Application of group v polyoxometalate as an efficient base catalyst: A case study of decaniobate clusters

Article abstract of DOI:10.1039/c6ra00338a

The base catalytic activity of the decaniobate cluster (TMA)₆[Nb₁₀O₂₈]·6H₂O (TMA⁺ = tetramethylammonium cation) was studied theoretically and experimentally. Density functional theory calculations showed that the oxygen atoms in the cluster are highly negatively charged and suggested the possibility that the cluster can act as a base catalyst. We demonstrated for the first time that [Nb₁₀O₂₈]^6- actually exhibits base catalytic activity for aldol-type condensation reactions including Knoevenagel and Claisen-Schmidt condensation reactions. The catalytic reactions proceeded under ambient conditions, suggesting that [Nb₁₀O₂₈]^6- holds promise as a practical strong base catalyst.

Full text of DOI:10.1039/c6ra00338a

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Application of group V polyoxometalate as an
efficient base catalyst: a case study of decaniobate
Cite this: RSC Adv., 2016, 6, 16239
clusters†
ab
Shun Hayashi,a Seiji Yamazoe,ab Kiichirou Koyasuab and Tatsuya Tsukuda
Received 6th January 2016
Accepted 1st February 2016
*
DOI: 10.1039/c6ra00338a
www.rsc.org/advances
The base catalytic activity of the decaniobate cluster (TMA)6[Nb10O28]$ (M ¼ V, Nb, Ta) are more promising than group VI POM (M ¼ Mo,
6H2O (TMA+ ¼ tetramethylammonium cation) was studied theoreti- W) for base catalysts. For example, polyoxoniobates (PONbs)10,11
cally and experimentally. Density functional theory calculations such as hexaniobate ([Nb6O19]8ꢀ) and decaniobate ([Nb10O28]6ꢀ
)
showed that the oxygen atoms in the cluster are highly negatively cluster have more negative ꢀn/y values (ꢀ0.42 and ꢀ0.21,
charged and suggested the possibility that the cluster can act as a base respectively) than polyoxotungstates (POWs) such as [HGeW10-
6ꢀ
catalyst. We demonstrated for the first time that [Nb10O28
]
actually O36]7ꢀ (ꢀ0.19), [W10O32]4ꢀ (ꢀ0.13) and [W6O19]2ꢀ (ꢀ0.11);
exhibits base catalytic activity for aldol-type condensation reactions although, [WO4]2ꢀ has a much more negative value of ꢀ0.50.
including Knoevenagel and Claisen–Schmidt condensation reactions.
In order to test the hypothesis, we studied in this work
The catalytic reactions proceeded under ambient conditions, sug- homogeneous base catalysis of PONb. As an initial target, we
8ꢀ
holds promise as a practical strong base focused on [Nb10O28]6ꢀ rather than [Nb6O19
]
although the
6ꢀ
gesting that [Nb10O28
]
catalyst.
latter has more negative ꢀn/y value. This is simply because the
crystal structure of tetramethylammonium (TMA) salt, that can
be dispersed in organic solvent, have been reported only for
1. Introduction
6ꢀ
12
[Nb10O28
]
.
In this communication, we theoretically
6ꢀ with
A polyoxometalate (POM),1 [MxOy]nꢀ, is an anionic metal-oxide
cluster that consists of a given number of [MO6] octahedral
units by sharing edges or corners. POMs have been widely used
as catalysts, especially for acid-catalyzed, oxidation, and pho-
tocatalytic reactions.2–4 Recently, Mizuno and coworkers re-
ported that [WO4]2ꢀ and [g-HGeW10O36]7ꢀ acted as base
catalysts for CO2 xation5–7 and Knoevenagel condensation
reaction,8,9 respectively. The base catalysis has been ascribed to
the negativity of the oxygen atoms, mainly due to the high
charge densities of the cluster.5 These studies have motivated us
to develop stronger base catalysts based on POMs.
Our simple working hypothesis is that POM having more
electronegative oxygen will exhibit stronger base catalysis. The
ꢀn/y value of [MxOy]nꢀ represents a negative charge averaged over
the oxygen atoms without assuming electronic charge transfer
from metal to oxygen and thus provides a lower limit of the
negativity of the oxygen atom. On the assumption that the ꢀn/y
value gives a measure of the basicity of the POMs, group V POMs
compared negative charges of oxygen atoms of [Nb10O28
]
those of POWs. Then, we applied (TMA)6[Nb10O28]$6H2O as
homogeneous catalyst for aldol-type condensation reactions
such as Knoevenagel and Claisen–Schmidt condensation reac-
tions and revealed for the rst time that it acted as a Brønsted
base catalyst whose strength is comparable to superbases. This
result will open up a new possibility of application of PONbs
since their catalytic application has been limited to electro-
catalysts13 and photocatalysts so far.14–17
2. Results and discussion
The structure of [Nb10O28]6ꢀ was optimized by density func-
tional theory (DFT) calculations. The structure obtained by
optimization (Table S1†)18 reproduced that determined experi-
mentally by single-crystal X-ray diffraction (XRD)12 except that
bond lengths were elongated by <3% relative to the experi-
mental values. The localized charge of the oxygen atoms ob-
tained by natural bond orbital (NBO) analysis is shown in Fig. 1;
this gives a measure of the basicity of the individual oxygen
atoms. The NBO charges of the oxygen atoms are much more
negative than ꢀ0.21, which is the lower limit of the negativity
without assuming electronic charge transfer between Nb and O.
This result indicates electronic charge transfer from Nb to O
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: tsukuda@chem.s.u-tokyo.ac.jp
bElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
Katsura, Kyoto 615-8520, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra00338a
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 16239–16242 | 16239

View Article Online
RSC Advances
Communication
Fig. 1 Optimized structure and NBO charges of oxygen atoms in
6ꢀ
[Nb10O28
]
(D2h).
atoms. Fig. 1 suggests that the four edge-sharing oxygen atoms
6ꢀ
of [Nb10O28
reaction.
]
are the most active sites for base catalytic
Fig. 3 Powder XRD pattern of the product and simulated pattern from
reported crystal structure.12
The electronic charge of the most negative oxygen atom in
6ꢀ
[Nb10O28
]
(ꢀ0.873) was compared with those of other typical
polyoxotungstates such as [W10O32
]
4ꢀ, [W6O19 2ꢀ, and [WO4]2ꢀ
]
(ref. 5) obtained from the same level of calculation (Fig. 2). The
charge of the most negative oxygen atom in [Nb10O28]6ꢀ
(ꢀ0.873) was much more negative than those of [W10O32]4ꢀ
(ꢀ0.753) and [W6O19]2ꢀ (ꢀ0.721), but was slightly less negative
than that of [WO4]2ꢀ (ꢀ0.934). This comparison suggests that
6ꢀ
[Nb10O28
]
will show base catalysis as in the case of POWs re-
ported so far.5–9
6ꢀ
To test the base catalytic activity of [Nb10O28
]
cluster
dispersed in organic solvent, we synthesized the TMA salt,
(TMA)6[Nb10O28]$6H2O, according to the reported method.12
The synthesized product was characterized by powder XRD,
negative-ion electrospray ionization mass spectrometry (ESI-
MS), and elemental analysis. The powder XRD pattern of the
product agreed well with that of (TMA)6[Nb10O28]$6H2O re-
ported previously (Fig. 3).12 The ESI-MS of the product in water–
MeOH (1 : 1) exhibited a series of mass peaks assigned to
Fig. 4 Negative-ion ESI-MS of (TMA)6[Nb10O28]$6H2O.
the reaction is proton abstraction from donors by base catalysts.
We studied whether (TMA)6[Nb10O28]$6H2O catalyzed the
coupling reaction between benzaldehyde (1) and various nitriles
having different pKa values. The results of the catalytic reactions
[TMAxH(5ꢀx)Nb10O28
]
ꢀ (x ¼ 2–5) (Fig. 4). The elemental analysis
of C, H, N, and Nb agreed with the calculated values for
[(CH3)4N]6[Nb10O28]$6H2O to an accuracy of <0.5% (calcd: C,
14.94; H, 4.36; N, 4.36; Nb, 48.2. Found: C, 14.75; H, 4.38; N,
4.27; Nb, 48.7). These results show that (TMA)6[Nb10O28]$6H2O
was successfully synthesized.
We applied (TMA)6[Nb10O28]$6H2O as a homogeneous cata-
lyst for the Knoevenagel condensation reaction. This is
a fundamental coupling reaction to form carbon–carbon double
bonds between active methylene compounds such as nitriles
(donors) and carbonyl compounds (acceptors). The key step in
are summarized in Table 1. [Nb10O28]
6ꢀ cluster afforded in good
yields coupling products with nitriles 2a and 2b (entries 1 and 2)
whose pKa values are 12.3 and 16.0 even at mild temperature T
¼ 313 K (entries 1-1 and 2-1). Negative-ion ESI-MS of the catalyst
aer the reaction (enꢀtry 2-1) exhibited a series of mass peaks of
[TMAxH(5ꢀx)Nb10O28
]
(x ¼ 1–3) (Fig. S1†),18 suggesting that the
catalysts did not decompose. Turnover frequencies for these
reactions (entries 1-2 and 2-2) were calculated to be 66 and 28
hꢀ1 at 343 K, respectively, from the kinetic data (Fig. S2†).18
[Nb10O28]
6ꢀ cluster showed base catalytic activity for nitriles 2c
(pKa ¼ 19.4, entry 3), 2d (pKa ¼ 19.5, entry 4), 2e (pKa ¼ 21.9,
entry 5) and 2f (pKa ¼ 23.8, entry 6), although the yields grad-
ually decreased with an increase in the pKa values of the nitriles.
However, [Nb10O28]6ꢀ cluster did not show any activity for the
reaction of 2g (pKa ¼ 32.5, entry 7).
Catalytic activity of (TMA)6[Nb10O28]$6H2O for Claisen–
Schmidt condensation reaction between 1 and acetophenone
(2h, pKa ¼ 24.7) was also studied. As shown in Scheme 1,
6ꢀ
[Nb10O28
]
also acted as a homogeneous base catalyst for
Claisen–Schmidt condensation.
Although the yields of the reactions with 2f (pKa ¼ 23.8) and
2h (pKa ¼ 24.7) were low compared to typical base catalysts such
Fig. 2 NBO charges of the most negative oxygen atoms.
16240 | RSC Adv., 2016, 6, 16239–16242
This journal is © The Royal Society of Chemistry 2016

View Article Online
Communication
RSC Advances
Table 1 Knoevenagel condensation catalyzed by (TMA)6[Nb10O28]$
showed base catalysis even when it was used in non-hydrated
solvent under ambient conditions. This novel feature suggests
that the PONbs hold promise as a practical strong base catalyst.
6H2Oa
3. Conclusions
We have reported for the rst time that decaniobate cluster
[Nb10O28]
6ꢀ acts as a strong and homogeneous base catalyst for
Entry
Donor
pKa
Temp. (K)
Yield (%)
Knoevenagel condensation reaction of benzaldehyde and p-
methoxyphenylacetonitrile (pKa ¼ 23.8) and Claisen–Schmidt
condensation reaction of benzaldehyde and acetophenone (pKa
¼ 24.7). This high catalytic activity is associated with the large
negative charge on the oxygen sites (ꢀ0.79 to ꢀ0.87) originating
mainly from the large total negative charge on the cluster.
1-1
1-2
12.3
313
343
61
88
2-1
2-2
16.0
313
343
77
88
4. Experiment and theory
4.1. Density functional theory calculations
3
19.4
19.5
343
343
61
39
DFT calculations were conducted using the Gaussian 09
program.22 The structure of [Nb10O28]6ꢀ was optimized at the
level of B3LYP/6-31G++(d). The absence of imaginary frequen-
cies was conrmed from vibrational analysis. Natural bond
orbital (NBO) analysis was conducted to obtain localized charge
on the oxygen atoms.
4
5
6
21.9
23.8
32.5
343
343
343
35
8
4.2. Chemicals
Niobic acid was supplied from CBMM (Nb2O5$nH2O, water 20%
w/w). Tetramethylammonium hydroxide pentahydrate (TCI,
97%), benzaldehyde (Wako, 98%), 4-nitrobenzyl cyanide (TCI,
98%), 4-cyanophenylacetonitrile (Aldrich, 97%), 3-bromobenzyl
cyanide (TCI, 98%), m-chlorobenzyl cyanide (Wako, 98%),
phenylacetonitrile (Wako, 98%), 4-methoxyphenylacetonitrile
(Aldrich, 97%), propionitrile (Wako, 98%), and acetophenone
(Wako, 98.5%) were used without further purication. Deion-
ized water (Milli-Q, >18 MU cm) was used in all experiments.
7
0
a
Reaction conditions: catalyst (1 mol% with respect to 1), 1 (1.0 mmol),
2 (1.0 mmol), MeOH 1 mL, 343 K, 24 h.
4.3. Synthesis
(TMA)6[Nb10O28]$6H2O was synthesized according to the liter-
ature procedure.12 In an autoclave, niobic acid (Nb2O5$nH2O,
water 20% w/w, 3.00 g, 9.02 mmol) was mixed with 30 mL of
EtOH solution of tetramethylammonium hydroxide (3.60 g, 19.9
Scheme
1 Claisen–Schmidt condensation catalyzed by (TMA)6-
[Nb10O28]$6H2O. aReaction conditions: catalyst (1 mol% with respect
to 1), 1 (1.0 mmol), 2 (1.0 mmol), MeOH 1 mL, 343 K, 24 h.
ꢁ
mmol). The autoclave was heated at 120 C for 18 h. Aer the
reaction, the orange solution was removed and a white precip-
itate was collected by centrifugation. The precipitate was
washed with EtOH (150 mL) and acetone (150 mL) and dried
well. The crude product was dissolved into 50 mL of MeOH and
centrifuged to remove unreacted niobic acid. The solution was
concentrated into 15 mL by evaporation and separated into
three vials. Recrystallization was conducted by adding 20 mL of
acetone slowly to each solution to form a second layer. Typical
yield was 2.01 g (1.04 mmol, 58% based on Nb).
6ꢀ
as hydroxide and alkoxide,19,20 the fact that [Nb10O28
]
cluster
could activate less acidic nitriles indicated its strong basicity. To
the best of our knowledge, [Nb10O28]6ꢀ cluster is the rst
example of a strong base catalyst that can activate 2f and 2h
among various POMs. The base strength of [Nb10O28]6ꢀ is as
high as 24.7 if we assume that the base strength of a catalyst is
comparable to the pKa values of methylene groups from which
protons can be abstracted. This value is comparable to those of
superbases, whose base strengths are dened to be larger than
26. However, those superbases are generally very sensitive to
water and CO2 and exhibit strong basicities only under rigor-
ously controlled conditions.21 In contrast, [Nb10O28]6ꢀ cluster
4.4. Characterization
The powder XRD pattern was collected with a Rigaku SmartLab
3 using nickel-ltered Cu-Ka radiation. Negative-ion ESI-MS was
measured with a JEOL JMS-T100LP AccuTOF LC-plus. (TMA)6-
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 16239–16242 | 16241

View Article Online
RSC Advances
Communication
[Nb10O28]$6H2O was dissolved into water–MeOH (1 : 1, 1 mg
mLꢀ1) and electrosprayed at a bias voltage of ꢀ2 kV. The mass
3 I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171.
4 N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199.
5 T. Kimura, K. Kamata and N. Mizuno, Angew. Chem., Int. Ed.,
2012, 51, 6700.
6 T. Kimura, H. Sunaba, K. Kamata and N. Mizuno, Inorg.
Chem., 2012, 51, 13001.
ꢀ
spectrum was calibrated by using CsnI(n+1) clusters generated
from CsI solution as a reference. Elemental analysis for C, H,
and N was conducted with an Elementar vario MICRO cube.
ICP-AES analysis for Nb was performed with a Thermo Scientic
iCAP DUO-6300 at a wavelength of 309.4 nm. GC analysis was
conducted with a Shimadzu GC-2014 with a Restek Rtx-1
(internal diameter ¼ 0.53 mm, length ¼ 30 m) capillary
column, and GC-MS analysis was conducted with a Shimadzu
GCMS-QP2010 Ultra with a Restek Rtx-1 (internal diameter ¼
0.32 mm, length ¼ 30 m) capillary column at an ionization
voltage of 70 eV.
7 K. Kamata, T. Kimura, H. Sunaba and N. Mizuno, Catal.
Today, 2014, 226, 160.
8 K. Sugahara, T. Kimura, K. Kamata, K. Yamaguchi and
N. Mizuno, Chem. Commun., 2012, 48, 8422.
9 K. Sugahara, N. Satake, K. Kamata, T. Nakajima and
N. Mizuno, Angew. Chem., Int. Ed., 2014, 53, 13248.
10 M. Nyman, Dalton Trans., 2011, 40, 8049.
11 H. Wu, Z. Zhang, Y. Li, X. Wang and E. Wang,
CrystEngComm, 2015, 17, 6261.
4.5. Catalytic test
12 C. A. Ohlin, E. M. Villa and W. H. Casey, Inorg. Chim. Acta,
2009, 362, 1391.
13 Y. Ye, C. Chen, H. Feng, J. Zhou, J. Ma, J. Chen, J. Yuan,
L. Kong and Z. Qian, Open J. Inorg. Chem., 2013, 3, 59.
14 L. Shen, Y. Xu, Y. Gao, F. Cui and C. Hu, J. Mol. Struct., 2009,
934, 37.
Benzaldehyde (1, 1.0 mmol), nitrile or acetophenone (2, 1.0
mmol), and biphenyl (internal standard, ca. 0.1 mmol) were
dissolved into MeOH, and (TMA)6[Nb10O28]$6H2O (10 mmol, 1
mol%) was added to start the reaction. The reaction mixture was
stirred for 24 h and then analysed by GC and GC-MS.
15 Z. Zhang, Q. Lin, D. Kurunthu, T. Wu, F. Zuo, S. Zheng,
C. J. Bardeen, X. Bu and P. Feng, J. Am. Chem. Soc., 2011,
133, 6934.
16 Z. Wang, H. Tan, W. Chen, Y. Li and E. Wang, Dalton Trans.,
2012, 41, 9882.
17 P. Huang, C. Qin, Z. Su, Y. Xing, X. Wang, K. Shao, Y. Lan and
E. Wang, J. Am. Chem. Soc., 2012, 134, 14004.
18 See ESI†
19 B. A. Bhat, K. L. Dhar, S. C. Puri, A. K. Saxena,
M. Shanmugavel and G. N. Qazi, Bioorg. Med. Chem. Lett.,
2005, 15, 3177.
Acknowledgements
We thank Dr Naoko Nonose (National Institute of Advanced
Industrial Science and Technology; AIST) for preliminary ICP
analysis and Dr Noritaka Mizuno, Dr Kazuya Yamaguchi, and Dr
Kosuke Suzuki (The University of Tokyo) for fruitful discus-
sions. This research was nancially supported by the Elements
Strategy Initiative for Catalysis and Batteries (ESICB) and by
a Grant-in-Aid for Scientic Research (No. 24750210) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of Japan.
´
´
´
´
´
20 A. Ramırez-Rodrıguez, J. M. Mendez, C. C. Jimenez, F. Leon
and A. Vazquez, Synthesis, 2012, 44, 3321.
Notes and references
21 L. Chen, J. Zhao, S. Yin and C. Au, RSC Adv., 2013, 3, 3799.
22 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, et al., Gaussian 09, Revision
D.01, Gaussian, Inc., Wallingford CT, 2013.
1 D. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int.
Ed., 2010, 49, 1736.
2 C. L. Hill and C. M. Prosser-McCartha, Coord. Chem. Rev.,
1995, 143, 407.
16242 | RSC Adv., 2016, 6, 16239–16242
This journal is © The Royal Society of Chemistry 2016