Synthesis, structure, and theoretical studies of bis(five-coordinate) -O-[CuL 2] 2: Predicting distortion toward trigonality

Article abstract of DOI:10.1080/00958972.2015.1110853

An oxo-bridged binuclear Cu(II) complex, [CuL2]2, has been synthesized and characterized by microanalysis, spectra (infrared and electronic), and X-ray diffraction crystallography. The theoretical properties of the complex were studied using the semi-empi

Full text of DOI:10.1080/00958972.2015.1110853

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, structure, and theoretical studies
of bis(five-coordinate) µ–O–[CuL2]2: predicting
distortion toward trigonality
A.O. Sobola, G.M. Watkins, B. Van Brecht & I.A. Adejoro
To cite this article: A.O. Sobola, G.M. Watkins, B. Van Brecht & I.A. Adejoro (2016)
Synthesis, structure, and theoretical studies of bis(five-coordinate) µ–O–[CuL2]2: predicting
distortion toward trigonality, Journal of Coordination Chemistry, 69:1, 81-89, DOI:
10.1080/00958972.2015.1110853
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Date: 05 February 2016, At: 01:50

Journal of Coordination Chemistry, 2016
Vol. 69, no. 1, 81–89
http://dx.doi.org/10.1080/00958972.2015.1110853
Synthesis, structure, and theoretical studies of bis(five-
coordinate) μ–O–[CuL2]2: predicting distortion toward trigonality
A.O. Sobolaa, G.M. Watkinsa, B. Van Brechtb and I.A. Adejoroc
adepartment of Chemistry, rhodes university, Grahamstown, south africa; bdepartment of Chemistry, nelson
mandela metropolitan university, Port elizabeth, south africa; cdepartment of Chemistry, university of ibadan,
ibadan, nigeria
ARTICLE HISTORY
received 11 march 2015
accepted 17 september
2015
ABSTRACT
An oxo-bridged binuclear Cu(II) complex, [CuL2]2, has been synthesized and
characterized by microanalysis, spectra (infrared and electronic), and X-ray
diffraction crystallography. The theoretical properties of the complex were
studied using the semi-empirical PM3 method. The optimized geometries,
dipole moment, thermodynamic parameters, and vibrational frequencies
were investigated. The crystal structure revealed that each Cu(II) was five-
coordinate with a distorted square pyramidal geometry (τ=0.324 and 0.307).
The two units of the complex were bridged by phenolic oxygens to give
a binuclear Cu(II) complex with the average bridging angle and Cu⋯Cu
distance being 98.49° and 3.413 Å, respectively. Theoretical studies revealed
that the geometric parameters are in agreement with the experimental data.
The compound was stable and has good non-linear optical properties.
KEYWORDS
Bridging ligands; trigonality;
metal complexes; semi-
empirical calculations; non-
linear properties
O
O
N
Cu
N
O
O
O
O
N
Cu
N
O
O
2Cu2+
+
4LH
[CuL2]2
+
4H+
1. Introduction
Polydentate ligands, especially N2O2 Schiff base ligands form stable five- or six-membered chelates with
transition metal ions [1–5]. Many salicylaldimine-based metal complexes have been evaluated for appli-
cations in organic synthesis [6, 7], catalytic activity [8, 9], electrochemistry [10, 11], corrosion inhibition
[12], and medicinal uses [13–15]. Complexes of copper(II) ions are known with coordination numbers of
four, five, or six. In the five-coordinate CuL5 chromophore, few regular trigonal bipyramidal and regular
CONTACT a.o. sobola
abdullahi.sobola@lasu.edu.ng, abdullahisobola@yahoo.com
supplemental data for this article can be accessed http://dx.doi.10.1080/00958972.2015.1110853
© 2015 taylor & francis

82
A. O. SOBOLA eT AL.
square pyramidal complexes are known because the vast majority of five-coordinate complexes involve
distorted stereochemistry [16], generally through the presence of non-equivalent ligands [17]. Thus,
there is a need to determine the degree of distortion from the regular geometry of the complexes. The
degree of distortion toward trigonality has been defined by Addison et al. geometric parameter [18].
Addison et al. introduced geometric parameter (τ) for distinguishing between square pyramidal and
trigonal bipyramidal geometries in five-coordinate metal complexes τ=(β−α)/60, where α and β are
the two largest basal trans angles [18].
Polynuclear Cu(II) complexes [4, 19–23] are of special interest because the magnetic spin interaction
between the metal centers provides information required for the fabrication of ferromagnetic materials.
In this article, we report the synthesis and structural characterization of an oxo-bridged binuclear
Cu(II) complex of LH, N-[(2-methylphenyl)-4-methoxysalicylaldimine, derived from condensation of
5-methoxyl-2-hydroxylbenzaldehyde (p-vanillin) with 2-methylaniline (o-toludine). The compound has
been characterized with a combination of analytical, spectral, and X-ray diffraction crystallography
data. The degree of distortion of the complex toward trigonality has been determined using τ [18].
Geometric properties (bond distances, bond angles, and dihedral), vibrational frequencies, thermody-
namic properties, heat of formation, and electronic properties of the molecule have been calculated
using the Spartan10 program package [24] with the semi-empirical PM3 (Parameterization Method 3).
The semi-empirical method (PM3) has been best in predicting the geometric properties and vibrational
frequencies of transition metal and organometallic complexes [25]. Several studies have shown that
the PM3 method gives data that agree with the experimental values [26–31].
2. Experimental
2.1. Materials and methods
All chemicals and reagents used were of analytical grade and used without purification. The 1H- and
13C-NMR spectra were recorded in CDCl3 with SiMe4 (TMS) as internal standard on a Bruker Avance NMR
operating at 400 MHz. The mid-infrared absorption frequencies (4000–700 cm−1) were recorded on a
Perkin-elmer Spectrum 100 FT-IR equipped with universal attenuated total reflectance (ATR) accessory,
while the far-infrared (700–30 cm−1) spectra were recorded in Nujol mull on a Perkin-elmer Spectrum
400 FT-IR. UV–vis spectra were obtained from a Perkin-elmer Lambda 25 spectrophotometer. The C, H,
N elemental analysis was done on a Vario MICRO V1.6.2 elemental analysen systeme GmbH, while the
percentage metal content was determined on a Perkin-elmer A Analyst atomic absorption spectrometer.
Molar conductivity measurement for the complex was done in DMF using Az® 86,555 pH/mV/Cond./
TDS/Temp. Melting points (uncorrected) of compounds were determined using a Galenkemp melting
point apparatus.
2.2. General procedure
The binuclear complex was prepared from copper(II) acetate monohydrate and LH in a ratio 1:2. LH
was synthesized by condensing equimolar mixture of 5-methoxyl-2-hydroxylbenzaldehyde (p-vanillin)
and 2-methylaniline (o-toluidine) in absolute ethanol.
2.2.1. Synthesis of LH
The Schiff base was synthesized according to the general synthetic procedure in the literature [32].
A hot 5 mmol solution (0.76 g) of 5-methoxyl-2-hydroxylbenzaldehyde (p-vanillin) in 5 mL ethanol
was mixed with 5 mmol (0.55 mL) of 2-methylaniline in 5 mL ethanol. The resulting yellow solution
was refluxed for 2 h in a 50-mL round bottom flask equipped with an air-cooled condenser to
obtain a yellow precipitate. The precipitate was filtered under suction, washed with ethanol, and
1
recryztallized from ethanol. It was dried over silica gel. Yield: 1.51 g (62.6%); m.p.: 92–94 °C; H-
NMR (400 MHz, CDCl3, 25 °C, TMS): δ = 14.04(s, 1H, –OH); 8.52(s, 1H, HC = N); 7.22(s, 1H); 7.18(d, 2H,

JOURNAL OF COORDINATION CHeMISTRY
83
Table 1. Crystal data and structure refinements for [Cul2]2.
empirical formula
formula weight
temperature (K)
Wavelength (Å)
Crystal system
space group
a (Å)
Cu2C60h56n4o8
1088.17
200(2)
0.71073
triclinic
P-1
11.4233 (2)
14.6534 (3)
16.6189 (3)
68.5070 (10)
86.0340 (10)
81.5850 (10)
2560.17 (8)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
D (gcm−3
)
1.412
0.892
μ (mm−1
)
f (000)
1132
Θ range for data collection (°)
no. reflections/observed
Rint
1.80–28.00
12,301/9743
0.0208
data/restraints/parameters
Goodness of fit
refinement method
final R indices [F> 4σ (F)]
R indices (all data)
12,301/0/707
1.016
full-matrix least-squares of F2
R1 = 0.0299, wR2 = 0.0720
R1 = 0.0463, wR2 = 0.0792
Table 2. selected calculated and experimental bond lengths and angles for [Cul2]2.
Bond distance
Calculated (Å)
experimental
Bond angle
Calculated (°)
experimental
Cu2–o21
o11–Cu2
Cu1–o11
Cu1–o12
Cu1–o22
Cu1–n11
Cu1–n12
Cu2–n21
Cu2–n22
Cu1–Cu2
1.84
2.11
1.87
1.90
1.95
1.93
1.91
1.91
1.90
3.37
1.88
2.55
1.90
1.89
2.59
2.01
2.00
2.00
2.01
3.41
o11–Cu1–n11
o22–Cu1–n12
o11–Cu2–n21
o21–Cu2–n22
o11–Cu1–o22
o12–Cu2–o22
Cu1–o22–Cu2
Cu1–o11–Cu2
n21–Cu2–n22
n11–Cu1–n12
93.06
105.88
112.07
85.13
73.04
88.52
114.36
119.17
143.67
166.91
93.11
113.91
115.45
90.23
77.57
87.64
97.89
99.09
145.18
146.49
d, J = 7.8); 7.14(t, 2H, J = 8.2); 6.54(t, 2H, J = 7.5); 3.86(s, 3H, –OCH3) and 2.42(s, 3H,–CH3); 13C-NMR
(400 MHz, CDCl3, 25 °C, TMS): δ = 164.74(Ar–OCH3); 164.49(Ar–OH); 161.43(HC = N); 147.63(Ar–N = C);
132.46(ArCH3); 133.88, 131.10, 127.42, 126.79, 118.06, 113.68, 107.53, 101.56(Ar–C); 55.90(–OCH3);
18.64(–CH3). IR (ATR): 3109–2339 cm−1 v(O–H); 1610 cm−1 v(C = N); 1292 cm−1 v(C–O). UV–vis (DMF):
285; 340; 420 nm. elemental Anal. Calcd (%) for C15H15NO2: %C: 74.60, %H: 6.27, %N: 5.80; found:
%C: 74.55, %H: 6.9, %N: 5.78.
2.2.2. Synthesis of [CuL2]2.
A hot ethanolic solution of copper(II) acetate monohydrate, Cu(OAc)2·H2O (0.624 mmol, 0.124 g), was
gradually mixed with hot ethanolic solution of the ligand (1.243 mmol, 0.30 g) in a 50-mL round bottom
flask. The resulting solution was refluxed overnight to obtain a green precipitate. The precipitate was
filtered under suction, washed with ethanol, and dried in a vacuum desiccator over silica gel. Yield: 0.27 g
(78%); m.p.:196–198 °C; IR (ATR): 1605 cm−1 v(C = N), 1310 cm−1 v(C–O); IR(Nujol): 522 cm−1 v(Cu–O),
465 v(Cu–N); UV-vis (DMF): 286, 351, 472, 548 nm. Conductivity (DMF): 2.6 Ω−1 cm2 mol−1. elemental
Anal. Calcd (%) for Cu2C60H56N4O8: %C: 66.22, %H: 5.19, %N: 5.15, %Cu: 12.04; found: %C: 65.55, %H:
5.40, %N: 4.96, %Cu: 11.85.

84
A. O. SOBOLA eT AL.
2.3. Crystallographic studies
Single crystals suitable for X-ray diffraction were obtained via slow evaporation of the saturated DMF
solution of the complex. A suitable single crystal of the complex was diffracted using a Bruker KAPPA
APeX II single-crystal X-ray diffractometer, with a four-circle Kappa goniometer.
Crystallographic data were collected at 200 K and 0.71073 nm (λ) on a sensitive CCD detector with
graphite-monochromated MoKα radiation; a total of 12,301 reflections were collected of which 9743
were considered observed [I > 2σ (I)]. The structure was solved by direct methods using SHeLXS-97
and refined anisotropically by full-matrix least-squares on F2 using SHeLXL-97 [33]. Details of crystallo-
graphic parameters, data collection, and refinements are listed in Table 1 with selected bond lengths
and angles presented in Table 2.
2.4. Computational details
Conformational search was conducted on the modeled structure using the molecular mechanic force
field calculation in aqueous medium which makes use of the systematic algorithm to obtain the struc-
ture with the lowest energy; this is to establish the most stable structure [29]. The best conformer has
energy of 2843.00 kJ mol−1. Quantum mechanical calculations on ground-state molecular geometry,
dipole moment, polarizability, energy, and frontier orbital energies (EHOMO and ELUMO) of the complex
were carried out using the PM3 semi-empirical method with SPARTAN10 software package [24] on a
1.70-GHz personal computer. PM3 has been proven to be the best method to account for the properties
of transition metal complexes [23, 24, 26, 27, 29].
3. Results and discussion
3.1. Elemental analysis and conductivity measurement
elemental analysis results for the complex suggested that the complex was of the form [CuL2]2, with the
Cu2+ coordinating to the Schiff base ligand in a ratio 1:2. The low conductivity value for the complex
(Table 1) implies that the Schiff base ligand is coordinated [34] via the deprotonated phenolic oxygens
and the imine nitrogens.
3.2. Spectroscopic characterization of compounds
3.2.1. 1H- and 13C-NMR
Assignments of the main NMR signals are given in the experimental section. evidence for the formation
of the Schiff base was indicated by the azomethine signal (HC = N) as one proton singlet at 8.52 ppm
1
in the H-NMR spectrum of the ligand. This was further confirmed by the appearance of the azome-
thine carbon at 161.43 ppm. The phenolic hydroxyl proton absorbed far downfield as a broad signal
at 14.04 ppm, due to a strong intramolecular hydrogen bonding between the imine nitrogen and the
hydroxyl proton. Strong signals at 3.86 and 2.42 ppm were assigned to the methoxy and methyl protons
of the p-vanillin and the o-toluidine, respectively. The purity of the ligands was, however, indicated by
the disappearance of the amino and aldehyde protons at 3–4 and 9–10 ppm, respectively.
3.2.2. Infrared study
The strong band at 1610 cm−1 was assigned to the imine (C = N) functional group of the free ligand.
As expected, the imine band was red-shifted (Δv = −5 cm−1) in the spectrum of the complex, indi-
cating coordination of the Schiff base via the azomethine nitrogen [35, 36]. The phenolic stretching
vibration v(C–O) of the free ligand appeared as a strong band at 1292 cm−1 and consequently blue-
shifted to 1310 cm−1 upon complexation [35, 36], suggesting the involvement of the phenolic oxygen
in coordination. Likewise, the O–H stretching vibration of the free Schiff base was observed as a broad
band at 3109–2339 cm−1, due to strong intramolecular hydrogen bonding [35, 37]. This band, however,

JOURNAL OF COORDINATION CHeMISTRY
85
Figure 1. labeled orteP diagram for [Cul2]2, showing 50% probability ellipsoids.
Table 3. summary of τ values for various five-coordinate Cu(ii) complexes.
Complexa
β (°)
α (°)
τ
Geometry
Refs.
i[Cu(salgly)(urea)]2
ii[Cu2lCl2(h2o)]
iii[Cu2Col2Cl6]
iv[lCu(nCs)]+
[Cul2]2
176.06
174.7
176.4
167.16
164.91
164.59
163.93
158.0
156.64
138.41
146.49
145.18
0.2
0.278
0.33
0.479
0.307
0.324
sPy
sPy
*sPy/tBPy
sPy/tBy
sPy
[22]
[21]
[18]
[43]
this work
sPy
a(i)salgly:n-salicylideneglycinato-;(ii)[Cu2lCl2(h2o)]:l =n,n-o-phenylenebis(salicylideneimine);(iii)[Cu2Col2Cl6]:l = 1,3,5-tris(pyra-
zol-l-ylmethyl)benzene (l); (iv) [lCu(nCs)]+: l = nC5h4C(Ch3)=n(Ch2)3 n = C(Ch3)C5h4 n.
*sPy/tBPy: intermediate geometry between square pyramidal and trigonal bipyramidal.
disappeared in the spectrum of the complex due to deprotonation and involvement of oxygen in the
coordination sphere. The coordination mode was further substantiated by the appearance of two new
bands in the far-infrared spectra of the complexes at 490–465 and 522 cm−1. These bands were assigned
to the v(Cu–O) and v(Cu–N) stretching vibrations, respectively [5, 38].
3.2.3. UV–vis study
The electronic transition study of the compounds was carried out in DMF. Three absorption maxima
were observed in spectra of the Schiff base ligand. The high-energy band at 285 nm was assigned to the
π →π* transition of the benzene ring, and the azomethine chromophore of the Schiff base ligand [39],
while the band at 340 nm was assigned to n→π* transition of the imine functional group [40]. The band
at 420 nm implies existence of the Schiff base ligand in keto-imine form [41, 42] due to tautomerism of

86
A. O. SOBOLA eT AL.
the orthohydroxyl Schiff base ligand. The π →π* absorption did not shift upon complexation, but rather
a shift to lower energy was observed for the n →π* band, thus indicating coordination via the imine
nitrogen. The spectrum of [CuL2]2 has a band at 472 nm corresponding to ligand-to-metal charge-trans-
fer transition [19]. The broad asymmetrical band centered at 621 nm was tentatively assigned to the
d–d transition of the Cu(II) complex.
3.3. Molecular structure of [CuL2]2
Selected bond angles and bond lengths of the compound are presented in Table 2, while the labeled
ORTeP diagram is presented in Figure 1. The complex crystallized in the triclinic system with a space
group of P-1 having a = 11.4233(2) Å, b = 14.6534(3) Å, c = 16.6189(3) Å, α=68.5070 (10)°, β = 86.0340
(10)°, and γ=81.5850(10)°. Two symmetric units of the Schiff base ligand coordinated to Cu(II) as mon-
obasic bidentate via the imine N and the phenolic oxygen. Two of the phenolic oxygens (O22 and O11)
were tridentate, bridging between the two Cu(II) centers, resulting in a binuclear complex. The crystal
structure of the complex indicates a distorted square pyramidal geometry around copper(II). The basal
plane consisted of the imine nitrogens (N11, N12; N21 N22) and the deprotonated phenolic oxygens
(O11, O12; O21, O211), while the apex was occupied by the bridging phenolic oxygens (O11; O22).
The axial Cu–O bond lengths were equivalent; Cu1–O22 (2.591 Å) and Cu2–O11 (2.549 Å) and longer
than the other Cu–O bonds; Cu1–O11 (1.903 Å), Cu1–O12 (1.889 Å), Cu2–O21 (1.880 Å), and Cu2–O22
(1.895 Å), in the equatorial plane. Valent et al. [23] reported values of 1.915(3), 2.406(3) Å, and 94.47(12)°
for equatorial Cu–O, apical Cu–O, and Cu–O1–Cu of a similar oxo-bridged binuclear Cu(II) complex. The
bridging Cu2O2 unit was constrained to be planar by the presence of the crystallographic center with
the Cu1⋯Cu2 distance of 3.413 Å while the bridging angles are 97.89(5)°–99.09(5)°.
The degree of distortion toward trigonality has been defined by Addison et al. [18] geometric param-
eter (τ) for distinguishing between square pyramidal and trigonal bipyramidal geometries in five-coor-
dinate complexes; τ=(β−α)/60, where α and β are the two largest basal trans angles. Generally, τ = 0 for
an ideal square-based pyramid (since α=β = 180°) and τ = 1 for an ideal trigonal bipyramidal geometry
(for α=120°; β = 180°). Maity et al. [22] and Chang et al. [19] assigned square pyramidal (τ = 0.28) and
square pyramidal/trigonal bipyramidal (τ = 0.33) geometries for the five-coordinate Cu(II) complex. A
summary of assigned geometries for some five-coordinate Cu(II) complexes with varying τ values is
given in Table 3.
In this complex, the values of τ were large (0.324 and 0.307) for the two copper(II) centers, indicating
distortion from square-based pyramidal geometry toward trigonal bipyramidal geometry. The deviation
from an ideal square pyramidal geometry was further observed from the basal trans angles: N11-Cu1–
N12 (146.49°); O11–Cu1–O12 (164.91°); N21–Cu2–N22 (145.18°); and O21–Cu2–O22 (164.59°), which
differed markedly from the ideal angle of 180° for a true square pyramidal geometry. In addition, the
values of α [146.49° (N11–Cu1–N12) and 145.18° (N21–Cu2–N22)] were between 180° (ideal square
pyramidal geometry) and 120° (ideal trigonal bipyramidal geometry) which shows that the geometry
was intermediate between the two extremes [19].
3.4. Theoretical calculations on the complex
3.4.1. Geometric properties
The optimized structure for the Cu(II) complex is presented in Figure 2. In order to study the theoretical
properties, geometric parameters were taken into consideration. Bond lengths and bond angles are
recorded in Table 2. Both the bond distances and bond angles were compared with the experimental
results.
Bond lengths Cu1–O1 (1.84 Å), Cu1–O3 (2.11 Å), Cu2–O2 (1.87 Å), Cu2–O3 (1.90 Å), Cu2–O8 (1.95 Å),
Cu1–N1 (1.93 Å), Cu1–N2 (1.91 Å), Cu2–N3 (1.91 Å), and Cu2–N4 (1.90 Å) in the calculated data compared
with the bond lengths in the experimental data Cu1–O1 (1.88 Å), Cu1-O3 (2.55 Å), Cu2–O2 (1.90 Å), Cu2–O3
(1.89 Å), Cu2–O8 (2.59 Å), Cu1–N1 (2.01 Å), Cu1–N2 (2.00 Å), Cu2–N3 (2.00 Å), and Cu2–N4 (2.01 Å) were in

JOURNAL OF COORDINATION CHeMISTRY
87
Figure 2. the structure of the Cu(ii) complex after optimization using the ball and wire model.
Table 4. electronic properties of [Cul2]2.
Property
[Cul2]2
−13.94
−17.16
3.22
lumo energy (eV)
homo energy (eV)
ΔE (eV)
Ħ = ΔE/2 (eV)
μ (debye)
Polarizability
1.61
13.71
40.61
Table 5. Vibrational frequencies with their corresponding vibrations.
Vibration
experimental (cm−1)
Calculated (cm−1)
v(C=n)str
v(C–o)str
v(o–h)str
v(Cu–o)str
v(Cu–n)str
1605
1310
3109–2339
490–465
522
1610–1601
1354–1294
3030–2898
–
543
agreement as shown in Table 2. Bond angles were also close except for the bond angles of Cu1–O3–Cu2
(114.36°), Cu1–O2–Cu2 (124.02°), N21–Cu2–N2 (98.99°), and N1–Cu1–N2 (92.11°) which gave Cu1–O3–Cu2
(97.37°), Cu1-O2–Cu2 (99.09°), N21–Cu2–N2 (145.18°), and N1–Cu1–N2 (146.49°), respectively. The Cu1⋯Cu2
interaction distance was 3.37 Å which agreed with 3.41 Å in the experiment.
However, there are significant differences between the found and calculated values of some bond
distances and angles (Table 2). This is owing to the fact that the interatomic potentials are based on
approximations and derived from different types of experimental data. Some existing energy functions
do not account for electronic polarization of the environment, an effect that can significantly reduce
electrostatic interactions of partial atomic charges, and the nature of the dielectric depends on the
model used. All types of van der Waals forces are also strongly environment-dependent because these
forces originate from interactions of induced and “instantaneous”dipoles [44].

88
A. O. SOBOLA eT AL.
3.4.2. Electronic properties and stability
energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) were determined. A band gap was obtained, indicating the complex has a good non-lin-
ear optical property. The calculated EHOMO and ELUMO, ΔE (band gap), hardness (Ħ), and dipole moment
(μ) are recorded in Table 4. The molecule was stable as shown by low energy gap and high polarizability.
energy band gaps obtained from band structure calculations for solids are analogous to HOMO–LUMO
energy differences in molecules [45].
Dipole moment is used to study molecular interactions involving the non-bonded-type dipole–
dipole interactions, which increase with low/decrease in band gap of molecules [46–48]. Polarizability
increases with the size of a molecule and/or low band gap. A molecule with a low band gap is polariz-
able and is generally said to be chemically reactive. In addition, such molecules possess high non-lin-
ear optical property and are said to be soft [42]. Non-linear optical properties are of interest due to
their potential usefulness for unique optical devices such as frequency-doubling devices, optical signal
processing, and optical computers [49]. The thermodynamic properties for the complex were: heat
of formation = 2903.62 kJ mol−1, free energy = 5433.75 kJ mol−1, enthalpy = 5775.00 kJ mol−1, and
entropy = 1130.43 J mol−1 K−1
.
3.4.3. Vibrational frequencies
Vibrational frequencies given in Table 5 are in agreement with the experimental values. The v(C = N)str
and v(C–O)str absorptions are at 1610–1601 and 1354–1294 cm−1, respectively. No absorption was
observed for v(Cu–O)str, although v(Cu–N)str appeared at 543 cm−1. The calculated IR spectrum for the
complex is presented in the Supplementary Data.
4. Conclusion
A new oxo-bridged binuclear Cu(II) complex, μ-O-[CuL2]2, was synthesized from p-vanillin Schiff base
ligand. The ligand was coordinated to the Cu(II) via the imine nitrogen and the phenolic oxygen. The
deprotonated phenolic oxygen bridged two units of the resulting complex to give a bis(five-coordi-
nate) binuclear Cu(II) complex. The binuclear Cu(II) complex has a low band gap which accounted for
its high dipole moment and polarizability. The complex is a probable non-linear optical material and
can be exploited for future research.
Supplementary material
X-ray crystallography data of the binuclear complex, μ-O-[CuL2]2, have been deposited with the Cambridge Crystallographic
Data Center (CCDC number 888458) and can be obtained free of charge on request at http://www.ccdc.cam.ac.uk/conts/
retrieving.html or from the Cambridge Crystallographic Data Center (CCDC), 12 Union Road, Cambridge CB2 1eZ, UK;
Fax:+44(0)1223 336033; e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
One of the authors, Dr Sobola A.O. is grateful to Rhodes University, South Africa, for providing facilities for this study.
Disclosure statement
No potential conflict of interest was reported by the authors.
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