Synthesis, Characterization, and DFT Calculations of New Fluorescent Cu(II) Complexes of Heterocyclic Ligands

Article abstract of DOI:10.1134/S0022476619020082

The synthesis, spectral studies, and quantum chemical investigation of two new copper(II) complexes of fluorescent heterocyclic ligands derived from benzimidazole are reported. The fluorescent heterocyclic ligands are synthesized by the reduction of imida

Full text of DOI:10.1134/S0022476619020082

Journal of Structural Chemistry. Vol. 60, No. 2, pp. 232-240, 2019.
Text © 2019 S. Rastegarnia, M. Pordel, S. Allameh.
SYNTHESIS, CHARACTERIZATION, AND DFT
CALCULATIONS OF NEW FLUORESCENT Cu(II)
COMPLEXES OF HETEROCYCLIC LIGANDS*
S. Rastegarnia, M. Pordel, and S. Allameh
The synthesis, spectral studies, and quantum chemical investigation of two new copper(II) complexes of
fluorescent heterocyclic ligands derived from benzimidazole are reported. The fluorescent heterocyclic
ligands are synthesized by the reduction of imidazo[4′,5′:3,4]benzo[1,2-c]isoxazole derivatives and
characterized by elemental analysis, IR, mass, and NMR spectra. Coordination of the bidentate ligands with
the Cu(II) cation produce orange complexes. The structures of the complexes are established by spectral,
analytical data and Job′s method. To gain insight into the geometry and spectral properties of the Cu(II)
complexes, DFT calculations are performed at the B3LYP/6-311++G(d,p) level. The DFT-calculated
spectral properties are in good agreement with the experimental values, confirming the suitability of the
optimized geometries for the copper complexes. The photophysical properties of the ligands and the Cu(II)
complexes are characterized by UV-Vis and fluorescence spectroscopy. An efficient charge transfer from
the ligand p orbital to the Cu(II) d orbital may be proposed as the main reason for the color of the
complexes.
DOI: 10.1134/S0022476619020082
Keywords: benzimidazole, fluorescent ligands, Cu(II) complex, UV-Vis and fluorescence spectroscopy,
DFT.
INTRODUCTION
Copper is an important bioelement responsible for many catalytic processes in living systems [1]. Copper is present
in enzymes of biological systems either alone [2] or in combination with some other metal ions [3] to discharge its function
by the redox reactivity. The copper complexes constitute a significant class of molecules from several points of view, viz.
bioinorganic, catalytic, and magnetic. Copper(II) complexes play an important role in the active sites of a large number of
metalloproteins in biological systems and can have the potential application for numerous catalytic processes in living
organisms that involve electron transfer reactions or the activation of some antitumor substances [4]. In fact, copper(II)
chelates have been found to interact with biological systems and exhibit antineoplastic [5], antibacterial, antifungal [6, 7], and
anticancer [8] activities. Some copper(II) N,S,O/N,N-donor chelators are good anticancer agents due to the strong binding
ability with a DNA base pair [9]. Also, copper(II) complexes find applications as catalysts for the oxidation of alcohols into
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran; mehdipordel58@mshdiau.ac.ir.
Zhurnal Strukturnoi Khimii, Vol. 60, No. 2, pp. 244-251, February, 2019. Original article submitted February 05, 2018;
revised August 23, 2018; accepted August 23, 2018.
* Supplementary materials are available for this article at doi 10.1134/S0022476619020082 and are accessible for authorized
users.
232
0022-4766/19/6002-0232 © 2019 by Pleiades Publishing, Ltd.

aldehydes and ketones in organic chemistry with the recognized industrial importance [10].
On the other hand, benzimidazoles are considered as significant heterocyclic compounds due to their presence in
a wide range of bioactive compounds such as antiparasitics [11], anticonvulsants [12], analgesics [13], antihistaminics [14],
antiulcers [15], antihypertensives [16], antiviral [17], anticancers [18], antifungals [19], anti-inflammatory agents [20], proton
pump inhibitors [21], and anticoagulants [22]. The optimization of substituents around the benzimidazole core has resulted in
many drugs: albendazole, mebendazole, thiabendazole as antihelmintics; omeprazole, lansoprazole, pantoprazole as proton
pump inhibitors; astemizole as antihistaminic; enviradine as antiviral; candesartencilexitil and telmisartan as
antihypertensives, and many leading compounds in a wide range of therapeutic areas. Also, benzimidazoles play an important
role in determining the function of a number of biologically important metal complexes [23].
Since, copper and benzimidazole play a key role in many disciplines, it is important to study the interactions of
benzimidazole ligands with copper. In this work, the synthesis of two new copper (II) complexes of fluorescent heterocyclic
ligands derived from benzimidazole is reported. Also, the optical properties of the compounds are investigated by UV-Vis
and fluorescence spectroscopy. Moreover, density functional theory (DFT) calculations are performed to provide the
optimized geometries, structural parameters, and vibrational frequencies of the investigated compounds.
EXPERIMENTAL
Equipment and materials. Melting points were measured on an Electrothermal type-9100 melting-point apparatus.
The FT-IR spectra were recorded for potassium bromide pellets using a Tensor 27 spectrometer and only noteworthy
1
absorptions are listed. The 13C NMR (75 MHz) and H NMR (300 MHz) spectra were obtained on a Bruker Avance DRX-
300 spectrometer. Chemical shifts are reported in ppm downfield from TMS as the internal standard; the coupling constant J
is given in Hz. The mass spectrum was recorded on a Varian Mat, CH-7 at 70 eV and the ESI mass spectrum was measured
using a Waters Micromass ZQ spectrometer. Elemental analysis was performed on a Thermo Finnigan Flash EA
microanalyzer. Absorption and fluorescence spectra were recorded on a Varian 50-bio UV-Visible spectrophotometer and
a Varian Cary Eclipse spectrofluorophotometer. UV-Vis and fluorescence scans were recorded from 200 to 1000 nm. The
Cu(II) percentage was obtained using a Hitachi 2-2000 atomic absorption spectrophotometer.
All solvents were dried according to the standard procedures. Compounds 1 [24] and 3a,b [25] were obtained
according to the published methods. Other reagents were commercially available.
Computational methods. All calculations have been performed using the DFT method with the B3LYP functional
[26] as implemented in the Gaussian 03 program package [27]. The 6-311++G(d,p) basis sets were employed, except for
Cu(II) where the LANL2DZ basis sets were used with considering its effective core potential. The geometry of the Cu(II)
complexes was fully optimized and confirmed to have no imaginary frequency of the Hessian. The geometry optimization
and frequency calculation simulate the properties in the gas/solution phases.
The fully-optimized geometries were confirmed to have no imaginary frequency of the Hessian.
The solute-solvent interactions have been investigated using one of the self-consistent reaction field methods, i.e.,
the sophisticated polarizable continuum model (PCM) [28]. The PCM calculations have been performed in the MeOH
solution.
General procedure for the synthesis of 4a,b ligands from 3a,b. Iron powder (0.89 g, 16 mmol) was added with
stirring to a solution of 3a,b (4 mmol) in EtOH (30 ml) and HCl (2M, 2 ml). Then the mixture was refluxed for 4 h and
poured into water. The precipitate was collected by filtration, washed with water, and air-dried to give crude 4a,b.
(5-Amino-1-propyl-1H-benzo[d]imidazol-4-yl)(phenyl)methanone (4a, L1) was obtained as shiny yellow needles
1
(EtOH). M.p.: 122-124 °C (Lit. m.p. 121-123 °C)] [ 25 ]; H NMR (CDCl3): δ 0.94 (t, J = 7.5 Hz, 3H, CH3), 1.75-2.10 (m,
2H, CH2), 4.04 (t, J = 7.5 Hz, 2H, NCH2), 4.12 (br s, 2H, NH2), 6.62 (d, J = 8.5 Hz, 1H, Ar H), 7.03 (d, J = 8.5 Hz, 1H, Ar
H), 7.11-7.52 (m, 5H, Ar H), 7.75 (s, 1H, Ar H) ppm; 13C NMR (CDCl3): δ 11.4, 23.2, 46.8, 111.3, 114.1, 114.8, 125.5,
233

–1 –1
127.1, 127.6, 127.9, 129.0, 132.3, 135.6, 142.2, 195.9(C=O) ppm. IR (KBr): 3315, 3260 cm (NH2), 1635 cm (C=O). MS
+
(m/z) 279 (M ). Anal. Calcd. for C17H17N3O (279.3): C 73.10, H 6.13, N 15.04. Found: C 73.45, H 6.15, N 14.87.
(5-Amino-1-butyl-1H-benzo[d]imidazol-4-yl)(phenyl)methanone (4b, L2) was obtained as shiny yellow needles.
1
M.p.: 117-119 °C. Yield: 79%. H NMR (CDCl3): δ 0.89 (t, J = 7.5 Hz, 3H, CH3), 1.21-1.33 (m, 2H, CH2), 1.77-1.88 (m, 2H,
CH2), 4.19 (br s, 2H, NH2), 4.31 (t, J = 6.9 Hz, 2H, NCH2), 6.67 (d, J = 8.5 Hz, 1H, Ar H), 7.01 (d, J = 8.5 Hz, 1H, Ar H),
13
7.09-7.47 (m, 5H, Ar H), 7.73 (s, 1H, Ar H) ppm; C NMR (CDCl3): δ 13.7, 20.01, 33.1, 44.5, 110.8, 114.3, 114.6, 125.3,
–1 –1
127.1, 127.4, 127.8, 128.8, 132.5, 135.9, 142.0, 195.3 (C=O) ppm. IR (KBr): 3320, 3263 cm (NH2), 1637 cm (C=O). MS
+
(m/z) 293 (M ). Anal. Calcd. for C18H19N3O (293.4): C 73.69, H 6.53, N 14.32. Found: C 74.01, H 6.57, N 14.09.
General procedure for the synthesis of complexes 5a,b from ligands 4a,b. To the yellow solution of ligands 4a,b
(2 mmol) in a metanolic solution (15 ml, MeOH:H2O 10:90) copper(II) sulfate pentahydrate (0.25 g, 1 mmol) was added,
resulting in a change in the color to shiny green. The reaction was carried out for another 1 h at room temperature. The
complex was isolated by evaporation of the solvent and washed with cold MeOH and then H2O.
Aqua bis (5-amino-1-propyl-1H-benzo[d]imidazol-4-yl)(phenyl)methanone) copper(II) sulfate [Cu(L1)2]SO4⋅(H2O)
–1
–1
(5a) was obtained as an orange powder. M.p. > 300 °C (decomp). IR (KBr): 3378, 3254 cm (NH2), 1624 cm (C=O). ESI–
2+
MS (+) m/z (%): 622 [Cu(L1)2] . Anal. Calcd. for C34H36CuN6O7S (736.3): C 55.46, H 4.93, N 11.41, Cu 8.63. Found: C
55.67, H 4.96, N 10.98, Cu 8.69.
Aqua bis(5-amino-1-butyl-1H-benzo[d]imidazol-4-yl)(phenyl)methanone) copper(II) sulfate [Cu(L2)2]SO4×⋅
–1
–1
×(H2O) (5b) was obtained as an orange powder. M.p. > 300 °C (decomp). IR (KBr): 3371, 3260 cm (NH2), 1625 cm
2+
(C=O). ESI–MS (+) m/z (%): 650 [Cu(L2)2] . Anal. Calcd. for C35H38Cl7CuN6O7S (764.4): C 56.57, H 5.27, N 10.99, Cu
8.31. Found: C 57.00, H 5.31, N 10.69, Cu 8.29.
RESULTS AND DISCUSSION
Synthesis and structure of new ligands 4a,b and complexes 5a,b. The reaction of 5-nitro-1H-benzimidazole with
propyl and butyl bromide in KOH and DMF leads to the formation of 1-propyl-5-nitro-1H-benzimidazoles (1a) and 1-benzyl-
5-nitro-1H-benzimidazoles (1b) [24]. 3-Propyl-8-phenyl-3H-imidazo[4′,5′:3,4]benzo[1,2-c]isoxazole (3a) and 3-butyl-8-
phenyl-3H-imidazo[4′,5′:3,4]benzo[1,2-c]isoxazole (3b) are obtained from the reaction of compounds 1a,b with benzyl
cyanide (2) in a basic MeOH solution [25]. The reduction of compounds 3a,b in EtOH with Fe/HCl gives 5-amino-1-propyl-
1H-benzo[d]imidazol-4-yl)(phenyl)methanone (4a) and (5-amino-1-butyl-1H-benzo[d]imidazol-4-yl)(4-chlorophenyl)
methanone (4b) in high yields (Scheme 1).
Scheme 1. Synthesis of ligands 4a,b.
The structural assignments of compounds 4a,b were based on the analytical and spectral data. For example, in the
1
H NMR spectrum of new compound 4b there is an exchangeable peak at δ 4.19 ppm attributed to NH2 group protons. Also,
there are two doublet signals (δ = 6.67 and 7.01 ppm), a multiplet signal (δ = 7.09–747 ppm), and a singlet signal
(δ = 7.73 ppm) assignable to eight aromatic ring protons. In addition, 16 different carbon atom signals are observed in the
13
C NMR spectrum of compound 4b. Furthermore, the IR spectrum of compound 4b in KBr showed broad absorption bands
234

–1
–1
at 3320 and 3263 cm assignable to NH2 and 1637 cm attributed to the C=O groups. The results of mass spectroscopy (m/z
+
293 [M] ) and elemental analysis support the structure of the new compound 4b.
New complexes 5a,b have been synthesized by treating ligands 4a,b and CuSO4⋅5H2O in the 2:1 molar ratio
obtained by Job′s method [29] in an aqueous metanolic solution. Nine aqueous methanolic mixtures of the ligand (0.6 mM)
and Cu (II) (0.6 mM) were prepared in an appropriate buffer at 25 °C. Sodium perchlorate was added to give a constant ionic
strength of 0.1 M. The volumes of the ligand solution used varied from 9 to 1 ml and those of the Cu(II) solution varied from
1 to 9 ml; the total volume was always 10 ml. The absorption spectra of the complex were achieved immediately after mixing
the ligand and Cu(II) solutions. The absorption spectra and Job′s plot of the nine aqueous methanolic mixtures of the ligand
and Cu(II) can be found in the Supplementary Data (Figs. S1–S3 of Supplementary Materials).
New complexes were characterized by the elemental and spectroscopic (IR, and mass) analyses. The elemental
analysis results for the Cu complexes confirm the proposed [Cu(L)2]SO4⋅(H2O) formulas. In addition, the molecular ion peak
2+
2+
at m/z 622 ([Cu(L1)2] ) and m/z 650 ([Cu(L2)2] ) strongly support the structure of new complexes 5a and 5b, respectively.
A square pyramidal geometry was proposed for Cu(II) complexes 5a,b based on our experimental results and those
reported in the literature [30, 31]. Complexes 5a,b can exist as two isomers, differing in coordination of ligands 4a and 4b to
the Cu atom (Scheme 2).
Scheme 2. Two possible structures of new Cu(II) complexes 5a,b.
In order to clarify the exact structure of 5a,b, an effort was taken up to crystallize the complexes in various solvent
systems under different experimental conditions. Unfortunately, in all our efforts, only amorphous compounds precipitated,
which prevented us from the analysis of the complexes by X-ray crystallization. However, the calculated data can help
identify the structure of the Cu complexes, revealing which of the two isomers is energetically more advantageous. Therefore,
the geometries of two isomers of the Cu(II) complexes were optimized in both gas phase and MeOH as the solvent (PCM
model) by DFT calculations at the B3LYP/6-311++G(d,p) level. By comparing the sum of the electronic and zero-point
energy (E) values of isomers I (–2148.306834 kJ/mol) and II (–2160.357074 kJ/mol), it can be concluded that the geometry
of isomer II is the most possible structure for complexes 5a,b. For example, the optimized geometry of complex 5a is
depicted in Fig. 1. Some of the calculated structural parameters of the Cu(II) complex are listed in Table 1. The optimized
geometries of ligands 4a,b are depicted in the Supplementary Data (Figs. S2 and S3 of Supplementary Materials). In the
optimized geometry of complex 5a, ligand 4a acts as a bidentate ligand and coordinates to Cu(II) via the nitrogen atom of the
amine functional group and the oxygen atom of the carbonyl group. The ligand aromatic rings are approximately in the same
plane. Another position of the complex is filled by a H2O molecule. The Cu–O and Cu–N bond lengths are collected in
Table 1.
The DFT-computed vibrational modes of Cu(II) complex 5a are listed in Table 2 together with the experimental
values for comparison. The atoms are numbered as in Fig. 1. As seen in Table 2, there is good agreement between the
experimental and DFT-calculated frequencies of complex 5a, confirming the validity of the optimized geometry as a proper
structure for complex 5a.
235

Fig. 1. Optimized geometry of complex 5a.
TABLE 1. Selected Structural Parameters of Cu(II) Complex 5a
Bond
Bond length (Å)
Angle
(deg.)
Dihedral angle
(deg.)
Cu–O2
Cu–O4
2.321
1.869
1.961
1.958
2.277
1.459
1.304
1.450
1.475
1.422
1.424
1.410
1.382
1.373
1.317
1.367
1.474
1.515
N1–Cu–N2
O3–Cu–O4
179.73
179.23
Cu–N2–C17–C19
Cu–O4–C18–C19
N1–Cu–N2–C17
N6–C27–C19–C18
N4–C11–C13–N3
N5–C30–N6–C27
C22–C24–C25–C23
O3–C2–C4–C6
1.076
22.555
–7.210
38.811
29.512
–9.110
0.421
Cu–N1
O2–Cu–O3
119.59
Cu–N2
Cu–O4–C18
Cu–N1–C1
129.21
Cu–O3
119.59
N1–C1
Cu–O3–C2
113.55
O4–C18
N2–C17
C27–C29
C27–C19
C25–C24
C20–C22
C27–N6
C14–N3
C14–N4
C29–N5
N3–C15
C15–C16
C1–C3–C2
119.59
O3–C2–C4
119.124
129.589
114.394
113.900
120.231
119.244
120.445
119.811
119.864
121.740
122.364
22.555
38.717
–0.155
25.511
–8.102
10.285
7.226
C3–C1–N3
C1–C10–C12–C13
C5–N2–C7–N1
N5–C30–N6
N3–C29–C28
C19–C18–C20
C31–C30–C31
C20–C22–C24
C10–C12–C13
O4–C18–C19
C15–C16–C34
C9–C7–C5
O2–Cu–O3–C2
N6–C30–N5–C29
O4–C18–C20–C22
C4–C6–C8–C9
N3–C13–C12–C10
C34–C16–C13–N3
Cu–N1–C2–C3
–179.400
–179.095
47.602
–179.501
C22–C20–C18–C19
Photophysical properties of the ligands and complexes. Fluorescent heterocyclic ligands 4a,b and Cu(II)
complexes 5a,b were characterized by UV-Vis and fluorescence spectroscopy in the wavelength range 200-1000 nm. The
absorption and fluorescence emission spectra of 4a,b and Cu(II) complexes 5a,b are depicted in Figs. 2 and 3, respectively.
Numerical spectral data are also presented in Table 3. Values of extinction coefficient (ε) were calculated as the slope of the
plot of absorbance vs. concentration. As seen in Fig. 2, the spectra of complexes 5a,b have an absorption maximum at
236

–1
TABLE 2. Selected Experimental and Calculated IR Vibrational Frequencies (cm ) of Cu(II) Complex 5a
Calculated
Experimental
Vibrational assignment
IR Intensity
frequencies
Frequency
–4
D(10 esu /cm )
2
2
673 (w)
721 (w)
959 (m)
1278 (m)
1393 (m)
1446 (s)
679
736
299
49
δυsym(Cu–N)
υasym(Cu–N)
976
190
140
1734
515
47
δop(C–H) aromatic
υ(C1–N3, C22–N6)
1255
1401
υ
asym(C4–C5–N2) + υasym(C25–C26–N5)
υ(C=C, C=N) of the aromatic rings
υ(C21–C9) + υ(C42–C30)
1413
1452
1474
256
66
υ(C=N) of the aromatic rings + υ(C42–N4, C21–N1)
υ(C=N) of the aromatic rings + υ(C22–N6, C1–N3)
υ(C=C) of the aromatic rings
1485
1624 (s)
1523
527
4101
322
19
1563
δsci(H–O–H) of the H2O ligands
1638
υ(C=C) of the benzene rings
1657
υ(C=C) of 1 moiety
2822 (m)
3066-3190
3079-3121
3185-3224
3549
23-12
24-6
12-5
25
υ
sym(C–H) aliphatic
υasym(C–H) aliphatic
υ(C–H) aromatic
3381 (vs,br)
υ(C28–H21) + υ(C7–H2)
3779
11
υsym(O–H) of the H2O ligands
3875
43
υasym(O–H) of the H2O ligands
A b b r e v i a t i o n s: op is out-of-plane; ip is in-plane; w is weak; m is medium; s is strong; vs is very strong; br is
broad; sh is shoulder.
Fig. 2. Absorption spectra of 4a,b and Cu(II)
complexes 5a,b in the MeOH solution
(10 M).
–4
425 nm at which the ligands have no absorbance. Absorption at 425 nm could be attributed to both ligand to-metal charge
transfer (LMCT) [32] bands and intraligand transitions.
237

Fig. 3. Fluorescence emission spectra of 4a,b (excitation
at 370 nm) and Cu(II) complexes 5a,b (excitation at
–6
425 nm) in the MeOH solution (3×10 M).
TABLE 3. Spectroscopic Data for 4a,b and 5a,b at 298 K
Dye
4a
4b
5a
5b
a
385
10.0
485
365
14.0
490
425
7.5
425
6.0
λ
abs (nm)
–3
–1
ε×10 [(mol/L) ⋅cm ]
–1 b
c
525
0.49
525
0.55
λ
flu (nm)
d
ΦF
0.66
0.73
a
b
c
Maximum absorbance wavelengths (λabs).
Extinction coefficient.
Fluorescence emission wavelengths (λflu) with excitation at 370 nm for 4a,b and 425 nm for 5a,b
compounds.
d
Fluorescence quantum yield.
Furthermore, ligands 4a,b and copper complexes 5a,b produced fluorescence in dilute MeOH solution (Fig. 3). The
fluorescence quantum yields of the compounds were also determined via comparison methods, using fluorescein as the
standard sample in a 0.1 M NaOH and MeOH solution [33]. The used value of the fluorescein emission quantum yield is 0.79
and the obtained emission quantum yields of the new compounds are around 0.49-0.73. As can be seen from Table 3,
extinction coefficient (ε), fluorescence intensity, and the emission quantum yield of 4b were the largest values. The excitation
spectra of ligands 4a,b and Cu(II) complexes 5a,b in the MeOH solution were also presented in Supplementary data (Fig. S6
of Supplementary Materials).
CONCLUSIONS
Coordination of the ligands derived from benzimidazole with the Cu(II) cation gives new fluorescent Cu(II)
complexes. The structures of the complexes have been established by spectral and analytical data and Job′s method, and
a square pyramidal geometry was proposed for the complexes. Also, the DFT methods were employed to gain a deeper
insight into the geometry and spectral properties of the new complexes. The DFT-calculated vibrational modes of Cu(II)
complexes are in good agreement with the experimental values, confirming the suitability of the optimized geometries for the
complexes. Fluorescent ligands and copper complexes were spectrally characterized by UV-Vis and fluorescence
spectroscopy. The results revealed that the ligands and Cu(II) complexes generated fluorescence in a dilute MeOH solution.
A great attempt to obtain the new complexes from the coordination of the ligands with other cations is in progress
and will soon be reported elsewhere.
238

We would like to express our sincere gratitude to Research Office, Mashhad Branch, Islamic Azad University,
Mashhad-Iran for the financial support of this work.
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