Synthesis, Characterization, Properties, and Theoretical Calculation of an Inorganic–Organic Hybrid Mononuclear Copper(II) Complex Containing 3-Hydroxy-2-methyl-quinoline-4-carboxylate

Article abstract of DOI:10.1007/s10870-019-00808-2

Abstract: A novel copper complex [CuL(bipy)(H₂O)]·H₂O was synthesized by solvothermal reaction and its crystal structure was determined by single-crystal X-ray diffraction technique. (HL = 3-hydroxy-2-methylquinoline4-carboxylic acid; bipy = 2,2′-bipyrdine) The title complex (1) crystallizes in the orthorhombic system of the Pbca space group, and exists as an isolated mononuclear structure. The intermolecular hydrogen bonds and the strong π···π stacking interactions yield a three-dimensional (3-D) supramolecular network. Solid-state photoluminescence spectrum uncovers that it shows an emission in the pale blue region of the light spectrum. It has remarkable CIE (Commission Internationale de I’éclairage) chromaticity coordinates of (0.2656, 0.3162), so it may be a very promising color lighting and display converter. Time-dependent density functional theory (TDDFT) calculation reveals that this emission is caused by to the ligand-to-ligand charge transfer (LLCT). Solid-state diffuse reflectance data shows that there is a narrow optical band gap of 1.38?eV. Therefore, it could be probably a candidate for narrow band gap semiconductors. Graphic Abstract: A novel copper (II) complex is synthesized via solvothermal reactions, which is characteristic of an isolated structure, at the same time, it displays a wide and intense photoluminescence emission band in the pale blue region and theoretical study reveals that the photoluminescence emission is originated from the ligand-to-ligand charge transfer transition.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10870-019-00808-2

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-019-00808-2
ORIGINAL PAPER
Synthesis, Characterization, Properties, and Theoretical Calculation
of an Inorganic–Organic Hybrid Mononuclear Copper(II) Complex
Containing 3‑Hydroxy‑2‑methyl‑quinoline‑4‑carboxylate
Xiu‑Guang Yi1 · Xiao‑Niu Fang1 · Yin‑Feng Wang1 · Jin Guo1 · Jia Li1,2
Received: 19 July 2019 / Accepted: 24 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A novel copper complex [CuL(bipy)(H2O)]·H2O was synthesized by solvothermal reaction and its crystal structure was deter-
mined by single-crystal X-ray difraction technique. (HL=3-hydroxy-2-methylquinoline4-carboxylic acid; bipy=2,2′-bipy-
rdine) The title complex (1) crystallizes in the orthorhombic system of the Pbca space group, and exists as an isolated mono-
nuclear structure. The intermolecular hydrogen bonds and the strong π···π stacking interactions yield a three-dimensional
(3-D) supramolecular network. Solid-state photoluminescence spectrum uncovers that it shows an emission in the pale blue
region of the light spectrum. It has remarkable CIE (Commission Internationale de I’Éclairage) chromaticity coordinates
of (0.2656, 0.3162), so it may be a very promising color lighting and display converter. Time-dependent density functional
theory (TDDFT) calculation reveals that this emission is caused by to the ligand-to-ligand charge transfer (LLCT). Solid-state
difuse refectance data shows that there is a narrow optical band gap of 1.38 eV. Therefore, it could be probably a candidate
for narrow band gap semiconductors.
Graphic Abstract
A novel copper (II) complex is synthesized via solvothermal reactions, which is characteristic of an isolated structure, at
the same time, it displays a wide and intense photoluminescence emission band in the pale blue region and theoretical study
reveals that the photoluminescence emission is originated from the ligand-to-ligand charge transfer transition.
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Journal of Chemical Crystallography
Keywords Copper (II) complex · Crystal structure · Photoluminescence · Semiconductor · TDDFT
carbon, hydrogen and nitrogen were performed on an Ele-
mentar Vario EL elemental analyzer. 1H NMR spectra were
measured on Bruker Avance 400 MHz instrument based on
deuterium DMSO as solvent. Photoluminescence measure-
ments were performed on a F97XP photoluminescence spec-
trometer. Solid-state UV/Vis refectance spectroscopy were
carried out with a TU1901 UV/Vis spectrometer equipped
with an integrating sphere. TDDFT investigations were per-
formed using the Gaussian 09 suite of program packages.
Introduction
In recent years, more and more attention has been paid to
supramolecular chemistry, which has been widely used in
the design of new crystal materials [1–6], owing to the self-
assembly to synthesize new compounds with weak interac-
tion. Supramolecular compounds linked by hydrogen bonds,
dipole–dipole interaction, ion–ion interaction, aromatic π–π
stacking interaction and ion–dipole interaction have been
reported extensively so far [7–10].
Synthesis of the HL
(HL = 3‑hydroxy‑2‑methylquinoline‑4‑carboxylic
acid)
As an inorganic–organic hybrid light-emitting material,
metal complexes have attracted much attention due to their
potential applications in optical sensors and light emitting
devices.
The synthesis of HL is based on relevant references [14, 15],
Quinoline carboxylic acid is not only a chelating agent,
but also a good chromophore, it has been widely used in the
feld of luminescent materials [11–13]. However, their cop-
per (II) complexes are less reported on photoluminescence
and solid-state difuse refectance in the region of transition
metal complexes.
as shown in Scheme 1.
Synthesis of Isatin
indigo (131 g, 0.5 mol), K2Cr2O7 (74 g, 1.0 mol) and dis-
tilled water (200 mL) were added into three fasks of 500 mL
and stirred, After cooling, dilute H2SO4 (10%, 250 mL) were
added and kept stirring at 43 °C for 1.5 h. The mixture was
diluted with twice its volume of distilled water, fltered of,
The residue dissolved 10% NaOH solution, fltered again,
acidifed with 10% HCl to pH=7 and refltered, and dried
to give the isatin as a reddish brown powder 116 g (90%);
m.p. 210 °C; HRMS m/z (ESI) calcd. for C8H5NO2 ([M+H)+
147.0320, found 147.0826.
Base on this, We are long interested in the crystal
engineering of copper(II)-containing compounds with
3-hydroxy-2-methyl-quinoline-4-carboxylic acid (HL) and
2,2′-bipyridine (bipy) as mixed ligands. In a continuation of
our work, we report the solvothermal synthesis, X-ray crys-
tal structure, photoluminescent, and semiconductor proper-
ties, and time-dependent density functional theory (TDDFT)
calculations of the title complex, which is an isolated mono-
nuclear zero-dimensional (0D) copper(II) complex.
Synthesis of HL
Experimental
isatin (73.5 g, 0.5 mol) and NaOH (20 g, 0.5 mol) were dis-
solved into a sufcient amount of distilled water and fltered.
The fltrate and NaOH (20 g, 0.5 mol) were added into chlo-
roacetone (92 g, 1.0 mol), heated under refux for 1.5 h until
the solution was completely yellow. Hydrochloric acid (10%)
was added dropwise to adjust pH=7, fltered, and dried to
give the HL as a yellow powder 96 g (95%); m.p. 225 °C;
HRMS m/z (ESI) calcd for C11H9NO3 ([M+H)+ 203.0582,
Materials and Physical Measurements
The reagents and chemicals for the synthesis of the title com-
pound were analytical reagent grade, commercially available
and applied without further purifcation. Infrared spectra
were obtained with a PE Spectrum-One Fourier transform
infrared (FT-IR) using KBr discs. Elemental analysis of
Scheme 1 Synthetic route of ligand HL
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Journal of Chemical Crystallography
Table 1 Crystallographic data and structural analysis for the title
complex (1)
found 203.0548. 1H NMR (400 MHz, DMSO) δ 9.15 (s, 1H),
7.93 (d, J=8.0 Hz 1H), 7.64 (t, J=8.0 Hz, 1H), 7.60–7.52 (m,
2H), 2.70 (s, 3H).
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C21H19N3O5Cu
456.93
292(2)
Orthorhombic
Pbca
9.7763(5)
17.1526(10)
23.0477(10)
90
Synthesis of the Compound [CuL(bipy)(H2O)]·H2O (1)
A mixture containing HL (203 mg, 1 mmol), CuCl2·2H2O
(170 mg, 1 mmol), bipy (156 mg, 1 mmol), H2O (9 mL), and
Triethylamine (1 mL) for adjusting Ph was placed in a 25 mL
Tefon lined autoclave and heated to 100 °C for 7 days under
autogenous pressure. Then it was allowed to cool to room
temperature, bright green block-like crystals were obtained
and used for the collection of single-crystal X-ray data. The
yield was 70% based on copper. IR peaks (cm−1): 3435(vs),
1609(w), 1567(m), 1531(w), 1452(s), 1342(w), 1310(w),
775(m).
β (°)
γ (°)
90
90
3864.8(3)
8
1.571
1.170
1880.0
0.14×0.12×0.02
MoKα (λ=0.71073)
Volume (Å3)
Z
ρcalc (g/cm3)
μ (mm−1
F(000)
)
Crystallographic Data Collection and Refnement
Crystal size (mm3)
Radiation
The single crystal data of the title complex were collected
on a SuperNova charge-coupled device (CCD) X-ray dif-
fractometer with a suitable single crystal (dimensions of
0.14 mm×0.12 mm×0.02 mm). The crystal was kept at
292(2) °C during data collection. Using Olex2 [16], the struc-
ture was solved with the ShelXT [17] the structure solution
program using Intrinsic Phasing and refned with the ShelXL
[18] refnement package using Least Squares minimisation.
All of the non-hydrogen atoms were generated based on the
subsequent Fourier diference maps and refned anisotropi-
cally. The hydrogen atoms were located theoretically and ride
on their parent atoms. The fnal refnement gave R=0.0468,
wR=0.0907(w=1/[σ2(F20)+(0.0391P)2 +1.6386P]), where
P=(F20 +2F2c)/3); S=1.021, (Δρ)max =0.27, (Δρ)min =−0.40
e/Å3 and (Δ/σ)max =0. A summary of the crystallographic
data, selected bond lengths and of the title complex is listed in
Tables 1 and 2, respectively.
2Θ range for data collection (°) 7.072 to 58.532
Index ranges
−9≤h≤13, −17≤k≤23,
−31≤l≤23
11,238
Refections collected
Independent refections
Data/restraints/parameters
Goodness-of-ft on F2
Final R indexes [I≥2σ (I)]
Final R indexes [all data]
Largest dif. peak/hole (e Å−3
4526 [Rint =0.0328, Rsigma =0.0513]
4526/0/281
1.021
R1 =0.0468, wR2 =0.0907
R1 =0.0788, wR2 =0.1023
0.27/−0.40
)
R1 =Σ||Fo|−|Fc||/Σ|Fo|; wR2 ={Σ[w(F2o −Fc2]/Σ[wFo2]2}1/2
Results and Discussion
Synthesis
Computational Details
The preparation process is divided into two parts. In the frst
step, isatin was obtained by oxidation of indigo with potas-
sium dichromate. In the second step, under alkaline condi-
tions, the target product (HL) was prepared by the reaction
of isatin with ketoacetone.
In order to reveal the nature of the photoluminescence emis-
sion of complex (1), we truncated ground state geometry from
its single-crystal X-ray difraction data set without optimiza-
tion and carried out its theoretical calculation in light of the
time-dependent density functional theory (TDDFT) based on
this ground state geometry. The TDDFT calculations were
performed using the B3LYP function [19, 20] with basis set
of SDD for Cu and 6-31G* for C, H, O, N and carried out by
means of the Gaussian09 program [21].
Crystal Structure
Single-crystal X-ray difraction measurement revealed that
the title complex (1) crystallizes in the space group Pbca of
the orthorhombic system. In the crystal structure of the title
complex (1), the metal Cu atom is situated at the inversion
center. The Cu (II) complex forms a rectangular pyramidal
geometry with O(2) and O(3) of HL, two nitrogen of bipy
and O(1) of the coordinated water molecule, as presented
in Fig. 1. The bond distances of Cu–O(1), Cu–O(2) and
1 3

Journal of Chemical Crystallography
Table 2 Selected bond lengths (Å) and bond angles (°) for the title complex (1)
Distance
(Å)
Distance
(Å)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(3)
2.252(2)
1.8872(12)
1.8960(18)
Cu(1)–N(1)
Cu(1)–N(2)
2.002(2)
2.034(2)
Angle
(°)
Angle
(°)
O(2)–Cu(1)–O(1)
O(2)–Cu(1)–O(3)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
O(3)–Cu(1)–O(1)
106.16(8)
92.88(7)
90.86(8)
163.30(9)
93.76(8)
O(3)–Cu(1)–N(1)
O(3)–Cu(1)–N(2)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–O(1)
170.79(9)
92.88(8)
93.28(9)
80.10(9)
99.44(8)
Fig. 1 An ORTEP drawing
of the title complex with 50%
thermal ellipsoids (Color fgure
online)
Cu–O(3) Å are 2.252(2) Å, 1.888(17) Å and 1.896(18) Å,
respectively, while those of Cu–N(1) and Cu–N(2) are
2.002(2) Å and 2.034(2) Å. These are comparable with that
reported in the references [22, 23]. Both the quinolinecar-
boxylate (L−) and bipy act as the bidentate ligand, which
occupy the basal position. The title complex (1) also con-
tains two water molecules, one of which is coordinated with
copper metal centers, which occupy the apical position, and
the other is a lattice water molecule.
weak intermolecular hydrogen bonds (O5–HA···N3i;
O5v–HA···N3). Those intermolecular hydrogen bonds like
O5–HB···O4 existing between the lattice water and the
quinoline carboxylic acid molecule, O1–H1A···O5ii exist-
ing between the coordination water and the another lattice
water molecule, O1–H1B···O4iii and O1iv–H1B···O4 existing
between the coordination water and the another carboxyl
oxygen atom, O5–HA···N3i and O5v–HA···N3 existing
between the lattice water and the nitrogen atom of quino-
line carboxylic acid molecule. They connect the molecules
together to form a three-dimensional structure, as pre-
sented in Fig. 2 and Table 3. In the title complex, there are
strong ofset face-to-face π···π stacking interaction between
Cg1···Cg4i, Cg2···Cg2ii and Cg3···Cg3iii [Cg1 is C6/C7/C8/
C9/C10/C11, Cg2 is N1/C17/C18/C19/C20/C21, Cg3 is
N2/C12/C13/C14/C15/C16, Cg4 is N3/C2/C3/C4/C5/C6;
The intramolecular week hydrogen bond can be found
between the carbon atom, carboxyl oxygen atoms, coor-
dination water and lattice water molecules (C8–H8···O4;
C12–H12···O3; C21–H21···O2). At the same time, the
title complex also includes four pairs of strong intermo-
lecular hydrogen bonds (O5–HB···O4; O1–H1A···O5ii;
O1–H1B···O4iii; O1iv–H1B···O4) and two pairs of
1 3

Journal of Chemical Crystallography
Fig. 2 The hydrogen bond
diagram of the title complex (1).
Hydrogen atoms not involved in
the motif show were removed
for clarity. Symmetric code: (i)
1+x, y, z; (ii) 1/2+x, 3/2x−y,
1−z; (iii) −1/2+x, y, 1/2−z;
(iv) 3/2−x, −1/2+y, z; (v)
1−x, 1−y, 1−z (Color fgure
online)
Table 3 Hydrogen bond lengths (Å) and bond angles (°)
the shift distance being of 1.783 Å and the twist angle being
of 0°. These π···π stacking interactions and via van de Waals
attraction yield the three-dimensional supramolecular struc-
ture, the crystal packing as presented in Figs. 3 and 4. There
are also two interactions between C–H···Cg (π···Ring) in the
three-dimensional structure, they are C5–H5···Cg (N3/C2/
C3/C4/C6/C7; 1/2+x, y, 1/2−z) and C14–H14···Cg (Cu1/
O2/C1/C2/C3/O3; 1−x, 1−y, 1−z) respectively.
D–H···A [ARU]
D–H (Å) H···A (Å) D···A (Å) D–H···A (°)
O5–H···O4 []
0.85
1.96
2.09
1.95
1.93
2.21
2.53
2.51
2.51
2.56
2.777(3) 160
2.874(3) 176
2.809(3) 165
2.804(3) 172
2.815(3) 122
3.020(3) 114
3.421(4) 167
3.431(4) 171
2.990(3) 108
O5–HA···N3 [6455] 0.79
O1–H1A···O5[1655] 0.88
O1–H1B···O4[3566] 0.88
C8–H8···O4 []
C12–H12···O3[]
C18–H18···O5[5666] 0.93
C19–H19···O4[7645] 0.93
C21–H21···O2[]
0.93
0.93
Photoluminescence Measurements
As a typical d9 transition metal polymer, the title complex
(1) exhibits potential photoluminescence property [24, 25].
In order to reveal the potential photoluminescent proper-
ties of complex (1), the photoluminescence spectra was
determined for solid state samples of (1) at room tempera-
ture and the results is presented in Fig. 5. It is obvious that
the photoluminescent spectrum of complex (1) displayed
an efective energy absorption in a wavelength range of
250–380 nm. Upon the emission of 477 nm, the excita-
tion spectrum showed a band at 341 nm. Upon excita-
tion at 341 nm, the emission spectrum was characterized
by a sharp band at 477 nm in the blue region. The emis-
sion band of complex (1) located in the blue violet light
0.93
Symmetric code: [1655]=1+x, y, z; [3566]=1/2+x, 3/2x−y,
1−z; [6455]=−1/2+x, y, 1/2−z; [7645]=3/2−x, −1/2+y, z;
[5666]=1−x, 1−y, 1−z
i=−1/2+x, +y, 1/2−z; ii=2−x, 1−y, 1−z; iii=1−x,
1−y, 1−z]. The centroid–centroid distance of Cg1···Cg4i
is 3.749(17) Å with the shift distance being of 1.353 Å and
the twist angle being of 4.358°. The centroid–centroid dis-
tance of Cg2···Cg2ii is 3.907(17) Å with the shift distance
being of 1.936 Å and the twist angle being of 0°. The cen-
troid–centroid distance of Cg3···Cg3iii is 3.967(16) Å with
1 3

Journal of Chemical Crystallography
Fig. 3 The π···π stacking
interactions diagram of the
title complex (1) (Color fgure
online)
region with the CIE1931 chromaticity coordinate (0.2656,
0.3162) Fig. 6. As a result, the title complex (1) is a poten-
tial pale blue photoluminescent material [26].
Refectance Measurements
To investigate the semiconductive properties of the
complex (1), the solid-state UV–Vis difuse refectance
spectra of powder sample of complex (1) was measured
at room temperature, using barium sulfate as the refer-
ence for 100% refectivity. After measuring the solid-state
difuse refectance spectra, the data was treated with the
Kubelka–Munk function that is known as α/S = (1 − R)2/
(2R). With regard to this function, the parameter α means
the absorption coefcient, S means the scattering coef-
cient, and R means the refectance, which is actually wave-
length independent when the size of the particle is larger
than 5 μm. From the α/S vs energy gap diagram, we can
obtain the value the optical band gap, which can be extrap-
olated from the linear portion of the absorption edges. The
solid-state UV–Vis difuse refectance spectrum reveals
that complex (1) has a narrow optical energy band gap of
1.38 eV, as shown in Fig. 8. As a result, the complex (1) is
a possible candidate for narrow band gap semiconductors.
The energy band gap of 1.38 eV of complex (1) is obvi-
ously close to those of GaAs (1.4 eV), CdTe (1.5 eV), and
CuInS2 (1.55 eV) [26], which are well known as highly
efcient band gap photovoltaic materials.
Theoretical Calculations
The characteristics of HOMO (the highest occupied molec-
ular orbital) and LUMO (the lowest unoccupied molecular
orbital) of the title complex (1) is shown in Fig. 7. It is
easy to fnd out that the electron-density distribution of
HOMO was totally resided at the coordinating π-orbital of
carboxylic acid HL with an energy of − 0.16604 Hartree;
while, the electron-density population of LUMO locates
at π-orbital of ligand bipy and the energy LUMO was cal-
culated to be − 0.11120 Hartree. The energy diference
between LUMO and HOMO is 0.05484 Hartree, and this
is small enough to allow the charge transfer from HOMO
to LUMO. In light of this observation, it is proposed that
the essence of the photoluminescence of the complex (1)
could be assigned to the ligand-to-ligand charge transfer
(LLCT; from the HOMO of the π-orbital of ligand HL to
the LUMO of the π-orbital of ligand bipy). This calcula-
tion result is in good agreement with the experimental
observations.
1 3

Journal of Chemical Crystallography
Fig. 4 The packing diagram of the title complex (1) with the dashed lines representing the hydrogen bonding interactions (Color fgure online)
Fig. 5 Solid-state photoluminescence spectra of the title complex (1)
(black curve: excitation; red curve: emission) (Color fgure online)
Fig. 6 The CIE chromaticity diagram and chromaticity coordinate of
the emission spectrum of the complex (1)
1 3

Journal of Chemical Crystallography
Fig. 7 HOMO and LUMO of the title complex (1) with isosurface of 0.003 a.u
Supplementary Materials
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center,
CCDC No. 1938345. Copies of this information can be
obtained free of charge from the Director, CCDC, 12 Union
Road, Cambridge, CBZ 1EZ, UK (fax: +44-1223-336033);
email: deposit@ccdc.cam.ac.uk or online at http://www.
ccdc.cam.ac.uk).
Funding This work was supported by the National Natural Science of
Foundation of China (No. 51363009), Jiangxi Province Department of
Education’s Item of Science and Technology & Higher Education and
Teaching Reform (GJJ160745, JXJG-17-9-14, 20194083), Doctoral
Research Startup Foundation and Natural Science Foundation Project
of Jinggangshan University (JZB1905, JZ1901).
Fig. 8 The solid-state UV–Vis difuse refectance spectrum of the
complex (1)
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In summary, a novel copper complex has been prepared under
solvothermal conditions using 3-hydroxy-2-methyl-quinoline-
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Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
Afliations
Xiu‑Guang Yi1 · Xiao‑Niu Fang1 · Yin‑Feng Wang1 · Jin Guo1 · Jia Li1,2
2
* Xiu-Guang Yi
School of Chemistry and Chemical Engineering, Xiamen
University, Xiamen 361005, Fujian, China
jayxgggchem@163.com
1
School of Chemistry and Chemical Engineering,
Jinggangshan University, Ji’an 343009, Jiangxi, China
1 3