Synthesis and characterization of lanthanide complexes prepared with 2-((E)-(1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-methoxyphenol

Article abstract of DOI:10.1080/00958972.2017.1319568

Rare-earth complexes of the general formula [Ln(H₂L₁)₂(NO₃)₃] [Ln?=?Gd (1), Ho (2) or Nd (3)] were prepared from an o-vanillin derived Schiff base ligand, 2-((E)-(1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-methoxyphenol (H₂L₁). The single-crystal X-ray diffraction studies and SHAPE analyses of the Gd(III) and Ho(III) complexes show that the complexes are ten-coordinate and exhibit distorted tetradecahedron geometries. The phenolate oxygen-bridged dinuclear complex, [Ce₂(H₂L₁)(ovan)₃(NO₃)₃] (4, ovan?=?monodeprotonated o-vanillin), was obtained from the reaction of Ce(NO₃)₃?6H₂O with H₂L₁. X-ray analysis revealed that hydrolysis of H₂L₁ occurred to yield o-vanillin, which bridged two cerium atoms with the Ce?Ce distance equal to 3.8232(6) ?. The Ce(III) ions are both ten-coordinate, but have different coordination environments, showing tetradecahedron and staggered dodecahedron geometries, respectively. With proton migration occurring from the phenol group to the imine function, complexation of the lanthanides to the ligand gives the Schiff base a zwitterionic phenoxo-iminium form.

Full text of DOI:10.1080/00958972.2017.1319568

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis and characterization of lanthanide
complexes prepared with 2-((E)-(1-hydroxy-2-
methylpropan-2-ylimino)methyl)-6-methoxyphenol
Abubak’R Abrahams, Tatenda Madanhire, Eric Hosten & Richard Betz
To cite this article: Abubak’R Abrahams, Tatenda Madanhire, Eric Hosten & Richard Betz (2017):
Synthesis and characterization of lanthanide complexes prepared with 2-((E)-(1-hydroxy-2-
methylpropan-2-ylimino)methyl)-6-methoxyphenol, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1319568
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1319568
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Date: 21 April 2017, At: 03:51

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2017.1319568
Synthesis and characterization of lanthanide complexes prepared with 2-((E)-
(
1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-methoxyphenol
ABUBAK’R ABRAHAMS*, TATENDA MADANHIRE, ERIC HOSTEN and RICHARD BETZ
Department of Chemistry, Nelson Mandela Metropolitan University, PO Box 77000,
6031 Port Elizabeth, South Africa
Rare-earth complexes of the general formula [Ln(H
were prepared from an o-vanillin derived Schiff base ligand, 2-((E)-(1-hydroxy-2-methylpropan-
-ylimino)methyl)-6-methoxyphenol (H ). The single-crystal X-ray diffraction studies and
2 1 2 3 3
L ) (NO ) ] [Ln = Gd (1), Ho (2) or Nd (3)]
2
2 1
L
SHAPE analyses of the Gd(III) and Ho(III) complexes show that the complexes are ten-
coordinate and exhibit distorted tetradecahedron geometries. The phenolate oxygen-bridged
dinuclear complex, [Ce
obtained from the reaction of Ce(NO
of H occurred to yield o-vanillin, which bridged two cerium atoms with the Ce∙∙∙Ce distance
2
(H
2
L
1
)(ovan)
3
(NO
3
)
3
] (4, ovan = monodeprotonated o-vanillin), was
3
)
3
∙6H
2
O with H . X-ray analysis revealed that hydrolysis
2 1
L
2 1
L
equal to 3.8232(6) Å. The Ce(III) ions are both ten-coordinate, but have different coordination
environments, showing tetradecahedron and staggered dodecahedron geometries, respectively.
With proton migration occurring from the phenol group to the imine function, complexation of
the lanthanides to the ligand gives the Schiff base a zwitterionic phenoxo-iminium form.
Keywords: Lanthanide; Bidentate; Crystal structure; Schiff base; o-Vanillin
1
. Introduction
Schiff bases are of great significance in coordination chemistry due to their applications in the
medical and pharmaceutical fields. They are known to exhibit biological antifungal, antibacterial
and antitumor activity [1-3]. Furthermore, Schiff bases are utilized as coordinating ligands, in
electrochemistry and as catalysts, for example in the hydrogenation of olefins [4-7]. Their
*
Corresponding author. Email: Abubakr.Abrahams@nmmu.ac.za

applications have been extended to ion-exchange, luminescence, magnetism and in biological
modelling [8]. Intramolecular hydrogen bonds between the oxygen and nitrogen atoms have been
found to exist and play an important role in stabilizing metal complexes [9]. Complexation of
Schiff bases to metal ions has been performed to enhance the biological activity of some Schiff
bases due to the synergistic effect [10, 11].
Antimicrobial studies performed on some compounds derived from 2-hydroxy-3-
methoxybenzaldehyde (o-vanillin) revealed a higher activity for the o-vanillin derivatives
compared to the salicylaldehyde-based ligands [12]. Zinc and nickel complexes with the
potentially
multidentate
Schiff
base
ligand,
2-((E)-(1-hydroxy-2-methylpropan-2-
2 1
ylimino)methyl)-6-methoxyphenol (H L , figure 1), have been studied due to their luminescent
and magnetic behavior. The crystal structure of the trinuclear Zn(II) complex revealed that the
monodeprotonated ligand acts in a tetradentate mode, coordinating through the phenolate
oxygen, the hydroxyl oxygen, the alkoxyl oxygen and the imino nitrogen [13]. In the dinuclear
2 1
Ni(II) complex, the monodeprotonated H L acts as a tridentate chelate with coordination
occurring via the imino nitrogen atom, the hydroxyl oxygen atom and one deprotonated
phenolate oxygen atom.
In this work, we report on the syntheses of four novel rare-earth complexes with H
2 1
L ,
viz. [Ln(H (NO ] [Ln = Gd (1), Ho (2), Nd (3)] and [Ce (H )(ovan) (NO ] (4, ovan =
L
1
)
)
3
2
2
L
1
3
3 3
)
2
2
3
monodeprotonated o-vanillin), and an investigation into their structures and spectroscopic
properties.
2
2
. Experimental
.1. Chemicals
The lanthanide salts, o-vanillin and 2-amino-2-methyl-1-propanol were sourced from Sigma
Aldrich and used as received. The solvents were obtained from Merck in analytical grade and
1
dried over 3 Å molecular sieves prior to use. Deuterated DMSO, used for H NMR spectroscopy,
was purchased from Merck.
2
.2. Synthesis
2
2 1 2 1
.2.1. Synthesis of H L . The Schiff base ligand H L was prepared using the procedure
described for related compounds [12-14].

2
.2.2.
methoxyphenol)trinitrato gadolinium(III), [Gd(H
solution of H (0.345 g, 1.55 mmol) was added Gd(NO
Synthesis
of
bis(2-((E)-(1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-
(NO ] (1). To a 15 mL methanolic
∙6H O (0.246 g, 0.545 mmol). The
2
1
L )
2
3 3
)
2
L
1
3
)
3
2
resulting mixture was stirred continuously for 30 minutes at room temperature, followed by
filtration of the yellow solution to dispose of undissolved material. Yellow single crystals
suitable for X-ray crystallography were grown within 3 days under vapour diffusion using
diethyl ether. Yield: 0.155 g (35.9%, based on Gd), m.p. = 251.6-255.5 C. Anal. Calcd. for
24 5
C H34GdN O15 (%): C, 36.50; H, 4.34; N, 8.87. Found: C, 36.19; H, 4.09; N, 8.84. Conductivity
-3
-1
2
-1
-1
-1
(
10 M, DMF): 124.9 ohm cm mol . UV-Vis (DMF, λmax nm (ε, M cm )): 329 (4280), 420
-1
-
-
-
(
4 3 1 3 6 3
1490). IR (cm ): v(N-H) 3495(w), v (NO ) 1550(s), v (NO ) 1299(m), v (NO ) 817(m),
v(Gd-O) 418(m).
2
.2.3.
methoxyphenol)trinitrato holmium(III), [Ho(H
was adopted. Single yellow X-ray quality crystals were obtained from the reaction of H
0.321 g, 1.44 mmol) and Ho(NO ∙6H O (0.226 g, 0.512 mmol). Yield: 0.138 g (33.8%, based
on Ho), m.p. = 230.1-232.2 C. Anal. Calcd. for C H HoN O (%): C, 36.15; H, 4.30; N, 8.78.
Synthesis
of
bis(2-((E)-(1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-
2
L
1
)
2
(NO ] (2). A similar procedure to that of
3 3
)
1
2 1
L
(
3
)
3
2
2
4
34
5
15
-3
-1
2
-1
Found: C, 36.14; H, 4.18; N, 8.71. Conductivity (10 M, DMF): 134.4 ohm cm mol . UV-Vis
-1
-1
-1
-
(
4 3
DMF, λmax nm (ε, M cm )): 298 (1530), 334 (1330). IR (cm ): v(N-H) 3495(w), v (NO )
-
-
1
550(s), v (NO ) 1299(m), v (NO ) 817(m), v(Ho-O) 418(m).
1 3 6 3
2
.2.4.
Synthesis
of
bis(2-((E)-(1-hydroxy-2-methylpropan-2-ylimino)methyl)-6-
(NO ] (3). To a 10 mL methanolic
(0.316 g, 1.42 mmol) was added a 5 mL methanolic solution of Nd(NO ∙6H
0.235 g, 0.536 mmol). The resulting mixture was stirred for 30 minutes at room temperature.
methoxyphenol)trinitrato neodymium(III), [Nd(H
solution of H
2
L
1
)
2
3 3
)
2
L
1
3
)
3
2
O
(
Diethyl ether was allowed to diffuse slowly into the brown solution at room temperature. Badly
twinned brown crystals were grown within 3 days. All attempts to produce reproducible X-ray
data were unsuccessful. Yield: 0.158 g (37.9%, based on Nd), m.p. = 235.0-237.8 C. Anal.
Calcd. for C24
5
H34NdN O15 (%): C, 37.11; H, 4.41; N, 9.02. Found: C, 37.14; H, 4.38; N, 9.12.
-3
-1
2
-1
-1
-1
Conductivity (10 M, DMF): 154.5 ohm cm mol . UV-Vis (DMF, λmax nm (ε, M cm )): 298

-1
-
-
-
(
4 3 1 3 6 3
2350), 370 (1980). IR (cm ): v(N-H) 3495(w), v (NO ) 1550(s), v (NO ) 1299(m), v (NO )
8
17(m), v(Nd-O) 418(m).
2
.2.5.
Synthesis
of
tri-µ-ortho-vanillin-2-((E)-(1-hydroxy-2-methylpropan-2-
[Ce (H )(ovan) (NO
(0.345 g, 1.55 mmol) was added a 5 mL methanolic
2
O (0.246 g, 0.567 mmol). The resulting mixture was stirred
ylimino)methyl)-6-methoxyphenol)trinitrato-di-µ-cerium(III),
4). To a 10 mL methanolic solution of H
solution of Ce(NO ∙6H
2
2
L
1
3
3 3
) ]
(
2 1
L
3
)
3
continuously for 30 minutes at room temperature, followed by filtration of the yellow solution to
get rid of undissolved material. Brown single crystals suitable for X-ray crystallography were
grown within 3 days under vapour diffusion using diethyl ether. Yield: 0.450 g (65.7%, based on
Ce), m.p. = 212.3-214.5 C. Anal. Calcd. for C36
2 4 4
H37Ce N O21,2(CH O) (%): C, 37.84; H, 3.76;
-3
-1
2
-1
N, 4.64. Found: C, 37.82; H, 3.71; N, 4.68. Conductivity (10 M, DMF): 184.0 ohm cm mol .
-1
-1
-1
UV-Vis (DMF, λmax nm (ε, M cm )): 340 (1030). IR (cm ): vas(N-O) 1549 (s), v(C-H) 1435(s),
(N-O) 1297(s), v(C-N) 1248(s), v(Ce-O) 420(m).
v
s
2
.3. Physical measurements
The melting points of complexes were determined using a Stuart® Melting Point Apparatus
SMP30, measuring and recording the temperatures of crystalline samples held within capillary
tubes. The microanalyses for carbon, hydrogen, nitrogen and sulfur were carried out using a
Vario Elementary ELIII Microcube CHNS analyser. Conductivity measurements were performed
at room temperature using HANNA instruments (HI) 2300 EC/TDS/NaCl Meter. The infrared
spectra were obtained from a Bruker Tensor 27 FT-IR spectrophotometer, equipped with the
Platinum ATR attachment. The samples were run neat via ATR and the recorded data was
analysed using the OPUS 6.5 software. UV-Vis spectroscopy was carried out on a Perkin Elmer
Lambda 35 UV/Vis spectrophotometer, and processing done using UV WinLab software. NMR
spectra were recorded at room temperature using a Bruker AvanceIII 400 Nuclear Magnetic
Resonance (NMR) spectrometer and acquisition done using TopSpin 3.0 software. Analysis of
spectra was performed using ACD/Labs software.
A Bruker Kappa Apex II X-ray Crystallography System was used for single crystal
diffractometry at 200 K using graphite monochromated Mo Kα radiation (λ = 0.71073 Å).
APEX2 was used for data collection and SAINT software for data reduction and cell refinement

[15]. The structures were solved by direct methods using SHELXT-2014 and refined by least-
squares procedures using SHELXL-2014 with ShelXle as a graphical interface [16, 17]. Data
were corrected for absorption effects using the numerical method implemented in SADABS [15].
All non-hydrogen atoms were refined anisotropically. Carbon-bound H atoms were placed in
calculated positions and were included in the refinement in the riding model approximation, with
Uiso(H) set to 1.2Ueq(C). The H atoms of the methyl groups were allowed to rotate with a fixed
angle around the C—C bond to best fit the experimental electron density (HFIX 137 in the
SHELX program suite), with Uiso(H) set to 1.5Ueq(C) [16]. The H atoms of the hydroxyl groups
were allowed to rotate with a fixed angle around the C—O bond to best fit the experimental
electron density (HFIX 147 in the SHELX program suite), with Uiso(H) set to 1.5Ueq(O) [16]. In
all three structures there is disorder of the hydroxyl-dimethylethyl groups of one of the ligands.
For the Gd and Ho structures one of the methoxy groups has a rotational disorder and the shelx
RIGU restraint was used. The disorder in the Ce structure is more extensive requiring restraints
in the bond distances (SADI and DFIX) and anisotropic displacement parameters (EADP).
Molecular graphics were obtained using ORTEPIII and Mercury softwares. Lanthanide geometry
analyses were performed by SHAPE 2.1 software. This program is based on the algorithm
defined by Pinsky and Avnir for the calculation of continuous shape measures (CShM values)
[
18]. The thermogravimetry (TG), differential scanning calorimetry (DSC) and derivative
thermogravimetry (DTG) were carried out at a heating rate of 10 °C/min under nitrogen on a TA
050 Thermogravimetric Analyser. Electrochemical analyses were done using C3 Cell Stand for
2
Bas Epsilon Version 1.30.64 system which consists of three electrodes: a platinum working
electrode, a platinum auxiliary electrode and a pseudo silver/silver chloride Re-5 reference
electrode. The redox behavior of the lanthanide complexes was investigated using 0.10 M
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The oxygen in sample
solutions was removed by aerating with nitrogen for 10 minutes before runs.
3
. Results and discussion
The introduction of a methanolic solution of Ce(NO
3
)
3
∙6H
2
O to H
2
L
1
yielded o-vanillin due to
-
hydrolysis of the imine bond, which then coordinated in its monodeprotonated form ovan
together with H (see figure 2).
2 1
L

-3
The molar conductance data of the metal complexes measured in DMF using 10 M
-1
2
-1
solutions at room temperature gave values in the range of 124.9 to 184.0 ohm cm mol . These
molar conductivity values adequately confirmed the 2:1 electrolytic nature of the metal
complexes [19].
3
.1. Infrared spectroscopy
-
1
The broad band appearing in the 3675-3149 cm region of the free ligand can be attributed to the
-
1
O-H stretching vibrations. In the 3034-2805 cm range is a series of weak bands which results
-1
from the C-H stretching of the ligand, while the vibrational band at 1438 cm can be assigned to
the C-H bending modes. The v(C=N), v(C-N) and v(C-O) bands are observed at 1635, 1229 and
-1
1
170 cm , respectively.
Complexes 1-3 have identical IR spectra, with characteristic bands having similar shifts
upon complexation from that of the free ligand – an indication of similar coordination
compounds. The replacement of the broad v(O-H) in the IR spectrum of the free ligand and
-1
subsequent appearance of the peak at 3495 cm upon complexation confirms the N-H vibration
+
in the C=N -H moiety of the ligand. This band shows that the hydrogen atom is still involved in
intramolecular hydrogen bonding with the phenolate oxygen atom, in accord with previous
observations of other rare-earth complexes of zwitterionic Schiff base ligands [20, 21]. This is
also further proof of proton migration from the phenolic group to the azomethine nitrogen atom.
The C-H bending mode stretch in the free ligand is shifted to a slightly lower energy on
-
1
-1
complexation with Gd(III), Ho(III) and Nd(III), viz. 1432 cm in 1 and 1428 cm in 2 and 3. The
-1
bands at 1550, 1299 and 817 cm are ascribed to the vibrations of the coordinated nitrate groups.
-1
4 1
The separation of the two highest frequency bands (v -v ) is approximately 251 cm , which
2
indicates η -chelation of the nitrate groups [21, 22]. Formation of the Ln-O bonds is further
-
1
confirmed by the appearance of bands at 418 cm , which is attributed to the v(Ln-O) [23].
In 4, the O-H and C-H vibrational bands become less intense upon coordination. The C-H
-1
-1
bending band is shifted to 1435 cm , and the v(C-N) is shifted to lower energy by about 19 cm .
-1
The appearance of the band at 420 cm has been assigned to the v(Ce-O), supporting the
-1
formation of Ce-O bonds [24]. The absorption bands at 1549 and 1297 cm may be attributed to
the N-O asymmetric (vas) and N-O symmetric (v ) stretches, respectively. The separation of the
s

-1
two frequency bands (vas-v
s
) is 252 cm , which is in accord with the bidentate character of the
nitrate groups [22, 24].
3
.2. NMR analysis
1
The six-proton singlet appearing at 1.26 ppm in the H NMR spectrum of the free ligand is
ascribed to the methyl protons, while the hydroxyl proton resonates at 2.08 ppm. The methylene
protons and the phenolic proton singlet signals are found at 3.37 ppm and 3.51 ppm,
respectively. A singlet observed at 3.78 ppm is due to the resonance of the methoxy protons
while that at the highest chemical shift (δ = 8.20 ppm) arises from the resonance of the imino
proton. The resonances of the aromatic protons are observed at δ = 6.58 ppm (triplet, 1H) and δ =
6
.74 ppm (doublet, 2H).
1
The H NMR spectra of 1-3 in DMSO-d
6
revealed signal broadening in the spectra of
complexes, with more intense broadening observed for the Gd(III) complex. These broader peaks
were expected since the lanthanides under investigation are paramagnetic. Broadening of the
signals is brought about by shortening of spin relaxation times [25]. When compared to free
2 1
H L , the resonances in the complexes show a downfield shift, with the greatest shift induced on
the methoxy proton signals since they are located in the vicinity of the metal center [26]. Upon
coordination of the ligand to the metal, formation of the Ln-O bonds results in a change in the
environment of the protons. The presence of the lanthanide ion accounts for the decrease in the
electron density and thus deshielding of these protons. As a result, the hydrogen atoms resonate
at lower field values. An insignificant shift is observed for the methyl protons relative to those of
the free ligand. The detected chemical shifts are a result of the interactions of the relevant
nuclear and electronic spins, with contributions mainly from through-space dipolar interactions
than through-bond Fermi contact interactions [26-28].
1
The difficulty in interpreting the H NMR spectrum of 4 arises from the peak broadening
as can be expected from the paramagnetic Ce(III) ion. The spectrum exhibits more peaks than are
expected for the Schiff base ligand. The additional signals can be attributed to the monoanionic
o-vanillin protons.

3
.3. UV-Vis spectroscopy
The electronic spectrum of the free ligand in the UV and visible region shows strong absorption
peaks at 329 and 420 nm, which are assigned to the π-π* and n-π* transitions, respectively
2 1
(figure 3) [24]. The molar absorption coefficients of H L are larger than those of the complexes,
which indicates ligand-centered solution absorption in the UV-visible region. There is no
noticeable wavelength shift in the electronic spectrum of 1 relative to the free ligand, whereas
absorptions are blue-shifted in 2 and 3. Higher energy absorptions are shifted by 31 nm while
lower energy absorptions are blue-shifted by 86 and 50 nm upon coordination to Ho(III) and
2 1
Nd(III), respectively. In 4, it is noted that the lower wavelength absorption band in H L is red-
shifted by about 11 nm upon complexation with Ce(III) and the n-π* transition band flattens out.
The shifts are attributed to the changes in energy levels of the ligand orbitals on coordination,
and the latter can be a result of interelectronic repulsions between the 4f electrons [24, 29].
3
.4. Shape analysis
The central Gd(III) and Ho(III) ions are bound to ten oxygen atoms in a tetradecahedron
structure and the deviation parameters (continuous shape measure values) are 4.8273 and 4.7649,
respectively. Polyhedral views of the lanthanide atoms are given in figure 4.
The deviation parameters of the two ten-coordinate cerium atom coordination spheres
were found to be 4.9327 and 4.2017 for Ce1 and Ce2, respectively. The geometries of the two
Ce(III) ions in the dinuclear complex were found to be different. The shapes around the Ce1 and
Ce2 atoms are tetradecahedron (TD-10) and staggered dodecahedron (SDD-10), respectively.
The SDD-10 shape has 8
3
+4
4
faces (8 triangles and 4 squares), 20 edges and exhibit D
2
symmetry while TD-10 displays 12
3
+2 faces (twelve triangles and two squares) and shows D2d
4
symmetry. A less distorted coordination sphere is observed for Ce2 compared to the Ce1 atom.
When taking into account steric effects, ligand type, size and shape are an important
consideration in geometry [30]. The Ce2 ion has a lower deviation from the ideal shape due to
the fairly rigid chelate rings resulting from small nitrate bite angles as well as shorter Ce(2)-O
bond lengths afforded by the carbonyl groups of the deprotonated o-vanillin. In contrast, the Ce1
atom is connected to the huge H
2 1
L ligand with a larger bite angle and has only one nitrate group
attached. In addition, Ce1 possesses longer Ce-Omethoxy bonds than Ce2.

It is also noted that there is a very small difference between the CShM values of TD-10
and SDD-10 shapes for the Ce2 atom. Thus, an interconversion between the two polyhedral
structures may occur in solution or in the solid state [31]. Rare-earth complexes show flexibility
and are in dynamic behavior especially in solution, due to lack of strong, directional bonding as
well as large ionic radii of the rare-earth ions and high coordination numbers. Therefore,
interconversion is brought about by rearrangements of ligands influenced by steric factors. It is
therefore necessary to describe the interconversion path between two ideal polyhedra by taking
into account the minimum distortion angle, θPT, or the related minimum distortion constant, kPT
,
which is obtained by calculation of the CShM of one shape (P) relative to the other (T) [31].
It is observed that there is a decrease in CShM values from Ce to Ho as the ionic radii
decreases, except for the Ce2 atom coordination sphere that deviates from the trend. The reason
could be that the Ce2 ion is not directly connected to the Schiff base ligand, but is only bonded to
-
small nitrate groups and ovan . Thus, the strength of ligand binding is probably responsible for
the decrease in deviation parameters.
3
.5. Crystal structures
The single-crystal X-ray crystallographic studies reveal that 1 and 2 are isostructural and
crystallize in the monoclinic P2 /c space group. The Gd(III) and Ho(III) ions are both ten-
1
coordinate with distorted tetradecahedron geometries. The crystal structures reveal that the
lanthanide ions exist in the +3 oxidation state and the cationic charges are neutralized by three
bidentately coordinated nitrate groups in each complex. Ten-coordination is achieved with four
2 1
oxygen atoms originating from the methoxy sidearms and phenoxo groups of H L , and the
remaining six oxygen atoms from three bidentate nitrato anions (figures 5a and 5b). The Ln-O
bonds involving the deprotonated phenol are shorter (range from 2.2502(15) to 2.3226(19) Å)
than the Ln-Omethoxy links that lie in the range 2.5352(16) to 2.582(2) Å. The Ln-Onitrate bonds
vary from 2.4551(18) to 2.578(2) Å.
The average nitrate bite angles were found to be 50.61 and 51.27 for 1 and 2,
respectively. It is also observed that the Ho(III) complex has larger ligand O-Ln-O bite angles
compared to the Gd(III) complex. Therefore, ligand bite angles and ionic radii are correlated.
There is an increase in bite angles as the ionic radii decreases. Relevant structural parameters of
1
, 2 and 4 are gathered in table 1a and refinement parameters are in table 2..

2 1
Both H L ligands remain protonated. However, proton migration occurred from the
phenolic oxygen to the azomethine nitrogen atom to form a zwitterionic ligand as shown in
2
figure 5c. The nitrogen atoms of the Schiff base ligand deviate from sp hybridization as
supported by C-N-C bond angles that are larger than the ideal 120, for example
C(12)-N(1)-C(13) angles equals to 126.5(2) and 126.4(2) in 1 and 2, respectively. The
zwitterionic phenoxo-iminium ligand coordinates to the metal centers forming five-membered
chelate rings with bite angles of approximately 64 and 65 for 1 and 2, respectively.
Complex 4 crystallizes in the monoclinic system with space group of P2
1
/c. The structure
] and two
consists of a neutral, well-separated dinuclear unit [Ce (H )(ovan) (NO )
2
2
L
1
3
3 3
crystallographic methanol molecules. As shown in figure 5d, the dinuclear Ce(III) complex
consists of two lanthanide ions, three bridging monodeprotonated o-vanillin molecules, three
2 1
bidentate nitrate groups and one Schiff base ligand (H L ), which acts in its bidentate O,O-donor
mode. Both metal ions exist in the +3 oxidation state and the total of 6+ cationic charges is
neutralized by the three nitrate groups and the three bridging phenolate oxygens. This scenario
2 1
shows that the Schiff base ligand remains protonated. The ligand H L adopted a zwitterionic
form as in 1 and 2 due to proton migration from the phenolic oxygen to the azomethine nitrogen
atom. It is interesting to note that each of the rare-earth ions is decacoordinated but they have
different coordination environments. The ten-coordinate centers, Ce1 and Ce2, adopt distorted
tetradecahedron and staggered dodecahedron geometries, respectively.
The ten coordination sites of Ce1 are occupied by two oxygen atoms from the bidentate
2 1
nitrate molecule, two oxygen atoms (from the alkoxyl and phenolic groups) from H L and six
oxygen atoms (two methoxyl, three deprotonated phenolate and one carbonyl oxygen) from
o-vanillin. The Ce2 center is coordinated to two bidentate nitrates and six oxygen atoms (three
phenolate, two carbonyl and one methoxyl oxygen) from three monodeprotonated tridentate
o-vanillin molecules. The two central Ce(III) ions are interconnected through oxygen bridges
afforded by three deprotonated o-vanillin groups.
The bridging Ce(1)-O-Ce(2) angles (in the range 99.04(12)-102.76(12)) are normal and
comparable to other Ln(III) centers bridged by the phenoxide groups. Examples of such
complexes are the Dy(III) complex with the Schiff base o-vanillin oxime and the dinuclear
Gd(III) complex with 2-[(2-hydroxypropylimino)methyl]phenol in which the Ln-O-Ln angles are
in the ranges 103.89(10)–105.27(10) and 95.76(12)-96.26(12), respectively [32, 33]. The

Ce-O-Ce bond angles are also in accord with those in previously reported phenolate oxygen-
bridged transition metal complexes, such as those in which the Zn-O-Zn angles were found to lie
in the range of 94.962(2)-103.665(3) [13]. The H L O-Ce-O bite angle is found to be
2
1
-
5
9.70(12) and the nitrate O-Ce-O bite angles range from 48.34(12)-49.19(13). For ovan
coordination, it is observed that the Obridging-Ce-O(C=O) angles (average = 69.03) are significantly
greater than the Obridging-Ce-Omethoxy (average = 60.80), with the former giving six-membered
chelate rings and the latter forming five-membered rings. Thus, there is a good correlation
between bite angle and size of the chelate ring. The bigger the ring, the wider the angle. A small
2 1
difference in bite angles is seen between the five-membered chelate rings formed by H L and
Obridging-Ce-Omethoxy.
An inspection of metric parameters shows that the average bond distances Ce-Oovan methoxy
2.651 Å, Ce-Onitrate = 2.619 Å, Ce-OH2L1 = 2.579 Å, Ce-Oovan C=O = 2.513 Å and Ce-Obridging
=
=
2
.460 Å are similar to those found in the literature [24]. The Ce∙∙∙Ce distance is equal to 3.823 Å.
This distance is longer than those found in other similar dinuclear rare-earth complexes [22, 33].
The crystal packings in 1 and 4 are shown in figures 5e and 5f, and the hydrogen bond
geometries are listed in table 1b. The crystal structures of the Gd(III) and Ho(III) complexes are
stabilized by classical hydrogen bonds of the N-H∙∙∙O and O-H∙∙∙O type. The first kind of
N-H∙∙∙O hydrogen bond is apparent between the azomethine hydrogen atom and the phenoxide of
H
2
L
1
which acts as the acceptor. The second kind of N-H∙∙∙O hydrogen bonds is facilitated by the
migrated proton attached to the azomethine which enables intramolecular hydrogen bonding with
the terminal hydroxyl group of H . Hydrogen bonds of the O-H∙∙∙O type appear between the
2 1
L
hydrogen atom on the terminal hydroxyl group of the Schiff base and the oxygen atom of the
coordinated nitrate. The second kind of O-H∙∙∙O interactions are supported by the hydrogen atom
of the hydroxyl group of one H
2 1
L ligand as the donor and the hydroxyl oxygen atom of another
2 1
H L
as the acceptor. In addition, weak C-H∙∙∙O interactions participate in maintaining the crystal
structure. Four kinds of these interactions exist, with the first kind maintained by one of the
hydrogen atoms of the methoxy group as donor and the oxygen atom of the nitrate as acceptor,
while the second kind is apparent between the azomethine hydrogen atom and the nitrate oxygen
atom. Intramolecular interactions exist between one of the hydrogen atoms of the methyl group
and the oxygen of the terminal hydroxyl group. Another kind of C-H∙∙∙O contact is a result of

weak interactions existing between the hydrogen atom (donor) of the methyl group and the
nitrate oxygen atom.
A detailed investigation of the packing diagram of the dimeric cerium complex shows the
existence of intra- and intermolecular hydrogen bonding interactions. The O-H group of H
hydrogen bonded to the coordinated oxygen atom of the nitrate with an H∙∙∙O bond distance of
.33 Å. An intramolecular hydrogen bond is seen in the ligand H , with the hydroxyl group
2 1
L is
2
2 1
L
interacting with the imino nitrogen atom (O∙∙∙N; 2.836(14) Å). Additionally, intermolecular
hydrogen bonds exist between the oxygen atoms of the lattice methanol molecules and the
hydroxyl groups of the crystallographic solvent.
The correlation between Ln-O bond lengths (or ligand bite angles) and the number of 4f
electrons is given by the plots in figures 5g and 5h. The Ln-Oall bond distances were obtained by
averaging all Ln-O bonds in the coordination polyhedra of all metal centers. These, together with
the Ln-Onitrate and Ln-OH2L1 bond lengths, were plotted to obtain evidence of the lanthanide
contraction. As expected, the Ln-O bond lengths decrease in a linear fashion from Ce to Ho as
the ionic radii decreases [34]. For the dinuclear cerium complex, the Ln-Oall lie below both
Ln-Onitrate and Ln-OH2L1. This is due to shorter Ce-O bond distances generated by the bridging
-
and carbonyl oxygens of ovan . It is observed that the ligand bite angles are increasing as the
number of 4f electrons increases, with a more linear graph observed for the nitrate angles.
3
.6. Thermal analysis
The thermogravimetric analysis and differential thermogravimetric analysis (TG-DTG) curves of
complexes 1-4 are shown in figures 6a-6d. Complex 1 is stable, showing a TG plateau up to
about 244-248 C. This may indicate the absence of solvent molecules or water in the
coordination environment of the rare-earth compound [35]. Degradation then starts occurring
from 248 C and peaks at 265.16 C as shown on the derivative curve.
For 2 and 3, first mass losses of 1.53% and 2.25% are observed, respectively, that could be
attributed to loss of moisture from the samples. For complex 2, the weight loss (7.52%) at the
second step (170-173 C) with TDTG at 171 C is assigned to the release of one coordinated
nitrate ligand from the complex (calculated = 7.77%). The complex then shows stability until
reaching 213 C, the temperature at which degradation continues. The Nd(III) complex is seen to
decompose in a different way from the Ho(III) complex. As the temperature is increased, the TG

curve of complex 3 exhibits weight loss (36.40%) in a wider temperature range (230-314 C),
which may be due to the decomposition of one H and one nitrate molecule. This is in
2 1
L
agreement with the theoretical (calculated) loss of 36.72%. Inspection of the DTG curve shows
that this weight loss follows a two-step decomposition pathway as shown by the two peaks at
2
70.45 and 288.65 C.
Thermal analysis of 4 was carried out from room temperature to 260 C. At the first step
(
22-106 C) there is a total mass loss of 5.49%, corresponding to loss of two methanol molecules
calculated = 5.31%). This mass loss has maximum rates, TDTG and TDTA at 44.36 and 47.77 C,
(
respectively. The complex is then thermally stable up to 215 C after which thermal degradation
continued.
3
.7. Cyclic voltammetry
Figures 7a-7c depict the voltammograms of the Gd(III), Ho(III), Nd(III) and Ce(III) complexes.
The cathodic peak potential (Epc) values are observed at -1.83, -1.87, -1.75 and -1.66 V for
complexes 1-4, respectively. These peak potentials of the reduction current were taken at the
-1
+
scanning rate of 200 mVs versus Ag/Ag and can be assigned to the reduction from Ln(III) to
Ln(II) [36, 37]. Ho(III) is the hardest to reduce. Anodic peak potentials are also observed for
Gd(III) and Ho(III) at 0.963 and 1.046 V, attributed to the oxidation of Ln(II) formed during the
preceding cathodic scan to Ln(III).
Some 4f elements such as gadolinium require a much larger potential to accept electrons
as evidenced by a very negative theoretical standard reduction potential of -2.28 V versus the
Normal Hydrogen Electrode. However, the presence of the aromatic π-system may ensure easy
reduction of the complex. This is made possible by the withdrawal of the electron density from
the metal, thus stabilizing the lower oxidation state [38].
2 1
Based on the above studies, it can be concluded that Nd(III) coordinates to H L in the
same manner as Gd(III) and Ho(III). The proposed coordination structure of the Nd(III) complex
with the o-vanillin derived Schiff base is presented in figure 8.
4
. Conclusion
Coordination of the lanthanides Gd(III), Ho(III), Nd(III) and Ce(III) to the Schiff base ligand
converted the ligand to its zwitterionic phenoxo-iminium form as a result of proton
2 1
H L

migration from the phenol function to the imine function. The o-vanillin derivative coordinated
bidentately via the phenolate oxygen and the methoxy oxygen atoms. SHAPE analyses of the
ten-coordinate Gd(III) and Ho(III) complexes indicated distorted tetradecahedron geometries.
The crystal structure of the Nd(III) complex could not be obtained due to badly intertwined
crystals. However, the use of other analytical techniques, such as microanalysis and IR, enabled
the prediction of the structure of the complex. It was concluded that the Nd(III) complex was
isostructural to the Gd(III) and Ho(III) compounds.
2 2 1 3 3 3
A novel dimeric cerium(III) complex, with the formula [Ce (H L )(ovan) (NO ) ], has
been successfully synthesized using the Schiff base ligand, 2-((E)-(1-hydroxy-2-methylpropan-2-
ylimino)methyl)-6-methoxyphenol. The complex has been fully characterized using various
physico-chemical techniques. Hydrolysis of the ligand occurred to yield o-vanillin, which
connected the two Ce(III) centers through bridging phenolate oxygens. The shape studies of the
coordination compound revealed different geometries adopted by the two metal centers.
The coordination mode of the isolated complexes is different from that of other vanillin
Schiff base complexes, such as homobinuclear lanthanide(III) complexes of multidentate
3
-methyloxysalicylaldoxime (MeOsaloxH
2
), [Ln
2
(MeOsaloxH)
(H O) ]∙2H
4 2 3
(PhCOO) (CH OH)] (Ln = Pr,
1
; Nd, 2; Sm, 3) and Gd (MeOsaloxH)(CH
2
3
COO)
2
2
2
2
O (4) [39]. In these complexes,
-
the two Ln(III) ions are bridged by two phenolate oxygens from two [MeOsaloxH] ligands to
give binuclear structures. In each case the methoxy oxygen atoms only participate in bonding
when the ligand acts as a bridge. The coordination modes are shown in figure 9. Almost the same
scenario is seen in a family of phenoxo-bridged heterometallic trinuclear complexes of the
polydentate Schiff base ligand 2-hydroxy-3-methoxy-phenylsalicylaldimine (H
A comparison can also be drawn between H and the ligand (E)-3-((2-hydroxy-3-
methoxybenzylidene)amino)propane-1,2-diol (H L) that was employed in the chemistry of
cubane-type {M } (M = Co(II), Zn(II), Cu(II)) clusters. The latter ligand adopts both chelating
2
L) [40].
2 1
L
3
4
O
4
and bridging modes and coordinates to the metal centers through the imine nitrogen,
deprotonated phenolic and alkoxo oxygen atoms. The methoxy oxygen atoms are left
uncoordinated [41].
Thermal stability studies and thermal degradation behaviors of the complexes were
studied. These studies (TG-DTG) played an important role in showing the upper limit of

stabilities of complexes as well as early stages of degradation. Furthermore, it was noted that the
complexes decompose via different pathways.
Appendix A. Supplementary information
Supplementary data for 1, 2 and 4 (CCDC 1472185-1472187, respectively) are available from
the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK on request. These data can be obtained
free
of
charge
from
the
Cambridge
Crystallographic
Data
Centre
via
http://www.ccdc.cam.ac.uk/data_request/cif.
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N
O
OH
OH
TINW=8a
Figure 1. The structure of H L .
2
1
N
Ce(III)/ Methanol
Hydrolysis
H N
2
OH
+
OH
OH
O
O
OCH3
OH
Ce(III)
Methanol
O
O
O
O
O
OH
Ce
Ce
Figure 2. Formation of o-vanillin from the hydrolysis of the Schiff base and its bridging mode to
the metal.

-5
-4
Figure 3. UV-Vis spectra of DMF solutions of H
room temperature.
2 1
L (6.25×10 M) and 1-4 (5.10×10 M) at

Figure 4. Polyhedral views of the ten-coordinate gadolinium (top left), holmium (top right), Ce1
bottom left) and the Ce2 atoms (bottom right).
(

Figure 5a. ORTEP diagram (thermal ellipsoids at 50% probability) with a partial atom-
numbering scheme for 1.
Figure 5b. ORTEP view of 2 showing 50% probability displacement ellipsoids and the atom-
labelling.

OH
(
116)
)
C
1.41
(
4
N(1)
H
5
8
6
1
.
3
(4
)
(
115)C
C(111)
)
4
(
4
0
.
4
1
(114)C
3
)
C(112)
8
(
.4
1
1
.
3
1
7
1.307 (
3
(
4
)
C(113)
3
O(11)
)
)
3
(
2
8
.
3
1
(12)O
Gd
Figure 5c. Bidentate coordination of the ligand H
2
L
1
in zwitterionic phenoxo-iminium form.

Figure 5d. The molecular structure of the dinuclear Ce(III) complex showing 50% probability
displacement ellipsoids and partial atom-labelling. The solvent of crystallization (methanol) was
omitted for clarity.

Figure 5e. Hydrogen bonding in 1.
2 2 1 3 3 3
Figure 5f. Unit cell structure of [Ce (H L )(ovan) (NO ) ].

Figure 5g. Graph of mean Ln-O bond distances versus the number of 4f electrons.
Figure 5h. Plot of average O-Ln-O ligand bite angles versus the number of 4f electrons.

Figure 6a. TG and DTG curves of 1 showing the upper limit of stability.
Figure 6b. TG-DTG curves of 2.

Figure 6c. TG-DTG curves of 3.
Figure 6d. The TG, DTG and DTA curves of 4.

Figure 7a. Cyclic voltammogram of a 1.90 mM DMF solution of 1.
Figure 7b. Cyclic voltammograms of 3.21 mM DMF solutions of 2 and 3.

Figure 7c. The cyclic voltammetry curve of the 2.07 mM DMF solution of 4.
OH
HN
O
N
O
N
O
O O
O
O
Nd
O
O
O
O
O
N
HN
O
HO
Figure 8. Proposed structure of the Nd(III)-H L complex.
2
1

O
O
O
Ln
O
N
N
OH
OH
Ln
Ln
-
Figure 9. Coordination modes of [MeOsaloxH] .