First report of singly phenoxo-bridged copper(II) dimeric complexes: Synthesis, crystal structure and low-temperature magnetic behaviour study

Article abstract of DOI:10.1039/b300217a

Three singly phenoxo-bridged dinuclear Cu(II) complexes [Cu ₂(L)₂(SCN)(H₂O)](ClO₄) 1, [Cu ₂(L)₂(N₃)(H₂O)](ClO₄) 2, [Cu₂(L)₂(NCO)(H

Full text of DOI:10.1039/b300217a

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First report of singly phenoxo-bridged copper(II) dimeric complexes:
synthesis, crystal structure and low-temperature magnetic
behaviour studyy
Chirantan Roy Choudhury,a Subrata Kumar Dey,a Ruma Karmakar,a Chuan-De Wu,b
Can-Zhong Lu,b M. Salah El Fallahc and Samiran Mitra*a
a
Department of Chemistry, Jadavpur University, Calcutta-700 032, India.
E-mail: smitra_2002@yahoo.com; Fax: +91-33-2414-6266
The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
b
c
´
Departament de Quımica Inorganica, Universitat de Barcelona, Martı i Franques,
1-11, 08028-Barcelona, Spain. E-mail: salah.elfallah@qi.ub.es
`
´
`
Received (in Montpellier, France) 3rd January 2003, Accepted 3rd June 2003
First published as an Advance Article on the web 22nd July 2003
Three singly phenoxo-bridged dinuclear Cu(II) complexes [Cu2(L)2(SCN)(H2O)](ClO4) 1,
[Cu2(L)2(N3)(H2O)](ClO4) 2, [Cu2(L)2(NCO)(H2O)](ClO4) 3 (HL is the Schiff base ligand derived from
O-vanillin and 2-aminomethyl ethylenediamine) have been synthesised and characterised by infrared, UV-Vis
spectra, electrochemical study and single crystal X-ray diffraction study. In all the complexes (1–3), there are
two geometrically different square pyramidal copper(II) centres, with N3O2 and N2O3 donor sets for the two
Cu(II) centres. In the dimeric unit, the copper atoms are held together by only one distinct rare m2-phenolate
˚
oxygen of one Schiff base. The Cu–Cu distances are 3.291(2), 3.244(1) and 3.244(1) A for complexes 1, 2 and
3, respectively, and the Cu–O–Cu angles are found to be 111.5(1), 109.3(2), 109.6(1)ꢀ for complexes 1, 2
and 3, respectively. Variable temperature magnetic susceptibility study on the complexes 1,2 and 3 indicate the
presence of antiferromagnetic coupling between the Cu(II) centres with 2J values ꢁ109.8, ꢁ103.9 and
ꢁ95.9 cmꢁ1, respectively.
Transition dinuclear metal complexes and ligands capable of
yielding them have been attracting increasing interest in the
field of synthetic and biological chemistry due to the key roles
they play in many applications.1–10 In fact, dinuclear metal
complexes have been used successfully for the recognition
and assembly of external species of different natures, such as
inorganic or organic substrates.1–4 Many metalloenzymes con-
tain two divalent transition metal ions in close proximity and
in most cases the two metal centres cooperate with each other11
and they have contributed to a better knowledge of oxygen
transport as well as of some industrial catalytic processes.12,13
The design of structural and functional model complexes of
such dinuclear centres has been the subject of very extensive
investigations.11,14,15
In this context, recent emphasis has been placed on the
detailed study of the properties of Cu(II) phenolate complexes
because of their postulated involvement in a range of biologi-
cal and catalytic processes. Thus, Cu(II) phenolate units have
been proposed as intermediates in the catalytic cycles of metal-
loenzymes e.g. galactose oxidase, tyrosinase,16–19 in the bio-
genesis of novel metalloenzyme cofactors (e.g., topaquinone
in amine oxidases),20,21 and in synthetic catalysis such as alco-
hol oxidation22–25 and phenol polymerization.26–28 The dis-
tance between the two metal centres is crucial to allow the
cooperation of both metal ions in the active (site) centre.29
The known Cu(II)–phenolate complexes exhibit coordina-
tion numbers ranging from 4 to 6, as is typical for the
coordination chemistry of Cu(II). The phenolates in most of
these compounds usually are incorporated as part of multiden-
tate ligand systems26–28 and complexes with simple, exogenous
phenolate ligands are less common.30
Sanmartin et al. have reported the synthesis and crystal
structure of one bishelical isoceles triangle core self-assembled
by two m-phenoxo bridges.31 Recently, the same group has
communicated the trinuclear Cu(II) bishelicate Cu3(H2L)(L)ꢂ
2H2O [H4L ¼ N,N0-bis(3-hydroxysalicylidene)-1,4-diaminobut-
ane] complex.12 Yang et al. have reported two compounds,
[Cu3(Sbal)2(Phen)(H2O)2](ClO4)2ꢂ3H2O and [Cu(H2Sbal)2-
(Phen)](ClO4)2 , where two m-phenoxo bridges are observed.32
Using an amino-phenolic ligand, Dapporto et al. have synthe-
sised dinuclear zinc complexes assembling butanolate and azide
anions, [Zn2(HL)(CH3CH2CH2CH2O)](ClO4)2 .1 The authors
also have done a CCDC search and shown that phenoxo brid-
ging is generally always associated with some other kind of
bridging featuring dinuclear metal complexes.1,33a So, dinuc-
lear singly phenoxo-bridged copper(II) complexes of Schiff base
ligands are still very sparse in the literature.33b,34
For the atoms which bridge between paramagnetic transi-
tion metal atoms, the magnetic interactions have been related
to the magnitude of the bridging angle (f) and other structural
features. This has been especially studied for oxygen-bridged
metal atoms such as Cu–O–Cu. Small f angles near 96ꢀ should
lead to strong ferromagnetic interactions, while larger values
should make the interaction increasingly strongly antiferro-
magnetic.35
In this paper, we describe the synthesis, characterisation and
crystal structure and low temperature magnetic behaviour of
three novel singly phenoxo-bridged dinuclear Cu(II) Schiff base
y Electronic supplementary information (ESI) available: structural
data for compounds 1, 3, 2. See http://www.rsc.org/suppdata/nj/
b3/b300217a/
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New J. Chem., 2003, 27, 1360–1366
DOI: 10.1039/b300217a
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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complexes. All the complexes show antiferromagnetic beha-
viour at low temperature.
Yield: 60%, Anal. calc. for C25H36ClCu2N5O9S, C, 40.26; H,
4.83; N, 9.39; Cu, 17.05. Found: C, 40.31; H, 4.86; N, 9.36; Cu,
16.97%.
Experimental
[Cu2(L)2(N3)(H2O)](ClO4) (2) and [Cu2(L)2(NCO)(H2O)]-
(ClO4) (3). The syntheses were carried out using the same
procedure as in 1, by using sodium azide and sodium cya-
nate instead of sodium thiocyanate for 2 and 3 respectively.
After 4 days single crystals suitable for X-ray diffraction were
collected and the rest were filtered and air-dried.
Yield: 65%, Anal. calc. for C24H36ClCu2N7O9 , (2) C, 39.49;
H, 4.94; N, 13.44; Cu, 17.43. Found: C, 39.67; H, 4.86; N,
13.66; Cu, 17.52%.
Materials and measurements
Reagent grade copper(II) perchlorate (Fluka), sodium thiocya-
nate, sodium azide, sodium cyanate (Aldrich), O-vanillin and
2-aminomethyl ethylenediamine (Fluka) were used as received.
The solvents used were of reagent grade.
Infrared spectra were recorded on a Perkin-Elmer FT-IR
spectrophotometer as a KBr pellet and electronic spectra on
a Perkin-Elmer Lambda-40 spectrophotometer using methanol
as solvent. Elemental analyses were carried out on a Perkin-
Elmer 2400-II instrument. Electrochemical studies were per-
formed on a CH 600A cyclic voltammeter instrument using
acetonitrile as solvent. EPR spectra were performed on a Bru-
ker ER 420 model. Variable temperature magnetic susceptibil-
ities were measured in the temperature range 5–300 K on a
model CF-1 superconducting extracting sample magnetometer
with powdered sample kept in a capsule for weighing. All data
were corrected for diamagnetism of the ligands estimated from
Pascal’s constants.36
Yield: 70%, Anal. calc. for C25H36ClCu2N5O10 , (3) C, 41.14;
H, 4.94; N, 9.60; Cu, 17.98. Found: C, 41.22; H, 4.86; N, 9.36;
Cu, 17.86%.
X-Ray crystallography
The determination of the unit cell and the data collection for
deep blue crystals of compounds 1, 2 and 3 were performed
on a Siemens SMART CCD diffractometer and the data were
collected using graphite-monochromatic MoKa radiation
ꢀ
˚
(l ¼ 0.71073 A) at 293 K in the range of 1.25 < y < 23.0
for 1, 1.98 < y < 25.10ꢀ, for 2, and 1.98 < y , 25.04ꢀ for 3
respectively. The data sets were corrected by the SADABS
program.37a The structures of 1, 2 and 3 were solved by direct
methods and refined by the full-matrix least-squares method
with the SHELXTL-9737b program package. The details of
X-ray data collection and structure refinement of 1, 2 and 3
are given in Table 1.
Synthesis of the ligand and complexes
Synthesis of the Schiff base ligand. The Schiff base
ligand [(CH3)2NCH2CH2N=CHC6H3(OH)(OMe)] (LH) was
obtained by refluxing a methanolic solution (25 ml) of O-vanil-
lin (1 mmol) and 2-dimethylaminoethylamine (1 mmol) for
30 minutes. The resulting orange–yellow solution containing
the ligand was used without further purification.
CCDC reference numbers 214016, 214955, 214017 for 1, 2
and 3, respectively. See http://www.rsc.org/suppdata/nj/b3/
b300217a/ for crystallographic in .cif or other elecrtonic
format.
Caution! Perchlorate salts are potentially explosive and
should be used in small quantity with much care.
Synthesis of the complexes
Results and discussion
[Cu2(L)2(SCN)(H2O)](ClO4) (1). To a methanolic solution
of copper perchlorate (0.370 g, 1 mmol), methanolic solution
of the Schiff base (1 mmol) was added. Then methanolic solu-
tion of sodium thiocyanate (0.081 g, 1 mmol) was added to the
mixture with constant stirring. The resulting solution was stir-
red for 5 minutes at room temperature. After 3 days crystals
suitable for X-ray diffraction were collected and the rest were
filtered and air-dried.
Infrared spectra
The infrared spectra of all the three complexes 1, 2 and 3 are
very much consistent with the structural data presented in this
paper. The bands in the range 3524 and 1631 cmꢁ1 for 1, 3547
and 1634 cmꢁ1 for 2 and 3548 and 1635 cmꢁ1 for 3 are attri-
butable to O–H stretching and bending of water ligands or
water of crystallisation. This also indicates the presence of
Table 1 Crystallographic data for complexes 1, 2 and 3
Compound
Chemical formula
M
1
C25H36ClCu2N5O9S
2
C24H36ClCu2N7O9
3
C25H36ClCu2N5O10
745.18
Monoclinic
P2(1)/c
729.13
Monoclinic
P2(1)/c
729.12
Monoclinic
P2(1)/c
Crystal system
Space group
Z
4
16.605(1)
4
16.675(1)
4
16.738(1)
˚
a/A
˚
b/A
13.775(1)
13.789(1)
13.319(1)
14.064(1)
13.286(1)
14.082(1)
˚
c/A
a/ꢀ
90
102.184(1)
90
104.699(1)
90
104.88(1)
b/ꢀ
g/ꢀ
90
293(2)
90
293(2)
90
293(2)
T/K
3
˚
V/A
3082.8(1)
8518
3021.2(1)
9445
3026.5(2)
9793
Total reflections collected
No. of independent reflections
Absorption coefficient m/mmꢁ1
F(000)
4281 [Rint ¼ 0.0807]
1.592
1536
5272 [Rint ¼ 0.0240]
1.558
1504
5279 [Rint ¼ 0.0210]
1.556
1504
y for data collection/ꢀ
R indices (all data)
1.25 to 23.00
R1 ¼ 0.1646, wR2 ¼ 0.1410,
R1 ¼ 0.0821, wR2 ¼ 0.1410
1.98 to 25.10
R1 ¼ 0.0732, wR2 ¼ 0.1369
R1 ¼ 0.0557, wR2 ¼ 0.1224
1.98 to 25.04
R1 ¼ 0.0664, wR2 ¼ 0.1253
R1 ¼ 0.0.0513, wR2 ¼ 0.1253
Final R indices [I > 2s(I)]
New J. Chem., 2003, 27, 1360–1366
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hydrogen bonding in the complex.32 The complexes 1, 2 and 3
show one strong band at 2078, 2068 and 2206 cmꢁ1, respec-
tively, which indicate the presence of NCS, N3 and NCO as
terminal ligands. The bands at 2936, 2946 and 2937 cmꢁ1 are
due to the imine stretching frequency for complexes 1, 2, 3
respectively. The phenolic C=O stretching is present at 1274
and 1286 cmꢁ1 for 1, 1268 and 1288 cmꢁ1 for 2 and 1270
and 1289 cmꢁ1 for 3.32 Bands in agreement with non-coordi-
nated anions could also be observed for perchlorate com-
pounds, 1172, 1161–1086, 969, 650, 455, 445 cmꢁ1 38,39
.
Ligand coordination to the copper centre is substantiated by
two bands appearing at 465, 464, 465 (medium) and 355,
378, 369 (strong) cmꢁ1, for complexes 1, 2 and 3, attributable
to n(Cu–N) and n(Cu–O), respectively.12
Electronic spectra
Although the electronic spectra of the copper complexes with
multidentate Schiff base ligands are not in general good indica-
tors of geometry,40 they support the structural data.
Fig. 1 ORTEP representations of the binuclear unit for 1 (30%
probability ellipsoids, for clarity hydrogen atoms were omitted).
(one ClO4 was omitted).
The complexes display two strong absorption bands in the
region 225, 276 nm for 1, 236, 277 nm for 2 and 237, 279 nm
for 3 respectively. These are clearly charge transfer in origin.
The UV absorption bands observed at 378, 379 and 382 nm
for complexes 1, 2 and 3, respectively, can be assigned to the
charge transfer from the ligands to the Cu(II), transition. The
band for an LMCT transition of the Cu–O–Cu skeleton is
masked here by the other bands. As usual, all the spectra show
very weak low-intensity absorption bands associated with d–d
transitions at 640, 644 and 646 nm for complexes 1, 2 and 3
respectively.1,11,30,32
ꢁ
double phenoxo-bridging, the Cu–Cu distances are in the
41
range 2.901–3.345 A. The Cu1–O5–Cu2 angle is 111.5(1)ꢀ
˚
taking Cu(1)–O(5) and Cu(2)–O(5) distances 1.996(6) and
˚
1.985(6) A respectively. This Cu–O–Cu angle value is very
small compared to similar complexes34 but larger than double
phenoxo-bridged complexes where the values generally lie
between 95.7–107.5ꢀ.41–44 The Cu–O distances are also
comparable to the similar complexes.41
In our case, the two Schiff base molecules behave in a differ-
ent way. One of the Schiff bases acts as a tetradentate one
while the other acts in a tridentate fashion with a non-bonded
(–OMe) group. One perchlorate is also present in the lattice
which is not shown in Fig. 1.
Electrochemical study
Electrochemical studies of the complexes were performed using
acetonitrile as solvent and tetrabutylammonium perchlorate
as supporting electrolyte at a scan rate of 50 mV secꢁ1. Cyclic
voltametry on the complexes 1, 2 and 3 show two reductive
responses on the negative side of SCE (at ꢁ0.67, ꢁ1.24 V
for complex 1, ꢁ0.69, ꢁ1.27 V for complex 2 and ꢁ0.71,
ꢁ1.30 V for complex 3, respectively) both of which are irrever-
sible in nature. The observed potential gaps (570, 580 and 590
mV for complex 1, 2 and 3, respectively) between the two
reductions are considerably large and is believed to be due to
the combined effect of the difference in coordination environ-
ment around the two copper centres and effective communica-
tion between them through the oxo-bridge. Two irreversible
oxidation responses are also observed on the positive side of
SCE and these are tentatively assigned to oxidation of the
coordinated ligands.
The geometry of the Cu(1) centre is best described as a dis-
torted (4 + 1) (NNNO + O) square-based pyramid. The four
atoms constituting the basal plane are the phenolic oxygen
(O5) atom, two nitrogen (N2 and N5) atoms of one Schiff base
and the N(1) atom of the monodentate thiocyanate ligand. The
axial site is occupied by the O(4) atom of the coordinated
water molecule. The Cu(1) atom lies not in the basal plane
˚
but slightly out of it at a distance of 0.1496 A. In the basal
˚
plane, the average bond distances (A) are Cu(1)–N(1) ¼
1.952(10), Cu(1)–N(2) ¼ 2.077(8), Cu(1)–N(5) ¼ 1.933(9),
Cu(1)–O(5) ¼ 1.996(6). The axial distance is Cu(1)–O(4) ¼
˚
2.260(6) A, which is slightly longer than the basal bond
lengths. The bond lengths are slightly higher than double
phenoxo-bridged complexes.41,43,44 The deviation from the
Description of the crystal structures of the complexes 1, 2 and 3
ꢀ
˚
Table 2 Selected bond lengths (A) and angles ( ) for complex 1
The structures of 1, 2 and 3 are identical, except for a slight
difference in the bond parameters and the standard deviation,
so we limit the description to structure 1.
Cu(1)–N(5)
Cu(1)–N(1)
Cu(1)–O(5)
Cu(1)–N(2)
Cu(1)–O(4)
1.933(9)
1.952(10)
1.996(6)
2.077(8)
2.260(6)
Cu(2)–O(3)
Cu(2)–N(4)
1.936(6)
1.937(9)
1.985(6)
2.070(8)
2.334(7)
3.291(2)
91.3(3)
Cu(2)–O(5)
Cu(2)–N(3)
Cu(2)–O(2)
Cu(1)–Cu(2)
Crystal structure of complex 1
N(5)–Cu(1)–N(1)
N(5)–Cu(1)–O(5)
N(1)–Cu(1)–O(5)
N(5)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(5)–Cu(1)–N(2)
N(5)–Cu(1)–O(4)
N(1)–Cu(1)–O(4)
O(5)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
Cu(2)–O(5)–Cu(1)
167.0(4)
O(3)–Cu(2)–N(4)
O(3)–Cu(2)–O(5)
N(4)–Cu(2)–O(5)
O(3)–Cu(2)–N(3)
N(4)–Cu(2)–N(3)
O(5)–Cu(2)–N(3)
O(3)–Cu(2)–O(2)
N(4)–Cu(2)–O(2)
O(5)–Cu(2)–O(2)
N(3)–Cu(2)–O(2)
The ORTEP representation of the binuclear unit for 1 is shown
in Fig. 1 with important bond lengths and angles summarised
in Table 2 . The crystallographic cell diagram viewed down the
c-axis is shown in Fig. 2. Complex 1 crystallises in a monoclinic
system with space group P2(1)/c. There are two geometrically
different square pyramidal copper(II) centres, with N3O2 donor
set for Cu(1) and N2O3 donor set for Cu(2). In the dimeric
unit, the copper atoms are held together by only one distinct
rare m2-phenolate oxygen of one Schiff base ligand. In the
91.1(3)
89.9(3)
84.9(4)
93.1(4)
174.2(3)
98.1(3)
94.9(3)
86.6(2)
98.0(3)
111.5(3)
90.6(3)
172.1(3)
172.8(3)
84.9(3)
94.0(3)
91.5(3)
96.7(3)
75.6(3)
95.0(3)
˚
present case, the Cu1ꢂ ꢂ ꢂCu2 distance is 3.291(2) A which is
lower than similar complexes found in the literature.34 For
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Fig. 2 Cell diagram of 1 viewed down the c-axis.
square pyramidal geometry is also indicated by the bond
angles involving the atoms in the cis positions which vary from
84.9(4)–93.1(4)ꢀ as well as the angles involving the trans posi-
tions that vary to a large extent from 167.0(4)–174.2(3)ꢀ. All
the angles are more or less comparable to those compounds
in the literature.28,31,32,34,41–46
Fig. 4 ORTEP representations of the binuclear unit for 3 (30%
probability ellipsoids, for clarity hydrogen atoms were omitted).
(one ClO4 was omitted).
ꢁ
Similarly, the geometry around Cu(2) can also be described
as a distorted (4 + 1) (NNOO + O) square-based pyramid. The
four atoms constituting the basal plane are the phenolic oxy-
gen [O(5)] atom, two nitrogens [N(3) and N(4)] of another
Schiff base and the O(3) atom of the second phenolate oxygen
atom which does not take part in bridging but remains bonded
with Cu(2) only. Here the axial site is occupied by the O(2)
atom of the coordinated methoxy (–OMe group) of the first
compounds 2 and 3 are given in Tables 3 and 4, respectively. A
comparison of the coordination environments for Cu(1) and
Cu(2) in 1, 2 and 3 is summarised in Table 5.
Magnetic properties and EPR spectral studies
Magnetic measurements were carried out on polycrystalline
powder samples ranging from 5.0 to 300 K at 10 KG applied
field. Compounds 1, 2 and 3 show an almost identical behavior
with very slight differences. In Fig. 5 we represent only the mag-
netic behavior of 1 in the forms of wMT vs. T and wM vs. T plots.
Fig. 6 and Fig. 7 represent the magnetic behavior of 2 and 3,
respectively, in the forms of wMT vs. T and wM vs. T plots.
At 300 K, the wMT values per dimer are 0.6733, 0.7063 and
0.7032 emu K molꢁ1 for complexes 1, 2 and 3, respectively;
these values are smaller than that expected for two uncoupled
S ¼ 1/2 spins (0.75 emu K molꢁ1). wMT decreases rapidly with
decreasing temperature to reach a plateau at ca. 20 K giving
values of 0.0161, 0.0133 and 0.0208 emu K molꢁ1 for 1, 2
and 3 respectively, which indicate the existence of antiferro-
magnetic coupling. The plateau observed at low temperature
may be due to the non coupled Cu(II) ions in the polycrystal-
line powder sample. The magnetic susceptibility data were
quantitatively analysed by simply treating as an interacting
dimer (eqn. (1)).
˚
Schiff base. As usual, here the Cu(2) atom also lies 0.0157 A
out of the mean basal plane. In the basal plane, the average
˚
bond distances (A) are, Cu(2)–N(3) ¼ 2.070(8), Cu(2)–N(4) ¼
1.937(9), Cu(2)–O(3) ¼ 1.936(6), Cu(2)–O(5) ¼ 1.985(6). The
˚
axial distance is Cu(2)–O(4) ¼ 2.260(6) A, which is slightly
longer than the basal bond lengths. The bond lengths are
slightly higher than double phenoxo-bridged complexes.41,43,44
The deviation from the square pyramidal geometry is also
indicated by the bond angles involving the atoms in the cis
positions which vary from 84.9(3)–94.0(3)ꢀ as well as the
angles involving the trans positions that vary from 172.1(3)–
172.8(3)ꢀ. All the angles are more or less comparable to those
compounds in the literature.28,31,32,34,41–46
Compounds 2 and 3 are iso-structural to that of the above-
described compound 1, except they contain the N3ꢁ and OCNꢁ
anion in 2 and 3, respectively, instead of the SCNꢁ anion in 1.
The bond lengths and angles in the counterpart are also com-
parable. The symmetry expanded structures and the coordina-
tion environments around CuII atoms in 2 and 3 are shown in
Fig. 3 and 4 respectively. Selected bond lengths and angles for
wM ¼ ð1 ꢁ rÞð2Ng2mB2=kTÞ½3 þ expðꢁ2J=KTÞꢃꢁ1
þ rNg2mB2=2kT
ð1Þ
Table 3 Selected bond lengths and angles for complex 2
Cu(1)–N(5)
Cu(1)–N(1)
Cu(1)–O(5)
Cu(1)–N(2)
Cu(1)–O(4)
1.944(4)
1.971(4)
1.975(3)
2.064(4)
2.318(4)
Cu(1)–Cu(2)
Cu(2)–O(3)
3.244(1)
1.941(3)
1.946(4)
Cu(2)–N(4)
Cu(2)–O(5)
2.003(3)
2.090(4)
2.370(4)
Cu(2)–N(3)
Cu(2)–O(2)
N(5)–Cu(1)–N(1)
N(5)–Cu(1)–O(5)
N(1)–Cu(1)–O(5)
N(5)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(5)–Cu(1)–N(2)
N(5)–Cu(1)–O(4)
N(1)–Cu(1)–O(4)
O(5)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
167.88(19)
90.67(16)
84.58(16)
85.32(19)
98.77(19)
175.01(16)
97.49(17)
93.38(17)
86.81(14)
96.65(16)
O(3)–Cu(2)–N(4)
O(3)–Cu(2)–O(5)
N(4)–Cu(2)–O(5)
O(3)–Cu(2)–N(3)
N(4)–Cu(2)–N(3)
O(5)–Cu(2)–N(3)
O(3)–Cu(2)–O(2)
N(4)–Cu(2)–O(2)
O(5)–Cu(2)–O(2)
N(3)–Cu(2)–O(2)
91.81(16)
89.44(14)
173.09(16)
176.10(16)
84.31(18)
94.45(16)
87.83(15)
99.05(16)
74.21(13)
93.12(16)
Fig. 3 ORTEP representations of the binuclear unit for 2 (30%
probability ellipsoids, for clarity hydrogen atoms were omitted).
(one ClO4 was omitted).
ꢁ
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Table 4 Selected bond lengths and angles for complex 3
Cu(1)–N(5)
Cu(1)–N(1)
Cu(1)–O(5)
Cu(1)–N(2)
Cu(1)–O(4)
1.936(4)
1.959(4)
1.982(3)
2.082(4)
2.324(3)
Cu(1)–Cu(2)
Cu(2)–N(4)
3.244(1)
1.934(4)
1.944(3)
Cu(2)–O(3)
Cu(2)–O(5)
1.989(3)
2.085(4)
2.367(3)
Cu(2)–N(3)
Cu(2)–O(2)
N(5)–Cu(1)–N(1)
N(5)–Cu(1)–O(5)
N(1)–Cu(1)–O(5)
N(5)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(5)–Cu(1)–N(2)
N(5)–Cu(1)–O(4)
N(1)–Cu(1)–O(4)
O(5)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
168.62(18)
90.65(15)
88.16(15)
84.68(17)
95.82(17)
174.42(14)
96.94(16)
94.31(16)
87.25(13)
96.33(15)
N(4)–Cu(2)–O(3)
N(4)–Cu(2)–O(5)
O(3)–Cu(2)–O(5)
N(4)–Cu(2)–N(3)
O(3)–Cu(2)–N(3)
O(5)–Cu(2)–N(3)
N(4)–Cu(2)–O(2)
O(3)–Cu(2)–O(2)
O(5)–Cu(2)–O(2)
N(3)–Cu(2)–O(2)
91.91(15)
173.64(14)
89.25(13)
84.14(16)
175.86(15)
94.81(14)
99.15(14)
88.31(14)
74.63(12)
93.52(15)
Fig. 5 The wMT vs. T and wM vs. T plots for complex 1.
where we take into account a proportion of a monomeric
impurity r, for which the susceptibility is assumed to follow
the Curie law w ¼ (NmB2g2/kT ). The parameters N, mB and
k in eqn. (1) have their usual meanings, 2J ¼ singlet–triplet
splitting. Least-square fitting of all experimental data leads to
the following parameters: g ¼ 2.05, 2J ¼ ꢁ109.8 cmꢁ1 and
r ¼ 0.019 for complex 1, g ¼ 2.08, 2J ¼ ꢁ103.9 cmꢁ1 and
r ¼ 0.015 for complex 2 and g ¼ 2.06, 2J ¼ ꢁ95.9 cmꢁ1
and r ¼ 0.022 for complex 3 with the agreement factor
P
P
R ¼ [(wM)obs ꢁ (wM)calcd]2/ [(wM)obs2] inferior in all the
cases to R ¼ 1.2 ꢄ 10ꢁ6
.
The EPR spectra of the three binuclear Cu(II) complexes
registered are exactly the same, because they have almost
identical coordination polyhedra. The polycrystalline EPR
spectrum at room temperature gives a broad signal for the
three complexes with g ¼ 2.098 and DH ¼ 516.67 G for 1,
g ¼ 2.1232 and DH ¼ 360 G for 2 and g ¼ 2.0909 and
DH ¼ 475 G for 3 respectively. These values are slightly high
compared to those found above.
Fig. 6 The wMT vs. T and wM vs. T plots for complex 2.
Magneto-structural correlation
Dinuclear copper(II) complexes in which the metals are bridged
by only a single hydroxo, alkoxo or phenoxo–oxygen group
are relatively rare,34,42,45–50 and a none straightforward corre-
lation between structure and magnetism has been developed.
However, similar compounds with double (OR) groups have
been extensively studied. It is well known that the magnetic
behavior of divalent copper complexes bridged equatorially
by
a
pair of hydroxide,51–54 alkoxide,55,56 or phen-
oxide41,43,44,57–64 oxygen atoms is highly dependent on the
Cu–O–Cu bridge angle. Also it can be influenced but in smaller
measure by the Cu–O(bridge) distance, the Cuꢂ ꢂ ꢂCu separa-
tion, the geometry around the copper(II) center, and the geo-
metry around the bridging oxygen atom. As in all the cases,
Fig. 7 The wMT vs. T and wM vs. T plots for complex 3.
the magnetostructural correlations in bis(m-phenoxide)-
bridged macrocyclic dinuclear copper(II) complexes have been
studied.,44 and a linear relationship between the exchange inte-
gral (2J) and the phenoxide bridging angle (y) was observed;
2J ¼ ꢁ31.95y + 2462. If this relationship holds for complexes
1, 2 and 3 (y ¼ 111.5(1), 109.3(2) and 109.6(1)ꢀ, respectively),
a 2J value of ca. ꢁ1200 cmꢁ1 is anticipated. The relatively
weak antiferromagnetic interaction (2J ¼ ꢁ109.8, ꢁ103.9
and ꢁ95.9 cmꢁ1 for 1, 2 and 3, respectively) found experimen-
tally appeared to be less than the prospective one using the
previous relationship.
The relatively small magnitudes of the exchange coupling
constants found can be understood in terms of the geometrical
distortions, which affect the copper coordination spheres, the
geometries of the bridges, and the planarity on the bridge
core. In 1, 2 and 3, the coordination geometry around each
Table 5 The comparison of the coordination environments for Cu(1)
and Cu(2) in 1, 2 and 3 respectively
1
2
3
Cu(1)–N(5) 1.933(9) Cu(1)–N(5) 1.944(4) Cu(1)–N(5) 1.936(4)
Cu(1)–N(1) 1.952(10) Cu(1)–N(1) 1.971(4) Cu(1)–N(1) 1.959(4)
Cu(1)–O(5) 1.996(6) Cu(1)–O(5) 1.975(3) Cu(1)–O(5) 1.982(3)
Cu(1)–N(2) 2.077(8) Cu(1)–N(2) 2.064(4) Cu(1)–N(2) 2.082(4)
Cu(1)–O(4) 2.260(6) Cu(1)–O(4) 2.318(4) Cu(1)–O(4) 2.324(3)
Cu(1)–Cu(2) 3.290(2) Cu(1)–Cu(2) 3.244(1) Cu(1)–Cu(2) 3.244(1)
Cu(2)–O(3) 1.936(6) Cu(2)–O(3) 1.941(3) Cu(2)–N(4) 1.934(4)
Cu(2)–N(4) 1.937(9) Cu(2)–N(4) 1.946(4) Cu(2)–O(3) 1.944(3)
Cu(2)–O(5) 1.985(6) Cu(2)–O(5) 2.003(3) Cu(2)–O(5) 1.989(3)
Cu(2)–N(3) 2.070(8) Cu(2)–N(3) 2.090(4) Cu(2)–N(3) 2.085(4)
Cu(2)–O(2) 2.334(7) Cu(2)–O(2) 2.370(4) Cu(2)–O(2) 2.367(3)
1364
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Table 6 Main molecular parameters which affect antiferromagnetic coupling
b
ꢀ
Compound
a(Cu1–OCu2)/ꢀ
t
Cu1/tCu2
/
d(ꢀ)a
l(ꢀ)
2J/cmꢁ1
ꢁ280
Ref.
6
[Cu2L(N3)2.5](ClO4)0.5ꢂ(H2O)c
134.5(4)
132.3(5)
111.5(1)
109.3(2)
109.6(1)
distorted (sp)
distorted (sp)
0.12/0.01
—
—
83.6(4)
71.4(4)
81.2(3)
81.6(2)
81.8(2)
Complex 1
Complex 2
Complex 3
18.2
17.1
16.5
ꢁ109.8
ꢁ103.9
ꢁ95.9
This work
This work
This work
0.10/0.05
0.10/0.04
LH ¼ 2,6-bis((N-methylpiperazino)methyl)–4-chlorophenol.a Out-of-plane shift angle: dihedral angle formed between the (m-phenoxide) and
b
c
Cu1–Cu2–O5 planes. Dihedral angle between two adjacent basal planes. This compound contains two different dinuclear molecules in the
asymmetric unit.
copper ion is slightly distorted square pyramidal (cf. structure
description). The dx2ꢁy2 magnetic orbitals (containing the
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