New Sensitive and Selective Chemical Sensors for Ni2+and Cu2+Ions: Insights into the Sensing Mechanism through DFT Methods

Article abstract of DOI:10.1021/acs.jpca.0c03834

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni2+ and Cu2+ ions over other metal cations in the CH3CN/H2O solution. Compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (A) and (E)-4-((3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV-vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π→ π? transition centered in the ligand. Instead of Ni2+ and Cu2+ ions, the transition is set toward the σ? molecular orbital with a strong contribution of the 3dx2-y2 transition (π→ 3dx2-y2). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.

Full text of DOI:10.1021/acs.jpca.0c03834

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
New Sensitive and Selective Chemical Sensors for Ni2+ and
Cu2+ Ions, Sensing Mechanism Insights through DFT Methods
Manuel A. Treto-Suarez, Jorge Tapia, Yoan Hidalgo-Rosa, Dayan Paez-
Hernandez, Elies Molins, Ximena Zarate, and Eduardo Enrique Schott
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.0c03834 • Publication Date (Web): 08 Jul 2020
Downloaded from pubs.acs.org on July 9, 2020
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New Sensitive and Selective Chemical Sensors for Ni and
2
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Cu ions, Sensing Mechanism Insights Through DFT
Methods
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a
b,f
a,f
a,e*
Manuel A. Treto-Suárez , Jorge Tapia , Yoan Hidalgo-Rosa , Dayan Páez-Hernández , Elies
c
d*
b,f*
Molins , Ximena Zarate and Eduardo Schott .
aDoctorado en Fisicoquímica Molecular, Universidad Andres Bello. Ave. República #275, Santiago de
Chile, Chile. dayan.paez@unab.cl, mtretosuarez@gmail.com.
bDepartamento de química inorgánica, UC Energy Research Center, Facultad de Química y de Farmacia,
Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Macul, Santiago, Chile.
maschotte@gmail.com
cInstitut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra. Spain.
elies.molins@icmab.es
dInstituto de Ciencias Químicas Aplicadas, Theoretical and Computational Chemistry Center, Facultad de
Ingeniería, Universidad Autónoma de Chile, Av. Pedro de Valdivia 425, Santiago, Chile.
ximena.zarate@uautonoma.cl
eCenter of Applied Nanosciences (CANS), Chile República #275, Santiago de Chile, Chile.
fMillennium Nuclei on Catalytic Processes towards Sustainable Chemistry (CSC), Chile.
Corresponding Author
c*
b,d
Ximena Zarate : ximena.zarate@uautonoma.cl and Eduardo Schott : maschotte@gmail.com
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Abstract
We report the synthesis and theoretical study of two new colorimetric chemosensors with especial
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2+
selectivity and sensitivity to Ni and Cu ions over other metal cations in CH CN/H O solution.
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Compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (A) and (E)-4-
(3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change
(
from yellow to colorless, which allows the detection of the mentioned metal cations through
different techniques. The interaction of sensors with these metal ions induced a new absorption
band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study
via Time-Dependent Density Functional Theory (TD-DFT) was performed. This method has
enabled us to reproduce the hypsochromic shift in the maximum UV-vis absorption band and to
explain the selective sensing of the ions. For all the systems studied, the absorption band is
2
+
2+
characterized by a →* transition centered in the ligand. Instead of Ni and Cu ions, the
transition is set towards the σ* molecular orbital with a strong contribution of 3d 2 transition
2
x -y
(
→3dx2-y2). These absorptions imply a Ligand to Metal Charge Transfer (LMCT) mechanism that
results in the hypsochromic shift in the absorption band of these systems.
Introduction
The detection of metals either in situ or in vivo in a simple, sensible and selective way is
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–7
challenging due to the complexity of the different biological and environmental matrices . This
8,9
topic has inspired scientists to implement diverse methods to accomplish these aims ⁠ . There are
many analytical methods for the qualitative and quantitative determination of metal ions10–13. The
most employed are atomic absorption spectroscopy, chromatography, inductively coupled plasma
atomic emission spectrometry, among others13–15, which need sophisticated and expensive
equipment10–13. In this regard, chemosensors have been proven as promising devices for the
detection of analytes⁠ due to their simplicity, sensitivity and selectivity12–16. This analysis
procedure can be performed in vivo, and in situ12–16
.
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Optical sensors are chemosensors that detect analytes through measurable changes in properties
such as absorbance, luminescence, light polarization, etc8,17. They are excellent for the detection
of analytes as distinct optical properties can be generated, increased, or diminished upon the
incorporation of the analyte, which makes the signal easily detectable8,17–20. Absorbance is the
principle behind the colorimetric sensors. The analyte produces a change in the UV-Vis absorption
spectra profile of the sensor, eventually leading to a color change. This is a clear advantage to
design devices with accessible cost and rapid tracking of analytes in vivo and in situ. These
peculiarities have increased the interest of researchers, notably in medical, biological and
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environmental science13–16,21,22
.
The nature of change in the UV-Vis absorption spectra is determined by the interaction of the
13
metal ion with the acceptor or donor group in the sensor . If a rich-electron metal ion interacts
with an acceptor group of the sensor (with low-lying empty orbitals), the Metal to Ligand Charge
Transfer (MLCT) mechanism can be activated. This mechanism causes a red-shift (bathochromic
shift) of the bands in the absorption spectra and the oxidation of the metal. If the orbitals of the
metal cation are empty or partially filled, they can interact with a donor group (with relatively
high-energy lone pairs) of the sensor. Therefore, the Ligand to Metal Charge Transfer (LMCT)
can occur and a hypsochromic shift (blue-shift) of the absorption bands is observed as well as the
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reduction of the metal .
The development of chemical sensors for Cu and Ni ions has been rather slow compared to
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+
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other metal ions . Copper is the third most abundant metal in the Earth's crust, one of the most
used metal in daily life and a vital nutrient for living organisms13,23. This metal has a significant
role in many physiological processes as it participates in the formation and performance of brain
tissues, the heart, and other organs13,24. However, when copper concentrations are deficient, less
1
00 mg in an adult person, or become excessive, they can produce alterations for the organism
mainly hematological, and also leukopenia and neuro disorders such as Wilson's, Menke's,
Alzheimer's and Parkinson's disease, among others13,23. For this reason, the determination of serum
ceruloplasmin concentration is used as a control method of this metal ion.25–27. Nickel, unlike
Copper, is required in very small amounts for the metabolic and synthetic processes in the
organisms. However, like Copper, the excess of Nickel can produce serious effects, such as
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disorders in the respiratory system, in the central nervous system, in the dermis, or even cancer,
among others28–31
.
The development of synthetic colorimetric sensors involves expensive and prolonged synthetic
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protocols . To reach this goal more efficiently, rational molecular engineering is a protocol to
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design molecular systems holding predetermined photophysical-properties. In this regard, as
colorimetric sensors performance depends on the electronic properties, the theoretical and
computational characterization constitutes a very helpful tool to explain the transduction
mechanism principle32,33
.
Density Functional Theory (DFT) is the most common theoretical approach implemented to study
metal ions sensors. The most consistent results are obtained by mixing Density Functional Theory
(DFT) and the adiabatic approximation through the Time-Dependent Density Functional Theory
(TD-DFT)34–38. In this sense, it is necessary to obtain the geometry of the energy minimum of the
sensor before and after the interaction with the analyte in the ground state. The vibrational
frequencies analysis is performed for all the optimized structures to confirm that they are minima
on the Potential Energy Surface (PES) and to analyze the coordination process34–37
.
2+
In this framework, two colorimetric sensors with high selectivity and sensitivity towards Ni and
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+
+
2+
2+
2+
2+
2+
2+
Cu ions over other metal cations (Ag , Ba , Pb , Zn , Co , Fe , and Mg ) in acetonitrile-
water (8:2) solution were synthesized and characterized. Metals show distinguishable chemistry
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+
2+
properties and characteristics, i.e. the electronic structure of alkaline earth metals (Mg and Ba )
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+
2+
2+
2+
is different from metals with open-shell d orbitals (Cu , Ni , Co , and Fe ) and with others with
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0
+
2+
2+
closed-shell d (Ag , Pb , and Zn ). These differences allow relating the behavior of the sensors
with the characteristics of metal ions, which were deeply analyzed through a theoretical
methods32,33
.
2. Experimental Section
2
.1. General
For the measurement of sensitivity, perchlorate salts with different cations were used. The
measured solutions contained concentrations of 10 µM for ligands and 50 µM of the perchlorate
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of the respective metal (Ag , Ba , Cu , Ni , Pb , Zn , Co , Fe , and Mg ). The sensor
solution was made in an 8:2 mixture of acetonitrile-buffer (50 mM HEPES at pH 7.2), while the
metal and capacity solutions of the measured solutions were made with an 8:2 acetonitrile-water
mixture. The stoichiometry of the generated complex was determined through the Jobs method,
stock solutions of 10 mM concentration of metal and ligand were used, increasing the molar
fraction of the ligand from 0-1. The competition experiments were carried out with the mixtures
already mentioned, the concentration of the ligation used was 10 µM and the concentration of the
metals used was 20 µM for both the sensor metal and the competitor metal.
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Synthesis of N,N-bis(pyridin-2-ylmethyl)aniline. The synthesis of N,N-bis(pyridin-2-ylmethyl)
aniline was performed with a modification of the method described by Hrishikesan and Kannan39.
In summary, in a reflux system and nitrogen atmosphere, a 2-neck round bottom flask equipped
with an addition funnel and condenser, 13 g (0.075mol) of K HPO and 25 mL of dry acetonitrile
2
4
were added. The solution was stirred at 0°C and 2.4 mL (0.026 mol) of aniline were added
dropwise, the solution was kept under stirring for half an hour after the addition was finished.
Afterward, 11.42 g (0.07mol) of picolyl chloride is added in small portions through the addition
funnel. The final mixture is kept under reflux for 3 days. The solution is cooled, filtered, and
washed with ethyl acetate. The solid is discarded and the solution is concentrated under vacuum
up to 1/3 of its original volume. 10 mL of distilled water are added, and the pH increased to 8 using
3M NaOH. The extractions are performed with DCM (3 times 50mL) and the organic layer is dried
with anhydrous sodium carbonate. The obtained red solution is dried under vacuum and the
obtained solid is recrystallized with hot ethanol.
Synthesis of compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline
(
A) and (E)-4-((3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B). The
respective nitroaniline (10.2 mmol) was dissolved in 10 mL of THF and a solution of 0.523 g (7.58
mmol) of NaNO2 in 10mL of water is added. This solution is cooled to 0 °C and 1 mL of
concentrated HCl is added dropwise under stirring during 30 min. The resulting solution is added
gradually to a solution of 20 mL of THF, 20 mL of water and 1.87g (6.8 mmol) of N, N-bis
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(pyridin-2-ylmethyl) aniline under stirring and at 0 ° C. The resulting mixture should be kept under
stirring and at 0 °C for 3 days. The compound is purified on a silica flash column using hexane
and ethyl acetate in polarity gradient (using each time 400mL of hexane:ethyl acetate in the
following proportions=1:0;9:1;8:2;7:3;6:4;1:1;4:6;3:7;2:8;1:9;0:1). The obtained solid is
recrystallized using hot methanol. It is possible to monitor the formation of product by TLC,
differentiating the precursor and the products with similar RF using the vanadate sulfide reagent
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presenting gray and pink tones caused by the change to acidic pH, respectively . Compound A:
4% yield, dark red semi-translucent crystals. Elemental Analysis: C, 67.91; H, 4.75; N, 19.80; O,
7.54. The mass calculated for: C H N O : 424.46. Mass found: 425.0. The numeration and
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assignation of the protons and carbos can be found in the SI. H NMR (400 MHz, Chloroform-d)
δ 8.62 (d, J = 4.9 Hz, 2H, H-6, H-12), 7.84 (s, 1H, H-25), 7.82 (s, 1H, H-14), 7.80 (s, 1H, H-16),
7.68 – 7.61 (m, 2H, H-1, H-2, H-10, H-11), 7.59 (t, J = 7.7 Hz, 1H, H-26 ), 7.46 – 7.42 (m, 1H, H-
2
7), 7.25 (d, J = 7.9 Hz, 2H, H-3, H-9), 7.20 (d, J = 4.6 Hz,1H, H-28)), 6.82 (d, J = 9.2 Hz, 2H,
1
3
H-13, H-17), 4.94 (s, 4H, H-21, H-22). C NMR (101 MHz, Chloroform-d) δ 157.6 (C-4, C-8),
51.9 (C-18), 150.0 (C-6, C-12), 147.3 (C-24), 146.0( C-29), 144.8(C-15), 137.1(C-2, C-10), 132.8
1
(C-26), 129.0 (C-27), 126.2 (C-14, C-16), 123.9 (C-25), 122.5 (C-28), 120.9 (C-3,9), 118.7 (C-1,
C-11), 112.5 ( C-13, C17), 57.4 (C-21, C-22). Compound B: Yield 53%, orange crystals. Elemental
Analysis: C, 67.91; H, 4.75; N, 19.80; O, 7.54. The mass calculated for: C H N O : 424.46. Mass
2
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found 424.6. The numeration and assignation of the protons and carbos can be found in the SI. H
NMR (400 MHz, Chloroform-d) δ 8.62 (s, 1H, H-29), 8.60 (d, J = 2.3 Hz, 2H, H-6, H-14), 8.18
(
d, J = 6.4 Hz,1H, H-25), 8.11 (d, J = 6.8 Hz, 1H, H-27), 7.83 (d, J = 8.5 Hz, 2H, H-2, H-12), 7.66
7.63 (m, 2H, H-18, H-20), 7.59 (t, J = 8.0 Hz, 1H, H-26), 7.25 (d, J = 7.9 Hz, 2H, H-3, H-11),
7.20 (dd, J = 7.5, 4.9 Hz, 1H, H-1, H-13), 6.83 (d, J = 8.9 Hz, 2H, H-17, H-21), 4.93 (s, 4H, H-7,
–
1
3
H-9). C NMR (101 MHz, Chloroform-d) δ 157.6 (C-16) , 153.7(C-24) , 151.8(C-4,C-10) ,
50.0(C-29), 149.1 (C-28) , 144.2 (C-19) , 137.1 (C-3, C-11) , 129.8( C-26) , 128.8 (C-27) , 125.8
C-2, C12) , 123.5( C-25) , 122.5( C-1, C-13) , 120.9 (C-18,C-20) , 116.5 (C-6, C-14), 112.5(C-
17, C-21) , 57.4( C-7, C-9).
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(
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Data Collection. Single crystals for A and B were obtained from the slow evaporation of ethanol
solutions. Single-crystal X-Ray diffraction data sets were collected at 294 K for A and B up to a
max 2θ of ca. 61° on a Bruker APEX II diffractometer, using monochromatic MoKα radiation, λ
=
0.71073 Å and a 0.3° separation between frames. Data integration was performed using SAINT
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V6.45A and SORTAV (Blessing, 1995) in the diffractometer package . Table 1 shows a selection
of bond lengths and angles, and torsion angles (A and B ). The data collection and structural
c
c
refinement parameters are given in Table S1. The structures were solved by direct methods using
SHELXT-2014 (Sheldrick, 2014) and Fourier's difference methods, and refined by least-squares
on F2 using SHELXL-2014/7 (Sheldrick, 2014) inside the WinGX program environment (Farrugia,
2
012). Anisotropic displacement parameters were used for non-H atoms and the H-atoms were
positioned in calculated positions and refined riding on their parent atoms. Figure 5a and b exhibit
ORTEP views of both compounds. Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication, CCDC No. 1998031 and 1998032 for A and B, respectively.
Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336 033. E-mail: data_request@ccdc.cam.ac.uk.
Web page: http://www.ccdc.cam.ac.uk.
2
.2. Computational Details.
We inform a set of calculations that account the structure and optical properties of the free sensors
(A and B) and coordinated with metals of interest (A/M and B/M). All the S state geometries were
0
optimized with ORCA 4.0 code at the Density Functional Theory (DFT) level with a B3LYP
42
functional and def2-TZVPP basis set for all atoms . The structures were optimized to study the
change in the UV-Vis spectra, through TD-DFT computations43,44. The optical properties were
analyzed through the Coulomb attenuating method (CAM-B3LYP) functionals, which was
developed to minimize deviations in charge-transfer excitation energies43,44. The solvation effects
were considered via a Conductor–like Screening Model (CPCM) using the parameters of
4
5
acetonitrile/water as solvents (Ɛ=36.6 and refraction=1.344) . The coordination number was
included explicitly, considering the most probable for each metal ion46–51, as perchlorate or water
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2
2
2
2
2
2
2
2
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3
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6
2
+
2+
2+
ligands. Due to Pb , Ba and Mg ions do not form coordination bonds, there were not considered.
In all cases, the frequencies were calculated for showing that the geometries obtained are at a global
minimum.
The nature of the interaction A/Mand B/M is studied through the Morokuma-Ziegler decomposition
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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6
7
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1
2
3
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1
2
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5
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7
8
9
0
scheme. The interaction energy (ΔE ) was analyzed by across fragments approximation to the
Int
molecular structure and the decomposition of the ΔE (Equation 1)52–54⁠ .
Int
훥퐸퐼푛푡 = 훥퐸푃푎푢푙푖 +훥퐸푑푖푠 +훥퐸퐸푙푒푐 +훥퐸푂푟푏 (1)
The ΔEPauli has a destabilizing role in the interaction and represents the contribution in energy due
to the antisymmetric of the wave function result. The ΔE is related to the distortion term and
dis
characterizes the quantity of energy required to promote the separated fragments from their
equilibrium geometry to the structure until it will occupy in the combined molecule. The ΔEElec is
associated with the electrostatic interaction nucleus–electron. The ΔEOrb is originated from the
relaxation of the system because of the mixing of occupied and unoccupied orbitals. This
parameter can involve donor-acceptor interactions, charge-transfer, polarization, or electron-pair
bonding. The ΔEOrb is investigated through the Natural Orbital of Chemical Valence (NOCV)
methodology proposed by Mitoraj54,55⁠ . The calculations of the Morokuma-Ziegler
decomposition scheme were carried out using the Amsterdam Density Functional (ADF) code56.
Agree to this scheme, A/M and B/M systems were studied as two fragments that make up all
system: M (Metal with ions perchlorate or water as coordination sphere) and sensor (A and B) (see
Figure 1).
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0
Figure 1. a) Molecular structure of A and B. b) Molecular structures of A/M and B/M systems with M:
+
2+
2+
2+
2+
2+
2+
2+
2+
Ag , Ba , Cu , Ni , Pb , Zn , Co , Fe , and Mg .
3
. Results and Discussion
3
.1. General
As shown in Table 1, a good agreement between the experimental and theoretical results was
obtained. A small deviation is observed in the torsion angle of the methylbenzene portion of the
molecule. The crystallization of the sensors bounded to the studied metals was not possible.
Experiments using metal-ligand concentrations 5:1 have demonstrated the chemosensing ability
2
+
2+
of compounds A and B of Cu and Ni cations. Furthermore, there is a drastic color change from
yellow to colorless, which allows the effortless detection of the presence of the mentioned metals
(see SI, Figure S1). This color change is explained as in the UV-Vis measurements one of the
absorption bands of the sensors at 428 and 420 nm, for A and B respectively, decreases its intensity
2
+
2+
in the presence of Cu and Ni (see Figure 2). Furthermore, a new absorption band belonging to
the sensor/metal interaction is observed. This absorption band has in all cases a hypsochromic shift
2+
to the characteristic signal of the free sensors. In the case of Fe , a new band is also observed for
the sensor/metal interaction, however, the color does not vanish, as for the two-other metal, due to
the small shift of the mentioned band, thus there is no pronounced change in color.
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0
−
5
Figure 2. UV-Vis absorption spectra of sensor a) A and b) B [1×10 M in CH CN/H O (80:20, v/v)] in the
3
2
+
2+
2+
2+
2+
2+
2+
2+
2+
presence of different cations (5 equiv.) of Ag , Ba , Cu , Ni , Pb , Zn , Co , Fe , Mg ions.
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2
+
2+
The determination of the sensor stoichiometry was performed for the metals Cu and Ni ; on the
2
+
other hand, Fe cation was not considered as the new absorption band does not allow for a color
change, additionally, it presents a small difference between the signals. The sensor/metal
stoichiometry was analyzed using the Jobs chart methodology, where the absorbance v/s molar
fraction is plotted. For both studied metals, it was determined that the λmax of the bands of each
sensor-metal complex were reached in the solution with 0.5 molar fraction, which corresponds to
a ratio of sensor/metal 1:1 (see Figure 3 and S2).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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9
0
1
2
3
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
+
Figure 3. Job plot of a 1:1 complex of sensor A and Ni ion, where the absorption was 345nm and was
plotted against the mole fraction of the metallic ions at an invariable total concentration of 10 μM in
CH CN/H O (80:20, v/v).
3
2
Furthermore, selectivity is one of the most important characteristics of a sensor, as it determines
its utilization efficiency in a real sample. Thus, to explore the practical applicability of A and B as
2+
2+
Cu and Ni selective sensors, competition experiments were also performed in 2.0 equiv. of
2
+
2+
+
2+
2+
2+
Cu or Ni mixed with 2.0 equiv. of background metal cations such as Ag , Ba , Cu , Ni ,
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+
2+
2+
2+
2+
Pb , Zn , Co , Fe , and Mg . The absorbance was measured at the λ
sensor-metal
max
in the UV-Vis spectrum. The ΔA(A-A ) parameter is used because the free sensors has
0
still small absorbance in λmax, and this procedure eliminates the signal of absorbance of
the free sensor, which allows a better interpretation of the cation’s interference. As can be
0
1
2
3
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8
9
0
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8
9
0
1
2
3
4
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6
7
8
9
0
2
+
2+
observed in Figure 4, the largest interference was observed for Fe and Co for both studied
2+
sensors. As discussed before, the UV-Vis profile of the sensor-Fe complex has a hypsochromic
shift of the λmax, however, there is no visual change upon sensor-metal interaction, thus competition
2
+
is observed. In the case of Pb , when sensor A is used a decrease in the absorption band of the
2
+
2+
complexes with Cu and Ni is observed (see Figure 4 and S3). In contrast, sensor B does not
2+
show Pb interference (see Figure S3).
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-
5
-5
Figure 4. Responses of A (1*10 M) in CH CN/H O (80:20v/v) to representative metal ions (2*10 M) and
3
2
2
+
Cu , molar fraction is 0.2.
The crystal structure for both studied sensors was determined in its free state. ORTEP views of
both structures are shown in Figure 5. Compound A crystallizes as two molecules per asymmetric
unit with similar but not identical conformations. As could be observed both structures show an
almost planar structure on the phenyl azo ring portion. Based on the crystal structure analysis and
theoretical calculations, a series of angles, torsion angles and bond lengths of interest are analyzed
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
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5
5
5
5
5
5
5
6
0
1
2
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7
8
9
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7
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9
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2
3
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
see Table 1). As compound A crystallizes in two molecules per asymmetric unit, he
conformational magnitudes in Table 1 are given as the average of both. On the other hand, the
skeleton of the B molecule exhibits a disorder following an approximate binary axis with unequal
populations of 62:38; in this case, only the major component has been listed. Table 1 shows good
agreement between the experimental and theoretical data for the structures. The variation is only
significant for the C -N -C -C of the B and C -N -C -C to both (A and B ). This difference is
1
4
3
6
C
1
4
2
4
C
C
due to the freedom degrees of rotation which these angles have in solution. Root Mean Square
Deviations (RMSD) provide a quantitative evaluation of the differences between two structures,
based on their atomic positions. Per this parameter, the differences are ~ 4% and ~ 6% to A and
B, respectively. Furthermore, a comparison between the PXRD pattern from the crystal and the
bulk sample was performed. Both samples show the same PXRD pattern, showing that both
samples have the same composition, see SI (Figure S14).
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0
1
2
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5
6
7
8
9
0
Figure 5. ORTEP diagrams with 50% of probability ellipsoids for compounds a) A, and b) B. Optimized
structure of sensors c) A and d) B.
*
Table 1. Structural parameters obtained through X-ray crystal structure analysis ( A and B ) and
C
C
theoretical calculated (A and B ).
T
T
System
AT
AC
BT
BC
d(C-N1)
d(C-N2)
d(C-N3)
d(C -N )
1.472
1.408
1.396
1.375
1.452
1.453
1.334
1.332
1.337
1.331
1.463(5)
1.4605(4)
1.46(4)
1.477(4)
1.414(4)
1.396(5)
1.375(4)
1.452(5)
1.453(5)
1.333(5)
1.332(6)
1.337(5)
1.331(6)
1.448
1.455
1.441
1.364
1.450
1.453
1.331
1.335
1.335
1.331
1.373(5)
1.456(5)
1.4455(5)
1.2995(5)
1.3615(6)
1.3225(5)
1.353(7)
1
4
d(C -N )
2
4
d(C -N )
3
4
d(C -N )
4
5
d(C -N )
5
5
d(C -N )
6
6
d(C -N )
7
6
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d(N =N )
1.255
114.7º
116.1º
120.7º
177.1º
37.5º
1.216(4)
109.7(3)º
112.0(3)º
120.4(4)º
178.75(3)º
-66.95(5)º
10.6(4)º
1.255(4)
114.9(4)º
116.3(4)º
118.8(3)º
-179.6(4)º
36.2(4)º
1.238
112.6
112.5
118.5
-179.8
157.8
174.8
76.7
2
3
α(C-N =N )
2
3
α(N =N -C)
2
3
α(N -C-C)
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
γ(C-N=N-C)
γ(N -C -C -N )
4
2
4
5
γ(N -C -C -N )
176.9º
91.0º
177.1(4)º
81.3(5)º
4
3
6
6
γ(C -N -C -C )
77.6(5)º
1
4
2
4
γ(C -N -C -C )
81.4º
79.2(5)º
91.1(5)º
76.9
1
4
3
6
*
Where α and γ are bond angles, and d is the bond lengths in Å. A is the average for the two molecules
c
per asymmetric unit
3
.2. Optical properties and sensing mechanisms
The UV-Vis absorption spectra of A and B are experimentally characterized by two bands at ~260-
28 nm and ~260-420 nm, respectively. The UV-vis absorption spectra show a blue-shift (~ 90
4
2
+
2+
nm) of the maximum absorption peak upon the interaction with Cu and Ni ions. To design
more efficient and effective systems in the future, the rationalization of the sensing mechanisms
of metal ions is important. In this sense, quantum chemical computations were also carried out for
+
2+
2+
2+
2+
2+
structures of A, B, and metal complex A/M and B/M, (where M: Ag , Ba , Cu , Ni , Pb , Zn ,
2+
2+
2+
Co , Fe , Mg ) in the ground state (S ). The structure geometries were optimized in the ground
0
state (S ) states and where the input data to calculate the excitations that constitute the UV-Vis
0
absorption spectra. The optimized geometries are shown in figures from 5 to 7.
2
+
2+
The coordination of the metal is quasi-planar square for the Cu and Ni ions, trigonal planar for
+
2+
2+
the Ag ions, hexagonal for the Co and Fe ions, and tetrahedral for other metals ions (see Figure
6 and 7). The coordination obtained are characteristic of these metal ions46,47,49,51,57. When A and
B interact with the metal ions, a structural change in the S state of these sensors is observed. As
0
this change is produced in all systems (although de sensors lose-more planarity in the complexes
2
+
2+
with Cu and Ni ions) (see Figures 6 and 7), it is not possible to directly correlate this change
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6
with the marked differences in their absorption spectra only through this analysis. In this sense, a
time-dependent analysis (TDDFT) was performed to see the nature of the transitions.
0
1
2
3
4
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7
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9
0
1
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2
3
4
5
6
7
8
9
0
+
2+
2+
2+
2+
2+
2+
2+
2+
Figure 6. Optimized structures of the A/M (M: Ag , Ba , Cu , Ni , Pb , Zn , Co , Fe , Mg )
complexes in S states.
0
+
2+
2+
2+
2+
2+
2+
2+
2+
Figure 7. Optimized structures of the B/M (M: Ag , Ba , Cu , Ni , Pb , Zn , Co , Fe , Mg )
complexes in S states.
0
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TDDFT allowed to reproduce the two main absorption bands observed experimentally for A and
B at ~260 and ~420 nm (these transitions are tabulated in Tables 2 and 3). As can be observed,
there is a small difference with the experimental values, around ~30 nm, which is within the error
range reported for TDDFT computations34,58. The experimentally blue-shift of the maximum UV-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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1
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1
2
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4
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6
7
8
9
0
2
+
2+
vis absorption band (~ 90 nm) induced by the interaction of A and B with Cu and Ni ions are
2
+
also well reproduced. The smallest blue-shift of the absorption band in the case of Fe (~30 nm)
systems are reproduced as well (see Table 2 and 3).
2+
2+
2+
In the B/M systems, a blue-shift can be seen for other metal ions (Zn , Mg , and Pb ) (see Table
3, Figure S4 and S5), although this behavior is not experimentally evidenced. However, the
calculated absorption band is characterized by the →* transition centered on the ligand for all
2
+
2+
systems (see Figure from S4 to S6), except for the complexes with Cu and Ni ions. For these
systems, the transition is characterized by a σ* molecular orbital with a strong contribution of the
2
2
2+
2+
3d
transition for the Cu and Ni ions (see Table 2, Figure 8 and 9). These absorptions imply
x -y
a ligand to metal charge transfer (LMCT) that results in a blue-shift in the absorption band, as
13
experimentally evidenced. The blue-shift is characteristic of the LMCT transition and is very
common in transition-metal complexes47,59,60. This charge transfer can be established by the
availability of its d orbitals for accepting electrons from some ligand rich in electrons46,59
.
Table 2. Singlet→Singlet absorption data in A and A/M considering the solvent effect (Acetonitrile, =36.6
and refraction=1.344).
CAM-B3LYP
Active MOs
B3LYP
*a
a
Systems
A
f
A
f
Active MOs
A
3
2
36.8 0.240
22.7 0.103
H→L+1
→*
→*
393.9 1.099
264.9 0.128
H→L+1
H→L+7
→*
→*
→3dx2-y2
→*
H-2→L+2
→3dx2-y2 320.9 0.436
→*
266.7 0.165
297.6 1.020
H-1→L+3
H-3→L+1
H-1→L+3
H-3→L+1
A/Ni2+
2
2
48.8 0.130
95.3 0.356
H
-1→L
+1 →3dx2-y2 302.7 0.435
H
-1→L
+1 →3dx2-y2




A/Cu2+
222.8 0.130 H -3→L +1
→*
→*
264.1 0.240 H -3→L +1
→*
→*




A/Co2+
309.8 0.727
412.8 0.545 H -1→L +1
α α
α
H →Lα
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
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4
4
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5
6
251.3 0.103
H -4→L
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
239.1 0.046 H -3→L +2
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
α
α
α
α
3
2
03.7 0.845
49.9 0.129
H→L
H-3→L
404.0 0.632
260.4 0.109
H-1→L
A/Zn2+
H-4→L+1
3
2
28.8 0.564
H -1→L
367.5 0.160
H -1→L
α α
α
α
A/Fe2+
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
25.1 0.100 H -1→L +2
243.3 0.279 H -1→L +5
α α
α
α
3
2
62.6 0.962
25.6 0.138
H→L+1
H-4→L
401.4 0.689
268.2 0.124
H-2→L+1
H-4→L+2
A/Ag+
296.1 0.694
H→L
367.5 0.284
304.8 0.539
H-1→L
A/Mg2+
A/Ba2+
A/Pb2+
*
2
3
49.3 0.132
61.2 0.245
H-3→L
H-1→L+1
H→L
391.2 0.484
269.4 0.197
H→L+1
319.2 0.326
H-1→L
H-5→L+1
3
3
20.4 0.132
01.5 0.887
H→L
384.5 0.183
314.3 0.178
H-1→L
H-1→L+2
H-1→L+2
Where  is the theoretical absorption wavelength in nm, A is the assignment of transitions, f is the
a
oscillator strength, H (HOMO) and L (LUMO).
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Table 3. Singlet→Singlet absorption data in B and B/M considering the solvent effect (Acetonitrile, =36.6
and refraction=1.344).
CAM-B3LYP
Active MOs
B3LYP
*a
a
Systems
B
f
A
f
Active MOs
A
→*
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
2
79.9 1.114
39.5 0.168
H→L
→*
→*
407.4 1.420
262.3 0.193
H→L+1
H→L+7
H→L+4
→*
→3dx2-y2 325.5 0.890
→3dx2-y2
295.5 1.318
H→L+1
H→L+2
H-1→L+1
H-4→L+2
B/Ni2+
B/Cu2+
B/Co2+
2
16.5 0.203
08.1 0.215
→*
251.0 0.111
311.2 0.663
→*
→3dx2-y2
→3dx2-y2
3
2
H

-1→L

+1
H -1→L +1
 
49.5 0.241 H -5→L +3
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
231.9 0.061 H -5→L +3
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*
→*




3
2
19.5 1.157
H →L
347.9 0.291
H →L +1
α α
α
α
54.1 0.263 H -6→L +1
275.0 0.132 H -4→L +1
α
α
α
α
297.0 1.370
H→L+2
H→L+5
336.9 1.257
268.7 0.180
H→L+1
B/Zn2+
2
3
33.6 0.352
21.5 1.030
H-4→L+1
H →L
371.6 0.208
H -2→L
α α
α
α
B/Fe2+
254.1 0.295
H -7→L
235.8 0.607 H -2→L +1
α α
α
α
3
2
62.7 1.187
38.5 0.144
H→L
412.3 0.839
257.4 0.159
H→L
B/Ag+
H→L+6
H→L+1
H-1→L+8
3
1
10.2 1.163
336.4 1.236
187.7 0.165
H→L+1
H-4→L
B/Mg2+
65.5 0.178 H-2→L+4
33.7 1.261 H→L
3
372.9 1.171
272.4 0.279
H→L+1
B/Ba2+
234.5 0.237 H-2→L+1
H-4→L+1
308.2 1.182
55.3 0.364
H→L
328.1 1.064
265.5 0.102
H-1→L+2
H-4→L+2
B/Pb2+
2
H-4→L
*
Where  is the theoretical absorption wavelength in nm, A is the assignment of transitions, f is the
a
oscillator strength, H (HOMO) and L (LUMO).
On the other hand, when the solvent effect is not considered, the relative energy of the transitions
displays significant change and the  are not closer to the experimental wavelength (Table S4 and
a
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S5). The computed data using B3LYP functional resemble the experimental results more than by
CAM-B3LYP values in terms of the relative energy of the transition (see Tables 2 and 3).
0
1
2
3
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3
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6
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9
0
Figure 8. Molecular orbital diagram based on the S state (absorption) in the free sensors (A and B), in
0
2
+
2+
the A/Ni and B/Ni . Where  is the theoretical absorption wavelength and f is the oscillator strength.
a
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1
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9
0
2
+
Figure 9. Molecular orbital diagram based on the S state (absorption) in the A/M and B/M (M: Cu and
0
2+
Co ) complexes in S states. Where  is the theoretical absorption wavelength and f is the oscillator
0
a
strength.
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The analysis of the Morokuma-Ziegler decomposition schema provides the compression of the
composition of the interaction energy. According to this method, the electron transfer is established
from the ligand to metal and the metal to ligand (Figures 10 and S7 to B/M). The interaction does
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
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+
not exhibit a marked difference between orbital or electrostatic character. Only, results for Ni ,
2
+
+
Cu and Ag ions, show a large electrostatic character. This implies that the LMCT can be more
favored in these systems than in the others (see it in Tables 4, S4 and S5).
Table 4. Morokuma-Ziegler EDA (contribution to the ΔEInt in %)*.
Systems
A/Ni2+
A/Cu2+
B/Ni2+
ΔEInt
-226.6
-184.0
-237.9
-185.5
ΔEPauli ΔEElec ΔEOrb ΔEDisp %Eelec %Eorb
%Edisp
k
259.0 -242.2 -228.4 -15.0
196.6 -203.2 -161.8 -15.6
236.3 -240.4 -218.3 -15.5
196.7 -201.9 -1617 -15.6
50
53
51
53
47
43
46
43
3
4
3
4
-120.0
-70.4
-104.3
-70.1
B/Cu2+
Figure 10. Contours of the NOCV deformation density  for A with metals.
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. Conclusions
Two new chemosensors were experimentally developed, they show high selectivity and sensitivity
2
+
2+
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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0
1
2
3
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9
0
1
2
3
4
5
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8
9
0
to Cu and Ni ions over other metal cations in CH CN/H O solution. These sensors exhibit a
3 2
drastic color change from yellow to colorless, which allows the detection of the presence of the
2
+
2+
mentioned metals. The sensor/metal interaction, to Cu and Ni ions, induced a new absorption
band with a hypsochromic shift respect to the characteristic signal of the free sensors. For both
studied metals correspond a ratio of sensor/metal of 1:1. The theoretical research has enabled us
to rationalize the behaviors of the two sensors. The blue-shift of the maximum UV-vis absorption
2
+
2+
band (~ 90 nm), by the interaction of the sensors with Cu and Ni ions, are reproduced through
2+
the TDDFT. The smallest blue-shift of the absorption band in the Fe (~30 nm) systems is
2
+
2+
2+
reproduced too. As the systems B/M (M: Zn , Mg , and Pb ) theoretically show also a blue shift
in the absorption band, it is necessary to analyze the nature of the transitions to correctly explain
2+
2+
the experimental behavior of the sensors. For all systems, except for complex with Ni and Cu
ions, the absorption band is characterized by the →* transition centered in the ligand. Instead
of Ni2+ and Cu ions, the transition is set towards the σ* molecular orbital with a strong
2+
contribution of 3d 2 transition for Ni+2 and Cu+2 ions. These absorptions imply an LMCT
2
x -y
mechanism that results in the blue shift in the absorption band of these systems. When the solvent
effect is not considered, the relative energy of the transition displays significant change. The
computed data using B3LYP functional resemble the experimental results more than by CAM-
B3LYP values in terms of the relative energy of the transition. According to the Morokuma-Ziegler
decomposition schema, the electron transfer is established from the ligand to metal and the metal
to ligand for all systems. This interaction does not present a marked orbital or electrostatic
2
+
2+
+
character. Only for the Ni , Cu and Ag ions are shown a large electrostatic character. This result
entails that the LMCT can be more favored in these systems than the others.
Supporting Information
Crystal data and structure refinement, job plots for all studied sensors, competition measurements,
TDDFT results, molecular orbital diagram for the studied sensors, Morokuma-Ziegler EDA, NMR
spectra, Mass spectra and PXRD.
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Acknowledgment
This work was supported by CONICYT-Chile under FONDECYT-Chile projects Nos. 1201880,
180565 and 1180017, PIA CCTEAFB170007 and FONDEQUIP EQM 160070. This work was
1
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also supported by ANID/FONDAP/15110019, CORFO 14ENI2−26862 projects and Millennium
Science Initiative of the Ministry of Economy, Development and Tourism-Chile, Nuclei on
Catalytic Processes towards Sustainable Chemistry (CSC)-Chile grant.
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