Nonredox metal-ions-enhanced dioxygen activation by Oxidovanadium (IV) complexes toward hydrogen atom abstraction

Article abstract of DOI:10.1021/acs.inorgchem.6b02277

Dioxygen activation toward efficient catalysis at ambient temperature is still a big challenge for industrial oxidations, while it proceeds smoothly in nature. This work presents an example of that adding nonredox metal ions as Lewis acid can enhance dioxygen activation by oxidovanadium(IV) complex, [VIV(O)Cl(TPA)]PF₆ (where TPA is tris-[(2-pyridy)methyl]amine), which leads to efficient hydrogen abstraction at ambient temperature, whereas, in the absence of a Lewis acid, the catalytic hydrogen abstraction of the oxidovanadium(IV) complex is very sluggish. Ultraviolet-visible light (UV-vis), electron paramagnetic resonance (EPR), mass, and nuclear magnetic resonance (NMR) studies have provided informative clues to indicate the interaction between the Lewis acid and vanadium complexes, including assisting the dissociation of the chloride from the oxidovanadium(IV) complex, interacting with the vanadium oxido group, and stabilizing the vanadium(V) superoxo species. These interactions enhanced the dioxgyen activation efficiency of oxidovanadium(IV) complex, and improved the hydrogen abstraction ability of vanadium(V) oxido species, which leads to efficient hydrogen abstraction in a catalytic process. A brief mechanism has also been proposed for dioxygen activation toward hydrogen abstraction by an oxidovanadium(IV) complex.

Full text of DOI:10.1021/acs.inorgchem.6b02277

Article
pubs.acs.org/IC
Nonredox Metal-Ions-Enhanced Dioxygen Activation by
Oxidovanadium(IV) Complexes toward Hydrogen Atom Abstraction
Jisheng Zhang, Hang Yang, Tingting Sun, Zhuqi Chen, and Guochuan Yin*
School of Chemistry and Chemical Engineering, Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry
of Education, and Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and
Technology, Wuhan 430074, People’s Republic of China
*
S Supporting Information
ABSTRACT: Dioxygen activation toward efficient catalysis at ambient temperature is
still a big challenge for industrial oxidations, while it proceeds smoothly in nature. This
work presents an example of that adding nonredox metal ions as Lewis acid can
IV
enhance dioxygen activation by oxidovanadium(IV) complex, [V (O)Cl(TPA)]PF6
(where TPA is tris-[(2-pyridy)methyl]amine), which leads to efficient hydrogen
abstraction at ambient temperature, whereas, in the absence of a Lewis acid, the
catalytic hydrogen abstraction of the oxidovanadium(IV) complex is very sluggish.
Ultraviolet-visible light (UV-vis), electron paramagnetic resonance (EPR), mass, and
nuclear magnetic resonance (NMR) studies have provided informative clues to indicate
the interaction between the Lewis acid and vanadium complexes, including assisting the
dissociation of the chloride from the oxidovanadium(IV) complex, interacting with the
vanadium oxido group, and stabilizing the vanadium(V) superoxo species. These
interactions enhanced the dioxgyen activation efficiency of oxidovanadium(IV)
complex, and improved the hydrogen abstraction ability of vanadium(V) oxido species, which leads to efficient hydrogen
abstraction in a catalytic process. A brief mechanism has also been proposed for dioxygen activation toward hydrogen abstraction
by an oxidovanadium(IV) complex.
•
−
INTRODUCTION
V(IV) + O ⇄ V(V) − O
(1)
■
2
2
Although the activation of dioxygen at ambient temperature
and its application in industrial oxidations are greatly attractive,
until now, most industrially aerobic oxidations are performed at
elevated temperature, which generally proceeds via a radical
chain process and leads to non- or low-selective products. In
contrast, utilization of dioxygen in nature proceeds smoothly
In the literature, vanadium(V) superoxo species was
generally proposed to be the first step in many vanadium-
based chemical transformations, such as aerobic oxidation. For
examples, in vanadium-dependent oxidation of NADP(H), it
was proposed that vanadium(IV) was autoxidized to vanadium-
(V) with superoxide radical formation, which next initiated the
free-radical chain oxidation of NAD(P)H,
(acac) initiated lipid peroxidation, oxidovanadium(IV) was
also first oxidized by dioxygen to the vanadium(V) superoxo
species via an inner sphere electron transfer. In particular,
1
−3
44,45
and is highly selectively at ambient temperature.
the mechanism of dioxygen activation by redox enzymes, many
To unveil
and in Cp V-
2
2
4
−10
11−18
synthetic models having metal oxido,
hydroxo,
1
9−24
25−28
29−32
46
hydroperoxide,
peroxo,
or superoxo
functional
groups were extensively developed, and direct synthesis of these
complexes from dioxygen had also been achieved.
̈
Kruger and Mayer independently reported that the vanadium-
3
3−37
(V) superoxo species are capable of hydrogen abstraction from
robust THF having a bond dissociation energy (BDECH) of 92
Generally, the formation of the redox metal superoxo species
is the first step in dioxygen activation. Delivering one electron
to this superoxo species can generate the peroxo species, and its
next protonation results in the hydroperoxide species which can
further go through the O−O bond cleavage to generate the
active metal oxido species. This is a typical haem enzymatic
kcal/mol, in which Kru
prepared by electrochemical oxidation of the corresponding
̈
ger’s vanadium(V) superoxo species was
31
vanadium(V)-oxido-peroxo complex, whereas, in Mayer’s
studies, the vanadium(V) superoxo species was proposed to
occur in direct reduction of dioxygen via an oxidovanadium(IV)
25
3
8,39
complex. However, the hydrogen abstraction from THF by a
vanadium(V) superoxo species is very sluggish, and the
induction period could be long, on the order of days. In
addition to the relatively weak hydrogen abstraction ability, the
sluggish hydrogen abstraction rate of the vanadium(V)
mechanism.
Alternatively, abstracting one H atom (con-
certed proton and electron transfer) from the substrate by the
metal superoxo species can also lead to the hydroperoxide
product directly, and this is the pathway of many non-haem
4
0,41
enzymes.
However, the hydrogen abstract ability of those
iron(III)-superoxo moieties in non-haem enzymes and related
synthetic models is relatively weak, which can only abstract
hydrogen from an activated C−H bond.
Received: September 21, 2016
41−43
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
IV
+
IV
+
V
+
V
+
Figure 1. Structures of (i) [V (O)Cl(TPA)] , (ii) [V (O)(OTf)(TPA)] , (iii) [V (O) (TPA)] , (iv) [V (O)(O )(TPA)] , and (v)
2
2
IV
2+
[
{V (O)(TPA)} (μ-O)] .
2
superoxo species is also possibly related to its low
concentration in solution, because of the unfavorable
system that was equipped with a UV detector and C18 column (250
mm × 4.6 mm).
General Procedure for Hydrogen Abstraction through
equilibrium in the initial binding of O to the oxidovanadium-
2
IV
Dioxygen Activation by [V (O)Cl(TPA)]PF in the Presence of
6
(IV) complex (eq 1). Accordingly, driving the chemical
IV
Al(OTf) . In a typical experiment, [V (O)Cl(TPA)]PF (1.67 mg,
3
6
equilibrium from oxidovanadium(IV) and dioxygen to the
vanadium(V) superoxo complex at ambient temperature may
have opportunities to offer efficient dioxygen activation, which
can be applied in a catalytic process. In the literature, Fukuzumi
even reported that the binding of a Lewis acid to the
0
.003 mmol), Al(OTf)3 (2.85 mg, 0.006 mmol), and 1,4-cyclo-
hexadiene (24.0 mg, 0.3 mmol) were dissolved in 3 mL of acetonitrile
in a glass tube, and then the reaction solution was magnetically stirred
at 303 K in a water bath with a dioxygen balloon for 5 h. After the
reaction, the quantitative analysis was performed by GC with the
•
−
superoxide, O2 , could make electron transfer from (TPP)
IV
internal standard method. Control experiments using [V (O)Cl-
Co to O become possible, that is, driving dioxygen activation
2
(
TPA)]PF or Al(OTf) alone were carried out in parallel, and similar
6 3
•
−
through stabilizing the O
Ca promoted dioxygen activation by their manganese(II) and
iron(II) complex to generate heterobimetallic complexes with
product; Borovik also reported
2
control experiments using commercial acetonitrile in place of dry
acetonitrile or using freshly distilled CHD. Reactions were performed
at least in duplicate, and average data were used in discussions.
2+
4
7−49
μ-O bridge.
Together with our understanding on Lewis-
General Procedure for the Hydrogen Abstraction Kinetics
50−57
IV
acid-promoted catalytic oxidations by redox catalysts,
here,
through Dioxygen Activation by [V (O)Cl(TPA)]PF in the
6
IV
we present a Lewis-acid-enhanced dioxygen activation via an
oxidovanadium(IV) complex, which leads to efficient hydrogen
atom abstraction, whereas, in the absence of Lewis acid, the
hydrogen abstraction efficiency is very low.
Presence of Al(OTf)
3
. In a typical experiment, [V (O)Cl(TPA)]PF
6
(1.67 mg, 0.003 mmol), Al(OTf)
(2.85 mg, 0.006 mmol) and 1,4-
3
cyclohexadiene (24.0 mg, 0.3 mmol) were dissolved in 3 mL of
acetonitrile in a glass tube. The reaction solution then was
magnetically stirred at 303 K in water bath with dioxygen balloon.
The product analysis was performed by GC using the internal standard
EXPERIMENTAL SECTION
■
IV
method at set intervals. Control experiments using [V (O)Cl(TPA)]-
Materials. All chemical reagents are commercially available and
used without further purification, except for acetonitrile, which was
dried by molecular sieves prior to use. 1,4-Cyclohexadiene (CHD),
cyclooctene, methyl phenyl sulfide, methyl phenyl sulfoxide, benzyl
phenyl sulfide, and triphenylphosphine were purchased from Alfa
Aesar. Sodium trifluoromethanesulfonate (NaOTf), magnesium
PF or Al(OTf) alone were carried out in parallel. Similar kinetics
6
3
V
V
were also conducted using [V (O) (TPA)]PF and [V (O)(O )-
TPA)]PF , instead of [V (O)Cl(TPA)]PF . In addition, the
2
6
2
IV
(
6
6
hydrogen abstraction kinetics was also recorded using a UV-vis
spectrometer at set intervals.
trifluoromethanesulfonate (Mg(OTf) ), and scandium trifluorometha-
General Procedure for Stoichiometric Hydrogen Abstrac-
2
V
nesulfonate (Sc(OTf) ) came from Aldrich. Other trifluoromethane-
tion by [V (O)(O
2
)(TPA)]PF
6
with Al(OTf)
3
. In a typical experiment,
3
V
sulfonates, including Zn(OTf) , Ca(OTf) , Al(OTf) , Y(OTf) , and
[V (O)(O )(TPA)]PF (10.68 mg, 0.02 mmol), Al(OTf) (18.96 mg,
2
2
3
3
2
6
3
Yb(OTf) , came from local chemical companies. Tris-[(2-pyridy)-
0.04 mmol) and 1,4-cyclohexadiene (1.6 mg, 0.02 mmol) were
dissolved in 1 mL of deaerated acetonitrile in a glass tube. The reaction
solution then was magnetically stirred at 303 K in water bath under an
argon atmosphere. The product analysis was performed by GC using
the internal standard method.
3
methyl]amine (TPA) with corresponding vanadium complexes
I V
I V
[
[
V (O)Cl(TPA)]PF6 , [{V (O)(TPA)} (μ-O)](PF ) ,
2 6 2
V
V
V (O) (TPA)]PF , and [V (O)(O )(TPA)]PF were synthesized
2
6
2
6
5
8−61
IV
according to the literature.
obtained by replacing the Cl ion of [V (O)Cl(TPA)]PF6 with
AgOTf in acetone solution (Figure 1).
[V (O)(OTf)(TPA)]PF was
6
−
IV
General Procedures for Sulfoxidation and Epoxidation
IV
Reactions by [V (O)Cl(TPA)]PF in the Presence of Al(OTf) .
6
3
Instrumentation. UV-vis spectra were collected on Analytikjena
specord 205 UV−vis spectrometer. Mass spectra were determined by a
Bruker SolariX 7.0T spectrometer. Gas chromatography−mass
spectroscopy (GC-MS) analysis was performed on an Agilent,
Model 7890A/5975C spectrometer, and NMR spectra were obtained
on a Bruker AV400 spectrometer. Electron paramagnetic resonance
IV
In a typical experiment, [V (O)Cl(TPA)]PF (1.67 mg, 0.003 mmol),
6
Al(OTf) (2.85 mg, 0.006 mmol), thioanisole, or cyclooctene (0.075
3
mmol) were dissolved in 3 mL of acetonitrile in a glass tube. The
reaction solution then was magnetically stirred at 303 K in a water bath
with a dioxygen balloon. The product analysis was performed by GC
using the internal standard method. The similar experiments by adding
1,4-cyclohexadiene (24.0 mg, 0.3 mmol) to the above reaction solution
were also conducted.
(
EPR) experiments were conducted at 298 K on a Bruker A200
spectrometer. Quantitative analysis of H O was performed using a
2
2
high-performance liquid chromatography (HPLC) (Model FL-2200)
B
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
RESULTS AND DISCUSSION
similar results (conversion, 91.6%; yield, 83.6%; in control
experiments: conversion = 6.1% and yield = 3.0%) as those
using commercial CHD as substrate (Table 1, entries 1 and
■
Lewis-Acid-Promoted Dioxygen Activation toward
Hydrogen Abstraction by the Oxidovanadium(IV)
Complex. As shown in eq 1, because of the instability of the
metal superoxo species, the low efficiency of dioxygen
activation by redox metal ions at ambient temperature (for
example, oxidovanadium(IV)) is plausibly related to the
unfavorable equilibrium between the metal complex and
dioxygen (eq 1). Therefore, driving the equilibrium from the
left to right may facilitate dioxygen activation. Fukuzumi even
found that the presence of the Lewis acid may stabilize the
superoxide species, thus promoting the dioxygen activation by
1
4), indicating the potential trace peroxide in CHD does not
affect the reaction in this system. In another control experiment
using commercial acetonitrile in place of dry acetonitrile, it also
provided results similar to those from dry acetonitrile, that is, a
conversion of 90.6% vs 92.8%, and a yield of 80.2% vs 82.5%,
indicating that the trace water does not affect the reaction
seriously if it were there in dry acetonitrile. Apparently, adding
IV
Lewis acid to [V (O)Cl(TPA)]PF can sharply improve its
6
dioxygen activation ability, leading to efficient hydrogen
abstraction from CHD, and the promotional effect is highly
dependent on the Lewis acidity, that is, a stronger Lewis acid
provided a higher efficiency.
Hydrogen abstract kinetics further confirmed the promo-
tional effect of Lewis acid to the oxidovanadium(IV) catalyst.
4
7,48
Co(TPP).
In present studies, using CHD as a substrate,
catalytic dioxygen activation toward hydrogen abstraction by
V (O)Cl(TPA)]PF complex was tested in dry acetonitrile, a
6
list of nonredox metal ions were scanned as Lewis acid to
promote dioxygen activation, and the results were summarized
in Table 1. It can be seen that, in the absence of Lewis acids, the
IV
[
As shown in Figure 2, in the absence of Al(OTf) , the
3
Table 1. Lewis-Acids-Promoted Hydrogen Abstraction from
IV
1
,4-Cyclohexadiene by [V (O)Cl(TPA)]PF under
6
a
Dioxygen
temperature
(K)
time
(h)
conversion
(%)
entry Lewis acid
1
yield (%)
313
313
313
313
313
313
313
313
313
313
303
303
303
303
303
24
24
24
24
16
16
8
30.3
10.4
2
3
4
5
6
7
8
9
NaOTf
32.2 (20.3)
62.2 (21.3)
70.1 (19.1)
71.3 (17.4)
99.2 (16.5)
99.3(12.1)
99.5(13.2)
99.6(11.1)
99.4 (11.2)
12.3
14.5 (3.2)
44.8 (3.1)
52 (3.3)
59.2 (3.0)
75.2 (2.9)
86.3(3.2)
82.3(2.8)
83.3 (3.2)
85.3 (2.8)
2.9
Mg(OTf)2
Ca(OTf)2
Ba(OTf)2
Zn(OTf)2
Y(OTf)3
Yb(OTf)3
Al(OTf)3
Sc(OTf)3
8
8
1
0
1
2
3
4
5
8
1
1
1
1
1
24
8
Figure 2. Kinetics of hydrogen abstraction from 1,4-cyclohexadiene by
Y(OTf)3
6.5
2.9
IV
[
V (O)Cl(TPA)]PF in the absence (red line) or presence (black
6
Yb(OTf)3
8
6.7
2.8
line) of Al(OTf) . Conditions: CH CN, 3 mL; 1,4-cyclohexadiene,
3
3
Al(OTf)3
5
92.8 (6.2)
93.2 (5.9)
82.5 (2.9)
IV
100 mM; [V (O)Cl(TPA)]PF , 1 mM; Al(OTf) , 2 mM, under
dioxygen balloon, 303 K. (Legend: ( ) conversion of 1,4-cyclo-
) yield of benzene.)
6
3
Sc(OTf)3
5
86.2 (3.0)
■
a
IV
hexadiene; (
●
Conditions: CH CN, 3 mL; 1,4-cyclohexadiene, 100 mM; [V (O)-
3
Cl(TPA)]PF , 1 mM; Lewis acid, 2 mM; under dioxygen balloon. The
6
yields were based on 1,4-cyclohexadiene conversion, and the data
shown in parentheses represent oxidations using Lewis acid alone in
control experiments.
oxidovanadium(IV) complex is very sluggish for hydrogen
abstraction from CHD, while in the presence of Al(OTf) , it
3
greatly accelerated the hydrogen abstraction rate, even though
it still demonstrated an induction period of ∼2 h. Figure 3
displayed the influence of Al(III)/V(IV) ratio on hydrogen
abstraction from CHD. One may see that, increasing the ratio
from 0 to 1 sharply improves the catalytic efficiency, and further
increasing the ratio to 4 just slightly improves the efficiency,
implicating an 1:1 ratio of interaction between Al(III) and
V(IV) in catalysis. Similar promotional effects were also
[
VIV(O)Cl(TPA)]PF6 complex alone is very sluggish in
dioxygen activation, leading to only 30.3% conversion of
CHD with a 10.4% yield of benzene after 24 h at 313 K. Adding
Lewis acids can sharply improve the hydrogen abstraction,
leading to a high yield of benzene. For example, adding 2 equiv
of Y(OTf) leads to 99.3% conversion with 86.3% yield after 8
3
IV
h at 313 K, and 2 equiv of Zn(OTf) lead to 99.2% conversion
observed with other vanadium complexes including [V (O)-
2
V
V
with 75.2% yield after 16 h at 313 K. In particular, adding
Al(OTf) or Sc(OTf) can provide 92.8% or 93.2% conversion
(OTf)(TPA)]PF , [V (O) TPA]PF , [V (O)(O )TPA]PF ,
6 2 6 2 6
IV
and [{V (O)(TPA)} (μ-O)](PF ) (Table 2), in which
3
3
2
6 2
V
with 82.5% or 86.2% yield of benzene, respectively, within 5 h
at 303 K. In control experiments, Lewis acid alone has no
catalytic activity, and the minor benzene detected in the
reaction mixture is due to the impurity of the commercial
CHD, which naturally contains ∼3% benzene. Sometimes,
commercial CHD may contain a trace amount of peroxide,
which may affect the oxidation reaction; here, the use of freshly
distilled CHD as the substrate was also tested, which provided
[V (O) (TPA)]PF demonstrated a very similar induction
2 6
IV
V
period to that of [V (O)Cl(TPA)]PF , while [V (O)(O )-
6
2
TPA]PF has an induction period as long as 10 h (Figure S1(b)
6
in the Supporting Information). However, in the absence of
Al(OTf) , all of these vanadium complexes demonstrated a very
3
sluggish catalytic efficiency in hydrogen abstraction with
dioxygen. Importantly, a comparable long induction period of
V
IV
[V (O) TPA]PF and [V (O)Cl(TPA)]PF complexes with
2
6
6
C
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
IV
one new peak at 506.04231, corresponding to the [V (O)-
+
(
OTf)(TPA)] species (Figure 4b), indicating the dissociation
of the chloride from the coordination sphere of oxidovanadium-
IV) complex. In UV-vis studies, adding Al(OTf) also caused
(
3
IV
the absorbance change of [V (O)Cl(TPA)]PF in acetonitrile.
As the Al(OTf) /[V (O)Cl(TPA)] ratio increased from 0 to
6
IV
+
3
2
, the absorbance at ∼731 nm decreased and shifted to 690 nm
gradually; meanwhile, the absorbance at ∼574 and 363 nm also
decreased (Figure 5a). However, the spectrum generated in situ
by adding Al3+ to [V (O)Cl(TPA)]PF is still different from
IV
6
IV
that of authentic [V (O)(OTf)(TPA)]PF , whose absorption
also changed in the presence of Al (Figure 5b), implicating
that Al not only assists the removal of chloride from
6
3+
3+
IV
+
[V (O)Cl(TPA)] species, but also has interaction with the
IV
+
resulting [V (O)(OTf)(TPA)] species. The final absorbance
at ca. 350 nm is very different in Figures 5a and 5b; this is
possibly, but not conclusively, related to their different ion
components in the solution. For example, in Figure 5a, there
exists Al(III)-bound chloride, which may interact with the
vanadium(IV) species, whereas in Figure 5b, no chloride exists.
For the C4v symmetric oxidovanadium(IV) complexes, it is
Figure 3. Influence of Al(III)/V(IV) ratio on hydrogen abstraction
from 1,4-cyclohexadiene. Conditions: CH CN, 3 mL; CHD, 100 mM;
3
IV
[
V (O)Cl(TPA)]PF , 1 mM; under dioxygen balloon, 303 K, 5 h.
6
Table 2. Al(OTf) -Promoted Hydrogen Abstraction from
3
a
generally regarded that the unpaired electron is in the d
1
,4-Cyclohexadiene by Other Vanadium Complexes
xy
2
2
2
orbital, and its transition to [d , d ] (ν ), d
(ν ), and dz
xz
yz
1
x −y
2
entry
vanadium source
conversion (%)
yield (%)
62,63
(
ν ) happens with increasing energies.
In the case of the
3
IV
IV
+
1
2
3
4
[V O(OTf)(TPA)]PF
95.2 (7.2)
95.0 (6.5)
94.6 (12.5)
92.2 (6.4)
85.1 (2.9)
88.1 (2.8)
83.5 (2.9)
84.1 (2.9)
6
[V (O)Cl(TPA)] species, the weak visible absorptions at
V
[V (O) (TPA)]PF
2
6
∼731 and 574 nm can be respectively assigned to the ν and ν
1
2
b
c
V
[V (O)(O )(TPA)]PF
2
6
bands, and the ν band has a low intensity at ∼363 nm, which
3
IV
[{V (O)(TPA)} (μ-O)](PF )
2
6
2
has been obscured by the intensive charge transfer transition
occurring at the same region (Figure 5a). In the literature, it
was reported that the O2 group competes for the same vacant
a
Conditions: CH CN, 3 mL; 1,4-cyclohexadiene, 100 mM; vanadium
3
−
complex, 1 mM; Al(OTf) , 2 mM; under dioxygen balloon, 303 K, 5 h.
3
−
IV
+
The data given in parentheses represent the use of vanadium
dxz V orbital with the Cl group in the [V (O)Cl(TPA)]
species.
b
complexes alone as control experiments. The reaction time is 24 h.
64,65
Accordingly, the UV-vis change at ∼731 nm is
likely to be related to the interaction of Al with both O and
c
IV
[
{V (O)(TPA)} (μ-O)](PF ) , 0.5 mM.
3+ 2−
2
6 2
−
̈
Cl groups. In investigating the interaction of a Bronsted acid,
V
IV
much longer induction period of [V (O)(O )TPA]PF clearly
such as p-toluenesulfonic acid and HClO , with the V O
2
6
4
V
IV
supports that the formation of the vanadium(V) oxido, V O,
group in the [V (O)Cl(TPA)] complex, Sasaki even reported
V
or vanadium(V) peroxo, V −(O ) functional group is not the
that an excess of n-Bu NCl is essential to avoid the dissociation
2
4
IV
+
origin of the induction period in oxidovanadium(IV)-catalyzed
hydrogen abstraction.
of the chloride from the [V (O)Cl(TPA)] species. Here,
3+
adding strong Lewis acids such as Al causes dissociation of the
3+
IV
+
Interaction between Al and Vanadium Complexes.
The interaction of Al with the [V (O)Cl(TPA)]PF complex
chloride from the [V (O)Cl(TPA)] species; meanwhile, it
3
+
IV
IV
6
may also interact with the V O group, as well as the proton
59
was first evidenced by mass spectra studies. As shown in Figure
(Bro
̈
nsted acid) disclosed by Sasaki. Similar interaction of
IV
3+
4
a, [V (O)Cl(TPA)]PF in acetonitrile revealed one major
Al with vanadium(V) oxido group was also indicated for the
6
IV
mass peak at 392.05912, corresponding to the [V (O)Cl-
dioxidovanadium(V) complexes (vide infra).
The interaction between [V (O)Cl(TPA)] species and Al
+
IV
+
3+
(
TPA)] species. Adding Al(OTf) to this solution disclosed
3
was also indicated in EPR studies. For the paramagnetic
mononuclear oxidovanadium(IV) complex, it has a character-
istic eight-line signal for the oxidovanadium(IV) species in the
EPR spectrum, and increasing the density of the unpaired
electron on the oxidovanadium(IV) center would lead to an
6
6,67
increased A value.
In Figure 6, curve (a) displays the EPR
0
IV
signal of [V (O)Cl(TPA)]PF6 in acetonitrile at 298 K,
revealing its characteristic eight-line signal with the g value
0
−4
of 1.984 and a hyperfine coupling constant of A = 89.6 × 10
0
−1
cm . Adding 1 equiv of Al(OTf) to the above solution leads
3
to the formation of a new oxidovanadium(IV) species with the
decay of the original one, while adding 2 equiv of Al(OTf)3
caused complete transformation of the original oxidovanadium-
(
IV) species to the new one having the g value of 1.986 with
0
−4
−1
A0 = 99.8 × 10 cm . The increased A value was likely the
result of the interaction of the [V (O)Cl(TPA)] species with
Al .
0
IV
+
3+ 68,69
IV
Figure 4. Mass spectra of (a) [V (O)Cl(TPA)]PF (1 mM) and (b)
Similar EPR signal changes were also observed by
6
IV
IV
[
V (O)Cl(TPA)]PF (1 mM) and Al(OTf) (2 mM) in acetonitrile.
adding Al(OTf)3 to [V (O)(OTf)(TPA)]PF , in which
6
3
6
D
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
IV
IV
Figure 5. UV-vis absorption change by adding Al(OTf) to (a) [V (O)Cl(TPA)]PF and (b) [V (O)(OTf)(TPA)]PF , using 3 mL of CH CN at
3
6
6
3
IV
2
98 K. Conditions for panel (a): [V (O)Cl(TPA)]PF = 1 mM, Al(OTf)3 = 0, 0.25, 0.5, 1, and 2 mM; conditions for panel (b):
6
IV
[
V (O)(OTf)(TPA)]PF = 1 mM, Al(OTf) = 0 and 1 mM.
6
3
another test, adding 3 equiv of HOTf as a Bro
̈
nsted acid to the
V
[
V (O) (TPA)]PF solution also leads to the formation of
2
6
precipitate, but it is much slower and much less than that of
adding Al(OTf) . In addition, the interaction was further
3
3+
confirmed by the Al -accelerated reduction of vanadium(V)
through hydrogen abstraction from CHD. As shown in Figure
7
, in the absence of Al(OTf) , there is no significant
3
IV
Figure 6. EPR spectra of [V (O)Cl(TPA)]PF with Al(OTf)3.
6
IV
Conditions: 3 mL of CH CN, 298 K. [V (O)Cl(TPA)]PF , 1 mM,
3
6
and Al(OTf) content = (a) 0, (b) 1 mM, and (c) 2 mM. A values:
3
0
−4
−1
−4
−1
−4
(
a) 89.6 × 10 cm , (b) 95.8 × 10 cm , and (c) 99.8 × 10
1
−
cm .
IV
[
V (O)(OTf)(TPA)]PF alone shows the g value of 1.984
6
0
−4
−1
3+
with A = 92.3 × 10 cm , while adding 1 equiv of Al
0
−4
−1
V
changed it to a g value of 1.984 with A = 95.8 × 10 cm
Figure 7. EPR spectra of [V (O) (TPA)]PF reacting with 1,4-
0
0
2
6
(
Figure S2 in the Supporting Information). The final EPR
cyclohexadiene in the absence (black) and presence (red) of Al(OTf) .
3
V
Conditions: CH CN, 3 mL; CHD, 100 mM; [V (O) (TPA)]PF , 1
spectra differences between the one observed by adding 2 equiv
3
2
6
IV
+
mM; Al(OTf) , 2 mM; 303 K, 10 min.
of Al(OTf) to the [V (O)Cl(TPA)] species (Figure 6) and
3
3
IV
that by adding 1 equiv of Al(OTf) to the [V (O)(OTf)-
3
(
TPA)] species (Figure S2) are possibly related with their
vanadium(IV) signal observed by EPR detection after mixing
V
different ion components in the solution, which causes the
coordination environment difference of the vanadium(IV)
species, as well as those in UV-vis studies (vide supra).
[V (O) (TPA)]PF with CHD in acetonitrile at 303 K. In the
2
6
presence of Al(OTf) , the clean vanadium(IV) signals occur in
3
3+
10 min, indicating the Al -promoted hydrogen abstraction by
IV
+
3+
V
Similar to the [V (O)Cl(TPA)] species, Al may also
the V O moiety. The two generated vanadium(IV) species
V
+
V
IV
+
interact with the [V (O) (TPA)] species via the V O
can be tentatively assigned to the [V (O)(OTf)(TPA)]
2
functional group, and in the literature, it was reported that
species and its interaction with the Al(OTf) , as well as that
3
adding a Bro
̈
nsted acid leads to the formation of the
in Figure S2. In the literature, similar enhanced hydrogen atom
abstraction ability of the active metal oxido moieties from C−H
bond was even observed by Lau, in which adding a Lewis acid
V
V (O)(OH) species in certain dioxidovanadium(V) com-
7
0,71
plexes.
Here, as shown in Figure S3(a) in the Supporting
V
72,73
Information, the UV-vis spectrum of [V (O) (TPA)]PF
changed gradually by adding 2 equiv of Al(OTf) , and an
clearly accelerated cyclohexane oxidation by permanganate.
2
6
V
For the [V (O)(O )(TPA)]PF complex, it was reported
3
2
6
orange precipitate was generated after 1 h, indicating the direct
interaction between the dioxidovanadium(V) complex with
Al in acetonitrile. This precipitate is not the vanadium(V)
oxide, because it is soluble in dimethylsulfoxide (DMSO) and
dimethyl formamide (DMF), and no free TPA ligand is
detected via thin layer chromatography (TLC) analysis. In
that the presence of Bro
the peroxo group, and the proton tending to activate the
peroxo group in the vanadium(V) center was also proposed in
many vanadium haloperoxidase (VHPO) models.
although the UV-vis absorption of [V (O)(O )(TPA)]PF in
̈
nsted acid leads to the protonation on
61
3
+
74,75
Here,
V
2
6
CH CN just changed slightly by adding Al(OTf) (Figure
3
3
E
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
IV
Figure 8. UV-vis changes of [V (O)Cl(TPA)]PF with Al(OTf) in the presence of 1,4-cyclohexadiene under dioxygen. Conditions: CH CN, 3
6
3
3
IV
mL; CHD, 100 mM; [V (O)Cl(TPA)]PF , 1 mM; Al(OTf) , 2 mM; under a dioxygen balloon, 303 K.
6
3
S3(b)), the interaction was clearly indicated by 27Al NMR
studies. As shown in Figure S5 in the Supporting Information,
(ε = 25 M cm ) and 731 nm (ε = 45 M cm ), while the
−1
−1
−1
−1
V
+
[V (O)(O )(TPA)] species shows characteristic absorption at
2
−1
−1
Al(OTf) alone displayed a major chemical shift at ∼0.5 ppm,
447 nm (ε = 287 M cm ) assigned to the peroxo-to-
vanadium charge transfer transition. Here, in UV-vis kinetic
studies (Figure 8), in the presence of Al(OTf) , the absorption
3
3
+
59
indicating that Al has an octahedral coordination environ-
5
6,76,77
V
ment.
By adding different amounts of [V (O)(O )-
2
3
(
TPA)]PF , it changed to a broadened resonance gradually,
at 447 nm gradually increased in the acetonitrile solution
6
V
+
IV
+
clearly indicating the interaction between [V (O)(O )(TPA)]
species and Al . This interaction was further supported by
containing both the [V (O)Cl(TPA)] species and CHD
(UV-vis data were collected from the reaction mixture under a
dioxygen balloon at set intervals), whereas it remains its original
2
3+
3
+
Al -enhanced stoichiometric hydrogen abstraction from CHD
Table S1 in the Supporting Information). In the absence of
IV
+
(
absorbance of [V (O)Cl(TPA)] species in the absence of
Al(OTf) . This transformation was also verified by mass studies
(Figure 9). In the absence of Al(OTf)
V
+
Al(OTf) , the [V (O)(O )(TPA)] species alone is incapable
of hydrogen abstraction from CHD under argon at 303 K,
3
3
2
IV
+
, [V (O)Cl(TPA)]
3
while adding Al(OTf) provided 31.0% yield of benzene within
3
2
4 h. In addition, this observation also supports that the
V
V (O ) functional group is very sluggish in hydrogen
2
abstraction, even though it was proposed as the active species
78,79
in VHPO and many other oxidation reactions.
In the complementary experiments with other Lewis acids, it
was found that adding Sc(OTf) caused similar dissociation of
3
IV
the chloride from the coordination sphere of the [V (O)Cl-
TPA)] species, which was evidenced by similar UV-vis and
+
(
EPR changes to those of adding Al(OTf) ; however, adding
3
Y(OTf) or others did not cause the dissociation of the chloride
3
at ambient temperature (303 K) (Figure S6 in the Supporting
Information). These interactions between different Lewis acids
and oxidovanadium(IV) complex are remarkably linked with
their promotional effects in dioxygen activation toward
Figure 9. Mass spectra of hydrogen abstraction from 1,4-cyclo-
IV
hexadiene by [V (O)Cl(TPA)]PF6 with Al(OTf) . Conditions:
3
IV
CH CN, 3 mL; CHD, 100 mM; [V (O)Cl(TPA)]PF , 1 mM;
3
6
Al(OTf) , 2 mM; under a dioxygen balloon, 303 K, 5 h.
3
3
+
3+
hydrogen abstraction (Table 1). That is, adding Al and Sc
IV
can efficiently promote hydrogen abstraction with the [V (O)-
Cl(TPA)]PF catalyst at 303 K; for other Lewis acids, higher
species shows its mass peak at 392.05912, and it does not
change after long-standing; in the presence of Al3+ and CHD,
this mass peak changed to 389.08087, corresponding to the
6
temperature with longer reaction time is needed to achieve
similar activity, implying that the dissociation of the chloride
may facilitate the attack of dioxygen on the oxidovanadium(IV)
V
+
V
+
[V (O)(O )(TPA)] species. Since the [V (O) (TPA)]
2
2
59
center to initiate dioxygen activation. As evidence, when
species has no absorption above 400 nm, the final absorption
at 447 nm indicated a crude 92% yield of [V (O)(O )(TPA)]
species formation in the solution, clearly indicating the Al -
promoted dioxygen activation via the oxidovanadium(IV)
complex. Note that, in the absence of CHD, no such
IV
V
+
[
V (O)Cl(TPA)]PF and Al(OTf) were premixed for 1 h in
6
3
2
3
+
advance, the hydrogen abstraction from CHD demonstrated a
relatively faster kinetic behavior, as shown in Figure S7 in the
Supporting Information.
IV
+
Roles of Lewis Acid in Dioxygen Activation by the
Oxidovanadium(IV) Complex. It has been reported that
oxidovanadium(IV) complexes can be oxidized by dioxygen to
vanadium(V)-oxido-peroxo species in the presence of THF or
transformation is observed for the[V (O)Cl(TPA)] complex,
if even Al(OTf) was added, as shown in Figure 5 (vide supra).
3
In dioxygen activation, the primary step is the formation of
the metal superoxo species, which further proceeds by electron
transfer to generate the metal peroxo species or proceeds by
hydrogen atom abstraction to generate the metal hydroperoxide
2
5,31,59,80,81
other ethereal solvents.
Here, similar conversion was
IV
also observed for the [V (O)Cl(TPA)]PF complex in the
6
3
8−41
presence of Al(OTf) and CHD in acetonitrile. As reported, the
species.
As shown in Figure 8, in the presence of CHD,
3
IV
+
IV
[
V (O)Cl(TPA)] species has very weak absorption at 574 nm
adding Al(OTf) to the acetonitrile solution of [V (O)Cl-
3
F
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Proposed Mechanism for Lewis Acid Enhanced Dioxygen Activation by Oxidovanadium(IV) Complex Towards
Hydrogen Abstraction from 1,4-Cyclohexadiene
V
of Al3+ with the vanadium(V) superoxo species may also drive
the equilibrium to the right side in eq 1, to a certain degree,
thus promoting dioxygen activation and leading to an efficient
hydrogen abstraction.
(
(
TPA)]PF complex leads to the formation of [V (O)(OOH)-
6
TPA)]2 species having absorbance at ∼550 nm, which was
+
81
V
+
gradually converted to the [V (O)(O )(TPA)] having
2
absorbance at 447 nm. In addition, the formation of
V (O)(O )(TPA)] species has also been evidenced by
V
+
[
Oxidative Reactivity of the Vanadium(V) Superoxo
2
3+
mass studies (Figure 9, vide supra). The formation of the
Species. As disclosed above, the presence of Al can promote
V
+
V
IV
[
(
V (O)(O )(TPA)] species, as well as the [V (O)(OOH)-
dioxygen activation via the [V (O)Cl(TPA)]PF complex,
2
6
2
+
TPA)] , species may release H O in the hydrogen
As evidence, along with the formation
of benzene and the [V (O)(O )(TPA)] species, the
leading to efficient hydrogen abstraction with the stoichio-
2
2
8
2,83
V
+
abstraction process.
metric formation of [V (O)(O )(TPA)] species (up to 92%
2
V
+
V
+
yield). Similar formation of the [V (O)(O )(TPA)] species
2
2
accumulation of H O , up to 40 mM, was also observed by
HPLC analysis during the hydrogen abstraction from CHD
could also be observed by adding 9,10-dihydroanthracene in
place of CHD; however, it is a much more sluggish process (see
Figure S9 in the Supporting Information, versus Figure 8).
2
2
(Figure S8 in the Supporting Information).
Although it is generally believed that formation of the metal
̈
Kruger and Mayer even reported that the vanadium(V)
superoxo species is the first step in dioxygen activation, here,
the attempts to directly observe the vanadium(V) superoxo
species were not successful by UV-vis, EPR, and mass studies,
superoxo species is also capable of hydrogen abstraction from
tetrahydrofuran (THF), which has a BDECH value of 92 kcal/
mol; however, the induction period could be long (on the order
25,31
even in the presence of Al(OTf) , suggesting that the lifetime of
of days).
These findings clearly support that the vanadium-
3
the superoxo species is very short, because the equilibrium is
very unfavorable for dioxygen activation, as shown in eq 1, and/
or it exhibits a rapid transformation to other products.
Alternatively, the superoxo species was indicated by chemical
scavenger tests in this work. In the literature, benzoquinone is a
popular scavenger for the superoxo species, or, for example,
(V) superoxide species is capable of hydrogen abstraction, and
the presence of a Lewis acid such as Al3+ can accelerate
dioxygen activation, leading to an efficient hydrogen abstraction
in a catalytic process.
However, in the case of using sulfide or olefin in place of
CHD, no oxygenation products were observed, which
suggested that the in situ-generated vanadium(V) superoxo
species is incapable of oxygenation to sulfide or olefin, even in
8
4−87
superoxide, in wastewater treatment.
Here, adding
benzoquinone to the acetonitrile solution containing CHD,
IV
Al(OTf) , and [V (O)Cl(TPA)]PF would completely quench
the presence of Al(OTf) . However, in the presence of both
3
6
3
V
the hydrogen abstraction reaction, and there is no [V (O)-
CHD and sulfides such as thioanisole, diphenyl sulfide, and
benzyl phenyl sulfide, adding Al(OTf)3 can promote the
formation of both benzene and sulfide oxidation products, but
the hydrogen abstraction efficiency is much poorer than that in
the absence of sulfide. For example, under the conditions of 1
(
OOH)(TPA)]2 or [V (O)(O )(TPA)] species formation
+
V
+
2
3+
observed by UV-vis studies, clearly supporting that, in Al -
promoted dioxygen activation by an oxidovanadium(IV)
complex, the superoxo species occurs in the solution and
plays the key role in hydrogen atom abstraction.
IV
mM [V (O)Cl(TPA)]PF , 2 mM Al(OTf) , and 100 mM
6
3
IV
In the control experiments shown in Table 2, [V (O)-
CHD with 25 mM thioanisole, after stirring at 303 K for 24 h,
only 42.2% conversion of CHD was achieved with a 37.2%
yield of benzene formation (comparing with 92.8% conversion
with a 82.5% yield in 5 h at 303 K, see Table 1, entry 14);
meanwhile, 80% of the sulfide was converted with a 62% yield
of sulfide oxide and 6% yield of sulfone formation. In the
(
OTf)(TPA)]PF alone is inactive for hydrogen abstraction
6
from CHD through dioxygen activation, whereas adding
IV
Al(OTf) to [V (O)(OTf)(TPA)]PF provided 95.2% con-
3
6
version with 85.1% yield of benzene, indicating the crucial role
of Al3 in dioxygen activation, not only for the removal of the
chloride from the oxidovanadium(IV) complex to generate a
vacation site. In the literature, Fukuzumi even reported that the
binding of Lewis acids to the superoxide species can make
+
V
control experiments, [V (O) (TPA)]PF is incapable of sulfide
2
6
V
oxygenation, and [V (O)(O )(TPA)]PF can only lead to trace
2
6
oxygenation product, even in the presence of Al(OTf) . These
3
4
7,48
electron transfer from (TPP)Co to O become possible.
results suggest that none of the vanadium(V) superoxo, oxido,
and peroxo species are efficient for sulfide oxygenation, while
the vanadium(V) hydroperoxide species, which was generated
from hydrogen abstraction from CHD by the vanadium(V)
superoxo species, is responsible for sulfide oxygenation in this
2
3
+
Alder also reported that Al played the important roles in the
vanadium-mediated NADH oxidation under dioxygen, and it
was suggested that the addition of Al enhances the
3
+
88
production of the superoxide radical. Here, the interaction
G
DOI: 10.1021/acs.inorgchem.6b02277
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
system. However, this vanadium(V) hydroperoxide species is
incapable of epoxidizing olefin to the corresponding epoxide,
even though similar hydroperoxide intermediates from
titanium, manganese, and iron have been reported for efficient
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02277.
■
*
S
8
9−91
olefin epoxidation.
Mechanism of Lewis-Acid-Promoted Dioxygen Acti-
vation by Oxidovanadium(IV) Complex toward Hydro-
gen Abstraction. Regarding the facts disclosed above,
Complementary data for hydrogen abstraction, EPR
tests, UV−vis studies, and H O2 formation analysis
(PDF)
2
3
+
including (1) the presence of Al can promote the dissociation
IV
+
of the chloride from [V (O)Cl(TPA)] species, and mean-
IV
while interacts with the V = O functional group, (2) the
AUTHOR INFORMATION
presence of Al3 can promote dioxygen activation by [V (O)-
+
IV
■
*
Corresponding Author
E-mail: gyin@hust.edu.cn.
Cl(TPA)]PF , leading to efficient hydrogen abstraction from
6
CHD, and (3) in addition to the formation of benzene as
hydrogen abstraction product, up to 92% yield of the
ORCID
V
+
[
V (O)(O )(TPA)] species were generated by UV-vis analysis
2
Guochuan Yin: 0000-0003-1003-8478
Notes
The authors declare no competing financial interest.
with large amount of H O (40 mM) release in the reaction
2
2
3
+
solution, a brief Al -enhanced dioxygen activation via the
oxidovanadium(IV) complex toward hydrogen abstraction was
proposed as depicted in Scheme 1. In this mechanism, the
conversion of vanadium(V)-oxido-hydroperoxide species can
proceed via three pathways, including the release of H O to
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21273086 and 21573082), and
Natural Science Foundation of Hubei Scientific Committee
(No. 2016CFA001). The mass studies were performed in the
Analytical and Testing Center of Huazhong University of
Science and Technology.
2
2
generate vanadium(V)-oxido-hydroxide species, the formation
of vanadium(V)-oxido-peroxo product by deprotonation, and
IV
via homolytic cleavage to generate the V O and HOO•,
which leads to a radical process in hydrogen abstraction, as
92−95
reported in the literature.
Both H O and vanadium(V)-
2 2
oxido-peroxo product have been detected by UV-vis and HPLC
analysis. According to Scheme 1, the formation of H O would
be equivalent to that of benzene formation, that is, under the
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