A combined experimental and DFT study of active structures and self-cycle mechanisms of mononuclear tungsten peroxo complexes in oxidation reactions

Article abstract of DOI:10.1016/j.molstruc.2011.02.023

Tungsten peroxo complexes have been widely used in olefin epoxidation, alcohol oxidation, Baeyer-Villiger oxidation and other oxidation reactions, however, there is still not a unanimous viewpoint for the active structure of mononuclear tungsten peroxo complex by now. In this paper, the catalysis of mononuclear tungsten peroxo complexes 0-5 with or without acidic ligands for the green oxidation of cyclohexene to adipic acid in the absence of organic solvent and phase-transfer catalyst has been researched in experiment. Then we have suggested two possible kinds of active structures of mononuclear tungsten peroxo complexes including peroxo ring (nA, n = 0-1) and hydroperoxo (nB, n = 0-1) structures, which have been investigated using density functional theory (DFT). Moreover, the calculations on self-cycle mechanisms involving the two types of active structures of tungsten peroxo complexes with and without oxalic acid ligand have also been carried out at the B3LYP/[LANL2DZ/6-31G(d, p)] level. The highest energy barrier are 26.17 kcal/mol (0A, peroxo ring structure without oxalic acid ligand), 23.91 kcal/mol (1A, peroxo ring structure with oxalic acid ligand), 18.19 kcal/mol (0B, hydroperoxo structure without oxalic acid ligand) and 13.10 kcal/mol (1B, hydroperoxo structure with oxalic acid ligand) in the four potential energy profiles, respectively. The results indicate that both the energy barriers of active structure self-cycle processes with oxalic acid ligands are lower than those without oxalic acid ligands, so the active structures with oxalic acid ligands should be easier to recycle, which is in good agreement with our experimental results. However, due to the higher energy of product than that of the reactant, the energy profile of the self-cycle process of 1B shows that the recycle of 1B could not occur at all in theory. Moreover, the crystal data of peroxo ring structure with oxalic acid ligand could be found in some experimental references. Thus, the viewpoint that the peroxo ring active structure should be the real active structure has been proved in this paper.

Full text of DOI:10.1016/j.molstruc.2011.02.023

Journal of Molecular Structure 992 (2011) 19–26
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
A combined experimental and DFT study of active structures and self-cycle
mechanisms of mononuclear tungsten peroxo complexes in oxidation reactions
a,
Peng Jin a, Donghui Wei a, Yiqiang Wen a, Mengfei Luo b, Xiangyu Wang a, , Mingsheng Tang
⇑
⇑
a Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, China
b Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Tungsten peroxo complexes have been widely used in olefin epoxidation, alcohol oxidation, Baeyer–
Villiger oxidation and other oxidation reactions, however, there is still not a unanimous viewpoint for
the active structure of mononuclear tungsten peroxo complex by now. In this paper, the catalysis of
mononuclear tungsten peroxo complexes 0–5 with or without acidic ligands for the green oxidation of
cyclohexene to adipic acid in the absence of organic solvent and phase-transfer catalyst has been
researched in experiment. Then we have suggested two possible kinds of active structures of mononu-
clear tungsten peroxo complexes including peroxo ring (nA, n = 0–1) and hydroperoxo (nB, n = 0–1) struc-
tures, which have been investigated using density functional theory (DFT). Moreover, the calculations on
self-cycle mechanisms involving the two types of active structures of tungsten peroxo complexes with
and without oxalic acid ligand have also been carried out at the B3LYP/[LANL2DZ/6-31G(d, p)] level.
The highest energy barrier are 26.17 kcal/mol (0A, peroxo ring structure without oxalic acid ligand),
23.91 kcal/mol (1A, peroxo ring structure with oxalic acid ligand), 18.19 kcal/mol (0B, hydroperoxo struc-
ture without oxalic acid ligand) and 13.10 kcal/mol (1B, hydroperoxo structure with oxalic acid ligand) in
the four potential energy profiles, respectively. The results indicate that both the energy barriers of active
structure self-cycle processes with oxalic acid ligands are lower than those without oxalic acid ligands, so
the active structures with oxalic acid ligands should be easier to recycle, which is in good agreement with
our experimental results. However, due to the higher energy of product than that of the reactant, the
energy profile of the self-cycle process of 1B shows that the recycle of 1B could not occur at all in theory.
Moreover, the crystal data of peroxo ring structure with oxalic acid ligand could be found in some exper-
imental references. Thus, the viewpoint that the peroxo ring active structure should be the real active
structure has been proved in this paper.
Received 22 December 2010
Accepted 10 February 2011
Available online 15 February 2011
Keywords:
Active structure
Tungsten peroxo complex
Density functional theory (DFT)
Hydrogen peroxide
Adipic acid
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
commonly prepared in industrial processes by oxidizing cyclohex-
anol or cyclohexanone using nitric acid. This reaction results in the
The peroxotungstates and peroxomolybates have been widely
used as high efficient catalysts for the oxidation of alcohols, sul-
fides and olefins [1–11], especially in the green oxidation reaction
with hydrogen peroxide as oxidant. Hydrogen peroxide has high
oxidation content and the only byproduct is water, so catalytic
oxidations of organic substrates by H2O2 have been of growing
interest [12–14]. Among these, the green syntheses of adipic acid
with H2O2 as oxidant in the presence of tungsten species (tungsten
peroxide species in particular) as catalysts are more attractive and
have been successfully demonstrated.
formation of environmentally unfriendly nitrous oxide (N2O) [16].
Thus, green routes to a direct synthesis of adipic acid are very
desirable [17–24]. In recent years, the one-pot synthesis of adipic
acid using H2O2 via homogeneous catalysis has been studied
(Scheme 1), which can avoid the formation of N2O [25]. According
to the proposed pathway (Scheme 1), it should be stepwise mech-
anism in the reaction for the direct oxidation of cyclohexene to adi-
pic acid with aqueous H2O2. From then on, many researchers have
made a series of work continuously to promote the development of
this method [30–34]. It has been well known that organic solvents
using as improving agents, such as t-BuOOH, are usually intro-
duced to increase reaction rates [26,27], and tungstates have re-
ceived extensive attention for reactions of oxidative cleavage of
alkenes in organic solvents [28,29]. However, organic solvents
are also harmful to the environment and difficult to be disposed
of. More recently, simple tungsten compounds [30–33] as catalysts
Adipic acid is an important organic biacid which is used in the
manufactures of nylon 6,6 and many other products [15]. It is
⇑
Corresponding authors. Tel.: +86 371 67766076; fax: +86 371 67762210.
E-mail addresses: xiangyuwang@zzu.edu.cn (X. Wang), mstang@zzu.edu.cn
(M. Tang).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.023

20
P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
(n = 0, Scheme 2) have been suggested and investigated by DFT
Na2WO4
orgainc solvent-and-halid-free
theory in this article, which has been widely used in explaining
many chemical problems [36–40]. Furthermore, in order to explain
the activities and stabilities of these peroxo complexes, the self-cy-
cle mechanisms of two active structures have also been investi-
gated using the DFT theory.
COOH
COOH
4H2O2
4H2O
75-90oC
[CH3(n-C8H17)3N]HSO4
The reported reaction of green synthesis of adipic acid
OH
O
[o]
olefin
[o]
H2O
2. Experimental and computational details
O
alcohol
oxidation
OH
O
OH
epoxidation
2.1. Reaction of green oxidation of cyclohexene to adipic acid
O
[o]
[o]
H2O
COOH
COOH
Reagents were purchased from Sinopharm Chemical Reagent
Co., Ltd. Under the condition of organic solvent- and phase-transfer
catalyst-free, the green synthesis of adipic acid via direct oxidation
of cyclohexene by 30% H2O2 in the presence of tungsten peroxo
complexes as catalysts have been investigated. Accurately mea-
sured the WO3ÁH2O, ligand (with or without) and H2O2 (30%) (the
molar ratio is 1:1.1:220) were introduced in a 500 mL round-
bottomed flask. The mixture was stirred 30 min for in situ prepara-
tion of catalyst and then cyclohexene (the molar ratio to H2O2 is
1:4.4) was added and the system was heated to keep reflux. After
reflux, the system was stirred 6 h at 80–90 °C. When the reaction
was over, the resulting hot solution was used for determination
of the adipic acid yield by high performance liquid chromatogra-
phy (HPLC).
O
O
Baeyer-Villiger
oxidation
alcohol
oxidation
OH
O
The proposed reaction pathway
Scheme 1. The reported reaction for the direct oxidation of cyclohexene to adipic
acid with aqueous H2O2 and proposed pathway [25].
have been used to synthesize adipic acid with H2O2 as oxidant with
the addition of quaternary ammonium compounds or acidic
compounds as co-catalysts under organic solvents free conditions.
Generally, acidic compounds are suggested as ligands with coordi-
nation effect on the stability and activity of tungsten compounds.
In addition, Usui and Deng et al. [31,34] suggested that the
oxidation actually occurred by the tungsten peroxide species, the
tungsten-containing anion was converted to peroxotungstates in
the course of the reaction. Thus, the tungsten peroxide species
are essential to oxidation reactions.
To the best of our knowledge, there is still not a unanimous
viewpoint for the active structure of mononuclear tungsten peroxo
complex by now. On the one hand, Rösch et al. calculated that the
tungsten peroxo complexes with bisperoxo groups instead of
monoperoxo ones had higher activities for olefin epoxidation reac-
tion using DFT/B3LYP method [35]. They thought W (O2) three-
membered ring formed the active group of catalyst. On the other
hand, Mares et al. proposed that the WAOAOAH group would be
the active group in alcohol oxidation reaction [1], which could effi-
ciently transfer active oxygen like peracid and hydroperoxo (or
alkylperoxo) groups, but this had not been proven by both of
experiments and calculations. Which one is the real active struc-
ture of catalyst in these oxidation reactions?
HPLC analyses were carried out using Agilent 1100 HPLC system
with a quaternary pump, an UV detector and a RID detector. HPLC
separations were carried out on an Agilent TC-C18 reversed phase
column (25 cm  4.6 mm, 5
lm particle size) using a 30% metha-
nol in 5 mM ammonium acetate buffer at pH 3.3 at 30 °C. UV data
was collected at 225 nm.
2.2. Computational details
Density functional theory as implemented in the restricted
B3LYP hybrid exchange correlation scheme was used to include
some effects of electron correlation with only a marginal increase
in computational cost over Hartree–Fock methods. All results have
been obtained for cluster geometries optimized. W atom was
modeled by a LANL2DZ effective core potential with additional
catalyst n (n=0-5)
COOH
In this work, our experimental study has been directed toward
the exploring of catalysis of tungsten peroxo complexes (catalyst
0–5, Table 1) for the green oxidation of cyclohexene to adipic acid.
We found that, under the condition of only adding inorganic or or-
ganic acids as ligands with organic solvent and phase-transfer cat-
alyst all free, the catalytic systems also have very excellent
activities and the yields of adipic acid are high. Based on the exper-
imental results and the reports [1,35], we are interested in study-
ing what the real active structures of tungsten peroxo complexes
used as catalysts are, so the two possible types of active structures
for tungsten peroxo complexes involving nA and nB with oxalic
acid ligand (n = 1, Scheme 2) and without oxalic acid ligand
+
4 H2O2
4 H2O
+
80-90oC
COOH
P
organic solvent and phase-transfer
catalyst all free
R
Our experiment for green synthesis of adipic acid
-
O
2-
O
O
O
O
O
O
O
H
Suggested catalyst
active structure
W
W
vs.
O
O
=
L (n)
L (n)
nA (n=0-1)
nB (n=0-1)
peroxo ring
hydroperoxo
active structure
active structure
-
H
O
O
2-
O
O
O
+
O
O
Table 1
O
O
O
O
O
W
O
O
H
Yields of adipic acid in the presence of different tungsten peroxo complexes 0–5 as
catalysts without organic solvent and phase-transfer catalyst.
W
O
H
O
O
W
W
O
O
O
O
O
O
O
O
C
C
C
Catalysts
Acidic ligand
Yield of adipic acid (wt.%)
H
O
C
O
O
0
1
2
3
4
5
–
81.0
92.0
59.5
86.7
81.7
45.2
O
Oxalic acid
Adipic acid
Phosphoric acid
Sulfuric acid
Hydrochloric acid
0A
1A
0B
1B
Scheme 2. The suggested two possible kinds of catalyst active structures partic-
ipating in olefin epoxidation, alcohol oxidation, Baeyer–Villiger oxidation reactions
and so on.

P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
21
polarization (d-type) and diffuse (p-type) functions as developed
by Check et al. [41]. A 6-31G(d, p) basis set was used for all other
atoms.
As Table 2 displays, the calculated geometry of the model 1A
performs satisfactorily for the experimental structure of this com-
plex. And 0A is the same kind of tungsten peroxo complex with 1A,
while nB (n = 0–1) is formed by acidizing nA (n = 0–1). Hence, all
the optimized geometric structures of nA and nB (n = 0–1) are ob-
tained at the B3LYP/[LANL2DZ/6-31G(d, p)] level, which have been
shown in Fig. 2. A vibrational frequency calculation was then per-
formed at the optimized geometry belonging to each structure. We
confirmed that nA and nB (n = 0–1) have no imaginary frequencies.
Some calculated important bond length of active structures of
complex nA and nB are given in Table 3. As shown in Table 3, for
each kind of active structure of tungsten peroxo complex (nA or
nB, n = 0–1), the important structural parameters are similar.
All stationary points have been characterized with a full vibra-
tional analysis, and all reported energy differences include zero
point energy corrections. The corresponding vibrational frequen-
cies were calculated at the same level to take account of the
zero-point vibrational energy (ZPVE) and to identify the transition
states. At the same time, the structures of intermediates and the
transition states were confirmed by using the intrinsic reaction
coordinate (IRC). All the theoretical calculations were performed
using the Gaussian03 [42] suits of programs.
3. Results and discussion
3.3. The self-cycle mechanism of active structure
3.1. Experimental results
For interpreting the activities and stabilities of these peroxo
complexes, the self-cycle mechanisms of two active structures
have been calculated. Considering the suggested two active struc-
tures of tungsten peroxo complexes, the research result of peroxo
ring active structure is firstly given in the following part.
In order to discover the activity of tungsten peroxo complexes,
under the condition of organic solvent and phase-transfer catalyst
all free, the green synthesis of adipic acid via direct oxidation of
cyclohexene by 30% H2O2 with tungsten peroxo complexes with-
out or with acidic ligand as catalysts 0–5 (Scheme 2) have been
examined and the results are listed in Table 1.
As shown in Table 1, the kind of ligand in catalyst has a signif-
icant effect on the corresponding yield of adipic acid. From catalyst
1 to 5, the corresponding yield of adipic acid reduces from
92.0 wt.% to 45.2 wt.%, and the trend is similar with the reported
one [30].
3.3.1. The self-cycle mechanism of peroxo ring active structure
From experimental result above, we realize that oxalic acid li-
gand plays a very important role in the catalytic activity of tung-
sten peroxo complex. The self-cycle efficiency of catalyst not only
influences the stability of its active structure, but also is closely re-
lated to the catalytic activity of catalyst. Thus, it is very necessary
to research the role of oxalic acid ligand in self-cycle efficiency of
active structure and to know the effect of structure on the perfor-
mance of active structure. For these reasons, the self-cycle mecha-
nisms of active structure involving with and without oxalic acid
ligand have been investigated. We set the E + ZPVE of M2 as
0.00 kcal/mol as reference in the energy profile of peroxo ring ac-
tive structure without oxalic acid ligand, while we set the E + ZPVE
of M6 as 0.00 kcal/mol as reference in the energy profile of peroxo
ring active structure with oxalic acid ligand (Fig. 4). The processes
could be illustrated as follows:
3.2. The active structures of tungsten peroxo complexes
Considering the experimental results by the catalysts (catalyst
n, n = 0–1) above, we are very interested in the role of oxalic acid
ligand, and we want to compare the activities and stabilities of
tungsten peroxo complexes with or without oxalic acid ligand (cat-
alyst n, n = 0–1), moreover, we can not find any powerful experi-
mental evidences to prove what the structure of complex 2, 3, 4,
5 is. Thus, we will concentrate on oxalic acid complex and the com-
plex without oxalic acid ligand only. Based on the reports [1,35],
the two possible kinds of active structures for tungsten peroxo
complexes (Scheme 2) have been suggested, which include peroxo
ring (nA, n = 0–1) and hydroperoxo (nB, n = 0–1) active structures.
Many experimentally characterized bisperoxo complexes of
tungsten that exhibited a pentagonal bipyramidal structure with
a seven-coordinated metal center, the oxo and one ligand (or donor
atom of a bidentate ligand) occupied axial positions, and the two
peroxo groups and the remaining ligand were equatorial positions
[43–45]. The calculated model complex 1A (Fig. 1) that we have
chosen also exhibits similar structure.
3.3.1.1. The self-cycle mechanism of active structure without oxalic
acid ligand (0A). The self-cycle mechanism of peroxo ring active
structure without oxalic acid ligand (0A) has been suggested and
the structures of intermediates and transition states are shown in
Fig. 3.
There are two stages in this mechanism. First, it can be seen that
after the peroxo ring group (O(1)AO(2)AW(3)) of 0A participating
in the catalytic oxidation reaction through a concerted oxygen-
transfer step forms O(1)@W(3) in intermediate M1, the H2O2 will
be close to M1 to form M2 via weak interaction in order to regen-
erate the peroxo ring active group. the O(4) atom of H2O2 attacks
To analyze the model complex 1A, we have compared the calcu-
lated geometry parameters to the results of X-ray analyses which is
available for 1A in Table 2.
Table 2
Comparison of calculated structures of 1A with available experimental X-ray crystal
data (distance unit: Å).
r
[WO(O2)2(C2O4)]2À, calcd
[WO(O2)2(C2O4)2À, expta
WAO(l)
1.94
1.99
1.99
1.94
1.73
2.24
2.07
1.48
1.48
1.29
1.32
1.93
1.97
1.97
1.94
1.72
2.24
2.14
1.51
1.50
1.29
1.29
WAO(2)
WAO(3)
WAO(4)
WAO(5)
WAO(6)
WAO(7)
O(l)AO(2)
O(3)AO(4)
C(l)AO(6)
C(2)AO(7)
a
Ref. [45].
Fig. 1. Model complex 1A.

22
P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
Fig. 2. The optimized active structures of complex nA and nB (n = 0–1, without 1A).
Table 3
The calculated structures of nA and nB (n = 0–1, without 1A, distance unit: Å).
26.35
23.91
30.00
20.00
10.00
0.00
TS42
TS32
r
OA
OB
IB
14.05
WAO(l)
1.92
1.90
1.90
1.92
1.70
–
1.95
–
1.98
–
12.30
M72
18.91
TS11
18.91 23.91
WAO(2)
WAO(3)
WAO(4)
WAO(5)
WAO(6)
WAO(7)
O(l)AO(2)
O(3)AO(4)
H(l)AO(2)
0.19
1.90
1.88
1.69
2.13
–
1.47
1.46
0.99
1.92
1.92
1.72
2.00
2.05
1.45
1.48
0.98
TS21
M21
M62
-8.56
M82
-10.00
-20.00
26.17
–
-20.73
M41
1.47
1.47
–
-25.98
M31
-30.00
Reaction coordinate
the W(3) atom in M2 and the H(4) atom of H2O2 transfer to the
O(1) atom of W(3)@O(1) bond to form M3 via a four-membered
ring (W(3)AO(1)AH(4)AO(5)) transition state TS1. The energy bar-
rier of this process is only 18.91 kcal/mol (M2 ? TS1, Fig. 4), which
is not a high barrier. In addition, the distances of O(1)AW(3),
H(4)AO(5), W(3)AO(5) and O(1)AH(4) change from 1.725 Å,
0.9760 Å, 2.172 Å and 2.923 Å in M2 to 1.803 Å, 1.243 Å, 2.104 Å
and 1.259 Å in TS1, respectively. At last, the distances of
W(3)AO(5) and O(1)AH(4) are 1.930 Å and 0.968 Å in M3, respec-
tively. During the process of M2 converting to M3 via TS1, double-
bond O(1)@W(3) become the single-bond while single-bond
H(4)AO(5) disappear, and new single-bonds (W(3)AO(5),
O(1)AH(4)) form. In other words, there are two bonds break so
as to generate two new bonds.
Fig. 4. The potential energy profiles of self-cycle mechanism of peroxo ring active
structures at the B3LYP/[LANL2DZ/6-31G(d, p)] level (unit: kcal/mol, the super-
script 1 represents active structures without oxalic acid ligand and the superscript 2
represents active structures with oxalic acid ligand).
(O(1)AW(3), O(6)AH(7)) break to generate two new bonds
(W(3)AO(6), O(1)AH(7)). The distances of the O(1)AW(3),
O(6)AH(7), W(3)AO(6) and O(1)AH(7) are 1.881 Å, 0.9760 Å,
2.459 Å and 3.847 Å in M3, and these distances change from
2.098 Å, 1.438 Å, 2.203 Å and 1.099 Å in TS2 to 2.196 Å, 2.487 Å,
1.922 Å and 0.9710 Å in M4, respectively. Attentively, O(5) and
O(6) atoms in M4 are corresponding to O(2) and O(1) atoms in
0A (Fig. 3), respectively.
Second, the O(6) atom of H2O2 attacks W(3) atom, and the H(7)
atom of H2O2 is close to O(1) atom in M3 to generate M4 through
transition state TS2. The energy barrier of this process is
26.17 kcal/mol (M3 ? TS2, Fig. 4), which is the highest energy bar-
rier in the mechanism. In this process, two single-bonds
3.3.1.2. The self-cycle mechanism of active structure with oxalic acid
ligand (1A). The self-cycle mechanism of this kind of complex with
oxalic acid ligand (1A) is shown in Fig. 5, which is similar with that
without oxalic acid ligand (Fig. 3).
4
O
H
O
O
5
O
1.725
O
+
6
1
2.923
1
O
2.172
1
W
O
Participating in the
catalytic reaction
O
O
7
W
O
W
H
Reactant
Product +
3
+
O
3
3
4
H
O
O
O
O
2
0.976
O
M1
5
0A
O
M2
6
7
H
H
4
-
1
O
7
H
O
O
O
O
O
O
O
H
4
H
O
1.881
H
4
1
4
O
2.196
O
2.098
O
W1.803
2.104 1.259
0.968
3.847
7
O
O
O
W
W
1
O
1
W
3
2.459
O
3
1
1.930
0.971
3
1.922
3 2.203
O
1.099
O
O
5
1.243
H
5
O
O
6
H
5
4
O
H
7
7
O
H
2.487
5
1.438
0.976
6
6
O 6
7
H
M4
TS2
M3
TS1
Fig. 3. The self-cycle mechanism of peroxo ring active structure without oxalic acid ligand (distance unit: Å, energy unit: kcal/mol).

P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
23
O
2-
O
2-
O
O
1
3
O
O
O
W
W
Participating in the
catalytic reaction
O
3
O
O
O
1
2
Reactant
+
Product
+
O
O
C
O
C
C
C
O
1A
O
O
M5
O
6
H
H
H
5
O
4
4
+
-
1
O
O
7
7
H
2-
O
O
4
2-
H
O
1.770
O
O
O
3.753
O
1
W
3.686
1.765
O
3
1
O
W
O
0.977
3
1.960
H
4
5
C
O
C
O
0.991
H
O
O
7
C
O
1.960
O
C
6
O
6
5
O
H
7
O
M8
M6
4
2-
2-
4
O
H
O
2-
H
O
O
O
1.862
O
O
O
C
O
O
2.394
10.970
O
W
1
1.987
O
11.217
O
C
W
O
3
1.185
4
O
C
W
2.061
2.450
3
2.322
O
3
3.599
2.926
O
H
O
5
O
H
O
1.246
7
O
O
C
6 1.190
C
O
O
H
O
C
O
7
O
6 0.978
5
5
6
O
O
O
H
7
M7
TS4
TS3
Fig. 5. The self-cycle mechanism of peroxo ring active structure with oxalic acid ligand (distance unit: Å, energy barrier unit: kcal/mol).
Fig. 6. The self-cycle mechanism of hydroperoxo active structure without oxalic acid ligand (distance unit: Å, energy barrier unit: kcal/mol).
The energy barrier of traversing the four-membered ring
(W(3)AO(1)AH(4)AO(5)) transition state TS3 is 23.91 kcal/mol
(M6 ? TS3, Fig. 4). Moreover, the distances of O(1)AW(3), H(4)A
O(5), W(3)AO(5) and O(1)AH(4) change from 1.770 Å, 0.9910 Å,
3.753 Å and 1.765 Å in M6 to 1.862 Å, 1.246 Å, 2.450 Å and
1.185 Å in TS3, respectively. While the distances of W(3)AO(5)
and O(1)AH(4) are 2.061 Å and 0.9700 Å in M7, respectively.
The energy barrier of the second step (traversing transition
state TS4) is only 14.05 kcal/mol (M7 ? TS4, Fig. 4), which is
12.12 kcal/mol lower than that without oxalic acid ligand. The dis-
tances of the O(1)AW(3), O(6)AH(7), W(3)AO(6) and O(1)AH(7)
are 1.987 Å, 0.9780 Å, 2.926 Å and 3.599 Å in M7, and these dis-
tances change from 2.394 Å, 1.190 Å, 2.322 Å and 1.217 Å in TS4
to 3.686 Å, 1.960 Å, 1.960 Å and 0.9770 Å in M8, respectively.

24
P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
3.3.2. The self-cycle mechanism of hydroperoxo active structure
9.61
TS72
For the same reason with active structure nA, the self-cycle
mechanisms of active structure nB containing with and without
oxalic acid ligand have also been investigated. We set the E + ZPVE
of M9 as 0.00 kcal/mol as reference in the energy profile of hydro-
peroxo active structure without oxalic acid ligand, while we set the
E + ZPVE of M12 as 0.00 kcal/mol as reference in the energy profile
of hydroperoxo active structure with oxalic acid ligand (Fig. 7).
8.11
1B2
10.00
5.00
6.45
4.84
TS61
TS51
M91
M122
3.71
4.84 9.61
TS82
0.00
-2.65
M152
13.10
18.19
-5.00
-9.39
M142
-10.00
-15.00
-20.00
-25.00
-11.74
M111
3.3.2.1. The self-cycle mechanism of hydroperoxo active structure
without oxalic acid ligand (0B). The structures of intermediates and
transition states in the self-cycle mechanism of active structure 0B
have been depicted in Fig. 6, there are also two stages in this
mechanism.
-26.58
0B1
-26.58
0B1
Reaction coordinate
First, the protonhydrate in the solution system is close to 0A
(Scheme 2) to form intermediate M9 and the H(8) atom of proton-
hydrate attacks the O(2) atom in M9, which generates 0B through
transition state TS5. The energy barrier of this process is only
4.84 kcal/mol (M9?TS5, Fig. 7), which is a very low barrier. In addi-
tion, the distances of O(2)AW(3), O(2)AH(8) and H(8)AO(9) change
from 1.977 Å, 1.345 Å and 1.095 Å in M9 to 2.069 Å, 1.047Å and
1.503 Å in TS5, respectively, while the distance of W(3)AO(9) is
2.127 Å in 0B. After the hydroperoxo (W(3)AO(1)AO(2)AH(8))
group of 0B participating in the catalytic oxidation reaction with
a concerted oxygen-transfer step to form W(3)AO(1)AH(8) in inter-
mediate M10, the H2O2 will close to M10 and the H2O will go away
to form M11 via weak interaction.
Second, the O(5) atom of H2O2 attacks W(3) atom in M11 and
the H(4) atom of H2O2 is close to O(1) atom in M11 to regenerate
0B via transition state TS6. The energy barrier of this process is
only 18.19 kcal/mol (M11 ? TS6, Fig. 7). During this process, two
single-bonds (O(1)AW(3), H(4)AO(5)) break so as to generate
two new bonds (W(3)AO(5), O(1)AH(4)). The distances of the
O(1)AW(3), H(4)AO(5), W(3)AO(5) and O(1)AH(4) change from
1.838 Å, 0.981 Å, 2.145 Å and 3.538 Å in M11 to 1.986 Å, 1.204 Å,
2.147 Å and 1.282 Å in TS6, respectively. Attentively, O(1), O(5),
Fig. 7. The potential energy profiles of self-cycle mechanism of hydroperoxo active
structures at the B3LYP/[LANL2DZ/6-31G(d, p)] level (unit: kcal/mol, the super-
script 1 represents active structures without oxalic acid ligand and the superscript 2
represents active structures with oxalic acid ligand.
Noteworthy, O(5) and O(6) atoms in M8 are corresponding to O(2)
and O(1) atoms in 1A (Fig. 5), respectively.
In addition, the highest energy barrier of cycle process of peroxo
ring active structure without oxalic acid ligand is 26.17 kcal/mol
(M3 ? TS2) in Fig. 4, while the highest energy barrier of cycle pro-
cess of peroxo ring active structure with oxalic acid ligand is
23.91 kcal/mol (M6 ? TS3) in Fig. 4. It can be seen that the energy
barrier of cycle process with oxalic acid ligand is 2.26 kcal/mol
lower than that of cycle process without oxalic acid ligand, which
shows that the cycle process of peroxo ring active structure with
oxalic acid ligand is easier to proceed. And it is beneficial to recycle
the active structure of complex as soon as possible in the catalytic
reaction, which is very important not only for keeping active struc-
ture of complex stable, but also for promoting the catalytic reac-
tion. Moreover, the computational trend of self-cycle processes
(Fig. 4) is agreement with that of experiment (Table 1).
-
-
1
O
O
O
3
O
1
3
W1.940
O
W 2.074
O
2
O
2
O
2.919
1.201
2.605
2.373
O
O
O
H
O
H
0.971 8
O
C
O
C
91.251
9
8
C
C
O
O
TS7
O
O
M12
-
O
H
O
-
O
O
O
8
3
O
O1
W
1
O2
O
W
3
Participating in the
catalytic reaction
O
2.199
O
8
+
Reactant
Product
+
O
H
O
C
C
9
C
O
C
O
O
M13
1B
O
O H
-
5
4
O
H
+
O
6
1
7
7
H
8
H
-
-
-
O
O
O
O
O
O H
O
O
H
4
O
H
O
O
O
O
O
3.9106
5
4
5
5
4
2.210
W 2.025
O
O
W
W
O
6
O
6
0.981
3
O
3
O
3
1.152
2.164
2.506
1.864
C
C
C
O
O 1.964
O
O
O
H
7
O
H
7
O
H
C
O
7
1.830
C
C
10.981
1
11.285
H
H
H
8
8
8
O
O
M15
TS8
M14
Fig. 8. The self-cycle mechanism of hydroperoxo active structure with oxalic acid ligand (distance unit: Å, energy barrier unit: kcal/mol).

P. Jin et al. / Journal of Molecular Structure 992 (2011) 19–26
25
O(6), H(4), H(7) and H(8) atoms in TS6 are corresponding to O(9),
O(1), O(2), H(11), H(8) and H(10) atoms in 0B (Fig. 6), respectively.
above, we think the peroxo ring active structure should be more
reasonable. Furthermore, we will provide the further conclusion
after we carefully research the complete mechanism of the green
oxidation of cyclohexene to adipic acid in future.
3.3.2.2. The self-cycle mechanism of hydroperoxo active structure with
oxalic acid ligand (1B). The self-cycle mechanism of hydroperoxo
active structure with oxalic acid ligand (1B) is given in Fig. 8, which
also has two stages as follows:
Acknowledgments
First, the dissociative H(8) atom which is derived from ligand is
in the vicinity of anionic groups, when the system is heated, the
H(8) atom will be close to 1A (Scheme 2) to form bond
(H(8)AO(9)) in intermediate M12. Then the H(8) atom attacks
the O(2) atom in M12 to generate 1B via transition state TS7. The
energy barrier of this process is only 9.61 kcal/mol (M12 ? TS7,
Fig. 7). Moreover, the distances of O(2)AW(3), H(8)AO(9),
W(3)AO(9) and O(2)AH(8) change from 1.940 Å, 0.971 Å, 2.605 Å
and 2.919 Å in M12 to 2.074 Å, 1.251Å, 2.373 Å and 1.201 Å in
TS6, respectively, while the distance of W(3)AO(9) is 2.199 Å in
1B. In this process, two bond (O(2)AW(3), H(8)AO(9)) break so
as to generate new bond (O(2)AH(8)). Then, the formed 1B will
participate in the catalytic reaction.
Second, the energy barrier of traversing the four-membered
ring (W(3)AO(6)AH(7)AO(1)) transition state TS8 is only
13.10 kcal/mol (M14 ? TS8, Fig. 7), which is 5.09 kcal/mol lower
than that without oxalic acid ligand. The distances of the
W(3)AO(6), O(1)AH(7), O(1)AW(3) and O(6)AH(7) are 3.910 Å,
1.830 Å, 1.964 Å and 0.981 Å in M14 and these distances change
from 2.210 Å, 1.285 Å, 2.164 Å and 1.152 Å in TS8 to 2.025 Å,
0.9810 Å, 2.506 Å and 1.864 Å in M15, respectively. Noteworthy,
O(6), O(5) and H(4) atoms in M15 are corresponding to O(1),
O(2) and H(8) atoms in 1B (Fig. 8), respectively.
Compared the highest energy barrier of cycle process of 0B
(18.20 kcal/mol (M11 ? TS6) in Fig. 7) with that of 1B
(13.10 kcal/mol (M14 ? TS8) in Fig. 7), the energy barrier of cycle
process with oxalic acid ligand is 5.10 kcal/mol lower than that
without oxalic acid ligand, which also indicates that the self-cycle
process of hydroperoxo active structure with oxalic acid ligand is
more energy favorable. However, the reaction trend should be
from right to left in the cycle process of 1B (Fig. 7), which indicate
that the self-cycle process of 1B could not proceed. Thus, the
hydroperoxo active structures may be unreasonable in theory.
We gratefully acknowledge the financial support from the inno-
vation fund for Elitists of Henan province, China (Grants No.
0221001200).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.02.023.
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