Photo-induced reduction of Cr6+ by the hybrid systems “CuII complex with Schiff base and TiO2”: dependence on irradiation wavelength

Article abstract of DOI:10.1007/s11172-017-1981-7

The synthesis of novel Cu^II complexes with a Schiff base obtained by condensation of salicylaldehyde and an l-aspartic acid ester are described. The physicochemical properties of the complexes were compared with those of related Cu^II complexes obtained earlier. All the complexes studied were characterized by elemental analysis as well as by IR, UV-Vis, and EPR spectroscopies. The activity of the complexes and their hybrid systems (HS) with TiO₂ in visible-light-driven photocatalysis in organic solvents was investigated. After irradiation with visible light, the complexes and corresponding HS reduce Cr⁶⁺ to Cr³⁺ more efficiently than bare TiO₂. To determine the molecular orbital compositions and energies and to explain the electronic spectra and redox properties of the systems studied, density functional calculations of the optimized structures of representative model complexes were performed.

Full text of DOI:10.1007/s11172-017-1981-7

Russian Chemical Bulletin, International Edition, Vol. 66, No.11, pp. 2057—2065, November, 2017
2057
6
+
Photoꢀinduced reduction of Cr by the hybrid systems
Cu complex with Schiff base and TiO ": dependence on irradiation wavelength
II
"
2
N. Yoshida, A. A. Tsaturyan,b T. Akitsu,а Y. Tsunoda, and I. N. Shcherbakovb
а
а
aDepartment of Chemistry, Faculty of Science, Tokyo University of Science,
1
ꢀ3 Kagurazaka, Shinjukuꢀku, Tokyo 162ꢀ8601, Japan.
Fax: +(81) 35261 4631. Eꢀmail: akitsu@rs.kagu.tus.ac.jp
b
Southern Federal University,
7
ul. Sorge, 344060 RostovꢀonꢀDon, Russian Federation.
Eꢀmail: caturyan@sfedu.ru
The synthesis of novel CuII complexes with a Schiff base obtained by condensation of
salicylaldehyde and an Lꢀaspartic acid ester are described. The physicochemical properties of
II
the complexes were compared with those of related Cu complexes obtained earlier. All the
complexes studied were characterized by elemental analysis as well as by IR, UVꢀVis, and
EPR spectroscopies. The activity of the complexes and their hybrid systems (HS) with TiO2
in visibleꢀlightꢀdriven photocatalysis in organic solvents was investigated. After irradiation
with visible light, the complexes and corresponding HS reduce Cr6 to Cr more efficiently
+
3+
than bare TiO . To determine the molecular orbital compositions and energies and to explain
2
the electronic spectra and redox properties of the systems studied, density functional calculaꢀ
tions of the optimized structures of representative model complexes were performed.
Keywords: copper, titania, photocatalyst, Cr6+ reduction, density functional theory, quanꢀ
tum chemical calculations.
Chromiumꢀcontaining compounds have a wide range
Copper, similarly to iron, also has a lower valence
form and can react with organic compounds to form comꢀ
plexes. Having in mind the previously reported photocatꢀ
of applications for military purposes, metal plating, tanꢀ
ning of leather and are also used as dyes and in refractory
1
6+
industry. Chromium salts easily dissolve in groundwater
alytic reduction of Cr by organic acids in the presence
III 15
I
when left unattended in soil and cause water pollution.
The toxicity of chromium depends greatly on its redox
form. In natural environment, chromium occurs in two
of Fe
,
we hypothesized that it is likely that Cu and
some reductive free radicals can also be produced in the
II
presence of both Cu and organic acids with light, thus
6
+
3+
6+
oxidation states, Cr and Cr . The solubility and toxicꢀ
leading to the rapid reduction of Cr .
ity of Cr6 is higher than those of Cr that is usually
+
3+
In recent years, the development of photocatalysts
II
immobilized through precipitation. Therefore, reduction
containing Schiff base Cu complexes takes a great part
of Cr6 to Cr is used as an effective approach to miniꢀ
+
3+
of scientific interest of our research group.
16—23
As a conꢀ
2
—5
mize chromium pollution. A number of studies
Cr reduction by organic and inorganic reductants were
aimed to treat Cr ꢀcontaminated wastewater.
Recently, a number of reductants including Fe , phoꢀ
on
tinuation of our previous research, in this work we invesꢀ
tigated visibleꢀlightꢀdriven photocatalytic activity of four
6
+
6
+
II
Schiff base Cu complexes 1—4 (Scheme 1) and their
II
hybrid systems (HS) with TiO2 (from this point on,
6
7,8
tocatalysts, and other compounds have been utilized
to convert Cr6 into Cr3+. Among them, FeII is considꢀ
ered to be the most feasible and efficient from the standꢀ
point of reducing chromium contamination. Reduction
HS1—HS4, respectively), which act as photosensitizers
+
6+
3+
in the reduction of Cr to Cr . The complexes studied
were characterized by elemental analysis as well as by IR,
UVꢀVis, and EPR spectroscopies. In addition to experiꢀ
mental studies, theoretical calculations were employed to
complete and confirm the experimental observations.
Quantum chemical calculations within the framework of
the density functional theory (DFT) were performed
to study the structural, electronic, spectroscopic, and
of Cr6 under visible light irradiation in the presence of
+
9
TiO was also reported.
2
Titanium dioxide (TiO ) is a good semiconductor for
2
photocatalysis, but it shows activity only after UV irradiꢀ
ation.6
,10
The photocatalytic activity of TiO2 can be
II
I
enhanced by changing the powder fineness or crystallite
thermodynamic properties of the Cu and Cu complexꢀ
es. Timeꢀdependent density functional theory (TDꢀDFT)
11,12
13,14
size
or using it as a coreꢀshell type nanocomposite.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2057—2065, November, 2017.
066ꢀ5285/17/6611ꢀ2057 © 2017 Springer Science+Business Media, Inc.
1

2
058
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017
Yoshida et al.
was used to explain the electronic properties of the comꢀ
plexes. A comparison of the experimental and theoretical
parameters could be used as a useful tool for structural
identification of the complexes.
1733 (C=O). UV–Vis spectrum (diffuse reflectance), λmax/nm:
686 (d—d), 378 (р—р*). ESR spectrum (Xꢀband, 1 mM of complex 1
in MeOH, 77 K): g = 2.039.
Copper complex with (E)ꢀ4ꢀ(benzyloxy)ꢀ2ꢀ(2ꢀhydroxybenzꢀ
ylideneamino)ꢀ4ꢀoxobutanoic acid (3). LꢀAspartic acid (1.33 g,
1
0 mmol) and benzyl alcohol (1.08 g, 10 mmol) were disolved
in 60% sulfuric acid (1.65 g) and heated at 70 °С for 45 min.
Clear gel gained by vacuum concentration at 70 °С was neuꢀ
tralized using sodium bicarbonate and the white sample thus
obtained was dried over a period of 16 h. The sample (0.60 g)
was mixed with salicylaldehyde (0.24 g, 2 mmol) in methanol
and the mixture was heated at 40 °С for 3 h. Copper(II) acetate
monohydrate (0.20 g, 1 mmol) was added, the mixture was
stirred for an additional 3 h, and a green sample was then obꢀ
tained by concentration. Complex 3 was recrystallized from
MeOH. The yield was 45%. Found (%): C, 54.14; H, 3.61;
Scheme 1
N, 3.71. C18H15CuNO . Calculated (%): C, 55.60; H, 3.89;
5
1
N, 3.60. IR spectrum (KBr), ν/cm : 1651 (C=N), 1731 (C=O).
UV–Vis spectrum (diffuse reflectance), λmax/nm: 689 (d—d),
3
92 (π—π*). ESR spectrum (Xꢀband, 1 mM of complex 3 in
MeOH, 77 K): g = 2.034.
Copper complexes with (E)ꢀ2ꢀ(2ꢀhydroxybenzylideneamino)ꢀ
pentanedioic acid (2) and (E)ꢀ2ꢀ(2ꢀhydroxylideneamino)butaneꢀ
dioic acid (4) were prepared following the method described
previously.2
1
—4
4,25
1
2
R
R
= CH , (CH )
2 2 2
= (CH ) COOBn (1), (CH ) COOH (2), CH COOBn (3),
Complex 2. The yield was 87%. Found (%): C, 43.12;
2
2
2 2
2
CH COOH (4)
H, 4.29; N, 5.81. C12H11CuNO + C H NO (unreacted preꢀ
2
5 5 9 4
cursor). Calculated (%): C, 44.40; H, 4.38; N, 6.09. IR specꢀ
trum (KBr), ν/cm : 1623 (C=N), 1725 (C=O). UV–Vis specꢀ
trum (diffuse reflectance), λmax/nm: 664 (d—d), 375 (π—π*).
ESR spectrum (Xꢀband, 1 mM of complex 2 in MeOH, 77 K):
g = 2.035.
Reagents and conditions: i. 70 °C, 45 min; ii. 1) 40 °C, 3 h,
) Cu(AcO) •H O, 40 °C, 3 h.
1
2
2
2
Experimental
Chemicals of the highest commercial grade available were
purchased from Aldrich (St. Louis, MO, USA), WAKO (Tokyo,
Japan), and TCI (Tokyo, Japan) and used as received without
further purification.
Complex 4. The yield was 17.4%. Found (%): C, 40.81;
H, 3.35; N, 5.58. C H CuNO + 0.5C H NO (unreacted preꢀ
1
1
9
5
4
7
4
cursor). Calculated (%): C, 42.74; H, 3.45; N, 5.75. IR specꢀ
1
trum (KBr), ν/cm : 1635 (C=N), 1715 (C=O). UV–Vis specꢀ
trum (diffuse reflectance), λmax/nm: 673 (d—d), 365 (π—π*).
ESR spectrum (Xꢀband, 1 mM of complex 4 in MeOH, 77 K):
g =2.035.
Elemental analysis (C, H, N) was carried out with a Perꢀ
kinꢀElmer 2400II CHNS/O analyzer (USA) at Tokyo Univerꢀ
sity of Science. Infrared (IR) spectra were recorded with
a JASCO FTꢀIR 2400 spectrophotometer (USA) equipped with
a polarizer in the range of 4000—400cm– at 298 K. Absorption
electronic spectra (UVꢀVis) were measured on a JASCO Vꢀ570
spectrophotometer (USA) in the range of 900—250 nm at 298 K.
Xꢀband EPR spectra of solutions were measured with a JEOL
JESꢀFA 200 spectrometer (Japan) at 77 K. Circular dichroism
Determination of photocatalytic activity. The photocatalytꢀ
ic activity of the copper complexes 1—4 and hybrid systems
HS1—HS4 in the reduction of Cr6 to Cr was measured
spectrometrically using 1,5ꢀdiphenylcarbazide (Japanese Inꢀ
dustrial Standards, JIS K 0400ꢀ65ꢀ20:1998).26 The measureꢀ
ment solutions were prepared by mixing a solution of complex
+
3+
1
1
—4 (1 mmol) and TiO2 (0.3 mmol) in methanol (25 mL)
and a solution of K Cr O (1 mmol) in water (1 mL). The
2
2
7
(
CD) spectra were measured on a JASCO Jꢀ725 spectropolariꢀ
same experiments were also carried out using the following
meter (USA) in the range of 900—250 nm at 298 K. Spectroꢀ
electrochemical measurements were carried out with a BAS
SEC2000ꢀUV/VIS spectrometer and ALS2323 potentiostat
with Ag/AgCl electrodes (both instruments from ALS Co., Ltd.,
Japan) in DMSO solutions. Two Hayashi lamps, LAꢀ310UV
and LAꢀ251Xe, with five filters for the wavelength ranges of
–
1
reference samples: a solution of TiO (0.3 mmol L ) in methaꢀ
2
–
1
nol, solutions of complexes 1—4 (1 mmol L ) in methanol,
and methanol with no admixtures. The reactions of Cr6 reꢀ
duction by HS1—HS4 were activated by irradiation with visiꢀ
ble light.
Even sole complexes 1—4 or MeOH could reduce Cr6+
under visible light irradiation.
+
4
5
00—440 (region I), 460—495 (region II), 515—560 (region III),
75—625 (region IV), and 600—690 nm (region V) were used as
The catalytic parameter αratios was calculated by the equation
the UV and visible light source, respectively.
Copper complex with (E)ꢀ5ꢀ(benzyloxy)ꢀ2ꢀ(2ꢀhydroxybenzꢀ
ylideneamino)ꢀ5ꢀoxopentanoic acid (1) was prepared as described
in a previous work. The yield was 24%. Found (%): C, 56.01;
Δαratios = (A540 – Aback)/(A540(t = 0) – Aback),
2
4
where A540 is the absorbance at 540 nm at instant t, Aback is the
backround absorbance (solvent absorbance), and A540(t = 0) is
the absorbance at 540 nm at t = 0.
H, 4.12; N, 3.60. C18H15CuNO . Calculated (%): C, 56.64;
5
1
H, 4.25; N, 3.48%. IR spectrum (KBr), ν/cm : 1653 (C=N),

Hybrid systems CuII complex/TiO2
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017 2059
Quantum chemical calculations were performed with the
A (rel. units)
.5
A (rel. units)
0.14
2
7
Gaussianꢀ09 program using the B3LYP and PBE0 hybrid
4
4
3
3
2
2
8—31
functionals,
which have been shown suitable for redox
.0
.5
.0
.5
3
2
0.10
0.06
potential calculations. The 6ꢀ311+G(d,p) basis set was used
for all atoms.
3
3,34
The geometries of all structures correspondꢀ
ing to stationary states were fully optimized. Correspondence
between the optimized structures and minima on the potential
energy surface was confirmed by vibrational frequency analyꢀ
ses at the same level of theory. The thermodynamic free enerꢀ
gies were extracted from the vibrational frequency calculations
for T = 298.15 K. The solvent effect was included within
the framework of the selfꢀconsistent reaction field (SCRF)
0
.02
6
00 625650 675700725 λ/nm
2.0
1.5
1
2
3
4
1
0
.0
.5
3
5—38
approach using the polarizable continuum model (PCM).
3
00
400
500
600
700
800 λ/nm
Fig. 1. UVꢀVis spectra of the hybrid system HS4 before (1) and
after UV irradiation for 20 (2), 40 (3), and 60 min (4).
Results and Discussion
Inset: fragments of the spectra in the λ region of 575—725 nm
(
magnified).
Activity of catalyst. As a control experiment, the UVꢀVis
spectra of the hybrid systems HS1—HS4 were measured
before and after UV irradiation. The samples represented
energy loss upon electron transfer from the complexes
to TiO2.
Solvent dependence of the photocatalytic activity. Going
from a protic solvent (methanol) to aprotic one (DMSO)
causes the Cr reduction rate to decrease (see Fig. 2).
This indicates that the reduction reaction does not reꢀ
quire sacrificial molecules (alcohols) but these molecules
might activate the reaction. This is the proof of the solꢀ
vent (methanol) coordination upon the formation of
–
1
solutions of complexes 1—4 (1 mmol L ) and TiO2
0.25 mmol L–1) in methanol. Measurements were carꢀ
(
ried out to check decomposition of the complexes under
UV irradiation by measuring the spectra each 20 min up
to 60 min. There were no significant changes in the specꢀ
tra (Fig. 1). A slight decrease in absorbance in the region
6
+
5
75—725 nm (see inset in Fig. 1) is caused by structural
II
I
changes due to the formation of Cu /Cu complexes
II
I
(
see below).
Cu /Cu complexes.
Complexes 2 and 4 exhibited higher reducing ability;
Wavelength dependence of the photocatalytic activity.
To elucidate the mechanism of the reaction, the photoꢀ
catalytic activity of the solutions under study was deterꢀ
mined after visible light irradiation at different waveꢀ
lengths (Fig. 3). Complex 4 and the hybrid system HS4
showed a higher photocatalytic activity compared to bare
hence the photocatalytic activity was activated by effiꢀ
cient energy transfer from the complexes to TiO due to
2
the fact that the complexes were adsorbed on TiO . Howꢀ
2
ever, complexes 1 and 3 having an ester moiety also reꢀ
duced Cr6 quickly within 20 min (Table 1). For the sake
+
of comparison, Table 1 also lists the data for TiO and
TiO and pure methanol throughout the wavelength range
2
2
MeOH.
investigated. The observed strong absorption of complex 4
in the regions near λ = 410 and 600 nm can be attributed
to chargeꢀtransfer (CT) or d—d transitions. The photoꢀ
catalytic activity of HS4 and complex 4 in region I is
almost identical and low. The highest photocatalytic
There were no large differences between individual
complexes 1—4 and the corresponding hybrid systems
HS1—HS4 in Cr6 reduction rates. Higher activity of the
former in the reactions studied can be explained by the
+
Table 1. Catalytic parameter Δαratios of the hydrid systems HS1—HS4 and complexes 1—4 deterꢀ
mined before and after irradiation with visible light (λ > 350 nm)
Hybrid system/
Complex
Irradiation time/min
0
20
40
60
HS1/1
HS2/2
HS3/3
HS4/4
TiO2
1.000/1.000
1.000/1.000
1.000/1.000
1.000/1.000
1.000
0.131/0.0314
0.124/0.053
0.097/0.020
0.147/0.044
0.192
0.060/0.010
0.054/0.060
0.062/0.000
0.043/0.005
0.113
0.054/0.001
0.049/0.003
0.063/0.000
0.028/0.002
0.068
TiO (UV)*
1.000
0.070
0.017
0.001
2
MeOH
1.000
0.202
0.135
0.092
*
TiO sample after UV irradiation at λ = 350 nm for 15 min.
2

2
060
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017
Yoshida et al.
N (%)
1
00
9
9
8
5
0
5
DMSO
MeOH
Fig. 4. Calculated structures of copper complexes 4 and 4´.
Without
catalyst
TiO2
4
HS4
II
I
formation of the Cu /Cu complexes with the coordinaꢀ
tion number of copper equal to three or four (Fig. 4),
where the condensation product of salicylaldehyde and
Lꢀaspartic acid ester is coordinated to copper through two
oxygen atoms and one nitrogen atom. In the typeꢀ4´ comꢀ
plexes with the coordination number of Cu equal to four
the remaining coordination site is occupied by the oxygen
atom of the methanol molecule.
Fig. 2. Reduction of Cr6+ in DMSO and MeOH in the absence
and in the presence of catalyst (TiO ), complex 4, and HS4.
N is the percentage of Cr ions reduced within 60 min.
2
6
+
activity of HS4 and complex 4 was observed in region II.
In region V, the photocatalytic activity of complex 4 is
similar to that observed in region II.
I
II
Both Cu and Cu can form complexes of different
structure, where the coordination number of copper can
Quantum chemical calculations. In the design of new
materials on a molecular level, it is useful to understand
the properties of a single molecule. The redox potential is
one of the most fundamental properties of transitionꢀ
metal complexes, which gives information whether the
target molecule can easily take or loose an electron. The
redox potential values provide important information necꢀ
essary for understanding the catalytic activity of metal
clusters. To explain a correlation between the structures
and redox potentials of the Cu /Cu complexes with the
condensation product of salicylaldehyde and Lꢀaspartic
acid ester, quantum chemical calculations were carried out.
Complexes with only three donor atoms around the
metal center are important building blocks in crystal enꢀ
gineering of coordination polymers and metalꢀorganic
frameworks. In this work, we confirmed the possibility of
II
also be different. Reduction of the Cu complexes in
MeOH may involve innerꢀsphere rearrangements.
The stabilities of complexes 4 and 4´ (Fig. 5) were
estimated on the basis of changes in the Gibbs energies of
I
reactions (1) and (2) for the Cu complexes and reactions
II
(
3) and (4) for the Cu complexes (n = 5, 6) (Scheme 2).
According to our calculations, the most stable form has
the most negative value of this thermodynamic parameter.
II
I
Scheme 2
L2– + [Cu(MeOH) ]+
[CuL]– + 4 MeOH
(1)
4
L2– + [Cu(MeOH) ]+
4
–
[
CuLMeOH] + 3 MeOH
(2)
(3)
Δαratios
.45
A (rel. units)
0.030
0
0
0
0
0
0
0
0
0
L2– + [Cu(MeOH)n]2+
[CuL] + n MeOH
I
II
III
IV
V
.40
.35
.30
.25
.20
.15
.10
.05
0
.025
HS4
4
L2– + [Cu(MeOH)n]2+
TiO
2
0.020
[CuLMeOH] + (n – 1) MeOH
L is (E)ꢀ2ꢀ(2ꢀhydroxylidenamino)butanedioic acid dianion;
(4)
MeOH
0
0
0
.015
.010
.005
–
[
CuL] and [CuL] are typeꢀ4 copper complexes;
–
[
CuLMeOH] and [CuLMeOH] are typeꢀ4´ copper complexes.
Table 2 lists the thermodynamic parameters of the
I
II
Cu and Cu complexes, viz., the changes in the Gibbs
energies (ΔG), enthalpies (ΔH), and entropies (ΔS), deꢀ
termined in the gas phase and in methanol.
4
50
500
550
600
650 λ/nm
The coordination environment of the metal center in
II
the stable forms of the Cu complexes consists of the
Fig. 3. Values of the catalytic parameter Δαratios determined after
irradiation of complex 4 and HS4 at different wavelengths (reꢀ
gions I—V). The solid line is the UV–Vis spectrum of complex 4.
tridentate ligand coordinated through two oxygen atoms
and one nitrogen atom and one methanol molecule. The

Hybrid systems CuII complex/TiO2
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017 2061
I
II
4´ for CuI
4´ for CuII
4
for Cu /Cu
Fig. 5. Optimized structures of copper complexes 4 and 4´.
Note. Figure 5 is available in full color on the web page of the journal (http://www.link.springer.com).
I
ΔG values for the Cu complex 4 (coordination number of
copper is equal to three) are somewhat higher than the
values obtained for complex 4´, where the coordinaꢀ
tion number of copper equals four. Interestingly, geoꢀ
metry optimization of the typeꢀ4´ CuI complexes
leads to a structure, where complex 4 and MeOH moleꢀ
cule are linked by a hydrogen bond formed between
the methanol hydroxyl group and salicyl oxygen atom
According to calculations, reactions (3) and (4) inꢀ
volving the Cu complex with the tetracoordinate copper
II
atom are characterized by the most negative ΔG values, so
this structure is the most stable. The calculated redox
potentials (Table 3) were determined from the changes in
the Gibbs energy of the halfꢀreactions represented by the
4
0
Born—Haber cycles shown in Fig. 6.
Although many theoreticians have reported their
methods4
1–50
for calculating the redox potentials, the
(
see Fig. 5).
The thermodynamic parameters of the Cu complexꢀ
es were estimated for reactions (3) and (4). Divalent copꢀ
per with d electronic configuration usually forms comꢀ
plexes with pseudoꢀoctahedral or squareꢀpyramidal geoꢀ
metry, where the coordination number of copper is equal
to six and five, respectively. In water and methanol, Cu
coordination compounds probably exist as a mixture of
pentaꢀ and hexacoordinate complexes.
II
differences between the calculated and experimental valꢀ
ues are quite large, viz., the mean absolute error can reach
up to 0.20—0.30 V. The deviation from the experimental
values originates mainly from the treatment of the stanꢀ
dard hydrogen electrode (SHE) potential, which was asꢀ
sumed to be constant and independent of the computaꢀ
tional method in many theoretical papers. The SHE poꢀ
tential values reported by different authors range from 4.2
9
II
3
9
Table 2. Thermodynamic parameters (kcal mol–1) of complexation for the reduced Cu (Red) and oxidized Cu (Ox) forms
I
II
in the gas phase and in methanol caclulated for reactions (1)—(4) using the B3LYP and PBE0 functionals
Complex
Computational
method
Gas phase
MeOH
–
ΔH
ΔS
–ΔG
ΔΔG
–ΔH
ΔS
–ΔG
ΔΔG
I
Cu complexes
–
[
[
[
[
CuL]
CuL]
B3LYP
PBE0
B3LYP
PBE0
167.14
167.17
176.29
177.48
173.63
171.71
146.23
144.31
189.10
188.55
190.07
190.69
0.97
2.14
0
26.01
25.00
29.60
29.68
175.46
173.45
147.02
145.23
48.51
46.90
43.61
43.16
0
0
4.9
3.74
–
–
CuLMeOH]
CuLMeOH]
–
0
CuII complexes (n = 5)
[
[
[
[
CuL]
CuL]
CuLMeOH]
CuLMeOH]
B3LYP
PBE0
B3LYP
PBE0
339.28
336.81
357.00
356.81
118.72
117.08
184.13
181.90
374.68
7.4
9.51
0
52.23
128.67
131.60
194.19
197.50
90.59
87.11
94.45
93.21
3.86
6.1
0
371.72
382.08
381.23
47.87
66.37
64.14
0
0
CuII complexes (n = 6)
[
[
[
[
CuL]
CuL]
CuLMeOH]
CuLMeOH]
B3LYP
PBE0
B3LYP
PBE0
317.45
313.86
335.17
333.86
152.86
154.23
118.26
119.04
363.03
7.4
9.51
0
45.29
164.51
160.96
130.04
126.85
94.34
89.24
98.20
95.33
3.86
6.09
0
359.84
370.43
369.35
41.24
59.43
57.51
0
0

2
062
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017
Yoshida et al.
ΔGgas(Ox/Red)
MeOHgas
+
[CuL]–
[CuLMeOH]gas
+
+
egas
gas
ΔGsolv
ΔGsolv(Red)
ΔGsolv(Ox)
ΔGsolv(e) = 0
ΔGsolv(Ox/Red)
MeOHsolv
+
[CuL]–
[CuLMeOH]solv
esolv
solv
E(Ox/Red) = (–ΔGgas(Ox/Red) – ΔGsolv(Ox) + ΔGsolv(Red) + ΔGsolv)/F – ESHE
ΔGgas(Ox/Red)
(5)
[
CuLMeOH]–
[CuLMeOH]gas
+
+
egas
gas
ΔGsolv(Red)
ΔGsolv(Ox)
ΔGsolv(e) = 0
ΔGsolv(Ox/Red)
[
CuLMeOH]–
[CuLMeOH]solv
esolv
solv
E(Ox/Red) = (–ΔGgas(Ox/Red) – ΔGsolv(Ox) + ΔGsolv(Red))/F – ESHE
(6)
Fig. 6. Thermodynamic cycles (5) and (6) (changes in the Gibbs energy in the gas phase, ΔGgas(Ox/Red), and in the Gibbs energy of
solvation of the oxidized and reduced forms, ΔG (Ox) and ΔGsolv(Red), respectively). In Eqs (5) and (6), ERed/Ox is given in V and
solv
–
1
–1
F is the Faraday constant equal to 23.06 kcal mol
V .
to 4.73 V depending on the experimental conditions, such
tions for the most intense transitions are presented in
Table 5. Based on the results obtained for the complex
[CuLMeOH] , the lowꢀenergy absorption at 322 nm was
5
1–56
as the temperature, electrode, etc.
We chose the valꢀ
–
ue 4.44 V as it is recommended in Ref. 52 and by the
IUPAC.57
attributed to the HOMOꢀ1 → LUMO transition, which
can be described as metalꢀtoꢀligand charge transfer
(MLCT). The highꢀenergy absorptions at 223 nm and
243 nm were assigned to the electronic transition from
deepꢀlying molecular orbitals to the LUMO or LUMO+4.
Other transitions in the UV region mainly belong to the
ligandꢀtoꢀligand type. The calculated spectra of the
Investigation of the frontier molecular orbitals. The reꢀ
activity and stability of chemical compounds are deterꢀ
mined by the energy and shape of the frontier molecular
orbitals (HOMO and LUMO). We calculated the
II
I
HOMO—LUMO energy gaps for the Cu /Cu complexes.
The energies of the frontier orbitals are listed in Table 4
while their shape is shown in Fig. 7. The LUMO is localꢀ
ized on the copper ion and the salicylic part of the ligand
while the HOMO is delocalized over the entire ligand
with a small contribution of the copper atomic orbitals.
For all complexes, the atomic orbitals of methanol pracꢀ
tically do not contribute to the frontier molecular orbitꢀ
als. The effect of methanol coordination manifests itself
in the change in the HOMO—LUMO energy gap, which
increases upon coordination of the solvent molecule.
–
–
[CuL] and [CuLMeOH] systems are fairly similar.
In the theoretical spectra of [CuLMeOH], there are
five intense absorption bands at 329, 275, 250, 220, and
203 nm. The [CuL] complexes are characterized by the
same number of bands at 308, 259, 235, 217, and 202 nm,
the longꢀwavelength absorptions being red shifted relaꢀ
tive to those of [CuLMeOH]. The band intensities calcuꢀ
lated for the [CuL] systems are higher than those obꢀ
tained for [CuLMeOH].
II
I
The absorption spectra of the Cu /Cu complexes
were modeled within the framework of the TDꢀDFT apꢀ
proach for solutions in methanol. The results of calculaꢀ
Summing up, we examined the photocatalytic activiꢀ
ty of CuII complexes 1—4 and their hybrid systems with
6
+
3+
TiO (HS1—HS4) toward the reduction of Cr to Cr
2
Table 3. Redox potentials of copper complex 4 calculated using the B3LYP and PBE0
functionals
Parameter
B3LYP
PBE0
Cycle (5)
Cycle (6)
0.093385
–0.030774
–0.080142
.—
Cycle (5)
Cycle (6)
ΔGgas(Ox/Red)/au
ΔGsolv(Ox)/au
ΔGsolv(Red)/au
ΔGsolv/au
0.091838
–0.030774
–0.083078
–0.006409
0.34
0.08154
–0.031255
–0.083703
–0.006309
0.62
0.084953
–0.031255
–0.080645
.—
E(Red/Ox)/V
0.55
0.78

Hybrid systems CuII complex/TiO2
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017 2063
Table 4. Calculated energies of the frontier molecular orbitals
in eV)
in MeOH solutions. It was established that UV irradiaꢀ
tion has no effect on the photocatalytic activity of the
hybrid systems. The photocatalytic activity of complex 4
and HS4 depends strongly on the wavelength of visible
light used for irradiation. The highest photocatalytic acꢀ
tivity of complex 4 and HS4 was observed in the λ region
of 460—495 nm. Quantum chemical calculations predict
(
Orbital
[CuL]– [CuLMeOH]– [CuL]* [CuLMeOH]*
LUMO+2
LUMO+1
LUMO
0.99
0.33
0.97
0.32
0.15
–1.03
–2.44
–7.68
–8.74
–9.43
5.24
0.20
–0.82
–1.46
–7.44
–8.60
–9.31
5.98
–0.098
–6.32
–6.73
–7.83
6.22
–0.15
–6.47
–6.87
–7.92
6.32
II
HOMO
the tetracoordinate squareꢀplanar structure for the Cu
HOMO–1
HOMO–2
Energy
complex 4. In the solid state, the coordination number of
I
copper in complex 1 is four. In the Cu complex 4, there
is a tricoordinate copper atom and the methanol moleꢀ
cule is linked by a hydrogen bond formed between the
methanol hydroxy group and salicyl oxygen atom. The
gap/eV
*
βꢀMolecular orbital.
II
Cu complexes investigated act as photosensitizers.
–
[CuLMeOH]– (LUMO+1)
[
CuL] (LUMO+1)
[CuL]* (LUMO+1)
[CuLMeOH]* (LUMO+1)
–
[CuLMeOH]– (LUMO)
[
CuL] (LUMO)
[CuL]* (LUMO)
[CuLMeOH]* (LUMO)
–
[CuLMeOH]– (HOMO)
[
CuL] (HOMO)
[CuL]* (HOMO)
[CuLMeOH]* (HOMO)
[
CuL]– (HOMO–1)
[CuLMeOH]– (HOMO–1)
[CuL]* (HOMO–1)
[CuLMeOH]* (HOMO–1)
Fig. 7. Graphical representation of the frontier molecular orbitals of complexes 4 and 4´ (isodensity contour is of magnitude 0.03).
βꢀMolecular orbitals.
Note. Figure 7 is available in full color on the webpage of the journal (http://link.springer.com).
*

2
064
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 11, November, 2017
Yoshida et al.
II
I
Table 5. Calculated electronic transitions Cu /Cu in the optiꢀ
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(
obtained from TDꢀDFT calculations)
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Excitation
energy/eV
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0.1493
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0.5103
HOMOꢀ1 → LUMO (33%)
HOMOꢀ3 → LUMO (10%)
HOMOꢀ5 → LUMO (15%)
HOMOꢀ1 → LUMO+4 (12%)
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0. R. A. Aziz, I. Sopyan, Recent Pat. Mater. Sci., 2009, 2, 88.
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[
CuL]–
2
014, 16, 19790.
3
.75
331
0.1281
HOMOꢀ1 → LUMO (24%)
HOMO → LUMO+1 (14%)
HOMOꢀ3 → LUMO (13%)
HOMOꢀ6 → LUMO (14%)
1
1
1
2. C. L. Bianchi, C. Pirola, F. Galli, M. Stucchi, S. Morandi,
G. Cerrato, V. Capucci, RSC Adv., 2015, 5, 53419.
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5
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225
204
0.1175
0.3800
0.1020 HOMOꢀ1 → LUMO+2 (10%)
[
CuLMeOH]
, 12685.
3
.77
329
0.1099
α HOMO → LUMO (43%)
β HOMO → LUMO+1 (47%)
β HOMOꢀ3 → LUMO (49%)
β HOMOꢀ2 → LUMO (53%)
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4
4
5
.51
.97
.65
275
250
220
0.1016
0.1406
0.4056 α HOMO → LUMO+2 (26%)
β HOMO → LUMO+3 (30%)
0.1285 β HOMO → LUMO+6 (12%)
6
.11
203
1
2
2
2
2
9. H. Nishizuru, N. Kimura, T. Akitsu, Asian Chem. Lett.,
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[
CuL]
2
4
4
.03
.79
308
259
0.1994 α HOMO → LUMO (16%)
0.1141 α HOMOꢀ1 → LUMO (14%)
β HOMOꢀ6 → LUMO (30%)
0.2271 β HOMOꢀ14 → LUMO (13%)
β HOMOꢀ6 → LUMO (13%)
1. Y. Takeshita, A. Nogami, T. Akitsu, World Sci. Echo, 2014,
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5
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β HOMOꢀ5 → LUMO (12%)
2
217
202
0.4182 α HOMO → LUMO+2 (36%)
β HOMO → LUMO+3 (31%)
0.1584 β HOMOꢀ1 → LUMO+3 (10%)
3. N. Yoshida, T. Akitsu, in Samarium, Chemical Properties,
Occurrence and Potential Applications, Ed. K. R. Danford,
Nova Sci. Publ., Inc., New York, 2014, p. 95.
6.15
2
2
4. N. Yoshida, T. Akitsu, in Integrating Approach to Photofuncꢀ
tional Hybrid Materials for Energy and the Environment,
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This work was carried out at the University of Tokyo
Advanced Characterization Nanotechnology Platform
supported by the "Nanotechnology Platform" of the Minꢀ
istry of Education, Culture, Sports, Science, and Technolꢀ
ogy (MEXT), Japan. Density functional calculations were
performed on computational facilities at the HighꢀPerꢀ
formance Computing Center, Southern Federal University.
Theoretical part of this research was financially supꢀ
ported by the Council on Grants at the President of the
Russian Federation (State Program for Support of Young
Researchers—Candidates of Science MKꢀ3173.2017.3).
2
2
6. N. A. Malakhova, A. V. Chernysheva, K. I. Brainina,
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Received December 28, 2016;
in revised form April 17, 2017