Three Pd-decavanadates with a controllable molar ratio of Pd to decavanadate and their heterogeneous aerobic oxidation of benzylic C-H bonds

Article abstract of DOI:10.1039/c7dt03898d

By the combination of Pd-complexes and [V₁₀O₂₈]^6-, three Pd-decavanadate compounds [Pd(NH₃)₄]₃[V₁₀O₂₈]·8H₂O (1), [Pd(deta)(H₂O)]₂(NH₄)₂[V₁₀O₂₈]·2H₂O (2) (deta = diethylenetriamine) and [Pd(dpa)₂](Hdpa)₂(Et₃NH)₂[V₁₀O₂₈]·2H₂O (3) (dpa = 2,2′-dipyridylamine) have been successfully synthesized and thoroughly characterized using single X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), infrared spectroscopy (FT-IR) and elemental analyses (EA). Interestingly, in the three compounds, the molar ratios of Pd to decavanadate vary from 3:1 to 1:1 by changing N-ligands. The three Pd-decavanadates as heterogeneous catalysts are active in the aerobic oxidation of benzylic hydrocarbons under solvent-free conditions without adding any additives and co-catalysts. Moreover, compound 1 can be reused three times without losing its activity.

Full text of DOI:10.1039/c7dt03898d

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Three Pd-decavanadates with a controllable molar
ratio of Pd to decavanadate and their
heterogeneous aerobic oxidation of benzylic C–H
bonds†
Cite this: DOI: 10.1039/c7dt03898d
Xianqiang Huang,
‡
a,b Jikun Li,‡b,c Guodong Shen,a Nana Xin,a Zhengguo Lin,b
b
Yingnan Chi,*b Jianmin Dou,a Dacheng Lia and Changwen Hu
*
6−
By the combination of Pd-complexes and [V10O28
] , three Pd-decavanadate compounds [Pd
(NH3)4]3[V10O28]·8H2O (1), [Pd(deta)(H2O)]2(NH4)2[V10O28]·2H2O (2) (deta = diethylenetriamine) and [Pd
(dpa)2](Hdpa)2(Et3NH)2[V10O28]·2H2O (3) (dpa = 2,2’-dipyridylamine) have been successfully synthesized
and thoroughly characterized using single X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), infra-
red spectroscopy (FT-IR) and elemental analyses (EA). Interestingly, in the three compounds, the molar
ratios of Pd to decavanadate vary from 3 : 1 to 1 : 1 by changing N-ligands. The three Pd-decavanadates as
heterogeneous catalysts are active in the aerobic oxidation of benzylic hydrocarbons under solvent-free
conditions without adding any additives and co-catalysts. Moreover, compound 1 can be reused three
times without losing its activity.
Received 16th October 2017,
Accepted 30th November 2017
DOI: 10.1039/c7dt03898d
rsc.li/dalton
selective oxidations.5 Importantly, by virtue of the strong
electrostatic interaction between POM and Pd-complexes, the
Introduction
Polyoxometalates (POMs), as a remarkable class of discrete solidification of POMs is easily realized and this provides a
anionic clusters, are mainly composed of early transition new approach for the reuse of POMs. Because the assembly
metals (Mo, W, V, Nb and Ta) and oxo ligands.1 As vanadium process of Pd-POMs is affected by several factors including
has excellent reversible redox properties, polyoxovanadates ligands, the type of POM, pH value and reaction temperature,
(POVs) have been explored as oxidation catalysts in recent until now, only a handful of Pd-POMs with determined struc-
years.2 In addition, the counter-cations play an important role tures have been reported6 and the construction of Pd-POMs
in the self-assembly of POMs, which influence not only the still remains a challenge.
solubility of POMs, but also their catalytic activities.3 Pd-com-
The selective oxidation of C–H bonds is a primary and
plexes have been proved to be active in some oxidation reac- essential transformation in the synthesis of high value chemi-
tions.4 In our previous investigations Pd-complexes have been cals.7 Due to the extensive use of aryl ketones as building
selected as counter-cations to combine with V-containing blocks in the manufacture of pharmaceuticals and agrochem-
phosphomolybdates or alkoxohexavanadates by electrostatic icals, the selective oxidation of benzylic hydrocarbons has
attraction and the formed Pd-POMs are highly active for the attracted much attention recently.8 To date, although a rather
high yield was achieved in the benzylic C–H bond oxidation, in
most cases, hydrogen peroxide,9 tert-butylhydroperoxide,10 and
PhI(OAc)2 or oxone11 were used as oxidants. Molecular oxygen
is regarded as an ideal oxidant as it is cheap and green.
However, because O2 is difficult to activate, the aerobic oxi-
dation of benzylic hydrocarbons is still a challenge. Some Pd
aShandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry & Chemical Engineering, Liaocheng University,
Liaocheng, 252059 Shandong, P. R. China
bKey Laboratory of Cluster Science Ministry of Education, Beijing Key Laboratory of
Photoelectronic/Electrophotonic, School of Chemistry, Beijing Institute of Technology
Beijing, 100081 P.R. China. E-mail: chiyingnan@bit.edu.cn, cwhu@bit.edu.cn
cCollege of Chemistry and Chemical Engineering, Taishan University, Tai’an 271021,
Shandong, P. R. China
complexes12
and
organic
molecules13
(such
as
N-hydroxyphthalimine) have been reported to be active in the
aerobic transformation of benzylic hydrocarbons, but they
have been used in a homogeneous system and in the presence
of additives (organic bases). From an economic and environ-
mental viewpoint, heterogeneous catalytic systems without
using any additives are more desirable. Several metal organic
†Electronic supplementary information (ESI) available: Characterization data,
copies of IR and PXRD spectra and single crystal data. CCDC 975175 (1), 975174
(2), 1495357 (3) and 974935 (4). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7dt03898d
‡These authors contributed equally to this work.
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frameworks (MOFs) were found to activate molecular oxygen in X-ray diffraction were obtained. Yield: ca. 0.042 g, 34.7%
the catalytic oxidation of benzylic C–H bonds, but some cases (based on Pd(OAc)2). Anal. calcd (%) (found) for
face a leaching problem.14 Recently, the investigation of H52N12O36V10Pd3: H, 3.23 (3.45); N, 10.34 (10.46); V, 31.35
Y. Wang et al. showed that Pd nanoparticles supported on (31.27); Pd, 19.65 (19.51). IR (KBr, cm−1): 3280 (N–H, m), 1616
N-doped carbon not only exhibited good catalytic performance (s), 1243 (s), 942 (s), 819 (s), 734 (s), 586 (s), 522 (w), 455 (Pd–
(conv. 8.4% of diphenylmethane) under solvent-free con- N, m).
ditions but also that this catalyst can be reused without
leaching.15
Synthesis of [Pd(deta)(H2O)]2(NH4)2[V10O28]·2H2O (2). The
synthesis procedure of compound 2 is similar to that of com-
Herein, two components, Pd-organic amine complexes and pound 1 except that NH4Cl (0.065 g, 1.2 mmol) was replaced
decavanadates, both of which have the potential ability to cata- by deta (0.062 g, 0.6 mmol). Yield: ca. 0.038 g, 22.9% (based
lyze the aerobic oxidation of benzylic hydrocarbons, were on Pd(OAc)2). Anal. calcd (%) (found) for C8H42N8O32V10Pd2:
coupled by self-assembly and as
a result, [Pd(NH3)4]3 C, 6.47 (6.52); H, 2.85 (2.94); N, 7.55 (7.43); V, 34.31 (34.38);
[V10O28]·8H2O (1), [Pd(deta)(H2O)]2(NH4)2[V10O28]·2H2O (2) Pd, 14.34(14.25). IR (KBr, cm−1): 3138 (N–H, m), 1623 (w), 1581
(deta = diethylenetriamine) and [Pd(dpa)2](Hdpa)2(Et3NH)2 (w), 1438 (w), 1265 (s), 1027(m), 942 (s), 821 (s), 747 (s), 557 (s),
[V10O28]·2H2O (3) (dpa = 2,2′-dipyridylamine) have been syn- 462 (Pd–N, m).
thesized and thoroughly characterized using SXRD, PXRD,
Synthesis of [Pd(dpa)2](Hdpa)2(Et3NH)2[V10O28]·2H2O (3).
FT-IR and EA. The three Pd-decavanadate compounds as V6O15(1-mIM)8 (0.42 g, 0.35 mmol), dpa (0.14 g, 0.8 mmol)
heterogeneous catalysts show catalytic activity in the aerobic and triethylamine (0.041 g, 0.4 mmol) were successively added
oxidation of benzylic hydrocarbons under solvent-free con- to 20 mL HAc-NaAc buffer (pH = 4.5) and the solution was
ditions without using any additives or co-catalysts.
stirred at room temperature for 60 min. To this, Pd(OAc)2
(0.045 g, 0.2 mmol) was added. The resulting mixture was
further stirred for 12 h under 50 °C, and then the precipitate
was removed by filtration. Red crystals suitable for X-ray diffr-
action were collected after evaporation for two weeks. Yield: ca.
0.068 g, 17.3% (based on Pd(OAc)2). Anal. calcd (%) (found)
for C52H72N14O30V10Pd: C, 31.40 (31.26); H, 3.65 (3.53); N, 9.86
(9.74); V, 25.61 (25.71); Pd, 5.35 (5.31). IR (KBr, cm−1): 3232
(N–H, m), 3023 (w), 1609(s), 1573 (s), 1484(s), 1406 (m), 1327
(s), 1218 (s), 956 (s), 821 (s), 748 (s), 591 (s), 455 (Pd–N, m).
Synthesis of [N(CH3)4]4[H2V10O28]·5H2O (4). V2O5 (0.16 g,
0.88 mmol) and 25% Me4NOH (0.32 g, 0.88 mmol) were added
to 6 mL H2O. The reaction mixture was stirred for 12 h. From
the resulting solution, light red crystals suitable for X-ray diffr-
action grew upon allowing them to stand under ambient con-
ditions for a week. Yield: ca. 0.083 g, 35.07% (based on V2O5).
Anal. calcd (%) (found) for C16H60N4O33V10: C, 14.27 (14.39);
H, 4.46 (4.35); N, 4.16 (4.35); V, 37.89 (37.67). IR (KBr, cm−1):
3434 (m), 1629 (m), 1481 (m), 1403 (m), 958 (s), 834 (s), 748 (s),
568 (m).
Experimental
Materials and methods
All reagents and solvents for synthesis were purchased from
commercial suppliers and used as received, except for V6O15(1-
mIM)8 (1-mIM = 1-methylimidazole) which was synthesized
according to the related literature.2c The metal content of the
compounds 1–4 was measured by inductively coupled plasma
(ICP) on an ICP-6000 analyzer. The FT-IR spectra were recorded
as KBr pellets in the range of 4000–400 cm−1 on a Nicolet 170
SXFT/IR spectrometer. The powder X-ray diffraction pattern
was collected by using a Rigaku D/max-2550 diffractometer
with CuKα radiation. After the catalytic reaction was com-
pleted, the resulting mixture was analyzed using GC-MS and
GC using naphthalene as an internal standard substrate. The
GC analyses were performed on a Shimadzu GC-2014C system
with an FID detector equipped with an Rtx-1701 Sil capillary
column. The GC-MS spectra were recorded on an Agilent
7890A-5975C at an ionization voltage of 1200 V. The C, H, and
N elemental analyses were conducted on a PerkinElmer 240C
elemental analyzer. The XPS spectrum was recorded on an
ESAY ESCA spectrometer with an Mg–Kα achromatic X-ray
source.
Crystallography
Single-crystal X-ray diffraction data of compounds 1–4 were col-
lected on a Bruker-AXS CCD diffractometer equipped with
graphite-monochromated MoKα radiation (λ = 0.71073 Å). All
absorption corrections were applied using a multi-scan tech-
nique. The structure solution and refinement were carried out
with SHELXS-2014 and SHELXL-2014 program packages.16,17
Synthetic procedures
Synthesis of [Pd(NH3)4]3[V10O28]·8H2O (1). V2O5 (0.5 g, The solvent molecules could not be refined satisfactorily;
2.75 mmol) and 25% Et4NOH (1.62 g, 2.75 mmol) were added hence, they were removed by applying the SQUEEZE option of
to 20 mL water. The resulting solution was stirred at room PLATON.18 The void volumes were estimated using the
temperature for 60 min, and then Pd(OAc)2 (0.05 g, PLATON SQUEEZE program19 in which the solvated molecules
0.223 mmol) and NH4Cl (0.065 g, 1.2 mmol) were successively in the compounds 3–4 were excluded. The hydrogen atoms of
added. The resulting mixture was stirred for 12 h and heated the organic ligands were refined as rigid groups.
at 50 °C for 30 min, and then the precipitate was removed by Crystallographic data of compounds 1–4 are summarized in
filtration. After leaving the resulting solution to stand under Table 1. The crystallographic data have been deposited with
ambient conditions for three days, red crystals suitable for the Cambridge Crystallographic Data Centre (CCDC) as entries
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Table 1 Crystallographic data of compounds 1–4
1
2
3
4
Formula
Mr
Crystal system
Space group
T (K)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
N12H52Pd3V10O36
1625.13
Triclinic
ˉ
P1
298(2) K
9.5424(8)
11.2379(9)
11.8608(11)
115.251(2)
108.0690(10)
98.8730(10)
1031.12(15)
1
C8H42N8Pd2V10O32
1484.69
Monoclinic
P2(1)/c
296(2) K
9.715(7)
10.642(7)
19.195(13)
90
93.083(12)
90
1982(2)
2
C52H70N14PdV10O28
1955.02
Triclinic
ˉ
P1
293(2) K
11.353(6)
13.219(7)
13.408(7)
89.754(8)
77.245(8)
64.976(7)
1769.2(15)
1
C16H50N4V10O28
1256.00
Triclinic
ˉ
P1
296(2) K
12.6170(12)
12.6550(12)
15.6028(15)
68.3170(10)
83.535(2)
78.824(2)
2268.8(4)
2
γ (°)
V (Å3)
Z
Dcalc.(g cm−3
F(000)
)
2.617
792
2.488
1448
1.867
998
1.839
1256
R1 [I > 2σ(I)]
0.0298
0.0702
0.0373
0.0733
0.0737
0.2254
0.0879
0.2518
0.0347
0.0711
0.0442
0.0759
0.0607
0.2033
0.0834
0.2797
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
GOOF
0.988
1.063
0.978
1.126
CCDC no.
975175
975174
1495357
974935
975175 (1), 975174 (2), 1495357 (3) and 974935 (4).† For the plane. The bond lengths and angles of [V10O28] are comparable
selected bond lengths and bond angles of compounds 1–4, to the reported ones.20 BVS calculations show that all the
please see Table S2.†
vanadium atoms are in the +V state (Table S1†).21 Interestingly,
we found that the ratio of the Pd-complex to [V10O28] varies
from 3 : 1 in 1, 2 : 1 in 2, to 1 : 1 in 3.
Aerobic oxidation of benzylic hydrocarbons
Compound 1 was isolated when using V2O5, Pd(OAc)2,
NH4Cl and Et4NOH as starting materials. Crystal structure ana-
In a 50 mL stainless-steel autoclave with a magnetic stirring
bar, catalyst (8 mol%) and benzylic hydrocarbons (10 mmol)
were added. The reaction was carried out under 8 atm O2 in
the absence of solvents, additives and co-catalysts. The reac-
tion was conducted with magnetic stirring for the appropriate
time upon heating at 90 °C. After the reaction was completed,
the solution was analyzed using GC-MS and GC.
ˉ
lysis reveals that 1 crystallizes in the triclinic P1 space group
and consists of three [Pd(NH3)4]2+ cations, one [V10O28]6− poly-
anion and eight crystallographic water molecules (Scheme 1).
In this case, the ligand NH3 is formed in situ by the acid–base
reaction between NH4Cl and Et4NOH. In three [Pd(NH3)4]2+
cations, each palladium center exhibits square-planar geome-
try with the Pd−N bond distance in the range of 2.034(4)–2.046
(4) Å, which is very close to the Pd–NH3 bond length of 2.039
(3) Å in the trans-[Pd(NH3)2(1-MeC)2]2+ complex.22
Reuse experiments
The reuse experiment was carried out under the conditions
listed in Table 3. After the reaction was conducted for 36 h,
compound 1 was filtered, washed with 10 mL methanol three
times, dried under ambient conditions, and used for the next
cycle.
When the organic ligand, deta, was added, compound 2
instead of compound 1 was obtained. Structural analysis
shows that 2 is composed of two [Pd(deta)(H2O)]2+ cations, two
NH4 cations, one [V10O28]6− polyanion, and two lattice water
+
molecules (Scheme 1). As deta has a chelating coordination
mode, Pd is apt to bond with deta rather than the NH3 ligand.
Pd cations exhibit a planar PdN3O model, where three coordi-
nation sites are occupied by the nitrogen atoms of the deta
Results and discussion
Synthesis and crystal structures
Compounds 1–3 were synthesized by the reaction of V2O5 or ligand, and the fourth site is occupied by one water molecule.
V6O15(1-mIM)8, Pd(OAc)2 and the N-containing compounds The Pd–N and Pd–O bond distances are comparable with
(NH4Cl, deta, or dpa) in Et4NOH or Et3N aqueous solution those found in the previous PdN3O model complexes.23
(Scheme 1). Although different vanadate sources were used in
Compound 3 was synthesized by the reaction of V6O15(1-
the synthesis of 1–3 (V2O5 was used for compounds 1 and 2, mIM)8, trimethylamine, dpa and Pd(OAc)2 in a HAc-NaAc
and V6O15(1-mIM)8 was used for compound 3), only the buffer. As shown in Scheme 1, compound 3 contains one
[V10O28] cluster is observed as a polyanion. The decavanadate [Pd(dpa)2]2+ cation, one [V10O28]6− polyanion, two protonated
cluster is built from ten highly condensed distorted {VO6} octa- Et3NH+ ions and two protonated Hdpa+ ions. When compound
hedra with a cage-like structure where six vanadiums are 3 was synthesized in HAc-NaAc buffer, the pH value in this
arranged in a plane with four vanadiums above and below the case was dramatically decreased. Although the formation of
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Scheme 1 The controlling synthesis of compounds 1–3. Color codes: Pd, orange; V, lavender; C, gray-40%; O, red; N, blue. All lattice solvent mole-
cules and hydrogen atoms are omitted for clarity.
the [V10O28]6− cluster is not affected, the coordination ability
of the dpa ligand decreases. Some dpa ligands coordinate with
the Pd cation, but the others are protonated and act as
counter-cations. The Pd cations showed a planar PdN4 coordi-
nate model where four coordination sites are occupied by the
nitrogen atoms of two dpa ligands. The bond angles of N–Pd–
Scheme 2 Transformation of diphenylmethane to benzophenone
using Pd-POVs as catalysts.
N are all 180°, which are identical to those found in 1.
Table 2 Aerobic oxidation of diphenylmethane to benzophenonea
FT-IR spectra and PRXD
The IR spectra of the compounds 1–3 are shown in Fig. S1–
Conv. Sele. Reaction
S3.† The absorptions in the region 455–465 cm−1 (455 cm−1
Entry Catalysts
(%)
(%)
system
for 1, 462 cm−1 for 2, and 455 cm−1 for 3) are characteristic
peaks of Pd–N bonds, which are in agreement with the
reports.24 The stretching vibrations of VvO in the decavana-
1
2
3
4
5
6
7
No
—
—
b
Pd(OAc)2
10.3
12.8
10.6
9.2
4.9
14.7
87.5 Homogeneous
98.8 Heterogeneous
98.4 Heterogeneous
97.9 Heterogeneous
97.2 Heterogeneous
94.3 Heterogeneous
Pd(OAc)2 and NH4Clc
Pd(OAc)2 and EDTAd
Pd(OAc)2 and dpae
date cluster are observed around 950 cm−1 25
The asymmetric
.
vibrations of V–O–V groups are observed at 734 cm−1 (com-
pound 1), 747 cm−1 (compound 2), and 748 cm−1 (compound
3), respectively, and the corresponding symmetric vibrations
are observed at 586 cm−1 (compound 1), 557 cm−1 (compound
2), and 581 cm−1 (compound 3), respectively.26 The experi-
mental and simulated PXRD patterns of three Pd-POV com-
pounds 1–3 are shown in Fig. S5–S7.† Their peak positions
were in good agreement with each other, indicating the phase
purity of the three compounds.
[NMe4]4[H2V10O28
Pd(OAc)2 and
[NMe4]4[H2V10O28
]
]
8
9
1
2
22.5
18.7
15.4
22.5
98.6 Heterogeneous
96.5 Heterogeneous
94.8 Heterogeneous
98.5 Heterogeneous
10
11
3
1f
a Reaction conditions: Diphenylmethane (10 mmol), catalyst
(0.8 mmol), 90 °C, O2 (8 atm), 36 h. b Pd(OAc)2 (2.4 mmol). c Pd(OAc)2
(2.4 mmol) and NH4Cl (9.6 mmol). d Pd(OAc)2 (2.4 mmol) and EDTA
(2.4 mmol). e Pd(OAc)2 (2.4 mmol) and dpa (4.8 mmol). f Reaction time
40 h.
Catalytic activities of Pd-POVs
Given the fact that both the cations, the Pd-complexes, and the
anion, [V10O28]
6−, of compounds 1–3 are active in aerobic oxi- formed in a 50 mL stainless steel autoclave containing diphenyl-
dation, the transformation of diphenylmethane to benzophe- methane (10 mmol) and the Pd-POV catalyst (0.8 mmol) under
none was used as a model reaction to evaluate the catalytic 8 atm O2 pressure at 90 °C. The results indicate that compounds
activities of three Pd-POVs (Scheme 2). The reactions were per- 1–3 (Table 2, entries 8–10) can successfully catalyze the aerobic
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oxidation of diphenylmethane under solvent-free conditions
and that 1 (conv. 22.5%, sele. 98.6%) is more active than 2
(conv. 18.7%, sele. 96.5%) and 3 (conv. 15.4%, sele. 94.8%).
The blank experiment indicates that no substrate is con-
sumed in the absence of catalysts (Table 2, entry 1). To assess
the influence of the cation and the anion of Pd-POVs on
aerobic transformation, a series of control experiments were
performed. Although Pd(OAc)2 as a homogeneous catalyst can
produce benzophenone (Table 2, entry 2), the conversion
(10.3%) is obviously lower than those of compounds 1–3 and
palladium black is observed after the reaction (Fig. S9†). When
Pd(OAc)2 and N-containing ligands such as NH4Cl, deta and
dpa were used together, the conversion of diphenylmethane
was slightly increased (Table 2, entries 3–5) but no palladium
black was found (Fig. S9†). To further explore the role of the
Fig. 2 Kinetics of the aerobic oxidation of diphenylmethane with com-
pound 1 (black); compound 1 was filtered from the reaction mixture
after 12 h (red). Reaction conditions: diphenylmethane (10 mmol), cata-
lyst (8 mol%), 90 °C, O2 (8 atm), 36 h.
decavanadate cluster in the reaction, compound
4
[NMe4]4[H2V10O28] (Fig. 1) was prepared, which converts 4.9%
diphenylmethane into benzophenone with the selectivity of
97.2% (Table 2, entry 6). In addition, the mechanical mixture
of Pd(OAc)2 and compound 4 leads to a yield comparable to
that of compound 3. The above results show that both the Pd-
complex and the decavanadate polyanion in 1–3 play some
roles in the aerobic oxidation of diphenylmethane and, com-
pared to each single component, their electrostatic combi-
nation significantly enhances the catalytic activity.
The heterogeneity of compound 1 was confirmed by a hot
leaching experiment. The solid catalyst of 1 was filtered from
the reaction after 12 h under the optimum conditions and the
filtrate was subsequently reacted at 90 °C for an additional
24 h. GC analysis showed a negligible conversion after fil-
tration (Fig. 2). In addition, mercury was added to the reaction
to check the heterogeneity of Pd. Under otherwise identical
conditions, the conversion of diphenylmethane is basically
unchanged, revealing that no active Pd nanoparticles are
formed in the aerobic transformation. In addition, atomic
Fig. 3 Recycle experiments of compound 1 in the aerobic oxidation of
diphenylmethane. Reaction conditions: diphenylmethane (10 mmol),
catalyst (8 mol%), 90 °C, O2 (8 atm), 36 h.
absorption analysis shows that there were no palladium and react under an argon atmosphere (8 atm), the detected product
vanadium ions in the filtrate of the reaction system, which is in trace amount, indicating that the oxygen atom of benzylic
further proves that the catalytic system using 1 is heterogeneous. ketone does not directly come from POM. By comparison, the
Moreover, the recycle experimental results display that after conversion of diphenylmethane reaches 22.5% under oxygen
three runs no obvious loss of activity was observed (Fig. 3)
atmosphere (8 atm, Table 2, entry 8). According to the results,
The oxygen atom of benzylic ketone has two possible we speculate that molecular oxygen is first activated by Pd-
sources from decavanadate or molecular oxygen. When catalyst POVs and then diphenylmethane is oxidized to benzophenone.
1 (10 mmol) and diphenylmethane (10 mmol) were mixed to In addition, we found that the catalytic activity of Pd-POVs
increases with the ratio of the Pd complex to [V10O28
]
(Fig. S11†). For example, the turnover number (TON) of 1 con-
taining three Pd centers is higher than that of 2 and 3. The
yield of benzophenone (22.2%) using 1 outperforms many
reported heterogeneous catalysts, such as ZJU-1827 and Pd@N-
doped carbon.15
Next, according to the previous investigations,28 we con-
sider that a radical process might be involved in the reaction.
To prove our speculation, Ph2NH as an oxygen-radical scaven-
ger was chosen and added in the oxidation of diphenyl-
methane catalyzed by compound 1. The results show that
Ph2NH could effectively retard the oxidation reaction under
standard reaction conditions and when 15 mmol Ph2NH was
Fig. 1 The crystal structure of compound 4. V, lavender; C, gray-40%;
O, red; N, blue, H, white.
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In preliminary experiments, tertralin oxidation proceeds with
19.5% conversion (Table 3, entry 2) with 2,3-dihydronaphtha-
lene-1,4-dione and α-tetralol as by-products. Moderate catalytic
activities for the oxidation of ethylbenzene (conv. 16.5%, sele.
89.2%) (Table 3, entry 3) and substituted ethylbenzene (conv.
13.7–17.6%, sele. 85.5–88.6%) (Table 3, entries 4–5) were
observed under the same reaction conditions. The different
activities observed for the substrates are mainly attributed to
the different C–H bond reaction activities.33–35
Scheme 3 The possible mechanism for the aerobic oxidation of diphe-
nylmethane to benzophenone using Pd-POVs as catalysts.
Conclusions
added into the reactor, the conversion of diphenylmethane
almost drops to zero, instead of 22.5% in the absence of
radical scavengers. Based on the above results and the
reported literature,29 a possible mechanism for the aerobic oxi-
dation of diphenylmethane to benzophenone is proposed. As
shown in Scheme 3, first molecular oxygen could be activated
by Pd and POMs to form an active species (•O2−).30–32
Subsequently, the formed O2 oxidizes diphenylmethane a to
produce the intermediate diphenylmethyl hydroperoxide b,
which transforms to diphenylmethanol c or benzophenone d.
In conclusion, compounds 1–3 with Pd-complexes as the
cation and [V10O28]6− as the anion were synthesized and inter-
estingly the molar ratio of Pd to decavanadate in the three
compounds is controlled by using different N-containing
ligands. The three Pd-POVs are active in the aerobic oxidation
of diphenylmethane to benzophenone under solvent-free con-
ditions and we found that compound 1 with the highest ratio
of Pd to [V10O28]6− can effectively catalyze the oxidation of a
series of benzylic hydrocarbons in a heterogeneous system and
its activity is basically maintained after three cycles.
Investigations on using these Pd-POV catalysts for other cata-
lytic reactions are in progress.
•
−
Moreover, the formed diphenylmethanol c could be further
−
oxidized to benzophenone d by O2
.
After the preliminary optimization (Table 2, Fig. S12 and
S13†), the solvent-free aerobic oxidation of other benzylic sub-
strates using the Pd-POV catalyst 1 was examined. As shown in
Table 3, the oxidations of different benzylic substrates to the
corresponding products were performed with high selectivity.
Conflicts of interest
There are no conflicts to declare.
Table 3 The aerobic oxidation of benzylic compounds catalyzed by 1a
Acknowledgements
Entry
1
Substrates
Products
Conv. (%)
22.5
Sele.b (%)
98.6
This work was financially supported by the National Natural
Science Foundation of China (No. 21401094, 21402079,
21671019, 21231002, 21371024 and 21771020), the 973
Program (2014CB932103) and the Program for Scientific
Research Innovation Team in Colleges and Universities of
Shandong Province.
2
19.5
80.1
17.5
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