Synthesis, crystal structure, photoluminescence and electrochemical properties of a sandwiched Ni2Ce complex

Article abstract of DOI:10.1016/j.molstruc.2017.08.091

In this study, a Ni₂Ce complex [(NiL)₂Ce(NO₃)₂](NO₃) was synthesized and characterized by spectroscopic and analytical methods. The structure of the complex was determined by single crystal X-ray diffraction study. In the structure of the complex, a Ce(III) ion is sandwiched between the two NiL units, which are virtually parallel to each other. The Ce(III) center is 12-coordinate, surrounded by 12 oxygen atoms; four are from phenolic groups, four from methoxy groups, and four from two bidentate nitrate ligands. Hirshfeld surface analysis was used to evaluate the inter-molecular interactions within the crystal packing. The complex molecules are linked by H?ONO₂ interactions. The largest contribution is H?O/O?H with 41.6% contribution and followed by H?H contacts with 39.1%. The complex showed an excitation band in the range of 510–580 nm. A band in the range of 520–580 nm observed in the emission spectrum almost completely overlapped. This suggests that the band in the emission spectrum of the complex is not the actual fluorescence emission and is assigned to the Rayleigh scattering band. Electrochemical and thermal behaviours of the complex were also investigated.

Full text of DOI:10.1016/j.molstruc.2017.08.091

Accepted Manuscript
Synthesis, crystal structure, photoluminescence and electrochemical properties of a
sandwiched Ni Ce complex
2
Seyit Ali Güngör, Muhammet Kose
PII:
S0022-2860(17)31171-7
DOI:
10.1016/j.molstruc.2017.08.091
MOLSTR 24229
Reference:
To appear in: Journal of Molecular Structure
Received Date: 29 June 2017
Revised Date: 16 August 2017
Accepted Date: 27 August 2017
Please cite this article as: S.A. Güngör, M. Kose, Synthesis, crystal structure, photoluminescence and
electrochemical properties of a sandwiched Ni Ce complex, Journal of Molecular Structure (2017), doi:
2
10.1016/j.molstruc.2017.08.091.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis, crystal structure, photoluminescence and electrochemical
properties of a sandwiched Ni2Ce complex
Seyit Ali Güngör, Muhammet Kose*
Chemistry Department, Kahramanmaras Sutcu Imam University, Kahramanmaras, 46050,
Turkey.
Abstract
In this study, a Ni2Ce complex [(NiL)2Ce(NO3)2](NO3) was synthesized and characterized by
spectroscopic and analytical methods. The structure of the complex was determined by single
crystal X-ray diffraction study. In the structure of the complex, a Ce(III) ion is sandwiched
between the two NiL units, which are virtually parallel to each other. The Ce(III) center is 12-
coordinate, surrounded by 12 oxygen atoms; four are from phenolic groups, four from
methoxy groups, and four from two bidentate nitrate ligands. Hirshfeld surface analysis was
used to evaluate the inter-molecular interactions within the crystal packing. The complex
molecules are linked by H····ONO2 interactions. The largest contribution is H····O/O····H
with 41.6% contribution and followed by H····H contacts with 39.1%. The complex showed
an excitation band in the range of 510-580 nm. A band in the range of 520-580 nm observed
in the emission spectrum almost completely overlapped. This suggests that the band in the
emission spectrum of the complex is not the actual fluorescence emission and is assigned to
the Rayleigh scattering band. Electrochemical and thermal behaviours of the complex were
also investigated.
Keywords: Heteronuclear complex; Ni2Ce complex; Molecular structure; Emission; Rayleigh
scattering
Corresponding author: M. Kose
E-mail address: muhammetkose@ksu.edu.tr (Muhammet Kose)
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1. Introduction
Lanthanide complexes which emit in the near-infrared region have found several
applications including fluoroimmuno assays, laser systems, and optical amplification [1]. Due
to their interesting spectroscopic and magnetic properties, trivalent lanthanide complexes have
received considerable attention [2]. Magnetic and spectroscopic properties of lanthanide
complexes can also be tuned by the change of ligand around the metal centres [3]. Therefore,
chemists have prepared several Ln(III) complexes with different coordinating ligands [1,4,5].
Recently, the preparation and magnetic properties of 3d-4f heterometallic Schiff base
complexes have taken considerable attention, however, there are a few studies on their
luminescent properties [6–11]. The reason for this may be because of the fact that some
lanthanide ions such as Eu(III) and Tb(III) show strongly emissive and long lived excited
states yet do not exhibit intense absorption bands [9]. In the lanthanide(III) complexes, the
absorption coefficients of the f-f transitions are usually very low because of parity forbidden
f-f transitions. However, this can be overcome by attaching suitable chromophores to the
ligands [8,9]. These chromophore groups used as sensitizers can transfer energy to the
lanthanide ions causing an efficient emission (the antenna effect). In literature, some 3d–4f
heterometallic Schiff base complexes such as CuII–GdIII, NiII–GdIII and ZnII–HoIII have
been synthesized and found to exhibit interesting magnetic and luminescent properties [6–
9,11–13].
In our previous studies, La(III) and Ce(III) complexes of multidentate Schiff base
ligands were prepared and their luminescence properties were investigated [14,15]. As part of
our investigations into the structure and applications of 3d–4f heterometallic Schiff base
complexes, we report here the synthesis and X-ray crystal structure analysis of a Ni2Ce
complex derived from a Schiff base ligand. Molecular structure of the complex was
determined by single crystal X-ray diffraction studies. Luminescence and electrochemical
behaviours of the complex were also investigated.
2. Experimental
2.1 General methods
All starting materials and organic solvents were purchased from commercial sources
and used as received, unless noted otherwise. IR spectrrum was performed on a Perkin Elmer
Paragon 1000 PC. CHN analyses were performed using a CE-440 Elemental analyser. The
electronic spectrum in the 200–900 nm range was obtained on a Shimadzu UV-1800 UV-Vis
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spectrophotometer. The single-photon fluorescence spectra were collected on a Perkin Elmer
LS55 luminescence spectrometer. The thermal analysis of the complex was performed on a
Perkin Elmer Pyris Diamond.
The cyclic voltammetry studies at the glassy carbon electrode were carried out using a
BAS 100W (Bioanalytical System, USA) electrochemical analyser. A glassy carbon working
electrode (BAS; Φ: 3mm diameter), an Ag/AgCl reference electrode (BAS; 3M KCl) and
platinum wire counter electrode and a standard one-compartment three electrode cell of 10
mL capacity were used in all experiments. The solution of the complex was prepared in DMF
(10–4 M) for electrochemical studies.
2.2 X-ray structures solution and refinement for the complex
Data collection and cell refinement for X-ray crystallography were completed using a
Bruker D8 QUEST diffractometer and data reduction was performed using Bruker SAINT.
Crystals of the complex exhibited very weak diffraction, satisfactory structure solution was
able to be attained. Diffraction data were measured at ambient temperature using Mo-K
α
radiation with a Bruker ApexII diffractometer [16]. SHELX-2014/6 was used to solve and
refine the structures [17,18]. The structure was solved by direct methods and refined on F2
using all reflections. All the non-hydrogen atoms were refined using anisotropic atomic
displacement parameters. Details of the crystal data and refinement are given in Table 1.
2.3 Synthesis of [(NiL)2Ce(NO3)2](NO3)
To a stirring solution of 2-hyrdroxy-3-methoxybenzaldehyde (0.152 gr, 1 mmol),
Ni(OAc)2.4H2O (0.124 gr, 0.5 mmol) and Ce(NO3)3.6H2O (0.217 gr, 0.5 mmol) were added
respectively. The resulting clear green reaction solution was refluxed for 2 hours and
ethylenediamine (0.06 g, 0.5 mmol) in methanol (5 mL) was added. As soon as
ethylenediamine was added, the colour of the reaction solution turned from green to orange-
red colour. The reaction solution was further refluxed for 24 h. Upon cooling to the room
temperature, the orange precipitate formed was filtered and dried in air. Crystals suitable for
X-ray diffraction study were obtained from slow diffusion of diethyl ether into a methanol
solution of the complex.
[(NiL)2Ce(NO3)2](NO3): Colour: Orange-red, Yield: 0.195 g (75%). Elemental analysis data:
Anal. (%) Calculated for C36H36N7O17Ni2Ce (1096.22 g/mol): C, 39.44; H, 3.31; N, 8.94.
Found (%): C, 39.18; H, 3.12; N, 8.65. IR (KBr, ν, cm−1): 2946(w), 1619(s), 1561(w),
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1456(s), 1294(s), 1284(s), 1232(s), 1172(w), 1081(s), 1036(s), 994(s), 958(s), 862(s), 818(s),
785(s), 735(s), 676(s), 625(w), 603(w), 581(w), 532(w), 491(w).
3. Results and Discussion
The reaction of two equivalents of 2-hyrdroxy-3-methoxybenzaldehyde with one
equivalent of Ni(OAc)2.4H2O and half equivalent of Ce(NO3)3.6H2O in the presence of one
equivalent of ethylenediamine resulted in a Ni2Ce sandwich complex [(NiL)2Ce(NO3)2](NO3)
(Fig. 1). The complex is soluble in most organic solvents such as methanol, ethanol,
dimetylformamide and slightly soluble in water.
The FT-IR spectrum of the complex were obtained on KBr disc. In the FT-IR spectrum of
free aldeyhde, the characteristic aldehyde group vibration ν(C=O) was observed at 1657 cm-1
[19] and the aldehyde group vibration ν(C=O) was completely absent in the spectrum of the
complex [(NiL)2Ce(NO3)2](NO3), instead a new band due to the imine group ν(C=N)
appeared which confirms the condensation reaction between the aldehyde and amine groups
[20]. In the spectrum of the complex, in addition to the ν(M-O) band strechings at 581 and
532 cm-1, a band due to ν(M-N) stretching was observed 491 cm-1.
The UV-Vis absorption spectrum of the complex was investigated in the 250-800 nm
range in DMF (10-5 M). In the UV–vis spectrum of the complex, there is only one absorption
band in the range of 330-460 nm which was assigned to the π-π* transition. In the spectrum
of the complex no M→L, L→M or d-d transitions were observed at the concentration studied.
The single-photon fluorescence spectra of the complex were collected on a Perkin Elmer
LS55 luminescence spectrometer in DMF solution (10-5 M). The emission and excitation
spectra of the complex are shown in Fig. 2. The complex showed an excitation band in the
range of 510-580 nm (λmax = 543 nm). In the emission spectrum of the complex showed a
band in the range of 520-580 nm which almost completely overlapped with the excitation
band. The almost completely overlapped with the excitation spectrum is not the actual
fluorescence emission and this was assigned to the Rayleigh scattering band.
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Cyclic voltammetry studies of the complex was performed in DMF solution (10-4 M)
using 0.1 M NBu4BF4 as supporting electrolyte [21]. The obtained electrochemical data are
summarized in Table 2. The electrochemical curves of the complex are shown in Fig 3. The
complex shows two anodic peak potential at about -0.36 and 0.53 V at all scan rates. The
oxidation process may be referred to Ce(III)→Ce(IV) + e- [5, 22] and the oxidation process is
not reversible. In the reverse scan, the complex in DMF solution exhibited two cathodic peak
potentials in the -0.54 – (+ 0.46) V range. While the anodic potential values almost remain
invariable upon the increase in the scan rate, the cathodic peak potentials shifted to the more
negative regions.
Thermal behaviour of the synthesized complex was studied under nitrogen atmosphere
in the temperature range of 20–1000 °C with a heating rate of 10 °C/min. The thermal curves
for the complex are shown in Fig. 4. The thermal decomposition of complex
[(NiL)2Ce(NO3)2](NO3) showed a mass loss between 50-150 °C with about 2% of the total
mass. This mass loss may be due to the moisture in the sample. The molecular skeleton of the
complex is thermally stable up to 260 °C and thermal decomposition of the complex
[(NiL)2Ce(NO3)2](NO3) occurred in three steps. In the first step, a sharp mass loss was
observed between 260 and 375 °C with approximately 26% of the total mass. This mass loss
was accompanied by an exothermic at about 290 °C. The mass loss in the first step can be
attributed to the removal of the coordinated and non-coordinated nitrate anions. In the second
decomposition step, almost 22% of the total mass was lost between 375 and 740 °C. The rest
of the organic moiety of the compound decomposed up to higher temperatures (1050 °C)
leaving metal oxides as final residue. At 1050 °C, the 45% of the total mass remained and this
mass corresponds to the metal oxide (NiO + Ce2O6).
3.1 Molecular structure of the complex [(NiL)2Ce(NO3)2](NO3)
Single crystals of the complex suitable for X-ray diffraction study were grown from
slow diffusion of diethyl ether into a DMF solution of the complex over two weeks. The
structure of the complex was solved in orthorhombic unit cell and C2221 space group. X-ray
structure of the complex is depicted in Fig. 5. The structure of the complex contains a
trinuclear Ni2Ce coordination cluster with the formula of [(NiL)2Ce(NO3)2](NO3). In the
complex, two Ni(II) ions are coordinated to the inner N2O2 cores of the two Schiff-base
ligands with square planar geometries. The Ce(III) ion is sandwiched between the two NiL
units, which are virtually parallel to each other. The molecule sits on two fold rotation axis
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passing through the Ce(III) atom. The asymmetric unit, thus, contains half of the structure.
The Ce(III) center is 12-coordinate, surrounded by 12 oxygen atoms; four are from phenolic
groups, four from methoxy groups, four from two bidentate nitrate ligands. There is also a
third NO3 anion present in the crystal lattice but is not within bonding distance with any of the
metal ions. The bond lengths and angles are given in the supplementary file. The Ce-O
distances are in the range of 2.539(13)-2.96(2) Å and Ce-Ophenol distances are shorter than Ce-
Omethoxy distances. The azomethine bond distances are within the range of C=N bond
distances.
Hirshfeld surface analysis was used to evaluate the inter-molecular interactions within
the crystal packing [23, 24]. The Hirshfeld surface for [(NiL)2Ce(NO3)2](NO3) was obtained
to determine the interaction sites and contribution of different intermolecular contacts within
the crystal structure. The fingerprint plot of the complex shows two spikes indexed as (1) and
(2) corresponding to H····O/O····H and H····H contacts, respectively (Fig. 6a). The complex
molecules are linked by H····ONO2 interactions and these contacts are observed in the dn
surface as intense red spots. Relative contribution of the different intermolecular contacts to
the Hirshfeld surface of the molecule is shown in Fig. 6b. The largest contribution is
H····O/O····H with 41.6% contribution and followed by H····H contacts with 39.1%. There
is also π-π (phenyl-pheyl) stacking interactions in the structure of the complex (Fig. 6c,d).
The π-π (phenyl-pheyl) stacking interactions are seen in the shape index.
4. Conclusions
In this work, an heteronuclear Ni2Ce complex was synthesized and characterized by
spectroscopic and analytical methods. The structures of the complex was evaluated by single
crystal X-ray diffraction study. The structure of the complex contains a trinuclear Ni2Ce
coordination cluster with the formula of [(NiL)2Ce(NO3)2](NO3). The Ce(III) ion is
sandwiched between the two NiL units, which are virtually parallel to each other. The
complex showed an excitation band in the range of 510-580 nm (λmax = 543 nm) which
almost completely overlapped with the band (520-580 nm range) in the emission spectrum.
This band was then assigned to the Rayleigh scattering band. Finally, the electrochemical
behaviour of the complex was also investigated in DMF solution.
Supplementary Information
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Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre,
CCDC number for the complex is 1559335. Copies of this information can be obtained from
The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 335033;
email deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
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List of Figs. and Tables
Fig. 1 Schiff base ligand and Ni2Ce complex. ........................................................................... 2
Fig. 2 UV-vis (a) and excitation-emission spectra (b) of the complex (10-5 M)........................ 3
Fig. 3 CV curves of the complex................................................................................................ 4
Fig. 4 Thermal curves for the complex. ..................................................................................... 5
Fig. 5 Molecular structure of [(NiL)2Ce(NO3)2](NO3), hydrogen atoms are not shown for
clarity.......................................................................................................................................... 6
Fig. 6 a) Finger print plot b) Contribution of different intermolecular contacts c) Shape index,
(white circle shows π-π interaction site) d) π-π interaction within the structure........................ 7
Table 1 X-ray data for the complex. .......................................................................................... 8
Table 2 Electrochemical data of the complex. ........................................................................... 9
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Fig. 1 Schiff base ligand and Ni2Ce complex.
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a)
b)
Fig. 2 UV-vis (a) and excitation-emission spectra (b) of the complex (10-5 M).
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Fig. 3 CV curves of the complex.
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8.00
95.00
90.00
85.00
80.00
75.00
70.00
65.00
60.00
55.00
50.00
45.00
2.000
1.500
1.000
0.500
0.000
-0.500
-1.000
6.00
4.00
2.00
0.00
-2.00
-4.00
-6.00
-8.00
-10.00
-12.00
100
200
300
400
500
600
700
800
900
1000
Temp Cel
Fig. 4 Thermal curves for the complex.
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Fig. 5 Molecular structure of [(NiL)2Ce(NO3)2](NO3), hydrogen atoms are not shown for
clarity.
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Fig. 6 a) Finger print plot b) Contribution of different intermolecular contacts c) Shape index,
(white circle shows π-π interaction site) d) π-π interaction within the structure.
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Table 1 X-ray data for the complex.
Identification code
Empirical formula
Formula weight
Crystal size (mm3)
Crystal color
[(NiL)2Ce(NO3)2](NO3)
C36H36N7O17CeNi2
1096.26
0.13 x 0.09 x 0.08
Orange
Crystal system
Space group
Orthorhombic
C2221
Unit cell
a (Å)
9.132(5)
21.223(12)
20.763(12)
90
b (Å)
c (Å)
α (°)
β (°)
γ (°)
90
90
3
4024(4)
4
Volume (Å )
Z
Abs. coeff. (mm-1)
2.124
Refl. collected
26421
Ind. Refl. [Rint]
4001 [0.1781]
Final R indices [I>2sigma(I)]
0.0921, 0.2358
R indices (all data)
CCDC
0.1493, 0.2758
1559335
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Table 2 Electrochemical data of the complex.
Scan rate(mV/s) Epa(V)
Epc(V)
Epa/Epc E1/2(V) ∆Ep(V)
100
250
500
750
1000
-0.57, 0.51
0.46, -0.54
0.41, -0.60
0.39, -0.65
0.35, -0.68
0.31, -0.70
1.05
0.96
0.90
0.88
0.87
0.56
0.59
-
-
-
-0.03
0.02
0.06
0.08
0.09
-0.58, 0.52
-0.59, 0.53
-0.60, 0.54
-0.61, 0.55
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Highlights
ꢀ A Ni2Ce complex was synthesized and characterized.
ꢀ The structure of the complex was determined by X-ray diffraction study.
ꢀ Inter-molecular interactions were determined by Hirshfeld surface analysis.
ꢀ Electrochemical and thermal behaviour of the complex were investigated.