Synthesis, spectral, electrochemical and magnetic properties of new phenoxo-bridged dicopper(II) complexes derived from unsymmetrical binucleating ligands

Article abstract of DOI:10.1016/S0277-5387(99)00014-5

New unsymmetrical binucleating ligands 4-bromo-2-[(4-methylpiperazin-1-yl)methyl]-6-[N-(3,5-dimethyl-2-hydroxy-benzyl)- N-methylaminomethyl]phenol (H₂L¹) and 4-bromo-2-[(4-methyl-piperazin-1-yl)methyl]-6-[N-(3,5-dichloro2-hydroxybenzyl)- N-cyclo-hexylminomethyl] phenol (H₂L²) and their copper(II) complexes [Cu₂L^1-2(X)]ClO₄, (X=OH, Br, OAc) have been prepared. Spectral, electrochenuical and magnetic behaviors have been studied. Electrochemical study of the complexes shows two distinct quasireversible redox peaks. (E_pc¹=-0.35 to -0.70 V, E_pc²=-0.75 to -0.97 V). Variable temperature magnetic susceptibility studies of the complexes show the presence of antiferromagnetic interaction between the copper atoms (-2J=69-124 cm^-1). A comparison of the electrochemical and magnetic behavior of the complexes derived from both the ligands is discussed on the basis of substitutional steric and electronic effects.

Full text of DOI:10.1016/S0277-5387(99)00014-5

Polyhedron 18 (1999) 1363–1369
Synthesis, spectral, electrochemical and magnetic properties of new
phenoxo-bridged dicopper(II) complexes derived from unsymmetrical
binucleating ligands
*
P. Amudha, M. Thirumavalavan, M. Kandaswamy
Department of Inorganic Chemistry, School of Chemistry, University of Madras, Chennai-25, India
Received 4 November 1998; accepted 5 January 1999
Abstract
New unsymmetrical binucleating ligands 4-bromo-2-[(4-methylpiperazin-1-yl)methyl]-6-[N-(3,5-dimethyl-2-hydroxy-benzyl)-N-
methylaminomethyl]phenol (H2L1) and 4-bromo-2-[(4-methyl-piperazin-1-yl)methyl]-6-[N-(3,5-dichloro2-hydroxybenzyl)-N-cyclo-hex-
ylminomethyl] phenol (H2L2) and their copper(II) complexes [Cu2L1-2(X)]ClO4, (X5OH, Br, OAc) have been prepared. Spectral,
electrochenuical and magnetic behaviors have been studied. Electrochemical study of the complexes shows two distinct quasireversible
redox peaks. (E1pc520.35 to 20.70 V, Ep2c520.75 to 20.97 V). Variable temperature magnetic susceptibility studies of the complexes
show the presence of antiferromagnetic interaction between the copper atoms (22J569–124 cm21). A comparison of the electrochemical
and magnetic behavior of the complexes derived from both the ligands is discussed on the basis of substitutional steric and electronic
effects.
1999 Elsevier Science Ireland Ltd. All rights reserved.
Keywords: Binucleating ligands; Binuclear copper(II) complexes; Electrochemical studies; Magnetic studies
Synthesis and characterization of new binuclear cop-
per(II) complexes have made an impact on bioinorganic
chemistry, since the spectral, electrochemical and magnetic
features provide new insight into the structure of naturally
occurring dicopper proteins such as hemocyanin, tyrosin-
ase, catechol oxidases, ceruloplasmin etc. [1–5]. In
physicochemical aspects these dicopper complexes have
noteworthy significance as new inorganic materials capable
of showing peculiar magnetic and redox properties and
hence these binuclear copper(II) complexes have wealth of
applications in magnetochemistry [6–8] and homogeneous
catalysis [9]. From Robson’s pioneering work on binuclear
copper(II) complexes with phenolate endogeneous bridge,
a lot of work has been carried out [10,11]. Most of the
work is concerned with the studies of symmetrical di-
copper complexes and the study on unsymmetrical di-
copper complexes is sparse. Synthesis of unsymmetrical
binucleating ligands and their copper(II) complexes gains
the attention in recent years with the establishment of
unsymmetrical nature of the active site of hymocyanin [12]
and tyrosinase [13] and the expectation of interesting redox
and magnetic behaviours associated with the chemically
different coordination environments around the two copper
atoms within the molecule. The present work deals with
the synthesis of two new unsymmetrical binucleating
ligands H2L1 and H2L2 their copper(II) complexes and the
study of their spectral, electrochemical and magnetic
behaviors.
1. Experimental
1.1. Materials and methods
All the chemicals and reagents were obtained from
commercial sources and used without further purification.
3,4-dihydro-3,6,8-trimethyl-2H-1,3-benzoxazine [14], and
2-cyclohexylaminomethyl-4,6-dichlorophenol [15] were
prepared by following the literature procedures. Elemental
analyses were carried out using a Carlo Erba Elemental
1
Analyzer Model 1106. H NMR (90 MHz) spectra were
recorded in CDCb or DMSO D6 using JOEL FX-90Q
Fourier Transform NMR spectrometer. 13C NMR spectrum
(100 MHz) of ligand H2L was recorded on JOEL GSX 400
NMR spectrometer. Mass (EI) spectra were recorded on
*
Corresponding author. Tel.: 191 044 2454515; fax: 191 044
2352494; e-mail address: mKands@unimad.ernet.in
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00014-5
1999 Elsevier Science Ireland Ltd. All rights reserved.

1364
P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
JOEL JMS-DX303 HF Mass spectrometer. IR spectra were
recorded on KBr disc using Hitachi 270-50 Spec-
trophotometer. Electronic spectra were recorded using a
Hewlett-Packerd 8452A Spectrophotometer. Conductivity
measurements were done using an Elico digital conduc-
tivity bridge model CM-88 using freshly prepared solution
of the complexes in dmf Cyclic voltammograms were
obtained on an apparatus comprising a PAR model 173
potentiostat/galvanostat, model 175 universal programmer,
model 176 current/voltage convertor, 179 coulometer and
Perkin Elmer Hitachi 057 X–Y recorder using platinum
foil as the working electrode and a saturated Ag/AgCl as
the reference electrode, using freshly distilled HPLC grade
dmf as the solvent. Magnetic susceptibility on powdered
samples of the complexes were measured in the tempera-
ture range 77–300 K using a PAR model 155 vibrating
sample magnetometer and the instrument was calibrated
with the use of metallic nickel. Room temperature mag-
netic moment of the complexes were recorded using a
Gucy balance, in which HgIICo(SCN)411 was used as the
calibrant. ESR spectra of the complexes were recorded
using a Varian EPR-E112 spectrophotometer using DPPH
as the reference.
1.4. 4-Bromo-2-[(4-methylpiperazin-1-yl)methyl]-6-[N-
(3,5-dimethyl-2-hydroxybenzyl)-N-methy-
laminomethyl]phenol. H2L1
Compound PC1 (3.35g, 0.01 mol) was dissolved in 100
ml of ethanol and N-methylpiperazine (1.2 ml, 0.01 mol)
was added. To the resulting mixture 37% formalin (1.2 ml,
0.015 mol) was added with cooling and stirred at room
temperature for 5 h. The mixture was refluxed on a water
bath for 24 h and the solvent was evaporated. The residue
was thoroughly washed with water. Column chromato-
graphic separation on silica gel (CH2Cl2 /MeOH 4:1)
offered the compound as yellow oil. Yield 2.2 g (67%), 1H
NMR (CDCl3) d52.2 (two close peaks, 9H), 2.3 (s, 3H),
2.6 (br, 8H), 3.6 (two close peaks, 6H), 6.5–6.8 (m, 4H),
9.8 (br. 2H). Mass (EI) m/z5461 (M11). Anal (%) Calc
for C2 3H3 0BrN3O2 C, 60.00; H,6.52; N,9.13. Found
C,61.09; 11,6.96; N,9.27.
1.5. 4-Bromo-2-[(4-methylpiperazin-1-yl)methyl]-6-[N-
(3,5-dichloro-2-hydroxybenzyl)-N-cyclo-
hexylaminomethyliphenol H2L2
The procedure adopted for the preparation of the ligand
H2L2 was same as that of H2L1, but instead of ethanol
medium the reaction was carried out in THF/ethanol(1:1).
After 24 h of refluxion the solvent was evaporated at room
temperature. The residue was extracted with CH2Cl2 (23
50 ml). To the extract 50 ml of methanol was added and
cooled at 08C for several days. Solid separated was filtered
and recrystallized from ethanol–hexane(1:1). m.p. 878C,
Yield 1.6 g, (34%), 111 NMR (CDCl3) d51–2 (two broad
mutiplets, 10H), 2.3 (s, 3H), 2.6 (br. 9H), 3.6 (s, 2H), 3.65
(s, 2H), 3.7 (s, 2H), 6.6–7.2 (m, 4H). 13C NMR (CDCl3)
d525.4, 25.7, 31.5, 47.02, 47.06, 57.68, 58.8, 60.6, 109.7,
117.8, 123.1, 124.7, 126.5, 126.6, 127.3, 127.4, 153.6,
156.4. Mass(EI) m/z5571 (M1). Anal (%) Calc for
C26H34BrCl2N3O2 C,54.74; H,5.96; N,7.37. Found
C,54.84; H,6.01; N,7.45.
1.2. N-(2-Hydroxy-5-bromobenzyl)-N-(2-hydroxy-3,5-
dimethylbenzyl)-N-methylamine PC1
3,4-Dihydro-3,6,8-trimethyl-2H-1,3-benzoxazine (8.85
g, 0.05 mol) and p-bromophenol (8.65 g, 0.05 mol) were
mixed by slight warming and kept stoppered at room
temperature. After 15 days the viscous mass was diluted
with 150 ml of methanol and kept at room temperature for
24 h. Colourless solid separated was filtered off and
recrystallized from benzene–methanol (1:1) mixture. m.p.
1
1588C, Yield 9 g, 48 %. H NMR (CDCl3) d52.2 (two
close peaks, 9H), 3.7 (s, 4H), 6.7–6.9 (m, 5H). Mass (EI)
m/z5336 (M1). Anal (%) calc. for C17H20BrNO2 C,
60.81; H, 5.96; N, 4.17. Found C, 61.18; H, 6.35; N, 4.29.
1.3. N-(2-Hhydroxy-5-bromobenzyl)-N-(2-hydroxy-3,5-
dichlorobenzyl)-N-cyclohexylamine PC2
The
m-phenoxo-m-(X)-dicopper(II)
complexes
[Cu2L1–2(X)]ClO4 (1–6) were prepared by following the
procedure reported by Reed and coworkers [16].
Paraformaldehyde (1.8 g, 0.06 mol) was taken in 75 ml
of glacial acetic acid, 2-cyclohexylaminomethyl-4-6-di-
chlorophenol (13.6 g, 0.05 mol) was added and stirred at
room temperature for 24 hours. Then 4-bromophenol (8.65
g, 0.05 mol) was added and stirred for 72 hours. The
reaction mixture was neutralized with solid Na2CO3 and
the residue was dissoluted in 150 ml of methanol. Colour-
less insoluble solid separated was filtered off and re-
crystallized from CHCl3. m.p. 1798C. Yield 7 g (69%), 1H
NMR (DMSO D6) d51–2 (two broad multiplets, 10H),
2.5 (br, 1H), 3.6 (s, 2H), 3.7 (s, 2H), 6.7–7.2 (m, 5H).
Mass (EI) m/z5460 (M12) Anal (%) Calc for
C2 0H2 2BrCl2NO2 C,52.40; H,4.80; N,3.06. Found
C,52.45; H,4.85; N,3.17.
The ligand H2L1 or H2L2 (1 mmol) was dissolved in 75
ml of distilled methanol by heating. To the resulting
solution copper(II) perchlorate hexahydrate (0.74 g, 2
mmol) dissolved in 30 ml of methanol was added and
refluxed on a water bath for 3 h. The reaction mixture was
then reacted with one equivalent of NaX (X5OH, Br,
GAc) dissolved in 5 ml of water–methanol (1:4) and
refluxed for further 1 h. Solid compound deposited on slow
evaporation of the solvent was filtered, washed with
diethyl ether and dried. Various attempts to develope
crystals suitable for X-ray diffraction studies such as slow
diffusion, crystallisation using mixture of solvents and low
temperature crystallisation were unsuccesful.

P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
1365
1.5.1. [Cu2L1(OH)]ClO4.2H2O (1)
assigned to the piperazinyl protons and multiplet observed
in the region d56.6 to 7.1 were assigned to the aromatic
protons. Broad bands observed in the region d51–2 and
2.5 for the compound PC2 and ligand H2L2 were assigned
to the cyclohexyl protons. Appearance of new benzylic
peak in the region d53.6 and 3.65 and broad peak in the
region d52.6 in the ligands H2L1 and H2L2 with respect
to their corresponding intermediates clearly indicates the
presence of N-methyl piperazinyl residue in the ligands.
IR spectra of the complexes shows a broad band
centered around 3400 cm21 indicating the presence of
coordinated or lattice water in the complexes along with a
Green solid. Anal (%) calc for C2 3BrCl Cu2N3O9 C,
37.35; H,4.74; N,5.68. Found C,37.45; H,4.74; N,5.59.
Selected IR (KBr): 3450, 1630, 1100 and 620 cm21
.
Conductance (Lm /S cm2 mol21) in dmf 74.,/H2.
1.5.2. [Cu2L1(Br)]ClO4.H2O (2)
Brown solid. Anal (%) calc for C23H3 2Br2Cl Cu2N3O7
C,35.23; H,4.08; N,5.36. Found C,35.11; H,4.13; N,5.28.
Selected IR (KBr): 3450, 1630, 1100 and 620 cm21
.
Conductance (Lm /S cm2 mol21) in dmf 83.
1.5.3. [Cu2L1(OAc)]ClO4.2MeOH (3)
strong symmetrical peak for the ClO4 anion at 1100 cm21
,
Green solid. Anal(%) calc for C2 7H4 1BrCl Cu2N3O1 0
C,41.67; H,5.27; N,5.40. Found C,41.45, H,5.24; N,5.48.
Selected R(KBr): 1630, 1100 and 620 cm21. Conductance
(LmS cm2 mol21) in dmf 78.
which indicates that the perchlorate anion is not coordi-
nated to the copper ions. The acetato bridged complexes 3
and 6 shows the characteristic peak for the acetate in the
region 1540 cm21. Electronic spectra of the complexes
were studied in dmf and the data are given in Table 1. The
electronic spectra of the complexes shows three main
features, a weak band in the region around 640–676 nm
(´¯150 M21 cm21) for the d–d transition, a peak or
1.5.4. [Cu2L2(OH)]ClO4.2H2O (4)
Green solid. Anal(%) calc for C26H37Cl4 Cu2N3O9
C,38.81; H,4.60; N,5.22. Found C,38.89; H,4.65; N,5.28.
Selected IR (KBr): 3450, 1630, 1100 and 620 cm21
.
shoulder in the region 410–428 nm (´¯1000 M21 cm21
)
Conductance (Lm /S cm2 mol21) in dmf 77.
for phenolate to copper charge transfer transition and one
or two strong peaks below 300 nm (´¯7000 M21 cm21
)
1.5.5. [Cu2L2(Br)]ClO4.MeOH (5)
for intraligand charge transfer transitions. A red shift in the
d–d band for the complexes of the ligand H2L2 when
compared to the complexes of the ligand H2L1 indicates
that the coordination geometry around the copper atom of
the former complexes is more distorted than the latter
complexes.
Brown solid. Anal(%) calc for Cr2 7H3 6BrCl4 Cu2N3O7
C,37.54; H,4.17; N,4.87. Found C,37.69; H,4.14; N,4.98.
Selected IR (KBr): 1630, 1100 and 620 cm21. Conduct-
ance (Lm/S cm2 mol21) in dmf 85.
1.5.6. [Cu2L1(OAc)]ClO4.2H2O (6)
Green solid. Anal(%) calc for C28H3 8C1 4 Cu2N3O8
C,41.33; H,4.67; N,5.17. Found C,41.33; H,4.45; N,5.12.
2.2. Electrochemical studies
Selected IR (KBr): 3450, 1630, 1100 and 620 cm21
.
Electrochemical behavior of the complexes was studied
by molar conductance measurement and cyclic voltam-
metry. All the complexes show a molar conductance in the
range 75–85 Lm /S cm2 M21 which indicates that the
complexes are 1:1 electrolytes [18] in which the per-
chlorate is not coordinated to the any of the copper atoms
(Table 2). The cyclic voltammogram of the complexes
generally show two redox peaks and the data are summa-
rized in Table 2. Cyclic voltammogram for the complexes
1 and 4 are reproduced in Fig. 3.
Coulometric analysis carried out for the complexes 1
and 4 at 100 mV to the first reduction peak consumes one
electron per molecule and consumes two electrons per
molecule of the complex when it was performed at 100 mV
more negative to the second reduction peak. This be-
haviour indicates that the two reduction peaks are due to
Conductance (Lm /S cm2 mol21) in dmf 79.
2. Results and discussion
2.1. Synthesis
Reaction of p-bromophenol and 3,4-dihydro-3,6,8-tri-
methyl- 2H-1,3-benzoxazine in 1:1 molar ratio offered the
intermediate compound PC1, whose X-ray structure has
been solved (Fig. 1) [17]. Reaction of p-chromophenol and
2-cyclohexylaminomethyl-4-6-dichlorophenol gave the in-
termediate PC2. Reaction of PC1 and PC2 with formalde-
hyde and N-methylpiperazine (1:1:1) offered the ligands
H2L1 and H2L2 respectively. Reaction of the ligands with
copper(II) perchlorate and appropriate sodium salt (1:2:1)
afibrded the binuclear copper(II) complexes (Fig. 2).
In all the compounds the peaks observed in the region
d52.2 to 2.3 were assigned to the aromatic methyl and
N-methyl protons, peaks observed in the region d53.6 to
3.9 were assigned to the benzylic protons, broad observed
in the ligands H2L1 and H2L2 in the region d52.6 were
two
single
electron
transfers
which
involves
Cu(II)Cu(II)àCu(II)Cu(I) and Cu(II)Cu(I)àCu(I)Cu(I)
conversions. The relative stability of the mixed valent
species Cu(II)Cu(I) in the equilibrium mixture of
Cu(II)Cu(II) and Cu(I)Cu(I) has been calculated using the
relationship log Kcon5DE/0.0591 and the values are given
in Table 2. Larger Kcon values observed for the complexes

1366
P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
Fig. 1. Schematic diagram for the synthesis of the ligands and complexes.
Table 1
Electronic spectral data for the complexes
Complex
lmax /nm (´/M21 cm21
)
d–d
C.T
1
2
3
4
5
6
640(148)
642(191)
647(160)
672(184)
676(162)
666(178)
420(648), 310(5202), 290(4169)
410(s), 310(7171), 291(7498)
410(1121), 320(9202), 293(8167)
410(700), 313(9152), 298(9093)
415(765), 3 10(6763), 297(6160)
410(531), 3 15(7610), 298(8031)
Fig. 2. ORTEP diagram for compound PC1.

P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
1367
Table 2
Electrochemical data for the complexes
Complex
Ep 1c /V
Ep 1a /V
E11 / 2
Ep 2c /V
Ep a2 /V
E12 / 2 /V
Kcon
(DE/mV)
(DE/mV)
1
2
3
4
5
6
20.70
20.66
20.55
20.42
20.39
20.35
20.52
20.54
20.45
20.32
20.23
20.13
20.61(180)
20.60(120)
20.50(100)
20.37(100)
20.31(160)
20.24(220)
20.97
20.93
20.85
20.85
20.80
20.75
20.85
20.62
20.66
–
20.62
20.65
20.91(120)
20.81(190)
20.76(190)
–
20.72(180)
20.70(100)
1.193105
4.323105
2.503104
–
7.793103
6.073107
Potential V vs Ag/AgCl, Electrodes: Pt vs Ag/AgCl. Sweep rate 50 mV/sec. Supporting electrolyte: TBAP, Concentration: Complex (131023 M); TBAP
(131021 M).
indicates the greater stability of the mixed valent species
under electrochemical conditions.
superexchange interaction within the complex molecule.
The extent of superexchange interaction has been evalu-
ated by performing variable temperature magnetic moment
measurements in the temperature range 77–3000 K and by
fitting the data to the Bleaney-Bowers equation [22].
A comparison of the electrochemical data of the com-
plexes of the ligands H2L1 and H2L2 shows the following
interesting feature. Complexes of the ligand H2L2 undergo
reduction at less negative potential (E1pc520.35 to 20.42
V and E2pc520.75 to 20.85 V) when compared to the
complexes of the ligand H2L1 (Ep1c520.55 to 20.70 V
and E2pc520.85 to 20.97 V). Since, the nature of the
donor atoms in both these ligands are the same, the
reduction observed at less negative potential for the
complexes 4–6 can be related to the electronic and steric
effect of the substituents of the ligand H2L2. The electron
withdrawing chlorine atoms positioned on the phenolic
residues of H2L2 will reduce the electron density on the
copper atoms, distortion will be induced in the copper
coordination geometry by the relatively bulky cyclohexyl
group. Further, cyclohexyl amine of the ligand H2L2 is less
basic than methyl amine [19] of the H2L1, hence the latter
will be a better electron donor than the former. Both
distortion in the coordination geometry [20] and less
electron density on the copper atom [21] in the complexes
4–6 of the ligand H2L2 will influence easy Cu(II)→Cu(1)
conversion at less negative potential relative to the com-
plexes of the ligand H2L1.
xm 5 (Ng2b2 /3kT)[3 1 exp(22J/kT)21(1 2 P)
1 0.45P/T 1 Na
where xm is the molar magnetic susceptibility of the
complex, P is the percentage of monomeric impurities and
other symbols have their usual meanings. Na has been
fixed as 12031026 cm3 mol21 and g has been fixed at
2.20 for all magnetic simulations. A best fit to the
experimental value for the complexes were summarized in
Table 3. The temperature dependance of the magnetic
properties for the complexes 1 and 5 are shown in Fig. 4.
Poor exchange interaction is the generally observed
phenomenon in unsymmetrical dicopper(II) complexes due
to ligand induced two different geometries around the two
copper atoms within the molecule [23,24], which may
result larger dihedral angle between the two copper
magnetic orbitals [25]. In accordance with these results,
the observed coupling constant values for the complexes
are low.
The electrochemical data indicates that the reduction
potential is exogenous donor dependent. The reduction
potential follows the order OAc,Br,OH. Although ace-
tate is better electron donor than hydroxide and bromide,
reduction was observed at less negative potential. This
behavior seems to be associated with the increased number
of atoms involved in the acetate bridge, which may offer
more flexibility for the acetate bridged complexes as
against the bromo and hydroxo bridged complexes. Reduc-
tion at less negative potential observed for the bromo
bridged complexes when compared to the hydroxide may
be due to the poor electron donating nature of the bromide
than the hydroxide.
Relatively larger spin exchange coupling values are
observed for the complexes of the ligand H2L1, when
compared to the complexes of the ligand of H2L2. This
behavior indicates that the extent of spin–spin interactions
in the former case is more than the latter. As explained in
the electrochemical properties of the complexes of the
ligands, generation of electropositive character on the
copper atoms due to effective electron withdrawal by the
chlorine atoms substituted in the ligand H2L2 and the
possible distortion in geometry due to the bulky cyclohexyl
group of the ligand are the most probable reasons for the
observed low 22J values of the complexes of H2L2 than
H2L1.. Reports suggest both these factors i.e. reduction in
electron density on the copper atoms [26] and distorted
structure [27–31] are unfavourable for effective coupling,
resulting a small 22J value for the complexes of the
ligand H2L2.
2.3. Magnetic properties
Low room temperature magnetic moment value ob-
served for the complexes suggests the operation of
Comparison of the complexes 1, 3 or 5, 6 of a particular

1368
P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
Table 3
Magnetic and ESR spectral data for the complexes
Complex
22J cm21
meff per mole-
g
cule (B.M.)
77K
298K
1
2
3
4
5
6
124
–
98
–
85
69
1.65
–
1.96
–
1.73
2.16
1.98
2.10
2.15
2.24
2.65
2.83
213
225
219
gu 52.24
gu 52.20
gu 52.25
g' 52.06
g' 52.01
g' 51.99
Fig.
3.
Cyclic
voltammogram
for
the
complexes
(a)
[Cu2L1(OH)]ClO4.2H2O 1 and (b) [Cu2L2(OH)]C1O4.2H2O 4.
ligand shows that the exchange interaction is influenced by
the exogenous donor. A lower 22J value was observed for
acetato bridged complexes (3 and 6) when compared to the
hydroxo or bromo bridged complexes. This behaviour of
the acetato complexes is possibly due to the counter-
complementary nature of the overlap of the magnetic
orbitals involving monciatomic and three atom bridging
ligands [32].
Fig. 4. Temperature dependence magnetic properties of the complex (a)
[Cu2L1(OH)]ClO4.2H2O 1 and (b) [Cu2L2(Br)]ClO4.MeOH 5.

P. Amudha et al. / Polyhedron 18 (1999) 1363–1369
1369
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Fig. 5. ESR spectra for the complexes (a) [Cu2L1(OH)]ClO4.2H2O 1 and
(b) [Cu2L2(OH)[ClO4.2H2O 4.
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2.4. ESR spectra
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The ESR spectra of the complexes at 298 K of the
complexes were obtained in the X-band region and the g
values were evaluated using the relationship hy 5gbH and
the g values are given in Table 3 and the ESR spectra of
the complexes 1 and 4 are reproduced in Fig. 5. Generally
all the complexes of the ligand H2L1 shows a broad band
and the complexes of the ligand H2L2 show broad band
with poorly resolved hyperfine splitting signals and the gu
values observed for the complexes are greater than the g'
values which indicates that the copper complexes of both
ligands display a signal for uDMs 52u transition at g54.35
indicating the presence of spin-exchange interaction in the
complexes [33,34]
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3. Summary
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New unsymmetrical binucleating ligands and their cop-
per(II) complexes have been prepared. Electrochemical
study of the complexes indicates that the reduction po-
tentials are strongly dependent on the nature of the
exogenous donor atoms and peripheral substuitutents.
Variable temperature magnetic and ESR spectral study of
the complexes indicates the presence of spin–spin inter-
action between the copper atoms.
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References
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