Effect of anionic co-ligands on structure and magnetic coupling of bis(μ-phenoxo)-bridged dinuclear copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2011.07.005

Two phenoxo bridged dinuclear Cu(II) complexes, [Cu₂L ₂(NO₂)₂] (1) and [Cu₂L ₂(NO₃)₂] (2) have been synthesized using the tridentate reduced Schiff-base ligand 2-[(2-d

Full text of DOI:10.1016/j.ica.2011.07.005

Inorganica Chimica Acta 376 (2011) 422–427
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Inorganica Chimica Acta
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Effect of anionic co-ligands on structure and magnetic coupling of
bis(l-phenoxo)-bridged dinuclear copper(II) complexes
Apurba Biswas a, Michael G.B. Drew b, You Song c, Ashutosh Ghosh a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG 66AD, UK
c State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two phenoxo bridged dinuclear Cu(II) complexes, [Cu2L2(NO2)2] (1) and [Cu2L2(NO3)2] (2) have been syn-
thesized using the tridentate reduced Schiff-base ligand 2-[(2-dimethylamino-ethylamino)-methyl]-phe-
nol (HL). The complexes have been characterized by X-ray structural analyses and variable-temperature
magnetic susceptibility measurements. The structures of the two compounds are very similar having the
same tridentate chelating ligand (L) and mono-dentate anionic ligand nitrite for 1 and nitrate for 2. In
both complexes Cu(II) is penta-coordinated but the square pyramidal geometry of the copper ions is
Received 21 January 2011
Received in revised form 1 July 2011
Accepted 6 July 2011
Available online 21 July 2011
Keywords:
Copper(II)
Reduced Schiff base
Crystal structure
Magnetic properties
Antiferromagnetic coupling
severely distorted (Addison parameter (s) = 0.33) in 1 while the distortion is quite small (average
s
= 0.11) in 2. These differences have marked effect on the magnetic properties of two compounds.
Although both are antiferromagnetically coupled, the coupling constants (J = ꢀ140.8 and ꢀ614.7 cmꢀ1
for 1 and 2, respectively) show that the coupling is much stronger in 2.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
(–CH@N–) fragment to less constrained –CH2–NH– moiety
[10,11] and that should have considerable effect on the coordina-
The chemistry of dinuclear Cu(II) complexes with endogenous
bridging phenoxo ligands are of ongoing interest due to their inter-
esting magnetic properties [1–3]. Phenoxo ligands have been em-
ployed in the synthesis of many complexes with different
transition metals, among which the most frequent ones are those
of copper(II). The magnetic properties of phenoxo-bridged com-
plexes containing the Cu2O2 core depend on the structural proper-
ties of the core. It has been observed that the major factor
controlling spin coupling between the metal centers in dinuclear
Cu(II) complexes is the Cu–O–Cu bridging angle [4–9]. However
other factors, such as the coordination geometry of the copper ions,
the Cu–O bond distances, the Cuꢁ ꢁ ꢁCu distances, Addison parame-
tion geometry of the metal ion.
In this paper we report the syntheses, crystal structures, and
magnetic properties of two phenoxo bridged dinuclear Cu(II) com-
plexes, [Cu2L2(NO2)2] (1) and [Cu2L2(NO3)2] (2) obtained from the
reduced Schiff base ligand 2-[(2-dimethylamino-ethylamino)-
methyl]-phenol (HL). We show here that the anionic co-ligand (ni-
trite in 1 and nitrate in 2) has considerable effect on the geometry
of the copper ions which is severely distorted square pyramid in 1
but the distortion is quite small in 2. These differences are well re-
flected in their magnetic properties.
2. Experimental
ter (s) and the out-of-plane shift of the phenyl group (Scheme 1)
are the parameters that have been also postulated to influence con-
siderably the J values of the spin coupling [2,3]. Therefore to design
a compound having desirable magnetic properties, it is important
to study the effect of various factors that have significant influence
on these structural parameters. Recently, reduced Schiff base li-
gands are gaining popularity among chemists for syntheses of
complexes as they are considered to be more flexible compared
to the Schiff bases due to the reduction of the rigid azomethine
The reagents and solvents used were of commercially available
reagent quality.
2.1. Synthesis of the reduced Schiff base ligand (HL) 2-[(2-
dimethylamino-ethylamino)-methyl]-phenol
The Schiff base ligand was synthesized by refluxing a solution of
salicylaldehyde (0.52 mL, 5 mmol) and N,N-dimethylethylenedi-
amine (0.54 mL, 5 mmol) in methanol (30 mL) for 1 h [10,11].
The solution was cooled to 0 °C and solid sodium borohydride
(210 mg, 6 mmol) was added slowly to this methanolic solution
⇑
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.2011.07.005

A. Biswas et al. / Inorganica Chimica Acta 376 (2011) 422–427
423
Table 1
Cu
Cu
Crystal data and structure refinement of complexes 1 and 2.
1
2
O
O
Formula
M
C
22H34Cu2N6O6
C22H34Cu2N6O8
637.63
605.64
Out-of-plane shift angle
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
triclinic
P1
ꢀ
ꢀ
Scheme 1. Out-of-plane shift of the phenyl ring.
7.9388(8)
8.9703(9)
9.7128(8)
67.108(9)
84.024(7)
86.714(8)
633.64(10)
1
7.3412(13)
13.3232(14)
13.6414(12)
88.979(8)
89.793(10)
82.980(11)
1324.0(3)
2
with stirring. After completion of the addition, the resulting solu-
tion was acidified with concentrated HCl (5 ml) and then evapo-
rated to dryness [10–12]. The reduced Schiff base ligand HL was
extracted from the solid mass with methanol and this methanol
solution (ca. 20 mL) was used for preparation of complexes.
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
1.587
1.728
1.599
1.663
l
(mmꢀ1
)
2.2. Synthesis of the complexes [Cu2L2(NO2)2] (1) and [Cu2L2(NO3)2]
(2)
F(0 0 0)
Rint
Total reflections
314
0.020
4528
660
0.021
9443
Unique reflections
3547
2900
0.0329, 0.0896
150
7429
5010
0.0341, 0.0663
150
An extracted methanol solution of HL as prepared above was
added to a solution of Cu(NO3)2ꢁ3H2O (1.208 g, 5 mmol) in metha-
nol (20 mL) and an aqueous solution (1 mL) of sodium nitrite
(0.690 g, 10 mmol) was added to this mixture with stirring. The
mixture was stirred for 1 h and filtered. The filtrate was kept undis-
turbed at room temperature. Green crystals of 1 suitable for X-ray
diffraction were collected after 3 days on slow evaporation of the
solvent. Complex 2 was synthesized as the same way as 1 but so-
dium nitrite was not added. Brown crystals of 2 suitable for X-ray
diffraction were obtained after 1 day on slow evaporation of the
solvent.
I > 2r(I)
R1, wR2
T (K)
atoms bonded to carbon were included in geometric positions
and given thermal parameters equivalent to 1.2 times those of
the atom to which they were attached. Absorption corrections
were carried out using the ABSPACK program [15]. The structures
were refined on F2 to R1 0.0329, 0.0341; wR2 0.0896, 0.0663 for
2900, 5010 data with I > 2r(I).
2.2.1. Complex 1
3. Results and discussion
(Yield: 1.090 g, 72 %), Anal. Calc. for C22H34Cu2N6O6: C, 43.63; H,
5.66; N, 13.88.ꢀF1ound: C, 43.55; H, 5.72; N, 13.84%. IR (KBr):
(dm3 molꢀ1 cmꢀ1)] (acetonitrile), 637 (194), 424 (66).
m(N–
[emax
3.1. Synthesis of the complexes
H), 3253 cm
,
m
(C–N), 1594 cmꢀ1
;
kmax (nm),
The condensation of N,N-dimethylethylenediamine in 1:1 M ra-
tio with salicylaldehyde afforded the Schiff base, 2-[(2-dimethyl-
amino-ethylimino)-methyl]-phenol which on reduction with
sodium borohydride readily produced the reduced Schiff base, HL
2.2.2. Complex 2
(Yield: 1.259 g, 79 %), Anal. Calc. for C22H34Cu2N6O8: C, 41.44; H,
5.37; N, 13.18.ꢀF1ound: C, 41.37; H, 5.42; N, 13.14%. IR (KBr): (N–
m
H), 3215 cm
,
m
(C–N), 1598 cmꢀ1
;
kmax (nm),
[emax
(dm3 molꢀ1 cmꢀ1)] (acetonitrile), 631 (313), 424 (2862).
CH3
HC
N
N
CH3
NaBH4
CH3
2.3. Physical measurements
OH
Elemental analyses (C, H and N) were performed using a Per-
kin–Elmer 240C elemental analyzer. IR spectra in KBr pellets
(4500–500 cmꢀ1) were recorded using a Perkin–Elmer RXI FT-IR
spectrophotometer. Electronic spectra in methanol (1200–
350 nm) were recorded in a Hitachi U-3501 spectrophotometer.
The magnetization data were recorded on a Quantum Design
MPMS-XL7 SQUID magnetometer with an external magnetic field
of 2000 G in the temperature ranges of 2–300 K. The experimental
magnetic susceptibility data are corrected for the diamagnetism
estimated from Pascal’s tables and sample holder calibration.
CH2
NH
OH
N
CH3
HL
1) Cu(NO3)2.3H2O
2) NaNO2
1) Cu(NO3)2.3H2O
(HL:Cu2+ = 1:1)
(HL:Cu2+:NO2- = 1:1:2)
CH3
CH3
N
CH2 NH
N
CH2 NH
2.4. Crystal data collection and refinement
CH3
CH3
ONO2
Cu
ONO
Cu
Crystal data for the two crystals are given in Table 1. 3547, 7429
independent reflection data for 1, and 2, respectively were col-
lected with Mo Ka 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
O
O
ONO
H3C
H3C
O
ONO2
O
Cu
Cu
H3C
H3C
NH CH2
N
NH CH2
N
1
2
NaNO2 (exess)
Scheme 2. Formation of the complexes.

424
A. Biswas et al. / Inorganica Chimica Acta 376 (2011) 422–427
(Scheme 2). HL on reaction with copper(II) nitrate 1:1 M ratios
yielded compound 2. Compound 1 was produced when sodium ni-
trite (excess) was also present in the reaction mixture. Compound
2 was converted into 1 when a methanol solution of 2 was treated
with an aqueous solution of sodium nitrite (excess) and the result-
ing solution was evaporated slowly at room temperature.
lating oxygen atom O(11) which may be considered as axially coor-
dinated. The Addison parameter (
) of the penta-coordinated CuII is
s
0.33 indicating that the geometry around the metal is best de-
scribed as a severely distorted square pyramidal with O(11) as
the axial atom. The two Cu atoms are separated by 3.187 Å and
the Cu(1)–O(11)–Cu(1)a angle is 100.94(8)°. There is a moderate
intermolecular hydrogen bond N(19)–H(19)ꢁ ꢁ ꢁO(33)b (b = 1 ꢀ x,
ꢀy, 2 ꢀ z) with dimensions 3.002(3), 2.21 Å, 145° (Table 3) to result
in a 1-D polymeric structure (Fig. 2).
3.2. IR and electronic spectra
The structure of [Cu2L2(NO3)2] (2) is shown in Fig. 3. Dimen-
sions in the metal coordination sphere are given in Table 2.
The structure of 2 contains nitrate rather than nitrite but other-
wise has the same stoichiometry as 1. While the connectivity in
A moderately strong, sharp peak due to N–H stretching vibra-
tion at 3253, and 3215 cmꢀ1 for complexes 1 and 2, respectively
shows that the imine group of the Schiff base is reduced. The
reduction of the imine group is also very clearly indicated by the
absence of the strong band due to imine vibration which appears
in the region 1620–1650 cmꢀ1 for the complexes of the corre-
sponding unreduced Schiff bases [16]. In complex 1, absorption
bands at 1282, 1201, and 877 cmꢀ1 are tentatively assigned to
Table 2
Bond distances (Å) and angles (°) for complexes 1 and 2.
m
s(NO2), mas(NO2), and d(NO2), respectively [17]. In complex 2, the
Complex 1
Complex 2
characteristic strong peak of the coordinated nitrate anion is ob-
Cu(1)–O(11)
Cu(1)–N(19)
Cu(1)–O(11)a
Cu(1)–N(22)
Cu(1)–O(33)
Cu(1)–O(31)
2.186(2)
1.992(2)
1.942(1)
2.076(2)
2.695(2)
2.066(2)
Cu(1)–O(11)
Cu(1)–O(31)
Cu(1)–O(71)
Cu(1)–N(19)
Cu(1)–N(22)
Cu(2)–O(11)
Cu(2)–O(31)
Cu(2)–O(81)
Cu(2)–N(39)
Cu(2)–N(42)
1.961(1)
1.939(2)
2.292(2)
1.995(2)
2.0569(2)
1.983(1)
1.977(1)
2.245(2)
1.985(2)
2.065(2)
served at 1319 cmꢀ1 for asymmetric stretching vibration.
The electronic spectra of these two compounds were recorded
in acetonitrile solution. The electronic spectra show a single
absorption band at 637 and 631 nm for compounds 1 and 2,
respectively. At higher energy region, the ligand to metal charge
transfer bands were located at 424 nm for the compounds.
O(11)–Cu(1)–N(19)
O(11)–Cu(1)–O(11)a
N(19)–Cu(1)–O(11)a
O(11)–Cu(1)–N(22)
N(19)–Cu(1)–N(22)
N(22)–Cu(1)–O(11)a
O(11)–Cu(1)–O(31)
N(19)–Cu(1)–O(31)
O(31)–Cu(1)–O(11)a
N(22)–Cu(1)–O(31)
89.11(5)
79.06(7)
166.82(7)
111.92(7)
85.05(7)
93.95(7)
100.89(7)
91.89(7)
95.98(7)
146.96(7)
3.3. Crystal structures of the complexes 1 and 2
O(31)–Cu(2)–O(11)
O(31)–Cu(2)–N(39)
O(11)–Cu(2)–N(39)
O(31)–Cu(2)–N(42)
O(11)–Cu(2)–N(42)
N(39)–Cu(2)–N(42)
O(31)–Cu(2)–O(81)
O(11)–Cu(2)–O(81)
N(39)–Cu(2)–O(81)
N(42)–Cu(2)–O(81)
75.98(6)
90.78(6)
155.98(6)
166.93(6)
102.50(6)
85.86(7)
98.72(6)
91.78(6)
110.23(6)
94.29(6)
The structure of [Cu2L2(NO2)2] (1) is a centrosymmetric dimer,
shown in Fig. 1 together with the atomic numbering scheme.
Dimensions in the metal coordination sphere are given in Table 2.
The metal atoms are five-coordinate being bonded to three do-
nor atoms of one ligand L together with an oxygen atom O(11)a
(a = 1 ꢀ x, ꢀy, 1 ꢀ z) of a second ligand L and an oxygen O(31) of
a monodentate nitrite group. Both oxygens of the two ligands L
bridge the two copper atoms. Bond lengths in the metal coordina-
tion sphere are Cu(1)–O(11) 2.186(2) Å, Cu(1)–N(19) 1.992(2) Å
Symmetry element.
a
(1 ꢀ x, ꢀy, 1 ꢀ z)
and Cu(1)–N(22) 2.076(2) Å to the tridentate ligand
L with
Cu(1)–O(11)a 1.9417(14) Å and Cu(1)–O(31) 2.0656(17) Å to the
nitrite group. The nitrite group is best considered as monodentate
the second oxygen O(33) is 2.695(2) Å from the metal. The devia-
tions of the coordinating atoms O(31), N(19), N(22), O(11)a from
the least-square mean plane through them are ꢀ0.3768(17),
0.4137(18), ꢀ0.409618, 0.3727(15) Å, respectively. The deviation
of Cu(II) from the same plane is 0.1823(3) Å in the direction of che-
Table 3
Hydrogen bonding distances (Å) and angles (°) for the complexes 1 and 2.
Complex
D–Hꢁ ꢁ ꢁA
D–H
(Å)
Aꢁ ꢁ ꢁH
Dꢁ ꢁ ꢁA
\D–H–A
(°)
(Å)
(Å)
1
2
N(19)–
0.91
2.21
3.002(3)
145
Hꢁ ꢁ ꢁO(33)b
N(19)–Hꢁ ꢁ ꢁO(74)
N(39)–Hꢁ ꢁ ꢁO(83)
N(39)–
0.91
0.91
0.91
2.27
2.28
2.22
3.031(3)
2.990(3)
2.992(2)
141
135
142
Hꢁ ꢁ ꢁO(83)c
Symmetry element.
b
(1 ꢀ x, ꢀy, 2 ꢀ z).
c
(2 ꢀ x, 1 ꢀ y, 2 ꢀ z).
Fig. 2. Hydrogen bonding polymeric structure of compound 1; hydrogen atoms
Fig. 1. The centrosymmetric structure of 1 with ellipsoids at 30% probability.
except H19 have been excluded for clarity.

A. Biswas et al. / Inorganica Chimica Acta 376 (2011) 422–427
425
dentate Schiff base complexes of Cu(II), the chelating Schiff base
is usually meridional and the Addison parameter is low [18,19].
However, between the two complexes, in 1, the chelating ligand
is facially coordinated with rather high value of s. The higher flex-
ibility of the reduced Schiff base seems to be responsible for that.
Moreover, the presence of additional hydrogen atom on the re-
duced Schiff base allow them to form H-bonded supramolecular
structure.
3.4. Magnetic properties
Magnetic susceptibility measurements for complexes 1, and 2
were obtained on the Quantum Design MPMS-XL7 SQUID magne-
tometer at temperatures ranging from 2 to 300 K with an applied
magnetic field of 2000 G. Plots of complexes 1 and 2 are shown
in Figs. 4 and 5, respectively, in both cases
vmT being the magnetic
susceptibility per Cu2 unit. At room temperature the
vmT values,
especially of 2 (0.62 cm3 molꢀ1 K for 1, and 0.17 cm3 molꢀ1 K for
2) are considerably less than the calculated one (0.75 cm3 molꢀ1 K)
for two uncoupled S = 1/2 spins (g = 2.00). When the samples are
cooled, the
vmT values decreases to zero at low temperatures.
The shape of the curves indicates strong antiferromagnetic cou-
Fig. 3. The structure of
2 with ellipsoids at 30% probability. Intramolecular
hydrogen bonds shown as dotted lines.
the structures is the same the conformation is very different. The
copper atoms are bonded in similar fashion to that in 1 with three
bonds to the ligand L, one to an oxygen atom of a second ligand
and one bond to a monodentate nitrate ion. However, the tridentate
ligand L coordinates to the metal ion in a facial configuration in 1 but
in a meridional configuration in 2. Bond lengths in the metal coordi-
nation sphere are Cu(1)–O(11) 1.9608(13) Å, Cu(1)–N(19)
1.9954(17) Å and Cu(1)–N(22) 2.0569(18) Å to the tridentate ligand
L with Cu(1)–O(31) 1.9387(14) Å and Cu(1)–O(71) 2.2921(16) Å to
the nitrate group and Cu(2)–O(31) 1.9768(14) Å, Cu(2)–N(39)
1.9853(16) Å and Cu(2)–N(42) 2.0648(17) Å to the tridentate ligand
L with Cu(2)–O(11) 1.9831(14) Å and Cu(2)–O(81) 2.2448(16) Å to
the nitrate group. However in 2, the copper atoms have slightly dis-
torted square pyramidal geometries (Addison parameter 0.05 and
0.18 for Cu(1) and Cu(2), respectively) with the monodentate nitrate
in the axial positions. Root mean square (r.m.s.) deviations of the
four donor atoms in the equatorial planes are 0.001, 0.069 Å with
the copper atoms 0.237(1), 0.298(1) Å from the plane in the direc-
tion of the nitrate ligand. A further distinguishing feature in 2 is that
the two coordinated nitrate groups are on the same side of the Cu2O2
ring in the dimer, rather than on opposite sides as the two nitrite
groups in the centrosymmetric 1. Unlike 1, the central Cu2O2 ring
in 2 has a slight tetrahedral distortion with an r.m.s. deviation from
the plane of 0.124 Å. Another interesting difference between the two
structures is the position of phenyl rings relative to the Cu2O2 plane.
In complex 1, one of the phenyl rings is above and the other is below
the Cu2O2 plane whereas in 2 both the phenyl rings are in the same
side of the Cu2O2 plane.
Fig. 4. Plot of the
vmT and vm (inset) vs. T in the range 2–300 K for 1.
Unlike 1, there is no weak contact between a second oxygen of
nitrate with the copper atom as distances are 3.132(2), 3.369(2) Å.
In fact the closest contact in an axial position for both copper
atoms is O(73) (ꢀ1 + x, y, z) with distances of 3.006(2), 3.186(2) Å
for Cu(1) and Cu(2), respectively. There are hydrogen bonds from
N(19)–H(19) to O(74) and from N(39)–H(39) to O(83) with dimen-
sions (Table 3) Nꢁ ꢁ ꢁO 3.031(3), 2.990(3) Å, Hꢁ ꢁ ꢁO 2.27, 2.28 and N–
Hꢁ ꢁ ꢁO angles 141°, 135°. In addition there is an intermolecular
hydrogen bond from N(39)–H(39) to O(83)c (c = 2 ꢀ x, 1 ꢀ y,
2 ꢀ z) with dimensions 2.992(2), 2.22 Å, 142° (Fig. S1).
It is worth comparing the geometry of the present complexes
with that of related unreduced Schiff base complexes. In the tri-
Fig. 5. Plot of the
vmT vs. T in the range 2–300 K for 2.

426
A. Biswas et al. / Inorganica Chimica Acta 376 (2011) 422–427
pling in complex 2 whereas relatively weak antiferromagnetic cou-
pling in complex 1. Analyses of the experimental magnetic data
were performed by using the Bleaney–Bowers expression for an
isotropically coupled pair of S = 1/2 ions, modified to take into ac-
count the paramagnetic impurities (Eq. (1)) [1,20], where the sym-
bols have their usual meaning based on the following isotropic
Hamiltonian: H = ꢀJ(S1ꢁS2)
plane shift of the phenyl ring (Scheme 1) can increase the ferro-
magnetic contributions but this trend may inverted upon the con-
formation of the phenyl groups [2]. It has been found that in the
anti conformation the out-of-plane shift of the phenyl groups has
a minor effect on J, while in syn conformation the same shift
strongly enhances antiferromagnetic coupling in certain limits. Fi-
nally, the Cu–O–Cu–O torsion angles are also important: for the in-
plane phenyl rings with a particular Cu–O–Cu bridging angle, it
was shown that higher torsion angles result in an increase of the
ferromagnetic contribution.
Ng2l2B 2 expðJ=kTÞ
Ng2l2B
2kT
vm
¼
ð1 ꢀ
qÞ þ
q
ð1Þ
kT 1 þ 3 expðJ=kTÞ
Complexes 1 and 2, have different structural parameters (see
Table 4) that can explain their significantly different J values
(ꢀ140.8 and ꢀ614.7 cm3 molꢀ1 for 1 and 2, respectively). The low-
The parameters N,
lB and k in Eq. (1) have their usual meanings,
J = singlet–triplet splitting and
q
is the percentage of non-coupled
impurity. The best-fit parameters for reproducing satisfactorily the
er value of
s (0.11 (average) for 2 compare to 0.33 for 1) the greater
experimental data, as shown in Figs. 4 and 5, are J = ꢀ140.8 cmꢀ1
,
,
value of the Cu–O–Cu angle (101.45° (average) for 2 compare
to100.94° for 1), the shorter Cu–O distances (1.950 Å (average)
for 2 compare to 2.064 Å (average) for 1) and finally syn conforma-
tion of the phenyl groups in 2 indicate that J must be more AF for
complex 2 than complex 1. The Cu2O2 torsion angle, which is equal
to 0° for complex 1 and 14.80° for complex 2, does not change this
tendency.
g = 2.06 and
g = 2.26 and
q
= 0.04 with R = 2.41 ꢂ 10ꢀ5 for 1, J = ꢀ614.7 cmꢀ1
q
= 0.00 with R = 7.32 ꢂ 10ꢀ4 for 2, (R = Ri
(vTi-
calc ꢀ
v (v
Tiexp)2/Ri Tiexp)2).
3.4.1. Magnetostructural correlations
In Table 4 we have indicated the main structural parameters of
complexes 1 and 2 that can influence the corresponding J values.
The structures and magnetic properties for dinuclear copper
complexes bridged equatorially by pairs of hydroxide [4–6,21], alk-
oxide [22,23], or phenoxide [2,3] oxygen atoms have been studied
extensively. Several studies that have appeared in the literature
[3,4,24] have shown that the magnetic properties of the dinuclear
copper complexes containing the Cu2O2 core depend on the struc-
tural properties of the core. The first magnetostructural correlation
dealing with Cu(II) dinuclear complexes was proposed by Hatfield
4. Conclusions
The tridentate reduced Schiff-base ligand HL (2-[(2-dimethyl-
amino-ethylamino)-methyl]-phenol) afforded two phenoxo
bridged dinuclear Cu(II) complexes with the monoanionic co-li-
gands, nitrite (complex 1) and nitrate (complex 2). The copper
ion is penta-coordinated in both complexes having the same phen-
oxo-bridging chelated ligand (L) along with the respective mono-
coordinated anion. The only apparent difference in coordination
behavior is that the second oxygen atom of the nitrite ion in 1 is
very close to the metal ion but this seems to be responsible for
the difference in conformations of the ligand L and the structural
parameters of two complexes. The magnitudes of the antiferro-
magnetic couplings that are drastically different in two complexes
can be explained with the help of existing magneto-structural cor-
relations by considering these variations of structural parameters.
and Hodgson [4,5]. The correlation concerns planar di(l-hydrox-
ide)-bridged dicopper(II) complexes. The Cu–O–Cu bridging angle
is the major factor controlling the magnetic interactions and a lin-
ear dependence of the bridging angle on the coupling parameter
was deduced. A similar relationship also holds for the analogous
di(l-alkoxo)-bridged dicopper(II) complexes. Thompson et al.
found strong differences in
l
-hydroxo, -alkoxo and -phenoxo
l
l
[25]. It is apparent that the slopes (of J versus Cu–O–Cu angle) of
the hydroxide and alkoxide cases are comparable, but absolute val-
ues of ꢀJ are larger for the alkoxide. In general, all the alkoxo and
phenoxo-bridged complexes show stronger antiferromagnetic cou-
pling than the hydroxo-bridged ones (from theoretical or experi-
mental point of view) [22]. Another important structural factor in
Acknowledgments
We thank CSIR, Government of India [Senior Research Fellow-
ship to A. Biswas, Sanction No. 09/028 (0717)/2008-EMR-I] and
EPSRC and the University of Reading for funds for the X Calibur
system and ‘‘National Natural Science Foundation of China
(20631030 and 20771057)’’.
pentacoordinated Cu(II) complexes is the Addison parameter (
s)
[26]. The increase of diminishes the antiferromagnetic coupling
s
[27]. It has also been shown that the overlap between the metal
and the phenoxo bridging oxygen is a factor that controls the spin
coupling and is correlated not only with bridging angle, but also
with the M–O bond distances [3]. A survey of the magnetic and
structural properties of the dinuclear phenoxo-bridged Cu(II) com-
plexes 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. The
conformation of the phenyl rings is also important: the out-of-
Appendix A. Supplementary material
CCDC 800255 and 800256 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
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.2011.07.005.
Table 4
Selected structural parameters for complexes 1 and 2 related to their magnetic data.
Complex 1
Complex 2
References
J value (cmꢀ1
(Addison parameter)
)
ꢀ140.8
ꢀ614.7
0.05
s
0.33
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