Synthesis, characterization and electrochemical properties of bis(μ2-perchlorato)tricopper(ii) complexes derived from succinoyldihydrazones

Article abstract of DOI:10.1039/c3ra42048e

Three new homotrimetallic copper(ii) complexes ([Cu₃(L ⁿ)(ClO₄)₂(H₂O)_m] with H₄Lⁿ = H₄L¹ - H₄L ³, m = 0, 3) have been synthesized from substituted succinoyldihydrazones (H₄Lⁿ) in methanol. The composition of the complexes has been established on the basis of data obtained from analytical and mass spectral studies and molecular weight determination in DMSO. The structure of the complexes has been discussed in the light of molar conductance, magnetic moment data and electronic, EPR, IR and FT-IR spectral studies. The molar conductance values for the complexes in the region of 1.2-1.7 Ω^-1 cm² mol^-1 in DMSO indicate that the complexes are nonelectrolytes. The magnetic moment values for the complexes suggest considerable metal-metal interaction in the structural unit of the complexes. Copper centres have square planar and square pyramidal stereochemistry. The EPR parameters of complexes 1 and 3 indicate that the copper centre has a doublet ground state, while for complex 2, the ground state is a mixture of both doublet and quartet states. Electron transfer reactions of the complexes have been investigated by cyclic voltammetry. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42048e

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PAPER
Synthesis, characterization and electrochemical
properties of bis(m2-perchlorato)tricopper(II) complexes
derived from succinoyldihydrazones3
Cite this: RSC Advances, 2013, 3,
15139
R. Borthakur,a A. Kumar,b A. Lemtura and R. A. Lal*a
Three new homotrimetallic copper(II) complexes ([Cu3(Ln)(ClO4)2(H2O)m] with H4Ln = H4L1 2 H4L3, m = 0, 3)
have been synthesized from substituted succinoyldihydrazones (H4Ln) in methanol. The composition of the
complexes has been established on the basis of data obtained from analytical and mass spectral studies
and molecular weight determination in DMSO. The structure of the complexes has been discussed in the
light of molar conductance, magnetic moment data and electronic, EPR, IR and FT-IR spectral studies. The
molar conductance values for the complexes in the region of 1.2–1.7 V21 cm2 mol21 in DMSO indicate that
the complexes are nonelectrolytes. The magnetic moment values for the complexes suggest considerable
metal–metal interaction in the structural unit of the complexes. Copper centres have square planar and
square pyramidal stereochemistry. The EPR parameters of complexes 1 and 3 indicate that the copper
centre has a doublet ground state, while for complex 2, the ground state is a mixture of both doublet and
quartet states. Electron transfer reactions of the complexes have been investigated by cyclic voltammetry.
Received 25th April 2013,
Accepted 10th June 2013
DOI: 10.1039/c3ra42048e
www.rsc.org/advances
plexes offer the opportunities to test magnetic exchange
models on more complicated systems. Copper(II) complexes
1. Introduction
Copper is an essential element with important biological
functions, and is responsible for numerous catalytic processes
in living systems1 where it is often present in different
nuclearities. Copper is present in enzymes in biological
systems either alone2 or in combination with other metal ions
to cooperatively realize its biological function. Copper occurs
alone in the active centre of particulate methane monoox-
ygenase (pMMO) coupled sites3 where it is often present in di-
or trinuclear assemblies. On the basis of magnetic suscept-
ibility and EPR studies, it has been proposed that the copper
centre of pMMO is organized into a trinuclear catalytic or
electron transfer cluster.4 However, a mononuclear active
centre as well has been supported by another interpretation of
EPR spectra from similarly isolated pMMO samples.5
Moreover, a crystallographic study of a membrane preparation
of pMMO at 2.3 Å resolution has identified only a mono-
nuclear and a dinuclear metal site per enzyme unit.6 The
importance of multimetallic copper complexes also lies in
their relevance in the development of novel functional
materials with molecular ferromagnetism7 and specific cata-
lytic properties.8 Homo- and heterotrimetallic copper com-
find application as catalysts for the oxidation of alcohols to
aldehydes and ketones. This makes studies on the hetero-
geneous catalytic activities of homo and hetero-metal copper
complexes attractive and challenging.9 Further, trinuclear
complexes with bridging oxo and carboxylato motifs attract
considerable attention due to the isolation and crystal-
lographic characterization of trinuclear metal constellations
in the active sites of metalloenzymes, such as P1 nuclease
(Zn3), phospholipase
(Zn2Mg).6c
C (Zn3) and alkaline phosphatase
Polyfunctional ligands serve as means for synthesizing
monometallic, homo- and heterobimetallic, trimetallic and
polymetallic complexes. Hydrazones are a special kind of
polyfunctional Schiff base ligands derived from condensation
of organic acid hydrazines and o-hydroxy aromatic aldehydes
and ketones10–13 which have potential for yielding homo- and
heterometallic complexes. Dihydrazones derived from con-
densation of succinoyldihydrazines and o-hydroxy aromatic
aldehydes have been selected in the present study. These
multidentate ligands possess as many as eight bonding sites
and can utilize up to six bonding sites simultaneously in
bonding to metal ions in polynuclear metal complexes. In
succinoyldihydrazones, the two hydrazone parts are joined
together through two methylene groups. These dihydrazones
are unique in the sense that their succinoyl fraction offers
greater flexibility in three dimensional space because of their
ability to freely rotate around the C–C single bond as
aDepartment of Chemistry, North-Eastern Hill University, Shillong-793022,
Meghalaya, India. E-mail: profralal@gmail.com
bDepartment of Chemistry, Faculty of Science and Agriculture, The University of
West-Indies, St Augustine, Trinidad and Tobago, West-Indies
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra42048e
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compared to those in which the two hydrazone groups are
joined together either directly (oxaloyl) or through phenyl or
pyridyl groups. The oxaloyl, phenyl or pyridyl fractions by
virtue of their planar characteristics impose planarity over the
two hydrazone parts reducing their flexibility and reactivity. By
virtue of their greater flexibility, succinoyl dihydrazones are
expected to be more reactive ligands than those derived from
oxaloyl, phenyl and pyridoyl dihydrazones, and to yield more
stable complexes. Although such dihydrazones might exist in
only one configuration in the free state, in metal complexes
the can exist in a staggered anti-cis or syn-cis configuration.13
The resulting dihydrazones contain salicylaldimine, 2-hydroxy-
naphthaldimine and 5-bromo-salicylaldimine in their mole-
cular skeleton. Further, as we go from salicylaldimine
dihydrazone to naphthaldimine dihydrazone to 5-bromo-
salicylaldimine dihydrazone, the bulkiness and electronega-
tivity increase in the same order. It would be interesting to see
how the properties of the complexes change as we move from
salicylaldimine to 2-hydroxy-1-naphthaldimine to 5-bromosa-
licylaldimine dihydrazones. Among the methodologies that are
used for the construction of polynuclear assemblies of
predesigned compositions, the most popular method is the
stepwise construction of mononuclear transition metal com-
plexes of polydentate ligands in which some of the donor sites
are unoccupied, and which often serve as efficient building
blocks.14 Although some of these compounds have been
prepared by using either well-designed organic ligands or
transition metal complex ligands,15 much remains to be
learnt.
A literature survey revealed that although some isolated
studies are available on metal complexes of succinoyldihy-
drazones13 and related dihydrazones,10–13 reports on the
synthesis and characterization of homo- and heterotrinuclear
complexes of dihydrazones are quite meagre.16 Moreover, a
systematic study on trinuclear metal complexes of succinoyl-
dihydrazones is absent to the best of our knowledge, in spite of
their highly flexible nature. In view of the above, the
importance of trinuclear metal complexes in general and
copper complexes in particular, the absence of work on
homotrinuclear complexes of succinoyldihydrazones and the
highly flexible polyfunctional nature of succinoyldihydra-
zones, it was in our interest to synthesize the homotrinuclear
copper complexes of the title dihydrazones (Fig. 1), and to
characterize them by various physicochemical and spectro-
scopic studies. The electron transfer reactions of the com-
plexes have also been investigated by cyclic voltammetry. The
results of these investigations are presented in this paper.
Fig. 1 R1 = H, R2 = H, X = H: Disalicylaldehydesuccinoyldihydrazone (H4L1); R1
=
H, R2 = 5,6-benzo, X = H: disalicylaldehydesuccinoyldihydrazone (H4L1); R1 = H,
R2 = H, X = Br: bis(5-bromosalicyladehyde)succinoyldihydrazone (H4L3).
melting points of the complexes are higher than those of the
corresponding parent ligand. This suggests that the metal–
ligand bonds in these complexes have a more ionic character
than those of the corresponding parent ligand. All complexes
are insoluble in water and common organic solvents, but are
soluble in highly coordinating solvents such as DMSO and
DMF. When heated in an electronic oven for four hours,
complexes 1 and 3 do not show any weight loss either at 110 uC
or 180 uC. This rules out the possibility of the presence of
water molecules in either the lattice structure or coordination
sphere. Only complex 2 shows weight loss at 180 uC, which
amounts to the equivalent of three water molecules, and
suggests their presence in the first coordination sphere
around the metal centre.
An effort was taken to crystallize the complexes in various
solvent systems under different experimental conditions.
Unfortunately, only amorphous compounds precipitated in
all our efforts which prevented analysis of the complexes by
X-ray crystallography.
2.1. Mass spectral studies
All of the complexes have been characterized by mass
spectrometry. The molecular ions along with their experi-
mental and theoretical mass, for all of the complexes have
been given in Table S3, ESI. Mass spectra of complexes 1 and
3
2 are shown in Fig. S1 and S2, ESI.
3
The mass spectrum of complex 1 has a peak at an m/z value
of 1094.77, which is close to the mass of the molecular ion
[Cu3(H5L1)(HClO4)2(DMSO)4(H2O)2]+ (1094.65). Hence, this
peak can be attributed to this molecular ion. This molecular ion
arises, most probably, from the coordination of four DMSO
molecules and two water molecules to the metal centres in the
complex.17 The peak at the m/z value of 1027.63 corresponds to the
existence of the molecular ion [Cu3(L1)(ClO4)2(DMSO)3(H2O)3]+
(1027.65). This molecular ion results from coordination of three
DMSO molecules and three water molecules to the metal centres.
Another molecular ion peak is obtained at the m/z value of 950.70,
which suggests that it corresponds to the molecular ion
[Cu3(HL1)(ClO4)2(DMSO)2(H2O)3]+ (950.65). A most prominent peak
at the m/z value of 559.04 is observed which is similar to the mass of
the molecular ion [Cu3(HL1)(H2O)]+ (559.65). This molecular ion most
2. Results and discussion
The complexes isolated in the present study have been
suggested to have the composition [Cu3(Ln)(ClO4)2(H2O)m]
(H4Ln = H4L1, H4L2 and H4L3; m = 0, 3) on the basis of
analytical data, molecular weight and mass spectral studies.
The complexes decompose without melting above 300 uC. The
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probably originates
Paper
from
the
molecular
ion
naphthaldimine complex to the 5-bromo-salicylaldimine com-
plex.
[Cu3(HL1)(ClO4)2(DMSO)2(H2O)]+ through the loss of two perchlorate
ions and two DMSO molecules.
2.5. Electron paramagnetic resonance spectra
The origin of different molecular ion peaks in the mass
spectra of complexes 2 and 3 can be explained in the same
way. Hence, their further discussion seems redundant. The
EPR spectra of the complexes in the present study are isotropic
at RT. The g-values of 2.038–2.051 (Table S1, ESI ) indicate that
3
different molecular ions presented in Table S3, ESI originate
3
the adjacent metal ions in the structural unit of the complexes
persistently show moderately strong interactions. The spectra
of complex 3 are isotropic both at RT and LNT in the
polycrystalline state, and the gav-value is 2.048 for both
conditions. The g-tensor does not change when the tempera-
ture is lowered to the LNT, indicating the non-flexional
geometry of the polyhedra. It appears that the room
temperature narrowing is of an intramolecular nature due to
thermal population of the two doublet and quartet states. This
is confirmed by the fact that the EPR spectrum at LNT for
complex 3, in which only the lowest doublets and quartet
states are populated, is narrowed by 60 G due to intramole-
cular interactions and not broadened. Further, the polycrystal-
line powder X-band spectra of complexes 1 and 3 at LNT show
no transition at low field with g-values of around 4. This
suggests that the complexes exhibit a ground state with a total
spin S = 1/2.21,22 However, the polycrystalline powder X-band
spectrum of complex 2 at LNT shows a transition at low field
with a g-value of 4.064. This indicates that the ground state of
complex 2 is a mixture of both the doublet and quartet states
at low temperature. Each of the complexes 1 and 2 shows three
hyperfine lines in the g| region, while only two hyperfine lines
are present in the g) region. The hyperfine coupling constant
A| at LNT is 180 in the g| region, while A) in the g) region is
equal to 55 G. The g| value is around 2.219 for both the
complexes while g) values are 2.032 and 2.026, respectively.
The magnetic parameters for the complexes fall in the order g|
. g) . 2.0023. This shows that in all complexes, copper has
(dx22y2) orbitals as the ground state.
from the loss of either coordinated water molecules or
coordinated perchlorate groups followed by coordination of
DMSO molecules. The mass spectral behavior of the com-
plexes suggests that all of them are monomeric in nature.
2.2. Molecular weight
The molecular weights of complexes 1 to 3 have been
determined in the highly coordinating solvent DMSO by the
freezing point depression method based on the insolubility in
non-coordinating solvents. The complexes have molecular
weights of 780 ¡ 35, 1060 ¡ 52 and 920 ¡ 45, respectively.
The experimental values of the molecular weights of the
monomers are very close to the theoretical values of 739.65,
893.65 and 897.65, rather than the values of 1479.24, 1687.24
and 1795.24 for the dimers. This suggests that the complexes
are monomeric in nature. However, the experimental mole-
cular weight of complex 2 is 1060 ¡ 52 which is slightly higher
than the theoretical value as calculated on the basis of
monomer formulation. This is, most probably, owing to the
replacement of H2O molecules in complex 2 by some DMSO
molecules in the solution.
2.3. Molar conductance
The complexes have a molar conductance in the region of 1.2–
1.7 V21 cm2 mol21. These values are consistent with their
nonelectrolytic nature in this medium.18
2.4. Magnetic moment
The complexes 1–3 have magnetic moments (meff) of 1.79, 1.87
and 2.45 mB, per respective molecular formula, and 1.04, 1.08
and 1.42 mB per empirical formula. These values are much
lower than the values of 2.99 mB for trinuclear copper(II)
complexes and 1.73 mB for mononuclear copper(II) complexes,
in which no metal–metal interactions are present. These meff
values suggest that there is a considerable metal–metal
interaction in the structural unit of complexes 1–3.19
The EPR spectrum of complex 1 (Fig. 2) in frozen DMSO
glass exhibits a resolved hyperfine structure in the g| region
with six discernible lines with an average splitting of 95 G.23
The hyperfine lines at higher field around 3100 G are stronger
than the hyperfine lines at lower field around 2700 G. The g|
values corresponding to the weak and strong hyperfine lines
The lowering of the magnetic moment values may be either
due to intramolecular superexchange arising from the transfer
of paramagnetic spin density from one metal ion through the
orbital overlap of the diamagnetic bridging oxygen atoms and
perchlorate anions to adjacent metal ions, or due to direct
metal–metal intramolecular interaction via the overlap of
suitable metal orbitals.20 The magnitude of the magnetic
moment increases from the salicylaldimine to the 2-hydroxy-1-
naphthaldimine to the 5-bromo-salicylaldimine complex. This
shows that the magnetic exchange is stronger in the
salicylaldimine complex than that in the 2-hydroxy-1-naphthal-
dimine complex in which it is stronger than that in the
5-bromo-salicylaldimine complex. This may be related to
decreasing metal–metal interactions with increasing steric
crowding in the ligand as the size of the aromatic ring
increases from the salicylaldimine to the 2-hydroxy-1-
Fig. 2 EPR spectrum of the complex [Cu3(L1)(ClO4)2] (1) in DMSO glass at LNT (f
= 9.1 Mz).
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are 2.399 and 2.153 with the average value for the six signals
being 2.281, while the g) values are 2.038 and 1.988 with an
average value of 2.013. The hyperfine coupling constants A| for
the weak and strong signals are 115 and 75 G, respectively,
with an average value of about 95 G. This feature may arise
from the overlap of two sets of peaks corresponding to two sets
of different but similar Cu(II) centres in the tetragonal
environments, preferably a square pyramidal environment
around a Cu(II) centre with g| . g) . 2. However, this would
imply a change in stereochemistry for the copper centre from
trigonal bipyramidal to square pyramidal.24 The EPR spectra
of the trigonal bipyramidal complexes are characterized by
axial symmetry with g) . g| . 2.00. Usually, a hyperfine
structure is seen in the g| region with an A| in the range of 60–
nm and appear in the regions of 330–441 and 405–470 nm,
respectively. Such a feature associated with the red shift of the
ligand bands in the solid state provides good evidence for the
chelation of the metal centre by dihydrazone. The magnitude
of the red shift of the ligand bands upon complexation in the
solid state suggests strong bonding between metal ion and
ligands. The band at 470 nm in complex 2 appears to have a
contribution due to charge-transfer from the ligand to the
metal atom, most probably from the naphtholate oxygen atom
to the metal atom. The spectra of complexes 1 and 3 show a
single broad band centred at 640 and 620 nm, respectively.
This band is assigned to a d–d transition. The d–d transition
occurs at around 800 nm in the octahedral copper(II)
2
complexes due to the T2g
r
2Eg transition. The position of
100 6 1024 cm21 24
.
The reverse pattern of g| . g) y 2
the d–d bands shifts to a lower wavelength due to Jahn–Teller
distortion in the complexes, and in extreme cases, it falls in
the region 600–700 nm, reported for square planar com-
plexes.28 The position of the d–d band and its shape suggests
that the copper centre has a square planar geometry in the
complexes.28
The spectrum of complex 2 shows an asymmetric broad
intense band in the region of 620–800 nm in the solid state
with a maximum intensity at 660 nm. The intensity of the
band at lower energy is lower than that at higher energy.
Hathaway et al.29 and others30 have reported that five
coordinated copper(II) complexes have a single rather intense
band in the region of 650–550, and 830 nm for square
pyramidal and trigonal bipyramidal stereochemistry, respec-
tively. In view of the broad character of the band in the region
of 620–800 nm and its high intensity at around 660 nm, it is
suggested that copper centres in the complex 2 have square
pyramidal stereochemistry.
The solution spectra of the complexes are better resolved
than those in the solid state. Complex 2 shows only two ligand
bands at 330 and 420 nm, while complexes 1 and 3 show three
bands each. The ligand band in the region of 292–327 nm in
the uncoordinated ligand shifts to a longer wavelength by 7–28
nm and appears in the region of 300–330 nm. This band splits
into two bands in complex 2. The ligand band in the region
330–382 nm shows a considerable red shift of about 38–55 nm
upon complexation and appears in the region of 375–420 nm.
The feature associated with the ligand band upon complexa-
tion in the solution state remains essentially the same as that
in the solid state. This indicates that the ligands remain
coordinated to the metal centre in the DMSO solution.31 Also
in the solution state, the spectra of complexes 1 and 3 have a
single band at 650 nm due to d–d transition. The molar
extinction coefficient of this band falls in the region of 65–77
dm3 mol21 cm21. This band is slightly red shifted compared to
that in the solid state. This can safely be attributed to the
solvation of the complexes in DMSO solution. The essential
feature of the band in DMSO solution suggests that the
complexes have square planar stereochemistry (Fig. 4).
In DMSO solution, complex 2 shows an asymmetric strong
intense band at 520 nm with a molar extinction coefficient
equal to 224 V21 cm2 mol21. The complex does not show any
other band in the visible region. Hence, this band may be
attributed to a d–d transition. This band is shifted to a higher
energy as compared to that in the solid state. This may be due
observed for complex 1 indicates a distorted trigonal bipyr-
amidal geometry with square pyramidal coordination of the
copper centre. This suggests that in this complex, the dz2
orbital of the copper atom has a significant contribution in the
ground state.25 The A| value calculated from the resolved
hyperfine structure of the compound is 89 6 1024 cm21 (1 G =
0.9323 6 1024 cm21), and the lowest principal g-value is 2.013.
This g-value is appreciably different from the values observed
for structurally analogous compounds where Cu has been
shown to be coordinated in a trigonal bipyramidal fashion by
X-ray crystallography.26 This fact clearly suggests that the
coordination geometry around the copper atoms in the
complexes lies in between the tetragonally distorted square
pyramidal and the distorted trigonal bipyramidal geometries.
Alternatively, it may be considered that the EPR signal may
originate from a weak exchange interaction between the
copper centres.27 The hyperfine splitting pattern would result
from coupling of two sets of Cu nuclei with four lines of the
weak set, near 2700 G, (most probably tetragonally distorted
square pyramidal) overlapping with the first line of the strong
set (most probably distorted trigonal bipyramidal) near 3100
G. The essential features of the EPR spectra support the
trinuclear interaction of the signal. The EPR spectra of the
frozen solution of complexes 2 and 3 essentially exhibit axial
signals (g| = 2.169, g) = 2.041, A| = 120 G, for complex 2; g| =
2.193, g) = 2.048, A| = 140 G, A) = 30 G for complex 3) with a
three line hyperfine splitting in complex 2 and a two line
hyperfine splitting in complex 3 in the g| region due to
coupling to Cu nuclei. The EPR spectrum of complex 3 also
shows splitting in two lines in the g) region with A) = 30 G.
2.6. Electronic spectroscopy
The electronic spectra of the complexes have been qualitatively
studied in the solid state and quantitatively in DMSO solution
(Table S1, ESI ) due to limited solubility. The spectrum of
3
disalicyldehyde succinoyldihydrazone shows bands at 293 nm
and 330 nm, while the remaining two dihydrazones have four
and three bands, respectively, in the region of 292–382 nm.
The bands around 300 and 330 nm are assigned to the intra-
ligand p A p* transition, while the bands around 360 and 380
nm are assigned to the n A p* transition.11
In the solid state, the complexes show two to three bands in
the region of 300–805 nm. The ligand bands appearing in the
region of 293–382 nm shift to longer wavelengths by 33–153
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the ligands are present in the enol form in the complexes. The
present ligands show a very sharp singlet or doublet in the
region of 1604–1623 cm21, which is assigned to the stretching
vibration of the .CLN group. This band shifts to a lower
frequency by 6–19 cm21, which suggests that the .CLN group
is involved in bonding to the metal centre. Another important
feature of the IR spectra of the complexes is a new intense
band in the region of 1521–1539 cm21
. This band is
characteristic of the presence of the NCO-group.11 The
presence of this band in the IR spectra of the complexes
suggests that the dihydrazone undergoes enolization. The
spectra of the ligands show a medium to intense band in the
region of 1267–1280 cm21. This band may be attributed to
n(C–O) (phenolate/naphtholate) vibrations. This band shifts to
a higher frequency by 9–21 cm21 and appears in the region of
1276–1301 cm21 upon complexation. The shift of this band to
a higher frequency by +9–21 cm21 indicates bonding of a
phenolic/naphtholic oxygen atom to the metal centre via
deprotonation.11
On examining the spectra of the ligands and their
complexes below 600 cm21, the weak bands appearing in the
ranges of 536–548 and 426–458 cm21 are tentatively assigned
to the n(M–O) (phenolic/naphtholic) and n(M–O) (enolate)
stretching vibrations,40 respectively. A doublet arising in the
region of 329–386 cm21 is attributed to the n(M–N) stretching
vibration from the coordination of the azomethine nitrogen
atom to the metal centre.33
Fig. 3 Cyclic voltammetry of the complex [Cu3(L1)(ClO4)2] (1) at 100 mV s21
.
to replacement of water molecules by DMSO molecules, and its
weak interaction with the metal centre compared to that of
coordinated water. However, this band maintains its separate
identity and does not merge with the strong ligand/charge-
transfer band. In view of such a feature of this band in DMSO
solution, it is quite reasonable to suggest a square pyramidal
stereochemistry for the complex in solution. The higher value
of the molar extinction coefficient than that observed for
square pyramidal complexes may be attributed to intensity
stealing either from an adjacent ligand or a charge-transfer
band.32
2.7. Infrared spectra
The complexes show medium intensity bands in the region
of 626–684 and 484–492 cm21. These bands are not visible in
the IR spectra of the free ligands. Hence, they arise due to
bridged metal atoms through oxygen atoms.34 The position of
these bands in the complexes suggests that they originate from
the formation of dibridges involving enolate oxygen atoms.
The band in the region of 626–684 cm21 is assigned to
antisymmetric vibrations while the band in the region of 484–
492 cm21 is attributed to symmetric vibrations of the
The present ligands show strong broad bands in the regions of
3190–3235 cm21 and 3421–3433 cm21 (Table S2, ESI ) which
3
arise from the stretching vibration of secondary –NH groups
and naphtholic/phenolic –OH groups,10–13 respectively. All of
the complexes have a strong broad band in the region of 3491–
3433 cm21. The essential feature of this band suggests that it
arises from a nOH band of either the lattice or coordinated
water molecules. The complexes do not show any band in the
region of 3190–3235 cm21 due to the nNH in free ligands
except complex 2 which shows a strong band at 3212 cm21
.
This band in complex 2 may be attributed to the symmetric
stretching vibration of coordinated water molecules. The
absence of a nNH band in the IR spectra suggests that the
ligand is present in the enol form in all of the complexes.
Another important and most characteristic feature of the IR
spectra of the complexes is the absence a the band in the
region of 1659–1675 cm21 due to the .CLO group in the
uncoordinated dihydrazone. This corroborates the fact that
group,34 respectively.
The complexes have four strong bands in the regions of
1143–1175, 1121–1140, 1100–1110 and 1080–1090 cm21. Out
of the four fundamental vibrations of the perchlorato group,
only n3 and n4 are infrared active. The triply degenerate IR
active n3 mode splits into three bands in the IR spectra of the
complexes. Moreover, the n1 band which is IR forbidden in
non-coordinated perchlorate35 appears in the region of 1080–
1090 cm21 as a strong band. The appearance of the n1 band
and frequencies of the n3 mode are in good agreement with
those normally associated with the bidentate bridging
perchlorato groups.35
2.8. Cyclic voltammetry
The assignments of redox couples (Table 1) have been made
excluding the waves of the free ligands. Complex 1 (Fig. 3)
shows three reductive waves while complexes 2 and 3 show
Fig. 4 Suggested structure of the copper(II) complexes [Cu3(Ln)(ClO4)2] (H4Ln
H4L1, H4L3).
=
3
only two reductive waves each (Fig. S5 and S6, ESI ). On the
other hand, complexes 1 and 2 show three oxidative waves
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Table 1 Electrochemical parameters for the copper complexes (potential vs. Ag/
AgCl)
Reduction
Ec (V)
Oxidation
Ea (V)
SI. No.
1
Compound
DE (V)
[Cu3(L1)(ClO4)2]
21.23
20.92
20.17
—
—
—
+0.14
+0.35
+1.04
[20.52]a
—
+0.05
+0.29
+1.06
[+0.24]a
—
310
—
[20.58]
20.79
20.18
—
2
[Cu3(L2)(ClO4)2(H2O)3]
[Cu3)(L3)ClO4)2]
230
400
—
[20.84]
20.88
20.39
—
Fig. 5 Suggested structure of the copper(II) complexes [Cu3(L2)(ClO4)2(H2O)3].
3
+0.01
+0.75
[+0.47]a
[20.88]
[Cu3(Ln)(ClO4)2(H2O)m] (m = 0, 3) in excellent yields. All of
the complexes are monomers and nonelectrolytes. The ligands
are present in the enol form in all the complexes and function
as tetrabasic hexadentate ligands coordinating the metal
centre through phenolate/naphtholate and enolate oxygen
atoms, and azomethine nitrogen atoms. The EPR parameters
of the complexes 1 and 3 indicate that the copper centre has a
doublet state as the ground state while for complex 2 the
ground state is a mixture of both the doublet and quartet
states. The electron transfer reactions of the complexes have
been investigated by cyclic voltammetry. On the basis of the
results and discussion of various physicochemical and spectro-
scopic studies, complexes 1 and 3 are suggested to have a
square planar structure, whereas complex 2 has a square
pyramidal structure. (Fig. 4 and 5).
a
Waves arising from uncoordinated ligands.
each while complex 3 shows only two oxidative waves. The
reductive waves centred in the region of 20.79 to 21.23 V do
not have their counterpart in the oxidative scan. Hence, these
waves are assigned to electron transfer reactions centred on
the ligand. Further, an irreversible oxidative wave in the region
of +0.75 to +1.06 V is observed for the complexes. This
oxidative wave may be attributed to the oxidation of the .CLN
group in the ligand.36 As the uncoordinated ligands do not
show any other redox reactions in the region of +2 to 22, the
remaining reductive and oxidative waves may be associated
with the electron transfer reaction centred on the metal ion.
The peak potential separation for the first redox couple (Epa
=
+0.14 V and Epc = 20.17 V for complex 1; Epa = +0.05 V and Epc
= 20.18 V for complex 2; Epa = +0.01 V and Epc = 20.39 V for
complex 3) is more than 200 mV which is much higher than
that for an uncomplicated one electron redox process (0.06 V).
Hence, this redox couple is suggested to be irreversible. With
the highly negatively charged dihydrazone ligand bonded to
the metal centre, it is expected to help make the reduction of
these metal centres unfavourable, leading to high, negative Epc
values.37 The high peak separation most probably originates
4. Experimental section
4.1. Materials
Copper acetate monohydrate, copper chloride dihydrate,
diethyl succinate, hydrazine hydrate, substituted salicylalde-
hydes and 2-hydroxy-1-naphthaldehyde were E-Merck,
Qualigens, Hi-Media or equivalent grade reagents. The
precursor complexes [Cu(H2Ln)(H2O)] were synthesized by the
literature method using succinoyldihydrazine instead of
oxaloyldihydrazine and salicylaldehyde and substituted salicy-
laldehydes in place of salicylaldehyde.38
from
a
slow heterogeneous electron exchange rate
rather than from an intervening homogeneous reaction.36
Complexes 1 and 2 show another irreversible oxidative
wave at +0.35 and +0.29 V, respectively. This may be attributed
to the oxidative couple corresponding to the oxidation of
CuIICuIICuII to CuIICuIICuIII. In view of this, the first redox
4.2. Instrumentation and measurement
couple may be attributed to the reduction of
ze
The amount of copper was determined by the standard
2z
z
I
II
II
½(L)CuIICuIICuIIꢀ
½(L)Cu Cu Cu ꢀ . The electron transfer
?
/
literature procedure.39 The C, H, and
N content was
{e
reactions centred on the metal ion are proposed to be as
follows: ½(L)CuIICuIICuIII
ze
ze
determined by microanalytical methods using a Perkin-
Elmer 2400 CHNS/O Analyser II. Perchlorate was reduced to
chloride with Ti2(SO4)3 and ammonium chloride in a porcelain
crucible in the presence of little platinum powder, and the
resulting chloride was detected as AgCl. The molar conduc-
tance of the complexes (1023 mol L21 in DMSO) was measured
on a Systronics Direct Reading Conductance Meter-303 with a
dip-type conductance cell at room temperature. Room tem-
perature magnetic susceptibility measurements were made on
3z
2z
II
II
II
II
?
?
½(L)Cu
ꢀ
½(L)Cu Cu Cu ꢀ
/
/
z
CuIICuIꢀ .
{e
{e
3. Conclusion
In the present study, we have synthesized three homotri-
nuclear copper(II) complexes of the composition
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Sherwood Magnetic Susceptibility Balance MSB-Auto.
Paper
a
4.4. Synthesis and characterization of the ligands
Diamagnetic corrections were carried out using Pascal’s
constant.40 Electronic spectra of the complexes were recorded
in DMSO solution at y1023 M concentration on a Perkin-
Elmer Lambda-25 spectrophotometer. Electron paramagnetic
resonance spectra of the complexes were recorded at the
X-band frequency on a Varian E-112 E-Line Century Service
EPR spectrometer using TCNE (g = 2.0027) as an internal field
marker. Variable temperature experiments were carried out
with a Varian Variable Temperature accessory. Infrared spectra
were recorded on a BX-III/FT-IR Perkin-Elmer spectrophot-
ometer in the range of 4000–400 cm21 in KBr discs. Far-IR
spectra were recorded in the range of 600–50 cm21 in CsI
discs. Mass losses were determined by heating the complexes
at 110 uC and 180 uC in an electronic oven. APCI mass spectra
of the complexes were recorded on a Waters ZQ 4000
Micromass Spectrometer in DMSO solution. Cyclic voltam-
metric measurements of the compounds in DMSO were
conducted using a CH Instruments Electrochemical analyzer
under dinitrogen. The electrolytic cell comprises three
electrodes, the working electrode was a Pt disk, the reference
and auxiliary electrodes were Ag/AgCl (3 M KCl) separated
from the sample solution by a salt bridge, and 0.1 mol L21
TBAP was used as the supporting electrolyte.
Succinoyldihydrazine was prepared by reacting diethyl succi-
nate (10.0 g, 49.50 mmol) with hydrazine hydrate (5.50 g,
110.00 mmol) under reflux for 4 h and recrystallized from
methanol. Dihydrazones were prepared by reacting a solution
of succinoyldihydrazine (10.37 mmol) in methanol with
aldehydes and ketones (25.93 mmol) in hot methanol and
refluxing the reaction mixture for 30 min. The precipitate
obtained on cooling the solution was thoroughly washed with
methanol and air dried to give H4L1, H4L2 and H4L3,
respectively.
H4L1: Colour: white; yield: 89%; m.p.: 250 uC; anal. calc. for
C18H18N4O4 (354 g mol21) (%): C, 61.02; H, 5.08; N, 15.82.
1
Found: C, 61.23; H, 5.06; N, 15.69. H NMR (400 MHz, DMSO-
d6) (d (ppm)):11.72 (d, 1H, J = 7.6 Hz, OH), 11.30 (d, J = 5.2 Hz,
1H, OH), 11.16 (s, 1H, NH), 10.15 (s, 1H, NH), 8.33 (s, 1H,
CHLN), 8.27 (s, 1H, CHLN), 6.8–7.6 (m, 8H, Ar-H), 2.91 (m,
2H), 2.53 (m, 2H); 13C NMR (100 MHz, DMSO-d6) (d (ppm)):
173.05, 167.8, 156.31, 131.13, 130.89, 126.68, 118.55, 119.38,
28.64.
H4L2: Colour: dark yellow; yield: 86%; m.p.: 245 uC; anal.
calc. for C26H22N4O4 (454 g mol21) (%): C, 68.72; H, 4.83; N,
1
12.33. Found: C, 68.99; H, 4.83; N, 12.65. H NMR (400 MHz,
DMSO-d6) (d (ppm)): 12.66 (d, 1H, J = 14.4 Hz, OH), 11.62 (d,
1H, OH), 11.3 (s, 1H, NH), 10.05 (d, 2H, NH), 9.17 (s, 1H,
CHLN), 8.98 (s, 1H, CHLN), 7.18–8.44 (m, 12H, Ar-H), 2.59 (m,
2H), 2.07 (m, 2H); 3C NMR (100 MHz, DMSO-d6) (d (ppm)):
167.29, 157.74, 144.81, 138.31, 132.34, 131.73, 127.69, 124.10,
123.33, 121.90, 118.25, 108.33, 30.58.
4.3. Molecular weight determination
The molecular weight of the complexes was determined in
DMSO (Kf = 4.07)41 as the solvent by lowering of the freezing
point of DMSO (f.p. 18.5 uC) with a Beckmann freezing point
H4L3: Colour: light yellow; yield: 87%; m.p.: 257 uC; anal.
calc. for C18H16N4O4Br2 (512 g mol21) (%): C, 42.19; H, 3.13; N,
depression instrument. The instrument consisted of
a
Beckmann thermometer and a stirrer with a non-conducting
handle of wood fitted into a tube through a rubber stopper
fixed at its upper end. This tube was supported by a rubber
stopper in a bigger glass tube thoroughly purged with dry
dinitrogen which ensured slow and uniform cooling of the
inner tube. The outer tube was immersed in a 1,2-dimethyl-3-
nitrobenzene (f.p. 15 uC) freezing bath contained in a glass jar.
The jar was covered with a lid of plastic with three holes, one
for inserting the inner tube, another for a large metal stirrer
and a third for inserting the ordinary thermometer. The
molecular weight apparatus was enclosed in a dry box that was
constantly purged with dry nitrogen to minimize the error due
to the highly hygroscopic nature of DMSO. Different samples
of the solutions were prepared in DMSO with different
amounts of complex. All of the samples were placed in an
entry port and purged overnight with dry nitrogen before
bringing them into the dry box. 20 g of DMSO were transferred
into the thoroughly cleaned and dried freezing point tube
purged with dry nitrogen. About 0.6 g of the complex was
accurately weighed and inserted into the DMSO solution. The
tube was heated to dissolve the compound and then brought
to room temperature. Finally, the upper surface of the liquid in
the tube was purged with dry nitrogen and then adjusted into
the outer tube. The freezing point temperatures of the solvent
and the solutions were directly measured with this apparatus
through a magnifying glass fitted into the dry box.
1
10.94. Found: C, 42.59; H, 3.11; N, 11.24. H NMR (400 MHz,
DMSO-d6) (d (ppm)): 11.75 (d, 1H, J = 7.8 Hz, OH), 11.36 (d, 1H,
J = 4.8 Hz, OH), 11.19 (s, 1H, NH), 10.2 (s, 1H, NH), 8.30 (s, 1H,
CHLN), 8.21 (s, 1H, CHLN), 6.8–7.7 (m, 6H, Ar-H), 2.92 (m,
2H), 2.53 (m, 2H); 13C NMR (100 MHz, DMSO-d6) (d (ppm)):
173.34, 168.08, 156.21, 138.39, 138.27, 132.99, 121.15, 110.29,
26.86.
4.5. Synthesis of [Cu3(Ln)(ClO4)2(H2O)m] (where m = 0, 3; H4Ln
= H4L1(1), H4L2(2), H4L3(3))
[Cu(H2L1)(H2O)] (2.0 g; 4.43 mmol) was suspended in methanol (30
mL) and stirred vigorously to get a homogeneous suspension. This
suspension was added to a copper perchlorate hexahydrate solution
maintaining a [Cu(H2L1)(H2O)] : Cu(ClO4)2?6H2O (1) molar ratio of
1 : 3 and was refluxed for 3 h. A green compound precipitated,
which was suction filtered under hot conditions and washed three
times with methanol (30 mL each time) followed by ether, and finally
dried over anhydrous CaCl2.
Complexes 2 and 3 were also prepared by following the
above procedure using [Cu(H2L2)(H2O)] (2.0 g; 3.28 mmol) and
[Cu(H2L3)(H2O)] (2 g; 3.63 mmol) respectively, instead of
[Cu(H2L1)(H2O)].
[Cu3(L1)(ClO4)2] (1): yield: 2.60 g (78%), m.p.: .300 uC, anal.
calc. for C18H14N4O12Cu3Cl2 (%): Cu, 25.78; C, 29.20; H, 1.89;
N, 7.57; Cl, 10.00. Found: Cu, 26.05; C, 28.70; H, 1.86; N, 7.42;
Cl, 9.87. Mol. wt.: theo.: 739.65, exp.: 780 ¡ 35. Molar
conductance (DMSO, V21 cm2 mol21): 1.7.
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[Cu3(L2)(ClO4)2(H2O)3] (2): yield: 2.23 g (72%), m.p.: .300
uC; anal. calc. for C26H14N4O15Cu3Cl2 (%): Cu, 21.33; C, 34.91;
H, 2.69; N, 6.27; Cl, 7.94. Found: Cu, 21.49; C, 34.72; H, 2.71;
N, 6.42; Cl, 7.81. Mol. wt.: theo.: 893.65, exp.: 1060 ¡ 52. Molar
conductance (DMSO, V21 cm2 mol21): 1.5.
RSC Advances
V. Niel, L. K. Thompson, Z. Xu, V. A. Milway, R. G. Harvey,
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[Cu3(L3)(ClO4)2] (3): yield: 2.45 g (85%), m.p.: .300 uC; anal.
calc. for C18H12N4O12 Br2Cu3Cl2 (%): Cu, 21.24; C, 24.06; H,
1.34; N, 5.74; Cl, 7.90. Found: Cu, 21.59; C, 24.38; H, 1.37; N,
5.74; Cl, 8.10. Mol. wt. theo.: 897.65, exp.: 920 ¡ 45. Molar
conductance (DMSO, V21 cm2 mol21): 1.2.
Acknowledgements
The authors are thankful to the HEAD, SAIF, North Eastern
Hill University, Shillong-793022, Meghalaya, India for record-
ing NMR and mass spectra and the Head, SAIF, IIT Bombay,
India for recording EPR spectra. The authors express their
heartfelt thanks to Prof. A. T. Khan, Department of Chemistry,
IIT Guwahati, Assam, India for helping with recording the
magnetic moment data. R.B. thanks the University Grant
Commission, New Delhi for the award of the Research
Fellowship in Sciences for Meritorious Students (RFSMS).
12 (a) O. B. Chanu, R. Borthakur, A. Koch, R. A. Lal and
A. Kumar, J. Indian Chem. Soc., 2013, 90, 1; (b) A. Kumar, O.
B. Chanu, A. Koch and R. A. Lal, J. Struct. Chem., 2013, 54,
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J. Struct. Chem., 2013, 54, 866; (d) N. K. Kar, M. K. Singh
and R. A. Lal, Arabian J. Chem., 2012, 5, 67; (e) A. Ahmed, O.
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