridge in compound 3 mediates a negligible exchange
interaction.
l-1,1 Single azido bridged
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
gand is chelating tridentate, the azide ligand bridges adjacent cop-
per(II) centres acting as a basal–apical bridge [27–30]. Regarding
In recent years, the synthesis of polynuclear complexes of Schiff
base ligands with polyatomic anions has witnessed a huge advance
thanks to their interesting applications in the field of structural
chemistry [1–5], magnetism [1–3,6–9], gas storage [10–12], catal-
ysis [13] and luminescence [14,15]. The steric and electronic ef-
fects of the Schiff base as well as the polyatomic bridging ligand
play a crucial role in constructing the polymeric structure. Among
the different transition metal and anions used, the Cu(II)-azide sys-
tem is the most popular one. A variety of copper–azido complexes
with discrete monomeric or one-, two-, and three-dimensional
polymeric structures have been reported, in which the azido ligand
the basal–basal bridging mode, it has been established that when
the azido ligand bridges two Cu(II) ions in an -1,1 manner, the
l
nature of the exchange coupling changes from ferromagnetic to
antiferromagnetic when the Cu–N–Cu angle increases, with a
crossing point at around 108° [25,26,31]. In contrast, basal–apical
bridges usually give rise to very small magnetic couplings since
the magnetic orbital containing the unpaired electron is mainly
of an x2 ꢁ y2 type lying in the basal plane of the copper atoms with
an almost negligible contribution on the axis perpendicular to the
basal plane. For such small coupling, the Cu–N–Cu angle is not
indicative of the magnetic interaction. Instead, several other fac-
tors, such as the steric and electronic factors of the blocking ligand,
exhibits diverse bridging modes ranging from
l
l
-1,1 (end-on, EO)
-1,1,1,1, -1,1,3,3,
-1,1,1,3,3,3, depending upon the steric and electronic de-
and
and
l
l
-1,3 (end-to-end, EE) to
l-1,1,1,
l-1,1,3,
l
the Addison parameter, s, the axial Cu–N bond distances, supramo-
lecular H-bonding interactions, etc. play an important role in con-
trolling the magnitude and sign of the coupling constants [30] and
therefore, contrary to their basal–basal analogues, there is no clear
magneto-structural correlation. To have a better insight into the
magnetic properties of these complexes, the synthesis of different
compounds using the same blocking ligand seems necessary,
although optimization of the synthetic conditions in order to ob-
tain the different compounds in pure form is still a challenge.
Herein, we present the syntheses, crystal structures, and
variable-temperature magnetic properties of three basal–apical
azide bridged compounds, [Cu2L2(N3)2] (1), [Cu2L2(N3)2]ꢀH2O (2)
mands of the co-ligands [16–24]. Among the various bridging
modes of the azide ion, -1,1 (end-on, EO) is the most common
l
one and can be either basal–basal [25,26] or basal–apical [27–
30]. In the literature, the basal–basal and basal–apical coordination
are usually termed respectively as symmetric and unsymmetric (or
asymmetric) bridging. It may be found that when the blocking li-
⇑
Corresponding authors.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.027

S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
27
Yield: 0.50 g, 80%. Anal. Calc. for C22H32Cu2N10O3 (2): C, 43.20;
H
C
H, 5.27; N, 22.90. Found: C, 43.27; H, 5.25; N, 22.85%. IR (KBr pellet,
cmꢁ1): 3244
599 nm.
m(NH), 2044 m(N@N) 1625 m(C@N), kmax (methanol),
CH3
N
N
H
2.4. Synthesis of [CuL(N3)]n (3)
OH
The procedure was the same as that used for complex 1 except
that a methanol–water solution (9:1, v/v) of excess NaN3 (0.260 g,
4 mmol) was added with slow stirring. The solution was left to
stand overnight in air to yield needle shaped deep green X-ray
quality single crystals of complex 3.
Scheme 1. Schiff base ligand HL.
and [CuL(N3)]n (3). These three derivatives have been prepared
using azide and a single N,N,O donor Schiff base ligand, 2-[(3-
methylaminopropylimino)-methyl]-phenol (HL, Scheme 1). Both
1 and 2 are double asymmetric
only difference in their composition is that 2 contains a water of
crystallization. Compound 3 is a rare example of a single asymmet-
Yield: 0.22 g, 73%. Anal. Calc. for C11H15CuN5O (3): C, 44.51; H,
5.09; N, 23.59. Found: C, 44.45; H, 5.13; N, 23.63%. IR (KBr pellet,
l
-1,1-azide bridged dimers; the
cmꢁ1): 3245
606 nm.
m(NH), 2043 m(N@N) 1624 m(C@N), kmax (methanol),
ric l-1,1-azido bridged polymer. All three complexes interconvert
2.5. Physical measurements
in solution. The factors that allow the conversion of one compound
into another in solution have been explored. Compound 2 under-
goes an irreversible solid state transformation to compound 1 upon
dehydration. During the course of this work, the crystal structures
of 3 and a compound related to 2 in which methanol was the sol-
vent molecule have been reported by others [28]. However, the
previous authors reported only the synthesis and crystal structures
of the compounds. The inter-conversion of the compounds in solu-
tion, the solid-state transformation of 2 to 1 and the magnetic
properties were not reported.
Elemental analyses (C, H and N) were performed using a Perkin-
Elmer 240C elemental analyzer. IR spectra in KBr pellets (4500–
500 cmꢁ1) were recorded using a Perkin-Elmer RXI FT-IR spectro-
photometer. Electronic spectra in methanol (1000–200 nm) were
recorded in a Hitachi U-3501 spectrophotometer. Thermal analyses
(TG-DTA) were carried out on a Mettler Toledo TGA/SDTA 851 ther-
mal analyser in
a dynamic atmosphere of dinitrogen (flow
rate = 30 cm3 minꢁ1). The samples were heated in an alumina cru-
cible at a rate of 10 °C minꢁ1. The magnetic susceptibility measure-
ments were carried out in the temperature range 2–300 K with an
applied magnetic field of 0.5 T on polycrystalline samples of
compounds 1–3 (with masses of 45.70, 56.69 and 34.63 mg,
respectively) with a Quantum Design MPMS-XL-5 SQUID suscep-
tometer (PPMS-9 equipment for sample 3). The isothermal magne-
tizations were performed on the same samples at 2 K with
magnetic fields up to 5 T (up to 8 T in PPMS-9 equipment for
sample 3). The susceptibility data were corrected for the sample
holders previously measured using the same conditions and for
the diamagnetic contributions of the salts as deduced by using
2. Experimental
Materials: The reagents and solvents used were of commercially
available reagent quality.
2.1. Synthesis of the Schiff base ligand 2-[(3-
methylaminopropylimino)-methyl]-phenol (HL)
The monocondensed Schiff base ligand HL (Scheme 1) was
synthesized by the condensation of salicylaldehyde (1.05 mL,
10 mmol) and N-methyl-1,3-propanediamine (1.04 mL, 10 mmol)
in methanol (10 mL) as reported earlier [5]. The resulting dark yel-
low solution was then used directly for complex formation.
Pascal’s constant tables (
v
dia = ꢁ345.4 ꢂ 10ꢁ6, ꢁ354.7 ꢂ 10ꢁ6 and
ꢁ172.7 ꢂ 10ꢁ6 emu molꢁ1 for 1–3, respectively).
Table 1
Crystal and refinement data for compounds 1 and 2.
2.2. Synthesis of [Cu2L2(N3)2] (1)
Compound
1
2
Formula
C
22H30Cu2N10O2
C22H32Cu2N10O3
611.68
Cu(ClO4)2ꢀ6H2O (0.582 g, 2 mmol), dissolved in 10 mL of meth-
anol, was added to a methanolic solution (10 mL) of the ligand (HL)
(2 mmol) with constant stirring. After 10 min, a methanol-water
solution (9:1, v/v) of NaN3 (0.130 g, 2 mmol) was added. The colour
of the solution turned to deep blue. By slow evaporation of the
resulting solution at room temperature, blue coloured X-ray qual-
ity, rectangular shaped single crystals were obtained in two days.
Yield: 0.45 g, 75%. Anal. Calc. for C22H30Cu2N10O2 (1): C, 44.51;
H, 5.09; N, 23.59. Found: C, 44.47; H, 5.15; N, 23.55%. IR (KBr pellet,
Formula weight
Temperature (K)
Space group
Crystal system
a (Å)
593.66
150
P1
150
ꢀ
ꢀ
P1
triclinic
7.658(1)
8.952(4)
9.619(3)
103.07(3)
96.25(2)
101.83(3)
620.3(3)
1
triclinic
7.8575(6)
9.6622(6)
17.6571(8)
81.285(6)
79.946(6)
75.217(6)
1268.18(16)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
cmꢁ1): 3436 (broad)
(C@N), kmax (methanol), 606 nm.
m(OH), 3181 m(NH), 2045 m(N@N) 1620
Z
m
Dcalc (g/cm3)
1.589
1.756
1.602
1.723
l
(mmꢁ1
)
F(0 0 0)
Total reflections
306
4431
632
8980
2.3. Synthesis of [Cu2L2(N3)2](H2O) (2)
Unique reflections
Observed data [I > 2
Number of parameters refined
Rint
3480
2564
164
0.0300
R1 = 0.0414
wR2 = 0.0843
7078
5105
342
0.0240
R1 = 0.0362
wR2 = 0.0802
The procedure was the same as that for complex 1, except that
triethylamine (0.28 mL, 2 mmol) was added to the reaction mix-
ture after the addition of NaN3 solution. The bluish-green X-ray
quality, plate shaped single crystals were obtained from the result-
ing blue solution on keeping overnight.
r
(I)]
R indices (all data)

28
S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
2.6. Crystal data collection and refinement
In the present investigation we have used this ligand for the
synthesis of complexes with Cu(II) and Nꢁ3 anions. This group of
mono negative N,N,O donor ligands usually produce double asym-
Crystal data for complexes 1 and 2 are given in Table 1. 3480,
and 7078 independent data for 1 and 2 were collected with Mo
metric [36–39]
l-1,1-azide-bridged dimers like complex 1 which
Ka
radiation at 150 K using an Oxford diffraction X-calibur CCD
has been prepared by reacting a
methanolic solution of
system. The crystals were positioned at 50 mm from the CCD.
321 frames were measured with a counting time of 10 s. Data anal-
ysis was carried out with the CRYSALIS program [32]. The structures
were solved using direct methods with the SHELXS97 program
[33]. The non-hydrogen atoms were refined with anisotropic ther-
mal parameters. The hydrogen atoms bonded to carbon were in-
cluded in geometric positions and given thermal parameters
equivalent to 1.2 times (or 1.5 times for methyl hydrogen atoms)
those of the atom to which they were attached. The hydrogen
atoms bonded to the water molecule in 2 were located in a differ-
ence Fourier map and refined with distance constraints. Empirical
absorption corrections were carried out with ABSPACK [34]. The
structures were refined on F2 using SHELXl97 [33] to R1 values
0.0362 and 0.0414; wR2 values 0.0843 and 0.0802 for 2564 and
Cu(ClO4)2ꢀ6H2O with a methanolic solution of HL followed by the
addition of an aqueous solution of NaN3 in 1:1:1 molar ratio. Com-
pound 2 on the other hand crystallized when triethylamine was
added to the reaction mixture. Compound 2 transforms into com-
pound 1 on recrystallization from methanol whereas addition of a
few drops of triethylamine to a methanolic solution of 1 followed
by slow evaporation of the solvent yields compound 2 (see Scheme
2).
On the other hand, when a methanolic solution of HL and
Cu(ClO4)2ꢀ6H2O is allowed to react with excess azide ions in the
molar ratio 1:1:2 at room temperature, the 1D polymer [CuL2N3)]n
(3), which is held together by single
l-1,1-azide bridges, is ob-
tained. Compounds 1 and 2 also convert easily into 3; when a
methanol-water (9/1, v/v) solution of sodium azide is added to a
methanolic solution of 1 or 2 (1:1 molar ratio or more), compound
3 starts to crystallize within an hour. The higher concentration of
azide ion seems to be the driving force for its formation. Again,
compound 3 transforms into compound 1 on recrystallization from
methanol and into 2 when few drops of triethylamine are added to
the methanol solution (see Scheme 2). From these experimental
observations, it is clear that compound 1 crystallizes from neutral
methanolic solution, 2 from a solution containing triethylamine
and 3 in presence of excess azide ions. It is to be noted that com-
5105 reflections with I > 2r(I) for complexes 1 and 2, respectively.
3. Results and discussion
3.1. Synthesis of the complexes and their inter-conversion in solution
The mono-condensed tridentate Schiff base ligand HL and its
hydroxo-bridged trinuclear complex obtained with Cu(II) and
ClOꢁ4 anion, has already been reported by us very recently [35].
NH
Me
N
HC
OH
HL
C
u
In methanol at r.t.
(
C
l
O
Cu2+: HL : A = 1:1:2
)
n
i
.
6
)
H
A
(
i
O
n
N
+
o
a
m
C
N
a
N
N
N
e
t
u
a
+
h
a
.
N
t
:
.
n
O
r
H
(
t
A
1
:
l
L
H
a
)
6
1
l
.
:
:
t
+
N
o
O
Cu
1
A
r
.
)
N
n
E
t
a
=
.
N
=
t
h
O
l
t
A
N
1
e
C
:
:
(
1
m
u
Cu
O
:
L
1
N
C
:
N
3
u
R
e
C
N
c
N
r
y
m
s
n
t
o
e
a
i
t
t
h
l
l
l
i
a
o
za
z
M
i
n
n
at
l
l
a
o
e
t
i
a
h
l
o
t
+
t
h
s
+
n
e
a
N
y
r
E
n
f
r
m
aN
o
t
o
ec
f
N
l
m
m
N
N
R
o
s
r
o
.
l
n
N
N
.
N
N
O
N
O
N
.H2O
Recrystallization from methaol + Et3N
Recrystallization from methanol
N
N
Cu
O
Cu
Cu
O
Cu
N
N
N
N
N
N
N
N
N
N
N
2
1
= L-
N
(Asymmetric dimer)
(Centrosymmetric dimer)
O
Heated at
60°C in vacuum
Scheme 2. Synthetic route of 1–3.

S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
29
Fig. 1. (a) Structure of 1 with ellipsoids at 30% probability. Hydrogen bonds shown as dotted lines. (b) Polymeric 1D structure of 1 formed by the interdimer C–H/
p
interactions, shown as blue coloured dotted lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
pound 3 has been synthesised recently using ethanol as solvent
and a similar compound to 1 in which methanol was the solvent
molecule, has been obtained using methanol as solvent [28].
compound 2 upon dehydration undergoes irreversible solid state
transformation into 1.
Solid state transformation from one state to other is a fast-
emerging topic in chemical sciences especially in solid-state syn-
thesis and crystal engineering. Such a phenomenon has potential
applications in catalysis, magnetism, and in the design of solid-
state sensors. However, inorganic coordination complexes showing
crystal-to-crystal transformations are scanty because these trans-
formations imply the cooperative movement of atoms or mole-
cules in the solid state, leading to the loss of structural identity.
Most of the known examples are 3D and 2D coordination polymers
with high stability and porous crystalline structures in which the
crystal transitions are usually triggered by light, temperature,
guest desorption/adsorption or ion exchange [40–46]. In the pres-
ent case, although the single crystalinity of 2 is lost on dehydration,
we were able to get the crystals of 2 from solution by varying the
reaction conditions.
3.2. Solid state transformation of 2 to 1
Transformation of compound 2 to 1 takes place not only in solu-
tion but also in the solid state. From the X-ray structure it is found
that the guest water molecule is stabilized by hydrogen bonds in
between two dimeric entities in compound 2. Upon heating at
60 °C in a high vacuum autoclave, bluish green crystals of 2 turn
into deep blue (Figure S1). However, on heating, the single crystal
of 2 turns opaque after losing one water molecule and it does not
diffract indicating that the single crystallinity is not retained on
thermal desolvation. The powder XRD data (Figure S2) indicate
that complex 2 on heating transform to 1 on losing the solvated
water molecule. The thermogravimetric analysis shows a weight
loss of 3.0% in the temperature range 60–90 °C, corresponding to
the loss of one crystallization water molecule (calc. 2.9%) (Fig-
ure S3). The dehydrated sample on exposure to open atmosphere
does not reabsorb the water molecule. Therefore, it is clear that
3.3. IR and UV–Vis spectra of complexes
Spectroscopic data and their assignments are given in Section 2.
The IR spectra of the complexes are similar and show a strong and
sharp peak for m
(C@N) at 1620, 1625 and 1624 cmꢁ1 for complexes
Table 2
Bond distances (Å) and angles (°) for compounds 1 and 2.
1–3, respectively, indicating the presence of the Schiff base. The
N–H stretching mode is seen at 3181, 3244 and 3245 cmꢁ1 as a
sharp band for complexes 1–3, respectively. The peaks at 2045,
2044 and 2043 cmꢁ1 in the IR spectra of 1–3, respectively, appear
1
2
X = N(1)0
Around Cu(1)
X = N(4)
Around Cu(2)a
X = N(4)
due to the stretching of the l-1,1 single or double azide bridges. In
Bond distances
Cu(1)–O(11)
Cu(1)–N(19)
Cu(1)–N(23)
Cu(1)–N(1)
Cu(1)–X
the IR spectra of complex 2, the appearance of a broad band near
3436 cmꢁ1 indicates the presence of a hydrogen bonded water
molecule, consistent with the single-crystal X-ray diffraction
results. The electronic spectra in methanolic solution of the three
complexes display a single absorption band at 606, 599 and
1.924(2)
1.997(2)
2.021(2)
2.059(2)
2.338(2)
1.908(2)
1.979(2)
2.023(2)
2.354(2)
2.060(2)
1.910(2)
1.969(2)
2.033(2)
2.339(2)
2.045(2)
Bond angles
O(11)–Cu(1)–N(19)
O(11)–Cu(1)–N(23)
O(11)–Cu(1)–X
O(11)–Cu(1)–N(1)
N(19)–Cu(1)–N(23)
N(19)–Cu(1)–X
N(19)–Cu(1)–N(1)
N(23)–Cu(1)–X
N(23)–Cu(1)–N(1)
N(1)–Cu(1)–X
90.95(9)
172.12(8)
89.84(8)
86.46(9)
96.68(9)
109.26(8)
162.52(9)
89.49(8)
85.67(9)
88.04(9)
91.95(9)
90.85(7)
166.46(7)
96.56(7)
86.50(7)
93.93(8)
108.42(7)
171.69(8)
93.94(7)
86.98(8)
79.73(7)
97.87(8)
92.39(7)
159.30(7)
100.78(7)
89.33(7)
92.09(8)
102.20(7)
176.56(8)
97.99(7)
85.27(8)
80.40(7)
97.93(9)
Table 3
Hydrogen bonds parameters (in
A = acceptor).
Å and °) in compounds 1 and 2 (D = donor,
Dꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA
<D–Hꢀ ꢀ ꢀA
Symmetry
element for A
Compound 1
N(23)–H. . .O(11)
2.982(3)
2.11
161
1 ꢁ x, 2 ꢁ y, 1 ꢁ z
2 ꢁ x, 2 ꢁ y, 1 ꢁ z
Cu(1)–N(1)–Cu(2)
Compound 2
N(43)–H. . .O(11)
O(100)–H(1). . .N(6)
O(100)–H(2). . .O(31)
N(23)–H. . .O(100)
2.914(2)
3.107(3)
2.722(2)
2.978(3)
2.21
2.34
1.90
2.10
134
154
173
162
0
Symmetry code, = 1 ꢁ x, 2 ꢁ y, 1 ꢁ z.
a
Dimensions around Cu(2) are given alongside the equivalent dimensions
around Cu(1). The coordination sphere around Cu(2) contains atoms O(31), N(39),
N(43), N(4) and N(1), respectively.

30
S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
Fig. 2. (a) The Cu(II) dinuclear structure of 2 with ellipsoids drawn at 30% probability. (b) The hydrogen bonding network. Hydrogen bonds are shown as dotted lines.
606 nm for complexes 1–3, respectively. These spectra are typical
of a square-based environment for Cu(II) [47].
The hydrogen bond between N(43) and O(11), is however main-
tained with a distance of 2.914(2) Å comparable to that in 1. The
Addison parameters for Cu1 and Cu2 are 0.09 and 0.29, respec-
tively. As in compound 1, the basal planes of the Cu(II) ions are
formed by the N,N,O-Schiff base ligand with Cu–O and Cu–N bond
distances very similar to those of compound 1 (see Table 2).
The azide ions are nearly linear with N(1)–N(2)–N(3) and N(4)–
N(5)–N(6) angles of 178.6(2)° and 177.7(3)°, respectively. The four
donor atoms in the basal plane show tetrahedral distortions with
r.m.s. deviations of 0.034 and 0.149 Å from planarity for Cu(1)
and Cu(2), respectively, being the copper atoms 0.177(1) and
0.195(1) Å above the averages planes in the direction of the axial
bonds. The dihedral angle between the two basal planes in 2 is
10.7(1)°, higher than in compound 1 (where the angle is 0.0° since
the compound is centrosymmetric). Another interesting difference
between compounds 1 and 2 is that in 2 the central Cu2N2 unit is
not planar (in contrast to 1). The deviations of the four atoms from
the mean plane passing through Cu(1)–Cu(2)–N(1)–N(4) are 0.144,
0.146, ꢁ0.145 and ꢁ0.145 Å, respectively. As a consequence of this
asymmetry, the two bridging Cu–N–Cu angles and the four Cu–N
distances are slightly different (Table 2).
3.4. Description of the structures
3.4.1. Structure of [Cu2L2(N3)2] (1)
The structure of 1 consists of a centrosymmetric Cu(II) dimer
with a double l-1,1-azido bridge (Fig. 1). Dimensions in the metal
coordination sphere are given in Table 2.
The Cu(II) ion shows a five-coordinate square pyramidal envi-
ronment with the tridentate ligand and with one bridging azide
in the basal plane and a further bridging azide in an axial site. Each
azide ion bridges the Cu(II) ions in basal-apical,
l-1,1 (end-on)
fashion. The di- -1,1-azido bridging in these complexes leads to
l
a perfectly planar Cu2N2 ring as the dimer sits on a crystallographic
inversion centre with a Cu–N–Cu bond angle of 91.96(9)°. The four
donor atoms in the basal plane exhibit a tetrahedral distortion with
an r.m.s. (root means square) deviation of 0.146 Å from planarity
with the copper atom located 0.159(1) Å above the average plane,
towards the axial bond. The bond length involving the axial azide is
2.338(2) Å, much longer than that of the basal azide 2.059(2) Å.
Bond lengths to the ligand are Cu(1)–O(11) = 1.924(2) Å,
Cu(1)–N(19) = 1.997(2) Å and Cu(1)–N(23) = 2.021(2) Å. The Addi-
3.4.3. Structure of [CuL(N3)]n (3)
son parameter value for complex 1 (
torted square pyramidal structure for the Cu(II) ion. (
as |b ꢁ |/60 where b and are the two trans-basal angles, with
= 0 and 1 for perfect square pyramid and trigonal bipyramid
s
= 0.16), confirms the dis-
As mentioned earlier, structure of 3 has already been reported.
So we do not describe the structural part here. The ORTEP diagram
(S4) and the table (S5) for bond angle and distances are presented
as supporting information.
s
is defined
a
a
s
geometries, respectively) [48]. The coordinated azide ion is nearly
linear with a N(1)–N(2)–N(3) angle of 178.5(5)°. The formation of
the centrosymmetric dimer is facilitated by hydrogen bonds from
N(23)–H to O(11) across the centre of symmetry with Nꢀ ꢀ ꢀO =
1.0
2.982(3) Å (Fig. 1b, Table 3). In addition, a C–H/p non-covalent
0.8
interaction is established between the H(20B) atom of the Schiff
base ligand belonging to one dinuclear unit and the aromatic ring
of the neighbouring one (1 ꢁ x, 1 ꢁ y, 1 ꢁ z) with dimensions
0.16
0.14
0.6
Hꢀ ꢀ ꢀCg = 2.63 Å, gamma angle (
c) = 6.29° to form a supra-molecular
0.12
1D architecture (Fig. 1b).
0.4
0.10
3.4.2. Structure of [Cu2L2(N3)2(H2O)] (2)
0.08
The structure of 2 is shown in Fig. 2a together with the atomic
numbering scheme. This compound has the same overall structure
as 1 but the dimer is asymmetric and there is one water molecule
per Cu(II) dimer. This water molecule is inserted within the dimer
forming a hydrogen bond (as a donor) to O(31) and as an acceptor
with N(23) (Fig. 2b, Table 3). Bond distances and angles are listed
in Table 2. The presence of the water molecule means that there
is no direct hydrogen bond between N(23) and O(31). Indeed the
distance has increased to 4.279(4) Å compared to 2.982(3) Å in 1.
0.2
0.06
2
3
4
5
6
7
8
9
10
T (K)
0.0
0
50
100
150
200
250
300
T (K)
Fig. 3. Thermal variation of the vmT product per Cu(II) dimer for compound 1. Inset
shows the thermal variation of vm in the low temperature region. Solid line shows
the best fit to the S = 1/2 dimer model (see text).

S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
31
Fig. 4. Coordination environment of the Cu(II) ions in compounds (a) 1, (b) 2 and (c) 3 showing the bridging bond distances (in Å) and angles (in degrees). Colour code:
Cu = green, N = blue, O = pink. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 4
Structural and magnetic parameters of compounds 1–3.
Structure
Bridge
Cu–N (Å)
Cu–N–Cu (°)
g
J (cmꢁ1
)
1
2
Cu-dimer
Cu-dimer
Double 1,1-N3 (asymmetric)
Double 1,1-N3 (asymmetric)
2.059/2.338
2.045/2.355
2.060/2.339
2.028/2.459
91.95(9)
97.87(8)
97.93(9)
104.4(4)
2.0999
2.0666
ꢁ2.59
ꢁ0.10
3
Cu-chain
Single 1,1-N3 (asymmetric)
2.0931
ꢁ0.01
It is noteworthy to recall that although there are ca. 30 Cu(II)
l-
8 T, not shown) and shows a value at 8 T of ca. 1.6
lB per Cu(II) di-
1,1-azido bridged dimers reported in the literature, only the re-
cently reported [28] complex related to 2 and compound 2 are
non-centrosymmetric, being all the other centrosymmetric, as
mer, below the expected one for two isolated Cu(II) ions with g = 2.
In fact, the Brillouin function for two isolated Cu(II) ions cannot
reproduce satisfactorily the isothermal magnetization unless the
temperature, T is replaced by a reduced T ꢁ h term to account for
the aniferromagnetic intra-dimer coupling (solid line in Fig. 5). In
contrast, compound 2 shows an isothermal magnetization at 2 K
close to the expected one for a paramagnetic S = 1/2 dimer with
a saturation value close to 2.0 lB per Cu(II) dimer and can be very
well reproduced with a Brillouin function for two isolated S = 1/2
ions (solid line in Fig. 5).
compound 1 [25–27,29,30]. On the other hand, single l-1,1 (end-
on) azido bridged Cu(II) complexes, like 3, are also rare. A search
in the CCDC database (updated Feb. 2011) shows the presence of
only three such complexes reported to date. One is a trimer [48]
and the other two are 1-D chains, as indeed is complex 3 [49–51].
3.5. Magnetic properties
The product of the molar magnetic susceptibility per Cu(II) ion
The thermal variation of the molar magnetic susceptibility
times the temperature (vmT) for compound 3 shows at room
per two Cu(II) ions times the temperature (
vmT) for compounds
temperature a value of ca. 0.41 emu K molꢁ1, which is the expected
1 and 2 are very similar: they show values of ca. 0.82 and
0.80 emu K molꢁ1 at room temperature for 1 and 2, respectively,
the expected ones for two isolated Cu(II) S = 1/2 ions with
g = 2.09 and 2.06, respectively (Figs. 3 and 6). When cooling down
value for one isolated Cu(II) S = 1/2 ions with g = 2.09 (Fig. 7).
When cooling down the sample, the vmT product remains constant
down to very low temperatures, indicating that compound 3 is also
essentially paramagnetic, as confirmed by the fit of the magnetic
properties to a simple S = 1/2 regular antiferromagnetic chain
model [54] in agreement with the structure of 3 (Fig. 4c). This
the sample, the
vmT product remains constant down to ca. 20 and
10 K for 1 and 2, respectively and below these temperatures,
vmT
shows a smooth and progressive decrease, reaching values of ca.
0.32 and 0.78 emu K molꢁ1 at 2 K for 1 and 2, respectively. This
behaviour indicates that compounds 1 and 2 present very weak
antiferromagnetic coupling, responsible of the decrease observed
at low temperatures (although that of compound 1 must be stron-
ger than in compound 2). In compound 1 this weak antiferromag-
2.0
2
1.5
netic coupling is also observed in the thermal variation of
shows a rounded maximum at ca. 3 K (inset in Fig. 3).
vm that
1
Since compounds 1 and 2 present isolated Cu(II) dimers with a
double asymmetric 1,1-N3 bridge (Fig. 4a and 4b), we have used a
simple Bleaney–Bowers dimer model for two S = 1/2 ions to fit the
magnetic data [52,53]. This model reproduces very satisfactorily
the magnetic properties of both compounds in the whole temper-
ature range (solid lines in Figs. 3 and 6) with g = 2.0999(2) and
J = ꢁ2.59(4) cmꢁ1 for 1 and g = 2.0666(3) and a negligible J value
of ꢁ0.10(1) cmꢁ1 for 2 (Table 4, the Hamiltonian is written as
H = ꢁJS1S2).
1.0
3
0.5
0.0
0.0
0.5
1.0
1.5
-1
2.0
2.5
H/T (T.K
)
The isothermal magnetization measurements at 2 K (Fig. 5) con-
firm the presence of a weak antiferromagnetic coupling in com-
pound 1 and an almost negligible one in compound 2. Thus, at
2 K, compound 1 has not reached saturation at 5 T (nor even at
Fig. 5. Isothermal magnetizations at 2 K for compounds 1–3. Solid lines are the best
fit to the modified Brillouin function for the corresponding S = 1/2 ions (two for 1
and 2 and one for 3).

32
S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
simple model reproduces quite satisfactorily the magnetic data of
compound 3 in the whole temperature range (solid line in Fig. 7)
with g = 2.0931(1) and a negligible antiferromagnetic coupling, J =
ꢁ0.010(4) cmꢁ1 (Table 4, the Hamiltonian is written as H = ꢁJS1S2).
The isothermal magnetization of compound 3 at 2 K shows a
saturation value close to 1.0 B, the expected value for an S = 1/2
l
Cu(II) ion with a g value close to 2.0 and it can be very well repro-
duced with a Brillouin function for an S = 1/2 ion, confirming the
paramagnetic behaviour of compound 3 (solid line in Fig. 5).
The magnetic couplings in compounds 1–3 can be very
well rationalized with the magneto-structural correlations and
theoretical calculations performed for single and double asymmet-
ric 1,1-N3 bridges [27]. Thus, DFT calculations show that the main
parameter governing the magnetic coupling through asymmetric
double 1,1-N3 bridges (as observed in 1 and 2) is the long Cu–N
bond distance. These calculations indicate that for compounds 1
and 2, where the 1,1-N3 bridges connect an axial position of one
Cu(II) ion with a basal position of the other one, the expected cou-
pling should be antiferromagnetic and very weak, in agreement
with the experimental J values. If we compare both structures in
order to explain the differences found in the coupling constants,
we can see that the Cu(II) ions in both complexes present slightly
distorted square pyramidal geometries with an Addison parameter
0.85
0.80
0.75
0.70
0
50
100
150
200
250
300
in compound 1 (
s
= 0.16) which is in between the
s values of Cu(1)
and Cu(2) in compound 2 (
s
= 0.09 and 0.29). Furthermore, the 1,1-
T (K)
N3 bridges connect in both cases an axial position of one Cu(II) ion
with a basal position of the other one and the Cu–N bond distances
are also very similar (Table 4 and Fig. 4a and b). In fact, the only
significant difference is the Cu–N–Cu bond angle. Thus, in com-
pound 2 the Cu–N–Cu bond angles are much larger (97.87° and
97.93°) than the corresponding one in compound 1 (91.95°). This
situation leads to a better overlap of the orbitals in compound 1
and, therefore, to a larger antiferromagnetic coupling in compound
1, in agreement with the experimental results.
Unfortunately, for the single asymmetric 1,1-N3 bridges there
are neither magneto-structural correlations nor calculations since
there are few examples and these examples do not show any cor-
relation between the magnetic coupling and any structural param-
eter (in particular neither Cu–N bond distances nor Cu–N–Cu bond
angles, Table 5). In any case, all the reported examples show a
weak magnetic coupling (mainly antiferromagnetic, with J values
in the range ꢁ0.12 and ꢁ4.06 cmꢁ1, except in one case, where
J = ꢁ11.4 cmꢁ1, probably due to an unusual short Cu–N long dis-
tance of 2.276 Å, compared to 2.343–2.598 Å in all the other exam-
ples, see Table 5). In summary, the negligible antiferromagnetic
coupling found in compound 3 is within the expected range for this
type of 1,1-N3 bridge even if the exact value of this coupling is dif-
ficult to predict from structural parameters.
Fig. 6. Thermal variation of the vmT product per Cu(II) dimer for compound 2. Solid
line shows the best fit to the S = 1/2 dimer model (see text).
0.414
0.412
0.410
0.408
0.406
0
50
100
150
200
250
300
T (K)
Fig. 7. Thermal variation of the
vmT product per Cu(II) ion for compound 3. Solid
line shows the best fit to the S = 1/2 chain model (see text).
Table 5
Magnetic and structural data of the known compounds presenting Cu(II) ions connected by single asymmetric
l
-1,1-N3 bridges.
Compound (CCDC code)
FISFAP
Positions
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Ax(C4v
Eq(C4v
Cu–N (Å)
Cu–N–Cu (°)
J (cmꢁ1
)
Reference
[55]
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
2.305(4)-ax
1.991(5)-eq
2.343(4)-ax
2.044(4)-eq
2.416(2)-ax
1.961(2)-eq
2.598(2)-ax
1.951(2)-eq
2.341(2)-ax
1.992(2)-eq
2.480(4)-ax
1.994(4)-eq
2.394(5)-ax
1.990(4)-eq
2.276(3)-ax
2.007(2)-eq
2.342(9)-ax
1.983(9)-eq
2.459(5)-ax
2.028(5)-eq
117.4(2)
+2.88
ꢁ4.06
ꢁ2.2
FODQIZ
130.78(19)
113.6(1)
[56]
[57]
[58]
[17]
[58]
GAMVOG
GAMVUM
IQASEY
107.01(10)
116.60(9)
126.33(19)
125.50(19)
129.98(14)
131.6(4)
ꢁ3.7
+14.1
ꢁ0.21
ꢁ0.12
ꢁ11.5
+1.91
ꢁ0.01
KAKTOG
VEVRET
WELPEJ
3
[59]
)
)
)
)
[60]
104.4(4)
This work

S. Naiya et al. / Inorganica Chimica Acta 377 (2011) 26–33
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