Organozirconium Complex with Keggin-Type Mono-Aluminum-Substituted Silicotungstate: Synthesis, Molecular Structure, and Catalytic Performance for Meerwein–Ponndorf–Verley Reduction

Article abstract of DOI:10.1007/s10562-016-1813-7

Abstract: The organozirconium complex with α-Keggin-type mono-aluminum-substituted silicotungstate, [(n-C₄H₉)₄N]₆[α-SiW₁₁Al(OH)₂O₃₈ZrCp₂]₂·2H₂O (TBA–Si–Al–Zr) was synthesized by the reaction of Cp₂Zr(OTf)₂·THF (or Cp₂ZrCl₂) with [(n-C₄H₉)₄N]₄K_0.5H_0.5[α-SiW₁₁{Al(OH₂)}O₃₉]·H₂O in acetonitrile. This compound showed high catalytic activities for Meerwein–Ponndorf–Verley reduction of ketones with 2-propanol in both homogeneous and heterogeneous system. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1813-7

Catal Lett
DOI 10.1007/s10562-016-1813-7
Organozirconium Complex with Keggin-Type Mono-Aluminum-
Substituted Silicotungstate: Synthesis, Molecular Structure,
and Catalytic Performance for Meerwein–Ponndorf–Verley
Reduction
Chika Nozaki Kato1,2 Wataru Unno1 Sakie Kato1 Tsukasa Ogasawara1
•
•
•
•
Toshifumi Kashiwagi1 Hidemitsu Uno3 Kosuke Suzuki4 Noritaka Mizuno4
•
•
•
Received: 20 November 2015 / Accepted: 14 July 2016
Ó Springer Science+Business Media New York 2016
Abstract The organozirconium complex with a-Keggin-
type mono-aluminum-substituted silicotungstate, [(n-
C4H9)4N]6[a-SiW11Al(OH)2O38ZrCp2]2Á2H2O (TBA–Si–
Al–Zr) was synthesized by the reaction of Cp2Zr(OTf)2-
THF (or Cp2ZrCl2) with [(n-C4H9)4N]4K0.5H0.5[a-
SiW11{Al(OH2)}O39]ÁH2O in acetonitrile. This compound
showed high catalytic activities for Meerwein–Ponndorf–
Verley reduction of ketones with 2-propanol in both
homogeneous and heterogeneous system.
Graphical Abstract
Keywords Organozirconium complex Á Polyoxometalate Á
X-ray crystal structure Á Meerwein–Ponndorf–Verley
reduction Á Heterogeneous catalyst
1 Introduction
Polyoxometalates (POMs) are discrete anionic metal–
oxygen clusters that can be regarded as soluble oxide
fragments. They have attracted substantial attention in the
fields of catalysis, surface science, and materials science
because of their chemical properties, including their redox
potentials, acidities, and solubilities in various media,
which can be finely tuned by choosing the appropriate
constituent elements and countercations. In particular, the
coordination of metal ions to the vacant site(s) of lacunary
POMs is one of the most effective techniques for con-
structing efficient and well-defined active metal centers
[1–4].
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1813-7) contains supplementary
material, which is available to authorized users.
& Chika Nozaki Kato
sckatou@ipc.shizuoka.ac.jp
1
Department of Chemistry, Faculty of Science, Shizuoka
University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
2
Green Chemistry Research Division, Research Institute of
Green Science and Technology, Shizuoka University, 836
Ohya, Suruga-ku, Shizuoka 422-8529, Japan
3
Department of Chemistry and Biology, Graduate School of
Science and Engineering, Ehime University,
Matsuyama 790-8577, Japan
Several strategies for expanding POM chemistry have
been considered, and covalent grafting reactions of
organic/organometallic fragments onto the surfaces of
POMs has been presented as an effective technique to
functionalize POM-based materials [5–7]. For grafting
4
Department of Applied Chemistry, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
123

C. N. Kato et al.
reactions, complete (plenary) POMs obtained by the sub-
stitution of d0 transition metals or reduction [5–16] and
lacunary POM ligands [5–7, 17–24] have been used as
supports, and numerous organic–inorganic hybrid materials
have been applied to catalysts and functionalized materials.
Thus, the development of polyoxoanion-supported organic/
organometallic compounds has great potential to further
expand POM chemistry.
method with modifications [36]. The synthesis and char-
acterization results of TBA–Si–Al are shown in the Sup-
porting Information. H3PW12O4023H2O was obtained from
Nippon Inorganic Colour & Chemical Co., Ltd (Japan).
The number of solvated water molecules in these com-
pounds was determined by TG/DTA analysis. TBA–P–Al,
-
TBA–P–Al–Zr, and Cp2Zr(OTf)2ÁTHF (OTf = CF3SO3
)
were synthesized by previously published methods
[25, 37]. All the reagents and solvents were obtained and
used as received from commercial sources. Acetonitrile
(containing B0.001 % water) was used for the synthesis of
TBA–Si–Al–Zr.
To explore POMs that could be used as supports for
grafting reactions, we focused on the aluminum-substituted
POMs and successfully synthesized an organozirco-
nium(IV)-supported polyoxometalate, [a-PW11Al(OH)O39-
ZrCp2]62- (Cp = g5-C5H5-) (P–Al–Zr), by a grafting
reaction of a-Keggin mono-aluminum-substituted phos-
photungstate with a bis(g5-cyclopentadienyl)zirconium(IV)
fragment [25]. Although the chemistry of organometallic
moieties possessing g5-cyclopentadienyl ligands has rapidly
expanded [26–28], new studies on their grafting reactions
onto POM surfaces have not been conducted: some examples
(determined by X-ray analysis) include {[Cp2U]2(l-j2O-
TiW5O19)2}4- [29, 30], [(PW11O39NbO)2ZrCp2]6- [31, 32],
[CpTiW5O18]3- [33], [Cp3U(MW5O19)2]5-, and [Cp3Th
(MW5O19)2]5- (M = Nb, Ta) [34].
2.2 Instrumentation/Analytical Procedures
Elemental analysis was performed using a Mikroanalytis-
ches Labor Pascher (Remagen, Germany) instrument. The
samples were dried overnight at room temperature under
10-3–10-4 Torr vacuum before analysis. Infrared spectra
were recorded on a Perkin Elmer Spectrum 100 FT-IR
spectrometer, in KBr disks, at room temperature. Thermo-
gravimetric (TG) and differential thermal analysis (DTA)
data were obtained using a Rigaku Thermo Plus 2 series TG/
DTA TG 8120. TG/DTA measurements were performed in
air while increasing the temperature by 4 °C per min from 20
In this study, we focused on the use of the tetra-n-buty-
lammonium/potassium salt of a-Keggin mono-aluminum-
1
substituted
silicotungstate,
[(n-C4H9)4N]4K0.5H0.5[a-
to 500 °C. Solution H (600.17 MHz), 13C (150.92 MHz),
SiW11{Al(OH2)}O39]ÁH2O (TBA–Si–Al), as a support and
synthesized the tetra-n-butylammonium salt of the
organozirconium(IV) compound, [(n-C4H9)4N]6[a-SiW11-
Al(OH)2O38ZrCp2]2Á2H2O (TBA–Si–Al–Zr). The com-
pound TBA–Si–Al–Zr was characterized by X-ray cry
stallography, elemental analysis, TG/DTA, potentiometric
titration, FTIR, cold-spray ionization mass (CSI-MS), and
31P-{1H} (242.95 MHz), 29Si (119.23 MHz), and 27Al NMR
(156.36 MHz) spectra were recorded in 5-mm-outer diam-
eter tubes on a JEOL ECA-600 NMR spectrometer. 1H and
13C NMR spectra were measured in acetonitrile-d3 with
reference to tetramethylsilane (TMS). Chemical shifts are
reported as positive for resonances downfield of TMS (d 0).
The 31P, 29Si, and 27Al NMR spectra were measured in
acetonitrile-d3 with reference to an external standard (a
substitution method) of 85 % H3PO4 in a sealed capillary,
TMS, and a saturated AlCl3–D2O solution, respectively.
Chemical shifts for 31P and 29Si NMR spectra were reported
as negative on the d scale for resonances upfield of H3PO4 (d
0) and TMS (d 0). The chemical shifts in the 27Al NMR
spectra were reported as positive on the d scale for reso-
nances downfield of AlCl3 (d 0). When a quartz tube was
used for 27Al NMR spectroscopy, a background signal was
also observed in the spectrum. Solid-state 27Al, 13C, and 29Si
NMR spectra were recorded at 400 MHz on a JEOL JNM-
ECA 400 FT-NMR spectrometer with a JEOL ECP-400
NMR data-processing system. The chemical shifts d for 27Al
and 13C NMR spectra were reported to be positive on the d
scale for resonances upfield of AlCl3 (d 0) and the methyl
group of C6(CH3)6 (d 17.4). The 29Si NMR chemical shifts
are reported as negative relative to polydimethylsilane (d
-31.0). Cold-spray ionization mass spectra (CSI-MS) were
recorded on a JEOL JMS-T100CS (acetonitrile, 1 mg/mL;
spray temperature, -20 °C) at The University of Tokyo.
1
solution (27Al, H, and 13C) and solid-state (27Al, 13C, and
29Si) NMR spectroscopy. Furthermore, TBA–Si–Al–Zr and
[(n-C4H9)4N]6[a-PW11Al(OH)O39ZrCp2]2 (TBA–P–Al–
Zr) were used as homogeneous and heterogeneous catalysts
in the Meerwein–Ponndorf–Verley(MPV) reduction of
cyclohexanone, 2-hexanone, and cyclopentanone with
2-propanol. Herein, we report the complete details of the
synthesis and molecular structure of complex Si–Al–Zr and
demonstrate the catalytic activities of the bis(g5-cyclopen-
tadienyl)zirconium(IV) complexes with a-Keggin mono-
aluminum-substituted polyoxotungstates.
2 Experimental
2.1 Materials
K8[a-SiW11O39]Á12H2O was synthesized according to the
literature [35]. [(n-C4H9)4N]4K0.5H0.5[a-SiW11{Al(OH2)}-
O39]H2O (TBA–Si–Al) was synthesized by a published
123

Organozirconium Complex with Keggin-Type Mono-Aluminum-Substituted Silicotungstate…
Potentiometric titration was carried out with 0.010 mol/L
tetra-n-butylammonium hydroxide as a titrant under Ar
atmosphere [38]. The compound TBA–Si–Al–Zr
(0.010 mmol) was dissolved in acetonitrile (20 mL) at
25 °C, and the solution was stirred for approximately 5 min.
The titration data were obtained with a pH meter (Mettler
Toledo). Data points were obtained in millivolt. A solution of
tetra-n-butylammonium hydroxide (0.010 mol/L) was syr-
inged into the suspension in 0.10-equivalent intervals.
NMR results gave 1H NMR (CD3CN, 25.4 °C): d 6.69
(singlet) (Cp), 6.64 (singlet) (Cp), 6.63 (singlet) (Cp), 6.59
(singlet) (Cp), 3.16 (triplet), 1.65 (multiplet), 1.42 (multi-
plet), 1.00 (triplet) ([(n-C4H9)4N]?), and 2.15 (singlet)
(H2O); 13C NMR (CD3CN, 26.7 °C): d 117.1 (Cp), 117.0
(Cp), 116.8 (Cp), 116.5 (Cp), 59.44, 24.51, 20.48, and
14.04 ([(n-C4H9)4N]?); 27Al NMR (CD3CN, 21.9 °C): d
15.5. Solid NMR results gave 13C NMR: d 116.2 (singlet)
(Cp), 58.6, 24.8, 20.5, and 14.8 ([(n-C4H9)4N]?); 27Al
NMR: d 9.84; 29Si NMR: d -81.9. Positive ion CSI-MS
(CH3CN, 253 K) m/z 3910.8 and 7577.4 (calcd m/z 3910.4
[{(n-C4H9)4N}8{SiW11Al(OH)2O38ZrCp2}2]2? and 7577.4
[{(n-C4H9)4N}7{SiW11Al(OH)2O38ZrCp2}2]?).
2.3 Synthesis of [(n-C4H9)4N]6[a-SiW11Al(OH)2
O38ZrCp2]2Á2H2O (TBA–Si–Al–Zr)
The following manipulations were performed in a dry box
filled with Ar ([99.9995 vol% purity). A solution of Cp2-
Zr(OTf)2ÁTHF (0.327 g; 0.54 mmol) dissolved in 10 mL of
acetonitrile was slowly added to a solution of TBA–Si–Al
(2.0 g; 0.54 mmol) dissolved in 15 mL of acetonitrile. The
color of the solution changed to yellow. After stirring for
4 h at 25 °C, the mixed solution was filtered through a
folded filter paper (Whatman #5). At this stage, the filtrate
was removed from the dry box, and combined with water
(400 mL) under ambient atmosphere. The yellow–white
precipitate was collected using a membrane filter (JG
0.2 lm) and washed with a small amount of ethanol. The
yield of the crude product was 1.721 g. The crude product
(1.721 g) was dissolved in acetonitrile (34 mL) and then
filtered through a folded filter paper (Whatman #5). The
yellow platelet crystals were obtained by vapor diffusion
from methanol in air. The yield of the product was 0.836 g
(the percentage yield of 21.3 % was calculated based on
the following: 2[moles of TBA–Si–Al–Zr]/[moles of
TBA–Si–Al] 9 100). Elemental analysis results showed:
C, 19.02, H, 3.30; Al, 0.79; N, 1.15; Si, 0.75; W, 54.50; Zr,
2.45; K\0.01 %. Calculations for [(n-C4H9)4N]6[a-SiW11
A(OH)2O38ZrCp2]2Á2H2O = C116H244Al2N6O82Si2W22Zr2
(7372.267): C, 18.90, H, 3.34; Al, 0.73; N, 1.14; Si, 0.76;
W, 54.86; Zr, 2.47; K 0 %. A weight loss of 0.18 % was
observed during drying overnight at room temperature,
under a vacuum of 10-3–10-4 Torr prior to analysis,
suggesting the removal of weakly solvated or adsorbed
acetonitrile molecules; this was also supported by the
absence of any signal attributable to acetonitrile molecules
in the 1H NMR spectrum of the sample acquired in DMSO-
d6 after drying overnight. TG/DTA performed under
atmospheric conditions showed a weight loss of 24.74 %
with exothermic points at 352.0 and 483.0 °C (observed
below 500 °C). No clear endothermic point was observed.
The calculated weight loss of 25.4 % was consistent with
the loss of four Cp ligands, six tetra-n-butylammonium
ions, and two water molecules. IR (KBr disk) in the
1300–400 cm-1 region (polyoxometalate region): 1012,
969, 921, 885, 799, 756, 733 s, and 669 m cm-1. Solution
2.4 X-ray Crystallography
A yellow platelet-shaped crystal of TBA–Si–Al–Zr
(0.140 9 0.090 9 0.040 mm) was mounted on a Micro-
Mount. Data were collected on a Rigaku VariMax instru-
ment with Saturn connected to a multi-layer mirror using
˚
monochromated Mo Ka radiation (k = 0.71075 A) at
154 1 K. Data were collected and processed using the
CrystalClear for Windows software. The structural analysis
was performed using the CrystalStructure for Windows
software. The structure was solved using SHELXS-97
(direct methods) and refined using SHELXL-97 [39]. For
the polyoxoanion Si–Al–Zr, 22 W atoms, 2 Zr atoms, 2 Si
atoms, 2 Al atoms, and 80 O atoms were clearly identified,
thereby clarifying the main features of the molecular
structure of the polyoxometalate. The six tetra-n-butylam-
monium ions and two water molecules could not be mod-
eled with disordered atoms. Accordingly, the residual
electron density was removed using the SQUEEZE routine
in PLATON [40].
2.5 Crystal Data for TBA–Si–Al–Zr
C116H244Al2N6O82Si2W22Zr2; M = 7372.47, monoclinic,
space group C2/c (#15), a = 25.252(8), b = 17.619(6),
3
˚
˚
c = 46.77(2) A, V = 20807(11) A , Z = 4, Dc = 2.353 g/
cm3, l(Mo-Ka) 123.080 cm-1. R1 = 0.0721 (I [ 2r(I)),
wR2 = 0.2308 (for all data). GOF = 1.126 (100,607 total
reflections, 21,524 unique reflections where I [ 2r(I)).
Some acetonitrile molecules were observed in a single
crystal of TBA–Si–Al–Zr; however, no acetonitrile solvent
molecules were observed by elemental analysis or 1H NMR
spectroscopy. Thus, the solvent molecules evaporate
gradually when crystals are removed from acetonitrile
solution. CCDC reference number 967942. The data for
TBA–Si–Al–Zr can be obtained free of charge at www.
ccdc.cam.ac.uk/conts/retrieving.html [or from Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
123

C. N. Kato et al.
CB2 1EZ, UK; Fax: ?44-1223-336-033; E-mail:
quite stable toward moisture; in contrast, the quality of
TBA–P–Al–Zr crystals degrades gradually upon exposure
to moisture. Thus, the internal Si and P atoms in poly-
oxoanions markedly affected the reactivity with zirconocene
dichloride and the resulting stability toward moisture.
Elemental analysis of TBA–Si–Al–Zr dried overnight at
room temperature under 10-3–10-4 Torr of vacuum prior
to analysis was consistent with the composition [(n-
C4H9)4N]6[a-SiW11Al(OH)2O38ZrCp2]2Á2H2O. The weight
loss observed during the drying of TBA–Si–Al–Zr prior to
analysis was 0.18 %, suggesting the absence of solvent
deposite@ccdc.cam.ac.uk.]
2.6 MPV Reduction
For the MPV reduction of cyclohexanone with 2-propanol
in a homogeneous system, a sample catalyst was placed in
a 60 mL Schlenk tube under Ar atmosphere. Cyclohex-
anone (1.9 mmol), 2-propanol (13 mmol), and acetonitrile
(4 mL) were added using a micropipette. In a heteroge-
neous system, a mixture of catalyst, ketones (cyclohex-
anone, cyclopentanone, and 2-hexanone; 1.6–2.2 mmol),
and 2-propanol (78 mmol) was placed in a Schlenk tube
under Ar atmosphere. The reaction mixture was heated in
an oil bath at 80 2 °C. The reaction solution was ana-
lyzed by gas chromatography (flame ionization detector, a
capillary column DB-WAX, 0.53 mm 9 15 m and a glass
column BX-10, 3.2 mm 9 3 m) and 1H NMR spec-
troscopy. Values of the products were assigned by com-
paring the obtained results with the analysis results
obtained from analyzing the authentic samples under the
same conditions. The conversion (%) and turnover number
(TON) was calculated as {[mol of substrate]0 - [mol of
substrate]t}/[mol of substrate]0 9 100 and [mol of corre-
sponding alcohol]t/[mol of catalyst], respectively.
1
molecules. This suggestion was also supported by the H
NMR spectrum, as shown in the Experimental section. A
weight loss of 25.4 % with exothermic points at 352.0 and
483.0 °C was observed from 17.5 to 500 °C in the TG/
DTA performed under atmospheric conditions; calculations
based on these data indicated the loss of six [(n-C4H9)4N]?
(calcd. 19.7 %) and four cyclopentadienyl groups (calcd.
3.5 %) (total: 23.2 %).
X-ray structural analysis revealed that the polyoxoanion
Si–Al–Zr is dimeric, containing two {SiW11AlO40} units
bridged by two ‘‘bent sandwich’’ Cp2Zr2? fragments, as
shown in Fig. 1. Selected bond distances and angles around
the Zr center in Si–Al–Zr are summarized in Table 1, and the
bond valence sum (BVS) calculations of the W, Si, Al, and O
atoms (Table S1) are provided in the Supporting Informa-
tion. Each Zr center was bound to the terminal O atoms of the
Al and W sites and to the edge-sharing O atom of the Al–O–
W linkage with 5-coordination geometry. The molecular
structure of Si–Al–Zr was quite similar to that of P–Al–Zr;
3 Results and Discussion
3.1 Synthesis and molecular structure of [(n-
C4H9)4N]6[a-SiW11Al(OH)2O38ZrCp2]2Á2H2O
(TBA–Si–Al–Zr)
˚
however, the AlÁÁÁAl (5.47 A) distance was larger than that of
˚
the polyoxoanion P–Al–Zr (5.32 A), and the angles of the
Al–O–Zr bond (109.7°), the O–Zr–O bond (73.5°), and the
W–O–Zr bond (163.6°) were larger than those of the poly-
oxoanion P–Al–Zr (105.3°, 69.1°, and 159.7°, respectively).
Thus, the two {SiW11AlO40} units were displaced from each
other relative to those of the polyoxoanion P–Al–Zr.
The bis(g5-cyclopentadienyl)zirconium(IV) compound with
a-Keggin mono-aluminum-substituted silicotungstate Si–
Al–Zr was obtained in 21.3 % yield based on the tetra-n-
butylammonium salt [(n-C4H9)4N]6[a-SiW11Al(OH)2O38-
ZrCp2]2Á2H2O (TBA–Si–Al–Zr). This compound was pre-
pared by the 1:1 stoichiometric reaction of [(n-
C4H9)4N]4K0.5H0.5[a-SiW11{Al(OH2)}O39]ÁH2O (TBA–
Si–Al) with Cp2Zr(OTf)2ÁTHF in acetonitrile under an Ar
atmosphere, followed by precipitation from water under
ambient atmosphere. Finally, yellow crystalline products
were obtained by crystallization via vapor diffusion from
acetonitrile/methanol at 25 °C in air. When Cp2Zr(OTf)2-
THF was added to the acetonitrile solution of H3PW12O40-
23H2O, a white precipitate formed and it was insoluble in
acetonitrile. It was noted that polyoxoanion Si–Al–Zr was
formed by the direct reaction of TBA–Si–Al with Cp2ZrCl2
in air, whereas the polyoxoanion P–Al–Zr was not obtained
by a direct reaction of [a-PW11{Al(OH2)}O39]4- (P–Al)
with zirconocene dichloride. In addition, single crystals
suitable for X-ray crystallography of TBA–Si–Al–Zr were
The values of the bond valence sums (BVSs) for TBA–
Si–Al–Zr [41–44], which were calculated based on the
observed bond lengths, were in the range of 5.615–6.107
(average 5.861) for the 11 W atoms, 4.067 for the one Si
atom, 2.712 for the one Al atom, and in the range of
1.623–2.063 (average 1.842) for the 38 O atoms, excluding
O(18) and O(21); these values were in reasonable agree-
ment with the formal valences of W6?, Si4?, Al3?, and
O2-, respectively (Table S1). The calculated BVS values
of O(18) and O(21) at the Al–O–Zr and Al–O–W linkages
were 1.033 and 1.249, respectively, suggesting that a pair
of protons were bound to each of the two bridging O atoms
between the Al and Zr atoms, and another pair of protons
was bound to the two bridging O atoms between the Al and
W atoms. From the potentiometric titration, three break
points at 2, 3, and 4 equivalents of added base were
123

Organozirconium Complex with Keggin-Type Mono-Aluminum-Substituted Silicotungstate…
Fig. 1 Molecular structure of
polyoxoanion Si–Al–Zr with
atom numberings
Table 1 Selected bond distances and angles around zirconium and
aluminum sites in TBA–Si–Al–Zr
formation of Zr–O linkages; this was observed at 631 cm-1
for TBA–P–Al–Zr [25]. A band assigned to the Cp ligands
was clearly observed at 3121 cm-1. These results sug-
gested that the g5-cyclopentadienylzirconium fragments
were coordinated to the [a-SiW11{Al(OH2)}O39]5-
surface.
˚
Distances (A)
Al(1)–O(4)
1.865 (11)
W(1)–O(1)
W(1)–O(2)
W(1)–O(4)
W(1)–O(5)
W(1)–O(6)
W(1)–O(7)
W(4)–O(17)
W(4)–O(19)
W(4)–O(20)
W(4)–O(22)
W(4)–O(30)
W(4)–O(31)
1.748 (10)
1.919 (12)
1.831 (10)
2.035 (9)
Al(1)–O(17)
2.035 (10)
1.873 (9)
1.948 (10)
1.903 (11)
1.885 (11)
2.230 (10)
2.164 (10)
2.239 (9)
1.635 (10)
7.950 (3)
5.465 (5)
2.273
Al(1)–O(18)
The cold-spray ionization mass (CSI-MS) spectrum of
TBA–Si–Al–Zr in acetonitrile exhibited peaks at m/
z = 3910.8 and 7577.4, as shown in Fig. 3. These peaks
were assigned to [{(n-C4H9)4N}8{SiW11Al(OH)2O38Zr
Al(1)–O(20)
Al(1)–O(21)
1.949 (11)
2.060 (10)
2.397 (9)
Al(1)–O(29)
Zr(1)–O(1)
Cp2}2]2?
and
[{(n-C4H9)4N}7{SiW11Al(OH)2O38Zr
Zr(1)–O(18)a
Zr(1)–O(20)a
Si(1)–O(17)
1.930 (11)
1.933 (8)
Cp2}2]?, respectively, clearly indicating that the dimeric
structure of Si–Al–Zr was maintained in acetonitrile
solution.
1.715 (11)
1.916 (10)
1.959 (10)
Zr(1)ÁÁÁZr(1)
Al(1)ÁÁÁAl(1)
Zr(1)–centroid Cp(1–5)
The solution 27Al NMR spectra of TBA–Si–Al–Zr and
TBA–Si–Al acquired at 25 °C in acetonitrile-d3 are shown
in Figs. 4 and S2. TBA–Si–Al–Zr showed a signal at
15.5 ppm, which was shifted relative to that of TBA–Si–Al
(d 16.0). The solid-state 27Al NMR spectrum of TBA–Si–
Al–Zr also showed a signal at 9.8 ppm, as shown in Fig-
ure S3. These results indicated that the g5-cyclopentadi-
enylzirconium fragment was grafted on the surface of Si–
Al and that the two Al sites in the {SiW11AlO40} unit were
equivalent in both the solution and solid phases. The
solution 29Si NMR spectrum was not obtained because of
the low solubility of TBA–Si–Al–Zr in acetonitrile. The
solid-state 29Si NMR spectrum of TBA–Si–Al–Zr was
quite noisy; however, a signal was observed at -81.9 ppm
(Figure S4). This also confirmed that the two {SiW11-
AlO40} units were equivalent.
Zr(1)–centroid Cp(6–10)
2.264
Angles (°)
Al(1)–O(18)–Zr(1)a
O(1)–Zr(1)–O(18)a
O(18)–Al(1)–O(20)
109.7 (5) W(1)–O(1)–Zr(1) 163.5 (6)
73.5 (4) W(1)–O(4)–Al(1) 145.3 (6)
79.1 (4) O(1)–W(1)–O(4) 100.7 (5)
Cp(1–5)–Zr(1)–Cp(6–10) 128.92
a
Symmetry operator: (1) -X?1/2?1, -Y?1/2?1, -Z?1
observed, as shown in Figure S1. This result also supported
the presence of four protons in Si–Al–Zr.
The FTIR spectra acquired in the POM region of TBA–
Si–Al–Zr and TBA–Si–Al from KBr disks of the com-
pounds are presented in Fig. 2. The IR spectrum of TBA–
Si–Al–Zr (1012, 969, 921, 885, 799, 756, 733, and
669 cm-1) was different from that of TBA–Si–Al (1006,
960, 916, 880, 799, and 740 cm-1). Specifically, a new
band appeared at 669 cm-1, probably because of the
The 1H NMR spectrum (Fig. 5a) of TBA–Si–Al–Zr
obtained at *25 °C in acetonitrile-d3 showed four signals
at 6.69, 6.64, 6.63, and 6.59 ppm, with an integrated
intensity ratio of approximately 1:1:1:1; these peaks were
shifted relative to those of Cp2Zr(OTf)2ÁTHF (d 6.52). No
123

C. N. Kato et al.
background
(a)
16.0
(b)
background
15.5
ppm
Fig. 4 27Al NMR spectra in acetonitrile-d3 of a TBA–Si–Al and
b TBA–Si–Al–Zr
Fig. 2 FTIR spectra in the polyoxoanion region (1800–400 cm-1), as
acquired from KBr disk of a TBA–Si–Al and b TBA–Si–Al–Zr
Fig. 3 CSI-MS spectrum of the acetonitrile solution of TBA–Si–Al–
Zr. Inset spectra in the range of m/z 3890–3930 and 7550–7620, and
the calculated patterns for [{(n-C4H9)4N}8{SiW11Al(OH)2O38Zr
Cp2}2]2? (m/z 3910.8) and [{(n-C4H9)4N}7{SiW11Al(OH)2O38Zr
Cp2}2]? (m/z 7577.4)
signal attributable to the Cp2Zr2? (d 6.44) formed by the
reaction of Cp2ZrCl2 with AgBF4 was observed in ace-
tonitrile-d3, suggesting that the Cp2Zr2? fragments were
not eliminated from the surface of [a-SiW11{Al(OH2)}-
O39]5- (Si–Al). Four signals were observed in the 13C
NMR spectrum at 117.1, 117.0, 116.8, and 116.5 ppm
123

Organozirconium Complex with Keggin-Type Mono-Aluminum-Substituted Silicotungstate…
(Fig. 5b). In contrast, the solid-state 13C NMR spectrum of
TBA–Si–Al–Zr showed a signal at 116.2 ppm that was
ascribed to Cp ligands, as shown in Fig. 6. Thus, the four
[(n-C4H9)4N]+
Cp ligands were not equivalent in acetonitrile solution
whereas they were equivalent in the solid phase.
Cp
3.2 Meerwein–Ponndorf–Verley(MPV) Reduction
of Ketones with 2-propanol Catalyzed by TBA–
Si–Al–Zr and TBA–P–Al–Zr in Homogeneous
and Heterogeneous Systems
First of all, the hydrogen-transfer reaction between cyclo-
hexanone and 2-propanol at 80 2 °C catalyzed by TBA–
Si–Al–Zr and TBA–P–Al–Zr was examined in a homo-
geneous system, as shown in Fig. 7. The catalytic activities
are summarized in Table 2. During the reduction reactions,
all the catalysts in Table 2 were soluble in acetonitrile.
Cyclohexanol and acetone were obtained with [99 %
selectivity. No reaction was observed in the absence of a
catalyst under the tested reaction conditions. A Brønsted
acid catalyst, H3PW12O40Á23H2O, also showed no reaction.
The conversions involving both TBA–Si–Al–Zr and
TBA–P–Al–Zr were [99 % complete after 24 h and both
turnover numbers (TON = [mol of cyclohexanol]/[mol of
ppm
Fig. 6 Solid-state 13C NMR spectrum of TBA–Si–Al–Zr
catalyst]) reached 280. As control experiments, TBA–Si–
Al and [(n-C4H9)4N]4[a-PW11{Al(OH2)}O39] (TBA–P–
Al) showed no reaction within 24 h. The conversions of
Cp2ZrCl2 and a mixture of Cp2ZrCl2 and TBA–P–Al after
24 h were 1.7 and 0.95 %, respectively; this supported the
result that TBA–P–Al–Zr was not obtained by a direct
reaction of TBA–P–Al with Cp2ZrCl2, as mentioned
above. In contrast, TBA–Si–Al–Zr was easily formed by
the reaction of TBA–Si–Al with Cp2ZrCl2 in acetonitrile,
and showed the high catalytic activities. Thus, the grafting
reaction of Cp2ZrCl2 onto the mono-aluminum-substituted
site in TBA–Si–Al caused the effective active centers.
Here, Cp2Zr(OTf)2THF showed faster initial rates, as
shown in Figure S5. The conversion after 3 h was 29 %,
exceeding those of TBA–Si–Al–Zr (18 %) and TBA–P–
Al–Zr (20 %), and after 24 h, the conversion was 82 %
(TON was 116). When TON was calculated on the basis of
[mol of cyclohexanol]/[mol of zirconium atom], both TON
after 24 h of TBA–Si–Al–Zr and TBA–P–Al–Zr was 140,
which was similar order to that of Cp2Zr(OTf)2THF (116).
Thus, a clear effect of grafting reactions of Cp2Zr(OTf)2-
THF onto the mono-aluminum-substituted sites in TBA–
Si–Al–Zr and TBA–P–Al–Zr was not observed under the
present reaction conditions.
However, TBA–Si–Al–Zr and TBA–P–Al–Zr can be
used as heterogeneous catalysts because they are insoluble
in 2-propanol; while, Cp2Zr(OTf)2ÁTHF is soluble in
2-propanol. The catalytic activities of TBA–Si–Al–Zr and
TBA–P–Al–Zr in a heterogeneous system, i.e., in the
absence of acetonitrile, were investigated for the MPV
reduction of cyclohexanone, cyclopentanone, and 2-hex-
anone with 2-propanol at 80 2 °C, as shown in Table 3.
For the all reactions, the corresponding alcohols and ace-
tone were formed with[99 % selectively. For the reaction
Fig. 5 a 1H NMR and b 13C NMR spectra in acetonitrile-d3 of TBA–
Si–Al–Zr
123

C. N. Kato et al.
(a)
(b)
100
100
80
80
60
60
40
40
20
20
0
0
16
20
4
24
0
8
12
4
20
8
16
24
0
12
Time/h
Time/h
Fig. 7 Time course for the Meerwein–Ponndorf–Verley reduction of cyclohexanone with 2-propanol catalyzed by a TBA–Si–Al–Zr and
b TBA–P–Al–Zr. Reaction conditions: see Table 2
Table 2 Meerwein–Ponndorf–Verley reduction of cyclohexanone with 2-propanol in a homogeneous system
Catalyst (lmol)
Reaction
time (h)
Selectivity of
cyclohexanol (%)
TONa
Conversion (%)b
TBA–Si–Al–Zr (6.8)
TBA–P–Al–Zr (6.8)
Cp2Zr(OTf)2THF (13.5)
3
[99
[99
[99
[99
[99
[99
[99
[99
–
51
18
[99
20
24
3
280
57
24
3
280
40
[99
29
24
24
24
24
24
116
2.3
1.3c
–
82
Cp2ZrCl2 (13.7)
1.7
0.95
–
A mixture of TBA–P–Al (13.5) and Cp2ZrCl2 (13.7)
TBA–Si–Al (6.7)
TBA–P–Al (6.8)
–
–
–
Reaction conditions: catalyst (6.7–13.7 lmol), cyclohexanone (1.9 mmol), 2-propanol (13 mmol), acetonitrile (4 mL), reaction temperature
80 2 °C, under argon
a
Turnover number (TON) = [mol of cyclohexanol]t/[mol of catalyst]
b
Conversion (%) = {[mol of cyclohexanone]0 - [mol of cyclohexanone]t}/[mol of cyclohexanone]0 9 100
c
TON was calculated on the basis of mol of TBA–P–Al
of cyclohexanone, the conversion after 24 h of TBA–Si–
Al–Zr reached [99 %; while, TBA–P–Al–Zr showed a
low conversion (46 %). A numerous homogeneous and
heterogeneous catalysts for MPV reduction have been
reported [45, 46]. Some of the recent examples containing
aluminum and zirconium, Cp2ZrH2 [47], ZrO2 [48], zir-
conium 1-propoxide grafted on SBA-15 [49], porous zir-
conium-phytic acid hybrid [50], partially crystalline
zirconosilicate [51], [CH2{Me(Me3Si)2Si}2SiO]2AlOPri(-
HOPri) [52], Mg/Al and Ca/Al mixed oxides [53, 54], and
aluminum isopropoxide-grafted mesoporous organosilica
[55], exhibited high conversions ([80 %) of cyclohex-
anone with 2-propanol under various reaction conditions.
Although it is difficult to draw a simple comparison, the
catalytic activities of TBA–Si–Al–Zr were by no means
inferior to them in both homogeneous and heterogeneous
system. TBA–Si–Al–Zr also showed the higher conversion
(55 % after 96 h) in the MPV reduction of cyclopentanone
than that of TBA–P–Al–Zr (30 %). When 2-hexanone was
used as a substrate, the conversion after 96 h of TBA–Si–
Al–Zr decreased to 24 %, which was similar to that of
TBA–P–Al–Zr (26 %).
123

Organozirconium Complex with Keggin-Type Mono-Aluminum-Substituted Silicotungstate…
Table 3 Meerwein–Ponndorf–Verleyreduction of ketones with 2-propanol in a heterogeneous system
Substrate (mmol)
Catalyst
Reaction time (h)
Selectivity of product (%)
TONa
Conversion (%)b
Cyclohexanone (1.9)
TBA–Si–Al–Zr
TBA–P–Al–Zr
TBA–Si–Al–Zr
TBA–P–Al–Zr
TBA–Si–Al–Zr
TBA–P–Al–Zr
24
24
96
96
96
96
Cyclohexanol ([99)
Cyclohexanol ([99)
Cyclopentanol ([99)
Cyclopentanol ([99)
2-hexanol ([99)
70
33
45
24
14
15
[99
46
Cyclopentanone (2.2)
2-hexanone (1.6)
55
30
24
2-hexanol ([99)
26
Reaction conditions: catalyst (27 lmol), substrates (1.6–2.2 mmol), 2-propanol (78 mmol), reaction temperature 80 2 °C, under argon
a
Turnover number (TON) = [mol of corresponding alcohol]t/[mol of catalyst]
b
Conversion (%) = {[mol of substrate]0-[mol of substrate]t}/[mol of substrate]0 9 100
After 24 h of reaction for cyclohexanone, the catalysts
were collected using a membrane filter (JG 0.2 lm) and
washed with a small amount of ethanol. No precipitation
was formed by the addition of excess water to the fil-
trates, suggesting no leaching of TBA–Si–Al–Zr and
TBA–P–Al–Zr into the reaction solutions. 31P NMR
spectrum in acetonitrile-d3 of the filtrate of TBA–P–Al–
Zr also showed no signals; this suggested that TBA–P–
Al–Zr did not leach into the solutions. The FT-IR
spectra of solid samples showed bands at (1010, 965,
921, 883, 800, 758, and 668 cm-1) and (1080, 1065,
969, 888, 810, 786, 749, 703, and 629 cm-1), respec-
tively, which were the same as those of as-prepared
samples [25]. The 31P NMR spectrum in acetonitrile-d3
of a solid sample of TBA–P–Al–Zr also showed two
signals at -12.25 and -12.35 ppm, which were the same
as those of as-prepared sample [25]. No other signals
were observed. These results suggested that the dimeric
structures of TBA–Si–Al–Zr and TBA–P–Al–Zr were
maintained under the present reaction conditions. For the
1H NMR spectrum in acetonitrile-d3 of a sample of
TBA–Si–Al–Zr, four signals assigned to Cp ligands were
observed at 6.69, 6.64, 6.63, and 6.59 ppm, which were
the same as those of as-prepared sample (Figure S8a).
4 Conclusion
The tetra-n-butylammonium salt of [a-SiW11Al(OH)2O38-
ZrCp2]62- (Si–Al–Zr), [(n-C4H9)4N]6[a-SiW11Al(OH)2-
O38ZrCp2]2Á2H2O (TBA–Si–Al–Zr), was obtained from a
1:1 stoichiometric reaction of [(n-C4H9)4N]4K0.5H0.5[a-
SiW11{Al(OH2)}O39]ÁH2O (TBA–Si–Al) with Cp2-
Zr(OTf)2ÁTHF in acetonitrile solution under an Ar atmo-
sphere, followed by precipitation from water and
crystallization via vapor diffusion from acetonitrile/
methanol in air. Single-crystal X-ray structure analysis of
TBA–Si–Al–Zr revealed that the two {SiW11AlO40} units
are bridged by two ‘‘bent sandwich’’ Cp2Zr2? fragments, in
which the four g5-cyclopentadienyl ligands were equivalent
in the solid phase but not equivalent in acetonitrile solu-
tion. The molecular structure of Si–Al–Zr was quite sim-
ilar to that of [a-PW11Al(OH)O39ZrCp2]62- (P–Al–Zr);
however, the partial structures around Al and Zr centers,
including the position and number of protons, and the
dynamic behaviors of the four Cp ligands in acetonitrile
solution were remarkably different from those of P–Zr–Al.
TBA–Si–Al–Zr was also formed by a 1:1 stoichiometric
reaction of TBA–Si–Al with Cp2ZrCl2 in air; this was
different from that of [(n-C4H9)4N]6[a-PW11Al(OH)O39-
ZrCp2]2 (TBA–P–Al–Zr). Furthermore, we investigated
the catalytic activities of TBA–Si–Al–Zr and TBA–P–Al–
Zr in the MPV reduction of cyclohexanone, cyclopen-
tanone, and 2-hexanone with 2-propanol in both homoge-
neous and heterogeneous system. In a homogenous system,
both compounds exhibited [99 % conversions after 24 h
for the MPV reduction of cyclohexanone; thus, a clear
effect of the internal atoms was not observed. In contrast,
TBA–Si–Al–Zr exhibited the higher activities than those
of TBA–P–Al–Zr for cyclohexanone and cyclopentanone
in a heterogeneous system. FT-IR and 31P NMR spectra
showed that the dimeric structures of these compounds
were maintained under the present reaction conditions.
Thus, the direct reaction of Cp2Zr(OTf)2THF (or
1
The H NMR spectrum of a sample of TBA–P–Al–Zr
showed two signals of Cp ligands at 6.67 and 6.56 ppm,
as shown in Figure S8b. The two signals were also the
same as those of as-prepared sample [25]. Thus, the
active centers would not be caused by cleavage of
linkages between the two Keggin mono-aluminum-sub-
stituted units. When a solid sample of TBA–Si–Al–Zr
was used as a catalyst in the second run for the MPV
reduction of cyclohexanone with 2-propanol at
80 2 °C, the conversion after 24 h was approximately
90 %, showing that TBA–Si–Al–Zr was basically recy-
clable. Further studies regarding the reaction mechanism
including determination of the active centers are in
progress and will be reported elsewhere.
123

C. N. Kato et al.
24. Proust A, Matt B, Villanneau T, Guillemot G, Gouzerh P, Izzet G
(2012) Chem Soc Rev 41:7605
25. Kato CN, Makino Y, Unno W, Uno H (2013) Dalton Trans
42:1129
Cp2ZrCl2) with TBA–Si–Al–Zr caused the high catalytic
activities for MPV reduction in both homogeneous and
heterogeneous system.
26. Raith A, Altmann P, Cokoja M, Herrmann WA, Ku¨hn FE (2010)
Coord Chem Rev 254:608
Acknowledgments This work was supported by a Grant-in-Aid for
Scientific Research of the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
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