A novel discrete dinuclear copper(II)–gadolinium(III) complex derived from a Schiff base ligand [Cu(salbn)Gd(NO3)3·H2O] (salbn): N,N′-butylenebis(salicylideaminato)
ABSTRACT The synthesis, X-ray and e.p.r. spectral studies of a 3d–4f couple are described here. The crystal structure of [Cu(salbn)Gd(NO3)3H2O], (2), salbn = N,N-butylenebis(salicylideaminato), has been determined by X-ray crystallography. Compound (2) crystallizes in the monoclinic system, space group p21/n, with a = 9.025(1), b = 22.912(1), c = 12.790(1) , = 99.36(1), Z = 4. The deviations of the four coordinating atoms (O(1)O(2)N(1) and N(2) of salbn and the copper atom is displaced from the plane in spite of the lack of any apical ligand. The gadolinium(III) ion is nine-coordinated by the two oxygen atoms of the salbn moiety, three bidentate nitrate ions and one water molecule. The geometry of GdIII can be described as a square antiprism, in which compound CuII and GdIII are bridged by the two phenolic oxygens of salbn. The CuII–GdIII distance is 3.269(1) . The bridging core CuO2Gd is a butterfly shape. Significant distortion was observed for the complex having the larger diamino string. The title compound exhibits seven e.s.r. transitions with |D| = 0.0467 cm–1, which demonstrates the existence of zero field splitting. This outcome indicates that compound (2) consists of a perfectly isolated dinuclear Cu–Gd core and steric bulk alters the dihedral angle in the Cu–O–Gd bridge.
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ABSTRACT: A tridentate ligand, N’-[1-(2-hydroxyphenyl)ethylidene]-1H-indole-3-carbohydrazide (H2L), reacts with vanadyl sulfate in methanol to give a centrosymmetric dinuclear complex, [VOL(OCH3)]2. The complex was characterized by physicochemical and spectroscopic methods. The crystal of the complex is monoclinic with space group P21/c, a = 8.345(3), b = 9.469(3), c = 24.087(4) Å, β = 96.290(3)°, V = 1892.1(9) Å, Z = 4. X-ray crystallography indicates that each V atom is in a distorted octahedral coordination. The two V atoms are bridged by two phenolate O atoms, with a distance of 3.540(2) Å.Synthesis and Reactivity in Inorganic Metal-organic and Nano-metal Chemistry - SYNTH REACT INORG MET-ORG NAN. 01/2011; 41(8):1052-1055.
A novel discrete dinuclear copper(II)–gadolinium(III) complex derived from a Schiff
base ligand [Cu(salbn)Gd(NO3)3ÆH2O] (salbn): N,N¢-butylenebis(salicylideaminato)
Thekkel M. Rajendiran*
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan – 48109-1055, USA
Ramu Kannappan, Rajaram Mahalakshmi, Rangarajan Venkatesan and Pillutla Sambasiva Rao
Department of Chemistry, Pondicherry University, Pondicherry – 605 014, India
Lakshmanan Govindaswamy and Devadasan Velmurugan
Department of Biophysics Crystallography, University of Madras, Guindy Campus, Madras – 600 025, India
Received 20 September 2002; accepted 26 November 2002
The synthesis, X-ray and e.p.r. spectral studies of a 3d–4f couple are described here. The crystal structure of
[Cu(salbn)Gd(NO3)3ÆH2O], (2), salbn ¼ N,N¢-butylenebis(salicylideaminato), has been determined by X-ray
crystallography. Compound (2) crystallizes in the monoclinic system, space group p21/n, with a ¼ 9.025(1),
b ¼ 22.912(1), c ¼ 12.790(1) A˚, b ¼ 99:36(1), Z ¼ 4. The deviations of the four coordinating atoms (O(1)O(2)N(1)
and N(2) of salbn and the copper atom is displaced from the plane in spite of the lack of any apical ligand. The
gadolinium(III) ion is nine-coordinated by the two oxygen atoms of the salbn moiety, three bidentate nitrate ions
and one water molecule. The geometry of GdIIIcan be described as a square antiprism, in which compound CuII
and GdIIIare bridged by the two phenolic oxygens of salbn. The CuII–GdIIIdistance is 3.269(1) A˚. The bridging
core CuO2Gd is a butterfly shape. Significant distortion was observed for the complex having the larger diamino
string. The title compound exhibits seven e.s.r. transitions with jDj ¼ 0:0467 cm)1, which demonstrates the existence
of zero field splitting. This outcome indicates that compound (2) consists of a perfectly isolated dinuclear Cu–Gd
core and steric bulk alters the dihedral angle in the Cu–O–Gd bridge.
The unique properties of copper(II)–gadolinium(III)
complexes have attracted increasing interest due to their
latent applications in the designs of bimetallic catalysts
, novel molecular based magnetism and molecule
devices . The administration of the contrasting agents
in magnetic resonance imaging (MRI) has greatly
improved the potentials of this modality [3, 4]. Lanth-
anide complexes of high stability could turn out to be
especially vital in two very different areas of research
where inert complexes are potentially useful; namely, for
the separation of the lanthanides as a set of metals and
for the design of gadolinium(III) contrast agents for
n.m.r. imaging . Many medicines require that the
complex be inert to metal ion release in water . The
MRI contrasting agents have an indirect mode of
action. Since they contain paramagnetic metal ions they
influence the signal intensity primarily by altering
proton relaxation rates in tissue. Gadolinium(III) is
the most effective relaxation enhancer and almost all
commercially available MRI contrast agents contain
gadolinium(III) complexes; important issues in the
development of gadolinium(III) containing MRI con-
trast agent are low toxicity, low osmolality, high
thermodynamic and/or kinetic stability and the presence
of at least one water molecule in the inner coordination
sphere of the Gd metal ion. The coordination chemistry
of lanthanides has become of increasing significance in
recent years due to the wide variety of potential
applications of lanthanide complexes. To date, most of
the studies with magnetic properties of 4f–3d complexes
have been limited to the copper(II)–gadolinium(III)
system. The e.p.r. studies have not been studied in detail
so far. In this paper, we describe the synthesis (Scheme
1), crystal structure and e.p.r. spectral studies of gado-
All chemicals were used as commercially purchased.
Gd(NO3)3Æ6H2O was prepared in the usual way by
evaporating a solution of the corresponding oxide in
HNO3to dryness. The ligand [N,N-1,4-butylethylene-
mixing warm EtOH solutions of salicylaldehyde and
1,4-diaminobutane in a 2:1 molar ratio according to the
* Author for correspondence
Transition Metal Chemistry 28: 644–649, 2003.
? 2003 Kluwer Academic Publishers. Printed in the Netherlands.
typical procedure [6, 7].1H-n.m.r. (400 MHz) in CDCl3,
dppm: 8.3(s,2H), 7.3(dd, 2H), 7.2(dd, 2H), 6.9(d, 2H),
6.8(t, 2H), 3.7(t, 4H), 1.9(m, 4H).13C-n.m.r. (100 MHz)
in CDCl3,dppm: 165,163, 161, 132, 131, 118, 117, 59 and
Metal presursor Cu(salbn) (1)
To a MeOH solution (50 cm3) of salbn (0.5 mmol) was
added a MeOH solution (25 cm3) of Cu(OAc)2Æ2H2O
(0.5 mmol) with stirring. The mixture was subsequently
boiled under reflux for 4 h. After cooling to room
temperature a green precipitate was collected by filtra-
tion, washed with MeOH and dried in air. Yield: 90%.
(Found: C, 60.4; H, 5.0; N, 7.8. C18H18N2O2Cu calcd.:
C, 60.4; H, 5.1; N, 7.8%). Mass spectrum (FAB, 3-
nitrobenzyl alcohol matrix): m=z ¼ 358 (100).
This compound was prepared by slowly adding a MeOH
solution of 0.225 g (0.5 mmol) of Gd(NO3)3ÆH2O to a
(0.5 mmol) sample of Cu(salbn) dissolved in 100 cm3of
hot CHCl3under constant stirring. The mixture was then
refluxed for 4 h and the solution was concentrated to
50 cm3. After slow evaporation, a dark green precipitate
was obtained. We attempted to obtain a single crystal
suitable for X-ray diffraction The compound was dis-
solved in hot CHCl3–MeOH mixture under diffusion
with Et2O at room temperature dark green single crystals
formed after 3 weeks. Yield: 67%. (Found: C, 29.3; H,
2.7; N, 9.7. C18H20N5O12CuGd calcd.: C, 30.1; H, 2.8; N,
FX-400 FT-n.m.r. spectrometer in CDCl3 solution,
using TMS as the internal standard. I.r. spectra of the
complexes and the ligands were recorded on a Hitachi
Infrared Spectrophotometer using the KBr pellet tech-
nique, in the 4000–200 cm)1range. U.v.–vis. spectra
13C-n.m.r spectra were recorded on a JEOL
were recorded with an Ocean Optics, Inc., SD 1000 fiber
Optic spectrometer in CHCl3solvent. FAB mass spectra
were recorded on a JEOL SX 102/DA-6000 mass
(6 kV, 10 mA) as the FAB gas and m-nitrobenzyl
alcohol (NBA) as the matrix. Elemental (C, H and N)
analyses were performed with a Heraus Rapid analyzer.
X-band e.s.r spectra were recorded using a JEOL JES-
TE100 ESR Spectrometer.
X-ray structure determination
The crystal of dimensions 0.40 · 0.32 · 0.26 mm (2) was
glued to a glass fibre. The intensity data were collected at
room temperature using the Siemens SMART CCD area
detector three-circle diffractometer equipped for graphite
monochromated MoKa (k ¼ 0.71073 A˚) radiation. The
data collection nominally covered a full hemisphere of
reciprocal space by a combination of three sets of
exposures, each set having a different / angle for the
crystal and each exposure covered 0.3? in x. The crystal
to detector distance was 5.89 cm. Coverage of the unique
set was over 86% complete to at least 25.6? in h. Crystal
decay was monitored by repeating the initial frames at
the end of the data collection and analyzing the duplicate
reflections. The substantial redundancy in data allows
empirical absorption corrections to be applied using
multiple measurements of equivalent reflections. Data
frames were collected for 10–30 s frames, depending on
the intensity of the data, giving an overall time for data
collection of 7–18 h. The data frames were integrated
using SAINT and were merged to give a unique dataset.
The structure was solved by automated Patterson
methods and subsequent difference Fourier technique
using DIRDIF 98.3. All hydrogen atoms were included
at calculated positions using a riding model. The Uisoof
H atoms of CH and CH2groups and the methyl group
were taken as 1.2Ueqof their carrier atoms, except for the
water hydrogen atoms, which could not be identified.
All non-hydrogen atoms were refined with anisotropic
thermal parameters. For the compound, the final R-
values is 0.063 for 4607 observed reflection with
I > 2rðIÞ and 0.089 for (6356) all data. Anomalous
dispersion effects for all atoms were included in the final
Results and discussion
The ligand (salbn) was synthesized by Schiff’s base
condensation. The spectral studies such as1H-n.m.r as
well as13C-n.m.r confirms the tetradentate nature of the
ligand. The electronic spectrum of the copper(II) com-
plex precursor (1) exhibits its higher energy band at
624 nm, while a band red shifted to lower energy at
688 nm for (2) seems to be due to a distortion of
geometry occurring at the copper center (Figure 1).
There is great similarity between the i.r. spectra of the
heterodinuclear complexes [8, 9]. They are almost
superimposible with the exclusion of the presence of a
mC@N¼ 1645 cm)1in the spectrum of (2). This absorp-
tion appears at mC@N¼ 1638 cm)1in the case of
compound (1). In compound (2) the energy discrep-
ancy between the asymmetric and asymmetric stretching
frequency for (NO?
indicates the nature of bidentate nitrate bridging.
3) occurs at Dm ¼ 153 cm)1, which
The unit cell contains four distinct entities of [LCuGd-
(NO3)3ÆH2O]. A view of the dinuclear unit is represent-
ed in (Figure 2) with crystal data; selected bond angles
and bond lengths given in Tables 1 and 2. The ORTEP
diagram of (2) is drawn at 50% probability displace-
ment thermal ellipsoids with atomic numbering scheme.
The coordination geometry about the Cu ion consists
of a chelate ring and a distorted coordination plane
composed of two imine N atoms and two phenol O
atoms. The copper(II) completes its coordination sphere
with two imine nitrogen atoms from the Schiff base. The
Fig. 1. Electronic spectrum of Cu(salbn)Gd(NO3)3ÆH2O (2).
Fig. 2. ORTEP diagram of (2) showing the 50% probability thermal ellipsoids.
CuAO bond lengths are 1.961(6) and 1.953(5) A˚
CuAO1 and CuAO2, respectively while the CuAN bond
lengths are 1.915(7) and 1.948(7) A˚
CuAN2 respectively, which are normal values for
[10–12]; they also agree with previously reported values
[13–15]. The most interesting comparative aspects of Cu
complexes with imine phenols involve the steric influ-
ence of the alkyl backbone upon the molecular struc-
ture. The Cu ion is coordinated by two imine nitrogens
for CuAN and
Schiff base complexes
and two oxygen from the Schiff base ligand. These four
atoms deviate significantly from the distorted coordina-
tion plane Cu1O1O2N1N2 and they are 0.451(7) and
0.031(1) respectively. The copper is displaced from the
plane in spite of the absence of any apical ligand. The
steric interaction of the propyl, butyl and phenyl
backbones affect the copper coordination geometry
significantly in many respects. In the five membered
ring systems with a two C atom backbone, the CuAN
distance is short (average 1.916 A˚) and the NACuAN
angle (82.7?) and the dihedral angle (5.3?) are small.
Addition of a third C atom to the backbone to make a
six membered chelate ring results in increased CuAN
lengths NACuAN angles and dihedral angles. Further
increase in the backbone size to give a seven membered
ring makes it more difficult to maintain the configura-
tion without considerable puckering of the ring. It seems
that tuning of the CuAN lengths, NACuAN angle and
dihedral contributes to the flexibility of the coordination
of copper by tetradentate iminephenol ligands.
Examination of the gauche conformation of the
butane bridge, which has often been found to be
unsymmetrical, can also provide some basis for com-
parison of the extent of distortion of copper(II) imine
phenol complexes. The butane bridging C atoms are
asymmetrical by buckling and its torsion angles are
)73(1) (N2AC8AC9AC10), 56(1) (C8AC9AC10AC11)
and 50(1)? (C9AC10AC11AN1) respectively. These
angles are comparable with similar types of copper(II)
coordination complexes . It is well known that
increasing the steric hindrance by elongation of the alkyl
bridge will result in a change in the chelate pattern from
planar to tetrahedral. The distortion of the inner
coordination sphere can be recognized by the magnitude
of the dihedral angle between the two planes defined by
Cu2N2O2. The dihedral angle between two planes is
The gadolinium environment is a distorted square
antiprism of oxygen atoms, two belonging to the salbn
ligand and six belonging to the three bidentate nitrato
ions and one water molecule orientated in axial position.
The ninth coordinated water oxygen is at 2.350(7) A˚
from the gadolinium atom and this value is comparable
with the metal H2O distance observed in other lantha-
nide complexes: (GdAO ¼ 2.39(1) A˚[7, 11]. The gado-
linium(III)–copper(II) distance is 3.269(1) A˚, which is
close to that 3.252(4) A˚, of the complexes of Kahn and
co-workers  but shorter than those 3.4275(9)–
3.5231(4) of the complexes of Costes et al.  which
is still greater than reports by Sasaki et al.  and
Kahn et al. , the CuAO2AGd bridge is asymmetric.
The CuAO bond distances are 0.82 A˚shorter than the
Gd ones. The two CuAOAGd angles are almost equal
to each other within an error of 97.3(2) and 98.4(2)?.
The three bidentate nitrato ions are bound to the
gadolinium(III) ions in a slightly asymmetric fashion.
All GdAO and NAO distances are in good agreement
with corresponding values in a similar type of Gd(III)–
Ni(II) complex . The evaluation of the dihedral angle
Table 1. Crystal data for complex (2)
Chemical formula weight (Wt)
Unit cell dimensions (A˚3)
a ¼ 9:025ð1Þ
b ¼ 22:912ð1Þ
c ¼ 12:790ð1Þ
0.40 · 0.32 · 0.26
Dc (calculated), mg m?3
Wavelength, (k, A˚)
Absorption coefficient (l, cm?1)
Temperature (T, K)
Crystal size (mm)
F(0 0 0)
CCD area detector
?11 ? h ? 12, ?30 ? k ? 25,
?16 ? l ? 12
Reflections with I > 2rðIÞ
Final R indices [I > 2rðIÞ]
Largest diff peak and hole (e A˚?3)
R1= 0.064 & WR2= 0.192
2.74 to ?3.95
Table 2. Selected bond distance (A˚) and angle (?) for complex (2)
and copper(II)–gadolinium(III) distance with previously
reported complexes is given in Table 3.
The bridging network GdO1O2Cu has a butterfly
shape taking O1O2 as the hinge, the GdO1O2 and
CuO1O2 planes forming a dihedral angle of 33.7(2)? and
the O1–O2 distance being equal to 2.635(8) A˚. These
values agree with previously reported results . The
OAO distances (O1AO2, O3AO4, O6AO7, O9AO10)
fall in the 2.161(10) to 2.635(8) A˚range. In accordance
with the bidentate nature of NO3 ligands, we note in
every case that NAO bond lengths are nearly equal.
Electron paramagnetic resonance
The e.p.r. spectrum of the polycrystalline sample of
complex (1) has been recorded at 77 K, yielding
parameters gk¼ 2:137, g?¼ 2:082, Ak¼ 142 ? 10?4
cm)1. This is typical of tetragonally coordinated mono-
meric copper(II) complex with the unpaired electron in
the dx2–dy2orbital . A polycrystalline powder e.p.r.
spectrum of complex (2) at room temperature is shown
in (Figure 3, insert 1). It shows a strong unique quasi-
isotropic broad signals centered at g ¼ 2:274 but no clear
characteristic peak in the g ¼ 2 region . The spectrum
of complex (2) at 77 K (Figure 3) exhibits an aniso-
tropic broad signal. The spectrum shows a striking
temperature dependence and broadens at 77 K . At
room temperature the spectrum is uninformative, but on
cooling to 77 K, the fine structure exhibits seven tran-
sitions around 99, 175, 241, 300, 311, 436 and 465 mT,
which may be generated by zero field splitting [29, 30].
This spectrum corresponds to the superposition of the
signal of the copper(II) ion, including the gadolini-
um(III) signal. The weak features 99, 175, 241 and
300 mT obviously leads to an increase in the intensity
and the characteristic peak moves toward lower fields,
whereas the intense feature at 241 mT becomes wider,
due possibly to a spin–spin relaxation effect . This
summing up reveals that the copper(II) (s ¼ 1/2) gado-
linium(III) (s ¼ 7/2) interaction is weak which gives rise
to an s ¼ 4 ground state; s ¼ 3 low lying excited state and
s ¼ 4 may be significantly populated at 77 K.
In this paper, we have described the successful
synthesis of a discrete dinuclear complex with increasing
backbone chain in the diamine arm which may result
from the larger distortion of the geometry around
copper(II) towards a tetrahedral structure. The steric
interaction of the ethylene, propyl and biphenyl back-
bones affect the copper coordination geometry signifi-
cantly in CuAN distance, NACuAN angle and the
dihedral angle. Further increase in the backbone size
(butyl) to give a seven membered ring makes it more
difficult to maintain the configuration without consid-
erable puckering of the ring. Which is tuning the CuAN
lengths, NACuAN angle and dihedral angle contributes
to the flexibility of the coordination of copper by
tetradentate iminophenol ligand. We observed seven
hyperfine lines at 77 K, which exhibits a week interac-
tion in the Cu(II)AGd(III) core. This data deserves
further investigations in order to authenticate this
hypothesis and we are at present found difficult to
obtain J value by magnetic measurements and single
crystal e.p.r. study.
Table 3. Comparison of structural parameters a and b for dinuclear
a is the dihedral angle between the O(1)CuO(2) and O(1)GdO(2)
planes in deg and b is the CuAGd separation in A˚. salen = N,N¢-
ethylenebis(salicylideaminato). MeIm = 1-methylimidazole. Salbn is
described in the text, O2COMe = monomethyl carbonate, hfac =
1,1,1,5,5,5-hexafluoroacetylacetone and salabza = N,N¢-bis(salicyli-
Fig. 3. E.p.r. spectrum of (2) in solid state at RT (inserted one) and 77 K; frequency ¼ 9.4023 GHz.
TMR thanks the CSIR [No: 01(1557/98/EMR-II] for
financial assistance. R.K and R.M. thank the CSIR for
Senior Research Fellowship awards.
Complementary data for the titled compound are
available from the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 1EZ, UK on
request, quoting the deposition number CCDC 176083
respectively (e-mail: email@example.com).
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