Imidazolidine ring as a reduced heterocyclic spacer in a new all-N-donor μ-bis (bidentate) Schiff base ligand: Synthesis, characterization and electron transfer properties of imidazolidine-bridged dicopper complexes

Abstract

Low-temperature stoichiometric Schiff base reaction in air in 3:1 mole ratio between benzaldehyde and triethylenetetramine
(trien) in methanol yields a novel tetraaza μ-bis (bidentate) acyclic ligand L. It was characterized by elemental analysis, IR, EI mass and NMR (1H and13C) spectra. The formation of a five-membered imidazolidine ring from the ethylenediamine backbone as a spacer-cumbridging
unit gives rise to a new type of imidazolidine-bridged ligand. A geometric optimisation was made of the synthesized ligand
and its complexes by the method of molecular mechanics (MM2) method in order to establish the stable conformations. This hitherto
unknown tetraaza acyclic ligand affords new cationic dicopper(I/I) and dicopper(II/II) complexes in good yield. Dicopper(II/II)
complex displays weakd-d transition bands in the visible region, while dicopper(I/I) complex displays strong MLCT band in the same region. Both the
dinuclear complexes are of non-intimate nature and show interesting solution electrochemical behaviour. EPR spectral study
of μ-bis (imidazolidino) bridged dicopper(II/II) complex also supports the non-communicative nature of the two copper centres within
the same molecule

Full text

Page 1

J. Chem. Sci., Vol. 116, No. 3, May 2004, pp. 151–158. © Indian Academy of Sciences.

151
*For correspondence
Imidazolidine ring as a reduced heterocyclic spacer in a new
all-N-donor mm -bis(bidentate) Schiff base ligand: Synthesis,
characterization and electron transfer properties of
imidazolidine-bridged dicopper complexes
MANINDRANATH BERA, PRASANT KUMAR NANDA, UDAY MUKHOPADHYAY and
DEBASHIS RAY*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
e-mail: dray@chem.iitkgp.ernet.in
MS received 9 January 2004; revised 6 April 2004
Abstract. Low-temperature stoichiometric Schiff base reaction in air in 3 : 1 mole ratio between benz-
aldehyde and triethylenetetramine (trien) in methanol yields a novel tetraaza m -bis(bidentate) acyclic
ligand L. It was characterized by elemental analysis, IR, EI mass and NMR (1H and 13C) spectra. The
formation of a five-membered imidazolidine ring from the ethylenediamine backbone as a spacer-cum-
bridging unit gives rise to a new type of imidazolidine-bridged ligand. A geometric optimisation was
made of the synthesized ligand and its complexes by the method of molecular mechanics (MM2) method
in order to establish the stable conformations. This hitherto unknown tetraaza acyclic ligand affords new
cationic dicopper(I/I) and dicopper(II/II) complexes in good yield. Dicopper(II/II) complex displays
weak d–d transition bands in the visible region, while dicopper(I/I) complex displays strong MLCT band
in the same region. Both the dinuclear complexes are of non-intimate nature and show interesting solu-
tion electrochemical behaviour. EPR spectral study of m -bis(imidazolidino) bridged dicopper(II/II) com-
plex also supports the non-communicative nature of the two copper centres within the same molecule.

Keywords. Schiff base; dicopper (II/II) complexes; imidazolidine-bridged; molecular mechanics; cy-
clic voltammetry; EPR.
1. Introduction
The design and synthesis of new dinuclear complex
requires good agreement between the stereochemi-
cal requirements of the metal and the special fea-
tures of the ligand such as geometry of the available
donor groups and spacers between the coordination
groups.1 Among various products from the conden-
sation of aromatic aldehydes with a,w-tetramine con-
taining both primary and secondary amino groups is
a binucleating Schiff base with an in-built spacer
imidazolidine ring, which can take up two same or
different metal ions.2 Recently we reported X-ray
structural characterization of one such heterodi-
nuclear complex of copper(II)–zinc(II) held by an
imidazolidine ring.3 The use of binucleating ligands
for the synthesis of new family of di-3d-metal com-
plexes has been receiving considerable attention in
recent years following the identification of similar
catalytically active biosites in living systems. In
case of dicopper complexes, during complex forma-
tion, the d10 Cu1+ ion can assemble two molecules of
any m -bis(bidentate) ligand in tetrahedral geometry.
Similar helical assembly is not feasible for d9 ion
copper(II) because its geometrical preference is dif-
ferent. Instead the copper(II) ions change the ligand
arrangement according to their choice for a tetra-
gonal coordination. The in-built unsubstituted imi-
dazolidine spacer, on the other hand, does not allow
the mononuclear N4 coordination around a single
copper(II) ion. Two m -bis(bidentate) ligands can
accommodate two copper ions in two cavities.
The p-acceptor features of the coordinating nitro-
gen atoms of the new ligand 2-phenyl-2,3-bis-[3′-aza-
4-(2″-phenyl)-prop-4′-en-1′-yl]-1,3-imidazolidine, L
stabilize the CuI state, while the phenyl substituted
imidazolidine spacer keeps the two metal centres
apart in a 2 : 2 metal–ligand stoichiometry. This
promotes the formation of the stable dicopper(I/I)

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Manindranath Bera et al

152
entity [Cu2
centres are only stabilized in a square ligating envi-
ronment, again in a dinuclear assembly. Therefore,
the reversible ligand twisting around the spacer imi-
dazolidine ring can accommodate both the copper(I)
and copper(II) centres within the same molecule.
The [Cu2
both in solution and in the solid state. In case of a
double-helical arrangement the flexible imidazolidine
ring is folded somewhat to introduce the required
twist in the ligand backbone for metal binding
through tetrahedral coordination, whereas for a non-
helical case it remains in planar form. Dicopper(I/I)-
promoted activation of molecular dioxygen and sub-
sequent hydroxylation reaction and mechanism of
the nearby phenyl ring is an important area of res-
earch4–9 using synthetic dicopper complexes. The
ineffectiveness of the imidazolidine bridges in im-
porting any magnetic communication between the
two copper(II) centres has also been here established
through EPR study.
I/IL2]2+ for 2b. The corresponding CuII
I/IL2]2+ and [Cu2
II/IIL2]4+ species are stable
2. Experimental
Triethylenetetramine (trien) was obtained from SD
Fine Chem, India. Benzaldehyde was purchased
from Qualigens, India. Cupric perchlorate hexahy-
drate was prepared by treating copper(II) carbonate
with 1 : 1 HClO4 and crystalling after concentration
on a water bath. Dry acetonitrile was prepared by
the published method.10 [Cu(MeCN)4]ClO4 was also
prepared by a published procedure.11 The prepara-
tion of tetraethylammonium perchlorate (TEAP) for
electrochemical work was performed as reported in
the literature.12 Microanalyses (C, H, N) were carried
out using a Perkin–Elmer 240 C elemental analyser.
The solution electrical conductivity and electronic
spectra were obtained using a Unitech type U131C
digital conductivity meter with a solute concentra-
tion of about 10–3 M and a Shimadzu UV 3100 UV-
Vis-NIR spectrophotometer respectively. Mass
spectra were obtained with a Finnigan MAT 8200
(electron ionisation, EIMS) instrument. Room tem-
perature magnetic susceptibilities in the solid state
were measured using a home-built Gouy balance fit-
ted with a polytronic DC power supply. The experi-
mental magnetic susceptibilities were corrected for
the diamagnetic response using Pascal’s constants.
IR spectra were recorded on a Perkin–Elmer 883
spectrophotometer. X-band EPR spectra were recor-
ded on a Varian E-109C spectrometer fitted with a
quartz Dewar flask for measurements at 77 K (liquid
nitrogen). The spectra were calibrated with diphenyl-
picrylhydrazyl(dpph) (g = 2⋅0037).The microwave
power level was maintained at ≈ 0⋅2 mW. Electro-
chemical measurements were made using a PAR
model 173 potentiostat/galvanostat, 175 universal
programmer, 178 electrometer, and 377-cell system.
A planar Beckman 39273 platinum-disk working-
electrode, a platinum-wire auxiliary-electrode, and
an aqueous saturated calomel reference electrode
(SCE) were used in a three-electrode configuration.
A digital series 2000 Omni Graphic recorder was
used to trace the voltammograms. Electrochemical
measurements were made under a dinitrogen atmos-
phere. Conformational analysis was done by using
Allinger’s MM2 method.13
2.1 Synthesis of ligand L
A solution of triethylenetetramine (5 g, 34⋅2 mmol)
in methanol (15 ml) was added drop wise to an ice-
cold methanolic solution (30 ml) of benzaldehyde
(10 g, 102⋅7 mmol) with stirring the yellow solution
was stirred for 1 h and the solvent was evaporated in
air. The yellow solid was separated by filtration
through G4 sintered bed and washed thoroughly
with hexane and water. Finally the isolated com-
pound was dried in vacuo over P4O10. Yield 8⋅4 g
(60%), m.p. 75–77°C. Analysis Calc. for C27H30N4:
C, 78⋅99; H, 7⋅36; N, 13⋅65%. Found: C, 78⋅76; H,
7⋅49; N, 13⋅89%. Mass spectrum (EI): m/z 410
(M+ = L+). Infrared spectrum (cm–1, KBr disk): 1639
(vs, nC=N). 1H NMR (200 MHz, CDCl3) dppm: 2⋅5–2⋅9
(8H, m, H8 and H9), 3⋅47–3⋅66 (4H, m, H10 and H11),
3⋅73 (H, s, H12), 7⋅25–7⋅28 (3H, m, H15, H16 and
H17), 7⋅34–7⋅36 (8H, m, H1, H2, H3, H14 and H18),
7⋅66–7⋅67 (6H, m, H4, H5 and H6), 8⋅17 (2H, s, H7).
13C NMR (50 MHz, CDCl3) dppm: 51⋅86 (C10 and
C11), 53⋅35 (C9), 60⋅61 (C8), 89⋅55 (C12), 96⋅14 (C1,
C5, C14 and C18), 127⋅81 (C2, C4, C15 and C17),
128⋅05 (C16), 128⋅44 (C3), 136⋅26 (C6), 161⋅70 (C7).
2.2 Synthesis of the complexes
[Cu2
Cu(ClO4)2.6H2O (0⋅542 g, 1⋅46 mmol) was slowly
added drop wise at ambient temperature to a stirred
methanolic solution (15 mL) of L (0⋅6 g, 1⋅46 mmol)
over a period of 0⋅5 h. The blue coloured compound
that formed was separated immediately. The mixture
was stirred for 1 h at room temperature and the blue
II/II(m -L)2] (ClO4)4 (2a): An aqueous solution of

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Study of imidazolidine-bridged copper complexes

153
precipitate was filtered through a glass frit, washed
with water followed by ethanol and hexane and
finally dried in vacuo over P4O10. The yield of the
compound was 0⋅69 g (70%). Analysis Calc. for
C54H60N8O16Cl4Cu2: C, 48⋅19; H, 4⋅49; N, 8⋅32; Cu,
9⋅44%. Found: C, 48⋅32; H, 4⋅66; N, 8⋅57, Cu,
9⋅58%. Infrared spectrum (cm–1, KBr disk): 1617
(vs, nC=N). Molar conductance, ΛM: (MeCN solution)
560 ohm–1 cm2 mol–1. UV-Vis spectra [lmax, nm (e,
l mol–1 cm–1)]: (MeCN solution) 576 (380), 277
(9875), 244 (25440). Mass spectrum (FAB): m/z
1247 (M+ = [Cu2

[Cu2
(0⋅32 g, 0⋅978 mmol) was added in portions to the
degassed acetonitrile (20 mL) solution of L
(0⋅976 mmol) in nitrogen atmosphere. Immediately
a yellow compound separated and the mixture was
stirred for 15 min. The compound was filtered,
washed and dried in vacuo over P4O10. A second
crop of the compound was obtained by evaporating
the yellow filtrate at reduced pressure. The yield of
the compound was 0⋅34 g (60%). Analysis Calc. for
C54H60N8O8Cl2Cu2: C, 56⋅54; H, 5⋅27; N, 9⋅77; Cu,
11.08%. Found: C, 56⋅17; H, 5⋅22; N, 9⋅52, Cu,
11⋅02%. Infrared spectrum (cm–1, KBr disk): 1611
(vs, nC=N).
2⋅49–3⋅45 (8H, m, H8 and H9), 3⋅51–3⋅68) (4H, m,
H10 and H11), 3⋅84 (H, s, H12), 7⋅33–7⋅39 (3H, m,
H15, H16 and H17), 7⋅45–7⋅59 (8H, m, H1, H2, H3, H14
and H18), 7⋅93–8⋅07 (6H, m, H4, H5 and H6), 8⋅62
(2H, s, H7). Molar conductance, ΛM: (MeCN solu-
tion) 255 ohm–1 cm2 mol–1. UV-Vis spectra [lmax,
nm (e, 1 mol–1 cm–1)]: (MeCN solution) 400 (4700),
257 (24330), 241 (24035).

Caution! Perchlorate salts of metal complexes are
potentially explosive and should be handled in small
quantities with care.
II/II(m -L)2](ClO4)3
+).
I/I(m -L)2] (ClO4)2 (2b): Solid [Cu(MeCN)4]ClO4
1H NMR (200 MHz, d6-DMSO) dppm:
3. Results and discussion
The ligand used in this work belongs to a new class
of m -bis(bidentate) type with a semi-rigid substituted
five-membered imidazolidine spacer and has easy
synthetic accessibility. The five-membered imida-
zolidine ring is introduced inside the tetradentate
precursor to act as a spacer-cum-bridging-cum-back-
bone unit. The synthesis and characterization of tri-
benzylidinetriethylenetetramine (L) ligand and its
dicopper(II/II) and (I/I) complexes are described in
this work. The ligand with bis imine groups sepa-
rated by a flexible semi rigid imidazolidine ring can
behave as a good bis(bidentate)-N,N′ donor ligand
to two metal ions for discrete binuclear metal com-
pounds. The conformationally and geometrically
well-defined phenyl substituted imidazolidine linker
can guide bis imine subunits to create a particular
coordination pocket for metal ion binding. Discrete
binuclear assembly of copper(II) ions as against
polynuclear assembly is formed in quantitative
yields. The ligand does not use all its coordination
sites to bind a single metal centre. Each copper(II)
centre is bonded to two different molecules of the
ligand in a CuN4 coordination mode. The preference
of the copper(II) ions for a tetragonal four-coordi-
nate geometry and the semirigidity of the imidazoli-
dine coordinating-cum-spacer group forces the ligand
to act as a bis(bidentate) rather than a tetradentate
one. The situation is also favourable for the forma-
tion of an infinite self-assembly1 which is not achie-
ved with our ligand system. These single imidazo-
lidine-bridged dimetal complexes are less intimate
and show magnetic features typical of mononuclear
complexes only. This non-phenolic all-N-donor
ligand system is effective in stabilizing the dicop-
per(I/I) complex also. Both the complexes show in-
teresting electron–transfer behaviour as revealed by
cyclic voltammetry.
In a one-pot reaction the ligand was prepared by
the condensation of 1 equivalent of trien and 3
equivalents of benzaldehyde (scheme 1).
Ligand L has been characterized by IR, 1H and
13C NMR and mass spectral analyses. In IR spec-
trum a strong band at 1639 cm–1 indicates the nC=N


H2N
+
3
CHO
N
N
N
N
1
2
3
4
5
10
11
13
14
15
16
L
17
18
8
9
7
6
12
N
H
N
H
NH2
MeOH
0 0 C - R.T.


Scheme 1. 2-Phenyl-2,3-bis-[3′-aza-4-(2″-phenyl)-prop-4′-
en-1′-yl]-1,3-imidazolidine, L.

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Manindranath Bera et al

154
stretching frequency. The 1H NMR spectrum of L in
CDCl3 shows a characteristic signal of the imida-
zolidine ring proton (H12) at 3⋅73 ppm. The aromatic
protons appear as a multiplet centred in the range of
7⋅25–7⋅67 ppm. The imine protons are observed at
8⋅17 ppm (H7). In the 13C NMR spectrum the imine
carbons (C7) appear at 161⋅70 ppm indicating the
characteristic imine carbons. In the mass spectrum
the molecular ion peak (m/z) is observed at 410
which indicates the formation of the desired imida-
zolidino N4 Schiff base ligand. The EI mass spectra
of the ligand is shown in figure 1.
The reaction of the methanolic solution of ligand
L with aqueous copper(II) perchlorate hexahydrate
in 1 : 1 mole ratio at room temperature in air leads to
the formation of a blue dicopper(II/II) complex (see
(1) below). The same ligand on reaction with tetra-
kis(acetonitrile)copper(I) perchlorate in 1 : 1 mole
ratio at room temperature in dinitrogen atmosphere
and dry MeCN, gives an orange dicopper(I/I) com-
plex ((2) below). Analytical, spectral and room tem-
perature magnetic moment values establish that the
dinuclear complexes are of composition [Cu2
L)2](ClO4)4 (2a) and [Cu2
pectively. The complex [Cu2
also characterized by solution (d6-DMSO) 1H NMR
spectra. The characteristic signals are slightly shifted
toward higher ppm values from their free ligand
values. The imine CH=N peaks are seen at 8⋅62
(8⋅17) ppm. In dicopper(I/I) complex ethylene hy-
drogen resonances of the amine back bone are
observed at 2⋅49–3⋅45 d and are broad in nature.
Imidazolidine proton signal is seen at 3⋅84 ppm and
slightly shifted downfield compared to the free
ligand position. The aromatic proton signals also
show downfield shift in the range 7⋅33–8⋅07 ppm.


II/II(m -
I/I(m -L)2](ClO4)2 (2b) res-
I/I(m -L)2](ClO4)2 was
N
N
NN
CuII/I
Cu II/I
Cu II/I
N
N
N
N
2a/2b

Cu(ClO4)26H2O + L
[Cu2

MeOH/H2O
RT stirring

II/II(m -L)2](ClO4)4 (1)
[Cu(MeCN)4](ClO4) + L
MeCN
RT stirring
N2 atm.

[Cu2

I/I(m -L)2](ClO4)2 (2)
3.1 MM2 calculation
Geometric optimisation of the synthesized ligand
and its complexes was done by the method of mole-
cular mechanics (MM2)13 in order to establish their
stable conformations. The data for the most stable
conformations of the ligand and the complexes are
listed in table 1. These three energy minimized con-
formations are shown in figure 2. The total energy
for the stable conformation of ligand is calculated to
be – 4⋅384 kcal/mol and the total energies for the
corresponding stable conformations of two com-
plexes [Cu2
are calculated to be 53⋅568 kcal/mol and 82⋅725 kcal/
mol respectively. The analysis of the total energies
for all the conformations have shown that the ener-
gies are primarily due to the torsional strain and van
der Waals interactions.
I/I(m -L)2](ClO4)2 and [Cu2
II/II(m -L)2] (ClO4)4
3.2 Conductivity measurement and IR spectra
All the crystalline complexes are soluble in MeCN,
DMF and DMSO. In acetonitrile solution the elec-
trical conductivities of the blue and orange solutions
of the complexes are close to the values for 1 : 4 and
1 : 2 (560 and 255 ohm–1 cm2 mol–1) electrolytic types
respectively. IR spectra of the complexes show strong
C=N stretching frequency of the terminal imine
functions at ∼1630 cm–1 which is shifted from the
free ligand value (for free ligand it is at 1639 cm–1).
The strong unsplit band (nClO4
suggests absence of coordination of perchlorate
ions.14
–) at around 1091 cm–1
3.3 Electronic absorption and mass spectra
The [Cu2
and shows a rather intense absorption band centred at
lmax = 400 nm (MLCT transition, e = 4700 M–1 cm–1),
whereas the [Cu2
centred transition, lmax = 576 nm, e = 380 M–1 cm–1).
I/I(m -L)2](ClO4)2 complex is orange in colour
II/IIL2] (ClO4)4 complex is blue (metal-
 4+/2+

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Study of imidazolidine-bridged copper complexes

155

Figure 1. EI mass spectrum of the ligand, L.


Table 1. Energies of the most stable conformations of the ligand and the complexes from the MM2 force field calcu-
lations (energies in kcal/mol).
Molecule Total Stretch bend Angle Torsional (dihedral) van der Waals Stretch Electrostatic

Ligand
Complex 2a
Complex 2b
– 4⋅384
82⋅725
53⋅569
0⋅245
0⋅474
0⋅433
9⋅449
63⋅052
34⋅689
–33⋅364
–2⋅224
–16⋅171
17⋅639
27⋅052
25⋅276
1⋅099
5⋅680
3⋅798
0⋅547
–11⋅484
5⋅442


The FAB mass spectrum of the compound 2a shows
the molecular ion peak at m/z 1247 for [Cu2
(ClO4)3
[Cu2
II/II(m -L)2]
+ corresponding to dinuclear formulation
II/II(m -L)2](ClO4)4 for the dicopper(II/II) complex.
3.4 Magnetic susceptibility measurements
Effective magnetic moment value of complex 2a in
the powdered state is 2⋅42 mB (1⋅71 mB/Cu) at room
temperature which is very close to the spin only
value (1⋅73 mB). This suggests that no spin exchange
is operative between the two metal centres in the
solid state.15 This is possibly due to the fact that the
imidazolidine group is not expected to contribute to
the magnetic exchange interaction in this molecule.
A diamagnetic correction of 278⋅43 × 10–6 cgsu per
complex, as calculated from the Pascal constants,16
was used. The orange complex 2b is diamagnetic in
nature.
3.5 EPR spectra
The polycrystalline EPR spectrum of [Cu2
(ClO4)4 at 296 K is typical for mononuclear cop-
per(II) complexes with no internuclear magnetic in-
teraction. The spectrum is axial with gII > g⊥
ordering for a dx
tion has high strain energy as obtained from MM2
calculations. The absence of any signal at g ≈ 4,
around 1500 G also shows that there is no weak
exchange interaction between the copper centres.
The results obtained are summarized as follows:
gII = 2⋅21, g⊥ = 2⋅02, giso = 2⋅03, AII = 122⋅0 G.17–19
Two imidazolidine bridges per molecule do not im-
port any magnetic communication between the two
copper(II) centres giving only spectra characteristic
for mononuclear ones. The frozen (77 K) MeCN-
toluene spectrum is also axial. Both the polycrystal-
line and frozen solution spectra at 296 K and 77 K
II/II(m -L)2]
2–y
2 ground state and this configura-

Page 6

Manindranath Bera et al

156
(shown in figure 3), are typical of a mononuclear
copper(II) complex. In both the cases the gII reso-
nance is split by the hyperfine coupling between the
unpaired electron on CuII and the I = 3/2 nuclear
spin of copper. The large line width in the g⊥ region
indicates that the copper centre is in a distorted
(rhombic) environment.







Figure 2. Calculated (MM2) conformation for (a) ligand
L (total strain energy – 4⋅384 kcal/mol); (b) compound
[Cu2
mol); (c) compound [Cu2
energy = 53⋅569 kcal/mol).
II/II(m -L)2](ClO4)4 (total strain energy = 82⋅725 kcal/
I/I(m -L)2](ClO4)2 (total strain
3.6 Electrochemistry
The electrochemical behaviour of both the Cu2
Cu2
mamide by cyclic voltammetry. In the cathodic
potential range (0 to –1⋅2 V) the Cu2
exhibits two reduction waves at Ep = –0⋅13 V and
–0⋅38 V vs SCE. This suggests that the reduction
processes may involve the following steps,

–0.13 V
II/II and
I/I complexes was investigated in dimethylfor-
II/II complex



No other reduction wave is observed at a more nega-
tive potential region due to any kind of reduction of
CuI–CuI to Cu0–Cu0 and subsequent deposition of
copper metal on the electrode surface. The respon-
ses observed are irreversible in nature. This irre-
versible nature of reductions observed may be
attributed to the fact that after reduction of CuIICuII
to CuICuI there may be a change in the coordination
geometry. In two cases, almost superimposable pro-
files are seen for the cupric and cuprous complexes
of the same ligand, and very similar profiles, as re-
gards shape and number of signals. Use of glassy
carbon electrode does not change the trace. For both
the complexes the initial potential was chosen at
0⋅0 V to check the identical electron–transfer behav-
iour. The successive addition of one electron to two


CuII–CuII
CuI–CuII
CuI–CuI.
–0.38 V

(3)

Figure 3. X-band (9⋅10 GHz) EPR spectrum of [Cu2
(m -L)2](ClO4)4 in solid state at 296 K.
II/II
(a)
(b)
(c)

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Study of imidazolidine-bridged copper complexes

157

Figure 4. Cyclic voltammogram (scan rate 50 mVs–1)
of [Cu2
ethyl ammonium perchlorate as supporting electrolyte at
a platinum electrode at 298 K.

I/I(m -L)2](ClO4)2 in dimethylformamide using tetra-

noncommunicating redox sites of a molecule should
lead to two essentially overlapping reduction pro-
cesses, the expected separation in redox potentials
being 36 mV.20 The voltammetric profile (figure 4)
for 2b on first scan displays two consecutive (one
electron transfer) oxidation waves at 0⋅39, 0⋅72 V,
which appear to be completely irreversible (i.e.,
these lack corresponding return waves), while a
separate, irreversible reduction at –0⋅14 V (again
without its corresponding oxidation wave) is observed
for two-electron transfer. The lack of corresponding
return waves in either direction is visualized from
the large separation in ∆Ep value. The CuII/CuI re-
duction step is followed by a very fast helical turn
of the ligands and the CuI/CuII oxidation process
occurs in two discrete steps for two metal centres.
The arrangement with coordinatively unsaturated
copper(II) ions allows one-step two-electron trans-
fer, whereas for the saturated copper(I) case two
discrete single electron transfer steps are observed.
The electronic interaction between the two centres is
the major source of wave splitting.21
4. Conclusions
The electrochemical process is believed to be rever-
sible, as there is no sign of dicopper(I/I) complex
catalysed activation of molecular dioxygen for ring
hydroxylation of the pendant phenyl ring on each
ligand. The change in metal oxidation state on the
other hand causes the transformation of planar di-
copper(II/II) complex to dicopper(I/I) complex with
some distortion from planarity. The change in ligand
conformation around the two metal ions is achiev-
able from the ligand consisting of two imine halves
linked by a 2-phenyl-1,3-imidazolidine spacer. The
coordinatively unsaturated copper(II) centres in
[Cu2
cent probe at the fifth coordination sites for quench-
ing/revival of a fluorescent signal during definite
change in ligand shape through change in oxidation
states of the metal centres. The new metal containing
assembly undergoing a change to two different mo-
lecular topologies during electron transfer. Absence
of any bridging donor group on the imidazolidine
ring of the ligand results in such controllable motion
in two different oxidation states of copper. Work is
now in progress in order to find other ligands not
featuring the terminal imine moieties but capable of
giving a similar molecular arrangement on changing
the oxidation state of the coordinated copper cation.
II/II(m -L)2] (ClO4)4 can bind any suitable fluores-
Acknowledgements
We thank the Council of Scientific & Industrial Re-
search, New Delhi for financial support.
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