Synthesis, crystal structures and magnetic properties of two bis(μ-phenoxido)dicopper(II) complexes derived from reduced Schiff base ligands

Article abstract of DOI:10.1016/j.ica.2011.09.026

Two phenoxido bridged dinuclear Cu(II) complexes, [Cu₂(L ¹)₂(NCNCN)₂] (1) and [Cu₂(L ²)₂(NCNCN)₂]·2H₂O (2) have been synthesized using the tridentate reduce

Full text of DOI:10.1016/j.ica.2011.09.026

Inorganica Chimica Acta 379 (2011) 28–33
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structures and magnetic properties of two
bis(l-phenoxido)dicopper(II) complexes derived from reduced Schiff base ligands
Apurba Biswas a, Michael G.B. Drew b, Joan Ribas c, Carmen Diaz c, Ashutosh Ghosh a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 70009, India
b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG 66AD, UK
c Departament de Química Inorgànica, Universitat de Barcelona, Marti i Franques, 1-11, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Two phenoxido bridged dinuclear Cu(II) complexes, [Cu2(L1)2(NCNCN)2] (1) and [Cu2(L2)2(NCNCN)2]ꢀ2H2O
(2) have been synthesized using the tridentate reduced Schiff-base ligands 2-[1-(2-dimethylamino-ethyl-
amino)-ethyl]-phenol (HL1) and 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL2), respectively.
The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic
susceptibility measurements. Both the complexes present a diphenoxido bridging Cu2O2 core. The geome-
tries around metal atoms are intermediate between trigonal bipyramid and square pyramid with the Addi-
Article history:
Received 4 November 2010
Received in revised form 7 September 2011
Accepted 13 September 2011
Available online 21 September 2011
Keywords:
Copper(II)
Reduced Schiff base
Crystal structure
Magnetic properties
Antiferromagnetic coupling
son parameters (s) = 0.57 and 0.49 for 1 and 2, respectively. Within the core the Cu–O–Cu angles are 99.15°
and 103.51° and average Cu–O bond distances are 2.036 and 1.978 Å for compounds 1 and 2, respectively.
These differences have marked effect on the magnetic properties of two compounds. Although both are
antiferromagnetically coupled, the coupling constants (J = ꢁ184.3 and ꢁ478.4 cmꢁ1 for 1 and 2, respec-
tively) differ appreciably.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
In this paper we report the syntheses, crystal structures, and mag-
netic properties of two phenoxido bridged dinuclear Cu(II) complexes,
Dinuclear copper(II) complexes with endogenous bridging phe-
noxido ligands are of ongoing interest due to their interesting mag-
netic properties [1,2]. These Cu(II) complexes also show
remarkable non-magnetic properties, making them useful in metal
extraction in hydrometallurgy [3], fluorescent sensors [4], hetero-
geneous catalysis [5], and show catechol oxidase activity [6]. The
magnetic properties of bridged dinuclear copper(II) complexes
have been extensively studied, most of them having a five-coordi-
nate geometry around each copper ion [2,7]. Five-coordinate Cu(II)
complexes are stereochemically flexible and they can be square
pyramidal or trigonal bipyramidal, or almost anything in between.
A considerable body of experimental evidence has been accumu-
lated to elucidate the magnetostructural relationship. The mag-
netic properties of phenoxido-bridged complexes containing the
Cu2O2 core depend on the structural properties of the core. Factors,
such as the coordination geometry of the copper ions, the Cu–O–Cu
angle, the Cu–O bond distances, the Cuꢀ ꢀ ꢀCu distances, the out-of-
[Cu2(L1)2(NCNCN)2] (1) and [Cu2(L2)2(NCNCN)2]ꢀ2H2O (2) obtained
from the reduced Schiff base ligands 2-[1-(2-dimethylamino-ethyl-
amino)-ethyl]-phenol (HL1) and 2-[1-(3-methylamino-propylamino)-
ethyl]-phenol (HL2), respectively. The geometry around copper ion is
intermediate between square pyramid and trigonal bipyramid for both
the complexes. However, they show appreciable differences in bridg-
ing Cu–O distances and Cu–O–Cu angles. These differences are well re-
flected in their magnetic properties.
2. Experimental
The reagents and solvents used were of commercially available
reagent quality.
2.1. Synthesis of the reduced Schiff base ligands (HL1)
2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol and (HL2)
2-[1-(3-methylamino-propylamino)-ethyl]-phenol
plane shift of the phenyl group, Addison parameter (s) and torsion
angle are the parameters that have also been postulated to influ-
The Schiff base ligand was synthesized by refluxing a solution of
2-hydroxyacetophenone (0.60 mL, 5 mmol) and N,N-dimethyleth-
ylenediamine (0.54 mL, 5 mmol) in methanol (30 mL) for 1 h [8,9].
The solution was cooled to 0 °C and solid sodium borohydride
(210 mg, 6 mmol) was added slowly to this methanolic solution
with stirring. After completion of the addition, the resulting solution
ence the J values of the spin coupling [2,7].
⇑
Corresponding author.
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.2011.09.026

A. Biswas et al. / Inorganica Chimica Acta 379 (2011) 28–33
29
Table 1
was acidified with concentrated HCl (5 mL) and then evaporated to
dryness [10,11]. The reduced Schiff base ligand HL1 was extracted
from the solid mass with methanol and this methanol solution (ca.
20 mL) was used for preparation of the complexes. HL2 was synthe-
sized in the same way as HL1 using N-methyl-1,3-propanediamine
(0.52 mL, 5 mmol) instead of N,N-dimethylethylenediamine [12].
Crystal data and structure refinement of complexes 1 and 2.
1
2
Formula
M
Crystal system
Space group
a (Å)
C
28H38Cu2N10O2
C28H42Cu2N10O4
709.80
monoclinic
P21/n
11.1055(10)
11.3769(9)
13.166(2)
90
102.410(16)
90
1624.6(3)
2
1.451
1.358
740
0.080
10641
4723
2072
0.0549, 0.1273
150
673.76
triclinic
P1
ꢀ
9.1566(4)
9.5443(4)
9.8732(4)
104.129(4)
108.360(4)
97.700(4)
773.03(6)
1
1.447
1.419
350
0.016
5465
4309
3201
0.0386, 0.0857
150
2.2. Synthesis of the complexes [Cu2(L1)2(NCNCN)2] (1) and
[Cu2(L2)2(NCNCN)2]ꢀ2H2O (2)
b (Å)
c (Å)
a
(°)
An extracted methanol solution of HL1 as prepared above was
added to a solution of CuCl2ꢀ2H2O (0.850 g, 5.00 mmol) in metha-
nol (20 mL) and an aqueous solution (1 mL) of sodium dicyanam-
ide (0.450 g, 5.00 mmol) was added to this mixture with stirring.
The mixture was stirred for 1 h and filtered. The filtrate was kept
undisturbed at room temperature. Green crystals of 1 suitable for
X-ray diffraction were obtained on standing overnight in air. Com-
plex 2 was synthesized as the same way of 1 by using HL2 instead
of HL1. Green crystals of 2 suitable for X-ray diffraction were ob-
tained after 1 day on slow evaporation of the solvent.
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
F(000)
Rint
Total reflections
Unique reflections
[I > 2
R1, wR2
T (K)
r
(I)]
2.2.1. Complex 1
(Yield: 1.3140 g, 78%), Anal. Calc. for C28H38Cu2N10O2: C, 49.91; H,
0.0857, 0.1273, for 3201, 2072 reflections for 1, 2, respectively with
I > 2 (1).
5.68; N, 20.79. Found: C, 49.85; H, 5.75; N, 20.75%. IR (KBr):
m(N–H),
r
3180 cmꢁ1 (C–N), 1594 cmꢁ1; kmax (nm), [emax (dm3 molꢁ1 cmꢁ1)]
,
m
(methanol), 638 (358), 422 (1683).
3. Results and discussion
2.2.2. Complex 2
3.1. Synthesis of the complexes
(Yield: 1.330 g, 75%), Anal. Calc. for C28H42Cu2N10O4: C, 47.38; H,
5.96; N, 19.73. Found: C, 47.31; H, 5.99; N, 19.66%. IR (KBr): (N–H),
3187 cmꢁ1 (C–N), 1592 cmꢁ1; kmax (nm), [ max (dm3 mol ꢁ1 cmꢁ1)]
m
The condensation of N,N-dimethylethylenediamine and
N-methyl-1,3-propanediamine in 1:1 M ratio with 2-hydroxyace-
tophenone afforded the Schiff bases, 2-[1-(2-dimethylamino-
ethylimino)-ethyl]-phenol and 2-[1-(3-methylamino-propylimi-
no)-ethyl]-phenol, respectively which on reduction with sodium
borohydride readily produced the reduced Schiff bases, HL1 and
HL2 (Scheme 1). HL1 and HL2 on reaction with copper(II) chloride
in presence of sodium dicyanamide in 1:1:1 M ratios yielded com-
pound 1 and 2, respectively (Scheme 1).
,
m
e
(methanol), 653 (391), 422 (2114).
2.3. Physical measurements
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 (1200–350 nm) were
recorded in a Hitachi U-3501 spectrophotometer. The magnetic
measurements were carried out in the ‘‘Unitat de mesures magnè-
tiques dels SCT (Universitat de Barcelona)’’ on polycrystalline sam-
3.2. IR and electronic spectra
A moderately strong, sharp peak due to N–H stretching vibra-
tion at 3180, and 3187 cmꢁ1 for complexes 1 and 2, respectively
shows that the imine group of the Schiff base is reduced. Both
the complexes exhibit several mC„N bands at 2275, 2227,
2163 cmꢁ1 for 1 and 2268, 2220, 2153 cmꢁ1 for 2 which are attrib-
ples (20 mg) with
a
Quantum Design SQUID MPMSXL
magnetometer in an applied field of 10000 and 500 G in the temper-
ature ranges of 2–300 and 2–30 K, respectively. The diamagnetic
corrections were evaluated from Pascal’s constants.
uted to msym
+ masym (CN), masym (CN) and msym (CN) modes of the
2.4. Crystal data collection and refinement
bridging dicyanamide ligand, respectively [16,17].
The electronic spectra of these two compounds were recorded
in methanol solution. The electronic spectra show a single absorp-
tion band at 638 and 653 nm for compounds 1 and 2, respectively.
At higher energy region, the ligand to metal charge transfer bands
were located at 422 nm for the compounds.
Crystal data for the two crystals are given in Table 1. 4309, 4723
data for 1 and 2, respectively were collected with MoKa radiation
at 150 K using the Oxford Diffraction X-Calibur CCD System. The
crystals were positioned at 50 mm from the CCD. Three hundred
and twenty-one frames were measured with a counting time of
10 s. Data analyses were carried out with the CRYSALIS program
[13]. The structures were solved using direct methods with the
SHELXS-97 program [14]. The non-hydrogen atoms were refined
with anisotropic thermal parameters. The hydrogen atoms bonded
to carbon and nitrogen were included in geometric positions and
given thermal parameters equivalent to 1.2 times those of the
atom to which they were attached. The hydrogen atoms of the sol-
vent water molecules in 2 were located in a difference Fourier
maps and refined with distance constraints. Absorption corrections
were carried out using the ABSPACK program [15]. The structures
were refined on F2 with SHELXS-97 [14] to R1, 0.0386, 0.0549; wR2,
3.3. Description of structures of complexes 1 and 2
The structure of [Cu2(L1)2(NCNCN)2] (1) is a centrosymmetric
dimer as shown in Fig. 1 with each metal atom in a five-coordinate
environment. Dimensions in the metal coordination sphere are gi-
ven in Table 2.
The metal atom is bonded to the tridentate ligand L1 via O(11)
at 2.128(2), N(19) at 1.986(2) and N(22) at 2.072(2) Å, together
with a bridging oxygen atom O(11)a, (a = 1 ꢁ x, 1 ꢁ y, 1 ꢁ z) from
a second ligand at 1.945(2) Å and a terminal NCNCN ligand via

30
A. Biswas et al. / Inorganica Chimica Acta 379 (2011) 28–33
Table 2
(CH2)n
H3C
Bond distances (Å) and angles (°) for complexes 1 and 2.
CH3
C
N
N
1 (n = 2)
2 (n = 3)
Cu(1)–O(11)a
Cu(1)–N(19)
Cu(1)–N(1)
Cu(1)–N(2n)
Cu(1)–O(11)
1.945(2)
1.986(2)
2.048(2)
2.072(2)
2.128(2)
2.005(2)
2.031(3)
2.246(4)
2.007(3)
1.952(2)
R
OH
NaBH4
(CH2)n
H3C
O(11)a–Cu(1)–N(19)
O(11)a–Cu(1)–N(1)
N(19)–Cu(1)–N(1)
O(11)a–Cu(1)–N(2n)
N(19)–Cu(1)–N(2n)
N(1)–Cu(1)–N(2n)
O(11)a–Cu(1)–O(11)
N(19)–Cu(1)–O(11)
N(1)–Cu(1)–O(11)
N(2n)–Cu(1)–O(11)
169.48(7)
96.39(7)
91.11(8)
94.61(7)
85.14(8)
135.31(8)
80.85(6)
90.17(7)
104.29(7)
120.20(7)
145.47(12)
100.99(11)
111.01(13)
99.88(11)
93.66(13)
88.65(13)
76.49(10)
91.31(11)
88.57(12)
174.92(12)
CH3
CH NH
OH
N
R
HL
CuCl2.2H2O
NaN(CN)2
HL:Cu2+:N(CN)2- = 1:1:1
1)
2)
(
)
Symmetry operation: a = (1 ꢁ x, 1 ꢁ y, 1 ꢁ z) in 1 and a = (ꢁx, 2 ꢁ y, ꢁz) in 2.
H3C
H3C
CH3
N
O
CH NH
O
HN
CH3
NCNCN
CH NH
O
CH3
NCNCN
mean plane through them are ꢁ0.4504(19), 0.4980(19),
ꢁ0.4845(19), 0.4369(15) Å, respectively. The deviation of Cu(II)
from the same plane is 0.3023(3) Å in the direction of chelating
oxygen atom O(11) which may be considered as axially coordi-
nated. However, the geometry around Cu(II) can be described bet-
ter as trigonal bipyramid with O(11), N(1), and N(22) make up the
equatorial plane which together with Cu(II) provide a r.m.s. devia-
Cu
Cu
Cu
Cu
O
NCNCN
H3C
NCNCN
NH CH
CH3
1 (n = 0, R = CH3)
N
NH CH
CH3
H3C
NH
H3C
tion of 0.0388(3) Å. The Addison parameter (s) of the penta-coordi-
2 (n = 1, R = H)
nated Cu(II) is 0.57 indicating that the geometry is a distorted
trigonal bipyramid with N(19) and O(11)a occupying axial posi-
tions with the trans N(19)–Cu–O(11)a angle of 169.48(7)°. The tri-
gonal bipyramidal geometry also explain the apparently
anomalous shorter Cu–O(11)a bond length in this compound, as
for the d9 system the axial bonds are longer in square pyramidal
or octahedral geometry but are shorter in trigonal bipyramidal
arrangement. While there is no short contact between N(5) and
metal atoms, N(5) does act as acceptor to form an intermolecular
hydrogen bond. Thus the proton on N(19) forms a hydrogen bond
to N(3)b (b = ꢁx, 1 ꢁ y, 1 ꢁ z) with dimensions (Table 3) Nꢀ ꢀ ꢀN
3.086(3) Å, Hꢀ ꢀ ꢀN 2.18 Å and N–Hꢀ ꢀ ꢀN 171° to result in a zig-zag
1-D supramolecular structure (Fig. 2).
Scheme 1. Formation of the complexes.
The structure of [Cu2(L2)2(NCNCN)2]ꢀ2H2O (2) contains a centro-
symmetric binuclear dimer with solvent water molecules shown in
Fig. 3.
The metal coordination spheres show the same bonding pattern
as 1, but the geometry is very different as indicated by the bond
angles compared in Table 2. The metal atom is bonded to the tri-
dentate ligand L2 which has one extra methylene group compared
to L1, via O(11) at 1.952(2), N(19) at 2.031(3) and N(23) at
2.007(3) Å, together with a bridging oxygen atom O(11)a (a = ꢁx,
2 ꢁ y, ꢁz) from a second ligand at 2.005(2) Å and a terminal
NCNCN ligand via N(1) at 2.246(4) Å. The two Cu atoms are sepa-
rated by 3.108 Å and the Cu(1)–O(11)–Cu(1)a angle is
103.52(11)°. If the geometry around Cu(II) is considered as square
pyramidal, the deviations of the coordinating atoms O(11), N(19),
N(23), O(11)a from the least-square mean plane through them
are 0.336(2), ꢁ0.276(3), 0.253(3), ꢁ0.313(2) Å, respectively. The
deviation of Cu(II) from the same plane is 0.2547(5) Å in the direc-
tion of N(1) of NCNCN ligand which may be considered as axially
coordinated. However, the geometry around Cu(II) can be de-
scribed better as trigonal bipyramid with N(1), N(19), and O(11)a
make up the equatorial plane which together with Cu(II) provide
Fig. 1. The structure of 1 with ellipsoids at 30% probability.
N(1) at 2.048(2) Å. The two Cu atoms are separated by 3.103 Å and
the Cu(1)–O(11)–Cu(1)a angle is 99.15(6)°. If the geometry around
Cu(II) is considered as square pyramidal, the deviations of the coor-
dinating atoms N(1), N(19), N(22), O(11)a from the least-square
a r.m.s. deviation of 0.1305(5) Å. The Addison parameter (s) of
the penta-coordinated Cu(II) is 0.49 indicating that the geometry
is a distorted trigonal bipyramid with O(11) and N(23) occupying
axial positions. As in 1, there is no short contact between N(5)

A. Biswas et al. / Inorganica Chimica Acta 379 (2011) 28–33
31
Table 3
Hydrogen bonding distances (Å) and angles (°) for the complexes 1 and 2.
Complex
D–Hꢀ ꢀ ꢀA
D–H (Å)
Aꢀ ꢀ ꢀH (Å)
Dꢀ ꢀ ꢀA (Å)
\D–H–A (°)
1
2
N(19)–H(19)ꢀ ꢀ ꢀN(3)b
N(23)–H(23)ꢀ ꢀ ꢀN(5)c
O(1)–H(1)ꢀ ꢀ ꢀN(3)a
O(1)–H(2)ꢀ ꢀ ꢀN(5)c
N(19)–H(19)ꢀ ꢀ ꢀO(1)
0.91
0.91
0.87
0.92
0.91
2.18
2.26
2.13
1.90
2.08
3.086(3)
3.159(6)
2.992(6)
2.757(6)
2.938(5)
171
168
172
156
158
Symmetry operation:
a
(ꢁx, 2 ꢁ y, ꢁz).
b
(ꢁx, 1 ꢁ y, 1 ꢁ z).
c
(x ꢁ 1/2, 5/2 ꢁ y, z + 1/2).
Fig. 2. Hydrogen bonding polymeric structure of compound 1; hydrogen atoms except H19 have been excluded for clarity.
first from N(19)–H(19) to the water molecule O(1) with dimen-
sions 2.938(6), 2.08 Å, 158° and second from O(1)–H(1) to the cen-
tral nitrogen N(3)a (a = ꢁx, 2 ꢁ y, ꢁz) of the NCNCN ligand with
dimensions 2.992(6), 2.13 Å, 172°. These hydrogen-bonding inter-
actions lead to the formation of a 2D network (Fig. 4).
3.4. Magnetic properties
Temperature-dependence molar susceptibility measurements
of powdered samples 1 and 2 were carried out in an applied field
of 10000 and 500 G in the temperature range of 2–300 and 2–
30 K, respectively. Plots of complexes 1 and 2 are shown in Figs.
5 and 6, respectively, in both cases vmT being the magnetic suscep-
tibility per Cu2 unit. At room temperature the
vmT values are far
from 0.75 cm3 molꢁ1 K (g = 2.00) for a non-interacting Cu2 unit
(0.60 cm3 molꢁ1 K for 1, and 0.31 cm3 molꢁ1 K for 2). When the
samples are cooled, the
vmT values decreases to zero at low tem-
peratures. The shape of the curves indicates strong antiferromag-
netic coupling in both cases.
Assuming the isotropic Hamiltonian H = ꢁJS1S2, the experimen-
tal data were fitted to the Bleaney–Bowers expression for an iso-
tropically coupled pair of S = 1/2 ions (Eq. (1)) [18], where the
symbols have their usual meaning.
Ng2l2B 2 expðJ=kTÞ
vm
¼
ð1Þ
kT 1 þ 3 expðJ=kTÞ
The best-fit parameters for reproducing satisfactorily the exper-
imental data, as shown in Figs. 5 and 6, are J = ꢁ184.3 cmꢁ1 and
g = 2.07 with R = 5 ꢂ 10ꢁ5 for 1, J = ꢁ478.4 cmꢁ1 and g = 2.22 with
Fig. 3. The structure of 2 with ellipsoids at 30% probability.
R = 3 ꢂ 10ꢁ5 for 2, R = Ri
(vTicalc
ꢁ
v (v
Tiexp)2/Ri Tiexp)2.
and metal atoms, but it acts as acceptor to two intermolecular
hydrogen bonds, one involving an amino hydrogen and the second
the solvent water molecule O(1). Dimensions for N(23)–
H(23)ꢀ ꢀ ꢀN(5)c (c = x ꢁ 1/2, 5/2 ꢁ y, z + 1/2) are 3.159(6) Å, 2.26 Å,
168° and for O(1)–H(2)ꢀ ꢀ ꢀN(5)c (c = x ꢁ 1/2, 5/2 ꢁ y, z + 1/2) are
2.758(6), 1.90 Å, 156°. There are two additional hydrogen bonds,
3.4.1. Magneto-structural correlations
In Table 4 we have indicated the main structural parameters of
complexes 1 and 2 that can influence the corresponding J values.
According to literature data the exchange coupling in dinuclear
complexes with Cu2O2 core depends on several factors: the Cu–
O(R)–Cu angle, the Cu–O distance and geometrical distortions.

32
A. Biswas et al. / Inorganica Chimica Acta 379 (2011) 28–33
Fig. 4. Hydrogen bonding polymeric structure of compound 2; hydrogen atoms except H19, H23, and those of water molecules omitted for clarity.
Table 4
Selected structural parameters for complexes 1 and 2 related to their magnetic data.
Complex 1
Complex 2
J value (cmꢁ1
(Addison parameter)
)
ꢁ184.3
0.57
ꢁ478.4
0.49
s
Distances (Cu–O) (Å)
1.945
2.128
99.15
1.951
2.005
103.51
Angles (Cu–O–Cu) (°)
Indeed, magnetostructural correlations in dinuclear copper(II)
complexes bridged equatorially by pairs of hydroxido [19,20],
alkoxido [21,22] and phenoxido [23] groups show that the most
important factor is the Cu–O(R)–Cu angle, having a quasi-linear
correlation between the experimentally exchange coupling con-
stant and the Cu–O(R)–Cu bond angle. In general the larger Cu–
O(R)–Cu angle favors large AF J values. There are, on the other
Fig. 5. Plot of the
vmT vs. T in the range 2–300 K for 1.
hand, strong differences in l-hydroxido, l-alkoxido and l-phenox-
ido: from theoretical or experimental point of view all the alkoxido
and phenoxido-bridged complexes show stronger antiferromag-
netic coupling than the hydroxido-bridged ones [23,24]. However,
several compounds have been found to deviate from this linear
relationship. The reasons that have been given for these deviations
are mainly the geometrical distortions [7], such as the variations in
Cu–O bond distances and/or the distortion of the square pyramidal
geometry towards trigonal bipyramid. A survey of the magnetic
and structural properties of the dinuclear phenoxido-bridged Cu(II)
complexes reveal that when the Cu–O bond distance is less than
1.98 Å strong antiferromagnetic coupling is observed and the
strength of the coupling is linearly dependent on the Cu–O bond
lengths.
In our present work, complex 2 shows strong antiferromagnetic
coupling with J = ꢁ478.4 cmꢁ1. Considering both the high Cu–O–
Cu angle (103.51°) and the short average Cu–O bond distance
(1.978 Å), such strong antiferromagnetic coupling is not surprising.
On the contrary, the coupling in 1 is much less (J = ꢁ184.3 cmꢁ1
)
and it can be well correlated with its smaller Cu–O–Cu angle
(99.15°) and longer average Cu–O distance (2.036 Å). Finally, in
Fig. 6. Plot of the
vmT vs. T in the range 2–300 K for 2.

A. Biswas et al. / Inorganica Chimica Acta 379 (2011) 28–33
33
penta-coordinated Cu(II) complexes the Addison parameter (
also important in determining the magnitude of coupling [25]:
increasing of diminishes the antiferromagnetic coupling [26].
The slightly higher value (0.57) of 1 compare to 2 (0.49) may also
s
) is
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.09.026.
s
s
contribute in diminishing the magnitude of coupling.
References
4. Conclusions
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The tridentate reduced Schiff-base ligands 2-[1-(2-dimethyl-
amino-ethylamino)-ethyl]-phenol (HL1) and 2-[1-(3-methyl-
amino-propylamino)-ethyl]-phenol (HL2) with Cu(II) and
dicyanamide anions have afforded two related phenoxido bridged
dinuclear Cu(II) complexes. The relatively high Addison parameter
(s
) of both complexes indicates that the environment around the
copper ions is an intermediate between the trigonal bipyramid
and a square based pyramid. The values of majority of the re-
s
ported literature structures of Cu2O2 type complexes show that
the copper environment exhibits a square pyramidal geometry
with slight distortion towards trigonal bipyramidal geometry.
The high
s value is rarer and is adopted from complexes containing
bulky and rigid ligands such as diketones, bipyridines or ligands
containing imine functional groups. The present study shows that
Cu(II) complexes with larger
more flexible reduced Schiff base ligand as is reflected from our
present work. The antiferromagnetic coupling in one of the com-
plexes (2) is rather strong in spite of the high s value. This apparent
anomaly can be explained considering the relatively wide bridging
Cu–O–Cu angle and short Cu–O bond distances.
s can be obtained with the help of
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We thank CSIR, Government of India [Senior Research Fellow-
ship to A. Biswas, Sanction No. 09/028 (0717)/2008-EMR-I]. We
also thank EPSRC and the University of Reading for funds for the
X Calibur System and Spanish Government (Grant CTQ2009-
07264) for their financial support.
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Appendix A. Supplementary material
[26] A. Rodríguez-Fortea, P. Alemany, S. Alvarez, E. Ruiz, Inorg. Chem. 41 (2002)
3769.
CCDC 798704 and 798705 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge