A Lewis acid catalytic core sandwiched by inorganic polyoxoanion caps: Selective H2O2-based oxidations with [Al III4(H2O)10(β-XW 9O33H)2]6- (X = AsIII, SbIII)

Article abstract of DOI:10.1039/c3cc44077j

The Al^III-containing polyanions [Al^III ₄(H₂O)₁₀(β-XW₉O ₃₃H)₂]^4- with X = As^III (1) and Sb^III (2) feature four aluminum(iii) centers sandwiched by two trivacant (β-XW₉O₃₃) Keggin units, and trigger peroxide catalysis as well as substrate coordination via multiple Lewis acid site interactions.

Full text of DOI:10.1039/c3cc44077j

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A Lewis acid catalytic core sandwiched by inorganic
polyoxoanion caps: selective H O -based oxidations
2
2
6À
Cite this: Chem. Commun., 2013,
III
III
III
4
9, 7914
with [Al (H O) (b-XW O H) ] (X = As , Sb )†
4
2
10
9
33
2
Received 30th May 2013,
Accepted 27th June 2013
a
b
a
a
Mauro Carraro,* Bassem S. Bassil,* Antonio Sorar u` , Serena Berardi,
b
b
a
Andreas Suchopar, Ulrich Kortz* and Marcella Bonchio*
DOI: 10.1039/c3cc44077j
www.rsc.org/chemcomm
III
III
4À
8
The Al -containing polyanions [Al
4
2 9
(H O)10(b-XW O
2
33H) ]
with POM epoxidation catalysts. Alcohol coordination by the tetra-
III
III
X = As (1) and Sb (2) feature four aluminum(III) centers sandwiched aluminate core has been highlighted herein with a chiral probe.
by two trivacant (b-XW 33) Keggin units, and trigger peroxide Polyoxoanions 1 and 2 were synthesized by the reaction of
catalysis as well as substrate coordination via multiple Lewis acid site Al ions (2.22 mmol) with 1 mmol of trivacant Keggin precursors [a-
9
O
III
9À
III
III
interactions.
XW
pH = 3) at 70 1C for 1.5 h. Crystals were obtained after a few days,
The coordination of one or more metal ions by totally inorganic upon addition of 0.5 mL of 1 M RbCl solution to 1 or 1 M NH Cl to
polyoxometalate (POM) ligands allows for the construction of 2. Single crystal XRD revealed that 1 and 2 are isostructural,
9 33
O ]
(X = As , Sb ) in 30 mL of an aqueous acidic medium
(
4
8À
III
III
oxidation catalysts with superior robustness, whose structural comprising two trivacant [b-XW O H] (X = As , Sb ) Keggin
features and solution chemistry merge, on the one hand, with units connected by four octahedrally coordinated Al ions (Fig. 1).
9
33
III
9
those of solid metal oxides and, on the other hand, with those Polyanions 1 and 2 have idealized C2h symmetry and they represent
1
,2
of natural enzymes. Although several main group III element- the Al-analogues of Kortz’s iron(III) and indium(III)-containing
3
,4
6À
III
III
III
containing polyoxotungstates are known, only a few reported polyanions [M
4
(H
and catalytic applica- Sb ). In full analogy, the coordination sphere of the two outer Al
tions of aluminum-containing heteropolyanions. ions in 1 and 2 is completed by three terminal aqua ligands each,
2
O)10(b-XW
9
O
33
)
2
]
(M = Fe , In ; X = As ,
4
,5
III
4
III
examples deal with the preparation
6
Herein we describe the synthesis of two novel, sandwich-type whereas the two inner Al ions have two terminal aqua ligands each.
polyoxotungstates encapsulating four aluminum(III) centers, which The presence of the terminal aqua ligands was confirmed by bond
7
10
can promote a synergic Lewis acid activation of both H O and the valence sum (BVS) calculations.
2
2
substrate for selective catalytic oxidation. Our study includes the
In 1, an Al–O–W oxygen bridge (O6WA) of each ‘outer’ Al ion is
structural characterization of the hydrated 18-tungsto-2-arsenate(III) monoprotonated, reducing the overall charge of the polyanion to
III
III
salt Rb
antimony(III)-analogue
0H O (NaNH -2) by single crystal X-ray diffraction (XRD), FT-IR, and NaNH
4
2
Na
2
[Al
4
(H
2
O)10(b-As W
9
O
33H)
2
[Al
2
III
]Á20H O (NaRb-1) and its 4À, whereas the exact protonation site in 2 could not be identified
2
III
(NH Na
4
)
2
4
(H
2
O)10(b-Sb W
9
O
33H)
2
]Á using single-crystal XRD. The complete formula units of NaRb-1
2
2
4
-2 were confirmed by elemental analysis (see ESI†).
TGA, and elemental analysis. Besides contributing to the formation
III
and stabilization of the dimeric structures, the Al ions provide
multiple Lewis acid sites for the coordination of suitable
substrates, in particular alcohols. This triggers an unprecedented
chemoselectivity towards alcohol oxidation with respect to
II
III
III
II
II
II
similar multi-Mn , Ru , Fe , Pd , Pt , or Zn -substituted
a
ITM-CNR and Department of Chemical Sciences, University of Padova,
via Marzolo 1, 35131 Padova, Italy. E-mail: mauro.carraro@unipd.it,
marcella.bonchio@unipd.it; Fax: +39 0498275239
b
School of Engineering and Science, Jacobs University, P.O. Box 750 561,
2
8725 Bremen, Germany. E-mail: b.bassil@jacobs-university.de,
u.kortz@jacobs-university.de; Fax: +49 421-200-3229
†
Electronic supplementary information (ESI) available: Experimental details,
Fig. 1 Ball-and-stick and polyhedral representations of polyanions 1 and 2. The
crystal data, CD, TGA, and CIF files with ICSD numbers 425246 and 425247.
See DOI: 10.1039/c3cc44077j
color code is as follows: W, black balls; O, red balls; Al, grey balls; As/Sb, turquoise
6 6
balls; WO octahedra, dark yellow; AlO octahedra, grey.
7
914 Chem. Commun., 2013, 49, 7914--7916
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As expected, the As–O bond lengths in 1 are shorter (1.797(8)– methyl-p-tolylsulfide is complete in 1 h at T = 30 1C, with
.814(9) Å) than those of Sb–O in 2 (2.004(10)–2.012(11) Å), and negligible overoxidation to sulfone (entries 10–12). On the other
1
the corresponding heteroatom separations (XÁÁÁX) in 1 (AsÁÁÁAs hand, terminal olefins (such as 1-octene and 1-decene) display a
ca. 5.92 Å) and 2 (SbÁÁÁSb ca. 5.54 Å) are consistent with this. The slower oxidation rate at 70 1C, leading to a modest epoxide
Al–O bond lengths in 1 (1.843(10)–1.940(10) Å) and 2 (1.843(12)– yield, due to concurrent peroxide decomposition (entries 6–9).
1.962(12) Å) are virtually identical. Tetra-n-butylammonium (TBA) Under the conditions explored, secondary and benzylic alcohols
salts of 1 and 2 were prepared by initial precipitation of the are efficiently converted to the corresponding carbonyl products,
1
2
polyanions from the reaction mixture (see above) by adding KCl with 99% selectivity and yield (entries 13–21).
0.5 g, 6.7 mmol), followed by redissolution of the white amorphous In particular, zero order kinetics were found for cyclohexanol
solids (ca. 1 g) in water and precipitation by addition of 1.67 g solid oxidation catalyzed by either TBA-1 or TBA-2, at 70 1C, with
(
III
III
À1
TBACl (6 mmol), leading to (TBA) [Al (H O) (b-As W O H) ]Á maximum turnover frequencies (TOFmax) of 263 and 375 h
,
4
4
2
10
III
9
33
2
III
4
4
H O (TBA-1) and (TBA) [Al (H O) (b-Sb W O H) ]Á4H O respectively. This behavior is likely ascribed to the in situ for-
2
4
2
10
9
33
2
2
(
TBA-2). As confirmed by FT-IR analysis, the polyanion structure mation of a POM–peroxide active complex, resulting from the
+
13
is maintained upon precipitation with K and after cation exchange association of H
with TBA (see Fig. S9, ESI†). maintaining a constant oxidation rate at high alcohol conversion
Despite the interest in aluminum-based Lewis acids and (Fig. S1, ESI†) is indicative of rather efficient equilibria involving
2 2
O to the pristine catalyst. Furthermore,
11
oxidation catalysis, the concept of using Al-substituted POMs for metal-alcoholate reactive groups, formed on the competent
H O activation has been scarcely explored. Table 1 summarizes the oxidant species. This mechanistic scenario is consistent with
2 2
6
8
results obtained with polyanions 1 and 2 as catalysts for the the high chemoselectivity observed for the oxidation of 2-cyclo-
oxidation of olefins, sulfides and alcohols with bulk H O in hexen-1-ol, leading to cyclohexenone in up to 99% yield (entries
2
2
acetonitrile. For this purpose, the TBA salts of 1 and 2 were used 22 and 23), whereby the CQC bond remains unaffected. The
Table 1, entries 1–24). A comparison with the indium analogue observed chemoselectivity stems from the POM-embedded tetra-
(
III
III
III
(TBA)
6
[In
4
(H
2
O)10(b-As W
9
O
33
)
2
]Á4H
2
O (TBA-3) was also performed Al core, which induces a definite reactivity ‘‘Umpolung’’ com-
with selected substrates.
pared to plenary polyoxotungstates or other transition metal-
1
4
The resulting reactivity signature parallels that expected for a substituted POMs (TMSPs). Al-free POMs are indeed known to
8
III
strong electrophilic activation of the peroxidic oxidant, whereby prefer the epoxidation route. Interestingly, the In derivative,
electron rich substrates, such as cyclic olefins and sulfides, undergo TBA-3, showed an intermediate behavior, being a less efficient
excellent to good conversion, forming the corresponding epoxides catalyst for alcohol oxidation (entry 21), and converting 2-cyclo-
À1
and sulfoxide with turnover frequencies (TOFs) of 68–128 h
(
hexenol with lower selectivity (entry 24).
The role of the Al centers, likely responsible for a Lewis
acid-directed coordination of hydroxylic functions,
III
Table 1, entries 1–5 and 10–12). Interestingly, the oxidation of
6
a,15
has
1
6
been established by using a tailored chiral probe. Indeed,
the interaction of a chiral organic molecule with the inorganic
framework can be revealed by induced circular dichroism (ICD)
a
2 2
Table 1 Catalytic activity of 1–3 in the presence of H O
b
#
Substrate
Product
POM salt t (h) Yield (%)
1
7
features. Thus the optically pure (S)-1-phenyl-1,2-ethanediol
has been selected as it displays UV absorption and a relevant
dichroic signal only up to 275 nm, whereas the POM absorption
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
cis-Cyclooctene Cyclooctene oxide TBA-1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
1
5
98
77
99
58
54
26
36
28
19
>99
99
99
73
99
81
90
60
81
96
99
67
90
99
65
>99
TBA-2
TBA-3
Cyclohexene
1-Octene
Cyclohexene oxide TBA-1
18
extends up to 375 nm. For both TBA-1 and TBA-2, upon
TBA-2
sequential addition of (S)-1-phenyl-1,2-ethanediol, a steady increase
in the dichroic signal was registered in the full range explored, with
additional positive Cotton effects between 275 and 350 nm, clearly
1,2-Epoxyoctane
1,2-Epoxydecane
TBA-1
TBA-2
TBA-1
TBA-2
1-Decene
1
9
c
c
c
indicative of the chirality transfer to the POM framework. In
particular, in the presence of 4 equivalents of S diol, two well
resolved signatures were observed for TBA-1 at 280 nm (molar
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
Me-p-tolylsulfide Me-p-tolylsulfoxide TBA-1
TBA-2
TBA-3
Benzylic alcohol Benzaldehyde
TBA-1
TBA-2
TBA-1
TBA-2
TBA-1
TBA-2
TBA-1
TBA-2
TBA-3
TBA-1
TBA-2
TBA-3
NaRb-1
3
2
À1
ellipticity y = 5.9 Â 10 deg cm dmol ) and at 307 nm (y = 7.6 Â
3
2
À1
2-Nonanol
2-Nonanone
10 deg cm dmol ) (Fig. S4, ESI†).
A CD titration experiment, performed on the chiral ligand-
centered signal at 260 nm, is typical of an association profile with
linearity deviation after addition of two equivalents of ligand (Fig. 2),
thus suggesting a major role of the two external and more accessible
Cyclopentanol
Cyclohexanol
Cyclopentanone
Cyclohexanone
2a,20
Al sites in the coordination equilibria.
The corresponding adduct
2-Cyclohexenol 2-Cyclohexenone
with TBA-2 led to a broader signal in the range 290–340 nm (molar
d
3
2
À1
e
ellipticity 5.6 Â 10 deg cm dmol at 314 nm, Fig. S5, ESI†).
As expected, mirror-like spectra were obtained for both TBA-1 and
TBA-2, by using the enantiomeric (R)-1-phenyl-1,2-ethanediol as a
ligand (Fig. S4 and S5, ESI†). Interestingly, the chiral probe was not
effective for the formation of adducts with TBA-3 (Fig. S6, ESI†).
Cyclohexanol
Cyclohexanone
a
Reaction conditions: H
2
O
2
(0.1 mmol, added as 35% aqueous
CN (0.6 mL), at
solution), POM (0.8 mmol), substrate (0.5 mmol), CH
3
b
c
d
T = 70 1C. Yields calculated with respect to H
2
O
2
.
T = 30 1C. 10%
e
epoxide was also observed. Reaction in H
2
O (1.2 mL).
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(
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7
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3
Fig. 2 Addition of (S)-1-phenyl-1,2-ethanediol (0–9 equiv.) to a CH CN solution
of TBA-1 (0.025 mM). Inset: absolute variation of the signal at 260 nm.
Analogous results have been obtained for the spectrophotometric
titration of TBA-1 and TBA-2 with 3,5-ditert-butylcatechol (DTBC),
where a marked increase in absorbance was observed at 385 nm,
likely resulting from chelate binding with an apparent 2:1 stoichio-
K. Micoine, P. Re
my, B. Hasenknopf, S. Thorimbert, E. Lac oˆ te,
´
M. Malacria, C. Afonso and J.-C. Tabet, Chem.–Eur. J., 2007, 13, 5426;
c) N. Dupr ´e , P. R ´e my, K. Micoine, C. Boglio, S. Thorimbert, E. Lac ˆo te,
B. Hasenknopf and M. Malacria, Chem.–Eur. J., 2010, 16, 7256.
8 (a) W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Mo
D. Seebach, A. K. Beck and R. Zhang, J. Org. Chem., 2003,
8, 8222; (b) W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-M ¨o ller,
(
2a
¨
ller,
metry, as for the chiral ligand (Fig. S7 and S8, ESI†).
Under the turnover regime, POM stability has been assessed
6
2
1
by FT-IR, after precipitation of the spent catalysts. The main
spectral features of the polyanions were maintained when
studying them in acetonitrile (see for example the spectrum
of recovered TBA-1 in Fig. S9, ESI†). The indium analogue TBA-3
displayed a decreased stability (Fig. S10, ESI†).
D. Sloboda-Rozner and R. Zhang, J. Org. Chem., 2003, 68, 1721.
9 Crystallographic data, for NaRb-1: block-shaped crystal with dimen-
3
sions 0.40 Â 0.16 Â 0.09 mm , triclinic system, space group P1%(2),
with a = 12.5177(3) Å, b = 12.6671(4) Å, c = 15.7160(5) Å, a =
3
8
6.132(2)1 b = 82.6840(10)1, g = 83.438(2)1. V = 2451.95(12) Å and
Z = 1. NaNH
0
4
-2: plate-like crystal with dimensions 0.14 Â 0.11 Â
3
.04 mm , triclinic system, space group P1%(2), with a = 12.5011(5) Å,
It is noteworthy that the catalytic activity of 1 was retained in
b = 12.5288(5) Å, c = 15.9248(7) Å, a = 73.995(2)1 b = 88.077(2)1, g =
77.976(2)1, V = 2344.16(17) Å and Z = 1.
3
H O, as shown for the oxidation of cyclohexanol (pH = 6.5) by the
2
22
water-soluble POM salt (entry 25 of Table 1). In aqueous media, 10 D. Altermatt and I. D. Brown, Acta Crystallogr., Sect. B, 1985, 41, 244.
1
1 (a) G. A. Olah, S. Kobayashi and J. Nishimura, J. Am. Chem. Soc.,
however, the reaction rates slowed down, which was not unexpected
considering competition with water in the binding equilibria.
In conclusion, the novel Al-containing polyanions 1 and 2
are example of molecular metal oxides with enhanced Lewis
acidic properties. Further possibilities for multi-metallic Lewis
1
1
973, 95, 564; (b) A. Corma and H. Garc ´ı a, Chem. Rev., 2002,
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2
Lett., 2008, 124, 330.
acid-directed catalysis are under consideration. We also plan to 12 The catalytic system turns out to be less effective with aliphatic
primary alcohols, as in the case of 1-nonanol (36% yield in 3 h).
prepare the gallium derivatives of 1 and 2.
1
3 M. Carraro, N. H. Nsouli, H. Oelrich, A. Sartorel, A. Sorar u` , S. S. Mal,
G. Scorrano, L. Walder, U. Kortz and M. Bonchio, Chem.–Eur. J.,
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6 The coordination of chiral molecules to POMs has been exploited in
catalysis (see ref. 8), and for the resolution of chiral POMs see:
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40, 2715; (b) J. F. Garvey and M. T. Pope, Inorg. Chem., 1978,
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U.K. thanks the German Science Foundation (DFG-KO-2288/3-2)
and Jacobs University for research support. We acknowledge the
ESF COST Actions D40 and CM1203 (PoCheMoN). The authors
thank Ms Valeria Feltrin for preliminary catalytic studies.
1
1
Notes and references
1
1
(a) Issue dedicated to polyoxometalates, U. Kortz (guest editor),
Eur. J. Inorg. Chem., 2009, 5055–5276; (b) Issue dedicated to poly-
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2
013, 7325–7648; (d) M. T. Pope and U. Kortz, Polyoxometalates,
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and D. I. Sinno, Angew. Chem., Int. Ed., 2002, 41, 4070. For a general
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Inorg. Chem., 2008, 5001.
1
2
(a) A. Sartorel, M. Carraro, G. Scorrano, B. S. Bassil, M. H. Dickman,
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(
b) M. Carraro, A. Sartorel, F. M. Toma, F. Puntoriero, F. Scandola,
3
2
À1
S. Campagna, M. Prato and M. Bonchio, Top. Curr. Chem., 2011, 1; 18 (S)-1-Phenyl-1,2-ethanediol has y = À4.7 Â 10 deg cm dmol at
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U. Kortz, J. Mol. Catal. A: Chem., 2007, 262, 36; (d) A. Sartorel, M. Truccolo, 19 (a) M. Carraro, G. Modugno, A. Sartorel, G. Scorrano and
(
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3
4
F. Zonnevijlle, C. M. Tourn ´e and G. F. Tourn ´e , Inorg. Chem., 1982, 20 A. Dolbecq, J.-D. Compain, P. Mialane, J. Marrot, E. Rivi `e re and
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(a) F. Hussain, M. Reicke, V. Janowski, S. de Silva, J. Futuwi and 21 Recharge of the reaction mixture containing benzyl alcohol with
2
U. Kortz, C. R. Chim., 2005, 8, 1045; (b) U. Kortz, M. G. Savelieff,
B. S. Bassil, B. Keita and L. Nadjo, Inorg. Chem., 2002, 41, 783.
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2 2
another portion of H O gave 88% yield.
22 Similar results were obtained for other water soluble salts of POMs 1
and 2. A decreased hydrolytic stability of the polyanions in aqueous
medium has been observed using FT-IR on recovered 1 and 2.
5
7
916 Chem. Commun., 2013, 49, 7914--7916
This journal is c The Royal Society of Chemistry 2013