Electrocatalytic property, anticancer activity, and density functional theory calculation of [NiCl(P^N^P)]Cl.EtOH

Article abstract of DOI:10.1002/aoc.6092

This study describes the electrocatalytic, anticancer, and density functional theory (DFT) studies of a nickel complex, [NiCl(P^N^P)]Cl.EtOH, based on a neutral P^N^P-type pincer ligand (P^N^P = bis[(2-diphenylphosphino)ethyl]amine). The ligand was synthesized without time-consuming and costly amine protection. It was characterized by ¹H NMR, ³¹P NMR, Fourier transform infrared (FT-IR), UV–vis, and single-crystal X-ray diffraction. The complex was isolated as a solvated chloride salt and characterized by FT-IR, UV–visible, ¹H NMR, ¹³C NMR, and ³¹P NMR spectroscopies as well as single-crystal X-ray diffraction and CHN analysis. The ligand and complex crystallized in a monoclinic P2₁/c space group. The molecular structure of the complex contains a four-coordinated distorted nickel ion with square-planar geometry. The electrocatalytic hydrogen ion reduction was studied for the nickel complex in an acidic non-aqueous medium. Cyclic voltammetry studies showed that this complex is an efficient electrocatalyst for hydrogen evolution at the potential of the Ni(II/I) couple. As a potential anticancer agent, the biological activities of the Ni complex were tested against two human cancer cell lines (MCF7 and HT29). The IC₅₀ results demonstrated that the nickel complex has better cytotoxic activity than cis-platin against the human breast cancer cell (MCF7) line. DFT calculations were performed to study the kinetics and thermodynamics of the pincer ligand's synthetic procedure and its Ni complex. Time-dependent DFT calculations were performed to calculate the pincer ligand's UV–vis spectra and the complex, which was in agreement with the experimental data. To assign the calculated UV spectra, molecular orbital calculations were performed. Finally, a modified mechanism was proposed for the electrocatalytic hydrogen ion reduction by [Ni(P^N^P)Cl]Cl.EtOH. The theoretical calculations showed that the cycle is thermodynamically favorable.

Full text of DOI:10.1002/aoc.6092

Received: 29 August 2020
DOI: 10.1002/aoc.6092
Revised: 19 October 2020
Accepted: 25 October 2020
F U L L P A P E R
Electrocatalytic property, anticancer activity, and density
functional theory calculation of [NiCl(P^N^P)]Cl.EtOH
1
1
1
Gholamhossein Mohammadnezhad
| Saeed Abad | Hossein Farrokhpour
|
2
2
Helmar Görls | Winfried Plass
1
Department of Chemistry, Isfahan
This study describes the electrocatalytic, anticancer, and density functional
theory (DFT) studies of a nickel complex, [NiCl(P^N^P)]Cl.EtOH, based on a
neutral P^N^P-type pincer ligand (P^N^P = bis[(2-diphenylphosphino)ethyl]
amine). The ligand was synthesized without time-consuming and costly amine
University of Technology, Isfahan,
84156-83111, Islamic Republic of Iran
2
Institute of Inorganic and Analytical
Chemistry, Friedrich-Schiller-Universität
Jena, Humboldtstr. 8, Jena, 07743,
Germany
1
31
protection. It was characterized by H NMR, P NMR, Fourier transform
infrared (FT-IR), UV–vis, and single-crystal X-ray diffraction. The complex
was isolated as a solvated chloride salt and characterized by FT-IR, UV–visible,
Correspondence
Gholamhossein Mohammadnezhad,
Department of Chemistry, Isfahan
University of Technology, Isfahan
1
13
31
H NMR, C NMR, and P NMR spectroscopies as well as single-crystal X-ray
diffraction and CHN analysis. The ligand and complex crystallized in a mono-
84156-83111, Islamic Republic of Iran.
clinic P2 /c space group. The molecular structure of the complex contains a
Email: mohammadnezhad@iut.ac.ir
1
four-coordinated distorted nickel ion with square-planar geometry. The
electrocatalytic hydrogen ion reduction was studied for the nickel complex in
an acidic non-aqueous medium. Cyclic voltammetry studies showed that this
complex is an efficient electrocatalyst for hydrogen evolution at the potential
of the Ni(II/I) couple. As a potential anticancer agent, the biological activities
of the Ni complex were tested against two human cancer cell lines (MCF7 and
HT29). The IC50 results demonstrated that the nickel complex has better cyto-
toxic activity than cis-platin against the human breast cancer cell (MCF7) line.
DFT calculations were performed to study the kinetics and thermodynamics of
the pincer ligand's synthetic procedure and its Ni complex. Time-dependent
DFT calculations were performed to calculate the pincer ligand's UV–vis
spectra and the complex, which was in agreement with the experimental data.
To assign the calculated UV spectra, molecular orbital calculations were per-
formed. Finally, a modified mechanism was proposed for the electrocatalytic
hydrogen ion reduction by [Ni(P^N^P)Cl]Cl.EtOH. The theoretical calcula-
tions showed that the cycle is thermodynamically favorable.
Funding information
Research Affairs Division of Isfahan
University of Technology (IUT); Iran
National Science Foundation (INSF),
Grant/Award Number: 94005007
K E Y W O R D S
aliphatic P^N^P ligand, anticancer, electrocatalyst, pincer complex, theoretical calculation
Appl Organomet Chem. 2020;e6092.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 15
https://doi.org/10.1002/aoc.6092

2
of 15
MOHAMMADNEZHAD ET AL.
1
| INTRODUCTION
Moreover, Lindley et al. synthesized rhenium
complexes based on an aliphatic PNP ligand
t
Pincer-type molecules/ions are an exciting class of
ligands that have received increasing attention in modern
organometallic chemistry.
pincer-type ligands have been successfully employed for
a wide variety of applications including supramolecular
chemistry,
(N(CH CH P Bu ) ) and studied the electrochemical
2
2
2 2
[
44]
splitting of N into the terminal nitride.
Also, there
2
[1–3]
Metal complexes of
are many reports about metal complexes with central
metal ions such as Co(II), Ni(II), Cu(II), and Zn(II) that
have been shown to display anticancer, antifungal,
[
4–6]
[7]
[8–12]
[45–47]
antibacteria,
anticancer,
antibacterial, and antiviral activity.
Among these,
[
13,14]
sensors,
heterogenic and homogenic).
of these ligands dates back to 1976 but it wasn't until in
and of course for catalytic reactions
there has been growing interest in using nickel com-
plexes in biological and pharmaceutical activities. Most
of the investigated complexes with tridentate PNP
[
15–24]
(
The earliest synthesis
1
989 that van Koten applied the term “pincer” word for
ligands showed square-planar structures with high
[25,26]
[48–53]
this group of ligands.
Since then, these ligands have
stability.
In 2014, Milenkovi ꢀc et al. synthesized
found wide usage for specific purposes in the chemical
square-planar Ni(II) complexes with P^N^O tridentate
[26]
sciences.
The formation of such complexes possessing
ligands which show moderate antibacterial and cytotoxic
[
54]
high thermal stability and reactivity is a consequence of
activity.
[27]
pincer ligand contribution.
Tridentate pincer ligands
In this paper, an aliphatic-centered pincer ligand
(P^N^P), as well as its nickel complex, was synthesized
and characterized. The nickel complex of the pincer
ligand acts as an efficient electrocatalyst for hydrogen ion
reduction. Also, kinetics, thermodynamics, and orbital
properties have been studied by theoretical calculation.
Furthermore, the anticancer activities of [NiCl(P^N^P)]
Cl.EtOH were investigated using MCF7 (breast) and
HT29 (colon) cancer cells.
having the description P^Y^P (where Y = heteroatom),
are tridentate and have been reported with meridional
[28,29]
geometry at the metal center.
In addition, the suc-
cessful synthesis of pincer ligands is dependent on factors
such as nature of heteroatom (Y), type of solvent, and
electron-donating or electron-withdrawing group on the
[30]
Y atom.
Some typical P^Y^P pincer ligands are shown
in Scheme 1. Among these, ligands with aromatic cen-
ters, P^C^P and P^N^P (Scheme 1a,b), represent the most
widely used structures where the aryl backbone leads to
[31–34]
stability and rigidity.
Reports indicated that the syn-
2 | EXPERIMENTAL
thetic mechanism of these ligands was followed by S 2
N
[30,35,36]
nucleophilic substitution.
Fryzuk first reported
2.1 | General procedures
the synthesis of a less common PNP pincer ligand
possessing a central amino group (Scheme 1c) in the
Anhydrous solvents, including dichloromethane, n-
[37]
1
980s.
were first reported by Danopoulos and Edwards
later developed for their unique application as cata-
Aliphatic P^N^P pincer ligands (Scheme 1d)
hexane, tetrahydrofuran (thf), and ethanol, were pre-
[
38]
[55]
and
pared by the standard methods
and degassed and
stored in Schlenk tubes (the air was rigorously removed
by three cycles of freeze–pump–thaw treatment). All
experiments were carried out using standard Schlenk
techniques under an argon atmosphere. Water was
degassed by bubbling with argon before use.
Chlorodiphenylphosphine and bis(2-chloroethyl)amine
hydrochloride were purchased from Merck and used
without further purification. Fourier transform infrared
[39,40]
lysts.
There is a substantial difference between
P^N^P and P^C^P ligands. For instance, nitrogen pos-
sesses a π-donor character resulting in the PNP ligands
having a weaker trans-effect than the PCP types with the
attendant result of increasing thermodynamic stability of
[36]
the ligands.
Nickel complexes of pincer ligands can
also act as electrocatalysts for hydrogen ion reduction
and are suitable for producing hydrogen as a green
(FT-IR) spectra were recorded on a Jasco-680 spectrome-
[
41–43]
−1
energy resource with low cost and high efficiency.
ter in the wavelength range between 400 cm
to
−1
4
4
000 cm , using a KBr pellet and making 60 scans at
−
1
cm resolution. The NMR spectra were recorded on a
1
31
Varian MERCURY plus-400 ( H: 300 MHz,
P:
1
31
1
1
21 MHz) and Bruker ultra shield ( H: 400 MHz, P:
62 MHz) spectrometer with chemical shifts reported in
ppm relative to the residual deuterated solvents.
Absorbance spectra were recorded on a UV–visible Jasco
V-570 spectrophotometer using quartz cells of 1.0-cm
optical path length in ethanol.
SCHEME 1 Some representative types of pincer ligands

MOHAMMADNEZHAD ET AL.
3 of 15
2.2 | Structure determinations
The intensity data for the compounds were collected on
a Nonius KappaCCD diffractometer using graphite-
monochromated Mo-K radiation. Data were corrected
α
SCHEME 2 Bis[(2-diphenylphosphino)ethyl]ammonium
chloride (P^N^P.HCl)
for Lorentz and polarization effects; absorption was taken
into account on a semi-empirical basis using multiple
[56–58]
scans.
The structures were solved by direct methods
[59]
(SHELXS ) and refined by full-matrix least-squares
Chlorodiphenylphosphine (4 mmol, 640 μl) was added
dropwise via a septum to a suspension of lithium
granules (10 mmol, 0.073 g) in thf (5 ml) under argon
2
[60]
techniques against Fo (SHELXL-2018 ). The hydrogen
atoms bonded to the amine-groups N1 of both com-
pounds were located by difference Fourier synthesis and
refined isotropically. All other hydrogen atoms were
included at calculated positions with fixed thermal
parameters. All nonhydrogen atoms were refined
atmosphere (Caution! The N atmosphere reacts with
2
lithium). The resulting solution was stirred vigorously at
room temperature for 24 h to yield a deep red solution of
lithium diphenylphosphanide. Then, this solution was
transferred to the suspension of bis(2-chloroethyl)amine
hydrochloride (1 mmol, 0.159 g) in thf (7 ml) by a can-
nula syringe. The reaction mixture was refluxed for 24 h.
The resulting red solution was cooled down to room tem-
perature. Subsequently, n-hexane (8 ml), 10% sodium
hydroxide (2 × 2 ml), and saturated sodium chloride
(2 × 2 ml) were added to the above solution, and the
aqueous layer was removed. The organic layer was stirred
for 45 min with HCl (2 M), and a white precipitate was
collected by filtration over sintered glass. The white solid
was purified in dichloromethane and diethyl ether.
[
60]
anisotropically.
MERCURY was used for structure
[61]
representations.
Crystal
data for
P^N^P.HCl:
C H ClNP ,
28 30 2
−1
Mr
0
=
477.92 g mol , colorless prism, size
3
.122 × 0.112 × 0.108 mm , monoclinic, space group
P2 /c, a = 10.1149(3), b = 26.7526(7), c = 10.2717(3) Å,
1
ꢀ
3
ꢀ
β = 114.210(1) , V = 2535.06(13) Å , T = −140 C, Z = 4,
ρcalcd. = 1.252 g cm , μ (Mo-K ) = 2.93 cm , multiscan,
−
3
−1
α
transmin: 0.6579, transmax: 0.7456, F(000) = 1008, 17,501
reflections in h(−13/13), k(−34/34), l(−13/13), measured
ꢀ
ꢀ
in the range 2.208 ≤ Θ ≤ 27.464 , completeness
Θmax = 99.3%, 5747 independent reflections, Rint = 0.0576,
−
1
−1
C H ClNP (477.92 g mol ). FT-IR (KBr, cm ): 3435
28
30
2
4
0
wR
749 reflections with F > 4σ(F ), 297 parameters,
( NH ), 3100 (aromatic CH), 2923 ( CH ). UV–vis
o
o
2
2
1
2
1
−4
−1
1
restraints, R
= 0.0607, wR
= 0.1401, R = 0.0760,
(1.0 × 10 mol L : EtOH): λ
= 248 nm. H NMR
max
obs
obs
all
2
ꢀ
= 0.1486, GOOF = 1.078, largest difference peak
(400 MHz, CDCl , 25 C, ppm): 2.65 and 2.85 (2 br s, 8H),
all
3
−3
31
and hole: 1.907/−0.499e Å .
Crystal data for
C H Cl NNiOP , Mr = 617.14 g mol , brown prism,
7.16–7.34 (m, 20H, Ph), 9.89 (s, 2H, NH ). P NMR
2
ꢀ
[Ni(P^N^P)Cl].EtOH:
(162 MHz, CDCl , 25 C, ppm): −21.
3
−
1
30
35
2
2
3
size 0.132 × 0.122 × 0.105 mm , monoclinic, space group
P2 /c, a = 15.4956(4), b = 9.1535(3), c = 23.7002(6) Å,
2.4 | Synthesis of [Ni(P^N^P)Cl]Cl.EtOH
1
ꢀ
3
ꢀ
β = 103.971(2) , V = 3262.17(16) Å , T = −140 C, Z = 4,
−
3
−1
ρcalcd. = 1.257 g cm , μ (Mo-K ) = 8.79 cm , multiscan,
The P^N^P.HCl (0.047 g, 0.1 mmol) was dissolved in eth-
anol (2 ml) and added dropwise into a solution of nickel
chloride (0.013, 0.1 mmol) in ethanol (2 ml) and stirred
at room temperature for 2 h. An orange crystalline
α
transmin: 0.6314, transmax: 0.7456, F(000) = 1288, 38,930
reflections in h(−20/20), k(−11/11), l(−30/30), measured
ꢀ
ꢀ
in the range 1.952 ≤ Θ ≤ 27.482 , completeness
Θmax = 99.7%, 7439 independent reflections, Rint = 0.0641,
ꢀ
product was obtained by slow evaporation at 5 C.
−
1
5
1
R
604 reflections with F > 4σ(F ), 351 parameters,
C H Cl NNiP .C H OH (617.14 g mol ). CHN: Calc: C,
28 29 2 2 2 5
o
o
1
2
8
restraints,
= 0.1230, wR
R
=
0.0922, wR
=
0.2390,
58.38; H, 5.72; N, 2.27 Found: C, 57.85; H, 5.32; N, 2.56.
obs
2
obs
1
−1
= 0.2663, GOOF = 1.104, largest
all
FT-IR (KBr, cm ): 3410 (NH), 3100 (aromatic CH), 2923
all
−3
difference peak and hole: 1.843/−0.726e Å .
( CH ), 1481, 1434, 1172, 1101.15, 839, 748, 724, 693,
2
−4
−1
5
47, 498. UV–vis (1.0 × 10
λmax = 471 nm. H NMR (300 MHz, CDCl , 25 C, ppm):
mol L : EtOH):
1
ꢀ
3
2
.3 | Synthesis of bis
2.16–2.52 (m, 4H, CH ), 2.98–3.22 (m, 4H, CH ), 7.4–8.3
2
2
13
[
(2-diphenylphosphino)ethyl]ammonium
(m, 20H, Ph), 10.44 (br s, NH). C NMR (75 MHz,
ꢀ
chloride (P^N^P.HCl)
CDCl , 25 C, ppm): 28.8 and 51.5 ( CH ), 129.3
3
2
(
d, JC,P = 31 Hz), 131.7 (d, J
= 50 Hz), 133.5
C,P
31
The ligand was synthesized by a modified reported
procedure.
(d, J = 81 Hz) (aromatic). P NMR (121 MHz, CDCl3,
25 C, ppm): 28.
C,P
[62]
ꢀ
The ligand has been shown in Scheme 2.

4
of 15
MOHAMMADNEZHAD ET AL.
2
.5 | Electrochemical experiments
The 96-well plates were then read at 490 nm by using a
Microplate Reader (Bio Rad, Model 680 instruments). For
each sample, the final results were expressed using IC ,
a parameter that indicated the anticancer potency of
chemicals.
The electrocatalytic properties of [Ni(P^N^P)Cl]Cl.EtOH
1 mM in acetonitrile) was studied in the presence of
NBu BF (0.1 M) as the supporting electrolyte. Cyclic
5
0
(
4
4
voltammograms (CVs) were recorded using OGF500
Origaflex–Origalys at a scan rate of 30, 50, 100, and
−
1
1
50 mV s . A typical three-electrode electrochemical
2.8 | Computational methods
system was used at room temperature and under argon
atmosphere. It consists of a glassy carbon electrode, a
platinum wire counter electrode, and a silver wire
reference. Catalytic hydrogen evolution experiments were
investigated after additions of 15, 65, 80, and 100 μl of
All calculations were carried out using the Gaussian
[
63]
09 program,
and the structures of ligand and nickel
complex were optimized using the density functional the-
ory (DFT) method employing B3PW91 functional. The
standard 6-31G(d) basis set was adopted for H, C, N, P,
and Cl atoms and LANL2DZ for the nickel atom.
Also, the kinetics and thermodynamics of ligand
synthesis were studied separately using the same compu-
tational method in the gas phase and thf, which was
considered a polarized continuum solvent. The reaction
rate was calculated by the equation presented in
1
M HCl via syringe, and the CVs were recorded under
an argon atmosphere. The agitation was provided by a
small magnetic stirrer.
2.6 | Biological studies
[
36]
The human colon adenocarcinoma cell line HT29 and
human breast adenocarcinoma cell line MCF7 were
cultured in Dulbecco's modified Eagle's medium
Mohammadnezhad et al.
The entropy of solvation (Ssolv) can be calculated by
Equation 1, which leads to the more accurate ΔG and
≠
[26,64]
(
DMEM-low glucose, Bioidea-Iran) under a humidified
ΔG of the reaction in the presence of thf.
ꢀ
atmosphere (95% air and 5% CO ) at 37 C. The medium
2
ꢀ
ꢁ
contained 10% fetal bovine serum (FBS) and 1%
penestrep (penicillin and streptomycin).
Vm,liquid
Vm,gas
ꢀ
Vm,liquid
Vm,gas
+ 7:98 calmol−1 K−1
,
ΔSsolv = Rln
−α S + Rln
ð1Þ
2
.7 | MTT cytotoxicity assay
where Vm,gas and Vm,liquid are the molar volume of the
solute in the gas phase and solvent, respectively. The
detailed information about Equation 1 has been given in
The aliphatic pincer complex's cytotoxicity was
investigated using adherent growing cell lines
[
64]
Wertz.
This equation for the solute in thf can be writ-
method
with
MTT
(3-(4,5-dimethylthiazol-2-yl)-
ten as follows (Equation 2).
2
,5-diphenyltetrazolium bromide) test. Three concentra-
THF : ΔSsolvation
Àcal mol− K−1Á
= −11:301−0:208 ðS −11:301Þ + 7:98:
1
ꢀ
tions, 50, 100, and 200 μM, of each powder (cis-platin or
Ni complex) was prepared in a culture medium-0.05%
volume of ethanol (medium containing DMEM-low + 10%
ð2Þ
ꢀ
FBS + 1% penestrep) at 37 C in a humidified atmosphere
with 5% CO . MCF7 and HT29 cells were cultured with a
2
4
growing density of 1.5 × 10 cells per well into a 97-well
plate in order to expand the cells. After a day of culture,
cis-platin and the pincer complex prepared in the culture
medium were added to MCF7 and HT29 cells and then
3 | RESULT AND DISCUSSION
3.1 | Synthesis and characterization
ꢀ
incubated at 37 C. MCF7 and HT29 cells were growing
on tissue culture polystyrene (TCP) dishes without any
additive in culture medium as control. After 24 and 48 h
of incubation, the cell culture medium was removed, the
pits were washed with phosphate-buffered saline, and
cell durability rate was measured using the MTT test. In
this regard, after incubation of the cells with MTT solu-
tion (0.5-mg ml− MTT reagent in PBS) for 4 h, DMSO
was added to dissolve the purple MTT formazan crystals.
In contrast to the usual synthetic methods, which are
based on time-consuming and costly amine protection
(Scheme 3a), this study avoids this by using LiPPh2
which has low basicity and high nucleophilicity thereby
[
38,65,66]
preventing the aziridine formation (Scheme 3).
The P^N^P ligand was then characterized by
NMR, P NMR, FT-IR, UV–vis, and single-crystal X-ray
1
H
1
31
diffraction. The calculated and experimental FT-IR

MOHAMMADNEZHAD ET AL.
5 of 15
SCHEME 3 Synthetic procedures of bis[(2-dialkyl/arylphosphino)ethyl]amine (P^N^P)
spectra of the Ni complex and the ligand are shown in
two signals at δ, 28.9 and 51.6 ppm, for two CH frag-
ments. The observed signals in the range of 129 to
2
−
1
Figure S1. In the ligand, the band at 3425 cm is related
to amine functional group. CH (aromatic) vibrational
135 ppm are assigned to aromatic carbons (Figure S3). A
−
1
31
modes appear at 3070 cm . The bands at 2850 and
single resonance at δ = 28 ppm was observed in the
P
400 cm− relate to CH (aliphatic) and C C stretching
1
{ H} NMR spectrum of the complex whereas the ligand
signal positioned at −21 ppm, which may suggest forma-
tion of the pincer complex (Figure S4). This value is in
accordance with similar complexes; for example,
Tamizmani and Sivasankar had synthesized [PN(H)
1
1
modes, respectively. Also, the complex FT-IR spectrum of
the complex shows vibrational bands of the ligand with a
few changes in intensities and band positions.
1
The H NMR spectrum of the ligand displays a signal
+
Ph
[68]
at 9.8 ppm for NH2 hydrogens. In addition, two signals
P NiCl] with P chemical shift at 27.6 ppm.
were observed at 2.8 and 2.4 ppm assigned to the methy-
lene groups. In the P NMR spectrum of the ligand
UV–vis spectra of the ligand and the Ni complex were
recorded in EtOH. The calculated and experimental
absorption spectra have been shown in Figure 1, and the
contribution of molecular orbitals in electronic transmis-
sions are shown in Tables 1 and 2. The experimental
spectrum of the ligand showed two bands at 230 and
248 nm, respectively. Upon formation of the Ni complex,
as well as a band at 224 nm, three additional bands
emerge at 264, 343, and 471 nm, respectively. There is a
relatively good agreement between the calculated and
experimental spectra. So DFT calculation helped to iden-
tify the type of transitions for each band.
31
showed a major signal can be seen at −21.5 ppm, which
[
62,66,67]
is consistent with reported values
and a minor
[62]
1
13
one at 34.5 ppm for the oxidized product.
The H, C,
P NMR of [NiCl(P^N^P)]Cl are shown in
31
and
1
Figures S2–S4. H NMR resonances of the complex also
displayed two multiplets, 2.14–2.49 and 2.74–3.39, related
to two chemically nonequivalent CH groups from the
2
ligand. The amine signal was observed at a higher value
(
(
10.44 ppm) in the complex compared to the ligand
9.8 ppm) (Figure S2). The C NMR spectrum showed
13
FIGURE 1 Density functional theory (DFT) calculation and experimental electronic spectra of (a) P^N^P.HCl (ligand) and (b) [Ni
P^N^P)Cl]Cl (complex)
(

6
of 15
MOHAMMADNEZHAD ET AL.
TABLE 1 Experimental and calculated electronic transitions by Time-Dependent DFT/Polarizable Continuum Model (TDDFT/PCM)
method of P^N^P.HCl (ligand) in ethanol
Transition
number
E
(eV)
excitation
λ
expt
λ
calc
(nm)
(nm)
Key transitions
1
5.81
210
213
2
3
5.43
232
250
228
269
4
.59
.77
259
4
In the calculated ligand spectrum, the band at
the ligand. The band located at 228 nm is due to two
orbital excitation, including HOMO ! LUMO + 4. The
HOMO ! LUMO + 4 excitation is due to the electron
excitation from Lp(P) and π(C C) of phenyl rings in part
B of the ligand to π*(C C) of the phenyl ring in the same
part of the ligand. The band located at 213 nm was due to
orbital excitation, including HOMO − 6 ! LUMO + 1.
The HOMO − 6 ! LUMO + 1 excitation is due to the
electron excitation from π(C C) of the phenyl ring in
part B of the ligand to π* of C C of the phenyl ring in
part A of the ligand. In the absorption spectrum of com-
plex, the absorption band located at 494 nm has the con-
tribution of HOMO ! LUMO (30.6%) assigned to the
excitation of Lp of Cl and σ of P Ni and P C bonds
to σ* of Ni Cl and Ni P bonds. Other orbital transitions
of these absorption bands are assigned to
2
59 nm originated from HOMO − 1 ! LUMO and
HOMO − 1 ! LUMO + 1. The HOMO − 1 ! LUMO is
related to the excitation of an electron from the lone pair
(
Lp) of P atom and π(C C) of the phenyl ring in part A
of the ligand to the π*(C C) of the phenyl ring in part B
of the ligand (both parts A and B are shown in Figure 2).
Similarly, the HOMO − 1 ! LUMO + 1 is the excitation
of Lp of P atom and π(C C) of the phenyl ring in part A
of the ligand to π*(C C) of the phenyl ring in part A of
the ligand. It can be said that these two transitions are
n ! π* transitions. The band at 269 nm originates from
the HOMO ! LUMO. The HOMO ! LUMO is also
related to the excitation of an electron from the lone pair
(
Lp) of P atom and π(C C) of the phenyl ring in part B of
the ligand to the π*(C C) of the phenyl ring in part B of

MOHAMMADNEZHAD ET AL.
7 of 15
TABLE 2 Experimental and calculated electronic transitions by TDDFT/PCM method of [Ni(P^N^P)Cl]Cl in ethanol
Transition
number
E
(eV)
excitation
λ
expt
λ
calc
(nm)
(nm)
Key transitions
1
5.54
224
223.95
2
4.87
264
254.71
3
2
.61
.51
343
471
343.47
494.55
3
3

8
of 15
MOHAMMADNEZHAD ET AL.
FIGURE 2 The optimized structures of (a) P^N^P.HCl and (b) [Ni(P^N^P)Cl]Cl in the gas phase
HOMO − 12 ! LUMO and HOMO − 9 ! LUMO. The
HOMO − 12 ! LUMO is related to the excitation of Lp
electrons of Ni, σ P1 C, and σ P2 Ni to σ* Ni Cl and σ*
P Ni. The HOMO − 9 ! LUMO is related to the excita-
tion of π(C C) of the phenyl ring of part A of the ligand
to σ* Ni Cl and σ* P Ni. The absorption band located at
HOMO − 9 ! LUMO and HOMO − 11 ! LUMO + 1
orbital excitations. The HOMO − 9 ! LUMO is related
to the excitation of an electron from π(C C) of the phe-
nyl rings of part A of ligand to π*(C C) of the phenyl
ring in the same part. The HOMO − 11 ! LUMO + 1 is
related to the excitation of Lp electrons of Ni to π*(C C)
phenyl ring in part B of the ligand (Figure 2).
2
64 nm is due to two orbital excitations, including
HOMO ! LUMO + 2 and HOMO − 1 ! LUMO + 1.
The HOMO ! LUMO + 2 is related to the excitation of
Lp of Cl, σ P Ni, and P C to π*(C C) of the phenyl ring
of part A of the ligand. The HOMO − 1 ! LUMO + 1 is
assigned to the excitation of an electron from the Lp of Cl
and Ni atoms and π(C C) of the phenyl ring of part A of
the ligand to the π*(C C) of the phenyl ring of part B of
the ligand. The highest intensity band at 223 nm in
the spectrum of the complex is assigned to
The X-ray structures and crystal data of the ligand
and the complex are shown in Figure 3 and Table 3,
respectively. This complex was first synthesized by
Walther et al. in 1999 and crystallized in thf in a triclinic
system with a = 11.745(1), b = 12.148(1), and c = 14.129
[
69]
(3) Å lattice parameters.
In this study, the single crys-
tal of the complex was obtained in ethanol with a mono-
clinic crystal system with a = 15.4956(4), b = 9.1535(3),
and c = 23.7002(6) Å lattice parameters. The ligand and
FIGURE 3 ORTEP diagram of (a) P^N^P.HCl and (b) [Ni(P^N^P)Cl]Cl.EtOH (some hydrogens are not shown for more clarity)

MOHAMMADNEZHAD ET AL.
9 of 15
TABLE 3 Crystal data and structural refinement
Crystal data
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
P^N^P.HCl
30ClNP
[Ni(P^N^P)Cl]Cl.EtOH
NNiOP
C
28
H
2
C
30
H35Cl
2
2
477.92
617.14
133(2)
0.71073
133(2)
0.71073
Monoclinic
Monoclinic
P2 /c
P2
1
/c
1
10.1149(3)
26.7526(7)
10.2717(3)
90
15.4956(4)
9.1535(3)
23.7002(6)
90
b (Å)
c (Å)
ꢀ
α ( )
ꢀ
β ( )
114.210(1)
90
103.971(2)
90
ꢀ
γ ( )
3
V (Å )
2535.06(12)
4
3262.17(16)
4
Z
Density
1.252
1.257
Absorption coefficient (mm−
1
)
0.293
0.879
F(000)
1008
1262
Crystal size (mm)
0.122 × 0.112 × 0.108
2.21–27.46
0.132 × 0.122 × 0.105
1.952–27.48
Θ Range for data
Reflection collected
Independent reflection
Max. and min. transmission
Data/restraints/parameters
17,501
38,930
5747 (Rint = 0.0641)
0.7456 and 0.6576
5747/0/297
7439 (Rint = 0.0641)
0.7456 and 0.6314
7439/18/3351
1.104
2
Goodness-of-fit F
1.076
Final R indices [I > 2σ(I)]
R indices (all data)
R
1
1
= 0.0607, wR
2
2
= 0.1401
= 0.1486
R
1
1
= 0.0922, wR
2
2
= 0.2390
= 0.2663
R
= 0.0760, wR
R
= 0.1230, wR
complex were crystallized in monoclinic P2 /c space
Cl]Cl.EtOH the obtained FCGP parameters is −43, which
1
groups. The molecular structure of the complex contains
a distorted four-coordinated square-planar geometry. The
Ni(II) coordination sphere includes one tridentate ligand
that forms two five-member rings and one Cl atom at
trans position to N atom of PNP ligand (Figure 3). The
suggest a distorted square-planar geometry.
3.2 | Electrocatalytic hydrogen ion
reduction
ꢀ
P1 Ni P2 angle is 170.86(6) , and the deviation from
the ideal value for square-planar coordination is due to
the steric constraints of the chelating ligand. The length
of the bonds for Ni P1, Ni P2, and Ni N are 2.1927
Although it is already well-known, no known studies into
the use of [Ni(P^N^P)Cl]Cl for electrocatalytic hydrogen
evolution have been carried out. Therefore, the electro-
chemical analysis was performed in a 0.1 M NBu BF
(16), 2.1930(16), and 1.934(5) Å, respectively, which
4
4
indicates that the phosphorus atoms are located at the
same distance from the central metal. Four coordinate
geometric parameter (FCGP) was also calculated for the
geometry determination of the pincer complex. For
tetrahedral and square planar geometry, the expected
acetonitrile solution under an Ar atmosphere containing
1 mM [Ni(P^N^P)Cl]Cl.EtOH. In CVs of 1 mM [Ni
(P^N^P)Cl]Cl.EtOH at different scan rates (30, 50,
−1
100, and 150 mV s ) reduction peaks could be observed
approximately −0.5 V versus NHE, which were subse-
[70]
II
I
FCGPs are 100 and −40, respectively.
For [Ni(P^N^P)
quently assigned to a Ni /Ni couple (Figure 4).

10 of 15
MOHAMMADNEZHAD ET AL.
FIGURE 4 Cyclic voltammograms of 1 mM [Ni(P^N^P)Cl]Cl.
FIGURE 6 The chronoamperometric curve of [Ni(P^N^P)Cl]
EtOH in 0.1 M NBu
scan rates
4
BF
4
in acetonitrile solution at different
Cl.EtOH
evaluated by the chronoamperometry (i–t) technique.
The data suggests that this complex is stable at a current
density of −16 mA cm− and continues this sustainability
for 300 s in the presence of 100-μl HCl.
As electrochemical methods are helpful in determin-
ing the potential of hydrogen evolution by Ni pincer com-
plexes, we investigated the electrocatalytic hydrogen ion
reduction activity of [Ni(P^N^P)Cl]Cl.EtOH in the pres-
ence of 1 M HCl in anhydrous acetonitrile. As shown in
Figure 5, the incremental addition of HCl solution
2
3.3 | In vitro cytotoxicity
(
15, 65, 80, and 100 μl) renders the reduction wave of
2+
+
Ni /Ni couple with an accompanying increase in the
current. This behavior is characteristic of electrocatalytic
hydrogen ion reduction and suggests hydrogen evolution
by the [Ni(P^N^P)Cl]Cl.EtOH catalyst. Figure 6 shows
stability test on [Ni(P^N^P)Cl]Cl.EtOH, which was
Cytotoxic effects of [Ni(P^N^P)Cl]Cl.EtOH on MCF7 and
HT29 cancer cells were determined after 1 and 2 days of
culture via MTT assay. Three different concentrations of
[Ni(P^N^P)Cl]Cl.EtOH (50, 100, and 200 μM) were pre-
pared and used for cell viability studies. Cell viability in
the presence of complex (50, 100, and 200 μM) treated for
1
day with MCF7 cancer cells was seen to increase from
89.96 to 127.59 (% control). Moreover, the relative cell
viability was enhanced from the least concentration
50 μM) to the highest (200 μM). The proliferation of
MCF7 cells was reduced in the second day for 100 and
(
200 μM to 89.47 and 105.55 (% contry (Figure 7).
For HT29 cancer cells, the MTT assay of the complex
after 1 day shows that the cell viability at 100 μM has the
highest value (83.10%), and 200 μM has the lowest one
(
71.26%). After 2 days, the cell viability dose not display
any significant changes for 50 and 100 μM of the complex
whereas at higher concentration i.e. 200 μM, it undergoes
a decrease. The proliferation of cells cultured on the
2
1
00 μM was seen to decrease from 71.26 (% control) (after
day) to 37.80 (% control) (after 2 days) (Figure 8). It
could be therfore concluded that the viability of the two
cancer cells in the presence of [Ni(P^N^P)Cl]Cl.EtOH
depends on culture time and concentration of the
complex.
FIGURE 5 Cyclic voltammograms of 1 mM solutions of
[
Ni(P^N^P)Cl]Cl.EtOH with various amounts of HCl in 0.1 M
4 4
NBu BF
in acetonitrile solution at a scan rate of 0.1 V s−
1

MOHAMMADNEZHAD ET AL.
11 of 15
3.4 | Computational results
The structures of the pincer ligand and its corresponding
nickel complex obtained from the single-crystal X-ray dif-
fraction were compared with their optimized structures,
with the latter shown in Figure 2. The experimental and
computational selected bond lengths and angles for the
ligand and the complex are also listed in Tables 5 and 6,
respectively.
From the tabulated data it can be surmised that there
is a good agreement between the calculated values in the
gas phase and the obtained experimental values in the
solid state, for both ligand and complex with any struc-
tural differences between the free and complexed ligand
being negligible. The experimental and theoretical values
of bond lengths and bond angles for the ligand were
compared and are shown in Table 5. The kinetics and
thermodynamics of the ligand synthesis with iso-propyl
substituents on the P atoms were already previously
FIGURE 7 A comparative plot of the cell viability of MCF7 in
the presence of 50, 100, and 200 μM of [Ni(P^N^P)Cl]Cl.EtOH after
1
and 2 days
[
30]
investigated in detail,
and we just briefly studied the
theoretical calculation of the phenyl substituents. The
optimized structures of the TSs related to the reaction of
lithium diphenylphosphanide (LiPPh ) reaction with bis
2
(
2-chloroethyl)amine are shown in Figure 9. The calcula-
≠
tions show that activation free energy (ΔG ) in the
−
1
absence of solvent is 31.05 kcal mol (rate constant
k) = 1.05 × 10− s−1) but, when the electrostatic fields
10
(
of thf were considered as a solvent, the activation free
−
1
energy changed to 20.14 kcal mol
(
(rate constant
k) = 6.62 × 10− s−1). It was then found that the synthe-
5
sis of the pincer ligand in the gas phase was undesirable,
whereas in the thf, the reaction became more thermody-
namically favorable. The calculated ΔG values for the
P^N^P ligand synthesis in the gas phase and thf for
FIGURE 8 A comparative plot of the cell viability of HT29 in
the presence of 50, 100, and 200 μM of [Ni(P^N^P)Cl]Cl.EtOH after
1
and 2 days
−
1
the first step were 121.70 and −25.34 kcal mol ,
The results from the cytotoxicity tests show IC50
values of 58.2 μM (1 day) and 193.2 (2 days), for [Ni
P^N^P)Cl]Cl.EtOH in the presence of MCF7 cells of
respectively.
(
which the former value is lower than that of than cis-
platin (Table 4). Using HT29 cells instead the same tests
reveal IC50 values of 205.3 and 79.0 μM after 1 and 2 days,
respectively, which are higher than cis-platin (Table 4).
These results suggest that the pincer complex displays
better cytotoxic activities than cis-platin against MCF7
cancer cells after 1 day.
3.5 | Proposed electrocatalytic and
thermodynamic cycle
Previously, various nickel complexes have been synthe-
sized and used as hydrogen ion reduction electrocatalysts
for hydrogen evolution. For example, Goh et al. reported
bis(cyclopentadienyl)nickel(II)-μ-thiolato complexes as
TABLE 4 IC50 values for
IC50 (μM)
1 day (3 points)
IC50 (μM)
2 days (3 points)
[
Ni(P^N^P)Cl]Cl.EtOH and cis-platin in
Sample
Cancer cell
MCF7
the presence of MCF7 and HT29 cell
lines
cis-Platin
67.0
101.8
58.2
111.9
23.3
HT29
[Ni(P^N^P)Cl]Cl.EtOH
MCF7
193.2
79.0
HT29
205.3

12 of 15
MOHAMMADNEZHAD ET AL.
TABLE 5 Experimental and
computational values of selected bond
ꢀ
Bond lengths (Å)
Bond angles ( )
Exp.
Comp.
Exp.
Comp.
115.19
117.28
98.62
lengths and bond angles of P^N^P.HCl
P(1)-C(1)
C(1)-C(2)
C(2)-N(1)
N(1)-H(1)
C(3)-C(4)
C(4)-P(2)
1.845(3)
1.527(4)
1.489(4)
0.86(4)
1.517(4)
1.851(3)
1.853
1.521
1.489
1.025
1.521
1.856
C(2)-N(1)-C(3)
C(3)-C(4)-P(2)
C(5)-P(1)-C(1)
C(6)-P(1)-C(1)
C(4)-P(2)-C(8)
C(4)-P(2)-C(7)
112.9(2)
117.37(18)
102.01(12)
101.64(14)
103.44(12)
100.09(12)
102.78
101.48
103.12
TABLE 6 Experimental and
computational values of selected
bond lengths and bond angles of
ꢀ
Bond angles ( )
Bond length (Å)
Exp.
Comp.
Exp.
Comp.
[Ni(P^N^P)Cl]Cl
Ni(1)-N(1)
Ni(1)-Cl(1)
Ni(1)-P(1)
Ni(1)-P(2)
N(1)-H(1 N1)
N(1)-C(2)
1.934(5)
2.1588(14)
2.1935(16)
2.1920(16)
2.06(10)
1.497(8)
1.976
N(1)-Ni(1)-Cl(1)
N(1)-Ni(1)-P(1)
Cl(1)-Ni(1)-P(1)
N(1)-Ni(1)-P(2)
P(1)-Ni(1)-P(2)
Cl(1)-Ni(1)-P(2)
174.83(18)
86.16(18)
93.80(6)
86.16(18)
170.86(6)
94.35(6)
174.67
87.26
92.74
87.15
170.53
93.54
2.1793
2.2481
2.2499
1.025
1.4896
FIGURE 9 TS structures related to
the reaction of bis(2-chloroethyl)amine
with lithium diphenylphosphanide
(
2
LiPPh ) (a) in the gas phase and (b) in
tetrahydrofuran (thf), which considered
as a polarized continuum solvent
hydrogen ion reduction electrocatalysts. The proposed
reaction pathways for reducing two hydrogen ions to
generate an dihydrogen molecule involve four steps in
the form of either chemical (C) or electrochemical
for the hydrogen ion reduction using the synthesized
complex in this work with some modifications (see
Figure 10). The optimized structures of the complex
at each step of the cycle are also shown in Figure 10.
[71]
+
(E) reactions.
In another study, Hong et al. synthesized
The first step is one-electron reduction of [1-Cl]
0
high-spin Ni(II) complexes with S N ligands. The pro-
complex to [2-Cl] which is a spontaneous reaction
2
2
−1
posed electrocatalytic mechanism involves the stepwise
(ΔG = −92.33 kcal mol ; step A, Figure 10). In the next
0
0
0
III
reduction of Ni(II) by two electrons to form [LNi ] in
step, the [2-Cl] is protonated to form Ni hydride
[72]
+
−1
acetonitrile.
A mechanism for the hydrogen ion reduc-
[1-H] (ΔG = −143.27 kcal mol , step B). In step C, the
III
+
tion using the PCP pincer complex has been reported by
subsequent Ni hydride [1-H] is reduced by one elec-
[42]
II
0
Luca et al.
A similar mechanism has been considered
tron to give Ni hydride, [1-H] . To complete the cycle,

MOHAMMADNEZHAD ET AL.
13 of 15
FIGURE 10 Mechanism of hydrogen evolution by [Ni(P^N^P)Cl]Cl.EtOH
0
+
with protonation of [1-H] , the [1-H ] complex was
efficient electrocatalyst for hydrogen evolution at the
potential of the Ni(II/I) couple. The crystal structures of
the ligand and the complex were determined by single-
crystal X-ray diffraction. DFT calculations were
performed to study the kinetics and thermodynamics of
the synthetic procedure of the pincer ligand and the Ni
complex. Theoretical calculations revealed that the
2
−1
formed as an intermediate (ΔG = −162.25 kcal mol ,
+
step D). Finally, the [1-H ] complex loses one H mole-
2
2
+
cule to revitalize the initial [1-Cl] complex. The calcu-
lated value of ΔG (total) for the electrocatalytic cycle is
522.02 kcal mol , which shows the cycle is thermody-
namically favorable and the [NiCl(P^N^P)]Cl can reduce
ꢀ
−1
−
hydrogen ion to H (Figure 10).
synthesis of the ligand follows an S 2 mechanism. Biolog-
2
N
ical activity studies of [NiCl(P^N^P)]Cl showed that the
death of cancer cells by pincer nickel complex depends
on two factors: concentration and time. In this way, the
highest efficiency for cancer cells' death was found for
MCF7 cancer cells in low concentration (50 μl) and less
time (24 h) thus suggesting that the complex is consider-
ably more cytotoxic than cis-platin against MCF7 cancer
cells within a 24-hour period.
4
| CONCLUSION
The P^N^P-type pincer ligand, bis[(2-diphenylphosphino)
ethyl]amine, has been synthesized simply and fully
characterized employing different analytical techniques.
It was then used to prepare its Ni(II) complex, [Ni
(
P^N^P)Cl]Cl.EtOH, which was analysed using FT-IR,
1
13
31
UV–visible, H NMR, C NMR, and P NMR spectros-
copies, single-crystal X-ray diffraction, and CHN
analyses. The molecular structure of the complex has a
four-coordinated distorted nickel ion with square-planar
geometry. CV studies revealed that the complex is an
ACKNOWLEDGMENTS
This research was supported by the Iran National Science
Foundation (INSF) (Grant No. 94005007) and Research
Affairs Division of Isfahan University of Technology
(IUT), Isfahan, I. R. Iran.

14 of 15
MOHAMMADNEZHAD ET AL.
AUTHOR CONTRIBUTIONS
[14] H. Tavallali, G. Deilamy-Rad, A. Moaddeli, K. Asghari, Sens.
Actuat B-Chem. 2017, 244, 1121.
Gholamhossein Mohammadnezhad: Conceptualiza-
tion; data curation; funding acquisition; investigation;
project administration. Saeed Abad: Conceptualization;
data curation; formal analysis; investigation; software;
validation. Hossein Farrokhpour: Conceptualization;
data curation; supervision. Helmar Görls: Data
curation; software; validation. Winfried Plass: Funding
acquisition; resources.
[
[
[
15] S. Sobhani, A. Habibollahi, Z. Zeraatkar, Org. Process Res. Dev.
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Yusufova, S. E. Shaner, J. A. Tendler, Organometallics 2014,
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2015, 119, 21567.
[
19] S. Chakraborty, P. O. Lagaditis, M. Förster, E. A. Bielinski, N.
SUPPORTING INFORMATION AVAILABLE
Crystallographic data (excluding structure factors) has
been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication CCDC-
Hazari, M. C. Holthausen, S. Schneider, ACS Catal. 2014,
4
, 3994.
[20] M. Nielsen, E. Alberico, W. Baumann, H. J. Drexler, H. Junge,
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2
020952 for P^N^P.HCl, and CCDC-2020953 for [Ni
[21] A. Staubitz, M. E. Sloan, A. P. Robertson, A. Friedrich, S.
(
P^N^P)Cl].EtOH. Copies of the data can be obtained free
Schneider, P. J. Gates, I. Manners, J. Am. Chem. Soc. 2010,
132, 13332.
of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Email: deposit@ccdc.
cam.ac.uk ). Additional supporting information may be
found online in the Supporting Information.
[
22] M. Käß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem.,
Int. Ed. 2009, 48, 905.
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DATA AVAILABILITY STATEMENT
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[
[
ORCID
2
Gholamhossein Mohammadnezhad https://orcid.org/
0
000-0003-1765-8063
2
Winfried Plass https://orcid.org/0000-0003-3473-9682
[
29] G. Zhu, L. Wang, H. Sun, X. Li, RSC Adv. 2015, 5, 19402.
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