Thermal behavior of new nickel(II) complexes with unsaturated carboxylates and heterocyclic N-donor ligands

Article abstract of DOI:10.1007/s10973-016-5445-3

Synthesis and characterization of four new Ni(II) complexes with mixed ligands have been reported in the present paper. As interest ligands, acrylate (acr) ion together with one N-donor heterocyclic ligand, namely benzimidazole (HBzIm), 2-methylbenzimidazole (2-MeBzIm), 5-methylbenzimidazole (5-MeBzIm) or 5,6-dimethylbenzimidazole (5,6-Me₂BzIm), was chosen. The syntheses afforded the complexes formulated as [Ni(HBzIm)₂(acr)₂(H₂O)]·3H₂O (1), [Ni(2-MeBzIm)₂(acr)₂(H₂O)]·1.5H₂O (2), [Ni(5-MeBzIm)₂(acr)₂(H₂O)] (3) and [Ni(5,6-Me₂BzIm)₂(acr)₂] (4). Complexes (1) and (2) contain crystallization water molecules and coordinated water molecules, both their nature and presence being confirmed on the basis of IR spectra and thermal analysis. In contrast to above-mentioned complexes, (3) contains only coordinated water molecule, while (4) is an anhydrous compound. Based on electronic spectra and magnetic measurements, a distorted octahedral stereochemistry was proposed for all Ni(II) complexes. Acrylate ions act both as unidentate and chelate ligands in complexes (1)–(3), while in complex (4) act only as chelate ligands. All used N-donor ligands function as monodentate in all Ni(II) complexes. Biological properties of complexes (1)–(4) were evaluated against several Gram(+), Gram(?) bacterial strains and against fungus Candida albicans.

Full text of DOI:10.1007/s10973-016-5445-3

J Therm Anal Calorim
DOI 10.1007/s10973-016-5445-3
Thermal behavior of new nickel(II) complexes with unsaturated
carboxylates and heterocyclic N-donor ligands
Rodica Olar1 Ioana Dorina Vlaicu1,2 Mariana Carmen Chifiriuc3
•
•
•
Coralia Bleotu4 Nicolae Stanica Gina Vasile Scaet¸eanu Luigi Silvestro7
Constant¸a Dulea8 Mihaela Badea1
5
6
•
•
•
•
˘
˘
˘
•
Received: 30 October 2015 / Accepted: 3 April 2016
´
´
Ó Akademiai Kiado, Budapest, Hungary 2016
Abstract Synthesis and characterization of four new Ni(II)
complexes with mixed ligands have been reported in the
present paper. As interest ligands, acrylate (acr) ion together
with one N-donor heterocyclic ligand, namely benzimida-
zole (HBzIm), 2-methylbenzimidazole (2-MeBzIm),
5-methylbenzimidazole (5-MeBzIm) or 5,6-dimethylbenz-
imidazole (5,6-Me2BzIm), was chosen. The syntheses
afforded the complexes formulated as [Ni(HBzIm)2(acr)2
(H2O)]Á3H2O (1), [Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2),
[Ni(5-MeBzIm)2(acr)2(H2O)] (3) and [Ni(5,6-Me2BzIm)2
(acr)2] (4). Complexes (1) and (2) contain crystallization
water molecules and coordinated water molecules, both their
nature and presence being confirmed on the basis of IR
spectra and thermal analysis. In contrast to above-mentioned
complexes, (3) contains only coordinated water molecule,
while (4) is an anhydrous compound. Based on electronic
spectra and magnetic measurements, a distorted octahe-
dral stereochemistry was proposed for all Ni(II) com-
plexes. Acrylate ions act both as unidentate and chelate
ligands in complexes (1)–(3), while in complex (4) act
only as chelate ligands. All used N-donor ligands function
as monodentate in all Ni(II) complexes. Biological prop-
erties of complexes (1)–(4) were evaluated against several
Gram(?), Gram(-) bacterial strains and against fungus
Candida albicans.
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-016-5445-3) contains supplementary
material, which is available to authorized users.
& Mihaela Badea
e_m_badea@yahoo.com
1
Department of Inorganic Chemistry, Faculty of Chemistry,
University of Bucharest, 90-92 Panduri Str.,
050663 Bucharest, Romania
Keywords Nickel complexes Á Thermal analysis Á
Acrylate Á Benzimidazole Á Antimicrobial activity
2
National Institute of Materials Physics, Atomistilor Str.
˘
405A, POB MG-7, Magurele, 077125 Ilfov, Romania
3
Department of Microbiology, Faculty of Biology, University
of Bucharest, 1-3 Aleea Portocalelor Str., 60101 Bucharest,
Romania
Introduction
The presence of benzimidazole and its derivatives in
coordination chemistry has gained a lot of importance and
received considerable attention because of their biological
significance and interesting structural aspects [1].
4
5
Stefan S Nicolau Institute of Virology, 285 Mihai Bravu
Ave., Bucharest, Romania
Romanian Academy, ‘‘Ilie Murgulescu’’ Physical Chemistry
Institute, 202 Splaiul Independentei, 77208 Bucharest,
Romania
Benzimidazole derivatives represent an important class
of bioactive molecules that play an important role in
medical area. The benzimidazole core is found in the
structure of some drugs (albendazole, fenbendazole, thi-
abendazole, inhibitors of proton pump) used in both human
and veterinary medicine [2]. Moreover, their metal com-
plexes exhibit anticancer [3, 4], antibacterial [5], SOD
activities [6], fluorescent [7] and magnetic [4] properties.
6
Department of Soil Sciences, University of Agronomical
˘ ˘
Sciences and Veterinary Medicine, 59 Maras¸ti Str., Sector 1,
011464 Bucharest, Romania
7
8
3S-Pharmacological Consultation and Research GmbH, 1
Koenigsbergerstrasse, 27243 Harpstedt, Germany
Pharma Serv. International SRL, 52 Sabinelor Str., Sector 5,
050853 Bucharest, Romania
123

R. Olar et al.
Synthesis of Ni(II) complexes with acrylate
and benzimidazole/benzimidazole derivatives
Besides above-mentioned properties, benzimidazole-
based complexes present higher catalytic effect on the
degradation of Congo red in the Fenton-like system [8] or
efficiently catalyze the controllable polymerization of
methyl methacrylate [9].
A simple procedure was approached in order to obtain new
complexes [23, 24]: Nickel(II) acrylate was freshly pre-
pared by direct reaction between nickel(II) carbonate
hydrate (4 g, 13 mmol) (molar ratio 1:6) and anhydrous
acrylic acid (5.43 mL, 80 mmol) in methanol (250 mL) at
100 °C. Then, the freshly prepared nickel(II) acrylate (1 g,
3.66 mmol) was mixed with benzimidazolic ligand
(L = benzimidazole (HBzIm, 0.866 g, 7.33 mmol),
2-methylbenzimidazole (2-MeBzIm, 0.969 g, 7.33 mmol),
5-methylbenzimidazole (5-MeBzIm, 0.969 g, 7.33 mmol)
and 5,6-dimethylbenzimidazole (5,6-Me2BzIm, 1.072 g,
7.33 mmol)) in 1:2 molar ratio. This method afforded four
blue nickel(II) complexes labeled (1)–(4) further in the
manuscript, which present weak solubility in methanol,
dimethylsulfoxide and N,N-dimethylformamide.
The literature data revealed that both type and sub-
stituent position (1-, 2- or 5-) on benzimidazole ring are
responsible for their wide range of pharmaceutical prop-
erties and the inclusion of benzimidazole moiety in various
complexes represents an efficient strategy for designing
biologically active compounds [4, 10, 11].
Benzimidazole ‘‘6 ? 5’’ heterocyclic structure is
encountered also in the case of purines and from this
structural similarity arises the biological activities of ben-
zimidazole and its derivatives. For instance, it is possible
that these antibacterial properties of benzimidazoles to be
attributed to their competition with purines, thus leading to
the inhibition of the synthesis of bacterial nucleic acids and
proteins [12].
The new synthesized compounds (1)–(4) are blue col-
ored, stable in air and light at room temperature. Results of
elemental analyses are in good agreement with the calcu-
lated values of elements abundances in the proposed
compounds, as follows:
Furthermore, in the nature appears the most important
benzimidazole derivative, namely N-ribosyl-dimethylben-
zimidazole, which acts as axial ligand for cobalt in B12
vitamin [13].
In the literature are reported two categories of com-
plexes that possess benzimidazole moiety and carboxylate
groups: (i) mixed ligand complexes based on benzimida-
zole/benzimidazole derivative and various carboxylic acids
[6, 7, 14–16] and (ii) complexes with ligands that contain
benzimidazole core with grafted carboxylate groups [17–20],
both categories presenting interesting architectures and
properties.
[Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1) Analysis, found: Ni,
11.38; C, 47.30; H, 5.03; N, 10.85 %; calculated for
NiC20H26N4O8 (F.W. = 509.13): Ni, 11.53; C, 47.18; H,
5.15; N, 11.00 %.
[Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2) Analysis, found:
Ni, 11.38; C, 51.50; H, 5.30; N, 11.13 %; calculated for
NiC22H27N4O6.5 (F.W. = 510.16): Ni, 11.50; C, 51.79; H,
5.33; N, 10.98 %.
In our ongoing efforts to synthesize and characterize
mixed complexes with N-donor ligands and carboxylic
acids [21–24], we report herein the results related to syn-
thesis, physicochemical characterization and biological
activity of four new nickel(II) complexes with acrylate
(acr) ion and benzimidazole (HBzIm) or benzimidazole
[Ni(5-MeBzIm)2(acr)2(H2O)] (3) Analysis, found: Ni,
12.08; C, 54.30; H, 5.20; N, 11.43 %; calculated for
NiC22H24N4O5 (F.W. = 483.14): Ni, 12.15; C, 54.69; H,
5.00; N, 11.60 %.
derivatives
(2-methylbenzimidazole
(2-MeBzIm)/5-
[Ni(5,6-Me2BzIm)2(acr)2] (4) Analysis, found: Ni, 12.08;
C, 58.30; H, 5.30; N, 11.23 %; calculated for NiC24H26N4O4
(F.W. = 493.18): Ni, 11.90; C, 58.45; H, 5.31; N, 11.36 %.
methylbenzimidazole (5-MeBzIm)/5,6-dimethylbenzimi-
dazole (5,6-Me2BzIm)) as ligands.
Experimental
Materials and methods
For synthesis of complexes, all reagents were purchased and
used as received from Sigma-Aldrich, Germany. The raw
materials involved in nickel acrylate synthesis are nickel
carbonate basic hydrate (NiCO3.Ni(OH)2ÁxH2O, 99.9 %) and
anhydrous prop-2-enoic acid (acrylic acid) (C3H4O2, 99 %).
As ligands are also used benzimidazole (C7H6N2, 98 %),
2-methylbenzimidazole (C8H8N2, 98 %), 5-methylbenzimi-
dazole (C8H8N2, 98 %) and 5,6-dimethylbenzimidazole
(C9H10N2, C99 %) (Scheme 1).
Chemical analysis has been performed using a EuroEA
elemental analyzer (for C, N and H) and a Shimadzu AA
6300 spectrometer (for metallic ion).
Infrared spectra were recorded on KBr pellets with a
Bruker Tensor 37 spectrometer in the range 400–4000 cm-1
.
Electronic spectra by diffuse reflectance technique, with
Spectralon as standard, were recorded in the range
200–1500 nm, on a Jasco V670 spectrophotometer.
123

Thermal behavior of new nickel(II) complexes with unsaturated carboxylates and heterocyclic…
H
N
H
N
H
N
H
N
O
CH3
CH3
CH3
OH
N
N
N
N
CH3
(a)
(b)
(c)
(d)
(e)
Scheme 1 Structural formula of a acrylic acid, b benzimidazole (HBzIm), c 2-methylbenzimidazole (2-MeBzIm), d 5-methylbenzimidazole (5-
MeBzIm), e 5,6-diemthylbenzimidazole (5,6-Me2BzIm)
Magnetic measurements were effectuated at room tem-
perature, on a Lake Shore’s fully integrated vibrating
sample magnetometer (VSM) system 7404 and calibrated
with a Ni sphere. The molar magnetic susceptibilities were
calculated and corrected for the atomic diamagnetism.
Mass spectra were recorded with API 5000 ABSCIEX-
Canada mass spectrometer with ESI source (Turbo spray
model ABSCIEX) operating in positive mode. Samples
were dissolved in methanol at 10 lg mL-1. Molecular ions
scanning range (m/z) was 0–1250.
only culture medium) and culture controls (wells contain-
ing culture medium seeded with the microbial inoculums)
were used. The influence of the DMSO solvent was
quantified also in a series of wells containing DMSO that
was diluted accordingly with the dilution scheme used for
the complexes. The plates incubated for 24 h at 37 °C, and
MIC values considered as the lowest concentration of the
tested compound that inhibited the visible growth of the
microbial overnight cultures.
The assessment of the complexes influence on the
microbial ability to colonize an inert substratum has been
carried out by the microtiter method. The absorbance read
at 490 nm with an ELISA reader Apollo LB 911. All
biological experiments performed in triplicates.
The X-ray powder diffraction patterns were collected on
a DRON-3 diffractometer with a nickel filtered Cu Ka
˚
radiation (k = 1.5418 A) in 2h range of 10°–115°, a step
width of 0.02° and an acquisition time of 0.5 s per step.
For thermal analysis the heating curves (TG and DTA)
were recorded using a Labsys 1200 SETARAM instrument,
with a sample mass of 6–12 mg over the temperature range
of 30–900 °C, using a heating rate of 10 K min-1. The
measurements were carried out in synthetic air (flow rate
16.66 cm3 min-1) using alumina crucibles.
Cytotoxicity assay
Cell cycle assay was studied on HCT-8 cell line
(ATCCÒ CCL-244TM). This was cultivated in RPMI 1640
(Gibco, NY, SUA) supplemented with 10 % heat-inacti-
vated bovine serum and penicillin/streptomycin at 37 °C
with 5 % CO2. The adherent cells were detached, cen-
trifuged, suspended in fresh medium, counted by trypan
blue exclusion and adjusted to 5 9 104 cells mL-1 and
then were cultivated in 6-well plate and treated for 24 h
with 100 lg mL-1 compound. After treatment period, cells
were taken from the substrate, fixed in 70 % cold ethanol
for at least 30 min at -20 °C, washed twice in phosphate-
buffered saline (PBS) and then incubated 15 min at 37 °C
with RNAse A (100 lg mL-1) and 1 h with propidium
iodide (100 lg mL-1). After staining of cells with pro-
pidium iodide, the acquisition was done using Epics
Beckman Coulter flow cytometer. Data were analyzed
using FlowJo software and expressed as fractions of cells in
the different cell cycle phases [25].
The antimicrobial activities of the complexes were
determined against ATCC reference and clinical microbial
strains, i.e., Gram-positive (Staphylococcus aureus ATCC
6538, Bacillus subtilis ATCC 6683, Enterococcus faecium
E5), Gram-negative (Klebsiella pneumoniae IC 13420,
Escherichia coli ATCC 25922, Pseudomonas aeruginosa
ATCC 27857) and fungus Candida albicans 1760.
Microbial suspensions [1.5 9 108 CFU mL-1; colony-
forming units (CFU)] corresponding to 0.5 McFarland
density obtained from 15 to 18 h bacterial cultures devel-
oped on solid media were used. The antimicrobial activity
was tested on Mueller–Hinton agar medium, while a med-
ium of yeast peptone glucose was used in case of C. albi-
cans. The compounds were solubilized in DMSO, and the
starting stock solution was of 1000 lg mL-1 concentration.
Quantitative assay of the antimicrobial activity was
carried out by liquid medium microdilution method, in
96-multiwell plates, in order to determine the minimal
inhibitory concentration (MIC/lg mL-1). In this purpose,
serial twofold dilutions of the compounds ranging between
1000 and 1.95 lg mL-1 were performed in 200 lL vol-
ume of broth and each well was seeded with 50 lL
microbial inoculums. Sterility control (wells containing
Results and discussion
Previously reported studies [23, 24] evidenced that com-
plexes with carboxylate and azole-type ligands present a
great interest, their chemistry correlated with their bio-
logical potential making them future candidates for more
123

R. Olar et al.
detailed studies. This is due to the fact that imidazole
nucleus is the ‘‘heart’’ of a large number of natural and
pharmacologically active products [26]. In light of the
above mentioned, herein are presented the synthesis and
characterization of four new nickel(II) complexes which
bear acrylate and azole ligands (benzimidazole (HBzIm)/2-
methylbenzimidazole (2-MeBzIm)/5-methylbenzimidazole
(5-MeBzIm)/5,6-dimethylbenzimidazole (5,6-Me2BzIm).
These new compounds were formulated using experi-
mental techniques like elemental analysis, NIR-UV–Vis,
mass and infrared spectroscopies, magnetic moments
measurements and thermal analysis. Moreover, there was
performed an evaluation for a possible use as pharmaco-
logical agents, by antimicrobial assays and cytotoxicity
effects investigations.
carboxylate characteristic bands is defined by D criterion.
The values calculated for D criterion in the case of these
new nickel complexes (1)–(4), and based on the values the
coordination modes assigned to acrylate ligands, are pre-
senting below, in Table 2:
Both unidentate and chelate bidentate coordination
modes were highlighted in complexes (1), (2) and (3),
while for complex (4) the acrylato anion acts only as
bidentate.
The presence of water molecules in complexes (1), (2)
and (3) was evidenced by the presence of the stretching
vibration mode m(OH) around 3400 cm-1. For complexes
(1), (2) and (3), the presence of the band around 830 cm-1
assigned for the vibration mode qr(H2O) indicated the
presence of water molecules in the coordination sphere of
nickel complexes [33].
Taking into account that thermal analysis is a useful and
accessible method for achievement of molecular compo-
sitions [23, 24], thermal analysis was employed in this
paper to find quantitative information about our new nickel
complexes.
Electronic spectroscopy
The absorption maxima, the band assignment and values of
magnetic moments are summarized in Table 3. The pres-
ence in electronic spectra (see supplementary material,
Fig. 2S) of the bands around 9300, 15,500 and
25,600 cm-1 in electronic spectra of all four Ni(II) com-
plexes (1)–(4) suggests an octahedral distorted stereo-
chemistry [38, 39]. The values of magnetic moments range
between 2.80 and 3.50 BM, behavior specific to the nickel
(II) ion found in an octahedral distorted stereochemistry
Infrared spectroscopy
The characteristic infrared bands for nickel complexes (1)–
(4) and their assignments, according to the literature data
regarding analogous Ni(II) complexes [27–32], are pre-
sented in Table 1. Infrared spectra (see supplementary
material, Fig. 1S) confirmed the presence in complexes of
some functional individual groups.
3
with the ground term A1g [40, 41].
The values for crystal field parameters were calculated
¨
based on the absorption maxima using Konig’s formulas
Thus, the bands characteristic to stretching vibration of
secondary amine group, m(N–H), together with that of
methine group, m(C–H), indicates the presence of benz-
imidazole derivatives in their neutral form [33]. Further-
more, the bands associated with the imidazolic m(C=N)
group in the free ligands (1587, 1585, 1587 and
1590 cm-1) are slightly shifted toward lower energies in
the spectra of complexes (1)–(4) (1595, 1593, 1592 and
1594 cm-1), indicating that the benzimidazoles are bound
to the metals through the nitrogen atom [27, 31]. The
negative shift of m(C=N) stretch in the complexes indicates
involvement of the azomethine nitrogen in complexation
[28]. The strong band from 1245 cm-1 assigned to the
benzimidazole ring is shifted to higher energies in all
complexes spectra; this aspect together with the decrease
of its intensity indicates that benzimidazole ligands are
coordinated to metal by pyridine-type nitrogen atom in
these complexes [28].
[42]. The calculated 10Dq values for complexes (1)–(4) are
around 9300 cm-1, which are close to 9400 cm-1 value
reported in the literature for the corresponding [NiN2O4]
chromophore [38, 43].
The presence of oxygen donor atoms (originates from
acrylates) is associated with a low field which generates
crystal field parameters with low values (Table 3).
nephelauxetic parameter (b) values correspond to a high
degree of ionic character due of the interactions with
acrylate anion [42, 43], this being in agreement with our
hypothesis.
In the electronic spectra of complexes (1)–(4), in the
range 36,000–38,000 cm-1, appear bands, which can be
assigned to p–p* transitions resulted from C=N group of
benzimidazole derivatives.
The intensities and positions of absorption bands
observed in NIR-UV–Vis spectra indicated that benzimi-
dazole is cis-positioned in complex (1). Furthermore,
considering the possible hindrance induced by methyl
substituents in complexes (2), (3) and (4), the trans posi-
tion of benzimidazole derivatives could be assumed.
Acrylate coordination mode was assigned based on the
presence of two characteristic modes associated with
antisymmetric mas(COO) and symmetric ms(COO) stretch-
ing vibration modes for carboxylate group. According to
literature data [34–37], the difference between these two
123

Thermal behavior of new nickel(II) complexes with unsaturated carboxylates and heterocyclic…
Table 1 Characteristic absorption bands (cm-1) in IR spectra of complexes (1)–(4)
NaAcr
HBzIm
2-MeBzIm 5-MeBzIm 5,6-diMeBzIm (1)
(2)
(3)
(4)
–
Assignments
–
–
–
–
–
–
3440 w 3468 m 3420 w
m (OH2)
3113 w 3062 m
3061 w 2995 m
3011 w
3016 w
3019 m
2963 m
3110 w 3197 m 3122 w 2968 m m (CH)
m (NH)
2973 w
2969 w
2920 w
–
–
–
–
–
–
2906 w 2900 w 2915 w 2930 m mas (CH2)
2880 w 2870 w 2880 w 2864 w ms (CH2)
2880 w
–
–
2539 w 2545 w
1587 m 1585 w
2535 w
1587 m
–
2535 w
1590 m
–
–
–
–
–
d (benzene ring)
–
1595 s
1593 s
1592 vs 1594 vs m (C=N)
1571 vs
–
–
–
–
1570 m 1557 vs 1575 s
1570 s
1525 s
1490 s
–
mas (COO)
1520 m
–
–
1537 m 1535 s
1492 s
1530 s
1491 s
–
m (C=C)
–
–
1495 w 1487 m
1477 m
1495 w
1475 vs
1444 vs
1415 s
–
1499 m
1473 vs
1440 vs
1406 s
–
–
c (CH from benzene ring)
m (ring)
1455 m 1456 m
1457 vs 1450 vs
1408 vs 1416 vs
1368 m
–
–
–
1423 vs 1426 s
1424 vs 1425 vs ms (COO)
1363 s
1354 s
1362 vs
1363 m 1361 m
1345 m
1339 s
1335 s
1303 m 1295 w 1299 s
1315 m m (ring)
1300 m 1290 m
1280 m
–
–
–
–
1270 m 1279 m 1275 m 1280 m d (CH2)
–
–
–
–
–
–
–
–
–
–
–
1271 m 1271 vs
1244 s 1219 s
1281 vs
1248 vs
1190 m
1135 m
954 vs
905 w
866 m
–
1268 vs
1239 vs
1195 w
1158 s
954 vs
900 w
864 vs
–
1264 m 1265 w 1254 m 1268 m d (benzene ring)
–
1222 w
–
–
–
m (ring)
1201 m 1200 w
1132 m 1130 w
1198 w
1193 s
1197 s
d (CH from benzene ring)
d (NH)
1154 w 1130 w 1125 w
–
957 m
932 m
886 m
–
950 w
905 w
885 w
–
963 m
–
957 m
930 w
870 w
835 m
–
961 m
–
961 m
d (CH from imidazole ring)
d (benzene ring)
d (imidazole ring)
qr (OH2)
–
875 w
834 m
770 w
744 s
653 m
865 m
830 m
770 w
–
870 m
–
768 m
745 vs
625 m
770 m
734 vs
635 w
770 m
740 m
632 s
770 m
745 w
647 m
–
d (imidazole ring)
c (CH from benzene ring)
745 vs
654 m
–
653 s
653 m
c (NH) ? d (imidazole ring) ?
s (imidazole ring)
617 w
577 w
541 w
603 s
617 w
622 m
613 m
–
537 w
–
555 m
525 w
550 w
500 m
560 w
s (ring)
*vs very strong, s strong, m medium, w weak (bands intensity in FTIR spectra), HBzIm benzimidazole, 2-MeBzIm 2-methylbenzimidazole, 5-
MeBzIm 5-methylbenzimidazole, 5,6-Me2BzIm 5,6-dimethylbenzimidazole, (1)—[Ni(HBzIm)2(acr)2(H2O)]Á3H2O; (2)—[Ni(2-MeBzIm)2(-
acr)2(H2O)]Á1.5H2O; (3)—[Ni(5-MeBzIm)2(acr)2(H2O)]; (4)—[Ni(5,6-Me2BzIm)2(acr)2]; m—stretching; d—in plane bending; c—out of plane
bending; s—torsion
Mass spectra
(Fig. 1). Some other resulted fragments from complexes are
evidenced: [NiL2acr]? (m/z: 366/394/394/422), [NiLacr2
?H]? (m/z: 320/334/334/348), [NiLacr]? (m/z: 248/262/
262/276) as well as the protonated ligands [L ? H]? (m/z:
119/133/133/147).
The ESI–MS spectra recorded in positive ionization mode
contain peaks corresponding to [NiL2acr2 ? H]? ion at m/z:
438/466/466/494 for complexes (1)/(2)/(3)/(4), respectively
123

R. Olar et al.
Table 2 Coordination mode assignment for acrylate anion in complexes (1)–(4)
Complex
D(COO-)/cm-1
Coordination mode for acrylate anions
[Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1)
147
207
131
203
151
213
145
Chelate bidentate
Unidentate
[Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2)
[Ni(5-MeBzIm)2(acr)2(H2O)] (3)
[Ni(5,6-Me2BzIm)2(acr)2] (4)
Chelate bidentate
Unidentate
Chelate bidentate
Unidentate
Chelate bidentate
Table 3 NIR–UV–Vis absorption bands and calculated magnetic moments for complexes (1)–(4)
Complex
Absorption
maximum/cm-1
Assignments
Crystal field parameters
lef (B.M.)
10Dq/cm-1
B/cm-1
b
[Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1)
36,360
25,640
15,500
13,500
9300
p ? p*
9302
883
0.85
3.20
3
3A1g ? T1g (P)
3A1g ? T1g
3
3A1g ? Eg
1
3A1g ? T2g
3
[Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2)
[Ni(5-MeBzIm)2(acr)2(H2O)] (3)
[Ni(5,6-Me2BzIm)2(acr)2] (4)
37,735
25,640
14,815
13,900
9260
p ? p*
9302
9216
9302
829
899
903
0.80
0.87
0.88
3.03
3.04
2.82
3A1g ? T1g (P)
3
3A1g ? T1g
3
3A1g ? Eg
1
3A1g ? T2g
3
36,360
25,315
14,815
13,500
9220
p ? p*
3A1g ? T1g (P)
3
3A1g ? T1g
3
3A1g ? Eg
1
3A1g ? T2g
3
38,460
36,360
25,975
15,500
13,515
9300
p ? p*
3A1g ? T1g (P)
3
3A1g ? T1g
3
3A1g ? Eg
1
3A1g ? T2g
3
These data are in good agreement with the results
obtained by elemental and thermal analysis, confirming the
proposed formulation of complexes.
arrangement around nickel ion. For all nickel complexes,
the blue color was ascribed to [NiN2O4] chromophore.
In the case of complex (1), two benzimidazoles that act
as monodentate are cis-positioned, one acrylate is uniden-
tate and the other one is bidentate chelate. One water
molecule completes the coordination sphere.
Proposed coordination for nickel(II) complexes
(1)–(4)
For complexes (2) and (3), the coordination sphere is
made up of 2-methylbenzimidazole/5-methylbenzimida-
zole trans-positioned units, one acrylate that acts uniden-
tate, other one bidentate chelate, and one water molecule.
Evaluating the results obtained from experimental data, the
most probable coordination for complexes (1)–(4) is that
presented in Fig. 2. All complexes present an octahedral
123

Thermal behavior of new nickel(II) complexes with unsaturated carboxylates and heterocyclic…
438.0
320.3
300.1
366.2
282.2
247.9
250
119.2
399.6
400
79.2
341.3
50
100
150
200
300
350
450
500
550
600
m/z/Da
Fig. 1 ESI–MS spectrum of complex [Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1)
Fig. 2 Proposed coordination
for nickel(II) complexes (1)–(4)
HN
CH3
CH3
HN
R'
HN
R
N
O
O
O
N
N
Ni
N
Ni
N
O
O
O
O
O
N
O
H2O
Ni
O
O
R
O
HN
R'
OH2
NH
H3C
H3C
NH
(2) R: CH3; R': H;
(3) R: H; R': CH3
(1)
(4)
Complex (4) is anhydrous and contains two 5,6-
dimethylbenzimidazole molecules trans-positioned and
two acrylates coordinated in a bidentate chelate fashion.
analysis of the final residue, cf JCPDS Card No. 47-1049
(see supplementary material, Fig. 3S).
Complex [Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2)
Thermal analysis
It can ascertain six well-defined decomposition steps in the
thermal behavior of complex (2) (Fig. 4). The first two
steps, in the 90–260 °C temperature range, correspond to
water molecules loss. First are released the uncoordinated
water molecules (calc. mass loss 5.3 %, exp. 5.4 %) [30, 31].
The compound resulted in the first step [Ni(2-Me
BzIm)2(acr)2(H2O)] is stable on a 30 °C temperature range.
The second decomposition step is assigned to the final
dehydration process (calc. mass loss 3.5 %, exp. 3.6 %),
and the anhydrous compound resulted in this step is
stable on a 20 °C temperature range. Above 280 °C, one
molecule of 2-methylbenzimidazole is released in an
oxidative degradation process accompanied by an
exothermic effect (calc. mass loss 25.9 %, exp. 25.8 %); in
the temperature range 380–545 °C, the second 2-methyl-
benzimidazole is released (calc. mass loss 25.9 %, exp.
25.7 %), process accompanied also by an exothermic
effect. Above 545 °C, the conversion of nickel acrylate to
nickel carbonate occurs (calc. mass loss 16.1 %, exp.
16.3 %). Starting with 700 °C, one can observe the last
thermal decomposition step being represented by decom-
position of nickel carbonate to nickel oxide (calc. mass loss
8.6 %, exp. 8.4 %), and the final residue (calc. mass
14.7 %, exp. 14.8 %) was confirmed by powder X-ray
diffraction analysis.
Thermal analysis was used in order to characterize the
thermal stability and to confirm the chemical formulas of
compounds (1)–(4). Table 4 contains information regard-
ing the thermal parameters and the temperature decompo-
sition ranges.
Complex [Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1)
Thermal decomposition of complex (1) occurs in several
well-defined steps as it can be observed in Fig. 3. First step
starts at 153 °C, and it corresponds to water molecules loss
(calc. mass loss 14.2 %, exp. 14.1 %), this step being
accompanied by an endothermic effect. The anhydrous
compound resulted in this first step is thermally stable in a
temperature range of 45 °C. The next two steps are starting
above 260 °C, with the oxidative degradation of one ben-
zimidazole molecule (calc. mass loss 23.2 %, exp. 23.1 %),
and above 350 °C the second benzimidazole molecule is
lost (calc. mass loss 23.2 %. exp. 23.4 %), both decom-
position steps being accompanied by exothermic effects, as
it can be observed from the DTA curve (Fig. 3). The last
step of thermal decomposition corresponds to nickel
acrylate conversion to nickel oxide (calc. mass loss 24.8 %,
exp. 24.5 %), which was confirmed by powder XRD
123

R. Olar et al.
Table 4 Thermal decomposition data (in air flow) for nickel complexes (1)–(4)
Complex
Step
Thermal effect
Temperature range/°C
Dmexp/%
Dmcalc/%
Evolved moieties
[Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1)
1
2
3
4
Endothermic
Exothermic
Exothermic
Exothermic
153–215
260–350
350–470
470–630
14.1
23.1
23.4
24.5
14.9
5.4
14.2
23.2
23.2
24.8
14.7
5.3
–4(H2O)
–(HBzIm)
–(HBzIm)
–(C6H6O3)
Residue (NiO)
Ni(2-MeBzIm)2(acr)2(H2O)]Á1.5H2O (2)
1
2
3
4
5
6
Endothermic
90–150
–1.5(H2O)
–(H2O)
Endothermic
Exothermic
Exothermic
Exothermic
Exothermic
180–260
280–380
380–545
545–700
700–880
3.6
3.5
25.8
25.7
16.3
8.4
25.9
25.9
16.1
8.6
–(2-MeBzIm)
–(2-MeBzIm)
–(C5H6O)
–(CO2)
Residue (NiO)
14.8
3.6
14.7
3.7
[Ni(5-MeBzIm)2(acr)2(H2O)] (3)
1
2
Endothermic
Exothermic
155–190
290–775
–(H2O)
80.7
80.8
–2(5-MeBzIm);
–(C5H6O);
–(CO2)
Residue (NiO)
15.7
29.1
46.6
15.5
29.6
46.3
[Ni(5,6-Me2BzIm)2(acr)2] (4)
1
2
Exothermic
250–410
410–710
–(5,6Me2BzIm);
–(5,6Me2BzIm)
–(C5H6O);
Exothermic
3
Exothermic
710–870
9.1
8.9
–(CO2)
Residue (NiO)
15.2
15.1
0.9
60
50
40
30
20
10
0
80
60
TG
0
TG
0
0.6
0.2
0.3
DTG
–20
–20
0.0
DTG
40
0.0
↑
↑
–0.3
–0.6
–0.9
–1.2
–1.5
–40
–0.2
–0.4
–0.6
–40
20
0
DTA
–60
DTA
–60
–10
–20
–30
–20
–80
–80
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature/°C
Temperature/°C
Fig. 4 Thermoanalytical (TG, DTG and DTA) curves of [Ni(2-
MeBzIm)2(acr)2(H2O)]Á1.5H2O (2) in air flow at 10 K min-1
Fig. 3 Thermoanalytical (TG, DTG and DTA) curves of
[Ni(HBzIm)2(acr)2(H2O)]Á3H2O (1) in air flow at 10 K min-1
Complex [Ni(5-MeBzIm)2(acr)2(H2O)] (3)
perature range of 100 °C. In the range of 290–775 °C,
several overlapped exothermic processes like oxidative
degradation of 5-methylbenzimidazole, acrylate con-
version to carbonate and the decomposition of nickel
carbonate are observed. The final product of the thermal
decomposition above 775 °C is nickel oxide (calc. mass
15.5 %, exp. 15.7 %), which was confirmed by powder
XRD analysis.
As it can be seen in Fig. 5, the thermal decomposition
of [Ni(5-MeBzIm)2(acr)2(H2O)] (3) occurs in two steps.
The thermal decomposition is starting at 155 °C with
the release of one water molecule (calc. mass loss
3.7 %, exp. 3.6 %), accompanied by an endothermic
effect. The anhydrous compound is stable in a tem-
123

Thermal behavior of new nickel(II) complexes with unsaturated carboxylates and heterocyclic…
0.4
0.2
0.0
–0.2
50
TG
0
$SRC
G1
40
30
Ni36...
Ni38...
Ni37...
Ni35...
HCT8
DTG
–20
–40
–60
–80
↑
20
10
0
–0.4
–0.6
DTA
–0.8
–1.0
–10
–20
G2
0
200
400
600
800
1000
Temperature/°C
S
Fig. 5 Thermoanalytical (TG, DTG and DTA) curves of [Ni(5-
MeBzIm)2(acr)2(H2O)] (3) in air flow at 10 K min-1
0
0
200
400
600
800
1K
FL3
0.2
60
50
40
30
20
10
0
TG
0
–20
Fig. 7 Flow cytometry overlapped histogram and percentages of
HCT-8 cells found in the different cell cycle phases, after treatment
with nickel (II) complexes
0.1
DTG
0.0
↑
–40
–0.1
–0.2
–0.3
–0.4
Table 5 Complexes effect upon the HCT-8 cell cycle progression
–60
Freq. G1
Freq. S
Freq. G2
DTA
–80
HCT-8
34.12
29.35
30.27
30.37
30.04
45.22
53.08
53.15
52.59
53.21
17.48
12.96
11.93
11.27
12.12
–10
–100
Ni35 (1)
Ni36 (2)
Ni37 (3)
Ni38 (4)
0
200
400
600
800
1000
Temperature/°C
Fig. 6 Thermoanalytical (TG, DTG and DTA) curves of [Ni(5,6-
Me2BzIm)2(acr)2] (4) in air flow at 10 K min-1
Complex [Ni(5,6-Me2BzIm)2(acr)2] (4)
dissolve all the complexes. All complexes presented superior
diameters of the inhibition zones compared with DMSO,
against all tested microbial strains. The next goal was to
determine quantitatively the antimicrobial activity by assessing
the minimum inhibitory concentration value MIC (lg mL-1).
The biological investigations allowed to conclude:
This compound is thermally stable up to 250 °C (as
depicted in Fig. 6); first decomposition step consists in
oxidative degradation of one 5,6-dimethylbenzimidazole
molecule (calc. mass loss 29.6 %, exp. 29.1 %), process
accompanied by an exothermic effect. The next step of
thermal decomposition is a sum of several processes like
the oxidative degradation of the other 5,6-dimethylbenz-
imidazole molecule and the conversion of nickel acrylate
into nickel carbonate (calc. mass loss 46.3 %, exp.
46.6 %), all these processes producing a strong exothermic
effect. Finally, nickel carbonate decomposes to nickel
oxide (calc. mass 15.1 %, exp. 15.2 %) whose formation is
confirmed by powder XRD analysis.
•
Complexes (1)–(4) exhibited moderated antimicrobial
activity with MIC values of 500 lg mL-1 against
Enterococcus faecium E5, Escherichia coli ATCC
25922 and Candida albicans 1760;
•
Complexes (1), (3) and (4) presented an inhibitory
effect of the microbial adherence on inert substrate
against Enterococcus faecium E5, and complex (2)
against Klebsiella pneumoniae with a biofilm minimum
inhibitory concentration BMIC of 250 lg mL-1
.
Microbiological assay
Literature data reveals low to moderate antimicrobial
activity of similar octahedral Ni(II) complexes [32, 44–47].
This behavior is due to nickel (II) strong preference for the
octahedral stereochemistry that limits the interaction with
bacterial or fungal strains.
The antimicrobial activity for nickel(II) complexes was quali-
tatively evaluated against several microbial strains, and it was
compared with DMSO activity which was the solvent used to
123

R. Olar et al.
´
´
5. Podunavac-Kuzmanovic SO, Cvetkovic DM. Antibacterial eval-
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All tested complexes slightly decreased the number of cells
found in G1 and G2 phases and increased the percentage of
cells found in the S phase. However, the tested complexes
proved to be no cytotoxic, as no apoptosis peak was
recorded on the flow cytometry histograms obtained for the
HCT-8 cells treated with the respective complexes (Fig. 7;
Table 5).
6. Devereux M, O’Shea D, O’Connor M, Grehan H, Connor G,
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crystal structures of [Cu(BZA)2(bipy)(H2O)], [Cu(SalH)2
(BZDH)2] and [Cu(CH3COO)2(5,6-DMBZDH)2] (SalH2 = sali-
cylic acid; BZAH = benzoic acid; BZDH = benzimidazole and
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Conclusions
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Four new and nickel (II) complexes with acrylate and
benzimidazole/benzimidazole derivatives were synthe-
sized, structurally and biologically characterized. Thermal
analysis indicated a great thermal stability for these nickel
compounds. Thermal decomposition trend is maintained
for this class of complexes, several steps being observed
like water molecules loss, benzimidazole oxidative degra-
dation, nickel acrylate conversion into nickel carbonate and
as a final step the nickel carbonate decomposition to nickel
oxide, which was the final residue for all four complexes.
The biological studies revealed that these new nickel
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and this fact can be ascribed to the coordinative saturation
of the nickel (II) complexes, the octahedral arrangement
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