HCl and O2 co-activated bis(8-quinolinolato) oxovanadium(iv) complexes as efficient photoactive species for visible light-driven oxidation of cyclohexane to KA oil

Article abstract of DOI:10.1039/c8cy01241e

This paper reports three bis(8-quinolinolato) oxovanadium(iv) complexes that are active for visible light-induced cyclohexane oxygenation under O₂ (1 atm) in acetonitrile with HCl participation, achieving greater than 18% conversion and 86% selectivity for KA oil (cyclohexanone and cyclohexanol), along with a small amount of chlorinated product. Notably, these V^IVOQ₂ complexes are not only basically comparable to V^IVO(acac)₂ and some typical V^V-compounds in photocatalytic activity but are also more selective for KA oil. Spectral characterizations (XPS, FT-IR and UV-vis) and DFT calculations support that V^IVOQ₂ complexes can be co-activated by HCl and O₂ to form the V^VO(OH)-containing photoactive (PA) species with one pendant ligand's phenoxyl radical and an axially coordinated chlorine, which are responsible for the present photocatalytic oxidation. Noteworthily, increasing the Cl-substituents of V^IVOQ₂ complexes and especially the amount of water in the reaction system obviously decelerated the generation of such PA species, which significantly improved the selectivity for KA oil.

Full text of DOI:10.1039/c8cy01241e

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HCl and O co-activated bisIJ8-quinolinolato)
2
oxovanadiumIJIV) complexes as efficient
Cite this: DOI: 10.1039/c8cy01241e
photoactive species for visible light-driven
oxidation of cyclohexane to KA oil†
a
b
b
a
b
a
Jialuo She, Xiangfeng Lin, Zaihui Fu, * Jianwei Li,* Senpei Tang, Ming Lei,
a
b
b
Xin Zhang, Chao Zhang and Dulin Yin
This paper reports three bisIJ8-quinolinolato) oxovanadiumIJIV) complexes that are active for visible light-
induced cyclohexane oxygenation under O (1 atm) in acetonitrile with HCl participation, achieving greater
2
than 18% conversion and 86% selectivity for KA oil (cyclohexanone and cyclohexanol), along with a small
IV
amount of chlorinated product. Notably, these V OQ
2
complexes are not only basically comparable to
IV
V
V OIJacac)
2
and some typical V -compounds in photocatalytic activity but are also more selective for KA
IV
oil. Spectral characterizations (XPS, FT-IR and UV-vis) and DFT calculations support that V OQ
2
complexes
V
Received 17th June 2018,
Accepted 6th October 2018
can be co-activated by HCl and O2 to form the V OIJOH)-containing photoactive (PA) species with one
pendant ligand's phenoxyl radical and an axially coordinated chlorine, which are responsible for the present
IV
photocatalytic oxidation. Noteworthily, increasing the Cl-substituents of V OQ
2
complexes and especially
DOI: 10.1039/c8cy01241e
the amount of water in the reaction system obviously decelerated the generation of such PA species,
which significantly improved the selectivity for KA oil.
rsc.li/catalysis
Additionally, harsh reaction conditions are still needed for
1
Introduction
most of the direct oxidations based on air and O , due to the
2
The selective oxygenation of hydrocarbons by molecular oxy-
2
inertness of triplet O . Singlet oxygen, obtained by thermal-
gen (O ) or air is a very important and challenging transfor-
mation technology, allowing for the wide use of their oxi-
excitation or photo-excitation, however, is usually unselec-
tive. Some effective catalytic methods have been developed
2
1
5
dized products. For example, cyclohexane oxidation plays an
important role in the chemical industry, since its products
cyclohexanol and especially cyclohexanone are crucial raw ma-
terials for producing plasticizers, organic solvents and key in-
termediates for the synthesis of pesticides, drugs and espe-
to achieve the highly selective oxidation of cyclohexane and
other organics under moderate conditions, and the use of
transition metal complexes is very effective and selective for
6
–14
these catalytic reactions.
Among these complexes,
vanadium-based complexes are popular research topics be-
cause of their high catalytic efficiency in the oxidation of al-
2
–4
cially caprolactam (the precursor to polyamides).
In
1
1–14
industry, generally, the oxygenation of cyclohexane in air is
catalyzed by cobalt salts under relatively high temperature
kanes, olefins, lignins and so on.
In recent years, photocatalytic oxidation has gained in-
creasing attention in green chemistry. It strives to conduct the
selective oxidation or complete oxidation of organic com-
pounds at normal temperature and pressure by using sunlight
3
and pressure, where the conversion should be lower than
4
1
0% to obtain an acceptable selectivity for target products.
1
5,16
as an energy source with O or H O as oxidants.
There-
2
2 2
fore, it is of significant scientific value and promising applica-
tion value to introduce photocatalysis to organic synthesis.
Scholars worldwide have conducted a lot of research on the
photocatalytic oxidation of cyclohexane. They have found
a
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, P. R. China. E-mail: lijw@mail.buct.edu.cn
b
National & Local United Engineering Laboratory for New Petrochemical
Materials & Fine Utilization of Resources, Key Laboratory of Resource Fine-
Processing and Advanced Materials of Hunan Province and Key Laboratory of
Chemical Biology and Traditional Chinese Medicine Research (Ministry of Educa-
tion of China), College of Chemistry and Chemical Engineering, Hunan Normal
University, Changsha 410081, P. R. China. E-mail: fzhhnnu@126.com
1
7
18
some efficient photocatalysts, including TiO , V O @TiO ,
2
2
5
2
19,20
20
Fe/N doped TiO2,
lloporphyrin@TiO
graphene oxide hybrid, Fe O nanoparticles/Ti-SBA-15, Au
BiOX (X = Cl, Br and I), meta-
2
1
22
2
(anatase),
DDQ,
2
TiO /reduced
2
3
24
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
x y
2
5
c8cy01241e
in CNTs,
metal nanoparticles/carbon quantum dots
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2
6
27
28
composites, g-C N based composites, metal chlorides,
to obtain solution B. Next, solution B was added dropwise to
solution A under magnetic stirring, and then the resulting
mixture was stirred continuously for 2 h at 30 °C. The formed
precipitate was separated by filtration and washed with
CH Cl at least 3 times. After vacuum-drying for 10 h, a dull
3
9,30
4
2
decatungstates,
and so on.
In our previous works, 8-quinolinolato complexes
containing a Mn or Fe metal center were found to effi-
ciently catalyze some selective oxidation reactions by H O ,
such as the epoxidation of alkenes, the oxidation of alco-
hols, thioanisole, cyclohexane,
III
III
2
2
2
2
3
1
IV
IV
IV
a 2 b 2 c 2
green solid (marked as V OIJQ ) , V OIJQ ) and V OIJQ ) )
32
33
34,35
36
and ethylbenzene.
was obtained in 88–95% yield.
Moreover, we found that bisIJ8-quinolinolato) oxovanadiumIJIV)
Elemental analyses were performed, respectively, by an
Elementar Vario EL III analyzer for H, C and N, and by a
PerkinElmer Optima 5300DV ICP-OES for V. The elemental
contents in the above complexes are listed in Table 1, which
are basically in agreement with their theoretical values based
on the tetradentate structures.
IV
2
(V OQ ) complexes combined with N-hydroxyphthalimide can
catalyze the oxygenation of ethylbenzene to acetophenone un-
3
7
der an O2 atmosphere.
Recently, we found that the
vanadiumIJV)-substituted tungsto- and molybdo-phosphoric
acids or their quaternary ammonium salts (POMs) combined
with concentrated HCl aqueous solution, can construct an ef-
X-ray photoelectron spectroscopy (XPS) of these complexes
was carried out using an ESCALAB 250Xi spectrometer and
then analyzed by the Avantage package. Transmission FT-IR
fective photocatalysis system for cyclohexane oxidation by O
ref. 38 and 39) and N O (ref. 40) to KA oil, providing a yield
2
(
2
−
1
of more than 18%. Furthermore, the preliminary mechanism
spectra (400–4000 cm ) were obtained using an FT-IR
spectrometer (Nicolet Nexus) with KBr discs. The liquid UV-
vis spectra (200–800 nm) were surveyed on a spectrophotome-
ter (UV 1102 II) in MeCN.
V
VI
shows that the V –O–Mo sites on POMs tend to be proton-
ated by HCl to yield photoactive species (PA),
POMs–(V OHMo) Cl , which can be responsible for such
a
V
+
−
3
9
photocatalytic oxidation.
It is interesting to use more
IV
available V OQ complexes to catalyze the photocatalytic oxi-
2
2.3 Typical photocatalysis procedures
dation promoted by HCl and especially to research how these
IV
The V OQ
2
complex (0.01 mmol) catalyzed cyclohexane (1
IV
V -containing complexes are converted into the correspond-
mmol) photo-oxygenation in MeCN (5.0 mL) with conc. HCl
0.1 mL) under O2 (1 atm) was conducted in sealed photo-
reactors furnished with a water condenser. One glass tube
containing a 35 W bromine tungsten lamp with an ultraviolet
filter was inserted in each reactor. After the reaction system
was irradiated for 10 h at 30 °C with magnetic stirring, the
resulting products were detected quantitatively on
Shimadzu GC-2010 Plus outfitted with a capillary column (30
m × 0.25 mm × 0.25 μm) and an FID using ultrapure N as
the carrier gas (1.5 mL min ) and cyclopentanone as the
internal standard.
V
ing V -containing PA species under the co-participation of
(
HCl and O
2
. The generation of vanadiumIJV) superoxide spe-
activa-
cies was generally proposed to be the first step in O
tion by oxidovanadiumIJIV) and the oxo-hydroxo interconver-
2
4
1
sion of its subsequent species also widely occurs in
4
2
solution. Usually, these two processes are not easy to carry
out at room temperature, but they can go on smoothly in our
reaction system. Additionally, the chlorine substituents on li-
gands influence the generation of these PA species and thus
adjust the selectivity for KA oil. Here, we report some initial
results in terms of the above scientific problems based on ex-
perimental and theoretical methods.
a
2
−
1
2
.4 Theoretical calculations
All the DFT calculations were performed with the Gaussian
9 program by using the M06 method combined with mixed
basis sets, i.e. 6-31+G(d,p) for main group elements, and
LANL2TZ(f) for V. The bulk solvent effects of the MeCN me-
dium were calculated via the polarizable continuum model
2
Experimental
0
2
.1 Reagents
In this work, the main reagents included 8-hydroxyquinoline
(
(
Q ), 5-chloro-Q (Q ), 5,7-dichloro-Q (Q ), dichloromethane
a
a
b
a
c
IV
CH
2
Cl
2
), vanadyl acetylacetonate (V OIJacac)
), ammonium metavanadate (NH
dium metavanadate (NaVO3), cyclohexane, acetonitrile
MeCN), cyclopentanone and concentrated HCl (conc. HCl),
2
), vanadium
(
PCM) in the SCRF method. The structures of key reaction
pentoxide (V
O
2 5
4
VO ), so-
3
species were optimized to have no imaginary frequency and
were drawn by the GaussView program. Natural charges,
Wiberg bond indexes (WBIs) and spin density distribution
were obtained based on these optimized structures. More de-
tails about the calculations can be seen in the ESI.†
(
all of them being analytical reagents. Distilled water was
employed in the whole experiment.
2
.2 Syntheses and characterizations of bisIJ8-quinolinolato)
oxovanadiumIJIV) complexes
According to our previous work, the preparation procedure
Table 1 The elemental analyses of the synthetic catalysts
3
7
Element content/wt% (theoretical values in brackets)
IV
for the title complexes is as follows: 1 mmol V OIJacac) was
2
Samples
H
C
N
V
dissolved in 2 mL CH
tom flask (25 mL), and 2 mmol of 8-hydroxyquinoline (Q ) or
2
Cl
2
to form solution A in a round bot-
IV
V OIJQ
a
)
2
3.44 (3.40)
2.46 (2.38)
1.68 (1.64)
58.83 (60.86)
49.73 (50.97)
42.92 (43.85)
7.51 (7.89)
6.38 (6.60)
5.74 (5.68)
14.23 (14.34)
11.99 (12.02)
9.95 (10.33)
IV
a
V OIJQ )
b 2
IV
its derivatives (Q and Q ) were dissolved in 18 mL of CH Cl
2
c 2
V OIJQ )
b
c
2
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3
Results and discussions
and the chlorination selectivity was restrained in the over-
all concentration range (see Fig. S1(a) in ESI†). Fig. 1(c) il-
lustrates that the conversion was sequentially and obvi-
ously improved from 10.8% to 20.0% as the catalyst
dosage increased from 0.25% to 0.75%. Thereafter, the
conversion slightly increased to 22.4% when its concentra-
tion was enhanced to 1.75%. However, using an excess of
catalyst (2%) conversely resulted in a decrease in conver-
sion, which is perhaps attributable to the decline in ab-
3
.1 Photocatalytic cyclohexane oxidation by O
2
V
IV
Table 2 lists the data for the various V /V -compounds-
catalyzed cyclohexane photo-oxidation assisted by conc. HCl
under O . Obviously, all these vanadium compounds were ac-
2
tive for this photo-oxygenation, achieving 18–26% cyclohex-
ane conversion and 78–95% KA oil selectivity after continu-
ous irradiation for 10 hours (entries 1–7), along with the
chlorinated side reaction (selectivity, 5–22%). They also pos-
sessed nearly equal photocatalytic efficiencies with a turnover
sorption efficiency of radiation as the color of catalyst so-
IV
lution became much deeper. Also, V OIJQ ) showed a
c 2
IV
number (TON) of 18.51–21.49, except for V
2
O
5
. Notably, the
similar concentration effect to V OIJQ
a
2
) , but the suitable
IV
V OQ complexes commonly exhibited a definite restraining
concentration range (0.5–1.5%, Fig. S1(b)†) was narrower
due to its darker color. Fig. 1(d) displays that the conver-
sion obviously increased from 2.7% to 24.8% as the illu-
mination time was extended from 2 to 8 h. Thereafter,
the conversion was slightly reduced as the time was fur-
ther extended to 10 h, which might be due to the deep
oxidation of the products. The selectivity for cyclohexa-
none continuously and slowly increased during the first
2
effect on the chlorination reaction and thereby achieved bet-
ter selectivity for KA oil than other V-compounds (entries 5–7
vs. 1–4). Such a restraining effect also became more notice-
able for the V OIJQ ) , although conversion over this complex
slightly decreased (entry 7 vs. entries 5–6). Two controlled ex-
periments show that V OIJQ ) -catalyzed cyclohexane oxida-
2
tion to KA oil by O did not happen without HCl (entry 8) or
IV
c 2
IV
a 2
under heating (entry 9), indicating that the current catalytic
oxidation should be induced by conc. HCl and visible light.
In the next experiments, we checked the influence of wa-
8
h, with
a
concomitant reduction in the chloro-
cyclohexane selectivity. Cyclohexanol selectivity was also
slightly affected by the time.
ter, HCl and catalyst dosages, as well as illumination time on
Noteworthily, the time to achieve the highest conversion
was prolonged to 10 h over V OIJQ ) and V OIJQ ) , and
b 2 c 2
moreover the restraint effect on the chlorination reaction was
largely improved during the whole time over the two com-
plexes (Fig. S2(a) and (b)†), indicating that the reaction rate
and the KA oil selectivity are greatly influenced by the substit-
IV
IV
IV
the current photocatalytic oxidation using V OIJQ ) as a cata-
a 2
IV
lyst (Fig. 1). Some conditional experiments by using V OIJQ
were also done for comparison (Fig. S1 and S2†).
)
c 2
Fig. 1(a) shows that when the water amount was be-
tween 0 and 0.05 mL, cyclohexane conversion slightly in-
creased by less than 3%. Meanwhile, the selectivity for
cyclohexanone continuously and significantly increased
with water amount and achieved 75.1% in 0.05 mL water,
concomitant with the reduction of cyclohexanol and espe-
cially chlorocyclohexane. On further increasing water dos-
age from 0.075 to 0.125 mL, the conversion was drastically
decreased from 22.9% to 6.5% and cyclohexanone selectiv-
ity was increased to 90%. Fig. 1(b) demonstrates that in-
creasing the amount of HCl resulted in a continuous in-
crease in cyclohexane conversion but a slight decrease in
uents on the ligands. Additionally, the recycling test showed
IV
that V OIJQ ) , as the more suitable one, can be used at least
c 2
three times without a significant decrease in the cyclohexane
conversion or KA oil selectivity (Fig. S3(a)†).
3
.2 Photocatalytic mechanism
3.2.1 Spectral characterizations
XPS spectra. To understand the ligand effect on these
IV
IV
a 2 c 2
complexes, the XPS of V OIJQ ) and V OIJQ ) were analyzed
cyclohexanone selectivity, along with an increase of chlori-
as well as their ligands. In each sample, the C 1s line of
aromatic carbon was taken as a reference to define the
binding energies (BEs) of other elements. Allowing for the
IV
nated product. In contrast, V OIJQ
)
c 2
can be effectively ac-
tivated only by adding high concentration HCl (>10 M),
Table 2 Varieties of vanadium-compounds that photocatalyzed cyclohexane oxidation by O
2
with or without conc. HCla
Selectivity of products (%)
Conv.
(%)
Entry
Catalyst
TON
K
A
Chlorocyclohexane
V
1
2
3
4
5
6
7
NH
4
V O
3
3
19.31
21.49
26.64
18.51
20.97
20.21
18.66
0
19.31
21.49
13.32
18.51
20.97
20.21
18.66
—
67.15
70.64
68.67
68.23
74.52
75.23
79.40
—
11.09
11.7
14.12
11.69
12.03
12.16
15.94
—
21.77
17.95
17.21
19.81
13.45
12.62
5.06
—
V
NaV O
2 5
V
V O
IV
V OIJacac)
V OIJQ
V OIJQ
V OIJQ
V OIJQ
V OIJQ
2
IV
a
)
2
IV
b
)
2
IV
c
a
a
)
2
b
IV
8
)
)
2
c
IV
9
2
0
—
—
—
—
a
b
c
Typical photo-oxidation conditions. Without conc. HCl. Thermal reaction (30 °C) in darkness.
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Fig. 1 The influence of water dosage (1 mmol HCl, (a)), HCl concentration (0.1 mL H
2
O, (b)), catalyst concentration (based on substrate, (c)) and
IV
reaction time (d) on the cyclohexane (1 mmol) oxidation photocatalyzed by V OIJQ
a
)
2
under O
2
.
IV
chemical shift induced by the chlorination effect, the
references in these samples, with and without Cl, were
and 0.4 eV compared to those of V OIJQ
2(e)†), allowing it to be more easily protonated.
To explore how the photoactive (PA) species are formed in
c 2
) (Fig. S4(f) vs.
4
3
IV
assigned to 285.0 and 284.6 eV, respectively. The V OIJQ )
c 2
IV
and its ligand were taken as examples here (Fig. 2) because
of their preferable regularity. Additionally, the samples with
2 2
the V OQ –HCl–O system and how the water dosage affects
their generation, the FT-IR and UV-vis spectra of these com-
plexes were measured with and without HCl. Some important
Q are listed in the ESI† file; please refer to this file for more
a
IV
IV
details.
spectral bands are described by using V OIJQ
)
a 2
and V OIJQ
)
c 2
IV
As shown in Fig. 2(a) and (b), V OIJQ
)
c 2
had five ele-
as examples in the following.
IV
ments (besides H) and presented the typical feature of the
FT-IR spectra. i) An FT-IR spectrum of the V OIJQ ) com-
plex exhibited some characteristic absorption bands of li-
gands in the range 400–2000 cm (curve 1 in Fig. 3(a)). The
bands at 1575, 1500 and 1466, as well as 1110 cm , are re-
spectively attributable to the ν(CN), ν(CC), and ν(C–O
stretching vibrations, and the other two at 1320 and 1265
cm originate from the in-plane bending vibration of Ar–
Notably, these characteristic bands had different de-
grees of shifts compared to those of the corresponding free li-
gand Q (Fig. S5†), which could be due to the coordination ef-
showed absorption peaks at 964
b
and 524 cm , which belong to the ν(V O) and ν(V –O )
a 2
4
4
reduction state (V 2p3/2 = 516.3 eV). Once the ligand Q
c
IV
−1
was coordinated to the V center, the BE of N 1s was in-
creased from 398.6 to 399.2 eV (Fig. 2(c)). Meanwhile, its O
−
1
1
s
peak was shifted from 532.4 to 530.6 eV
b
)
4
4
(Fig. 2(d) and (e)), and this change was more pronounced
−
1
than that of N 1s. These findings indicated the co-existence
of electron donation from N 2p to V 3d and the back dona-
4
5,47
O
b
.
4
5
tion from V 3d to O 2p in the above complex, and its
V–O might be more stable than the V–N. Furthermore, a
peak arose at 532.2 eV (higher than that of V–O) by the
deconvolution of the O 1s peak in the complex, which was
a
IV
fect. In addition, V OIJQ
)
a 2
−
1
IV
IV
4
6
45,47
assigned to VO. Therefore, the O in V–O–C should have
more electrons and then prefer to be protonated when
interacting with HCl. Fig. 2(f) reveals the electron withdraw-
ing effect of the Cl substituent as the BE of Cl 2p3/2 was
stretching vibrations,
respectively. Furthermore, a split-
ting of the ν(V O) band occurred at 953 cm , consistent
with six-coordinated vanadium complexes. Another com-
plex, V OIJQ ) (curves 1 in Fig. 3(b)), showed very similar FT-
IV
−1
4
8
IV
c 2
IV
IV
decreased from 200.2 to 200.1 eV after the coordination.
IR characteristic bands to the V OIJQ
a
2
) , but its ν(V O) and
IV
IV
−1
The V OIJQ
)
a 2
presented the aforementioned regularities
ν(V –O
b
) vibrations at 975 (splitting at 963) and 598 cm
had different degrees of blue-shift compared to V OIJQ ) .
Also, its ν(V O) vibration was split into two symmetrical
IV
in the XPS spectra but its V–O and VO showed relatively
lower BEs for O 1s, which were respectively decreased by 0.2
a 2
IV
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IV
c 2 c
Fig. 2 The XPS survey (a), V 2p (b), N 1s (c), O 1s ((d) and (e)) and Cl 2p (f) spectra of V OIJQ ) and or its ligand Q .
peaks. This indicates that the ligand's chlorine substituent
can strengthen the coordination capacity of the ligand to V
ion; on the other hand, it shows a significant promotion ef-
fect on the formation of six-coordinated vanadium
complexes.
can be partly cleaved via HCl protonation so that its
coordinatively unsaturated bond was formed with a pendant
phenolic hydroxyl. On the other hand, such a new band gen-
IV
V
erated with HCl treatment is likely assigned to the ν(V O)
stretching vibrations that usually appear at
a higher
IV
47
ii) After treatment with a drop of concentrated hydro-
wavenumber than that of ν(V O). This likely indicates
IV
V
chloric acid (HCl), the ν(C–O ) band of V OIJQ ) became
that the V -complexes can be generated by oxidizing these
b
a 2
IV
broader and asymmetric (curve 2 in Fig. 3(a)). Moreover, a
unsaturated species with O
2
. Notably, the V OIJQ
a
)
2
is more
−
1
split peak arose at 1099 cm , which approached the position
sensitive to HCl and carries out the above transformations
−
1
IV
of the ν(C–O ) band (1094 cm ) in the free-ligand, after two
more easily than the V OIJQ ) .
b
c 2
IV
more drops of HCl were added (curve 3 in Fig. 3(a)). The
UV-vis spectra. i) A low concentration of V OIJQ
(2.4 × 10
)
a 2
complex
IV
IV
−4
ν(V O) and ν(V –O
b
) bands became gradually weaker with
M) in MeCN exhibited some characteristic
increasing HCl amount, along with the appearance and en-
absorption bands in its UV-vis spectrum (curve A in Fig. 4(a)).
Among such bands, the strong one near 320 nm is assigned
to intraligand transitions (π–π*) and another obvious absor-
bance in 600–800 nm is attributed to the d–d transition of
−
1
hancement of a new band at 997 cm . Similar changes were
IV
c 2
also observed in the FT-IR spectra of V OIJQ ) , but a larger
amount of HCl was required to induce such changes (3–6
drops, curves 2 to 3 in Fig. 3(b)).
IV
48,49
V
ions.
ii) When HCl was added to the A solution, the
The appearance of the split peak attributed to the free
two bands described above gradually decayed with prolonged
time (curve B1–B5). Meanwhile, the new band near 360 nm,
which originates from charge transfer transitions from the
IV
ligand's ν(C–O) and the reduction of the ν(V –O ) band under
b
IV
IV
b 2
HCl treatment likely imply that the V –O bond of V OQ
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However, an opposite trend in the intensity of the other band
near 475 nm was noticed. In addition, the V O –HCl system
2
5
only exhibited a band near 475 nm in the visible spectrum
and there was only the band near 400 nm in the visible spec-
IV
trum of the V OIJQ ) –H SO system (Fig. S7†). Hence, the
a 2
2
4
band near 400 nm, related to the ligand, is assigned to the
5
0,51
n–π* transition of the ligand's phenoxyl radical
and the
chlorine substituents, as supported by the following DFT cal-
culations, can promote the occurrence of such transitions,
leading to the enhancement of such a band and the obvious
IV
IV
red-shift over V OIJQ ) and especially V OIJQ ) . The band
b 2
c 2
−
near 475 nm is related to the Cl ions, as previously reported
3
8,39
by us in the POMs–HCl photocatalysis system,
from the charge transfer transitions (LMCT) of Cl to V
ions, and was regarded as the characteristic band of the
photoactive (PA) species. A decay in the intensity of such a
band from V OIJQ ) to V OIJQ ) could be due to a reduction
originates
−
5+
3
9
IV
IV
a 2
c 2
IV
2
in the activation effect of HCl on V OQ complexes with
chlorine substituents, as supported by the results of Fig. 3–6
and the accompanying DFT calculations. Additionally, the
used catalysts exhibited the clear characteristic bands of the
IV
phenoxyl radical and V in the visible spectra. However, the
LMCT band assigned to PA species was remarkably weakened
IV
IV
a 2 c 2
Fig. 3 FT-IR spectra of two complexes V OIJQ ) (a) and V OIJQ ) (b)
in the absence and the presence of HCl aq.
V
48,49
ligand to the V cation (LMCT),
was generated and gradu-
ally increased in intensity with the interaction time, reaching
a maximum at 25 min. These spectral changes further sup-
port the assertion that HCl can significantly promote the oxi-
IV
V
2
dation of the central V to its V ions by O . Notably, a real
reaction solution, illuminated by visible light for 8 h, still
showed a stronger d–d transition band than the solution B5
(
curves C vs. B5), which could be due to the photocatalytic cy-
IV
V
cle between V and V ions. Also, the aforesaid UV-vis spec-
tral changes could be observed in the solution of the
IV
V OIJQ
c
)
2
complex (Fig. 4(b)), but the increasing trend in the
V
intensity of the LMCT band of V ions (vs. interaction time)
was obviously weakened, indicating that the chlorine substit-
uent had a restraining effect on the HCl-promoted oxidation
of V to V by O , which was further confirmed by the decay
rate of V ions based on the plot of the absorbance at 650
IV
V
2
IV
nm vs. interaction time (Fig. 5).
iii) Interestingly, when the concentrations of these HCl-
IV
−3
2
treated V OQ complexes were improved to 1.0 × 10 M, two
broad absorption bands near 400 and 475 nm appeared in
the visible spectra of these V-complexes (Fig. 6) and the latter
gradually increased with increasing HCl amount (Fig. S6†),
which was due to the strong activation effect of HCl. Notably,
the band near 400 nm gradually increased in intensity and
IV
−4
Fig. 4 Liquid UV-vis spectra of V OIJQ
a
)
2
(2.4 × 10 M, (a)) and
IV
−4
−4
V OIJQ
c
)
2
(2.4 × 10 M, (b)) in 5.0 mL MeCN with HCl (10 M). A:
Without HCl; B1–B5: after adding HCl aq. for 5, 10, 15, 25 and 35 min.
C: After visible light illumination for 8 h.
IV
IV
an obvious red-shift occurred from V OIJQ
)
a 2
c 2
to V OIJQ ) .
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Fig. 5 Dependency of the absorbance at 650 nm vs. the interaction
time of HCl.
IV
Fig. 7 Liquid visible spectra of V OIJQ
a
)
2
(0.01 mmol) in 5.0 mL MeCN
containing 1 mmol HCl and different water amounts.
due to the considerable conversion of the PA species to its re-
duced states. The catalysts recovered by vacuum-rotary evapo-
ration still maintained similar spectral character, although
their LMCT bands were further weakened (Fig. S3(b)†). Nota-
bly, the recovered catalysts can be easily dissolved in pure
MeCN but the fresh ones cannot, indicating that the former
cannot return to their initial states even though HCl was
removed.
3
.2.2 Proposed mechanism. According to the above re-
sults, a reasonable mechanism for the generation of such a
PA species and its further transformation is proposed as fol-
lows by using V OIJQ
the complex V OIJQ ) (species 1a) is pre-activated to give an
intermediate 2a via protonation of one of its V–O
HCl (eqn (1)), followed by the dissociation of another V–O
bond via protonation by the second HCl to form an unsatu-
rated V O species with one pendant phenolic hydroxyl and a
downward axially coordinated Cl anion (species 3a-1, eqn
2)). Then, 3a-1 combines with O to form a V O-superoxide
radical species 4a-1 (eqn (3)) with the ligands' positions ad-
justed properly. The species 4a-1 can undergo an intramolec-
ular hydrogen atom transfer (HAT) from phenolic hydroxyl to
IV
)
a 2
as an example (Scheme 1). Firstly,
IV
a 2
b
bonds by
b
In the following spectral monitoring, the effect of water
amount on the characteristic band of PA species (475 nm)
IV
IV
was checked using the V OIJQ
)
a 2
complex as an example. As
shown in Fig. 7, such a band was gradually and significantly
weakened with increasing water amount and almost
disappeared when the amount was higher than 0.1 mL, indi-
cating that the additive water can significantly hamper the
formation of PA species. Notably, the conversion curve (vs.
water amount) shown in Fig. 1(a) illustrates that the
restraining effect of water on the photocatalytic reaction obvi-
ously lagged behind such an effect on the generation of PA
species; this may imply that the generation of PA species is
not the rate-determining step for the present photo-
oxidation.
V
(
2
V
V
2
V O–O ˙ to yield a V O-hydroperoxide species with one pen-
dant phenoxyl radical (species 5a-1, eqn (4)). The existence of
the phenoxyl radical has been confirmed by the above UV-vis
spectra. Moreover, such HAT should easily proceed due to
the weak O–H bond of phenolic hydroxyl, which can be fur-
ther supported by the similar processes in the dehydrogena-
tion of 1,4-cyclohexadiene and 9,10-dihydroanthracene with
stronger C–H bonds by V O–O
acids. The species 5a-1 can be decomposed into species 6a-
(eqn (5)) in the presence of water with hydroperoxide re-
leased. Moro-oka et al. reported a kind of V OIJOH) com-
V
2
˙ in the presence of Lewis
4
1
1
4
1
IV
plex that can be oxidized by O to form its vanadiumIJV)-oxo-
2
V
52
2
monoperoxo species V OIJO ), but the following proofs can
rule out the possibility that species 6a-1 is assigned to the
V
V OIJO ): i) the characteristic ν(O–O) stretching vibration
2
−
1 53
V
(
915–950 cm ) of V OIJO
spectra (Fig. 3(a)). ii) By respectively treating V OIJQ ) with
2
) was not found in the above IR
IV
a 2
H O (in MeCN) and O (in MeCN/THF), the in situ generated
2
2
2
V
V OIJO
4
2
) showed the characteristic absorption (usually 400–
5
4
50 nm) centered around 450 nm in the visible spectra
(
Fig. S8†), which was inconsistent with that of 6a-1 (near 475
nm). Afterwards, 6a-1 as the PA species was most likely stim-
IV
Fig. 6 Liquid visible spectra of three kinds of V OQ
2
complexes in the
ulated by radiation to undergo an intramolecular electron
doubly diluted MeCN solution containing 1-fold catalysts and 1.6-fold
conc. HCl compared to the reaction concentration so that all the cata-
lysts were properly dissolved.
−
5+
transfer (ET) process from Cl to V , thus leading to the gen-
IV
t
eration of the triplet V OIJOH) species 7 a and Cl˙ radical
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IV
Scheme 1 Proposed photocatalytic mechanism for the V OIJQ
a
)
2
(1a) catalysis system. (The species with tab “-1” represent the Cl-axially coordi-
nated mode).
(
eqn (6)). The following reactions induced by Cl˙ can generate
substituting on ligands, supporting the assertion that the
bridging Ob atoms are protonated preferentially by HCl and
such protonated ability is weakened from 1a to 1c (Table
S1†), consistent with the above spectral results. ii) According
to the changes in WBIs and natural charges between species
KA oil and chlorocyclohexane, as previously reported by us in
the POMs–HCl–O
can act as a good hydrogen atom donor due to its relatively
weak O–H bond and transfer this H atom to the phenoxyl
3
8,39
IV
t
2
system.
Notably, the V OIJOH) of 7 a
4
2
V
radical, thus giving the V O
2
species 8a (eqn (7)). Then, 8a
1 and 2 (Fig. S9, Tables S1 vs. S2-1 and -2†), one of their V–O
b
captures one HCl to give another PA species 9a (eqn (8)). In
bonds is weakened significantly (WBIs decrease by 37.5, 36.7
and 32.8%, respectively) owing to the strong interactions in-
duced by one HCl, along with a slight reinforcement of three
other coordinated bonds. Hence, such interactions lead to
the structural distortions of these complexes by different de-
grees, and the distortion effect is gradually reduced with in-
creasing the Cl substitution on ligands. The vanadium com-
plexes with the Cl-axially coordinated mode were widely
fact, the following DFT calculations support the assertion
t
that 7 a more likely undergoes an ET and then HAT pathways
to directly yield 9a under HCl-interaction (eqn (9)). The spe-
−
V
cies 9a can also realize the ET process from Cl to V ions un-
der visible light irradiation, thus generating Cl˙ and the re-
duced species 10a (eqn (10)). Finally, 10a conducts an
intramolecular dehydration induced by the HCl interaction to
form the species 3′a-1, which has the Cl anion bonding to
the equatorial site (eqn (11)), thus completing the entire
photocatalytic cycle. Additionally, the weak O–H bonds of the
1
2–14,41,55,56
reported,
however, this mode cannot be obtained
in species 2 by calculations, thus we propose that 2 needs to
be further activated by other HCl molecules.
iii) After being attacked by another HCl, species 2 is trans-
formed into two kinds of isomers, the Cl-axially (species 3a-1
IV
pendant phenolic hydroxyl and the V –OH group likely trans-
fer their H atoms to the cyclohexyl peroxy radical (CyOO˙) to
s
t
give 7 a (one singlet species from 7 a) and 8a, respectively,
to 3c-1) and the Cl-nonaxially (species 3a-2 to 3c-2) coordi-
IV
along with the formation of CyOOH (eqn (12)). To supple-
nated V OQ
2
complexes (Fig. S9, Tables S2-1 and -2†). As a
bonds of the Cl-axial species 3 (WBI
≈ 0.30–0.35) are even weaker than the first V–Ob bonds of
species 2 (WBI ≈ 0.38–0.40). However, both the V–O bonds
s
ment, the new species 7 ′a found by calculations can be
result, the second V–O
b
t
regarded as another singlet species of 7 a with a special coor-
s
dination mode, and the pathway containing 7 ′a is discussed
b
in details below.
of the Cl-nonaxial species 3, compared with 2, get a little
stronger (WBI ≈ 0.42–0.44). The data suggest that species 1
can be efficiently transformed into more actively photoactive
precursors (species 3a-1 to 3c-1) under the interactions with
at least two HCl molecules. Additionally, we propose from
the differences in the Gibbs free energies (ΔG298, Table S3†)
3
.2.3 DFT calculations. The formation of some key inter-
mediates and their protonation processes in the above mech-
anism were further studied by DFT calculations, and the
main results, mostly listed in the ESI,† are summarized as fol-
lows: i) these complexes (species 1a–1c) in MeCN all have
centrosymmetric structures (Fig. S9(a)†) and similar coordi-
nation capacities according to their Wiberg bond indexes
between the two modes that the Cl-nonaxial structure should
IV
be dominant in V OIJQ ) activated by two HCl, but the ratio
c 2
IV
IV
(
WBIs in Table S1†) of the V–N and V–O bonds. Among the
various atoms of species 1, the coordinated O atoms possess
the most negative natural charges and such negative charges
NC) gradually decrease with increasing the amount of Cl
a 2 b 2
is reduced to some extent upon V OIJQ ) and V OIJQ ) com-
b
plexes. iv) The species 3 then, by several steps, transforms
into species 6, which can interact with HCl (Fig. 8 and S10;
Tables S4-1 to S5-2†). Both ligands of the Cl-axial species 6
(
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Fig. 8 Spin density distribution (positive in blue, negative in green, isoval. dens. = 0.002) in the Cl-axially (a) and Cl-nonaxially (b) coordinated
modes of species 6a, i.e. species 6a-1 and 6a-2. The colors of elements, white, grey, blue, red, green and dark yellow balls, represent hydrogen,
carbon, nitrogen, oxygen, chlorine and vanadium atoms, respectively.
almost stay in the same plane (e.g. 6a-1 in Fig. 8(a)), and the
HCl molecules tend to attack O(3) firstly and then attack
fect becomes outstanding for 6a-1 and 6b-1, indicating that
the ratio of the Cl-axial structure can be improved signifi-
cantly with such two kinds of multi-HCl-activated complexes,
in accordance with the photoreaction performances and spec-
tral characterizations mentioned above (Fig. 6 and S6†). Nota-
bly, the spin density distributions over both kinds of Cl-
coordinated structures show that the single electron mainly
O IJ1) above this plane. After having interacted with two HCl,
b
the H–Cl bond of the first additional HCl that does not be-
long to 6 is weakened significantly, with the V–O
ing even stronger than that of 1 (Tables S5-1 vs. S1†). Though
the V–O IJ2) bond of 6 becomes stronger after this interaction,
b
IJ1) bond be-
b
it is still much weaker than that of 1 (the corresponding
b
delocalizes in the entire phenolic ring containing O IJ2)
WBIs in these Cl-axial structures decrease by 27.2, 24.7 and
(Fig. 8), thus forming a phenoxy radical. Furthermore, in-
creasing the Cl substituents of phenolic ring leads to a grad-
ual increase in the electron density on the phenoxy oxygen,
but a decrease in that of the phenolic ring (Table S7†). There-
fore, a red-shift and an enhancement of the n–π* transitions
of these phenoxyl radicals can be observed, which are consis-
tent with the above visible spectral results (Fig. 6).
2
3.9%, respectively), which indicates the dissociated state of
this bond. Notably, the Cl-axial species 6, compared with
their Cl-nonaxial mode (e.g. 6a-2 in Fig. 8(b)), show much
stronger V–ClIJ1) bonds (more than 34-fold in WBI) and much
V
lower positive charges (PC) for the central V cations
(Table 3). Moreover, their V–ClIJ1) bonds in the axial modes
t
are gradually strengthened with increasing the extra-
interacting HCl, along with the further decrease in the PC of
v) The direct conversion of the 7 into the 8 via a HAT
pathway is expected to have a relatively high energy bar-
V
42,57
IV
the central V ions and the gradual changes in the charges of
rier.
However, when the V ion transfers its electron
s
HCl(1) from nearly neutral to positive (Table 3, see Tables S4-
to the phenoxyl radical to give a singlet intermediate 7
(Fig. S11, Tables S8-1 and -2†) with the O
down to the axial coordination site, such an intermediate
could spontaneously convert to with HCl-interaction
1
to S5-2† for more details). These results support the idea
b
IJ2) flipping
that an ET process directly from the Cl atom of HCl(1) to-
V
wards the center V ion can be efficiently conducted in the
9
Cl-axial mode, and the structures in this mode with the char-
acter of the PA species should be mainly responsible for such
ET capacities although they are not dominant according to
the ΔG298 (Table S6†) between the two coordination modes.
Interestingly, such ΔG298 are obviously reduced upon increas-
ing the number of interacting HCl and thus this reduced ef-
(Fig. S11, Tables S9-1 to S11†). Interestingly, the HCl acts
as a proton relay that protonizes the unsaturated phenoxy
b
O IJ2) and meanwhile forms a hydrogen bond with the
V
V O–H group. Moreover, the ET and HAT processes may
participate simultaneously in such conversion, and the ac-
tivation effect of HCl is weakened by the Cl-substituents
Table 3 Calculated Wiberg bond indexes (WBIs) and natural charges in species 6 and two HCl-interacting species 6 in MeCN
WBIs
Natural charges
Original
Original
H–ClIJ1)
Two HCl interacting
Two HCl interacting
a
Species
V–ClIJ1)
H–ClIJ1)
V–ClIJ1)
V
Cl(1)
HCl(1)
V
Cl(1)
HCl(1)
6a-1
6a-2
6b-1
6b-2
6c-1
6c-2
0.001
0.128
0.001
0.132
0.002
0.143
0.603
0.017
0.618
0.018
0.655
0.019
0.004
0.160
0.004
0.164
0.005
0.127
0.782
0.020
0.781
0.022
0.794
0.016
0.666
0.871
0.660
0.863
0.651
0.861
−0.559
−0.895
−0.541
−0.892
−0.510
−0.884
−0.028
−0.365
−0.009
−0.369
0.024
0.565
0.878
0.563
0.876
0.556
0.865
−0.409
−0.872
−0.398
−0.867
−0.385
−0.775
0.165
−0.344
0.176
−0.340
0.190
−0.362
−0.247
a
a b c
The letters a, b and c indicate the ligands Q , Q and Q in the complexes. 6a-1 to 6c-1 represent the Cl-axially coordinated species 6; 6a-2 to
6c-2 represent their Cl-nonaxial mode. H–ClIJ1) and HCl(1) represent the HCl belonging to species 6. The WBI of free HCl here is 0.896 in
MeCN.
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on ligands (Table S12†). vi) The pendant phenoxy O–H
bond of 10 (Fig. S12(a)†) formed by the photoexcitation
of 9 is much weaker than its V O–H bond (WBIs in
they can also promote the interconversion between different
vanadium species. This photocatalysis system has the follow-
IV
IV
ing advantages: i) using more available V OQ
2
complexes as
1
0a–10c decrease by more than 21%, Tables S13-1 and
photocatalysts; ii) giving good photocatalysis activity and se-
-
2†), so this H atom should be preferentially deprived by
lectivity for KA oil, and such selectivity can be improved via
t
IV
CyOO˙ with 10 going back to the stereostructure of 7 .
Similarly, the HCl also can serve as the proton relay when
interacting with 10 to form the hydrogen bonded system
2
increasing the Cl substituents of V OQ . Having found this
system, we are interested in exploiting its potential applica-
tion in the photocatalytic oxygenation of other organics by
(Fig. S12(b), Table S14†), and increasing the amount of
2
O .
HCl and the Cl-substituents on ligands can activate two
such O–H bonds (Tables S15-1 to S16-2†), thus accelerat- Conflicts of interest
IV
ing these dehydrogenation processes. Moreover, the V –
OH bond of 10 is weakened drastically under multi-HCl
There are no conflicts to declare.
interactions (Tables S13-1 vs. S16-1†), implying that this
−
Acknowledgements
2
–OH is easily removed as H O with the Cl of the inter-
acting HCl bonding to the equatorial coordination site to
We thank for the financial aid provided by the National Natural
Science Foundation of China (21546010, 21676079), the Natural
Science Foundation of Hunan Province (2018JJ3335), and the
Hunan Provincial Innovation Foundation for Postgraduate
t
give the species 3′. vii) Particularly, the 7 could also auto-
s
matically convert to the species 7 ′ (new species found by
calculations, eqn (13)) via an ET process without an en-
ergy barrier, shown by the closed shell calculations. In
such a conversion, the coordinated O IJ1) atom flips down
b
(
CX2017B178). Also, we would like to appreciate Prof. Steven
Robert Kirk for kind advice to polish this paper and thank
Yecheng Fan from Shiyanjia Lab for support of XPS analyses
to the axial coordination site with the angle of NIJ1)–V–
NIJ2) nearly being vertical, so that the approximate plane
coordination structure converts to the stereostructure with
two ligands being perpendicular to each other (Fig. S13†).
(
www.shiyanjia.com).
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ative stability can be reversed by interaction with HCl (see
ΔG298 in Table S17†). In addition, the species in the path-
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