Complexes containing benzimidazolyl-phenol ligands and Ln(III) ions: Synthesis, spectroscopic studies and preliminary cytotoxicity evaluation

Article abstract of DOI:10.1016/j.jinorgbio.2019.110842

Fourteen new complexes were obtained from Ln(III)(NO₃)₃?n-H₂O and the chromophores 2-(1H-benzo[d]imidazol-2-yl)-phenol (Bzp1) or 2-(5-methyl-1H-benzo[d]imidazol-2-yl)-phenol (Bzp2). The complete characterization allowed us

Full text of DOI:10.1016/j.jinorgbio.2019.110842

JournalofInorganicBiochemistry201(2019)110842
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Complexes containing benzimidazolyl-phenol ligands and Ln(III) ions:
Synthesis, spectroscopic studies and preliminary cytotoxicity evaluation
Andrea Hernández-Moralesa, José María Riveraa, Aracely López-Monteona,
Soledad Lagunes-Castroa, Silvia Castillo-Blumb, Karla Cureño-Hernándezb,
⁎
Angelina Flores-Parrac, Osvaldo Villaseñor-Granadosc, Raúl Colorado-Peraltaa,
a Maestría en Ciencias en Procesos Biológicos, FCQ-UV, C.P. 94340 Orizaba, Ver., Mexico
b Departamento de Química Inorgánica, FQ-UNAM, C.P. 04510 CDMX, Mexico
c Departamento de Química, CINVESTAV-IPN, C.P. 07360 CDMX, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fourteen new complexes were obtained from Ln(III)(NO3)3∙n-H2O and the chromophores 2-(1H-benzo[d]imi-
dazol-2-yl)-phenol (Bzp1) or 2-(5-methyl-1H-benzo[d]imidazol-2-yl)-phenol (Bzp2). The complete characterization
allowed us to assign unequivocally the structures of all the complexes. The techniques used for this purpose were
Ultraviolet-Visible (UV–Vis) and Fourier-Transform Infrared (FT–IR) spectroscopies, High-Resolution Mass
Spectrometry (HRMS), Magnetic Susceptibility (MS), Elemental Analysis (EA) and Molar Conductivity (MC).
HRMS allowed us to find the molecular ion and its isotopic pattern. The FT–IR spectral data suggested that
benzimidazolyl-phenol ligands coordinate with Ln(III) ions through iminic nitrogen and phenolic oxygen.
Thermogravimetric Analysis (TGA) studies of NdBzp1 and GdBzp2 complexes indicate the presence of lattice
water along with three nitrates and two benzimidazolyl-phenol ligands; the thermal decomposition was con-
sistent with the minimal formula suggested by EA. The coordination type of the benzimidazolyl-phenol ligands,
the geometry and the structural organization of these coordination complexes have been interpreted by Density
Functional Theory (DFT) calculations, and they coincided with the physicochemical data suggesting a co-
ordination number eight for the Ln(III) ions. The cytotoxicity of the chromophores and their coordination
complexes was tested against a cancer cell line (HeLa), as compared with structure/support cells (NIH-3T3) and
defense cells (J774A.1), revealing that three coordination complexes showed moderate cytotoxicity against the
cell lines studied.
Cytotoxicity
Lanthanides
Benzimidazoles
Bidentate ligands
DFT calculations
HeLa cells
1. Introduction
synthesizing and characterizing metal complexes of the p, d and f block
with benzimidazolic ligands [8,9].
Benzimidazole derivatives have an extensive diversity of pharma-
cological and biological applications due to their structural relationship
with purines and pyrimidines [1,2]. In addition, vitamin B12 contains a
5,6-dimethyl-benzimidazole fragment coordinated to the Co(III) ion
[3,4]. Literature data describe a large amount of properties for benzi-
midazole derivatives, such as antiparasitic, antioxidant, anti-in-
flammatory, analgesic, antihypertensive, anthelmintic, antiprotozoal,
anticancer, antimicrobial, among others [5]. In addition, they are active
constituents of biocides, such as fungicides and insecticides, and they
have formed important compounds in photochemical, photophysical
and bioinorganic chemistry [6]. Benzimidazole derivatives are ligands
extensively used in coordination chemistry, due to their structural
stability and low reactivity [7]; therefore, there is a great interest in
Additionally, the benzimidazolyl-phenol derivatives act as a bi-
dentate (N,O) ligand [10–12]. The complexes with these ligands are
important, due to their thermal stability, optical properties, catalytic
efficiency and their biological applications [13–26]. The benzimida-
zolyl-phenol complexes are a novel category of photochemical com-
pounds that can be handled as electroluminescent materials, whose
properties can be delimited using diverse metal ions [27–29]. In this
sense, metal complexes are of unlimited importance for OLED tech-
nology, as specified by numerous patents [30]. The benzimidazolyl-
phenol complexes are also useful as high-energy radiation detectors,
laser dyes, molecular energy storage systems, fluorescent probes and
selective complexes for determining Hg(II) [31–36]. The deprotonated
species of these organic ligands are appropriate for the creation of
⁎ Corresponding author.
E-mail address: racolorado@uv.mx (R. Colorado-Peralta).
https://doi.org/10.1016/j.jinorgbio.2019.110842
Received 30 April 2019; Received in revised form 6 September 2019; Accepted 7 September 2019
Availableonline11September2019
0162-0134/©2019ElsevierInc.Allrightsreserved.

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
different metal complexes with diverse coordination modes, structural
arrangement and geometry [37–45]. The oxygen atoms of the depro-
tonated hydroxyl group generally act as a terminal ligand or a bridge
ligand, although there are reports in which the proton is not lost by
coordinating different transition metals [46–50].
electrospray (ES) as ionization source and acetonitrile as solvent.
Electronic absorption spectra (200–1200 nm) of solid samples were
recorded on a Varian Cary (model 6000i) spectrometer by diffuse re-
flectance. Emission and excitation spectra (290–950 nm) of solid sam-
ples were acquired in a Horiba Scientific (model FluoroMax-4p)
equipment, with a 150 W continuous emission xenon lamp. Effective
magnetic moments of the solid samples were determined on a Johnson
Mathey (model MKII) balance by the Gouy method. Thermograms were
obtained in a Perkin Elmer (model TGA 4000) thermogravimetric
analyzer with a heating rate of 10 °C/min, under controlled nitrogen
atmosphere and temperature range of 25–500 °C. Molar electrical
conductivity was determined from methanol 1 × 10−3 M solutions
using an ORION (model 140) conductivity meter.
On the other hand, the coordination chemistry of the Ln(III) ions is
growing speedily, due to the importance of these complexes in basic
and applied research in different technological and scientific fields,
such as the natural sciences, engineering, medical and health sciences
[51–55]. Ln(III) complexes have attracted attention to the field of
bioinorganic chemistry, since some of their compounds are being used
in biomedical assays as MRI contrast agents [56,57]. They can be em-
ployed as biological probes in clinical chemistry and molecular biology
due to their exceptional biological and photophysical properties
[58,59]. The Ln(III) complexes have motivated countless attempts in
the design of potential anticancer agents, due to their electronic con-
figuration [Xe]4f1-145d0-16s2and its analogy with Ca(II). The Ln(III) ions
have ionic radii similar to that of Ca(II) ions, but due to their larger
charge, they show a high affinity for the sites that Ca(II) occupies in
biological systems and generate strong bonds with the water molecules
[61–66]. The Ca(II)-dependent enzymes can be inhibited by the Ln(III)
ions, but in some cases they have also been activated by them. Thus, the
inhibitory or activating effect of the Ln(III) ions could be related to the
particular function of the Ca(II) ions in the native enzymes [67,68].
The Ln(III) ions have diverse biological effects, it has been demon-
strated that in some cases they promote cell proliferation and in others,
they induce apoptosis. Therefore, there are some reports with com-
pounds that have shown a cytotoxic effect when studying several cancer
cell lines [69–73]. Among other effects, it has been shown that Ln(III)
ions can induce cell membrane perforation and apoptosis, they also
seem to have some influence in the control of oxidative damage pro-
duced by free radicals and other reactive oxygen species, these being
the main mechanisms of anti-cancer activity. Some authors emphasize
that Ln(III) complexes have the ability to interact directly with DNA,
preventing their relaxation through the inhibition of topoisomerases
[74,75].
2.2. Synthesis
2.2.1. Preparation of 2-(1H-benzo[d]imidazol-2-yl)-phenol (Bzp1)
The procedure reported by Quezada-Buendía et al., in 2008 was
utilized with some modifications that are mentioned below [22]. In a
round flask with magnetic stirring were placed: 1 g of 1,2-diamino-
benzene (9.2472 mmol), 1.2772 g of 2-hydroxybenzoic acid
(9.2472 mmol) and 10 g of polyphosphoric acid, which were heated for
6 h at a temperature of 220 °C. The reaction mixture was slowly cooled
to room temperature, and then 100 mL of distilled water was added and
neutralized by the addition of reagent grade ammonium hydroxide
dropwise. A gray precipitate was separated from the reaction mixture,
which was filtered and dissolved in 100 mL of methyl alcohol. A yellow
product crystallized slowly from the solution and was obtained with a
yield of 93% (1.8079 g, Mp 268 °C). NMR (δ ppm, DMSO‑d6, 25 °C) 1H:
8.80 (sa, 1H, OH), 8.12 (d, 1H, H-9, 3J 7.9 Hz), 7.67 (d, 2H, H-4, H-7, 3J
6.0 Hz), 7.37 (dd, 1H, H-11, 3J 7.5, 8.9 Hz), 7.25 (dd, 2H, H-5, H-6, 3J
6.0 Hz), 7.04 (d, 1H, H-12, 3J 7.5 Hz), 7.00 (dd, 1H, H-10, 3J 7.9,
8.9 Hz). 13C: 158.0 (C-13), 151.9 (C-2), 137.2 (sa, C-3a, C-7a), 132.5 (C-
11), 126.9 (C-9), 123.6 (C-5, C-6), 120.1 (C-10), 117.7 (C-12), 115.3 (C-
4, C-7), 113.1 (C-8). FT-IR (ATR, ν cm−1): 3324.9 (NeH), 3100.0
(OeH), 1603.3, 1562.4, 1489.8, 1461.8 (C=C)arom, 1603.3 (C=N),
1262.2 (CeN), 1077.9 (CeO), 749.6 (CeH)arom. (+)TOF m/z (amu):
Calc. for (C13H11N2O)+: 211.0871. Found: 211.0866. Anal. Calc. for
C13H10N2O: C, 74.3; H, 4.8; N, 13.3. Found: C, 74.2; H, 4.8; N, 12.9.
Finally, the Ln(III) complexes coordinated to benzimidazolyl-phenol
ligands are an area of interest, because they reveal ligand-to-metal
charge transfer processes (LMCT), as well as a supramolecular dis-
position built on hydrogen interactions and π-π stacking [76–80].
However, there are no studies describing the cytotoxic activity of this
class of benzimidazolic complexes with Ln(III) ions; therefore, we have
synthesized Ln(III) complexes with benzimidazolyl-phenol ligands and
we discuss their selective cytotoxicity between cancer cells, structure/
support cells and defense cells.
2.2.2. Preparation
of
2-(5-methyl-1H-benzo[d]imidazol-2-yl)-phenol
(Bzp2)
In a round flask with magnetic stirring were placed: 1 g of 3,4-dia-
minotoluene (8.1853 mmol), 1.1306 g of 2-hydroxybenzoic acid
(8.1853 mmol) and 10 g of polyphosphoric acid, which were heated for
6 h at a temperature of 220 °C. The reaction mixture was slowly cooled
to room temperature, and then 100 mL of distilled water was added and
neutralized by the addition of reagent grade ammonium hydroxide
dropwise. A gray precipitate was separated from the reaction mixture,
which was filtered and dissolved in 100 mL of methyl alcohol. A pink
product crystallized slowly from the solution and was obtained with a
yield of 72% (1.3217 g, Mp 310 °C). NMR (δ ppm, DMSO‑d6, 25 °C) 1H:
8.71 (sa, 1H, OH), 8.03 (d, 1H, H-9, 3J 7.9 Hz), 7.54 (s, 1H, H-4), 7.44
(d, 1H, H-7, 3J 7.5 Hz), 7.36 (dd, 1H, H-11, 3J 7.0, 7.9 Hz), 7.09 (d, 1H,
H-6, 3J 7.5 Hz), 7.02 (d, 1H, H-12, 3J 7.0 Hz), 7.00 (dd, 1H, H-10, 3J 7.9,
7.9 Hz), 2.25 (s, 3H, H-14). 13C: 158.4 (C-13), 151.9 (C-2), 137.0 (sa, C-
3a, C-7a), 132.8 (C-5), 132.1 (C-11), 126.6 (C-9), 124.6 (C-6), 119.6 (C-
10), 117.6 (C-12), 115.3 (C-4, C-7), 113.2 (C-8), 21.7 (C-14). FT-IR
(ATR, ν cm−1): 3211.9 (NeH), 3100.0 (OeH), 1625.4, 1561,5, 1486.9,
1457.0 (C=C)arom, 1623.8 (C=N), 1257.8 (CeN), 1070.8 (CeO), 722.3
(CeH)arom. (+)TOF m/z (amu): Calc. for (C14H13N2O)+: 225.1027.
Found: 225.1022. Anal. Calc. for C14H12N2O: C, 75.0; H, 5.4; N, 12.5.
Found: C, 74.9; H, 5.4; N, 12.9.
2. Experimental
2.1. Materials and instrumentation
In this project, reagents and solvents purchased from Sigma-Aldrich
Chemical Co. were employed. The solvents used were purified ac-
cording to the literature [81]. Melting points were determined in a
Thermo Scientific Electrothermal (model IA9100) instrument in sealed
capillaries. Elemental analyses were quantified in the analyzers: Flash
Thermo Finnigan (model 1112) and Perkin Elmer (model 2400). NMR
spectra of the samples in DMSO‑d6 solutions were acquired in the
spectrometers: Bruker Avance 300
(
13C: 75.47 MHz and 1H:
300.13 MHz) and Jeol GSX-Delta 270
(
13C: 67.94 MHz and 1H:
270.17 MHz) with 5 mm diameter tubes. Chemical shifts expressed in
ppm were corrected using tetramethylsilane as reference; coupling
constants were informed in Hertz. IR spectra (400–4000 cm−1) of the
solid samples were acquired on a FT-IR Perkin Elmer (model Spectrum
100) spectrometer with a universal ATR polarization accessory. High
resolution mass data were recorded in an Agilent Technologies LC/
MSD–TOF (model 6210, G1969A) spectrometer coupled to HPLC, with
2.2.3. General method for the preparation of the lanthanide(III) complexes
A solution with one equivalent of Ln(III)(NO3)3∙n-H2O (Ln(III) = La,
2

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
Nd, Sm, Eu, Gd, Tb, Dy) in 20 mL of methyl alcohol was prepared and it
was named solution A. Another solution was prepared with two
equivalents of the ligand (Bzp1 or Bzp2) in 20 mL of methyl alcohol
and it was named solution B. Solution A was slowly added dropwise to
solution B, in a round flask with magnetic stirring, subsequently it was
refluxed for 2 h. The reaction mixture was filtered and evaporated using
vacuum.
3.4; N, 11.7. Found: C, 37.4; H, 3.6; N, 11.4.
2.2.3.6. [Tb(III)(Bzp1)2(NO3)2]NO3∙3 H2O (TbBzp1). The compound
TbBzp1 was synthesized following the procedure described in Section
2.2.3. from 107.73 mg of Tb(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 90.6% (0.1766 g, Mp 196 °C). FT-IR (ATR, ν cm−1): 3065.8
(NeH), 3225.8 (OeH), 1625.0, 1610.0, 1561.4 (C=C)arom, 1625.0
(C=N), 1289.9 (CeN), 1241.3 (CeO), 750.8 (CeH)arom; NO3 bands:
1459.0 (ν1), 1390.1 (ν0), 1289.9 (ν4), 1022.0 (ν2), 830.0 (ν6), 814.3 (ν3),
799.2 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Tb)+: 703.0596.
Found: 703.0601. Anal. Calc. for C26H26TbN7O14: C, 38.1; H, 3.2; N,
12.0. Found: C, 37.8; H, 3.5; N, 11.8.
2.2.3.1. [La(III)(Bzp1)2(NO3)2]NO3∙5 H2O (LaBzp1). The compound
LaBzp1 was synthesized following the procedure described in Section
2.2.3. from 102.97 mg of La(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 83.0% (0.1684 g, Mp 225 °C). FT-IR (ATR, ν cm−1): 3064.9
(NeH), 3200.0 (OeH), 1624.3, 1609.9, 1561.7, 1498.3 (C=C)arom
,
1624.3 (C=N), 1289.4 (CeN), 1241.2 (CeO), 750.6 (CeH)arom; NO3
bands: 1458.4 (ν1), 1393.4 (ν0), 1289.5 (ν4), 1022.5 (ν2), 840.0 (ν6),
2.2.3.7. [Dy(III)(Bzp1)2(NO3)2]NO3∙4 H2O (DyBzp1). The compound
DyBzp1 was synthesized following the procedure described in Section
2.2.3. from 108.58 mg of Dy(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 86.8% (0.1737 g, Mp 247 °C). FT-IR (ATR, ν cm−1): 3035.9
815.3 (ν3), 798.7 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8La)+
:
683.0403. Found: 683.0407. Anal. Calc. for C26H30LaN7O16: C, 37.4; H,
3.6; N, 11.7. Found: C, 37.7; H, 3.5; N, 11.9.
(NeH), 3250.0 (OeH), 1622.7, 1608.7, 1558.5, 1498.7 (C=C)arom
,
2.2.3.2. [Nd(III)(Bzp1)2(NO3)2]NO3∙3 H2O (NdBzp1). The compound
NdBzp1 was synthesized following the procedure described in Section
2.2.3. from 104.24 mg of Nd(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 89.6% (0.1714 g, Mp 288 °C). FT-IR (ATR, ν cm−1): 3065.4
1622.7 (C=N), 1307.1 (CeN), 1242.1 (CeO), 751.7 (CeH)arom; NO3
bands: 1410.0 (ν1), 1386.8 (ν0), 1242.2 (ν4), 1020.0 (ν2), 840.0 (ν6),
817.0 (ν3), 798.0 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Dy)+
:
708.0634. Found: 708.0628. Anal. Calc. for C26H28DyN7O15: C, 37.1; H,
3.4; N, 11.7. Found: C, 37.2; H, 3.8; N, 11.3.
(NeH), 3236.3 (OeH), 1624.9, 1609.9, 1562.5, 1499.1 (C=C)arom
,
1624.9 (C=N), 1289.2 (CeN), 1241.1 (CeO), 751.1 (CeH)arom; NO3
bands: 1457.1 (ν1), 1393.8 (ν0), 1289.3 (ν4), 1022.3 (ν2), 839.7 (ν6),
2.2.3.8. [La(III)(Bzp2)2(NO3)2]NO3∙4 H2O (LaBzp2). The compound
LaBzp2 was synthesized following the procedure described in Section
2.2.3. from 96.56 mg of La(III)(NO3)3∙6 H2O (0.2230 mmol) and 100 mg
of Bzp2 (0.4460 mmol). A pink product was obtained with a yield of
87.8% (0.1655 g, Mp 246 °C). FT-IR (ATR, ν cm−1): 3150.0 (NeH),
3200.0 (OeH), 1625.3, 1600.0, 1561.5, 1483.9 (C=C)arom, 1625.3
(C=N), 1299.3 (CeN), 1242.8 (CeO), 743.8 (CeH)arom; NO3 bands:
1408.2 (ν1), 1395.0 (ν0), 1241.9 (ν4), 1042.2 (ν2), 849.7 (ν6), 820.0 (ν3),
805.8 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8La)+: 711.0719.
Found: 711.0723. Anal. Calc. for C28H32LaN7O15: C, 39.8; H, 3.8; N,
11.6. Found: C, 39.6; H, 4.2; N, 11.5.
815.2 (ν3), 799.1(ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Nd)+
:
686.0420. Found: 686.0417. Anal. Calc. for C26H26NdN7O14: C, 38.8; H,
3.3; N, 12.2. Found: C, 39.1; H, 3.0; N, 12.3.
2.2.3.3. [Sm(III)(Bzp1)2(NO3)2]NO3∙3 H2O (SmBzp1). The compound
SmBzp1 was synthesized following the procedure described in Section
2.2.3. from 105.69 mg of Sm(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 90.5% (0.1746 g, Mp 192 °C). FT-IR (ATR, ν cm−1): 3065.2
(NeH), 3200.0 (OeH), 1624.4, 1610.1, 1499.0, 1479.6 (C=C)arom
,
1624.4 (C=N), 1290.1 (CeN), 1241.8 (CeO), 750.7 (CeH)arom; NO3
bands: 1459.8 (ν1), 1392.6 (ν0), 1290.1 (ν4), 1022.3 (ν2), 839.2 (ν6),
2.2.3.9. [Nd(III)(Bzp2)2(NO3)2]NO3∙4 H2O (NdBzp2). The compound
NdBzp2 was synthesized following the procedure described in Section
2.2.3. from 97.75 mg of Nd(III)(NO3)3∙6 H2O (0.2230 mmol) and
100 mg of Bzp2 (0.4460 mmol). A pink product was obtained with a
yield of 86.9% (0.1649 g, Mp 189 °C). FT-IR (ATR, ν cm−1): 3200.0
818.9 (ν3), 798.6 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Sm)+
:
688.0463. Found: 688.0469. Anal. Calc. for C26H26SmN7O14: C, 38.5;
H, 3.2; N, 12.1. Found: C, 38.6; H, 3.3; N, 12.4.
(NeH), 3300.0 (OeH), 1625.3, 1600.0, 1561.1, 1487.6 (C=C)arom
,
2.2.3.4. [Eu(III)(Bzp1)2(NO3)2]NO3∙4 H2O (EuBzp1). The compound
EuBzp1 was synthesized following the procedure described in Section
2.2.3. from 101.79 mg of Eu(III)(NO3)3∙5 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 86.8% (0.1714 g, Mp 297 °C). FT-IR (ATR, ν cm−1): 2942.8
1625.3 (C=N), 1300.7 (CeN), 1242.9 (CeO), 743.8 (CeH)arom; NO3
bands: 1403.9 (ν1), 1376.4 (ν0), 1243.0 (ν4), 1043.1 (ν2), 850.0 (ν6),
830.0 (ν3), 804.6 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8Nd)+
:
714.0733. Found: 714.0737. Anal. Calc. for C28H32NdN7O15: C, 39.5; H,
3.8; N, 11.5. Found: C, 39.8; H, 4.0; N, 11.2.
(NeH), 3200.0 (OeH), 1624.4, 1609.8, 1562.1, 1458.7 (C=C)arom
,
1624.4 (C=N), 1289.6 (CeN), 1241.2 (CeO), 751.0 (CeH)arom; NO3
bands: 1458.7 (ν1), 1392.8 (ν0), 1289.7 (ν4), 1022.2 (ν2), 835.0 (ν6),
2.2.3.10. [Sm(III)(Bzp2)2(NO3)2]NO3∙2
H2O
(SmBzp2). The
compound SmBzp2 was synthesized following the procedure
described in Section 2.2.3. from 99.12 mg of Sm(III)(NO3)3∙6 H2O
(0.2230 mmol) and 100 mg of Bzp2 (0.4460 mmol). A pink product
was obtained with a yield of 93.7% (0.1715 g, Mp 141 °C). FT-IR (ATR,
ν cm−1): 3200.0 (NeH), 3300.0 (OeH), 1625.7, 1600.0, 1560.2,
1487.9 (C=C)arom; 1625.7 (C=N), 1314.1 (CeN), 1243.4 (CeO),
743.7 (CeH)arom; NO3 bands: 1402.9 (ν1), 1375.9 (ν0), 1242.8 (ν4),
1043.8(ν2), 848.8 (ν6), 822.4 (ν3), 804.3 (ν5). (+)TOF m/z (amu): Calc.
for (C28H24N6O8Sm)+: 716.0776. Found: 716.0779. Anal. Calc. for
C28H28SmN7O13: C, 41.0; H, 3.4; N, 11.9. Found: C, 41.3; H, 3.2; N,
11.9.
817.6 (ν3), 798.2 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Eu)+
:
697.0555. Found: 697.0549. Anal. Calc. for C26H28EuN7O15: C, 37.6; H,
3.4; N, 11.8. Found: C, 37.2; H, 3.8; N, 11.9.
2.2.3.5. [Gd(III)(Bzp1)2(NO3)2]NO3∙4 H2O (GdBzp1). The compound
GdBzp1 was synthesized following the procedure described in Section
2.2.3. from 107.33 mg of Gd(III)(NO3)3∙6 H2O (0.2378 mmol) and
100 mg of Bzp1 (0.4757 mmol). A yellow product was obtained with
a yield of 87.7% (0.1744 g, Mp 270 °C). FT-IR (ATR, ν cm−1): 3152.9
(NeH), 3200.0 (OeH), 1610.6, 1602.4, 1596.1, 1561.4 (C=C)arom
,
1602.4 (C=N), 1289.0 (CeN), 1259.2 (CeO), 737.6 (CeH)arom; NO3
bands: 1462.5 (ν1), 1401.3 (ν0), 1304.9 (ν4), 1022.0 (ν2), 839.9 (ν6),
2.2.3.11. [Eu(III)(Bzp2)2(NO3)2]NO3∙5 H2O (EuBzp2). The compound
EuBzp2 was synthesized following the procedure described in Section
2.2.3. from 95.46 mg of Eu(III)(NO3)3∙5 H2O (0.2230 mmol) and
818.0 (ν3), 800.3 (ν5). (+)TOF m/z (amu): Calc. for (C26H20N6O8Gd)+
:
702.0583. Found: 702.0579. Anal. Calc. for C26H28GdN7O15: C, 37.4; H,
3

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
100 mg of Bzp2 (0.4460 mmol). A pink product was obtained with a
yield of 85.2% (0.1666 g, Mp 247 °C). FT-IR (ATR, ν cm−1): 3236.3
sulphoxide (DMSO) and were applied to the cell monolayer by dupli-
cate. The complexes that showed moderate selectivity were also studied
at higher concentrations (150, 200, 250, μM). 1% Sodium dodecyl
sulfate (SDS) was employed as positive control of cell death (0% of cell
viability) and 1% DMSO was used as the negative control (100% of cell
viability). The incubation was carried out at 37 °C with 5% CO2 for
72 h, and subsequently the cell growth was evaluated by the sul-
phorhodamine B assay (SRB).
(NeH), 3410.2 (OeH), 1625.9, 1612.8, 1584.4, 1560.4 (C=C)arom
,
1625.9 (C=N), 1300.2 (CeN), 1243.0 (CeO), 744.8 (CeH)arom; NO3
bands: 1403.9 (ν1), 1376.4 (ν0), 1242.8 (ν4), 1044.3 (ν2), 849.2 (ν6),
822.3 (ν3), 803.1 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8Eu)+
:
725.0868. Found: 725.0873. Anal. Calc. for C28H34EuN7O16: C, 38.4; H,
3.9; N, 11.2. Found: C, 38.2; H, 3.7; N, 11.0.
The SRB assay was carried out as previously described by Papazisis
et al. [82]. Briefly, the culture medium was suctioned prior to fixation
and 50 μL of 10% cold trichloroacetic acid (4 °C) was gently added to
the wells. Microplates were left for 30 min at 4 °C, washed five times
with deionized water and left to dry at room temperature for at least
24 h. Subsequently, 50 μL 0.4% (w/v) SRB (Sigma) in 1% acetic acid
solution was added to each well and left at room temperature for
20 min. The SRB was removed and the plates were washed five times
with 1% acetic acid before air-drying. The bound SRB was dissolved
with 50 μL of 10 mM non-buffered Tris-base solution, and microplates
were placed on microplate shaker for at least 10 min. Absorbance was
read at 530 nm by subtracting the 620 nm background measurement.
The results obtained define the viability percentage. The half maximal
inhibitory concentration (IC50) was expressed as the concentration that
reduces the optical density (OD530) of treated cells to 50% with respect
of the untreated cells. The IC50 value was established as the con-
centration of the Ln(III) complex where percent inhibition was equal to
50% and was the mean of at least two independent experiments.
2.2.3.12. [Gd(III)(Bzp2)2(NO3)2]NO3∙3 H2O (GdBzp2). The compound
GdBzp2 was synthesized following the procedure described in Section
2.2.3. from 100.65 mg of Gd(III)(NO3)3∙6 H2O (0.2230 mmol) and
100 mg of Bzp2 (0.4460 mmol). A pink product was obtained with a
yield of 90.8% (0.1712 g, Mp 194 °C). FT-IR (ATR, ν cm−1): 3100.0
(NeH), 3355.1 (OeH), 1624.4, 1609.8, 1561.2, 1485.3 (C=C)arom
,
1624.4 (C=N), 1296.3 (CeN), 1242.8 (CeO), 743.8 (CeH)arom; NO3
bands: 1402.8 (ν1), 1375.4 (ν0), 1242.6 (ν4), 1041.9 (ν2), 848.8 (ν6),
822.4 (ν3), 807.0 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8Gd)+
:
730.0896. Found: 730.0901. Anal. Calc. for C28H30GdN7O14: C, 39.8; H,
3.6; N, 11.6. Found: C, 39.7; H, 3.8; N, 11.7.
2.2.3.13. [Tb(III)(Bzp2)2(NO3)2]NO3∙3 H2O (TbBzp2). The compound
TbBzp2 was synthesized following the procedure described in Section
2.2.3. from 101.02 mg of Tb(III)(NO3)3∙6 H2O (0.2230 mmol) and
100 mg of Bzp2 (0.4460 mmol). A pink product was obtained with a
yield of 90.8% (0.1716 g, Mp 249 °C). FT-IR (ATR, ν cm−1): 3233.3
(NeH), 3355.1 (OeH), 1625.2, 1612.1, 1561.3, 1487.7 (C=C)arom
,
1625.2 (C=N), 1314.1 (CeN), 1242.9 (CeO), 743.8 (CeH)arom; NO3
bands: 1403.5 (ν1), 1375.9 (ν0), 1242.8 (ν4), 1043.2 (ν2), 849.7 (ν6),
2.3.3. Erythrocytes lysis assay
The procedure reported by Ramos-Ligonio et al., in 2012 was uti-
lized with some modifications that are mentioned below. Human blood
was extracted from a healthy patient by peripheral venous puncture.
Subsequently, each of the samples was centrifuged, separating the
plasma and blood cells. The red blood cells were rinsed twice with PBS
(phosphate buffer solution, pH 7.4). A 2% suspension of human red
blood cells was prepared and 200 μL were incubated at 37 °C for 6 h
with the tested compounds: NdBzp2, GdBzp2, TbBzp2 and DyBzp2, at
different concentrations (1, 10, 25, 50, 100 μM). All samples were
centrifuged at 1000 ×g for 10 min. The absorbance of the supernatant
was estimated at 540 nm. Haemolysis inhibition was calculated ac-
cording to the equation described below [83].
822.9 (ν3), 804.2 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8Tb)+
:
731.0909. Found: 731.0913. Anal. Calc. for C28H30TbN7O14: C, 39.7; H,
3.6; N, 11.6. Found: C, 39.5; H, 3.5; N, 11.9.
2.2.3.14. [Dy(III)(Bzp2)2(NO3)2]NO3∙4 H2O (DyBzp2). The compound
DyBzp2 was synthesized following the procedure described in Section
2.2.3. from 101.82 mg of Dy(III)(NO3)3∙6 H2O (0.2230 mmol) and
100 mg of Bzp2 (0.4460 mmol). A pink product was obtained with a
yield of 88.0% (0.1706 g, Mp 192 °C). FT-IR (ATR, ν cm−1): 3200.0
(NeH), 3413.0 (OeH), 1625.2, 1561.3, 1487.5 (C=C)arom, 1625.2
(C=N), 1303.6 (CeN), 1243.4 (CeO), 745.1 (CeH)arom
; NO3
bands:1403.4 (ν1), 1376.0 (ν0), 1242.9 (ν4), 1042.6 (ν2), 849.7 (ν6),
Ap As
Ap An
822.4 (ν3), 806.2 (ν5). (+)TOF m/z (amu): Calc. for (C28H24N6O8Dy)+
:
Haemolysis inhibition (%) =
× 100
736.0947. Found: 736.0954. Anal. Calc. for C28H32DyN7O15: C, 38.7; H,
3.7; N, 11.3. Found: C, 38.7; H, 3.3; N, 11.1.
where As, Ap and An were the absorbance of the test sample, the po-
sitive control (1% Triton X-100), and the negative control (1% DMSO)
respectively. These results were the mean of at least two independent
experiments.
2.3. Cytotoxicity assays
2.3.1. Cell lines
Cytotoxic studies were executed in three cell lines: HeLa (ATCC CLL-
2) cervical adenocarcinoma epithelial, NIH-3T3 (ATCC CRL-1658)
embryonic fibroblast and J774A.1 (ATCC TIB-67) monocyte macro-
phages. Cells were placed in DMEM (Dulbecco's modified Eagle's
medium). The medium was supplemented with L-glutamine (2 mM) and
10% fetal bovine serum. The cells were incubated at 37 °C for six days,
in 5% CO2 and 95% relative humidity.
2.3.4. Statistical analysis
Two replicates were performed for each experiment in each of the
conditions tested, and to ensure reproducibility the experiments were
made twice. In the statistical analysis, the Dunnet tests and the one-way
analysis of variance (ANOVA) were employed and the statistical sig-
nificance was defined as p < 0.05. Statistical analysis for the calcula-
tion of the IC50, as well as the comparison of cell growth inhibition
curves, were performed with the GraphPad Prism version 5.04 program
applying a non-linear regression analysis [84].
2.3.2. Sulphorhodamine (SRB) assay
Biological studies were performed on a NuAire IR autoflow 5500
water-jacketed automatic CO2 incubator. The photometric microplates
were read in a photometer for Thermo Scientific Multiskan EX micro-
plates.
2.4. Computational details
The optimized geometries and vibrational modes of the La(III)
complexes were obtained using the hybrid method B3LYP with the
pseudopotential LANL2DZ [85–87] employing Gaussian 03 software
package [88]. All the frequencies presented positive values, so we ob-
tained equilibrium conformers for the considered complexes. The ana-
lysis of the IR vibration frequencies for the La(III) complexes was
96-well microtiter plates were used, where the cells were seeded
(5 × 106 cells per well) and allowed to proliferate for 48 h in dulbecco's
modified eagle medium (DMEM) containing 10% fetal bovine serum.
The two ligands and their fourteen Ln(III) complexes were studied using
different concentrations (1, 10, 25, 50, 100 μM) in 1% dimethyl
4

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
such as: HETCOR, COSY and APT. Chemical shift, integration and
constant couplings were not discussed here, since other authors had
previously reported them [10–50].
The IR absorptions of the coordination complexes showed slight
changes with respect to those of the ligands, indicating that they are
coordinated to the lanthanides through iminic nitrogen and phenolic
oxygen. The most representative stretching bands are: ν (C=N) ~
1624 cm−1, ν (CeN) ~ 1281 cm−1 and ν (CeO) ~ 1241 cm−1 for the
complexes containing Bzp1; ν (C=N) ~ 1625 cm−1, ν (CeN) ~
1300 cm−1 and ν (CeO) ~ 1242 cm−1 for the complexes containing
Bzp2. The strong vibration due to ν (OeH) bond was observed between
3200 and 3400 cm−1, confirming that the hydroxyl groups do not de-
protonate. The spectra of the compounds displayed the characteristic
absorptions of the bidentate NO3 groups (C2ν): ~ 1458 cm−1 (ν1), ~
1289 cm−1 (ν4), ~ 1022 cm−1 (ν2) and ~ 815 cm−1 (ν3) for the com-
plexes containing Bzp1; ~ 1408 cm−1 (ν1), ~ 1242 cm−1 (ν4), ~
1043 cm−1 (ν2) and ~ 822 cm−1 (ν3) for the complexes containing
Bzp2. The difference in wave number between the two most intense
absorptions of the bidentate NO3 group (ν1 and ν4) provided values
between 160 and 170 cm−1, confirming that two NO3 groups (C2ν) are
coordinated to the Ln(III) ions in a bidentate mode [51–53,93]. The
intense stretching band due to ν4 (NO3 group) overlaps the less intense
stretching bands of ν (CeN) and ν (CeO). In comparison to an unbound
NO3 group (D3h), the characteristic absorptions can be seen in the
spectra of the complexes: ~ 1393 cm−1 (ν0), ~ 840 cm−1 (ν6) and ~
798 cm−1 (ν5) for the complexes containing Bzp1; ~ 1376 cm−1 (ν0), ~
849 cm−1 (ν6) and ~ 804 cm−1 (ν5) for the complexes containing Bzp2,
Table 1. These three less intense bands denote that one NO3 group (D3h
)
is counterion in the Ln(III) complexes. Therefore, the results obtained
by FT-IR agree with the experimental evidence obtained by molar
conductivity measured in methyl alcohol, characteristic for 1:1 elec-
trolytes, Table 2.
The solid-state emission, excitation and absorption spectra at room
temperature of the chromophores (Bzp1 and Bzp2) were recorded and
the latter two are shown in Fig. 1. The excitation spectra showed two
bands at 348 nm (28,735 cm−1) and 397 nm (25,189 cm−1) for Bzp1,
347 nm (28,818 cm−1) and 396 nm (25,252 cm−1) for Bzp2. The ab-
sorption spectra of the Ln(III) complexes presented a broad band in the
UV-region similar to the absorption maxima of the ligands. The Nd(III),
Sm(III) and Dy(III) intraconfigurational 4f–4f transitions were observed
in the diffuse reflectance for the complexes with Bzp1 and Bzp2, the
assignments are displayed in Table 3 and Fig. 2. The emission spectra of
the ligands showed the band assigned to the 1S1 → 1S0 transition:
455 nm (21,978 cm−1) for Bzp1, and 440 nm (22,727 cm−1) for Bzp2.
The maximum emission does not change for each ligand, whether the
sample is excited at 348 or 397 nm for Bzp1, or either at 347 or 396 nm
for Bzp2; however, the intensity of the luminescence varies upon the
Scheme 1. Reaction of the benzimidazolyl-phenol ligands with Ln(III)(NO3)3∙n-
H2O.
performed using the same level of theory (B3LYP/LANL2DZ), com-
paring theoretical versus experimental frequencies using vibrational
scale factors [89,90]. The graphical results were obtained with the
programs: Chemcraft version 1.8 (compilation 536b) [91] and Diamond
version 4.5.1 [92].
3. Results and discussion
3.1. Synthesis of the compounds
Fourteen novel complexes of formula [Ln(III)(L)2(NO3)2]NO3∙n-H2O
(L = Bzp1 or Bzp2; Ln(III) = La, Nd, Sm, Eu, Gd, Tb, Dy) were syn-
thesized in high yields from the reaction between the chromophores
(Bzp1 or Bzp2) and seven Ln(III)(NO3)3·n-H2O in a [2:1] stoichiometry,
Scheme 1. The Ln(III) complexes are reported for the first time in this
article; however, the synthesis of the ligands (Bzp1 and Bzp2) had al-
ready been previously reported in the literature in different reaction
conditions by different authors [10–50]. A proposed reaction me-
chanism for the synthesis of the chromophores is included in the sup-
plementary information, Scheme S1. All Ln(III) complexes are com-
pletely soluble in solvents such as dimethyl sulfoxide, ethanol and
methanol. In addition, they are partially soluble in solvents such as
acetone, acetonitrile, chloroform, dichloromethane and water.
Table 1
NO3 stretching bands (cm−1) for all Ln(III) complexes.
Compound NO3−, C2ν (cm−1
)
NO3−, D3h (cm−1
)
ν1
ν4
ν2
ν3
ν1–ν4 ν0
ν6
ν5
LaBzp1
NdBzp1
SmBzp1
EuBzp1
GdBzp1
TbBzp1
DyBzp1
LaBzp2
NdBzp2
SmBzp2
EuBzp2
GdBzp2
TbBzp2
DyBzp2
1458.4 1289.5 1022.5 815.3 169
1457.1 1289.3 1022.3 815.2 168
1459.8 1290.1 1022.3 818.9 170
1458.7 1289.7 1022.2 817.6 169
1462.5 1304.9 1022.0 818.0 158
1459.0 1289.9 1022.0 814.3 169
1410.0 1242.2 1020.0 817.0 168
1408.2 1241.9 1042.2 820.0 166
1403.9 1243.0 1043.1 830.0 161
1402.9 1242.8 1043.8 822.4 160
1403.9 1242.8 1044.3 822.3 161
1402.8 1242.6 1041.9 822.4 160
1403.5 1242.8 1043.2 822.9 161
1403.4 1242.9 1042.6 822.4 161
1393.4 840.0 798.7
1393.8 839.7 799.1
1392.6 839.2 798.6
1392.8 835.0 798.2
1401.3 839.9 800.3
1390.1 830.0 799.2
1386.8 840.0 798.0
1395.0 849.7 805.8
1376.4 850.0 804.6
1375.9 848.8 804.3
1376.4 849.2 803.1
1375.4 848.8 807.0
1375.9 849.7 804.2
1376.0 849.7 806.2
3.2. Spectroscopic studies of the compounds
The chemical structure of the chromophores (Bzp1 and Bzp2) were
confirmed by NMR at room temperature with DMSO‑d6, 1H and 13C
chemical shifts were unmistakably labelled by specific experiments,
5

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
Table 2
diamagnetic, due to the absence of electrons in the 4f orbitals, while the
other coordination complexes are paramagnetic, as expected. The
magnetic moment values obtained agree with those described for ana-
logous complexes previously reported, confirming the non-participation
of the 4f electrons in the coordination bonds, due to the shielding fa-
cilitated by the 5s25p6 electrons [55,72].
Effective magnetic moment and electrical conductivity in solution for all Ln(III)
complexes.
Compound
μeff (B.M.)
μS/cm (10−3 M)
LaBzp1
NdBzp1
SmBzp1
EuBzp1
GdBzp1
TbBzp1
DyBzp1
LaBzp2
NdBzp2
SmBzp2
EuBzp2
GdBzp2
TbBzp2
DyBzp2
0
87.42
82.73
95.08
91.13
96.36
95.07
97.45
89.77
85.48
93.34
90.55
95.33
94.91
96.25
3.55
1.58
3.42
7.93
9.66
10.42
0
3.4. Cytotoxic activity of the compounds
The ability of the fourteen complexes and the two ligands to inhibit
cell growth was evaluated by in vitro assay performed on HeLa (cervical
adenocarcinoma epithelial) cells. These cells are currently studied as
potential target of several cytotoxic metal complexes different from
platinum, for example transition metals and rare earths. The cytotoxic
effect on HeLa was evaluated using the SRB assay for all the com-
pounds, comparing the results with those of two other cell lines: fi-
broblasts (NIH-3T3) and macrophages (J774A.1). The two ligands
(Bzp1 and Bzp2), all the Ln(III) complexes derived from Bzp1 (with La,
Nd, Sm, Eu, Gd, Tb and Dy), as well as three of the complexes derived
from Bzp2 (with La, Sm and Eu), showed cell viability lower than 50%
for HeLa and NIH-3T3 mainly, indicating that these compounds are
highly cytotoxic. Figs. S1–S4.
3.60
1.42
3.31
7.97
9.73
10.54
excitation wavelength, as shown in Fig. 1. The spectra of most Ln(III)
complexes obtained herein only show broad emission bands similar to
those of the ligands, see Table 3. This result indicates that there is no
energy transfer from either of the ligands to the metal ions.
Four compounds derived from Bzp2 (NdBzp2, GdBzp2, TbBzp2 and
DyBzp2) showed interesting cytotoxic behavior. Three of them
(GdBzp2, TbBzp2 and DyBzp2) were moderately selective against HeLa
cells and did not cause death of NIH-3T3 and J774A.1 cells. The above
is an important advantage, since these compounds could be employed
as adjuvants in chemoterapy treatments, as long as their activity was
maintained in vivo. They would not induce tissue damage and would
keep the macrophage population intact, decreasing the side effects
shown by the treatments currently employed. It was observed that the
cytotoxic effect of these three complexes in the HeLa cell line does not
show a dose-dependent behavior. In addition, NdBzp2 was not selective
and lysed HeLa and J774A.1 cells indistinctly, which is not convenient,
since it destroys both adenocarcinoma cells and defense cells, Fig. 3.
The stability in solution of these four complexes (NdBzp2, GdBzp2,
TbBzp2 and DyBzp2) was evaluated in order to support the results of
cytotoxicity. The details are shown in the supplementary information.
Tables S1–S4.
3.3. Thermogravimetric analysis and magnetic susceptibility
The thermal stability of the compounds was determined by choosing
a representative compound of each series of isostructural complexes
derived from Bzp1 and Bzp2. The selected compounds were the Nd(III)
and Gd(III) derivatives respectively (NdBzp1 and GdBzp2), the ther-
mogravimetric analysis (TGA) data were obtained by gradually in-
creasing the temperature 10 °C per minute. The thermograms of the Ln
(III) complexes are listed in Table 4.
The complexes NdBzp1 and GdBzp2 undergo a similar disintegra-
tion, which is carried out in three steps. The first step occurs in the
range of 70–105 °C and corresponds to the loss of three water molecules
of the crystal lattice. The second and the third step of the decomposition
correspond to the loss of two NO3 coordinated to the Ln(III) ion and one
NO3 of the crystal lattice in the temperature ranges of 175–230 °C and
230–275 °C respectively. Finally, the Ln(III) complexes decompose
completely losing the benzimidazolyl-phenol ligands in the temperature
range of 300–500 °C and the residues are rare earth oxides. The results
showed a thermal stability similar to other compounds reported in the
literature, and this discard the presence of water molecules coordinated
to the Ln(III) ion [55–57,72–74].
According to the selectivity shown, an effective response was ob-
served for NdBzp2, GdBzp2, TbBzp2 and DyBzp2 in HeLa cells, while
the effect was smaller in the NIH-3T3 and J774A.1 cell lines. It is im-
portant to note that the four compounds that displayed cytotoxic ac-
tivity in the SRB assay are functionalized in the C5 position, as in
several commercial benzimidazole-derived drugs. The selectivity allows
us to suppose that the activity shown would favor the activation of the
immune system, capable of attacking only damaged tumor cells in the
same way that occurs in vivo; however, this could only be confirmed by
evaluating the effects of the Ln(III) complexes in the cell cycle and their
The effective magnetic moments of the Ln(III) complexes were de-
terminate by the Gouy's method and correspond to the anticipated
values for the Ln(III) ions [51–54], see Table 2. The values of the ex-
perimental magnetic moment indicate that the complexes of La(III) are
Fig. 1. Solid-state excitation and emission spectra of the chromophores Bzp1 (left) and Bzp2 (right).
6

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
Table 3
Absorption and emission data for all Ln(III) complexes.
Compound
Absorption
Emission
λ (nm)
Compound
Absorption
Emission
λ (nm)
λ (nm)
λ (nm)
L1
362, 1S1
364, 1S1
←
←
←
1S0
455, 1S1
393, 1S1
395, 1S1
→
1S0
1S0
1S0
L2
368, 1S1
←
←
←
1S0
440, 1S1
420, 1S1
431, 1S1
→
1S0
1S0
1S0
LaBzp1
NdBzp1
1S0
→
→
LaBzp2
NdBzp2
371, 1S1
1S0
→
→
361, 1S1
1S0
367, 1S1
1S0
4
4
742, F7/2
,
4S3/2
2H9/2
←
←
4I9/2
741, F7/2
,
4S3/2
2H9/2
←
←
4I9/2
4
4
797, F5/2
,
4I9/2
797, F5/2
,
4I9/2
4
4
865, F3/2
←
4I9/2
863, F3/2
←
4I9/2
SmBzp1
364, 1S1
←
1S0
438, 1S1
→
1S0
SmBzp2
371, 1S1
←
1S0
441, 1S1
→
1S0
6
6
941, F11/2
←
6H5/2
938, F11/2
←
6H5/2
6
6
1086, F9/2
←
6H5/2
1079, F9/2
←
6H5/2
EuBzp1
GdBzp1
TbBzp1
DyBzp1
367, 1S1
383, 1S1
385, 1S1
←
←
←
←
1S0
1S0
1S0
1S0
437, 1S1
446, 1S1
445, 1S1
447, 1S1
→
→
→
→
1S0
1S0
1S0
1S0
EuBzp2
GdBzp2
TbBzp2
DyBzp2
378, 1S1
367, 1S1
378, 1S1
←
←
←
←
1S0
1S0
1S0
1S0
438, 1S1
401, 1S1
422, 1S1
426, 1S1
→
→
→
→
1S0
1S0
1S0
1S0
362, 1S1
376, 1S1
6
6
798, F5/2
←
←
6H15/2
6H15/2
797, F5/2
←
←
6H15/2
6H15/2
6
6
903, F7/2
900, F7/2
6
6
1098, F9/2
,
6H7/2
←
6H15/2
1095, F9/2
,
6H7/2
←
6H15/2
possible immunomodulatory activity.
Table 4
Thermogravimetric data for NdBzp1 and GdBzp2.
In order to evaluate the cytotoxicity of the four complexes in detail,
their IC50 values were determined. The IC50 value (half maximal in-
hibitory concentration), expressed in μM, is the concentration of the Ln
(III) complex that affords a 50% reduction in cell growth and the results
were displayed in Table 5. The IC50 value of the new complexes in HeLa
cells is higher compared to the IC50 of some commercial anticancer
agents, such as noscapine (22 μM), estramustine (1.5–3.0 μM) and cis-
platin (8 μM) [55–57]. However, their cytotoxic effect is equal or better
than some shown by other previously reported Ln(III) complexes
[60–62,67–74]. This improvement can be attributed to the presence of
the benzimidazolic group in the compounds synthesized herein. In
addition, with the exception of cisplatin, there is little information on
how metal drugs work against cancer, but it is clear that the presence of
different metal ions favors the cellular response [68]. Ln(III) ions can
suppress the absorption of Ca(II), Mg(II), Fe(II) or Mn(II), thus in-
hibiting Ca(II)-dependent enzymes, or else inhibiting the formation of
free radicals or ROS, as previously mentioned [69–75,94]. However,
additional experiments would be necessary to support this assumption.
Compound
Temperature
% Weight
Process
range (°C)
loss obs. (calcd)
NdBzp1
70–105
175–230
230–300
> 300
6.95 (6.71)
Loss of three lattice H2O
Loss of one lattice NO3
Loss of two coord. NO3
Loss of organic moiety
Loss of three lattice H2O
Loss of one lattice NO3
Loss of two coord. NO3
Loss of organic moiety
7.85 (7.70)
14.88 (15.40)
51.93 (52.24)
6.50 (6.37)
GdBzp2
70–105
175–230
230–300
> 300
6.82(7.31)
14.90 (14.62)
52.55 (52.90)
1% Triton X-100 exhibited 100% haemolysis, whereas 1% DMSO did
not cause haemolysis. Fig. 4.
3.6. Optimized structures and vibrational frequencies calculated by density
functional theory (DFT)
The structural arrangement and geometry were determined for a
representative compound of each series of the isostructural complexes
derived from Bzp1 and Bzp2. The compounds selected were LaBzp1
and LaBzp2. DFT calculations indicate that coordination complexes
contain an octacoordinated La(III) ion bonded to two bidentate ligands
(Bzp1 and Bzp2) and two bidentate nitrate ions, with a distorted do-
decahedral geometry (snub disphenoid) commonly observed for La(III)
ions. The local coordination environment around the La(III) ion is de-
picted in Fig. 5, where five of the six oxygen atoms are placed in the
same plane, while the two nitrogen atoms are placed in the apical po-
sitions of the dodecahedron. The data obtained by DFT calculations
allow proposing a structure for the Ln(III) complexes, supported by
3.5. Percentage of haemolysis inhibition of the compounds
It is important to note that when looking for new compounds that
have a potential anticancer effect, these must have high cytotoxic se-
lectivity and should not cause damage to mammalian host cells. In
order to validate the selectivity in the cytotoxic assay, a cytotoxicity test
was implemented in human red blood cells. The results show that the
Ln(III) complexes used (NdBzp2, GdBzp2, TbBzp2 and DyBzp2) do not
affect the red blood cells in the concentrations in which they cause
cytotoxicity in the HeLa cell line. These four compounds did not show
cytotoxic effect against human erythrocytes. In addition, the solution of
Fig. 2. Diffuse reflectance spectra of the complexes with Nd(III), Sm(III) and Dy(III). See Table 3 for the assignment of the 4f–4f transitions.
7

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
Fig. 3. Effect of Ln(III) complexes on cell viability. The
NIH-3T3, J774A.1 and HeLa cell lines were cultured with
different concentrations of NdBzp2, GdBzp2, TbBzp2 and
DyBzp2. 1% SDS corresponds to 0% viability, while 1%
DMSO corresponds to 100% viability. Values shown are
the mean SD of duplicate cultures. The data are re-
presentative of the results of two independent experi-
ments. *Displays statistically significant differences
(Dunnet, p < 0.001).
analytical and spectroscopic evidence.
Table 5
Half-maximal inhibitory concentration (IC50) for NdBzp2, GdBzp2, TbBzp2
The calculated IR spectra of the complexes displayed the vibration
frequencies of the bidentate NO3 groups: 1489 cm−1 (ν1), 1267 cm−1
(ν4), 1098 cm−1 (ν2) and 809 cm−1 (ν3) for LaBzp1; 1488 cm−1 (ν1),
1267 cm−1 (ν4), 1097 cm−1 (ν2) and 808 cm−1 (ν3) for LaBzp2. In
addition, the vibration frequencies of the anionic NO3 groups were
showed in: 1403 cm−1 (ν0), 841 cm−1 (ν6) and 798 cm−1 (ν5) for
LaBzp1; 1398 cm−1 (ν0), 850 cm−1 (ν6) and 799 cm−1 (ν5) for LaBzp2.
and DyBzp2.
Compound
Cell lines, IC50 (μM)a
NIH-3T3
J774A.1
HeLa
NdBzp2
GdBzp2
TbBzp2
DyBzp2
16.01
85.30
72.85
59.26
2.79
3.35
3.64
5.43
57.16
24.92
84.13
67.99
6.59
4.47
3.74
5.36
30.90
13.54
7.90
4.01
1.96
1.21
13.00
4.01
4. Conclusions
a
IC50 values are presented as the mean
from two separate experiments.
standard deviation of the mean
An efficient and simple method for the synthesis of [Ln(III)
(L)2(NO3)2]NO3∙n-H2O complexes with benzimidazolyl-phenol ligands
is discussed herein. The Bzp1 and Bzp2 chromophores and the Ln(III)
complexes were identified by several analytical and spectroscopic
methods. The comparative analysis of the FT-IR spectra of the chro-
mophores and their Ln(III) complexes shows that the stretching bands
assigned to the organic fragment move when they coordinate with the
Ln(III) ions, confirming the bidentate nature of the ligands. All com-
pounds are ionic. The TGA data showed that two NO3 groups are co-
ordinated of bidentate manner to the Ln(III) ion and are part of the
coordination sphere, while one NO3 group is outside the coordination
sphere and compensates the charge of the complex. In addition, a de-
composition process of the Ln(III) complexes was observed in three
stages.
The cytotoxic activity of the Ln(III) complexes and their benzimi-
dazolyl-phenol ligands was tested by the SRB assay in order to have
new drugs candidates against cancer. Cell cytotoxicity studies of these
complexes on HeLa, NIH-3T3 and J774A.1 cells indicate that they have
the potential to act as effective anticancer drugs. The results indicate
that only four (NdBzp2, GdBzp2, TbBzp2 and DyBzp2) of the fourteen
rare earth complexes possess different degrees of inhibition in HeLa
cells. The Ln(III) complexes (GdBzp2, TbBzp2 and DyBzp2) exhibited
cytotoxic activity towards cervical adenocarcinoma epithelial cells
(HeLa), and showed a minor cytotoxic effect in structure/support cells
(NIH-3T3) and defense cells (J774A.1), but only TbBzp2 had lower IC50
values than cisplatin. The results obtained in this study provide an
experimental basis for the development of new anticancer drugs based
on rare earths with low cytotoxicity.
Fig. 4. Percentage of haemolysis inhibition in the interaction of the NdBzp2,
GdBzp2, TbBzp2 and DyBzp2 complexes in different concentrations with
human erythrocytes. 1% DMSO corresponds to 0% haemolysis, while 1% Triton
X-100 corresponds to 100% haemolysis. The data are representative of the re-
sults of two independent experiments.
8

A. Hernández-Morales, et al.
JournalofInorganicBiochemistry201(2019)110842
Fig. 5. Lowest energy optimized structures with symmetry C2 of LaBzp1 and LaBzp2 by B3LYP/LANL2DZ.
Acknowledgements
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We greatly appreciate the academic support of our institutions: UV,
UNAM, and CINVESTAV-IPN, as well as the financial support of our
projects: PRODEP/103.5/10595, CONACyT/CB/178851 and UNAM/
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Supplementary data to this article can be found online at https://
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