Enhanced photocatalytic degradation of 2-thiobenzimidazole by the tris(8-quinolinolato)cobalt(iii) complex through peroxide adduct formation: Theoretical and experimental investigations

Article abstract of DOI:10.1039/c9nj04027g

Photo-initiated oxidative degradation of 2-thiobenzimidazole (2-TBI) was studied using different semiconducting metal complexes derived from 8-hydroxyquinoline with the general formula, [MQ₃C₂H₅OH] [Q = 8-quinolinolato anion; M = Cr³⁺, (CrQ3); Co³⁺, (CoQ3); Fe³⁺, (FeQ3)]. The degradation kinetics was monitored via UV-vis spectroscopy perceiving the changes in absorbance to subtract. The effect of pH and visible light irradiation on degradation was studied observing a significant increment in the oxidation rate with the presence of visible light whenever CoQ3 is used as a catalyst. By using density functional theory (DFT), it was found that the transition state energy of the adduct formed between CoQ3 and H₂O₂ is very low compared with other studied catalysts, which activates the formation of OH radicals from H₂O₂ increasing the 2-TBI degradation rate. In addition, a thermodynamically feasible degradation mechanism was proposed by calculating energy barriers involved in the 2-TBI oxidation pathway.

Full text of DOI:10.1039/c9nj04027g

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DOI: 10.1039/C9NJ04027G
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Enhanced photocatalytic degradation of 2-thiobenzimidazole by
Tris(8-quinolinolato)cobalt(III) complex through peroxide adduct
formation: Theoretical and experimental investigation
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
Carlos Alberto Huerta Aguilar , Jayanthi Narayanan *, José Abraham Balderas López , Marcos
DOI: 10.1039/x0xx00000x
c
d
d
Flores-Alamo , Enrique Fernando Velázquez Contreras , Karla Alejandra López Gastélum ,
Fernando Rocha Alonzo , Cristian Brayan Palacios Cabrera , Alan Javier Santiago Cuevas
d
a
a
Abstract
Photo-initiated oxidative degradation of 2-thiobenzimidazole (2-TBI) was studied using
different semiconducting metal complexes derived from 8-hydroxyquinoline of the
3+
3+
general formula, [MQ C H OH] [Q= 8-quinolinolato anion; M= Cr , (CrQ3); Co , (CoQ3);
3
2 5
3
+
Fe , (FeQ3)]. The degradation kinetics was monitored by UV-vis spectroscopy perceiving
the changes in absorbance of subtract. The effect of pH and visible light irradiation on
degradation was studied observing a significant increment in the oxidation rate with the
presence of visible light whenever CoQ3 is used as a catalyst. By using density functional
theory (DFT), it was found that the transition state energy of the adduct formed between
CoQ3 and H O is very low compared with other studied catalysts, which activates the
2
2
formation of ·OH radicals from H O increasing 2-TBI degradation rate. In addition, a
2
2
thermodynamically feasible degradation mechanism was proposed by calculating energy
barriers involved in the 2-TBI oxidation pathway.
1. Introduction
show that significant modifications in the structure of metal
complex exhibit a considerable advancement for the oxidation
1
3-17
processes.
For example, Schiff base metal complexes which
Metal complexes of Schiff base ligands are considered in the are highly photoactive, involve very strongly in the production
photocatalytic reactions owing to their interesting photo- of Reactive Oxygen Species (ROS) that can be employed in the
1
8
physical properties that indorse the excited-state metal degradation of organic contaminants. Therefore, it has been
complexes experience unusual structural and electronic considered that the catalytic oxidation is highly related with
1-7
changes generating Fenton-like catalytic reactions. Thus, in structure of the metal complex, type of ligands, and oxidation
photo/Fenton reactions, metal complex reacts with H
state of metal ions; besides pH, solvent medium, the
2 2
O
1
9-22
under UV light to generate ·OH radicals, which induce the concentration of substrate or catalyst are crucial factors.
3
, 8-12
oxidation process increasing its velocity.
Recent studies However, bandgap energy of the metal complex plays a crucial
role since that determine the amount of energy required to
produce photo-excited electron-hole pairs; it has not been
2
3-27
much focused in the literature.
Therefore, the study of
HOMO-LUMO influenced photocatalytic efficiency of the metal
complex with activation energy barriers involved in the
oxidation process is still a topic of interest. Hence, to
understand the mechanism of photo-induced electron/ energy
transfer in the photocatalytic degradation process, study of CB
and VB alignment of the photocatalysts with reference to the
activation energy that involved in the formation of ROS is
a
División de Ingeniería en Nanotecnología, Universidad Politécnica del Valle de
México, Av. Mexiquense s/n esquina Av. Universidad Politécnica, Tultitlan,
Estado de México, C.P. 54910, México.
Instituto Politécnico Nacional-UPIBI, Avenida Acueducto S/N, Col. Barrio la Laguna,
Del. Gustavo A. Madero, C. P. 07340, CDMX, México.
Facultad de Química, Universidad Nacional Autónoma de México,
Ciudad Universitaria, Coyoacán 04510, México D.F., México.
Departamento de Investigación en Polímeros y Materiales, Universidad de Sonora,
Blvd. Luis Encinas y Rosales S/N, Col. Centro, Hermosillo, Sonora, México.
b
c
d
2
3, 28
important.
Thus, the present work deals with the oxidation
of 2-thiobenzimidazole (2-TBI) by different metal complexes
Contact e-mail address: jnarayanan@upvm.edu.mx
xxxx | New J. Chem., 20xx, xx, xxxx--xxxx
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under visible light irradiation, and bandgap energy of the using a CCD area detector (λMoKɑ = 0.71073VieÅw, ArgtircalepOhnilitnee
metal complexes is correlated with their catalytic efficiency monochromator) equipped with a sealedDOtuI:
using DFT. The use of 2-TBI has been increased in several 130 K. The crystal-to-detector distance was 55 mm. Crystal
industries such as pharmaceuticals, pesticides, rubber, etc., unit cell was determined with a set of 15/3 narrow frame/runs
consequently, a large quantity of this compound is being (1° in   scans. A data set of 225 frames of intensity data was
released into the surroundings, and it is persistent in the collected, and integrated by using an orientation matrix by
b
10
e
.1
X
0
-
3
r
9
a
/C
y
9
s
N
o
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u
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r
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c
0
e at
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G
29-33
environments due to its poor water-solubility.
Schiff base CrysAlisPro and CrysAlis RED software packages employing the
complexes of Cr , Co , and Fe with 8-hydroxyquinoline were narrow frame scans. Final cell constants were determined with
prepared and used for the oxidation of 2-TBI by H under using total 3486 reflections for the structural refinement ( <
3+
3+
3+
0
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2 2
O
visible light. A mechanism for this degradation process is 29.461°). Collected data were corrected using analytical
proposed by analysing the degraded products. In addition, the numeric absorption by considering a multifaceted crystal
thermodynamic feasibility of the mechanism was analysed by model. The structure was solved using a dual-space
DFT. algorithm method with SHELXT-2014 and by full-matrix least
3
4
squares refinement was performed by minimising (Fo
using SHELXL-2014 in the WinGX program packages.
2
–Fc
2
)
2
3
5-37
All
2
. Experimental
non-hydrogen atoms were refined anisotropically; the H atoms
bound to ethanol molecule were located from difference
Fourier maps and refined as riding with Uiso(H) = 1.5Ueq(-OH).
The H atoms attached to the C atoms were placed in
geometrically idealized positions and refined using riding
model with isotropic thermal parameters: Uiso(H) = 1.2Ueq(-
2
.1. Materials and instruments
All reagents purchased from Aldrich Chemical & Co used as
received, and solvents were distilled before using in the
experiments. Elemental (C, H and N) analyses were carried out
on using Fissions (Model EA 1108 CHNS-O). For all samples,
Fourier transformed infrared (FTIR) spectra in the range of
2 3
CH,-CH ) and Uiso(H) = 1.5Ueq(-CH ). Crystallographic data
have been deposited with Cambridge Crystallographic Data
Centre as supplementary material number CCDC 1904242.
Copies of the data can be obtained free of charge from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; P: +44 (0)1223
4
000-400 cm-1 were recorded on a Perkin Elmer Frontier FTIR
3
36408;
F:
+44
(0)1223
336033;
e-
instrument. 1H NMR spectra was recorded on Varian VNMRS
equipment (300 MHz) by using DMSO as an internal standard.
Powder X-ray diffraction (XRD) analysis were performed on a
D8 Advance Da Vinci equipped with a Bruker AXS theta-theta 2.4. Computational studies
configuration with CuKα (l= 1.54 Å). UV-Vis spectrophotometer
mail:deposit@ccdc.cam.ac.uk.
(Perkin-Elmer Lambda 25) was employed to analyse the
To understand the electron migration mechanism from the
metallic centre of the complex to H , and the formation of
intermediates from the substrate, theoretical calculations
based on Density Functional Theory (DFT) were carried out
electronic properties for the complexes and used to monitor
the oxidation of 2-TBI. The intermediates formed from the
oxidation of 2-TBI were identified by a chromatographic
technique (Agilent 1200 HPLC equipped with a 150 mm Zorbax
C-18 column) coupled to a 6410 MS ESI+ detector.
2 2
O
38
using Gaussian 16 software. The geometry of all the metal
complexes was optimized; the energy of its ground state and
the first excited state was calculated by using its corresponding
spin multiplicity. All ground-state calculations were performed
using a uB3LYP/LANL2DZ hybrid basis set while the Time-
Dependent (TD-DFT) calculations with the aforementioned
methods were used for the first (S0→S1 excited state
2.2. Degradation of 2-TBI
The degradation of 2-TBI was performed by different
complexes under visible light, the results were compared with
those obtained without the light irradiation, and the effect of
pHs (7± 0.5 and 4± 0.5) was studied. Typically, metal complex
39
processes. The structure of the ligand was fully optimized
and the corresponding data were then used as input for the
optimization of each metal complex. To understand the
molecular orbital contribution to the electronic absorption
bands for the metal complexes, TD-DFT was employed using
water as a solvent given by IEF-PCM electrostatic solvent
model. Additionally, the charge distribution in the structures
(
0.05 mM) used to oxidize the substrate (2-TBI, 0.05 mM), in
H2O2 (0.5 % v/v) in an aqueous solution under visible light.
The concentration of the substrate was monitored by UV-Vis
absorption profile by measuring the intensity of 2-TBI and
recorded in the range between 200 and 350 in different time
intervals. The light source for the irradiation was 800 lm, 60 W
tungsten lamp. During the oxidation study, characterization of
the reaction intermediates were carried out by NMR and
GC/MS techniques.
40
was analysed by Natural Population Analysis (NPA). Internal
Reaction Coordinate (IRC) parameters such as Activation
Energy (E ), and Transition States (TS) were elucidated using a
a
BBK95 functional and a QST3 guess method with a subsequent
TS optimization. The thermodynamic feasibility of the
41
optimized TS was determined through IRC calculation.
2
.3. Crystallographic studies
2
.5. Synthesis of metal complexes of 8-hydroxyquinoline
Crystal structure of CoQ3 was resolved by X ray diffraction
technique on Oxford Diffraction Gemini "A" diffractometer
[
2 5
CrQ3·C H OH] (CrQ3)
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The preparation of CrQ3 is followed as reported previously.42 quintet, m: multiplet): 9.01 (d, 1H), 8.92 (d, 1H),V7iew.9A8rtic(tle,O1nHlin)e,
To a ligand 8-HQ (0.436 g, 3.0 mmol) dissolved in an ethanol 7.75 (t, 1H), 7.59 (d, 1H), 7.45 (d, 1H), 4.9DO(qI: .10), 7G),
,
1
2
0
H
3
3
9/
.
C
3
9
0
N
(
J
m
04
, 1H
0
2
(
200 mL) containing NaOH (0.12 g, 3.0 mmol), CrCl
3
·6H
2
O
1.01 (t, 3H).
(0.266 g, 1.0 mmol) in ethanol was added; the resulting
solution was kept at 30°C for 24 hrs, observing a colour change
from green to dark red. A reddish brown precipitate was 3. Results and discussions
obtained after removing solvent by roto-evaporator; it was re-
crystallized from a solvent mixture of acetonitrile and absolute
ethanol (1:1, v/v) to yield bright reddish-brown crystals. 3.1. FTIR spectra
Elemental analysis: anal. calc. (%) for C29H24CrN3O4: C, 65.65;
0
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H, 4.56; N, 7.92; found (%): C, 65.60; H, 4.44; N, 7.89. FT-IR
-
1
−1
The IR frequencies (λmax/cm ) of ligand 8-HQ determined with
those of metal complexes (Table S1), observed a remarkable
shift in the frequency for the complex from the ligand (Fig. S1).
(ν
max/cm ) (Fig. S1): bending vibrations at 520vs, 616
in coordination ring formation), 1373vs δ[O-H], 1462vs δ[α-CH
and pyridine ring stretching vibration at 1497vs were assigned.
Other signals in the FT-IR spectrum were 1571 [ν(C=C], 1321
s
(pyridine
2
],
-1
For example, υ(O-H stretching vibrations at 3034 and 709 cm
s
s
6
1
from the phenolic group observed for 8-HQ were disappeared;
OH stretching vibrations due to the intramolecular hydrogen
[
3
(ν(C-H], and 1277
00MHz) δ (s: singlet, d: doublet, t: triplet, q: quintet, m:
multiplet): 8.75 (d, 1H), 8.23 (d, 1H), 7.45 (t, 1H), 7.40 (t, 1H),
.28 (d, 1H), 7.05 (d, 1H), 4.87 (q, 2H), 3.31 (m, 1H), 1.2 (t, 3H).
m
[(ν(C=N]. H NMR (Fig. S2) (DMSO-d ,
−
1
bonding were observed in the range, 3200–3500 cm for the
4
3-44
complex systems.
Similarly, [υ(C=Npy] stretching vibration
7
-1
-1
detected at 1580 cm for 8-HQ was shifted to 1572-1574 cm
for the metal complexes, revealing the involvement of imine
4
5-46
[
CoQ3·C
2 5
H OH] (CoQ3)
nitrogen in chelation with the metal ion.
Also, the aromatic
-
1
C=C stretching vibration of benzene at 1501 cm for 8-HQ was
shifted to 1495-1497 cm- in the presence of metal ions,
suggesting the presence of distortion in the benzene ring
caused by the coordination of hydroxyl oxygen atom with the
metal ion. Besides, all the metal compounds showed the
1
To an ethanolic solution (200 mL) of the ligand 8-HQ (0.436 g,
.0 mmol) containing NaOH (0.12 g, 3.0 mmol), CoCl2·6H2O
0.238 g, 1.0 mmol) in ethanol was added, and the resulting
3
(
solution was reflexed for 24 hrs at 30 °C, observing a colour
change from violet to red. A red precipitate obtained was
further recrystallized from a solvent mixture of acetonitrile
and absolute ethanol (1:1, v/v) to obtain the deep red crystals,
which was suitable for X-ray analysis. Elemental analysis: anal.
-1
following ring deformation vibrations: α(-CH
2
), 1462 cm ; υ(C-
-1
H), 1318 to 1321 cm ; υ(=C-H and =CH
2
out-of-plane), 800-821
cm , which were not observed in the ligand alone confirms the
involvement of 8-HQ in the metal chelation.
-1
calc. (%) for C29
3 4
H24CoN O : C, 64.80; H, 4.50; N, 7.82; found
−1
(
%): C, 64.86; H, 4.46; N, 7.90. FT-IR (νmax/cm ) (Fig. S1): 3.2. X-ray structure analysis of [Co(8-quinolinolato)₃·C H OH]
2 5
bending vibrations at 819m [=C-H & =CH2]out of plan, and very
weak ring wagging vibrations at 910vw [CH2=CH- and -CH2]out of
3 2 5
Crystals of [Co(8-quinolinolato) ·C H OH] were grown in
plane were consigned. Also, bending vibrations at 733
78 (α-CH
bending), and a strong pyridine ring stretching vibrations at 1497
m
and
ethanol at room temperature; the crystal structure was
resolved by single crystal X-ray diffraction analysis at T = 130 K.
A perspective view of the molecular structure of CoQ3 is
shown in figure 1, and the crystal data and refinement details
are given in Table 1. Selected bond lengths and bond angles
are shown in table 2 that are consistent with the previously
7
m
(O-Hout of plan), 1318w (O-Hbending in plan), 1462
m
2
s
6
1
m
and 1574 were observed. H NMR (Fig. S2) (DMSO-d ,
300MHz) δ (s: singlet, d: doublet, t: triplet, q: quintet, m:
multiplet): 8.75 (d, 1H), 8.27 (d, 1H), 7.50 (t, 1H), 7.40 (t, 1H),
7
.3 (d, 1H), 7.1 (d, 1H), 5.1 (q, 2H), 3.27 (m, 1H), 1.2 (t, 3H).
4
7
reported data at room temperature. In complex CoQ3, the
III
six-membered chelate ring has been created around the Co
[
2 5
FeQ3·C H OH] (FeQ3)
atom by three 8-quinolinolato anions through an asymmetric
meridional arrangement along with one ethanol molecule
forming a slightly distorted octahedral structure.
To a ligand 8-HQ (0.436 g, 3.0 mmol) dissolved in an ethanol
(200 mL) containing NaOH (0.12 g, 3.0 mmol), FeCl ·6H
0.270 g, 1.0 mmol) in ethanol was added; the resulting
3
2
O
(
solution was stirred for 30 min, observing a colour change
from brown to black. A black precipitate after removing the
solvent by roto-evaporator was re-crystallized from absolute
ethanol. Elemental analysis: anal. calc. (%) for C29
3 4
H24FeN O : C,
6
5.18; H, 4.53; N, 7.86; found (%): C, 65.01; H, 4.56; N, 7.88.
−1
FT-IR (νmax/cm ) (Fig. S1): very strong stretching vibrations at
5
1
62vs [pyridine ring deformation], 1276vs [(ν(C=N], and 1320vs
,
572 [(ν(C-H] were observed. Also, the following strong
m
bending vibrations, 736vs δ[O-H]out of plan, 821vs δ[CH2=CH],
1105vs δ[C-O-H]quinoline, and 1376vs δ[O-H] confirm the
1
formation coordination sphere in FeQ3. H NMR (Fig. S2)
6
(
DMSO-d , 300MHz) δ (s: singlet, d: doublet, t: triplet, q:
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DOI: 10.1039/C9NJ04027G
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Fig. 1 ORTEP drawing of the asymmetric unit of [Co(8-
q uinolinolato₃·C OH] with thermal ellipsoids drawn at the Fig. 2 Crystal array of [Co(8-q uinolinolato₃·C
0% of probability.
H
2 5
2 5
H OH], view
5
along the b-axis and with perspective to plane formed by a-c
axes emphasizing the C11(9), R11(6) motif’s.
The average N, O bite angle 85.90°, the exclusive meridional
geometry of the isolated CoQ3 is similar to other chelates of Table 1 Crystal data and structure refinement for compound
this type.48 The observed Co –N bond distances [1.918(2)– [Co(8-q uinolinolato₃ C
III
2 5
H OH].
III
1
.937(2) Å] are longer than the Co –O [1.898(1)–1.906(1) Å]
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
29 3 4
C H24CoN O
4
9
and are comparable to other 8-quinolinolate complexes. The
planar 8-quinolinolato monovalent anions are nearly
perpendicular to each other where the chelate rings,
Co1/O1/C1/C22/N1,
Co1/O3/C19/C20/N3 make dihedral angles of 87.05(7),
87.67(9) and 87.23(7) °, respectively.
537.44
130(2) K
0.71073 Å
Co1/O2/C10/C11/N2,
and
Monoclinic
P 21/n
Unit cell dimensions
a = 11.1941(6) Å
b = 12.9126(7) Å
c = 16.5167(9) Å
β= 93.965(5)°.
3
In the crystalline arrays (Fig. 2), intermolecular interactions of
type π-π through hydrogen bonds, O—H···O and C—H···O are
observed. For example, an oxygen atom (ethanol molecule)
involves hydrogen bond formations such as O4—H4···O2
Volume
2
4
381.7(2) Å
Z
(
(
1.858 Å), C8—H8···O4 (2.717 Å), and C18—H18···O4 (2.650 Å)
Table 3). The overlapping arrangement of neighbouring
Density (calculated)
3
1
.499 Mg/m
-1
Absorption coefficient
parallel 8-quinolinolato rings was seen at 4.608 Å for Cg5-Cg8,
and 4.045 Å for Cg9—Cg9 (Cg5= centroid C11—N12/C15, Cg8=
centroid C10/C18 and Cg9= C19/C27), with the existence of π-
π stacking between the neighbouring complex molecules.
0.764 mm
1112
F(000)
Crystal size
0.500 x 0.130 x 0.080
3
mm
Theta range for data collection
Index ranges
3.363 to 29.461°.
-14<=h<=13, -
17<=k<=15, -22<=l<=20
Reflections collected
13622
Independent reflections
Completeness to theta = 25.242°
Absorption correction
Max. and min. transmission
Refinement method
5633 [R(int) = 0.0440]
99.8 %
Analytical
0.942 and 0.775
Full-matrix least-squares
2
on F
Data / restraints / parameters
5633 / 1 / 338
1.027
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R1 = 0.0539, wR2 =
0.1116
R1 = 0.0877, wR2 =
R indices (all data)
0
.1298
Largest diff. peak and hole
-3
0.915 and -0.723 e.Å
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Ple Na es we dJ oo u nr no at l ao df jCu hs et mm i as tr rgy ins
Journal Name
ARTICLE
1
2
3
4
5
6
7
8
9
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
Table 2 Selected bond lengths [Å] and angles [°] for compound α=γ≠β for CrQ3, CoQ3, and FeQ3 suggest monoVcielwinAircticclerOysnltinael
[
Co(8-q uinolinolato₃·C
2
H
5
OH].
structure of P type, and these results areDO
ray structural data of these similar type compounds.
w
I:
e
1
l
0
l
.
c
10
o
3
in9/
c
C
4
id
9
e
NJ
w
04it
0
h X-
27
G
2-44, 53-54
Bond
lengths
Bond angles
°]
[
[
Å]
100000
C(1)-O(1)
1.328(4)
1.338(3)
1.322(3)
1.368(5)
1.569(7)
1.890(2)
1.902(2)
1.913(2)
1.918(2)
1.934(3)
1.937(2)
O(3)-Co(1)-O(1)
O(3)-Co(1)-O(2)
O(1)-Co(1)-O(2)
O(3)-Co(1)-N(2)
O(1)-Co(1)-N(2)
O(2)-Co(1)-N(2)
O(3)-Co(1)-N(1)
O(1)-Co(1)-N(1)
O(2)-Co(1)-N(1)
N(2)-Co(1)-N(1)
O(3)-Co(1)-N(3)
90.22(9)
CrQ3
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
80000
C(10)-O(2)
C(19)-O(3)
C(28)-O(4)
C(28)-C(29)
Co(1)-O(3)
Co(1)-O(1)
Co(1)-O(2)
Co(1)-N(2)
Co(1)-N(1)
Co(1)-N(3)
O(4)-H(4D)
C(1)-C(2)
92.05(9)
176.74(8)
86.70(10)
91.94(10)
85.87(10)
175.92(10)
85.71(10)
91.99(10)
93.09(10)
86.12(10)
91.98(9)
60000
40000
20000
10
20
30
40
2
 (degrees)
0.914(19) O(1)-Co(1)-N(3)
1.420(4)
1.391(4)
1.368(5)
O(2)-Co(1)-N(3)
N(2)-Co(1)-N(3)
N(1)-Co(1)-N(3)
90.50(9)
CoQ3
1
2000
0000
8000
C(1)-C(9)
171.83(11)
94.35(10)
C(7)-C(8)
1
Table 3 Geometrical parameters for the intermolecular
hydrogen bonding interactions in the crystal structure of
[Co(8-q uinolinolato₃˖C
2
H
5
OH] [Å].
D-H...A
d(D-H)
d(H...A)
d(D...A)
<(DHA)
6000
C(3)-H(3)...O(2)
0.95
2.52
2.994(4) 110.8
3.270(4) 127.3
2.945(4) 110.9
3.284(4) 124.6
3.100(4) 110.5
2.741(3) 163(5)
10
20
30
40
C(4)-H(4)...O(2)#1 0.95
C(12)-H(12)...O(1) 0.95
C(18)-H(18)...O(4) 0.95
C(21)-H(21)...N(1) 0.95
2.61
2 (degrees)
2.47
1
50000
00000
50000
0
2.65
FeQ3
2.64
O(4)-H(4D)...O(2)
0.914(19)
1.86(2)
1
Symmetry transformations used to generate equivalent atoms: #1 -x+3/2,y-
1
/2,-z+1/2.
3
.3. XRD patterns of synthesized metal complexes
Powder XRD patterns of the complexes were performed in the
range (2θ = 10–80°) (Fig. 3). The phase purity and sharp peaks
for all the compounds indicating the crystalline nature of these
complexes. The independent indexing of major refluxes was
carried out using least square method. The miller indices h, k, l
were calculated and refined by using Match software. The d
values were obtained by Bragg’s equation (nλ = 2dsinϴ) using
10
20
30
40
2 (degrees)
Fig. 3 XRD pattern of CrQ3, CoQ3, and FeQ3.
5
0-52
the values of sin 2ϴ considering h, k, l data.
The correction
of d–values was confirmed by comparing the observed density
with that calculated from the X–ray powder diffractogram. The
lattice constants a, b and c for each unit cell were found out
and are given in Table 4. The lattice parameters as a≠b≠c and
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Page 6 of 13
ARTICLE
Journal Name
1
2
3
4
5
6
7
8
9
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
Table 4 Lattice constant, unit cell volume, crystal system, inter- To analyse the semiconductor nature of the meVtieawl Acrtoicmle pOnlelinxe,
planar spacing of metal complexes.
direct and indirect bandgap (E
g
) w
D
e
O
reI: 10
c
.
a
10lc
3
u
9/la
C
t
9
e
N
d
J04
f
0
r
27G
o
m
absorption spectra by plotting electronic parameters for
Unit
Inter axial cell
59
Lattice constants (Å)
d
Tauc´s exploration (Tauc´s expression vs. energy), (Fig. 5).
Metal
Crystal
Angle
(2θ
value
complex
angle
volume System
(Åᵌ
(Å)
A
b
c
Direct bandgap: (αhυ)2
Indirect bandgap: (αhυ)1/2
(1)
CrQ3
CoQ3
FeQ3
10.98 11.43 12.67 α=γ=90≠β 1590.1
10.14 10.87 11.76 α=γ=90≠β 1296.21 Monoclinic 10.55 8.38
9.96 10.27 11.23 α=γ=90≠β 1148.71 Monoclinic 10.27 8.6
Monoclinic 12.21 7.24
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
(2)
The absorption coefficient (α, Plank constant (h), and
3.4. Electronic spectra
photon’s frequency (υ) were used to determine E from the
g
extrapolated linear plot. The absorption coefficient (α is
determined by relating photon energy (h*neo) as follows:
Electronic absorption spectra were recorded for all the metal
complexes in ethanol or in DMF (1.0 x 10 M, at 25 °C) as
shown in figure 4.
-
3
α = β/(hυ)(hυ-Eg)n
Or
(αhυ)1/n =β(hυ-Eg)
g
E : optical bandgap energy and n: power factor of the
transition mode depending upon the nature of the material
whether it is crystalline or amorphous. Tauc's plotting for
αhυ) versus the photon energy (hυ) results a straight line at
specified region, and the extrapolation of that straight line
intercepting the (hυ)-axis was considered as indirect optical
1
/2
(
g
energy gap (E ).
Fig. 4 UV-Vis absorption spectra for the metal complexes (1.0 x
-3
1
0 M) at 25° C. [DMF = N,N-dimethylformamide].
UV-Vis spectra recorded for the metal complexes shows
intense absorption bands in the range of 206 to 251 nm
corresponding to n→π* (C=N)py, and π→π* transitions; the
ligand-to-metal charge-transfer (LMCT) bands at 250-300 nm
Fig. 5 Bandgap energies for the metal complexes by Tauc’s
plot. a) direct; b) indirect.
which were originated by coordination of 8-HQ with metal
center.5
5-57
In addition, d-d transitions for all the complexes
The observed variation in the direct and indirect bandgap (Fig.
) of metal complexes indicating the amount of energy
required to produce photo-excited electron–hole pairs and its
degree of delocalization of each metal complexes that involve
in the photo-reduction of H2O2.
were observed in the visible region, for example, CrQ3
presents three bands ( 260, 316 and 398) corresponding to the
5
4
4
4
4
transitions,
A
2g→ T1g(P(υ
3
),
A
2g→ T1g(F(υ
2
),
and
4
4
A
2g→ T2g(F(υ
1
) respectively. Similarly, for CoQ3, typical d-d
1
1
transitions, A1g→ T1g(D)( υ
1
) (312 nm) and 1A1g  1ꢀ2ꢁ(ꢂ( υ 2)
3+
6
(386 nm) for low spin octahedral Co (d ) were observed. In
the case of FeQ3, an intense charge transfer band associated
with bathochromic shift of ligand to metal charge transfer
(LMCT) at 291 nm for T2g→ T2g(ꢂ(υ ) which almost mask the
3
d-d bands was observed. In addition, spin and multiplicity
forbidden transitions exhibit as weak bands at 360, 450, and
3
.5. Photocatalytic studies
2
4
5
8
The photocatalytic activities of the metal complexes were
studied on 2-TBI oxidation by using hydrogen peroxide as
oxidant. Typically, oxidative degradation of an aqueous
solution of 2-TBI (50 mL, 0.05 mM) using each complex (5.0
mM) along with H2O2 (3mL of 30% soln.) was studied under
visible light (40W bulb). The observed absorbance changes
during 2-TBI degradation were recorded at λmax/300 nm for 3
2
4
5
2
90 nm which are corresponding to
ꢀ2ꢁ→4ꢀ2ꢁ(G(υ
T
2g→ T1g(G(υ
1
),
2
4
4
1
), and
T
2g→ A1g(G), E
g
(G(υ
2
+
) respectively
3
5
confirm the presence of low spin octahedral Fe (d ).
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Ple Na es we dJ oo u nr no at l ao df jCu hs et mm i as tr rgy ins
Journal Name
hours (Fig. 6). The concentration of substrate was measured in
constant time intervals, and it was plotted against time. The
same experimental procedure was followed when the 2-TBI
degradation was carried out without the presence of light
irradiation. The rate constant for the oxidation was
determined in the presence of each complex, and compared
each other. Furthermore, the effect of pH on the determined
rate constant was analysed using neutral pH=7.0±0.5, and
ARTICLE
1
2
3
4
5
6
7
8
9
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
1
0
1
View Article Online
DOI: 10.1039/C9NJ04027G
-
-2
-3
-4
-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
acidic pH=4.0±0.5 conditions using HNO . Similarly, the effect
of temperature on the oxidation was studied observing the
decomposition of hydrogen peroxide at the higher
temperatures; therefore, 25 °C was considered as optimum
reaction temperature for 2-TBI degradation.
catalyst alone
H₂O₂+Coq3
H₂O₂+Crq3
A decrease of band intensity at 300 and 290 nm corresponding
to the transitions, n→π* and π→π* respectively for 2-TBI was
observed during the oxidation, and a new signal at 276 nm was
appeared simultaneously with an isosbestic point at 282 nm,
indicating the formation of new degraded products (Fig. 6).
Kinetic parameters obtained for 2-TBI degradation under
different experimental conditions are summarized in Table 5.
The photocatalytic efficiency of metal complexes were
H₂O₂+Feq3
H₂O₂+Fenton
H₂O₂ alone
-
7
0
1000
2000
3000
4000
5000
6000
Time (s)
compared with Fenton reagent using H
the same experimental conditions.
O
2 2
as oxidant under Fig. 7 Pseudo first order kinetics for 2-TBI during the
degradation by different metal complexes at neutral pH
(
pH=7.0± 0.5) without the visible light irradiation.
2
1
0
-
-
-
1
2
3
catalyst alone
H₂O₂+Coq3
H₂O₂+Crq3
-4
-5
H₂O₂+Feq3
H₂O₂+Fenton
H₂O₂ alone
Fig. 6 Changes in absorption profile of 2-TBI during the
oxidative degradation.
-
6
0
1000
2000
3000
4000
5000
6000
Time (s)
Fig. 8 Pseudo first order kinetics for 2-TBI during the
degradation by different metal complexes at neutral pH
(
pH=7.0± 0.5) under visible light irradiation.
In both the studied pH conditions (pH7light, Fig. 7; pH7without light
Fig. 8; pH4light, Fig. S3; pH4without light, Fig. S4), catalyst alone
cannot oxidize 2-TBI without using H as oxidant; the fitted
,
2 2
O
linear lines (R2 > 0.9) indicate that 2-TBI degradation follows
pseudo-first-order kinetics. In the observed 2-TBI oxidation
kinetics, pH of the reaction medium and visible light irradiation
have a significant effect on its degradation. Figures, 9, 10, and
1 show the comparison of catalytic efficiency of all the
studied metal complexes at different experimental conditions.
1
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Page 8 of 13
ARTICLE
Journal Name
1
2
3
4
5
6
7
8
9
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
Generally, acidic pH showed a shrink in the 2-TBI degradation
for all the metal complexes due to the protonation of electron-
donating groups in the metal complex slowing the excitation of
electrons from metal ion. At neutral pH conditions, all the
metal complexes showed an improved 2-TBI degradation rate
along with percentage removal. Similarly, visible light
irradiation favours the oxidation to take place rapidly
increasing degradation percentage for all the studied catalysts.
Among the studied catalysts, CoQ3 showed a higher velocity
View Article Online
DOI: 10.1039/C9NJ04027G
Fig. 9 Comparison of rate constants for the oxidation of 2-TBI
by metal complexes at different experimental conditions.
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
-1
rate (1.06 x 10 s with t1/2 of 653.91 s) with 99.1% removal of
-TBI, indicating the degradation is more effective over CoQ3
2
under visible light irradiation.
Table 5 Kinetic parameters for the degradation of 2-TBI by
metal complex catalysts under different experimental
conditions.
2
-TBI
Removal
%)
Oxidation
conditions
-
1
R2
Catalyst
T
½
(s)
k (s )
(
Fig. 10 Calculated half-life (t1/2) for the oxidation of 2-TBI by
metal complexes at different experimental conditions.
CrQ3
FeQ3
956.0651 7.25E-04
780.2783 8.88E-04
0.9491
0.9451
68.7
83.3
pH4
With Light
CoQ3
639.8282 1.08E-03
0.9558
83.8
H
2
O
2
1248.5
5.55E-04
0.9235
0.921
85.6
89.5
68
Fenton
CrQ3
1252.676 5.53E-04
753.4208 9.20E-04
812.2819 8.53E-04
732.1977 9.47E-04
0.9199
0.9463
0.9606
Fig. 11 2-TBI removal by the metal complexes at different
FeQ3
CoQ3
83.7
88.9
experimental conditions.
pH4
Without
light
3
.6. Mechanism of photocatalysed degradation of 2-TBI
H
2
O
2
1542.9
4.49E-04
0.901
79.6
Fenton
CrQ3
1200.601 5.77E-04
714.5847 9.70E-04
850.4873 8.15E-04
0.8789
0.9806
0.9396
83
2
-TBI oxidation products were analysed by HPLC-MS technique
87.7
80.4
to propose a mechanism for its degradation. Figure 12 shows
HPLC-MS chromatogram with three different fragments
obtained from the degradation of 2-TBI.
FeQ3
pH7
With light
CoQ3
653.9124 1.06E-03
0.9427
99.1
H
2
O
2
812.2819 4.80E-04
558.2393 1.24E-03
709.707 9.77E-04
896.311 7.73E-04
0.9991
0.9572
0.9074
0.9307
0.9969
0.9423
0.9649
81.6
92.3
75.4
73.2
93.5
93.8
90.65
Fenton
CrQ3
FeQ3
CoQ3
pH7
Without
light
859.2
8.07E-04
H
2
O
2
1386.294 5.00E-04
1551.81 4.47E-04
Fenton
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Ple Na es we dJ oo u nr no at l ao df jCu hs et mm i as tr rgy ins
Journal Name
ARTICLE
1
2
3
4
5
6
7
8
9
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
e-+ H
O
→ OH + ·OH
-
(4)
2
2
View Article Online
DOI: 10.1039/C9NJ04027G
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
Fig. 12 Combined HPLC-MS chromatogram of the degradation
of 2-TBI.
Fig. 14 Proposed mechanism for 2-TBI oxidation by ·OH
radicals.
Fig. 13 Rout of the fragmentation of 2-TBI.
The degraded products I, II and III with their corresponding
m/z, 119, 135, and 136 respectively (Fig. 12) were
characterized by MS spectra. In addition, 1H NMR (400 MHz)
spectra (Fig.S5) for 2-TBI was analysed before and after the
catalysed oxidation; the aromatic and aliphatic region of the
oxidized products were identified. According to the results
obtained from the analyses of degraded products, a possible
degradation rout (Fig. 13) for 2-TBI, and its oxidation
mechanism (Fig. 14) was proposed considering the presence of
metal complex catalyst and visible light irradiation. Under the
visible light irradiation, excitation of electron (e ) from the
valence band (VB) of metal ion to the conduction band (CB)
leaving behind a hole (h ) in VB (Eqn. 1) takes place (Eqn. 3).
This photo-generated electron in excited state of the metal
complex interact with hydrogen peroxide to generate hydroxyl
radicals that involve in the 2-TBI oxidation.
3
.7. DFT calculations
To unveil the mechanism of hydroxyl radical formation from
H2O2 in the presence of metal complex catalysts, and also to
simulate the electron migration from metallic centre to
hydrogen peroxide, molecular energy calculations by DFT were
performed. In the first step, to understand the ground state
3
+
nature, molecular optimization of the metal complexes [M (L-
3
+
3+
3+
-
1₃] (M=Cr , Fe , Co ) was carried out in ground state using
an unrestricted uB3LYP functional with LANL2DZ basis set in an
aqueous medium through an IEF-PCM model. With the
+
0
collected information from ground state (S ), its excitation
process was studied via unrestricted TD-DFT calculations. Since
its application as visible light photocatalysts is being studied, it
is important to know the behaviour of these molecules upon
excitation by low energy photons, γ (6.0≥Eγ≥1.54 eV) present
in visible light. As the visible light limits the higher energy
+
-
[
CoQ3] + light → [CoQ3] (h + e )
(3)
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
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5
5
5
6
excitations, excitation process from ground state (S
first excited state (S →S ) was analysed. To determine the sp
0
) to the
View Article Online
DOI: 10.1039/C9NJ04027G
0
1
and d orbital hybridization in metal complexes, different
multiplicity have been previously executed since the individual
electron transfer reactions depend strongly on the spin and
energy state of the electrons. Since during the excitation
process, unpaired electrons may be easily excited because
paired electrons are intrinsically bound to its orbital;
nevertheless, excitation of single electrons could undergo
rapid relaxation processes included photon emission, electron
injection and electron transfer are dependent on energetic
limitations.
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
Scheme 1 Spin dependent ·OH radical formation.
Fig. 15 Frontier Molecular Energies for HQ complexes.
Fig. 16 Density of states (DOS) plot for the metal complexes of
-HQ.
8
As can be inferred from figure 15, the energy gap (ΔEHOMO-
LUMO) observed among the studied complexes was, ΔE=3.17
eV for CoQ3; ΔE=3.7 eV for FeQ3 and finally ΔE=3.0 eV for
CrQ3. In order to explain a little controversy observed
between the HOMO-LUMO energy where the lower value of
band gap lies in CrQ3 and the experimental results where
CoQ3 showed a higher oxidization rate, spin coupling and
Density of States (DOS) of the catalysts were analysed. From
The viability of the electron migration pathway, transition
state (TS) within the electron transfer process between the
metallic centre and H O in order to form hydroxyl radicals was
2 2
determined, and optimized by QST3 approach (Fig. 17). In
addition, thermodynamic feasibility of the interaction of metal
3
+
complex with H
2
O
2
was analysed through intrinsic reaction
),
the electronic configuration (Fig. S6) it is clear that Cr and
3
+
coordinates (IRC) parameters such as a activation energy (E
a
Fe complexes are in a doublet state with only one α electron
(+1/2) which would leave an empty HOMO during the
excitation process, which is thermodynamically viable;
however, resulting excited state would not be stable and thus
rather undergo vibrational relaxation (Fig. 16). On the other
hand, for CoQ3, a singlet configuration is observed; after light
absorption, single electronic excitation is observed (Fig. 16)
which would result in a triplet state with a change of spin
polarization from α→β (-1/2). This configuration is fairly stable
that endures an intermolecular electronic transfer from
complex to hydrogen peroxide (Scheme 1).
transition states (TS) and free energy (∆E that were
determined using a uBB1K/LANL2DZ basis set in a full aqueous
media given by a IEF-PCM model. Based on the experimental
evidence, the formation of •OH radicals is triggered and
depends on light irradiation (Scheme 2). For explaining this
behaviour, a mechanism has been proposed based on the
excitation of the complex followed by a multistep relaxation
(Eqns. 5-7) that allows the final formation of two •OH radicals
through a single electron transfer process (SET), and also
permits regeneration of metallic complex catalyst (Eqn. 7)
3
+
-1
3+ -1
[M3+(L-1₃] + γ [M (L ₃] + γ  ([M (L ₃]
t
*
(Phase 1) (5)
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2
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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
3
+
-1
4+ -1
+
•
•
-
(
[M (L ₃]
t
* + H
2
O
2
 ([M (L ₃] )
s
+ OH + OH (Phase 2) (6)
As can be observed in figure 17, excitation eneVrigewieAsrtifcolerOntlhinee
studied complexes vary from low ener
D
g
O
y
I:
e
10
x
.
c
1
i
0
t
3
a
9
t
/
io
C
n
9N
p
J0
r
4
o
0
cess
2
7
G
4
+
-1
+
•
-
3+ -1
•
(
[M (L ₃] )
s
+ OH  ([M (L ₃] + OH
(Phase 3)
(7)
(E=32.58 kcal/mol) for the case of CoQ3 to high energy for the
case of FeQ3 (E=64.93 kcal/mol) and CrQ3 (E=70.86 kcal/mol).
Interestingly there are no important changes in the energy of
formation after the excitation process was observed among
the studied complexes, indicating that the crucial step is the
light-induced excitation process, which has also been
demonstrated experimentally observing increased rate of 2-
TBI degradation under visible light irradiation.
Global:
3
+
-1
3+ -1
•
[M (L ₃] + H₂O₂ + γ  [M (L ₃] + 2 OH
(8)
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
According to this proposal, a low energy-visible light photon (γ
collides with [M (L ₃] complex which will excite one of its
electrons from basal (S ) to its first excited state (S ).
3
+
-1
0
1
In addition, to recognize the processes that lead to the
degradation of 2-TBI and their corresponding degradation
intermediate products, intrinsic reaction coordinates (IRC)
a
), transition states
TS), and free energy (∆E were determined using a uBB95K/6-
311G(d,p) basis set in aqueous media. Figures 18 and 19 show
computationally determined thermodynamical parameters
including solvent effects for the above-proposed mechanism in
figure 14 for 2-TBI degradation. The negative value of ∆E (-7.8
kcal/mol) observed for step 1 indicates that the interaction of
OH radicals with sulphur atom of the thiol group in 2-TBI (Fig.
parameters such as a activation energy (E
(
1
4) is thermodynamically feasible. Step 1 is followed by a
gradual decrease of ∆E up to the formation of stable
intermediate in step 4 through which the sulfuric acid formed
as a by-product is eliminated. In step 5, OH radical bonds with
N atom of the imidazole ring breaking the π bond in C=N, and
this process is followed by few fast steps up to the formation
of stable intermediates in steps 11 and 12. The most stable
intermediate formed in 2-TBI degradation process is found in
2 2
Scheme 2 Radical Oxygen Species production (ROS) from H O
by photoexcitation of [M (L ₃].
3
+
-1
Thermodynamic parameters for the metal complex catalysed
generation of ·OH radicals from H are shown in table 6. It
can be observed that the energy needed for the formation and
deformation of the adduct between CoQ3 and H is lower
than the other studied catalysts, which agrees with the
experimentally obtained kinetic data.
2 2
O
a
step 16 since the activation energy (E ) needed for this
2 2
O
intermediate to follow the next step 17 is maximum ∆E (-155.7
kcal/mol). As can be seen in steps 18 and 19, there is no higher
energy process has observed after step 17, indicates that the
final product formed in the degradation of 2-TB can be
Table 6 Thermodynamic parameters for electron transfer for considered in step 17. By relating this IRC profile with the
·OH formation by 8-HQ complexes.
experimentally obtained 2-TBI degraded products by HPLC-MS,
it can be concluded that the peaks I, II and III in figure 12 are
corresponding to the stable by-products observed in steps 11,
12 and 16 respectively.
Total
ΔE (kcal/mol)
Energy
System
M³⁺L-
- • -
(
(M³⁺L ₃ +
OH+ OH
(10⁻⁶
-
•
TS1
TS2
(M³⁺L ₃+ OH
•
₃*+H₂O₂
kcal/mol)
-
[
Cr³⁺L ₃]+H₂O₂ -1.05
37.58
35.88 26.56
60.21 49.89
31.1
16.65
-
[
[
Fe³⁺L ₃]+H₂O₂ -1.07
64.93
70.76
54.65 41.09
59.36 45.93
-
Co³⁺L ₃]+H₂O₂ -1.09
64.4
54.38
Fig. 17 IRC profile for ·OH radical formation by different metal
complexes.
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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
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
5
6
geometry along with an ethanol molecule bondedViewwiAtrhticolenOenlionef
the oxygen atoms of 8-quinolinolato anio
bond. The catalytic activity of synthesized metal complexes
was investigated over the oxidation of 2-TBI with H by UV-
D
n
O
t
I:
h
1
r
0
o
.1
u
0
g
3
h
9/
a
C9
h
N
y
J
d
0
r
4
ogen
0
2
7G
2 2
O
vis spectroscopy at different experimental conditions. Among
the all studied catalysts, CoQ3 showed a better catalytic
activity towards 2-TBI removal with 99.1% under the visible
light irradiation at neutral pH. The experimental results were
corroborated with theoretically obtained energy components
comprising in the activation energy of 2-TBI degradation
pathway. It was found that the activation energy involved for
the formation of hydroxyl radical from H O that is responsible
2 2
for 2-TBI oxidation is diminished with CoQ3 due to the
increased photo-excited electron transfer favoured by lower
transition state energy of the adduct CoQ3-H O . In addition, a
2 2
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
thermodynamically feasible mechanism for 2-TBI degradation
is proposed according to the experimental results, and
analysed by DFT.
Conflicts of interest
The authors declare no conflict of interest.
Fig. 18 Thermodynamic parameters for the oxidation of 2-TBI
Acknowledgements
by hydroxyl radicals a) Activation energies (E
∆E profile.
a
); b) Free energy
(
The authors acknowledge PRODEP (Programa para el
Desarrollo Profesional Docente en Educación Superior,
DSA/103.5/15/14210), Mexico, for financial support through
the programe “Red Temática de Colaboración Academica-
This observation is affirmed by the internal reaction profile
(IRC) (Fig. 19) that performed for the oxidation of 2-TBI by OH
radicals using uB3LYP/BB1K in aqueous media. A gradual
decrease in the activation energy for each transition state (TS)
observed in figure 18 confirms the thermodynamic feasibility
of the proposed degradation mechanism in figure 14.
2
015”. Also acknowledge DGTIC-UNAM (Dirección General de
Cómputo y de Tecnologías de Información y Comunicación,
Universidad Nacional Autónoma de México) for the
computational facilities and USAI (Unidad de Servicios de
Apoyo a la Investigación, Facultad de Química, UNAM) for
analytical services.
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Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L.
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