New porphyrin-polyoxometalate hybrid materials: Synthesis, characterization and investigation of catalytic activity in acetylation reactions

Article abstract of DOI:10.1039/c2dt31553j

New hybrid complexes based on covalent interaction between 5,10,15,20-tetrakis(4-aminophenyl)porphyrinatozinc(ii) and 5,10,15,20- tetrakis(4-aminophenyl)porphyrinatotin(iv) chloride, and a Lindqvist-type polyoxometalate, Mo₆O₁₉^2-, were prepared. These new porphyrin-polyoxometalate hybrid materials were characterized by ¹H NMR, FT IR and UV-Vis spectroscopic methods and cyclic voltammetry. These spectro- and electrochemical studies provided several spectral data for synthesis of these compounds. Cyclic voltammetry showed the influence of the polyoxometalate on the redox process of the porphyrin ring. The catalytic activity of tin(iv)porphyrin-hexamolybdate hybrid material was investigated in the acetylation of alcohols and phenols with acetic anhydride. The reusability of this catalyst was also investigated.

Full text of DOI:10.1039/c2dt31553j

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PAPER
New porphyrin–polyoxometalate hybrid materials: synthesis, characterization
and investigation of catalytic activity in acetylation reactions†
a
b
b
b
Mehdi Araghi, Valiollah Mirkhani,* Majid Moghadam,* Shahram Tangestaninejad and
b
Iraj Mohammdpoor-Baltork
Received 13th July 2012, Accepted 25th July 2012
DOI: 10.1039/c2dt31553j
New hybrid complexes based on covalent interaction between 5,10,15,20-tetrakis(4-aminophenyl)
porphyrinatozinc(II) and 5,10,15,20-tetrakis(4-aminophenyl)porphyrinatotin(IV) chloride, and a Lindqvist-
2
−
type polyoxometalate, Mo O
, were prepared. These new porphyrin–polyoxometalate hybrid materials
6
19
1
were characterized by H NMR, FT IR and UV-Vis spectroscopic methods and cyclic voltammetry. These
spectro- and electrochemical studies provided several spectral data for synthesis of these compounds.
Cyclic voltammetry showed the influence of the polyoxometalate on the redox process of the porphyrin
ring. The catalytic activity of tin(IV)porphyrin–hexamolybdate hybrid material was investigated in the
acetylation of alcohols and phenols with acetic anhydride. The reusability of this catalyst was also
investigated.
Introduction
associations may stabilize the metalloporphyrins against deacti-
3
vation during the catalytic cycles. Such assemblies can be pre-
4
The interest in possible developments of mixed organic–
inorganic materials with diverse properties that may lead to new
applications has been continuously increasing. In a typical set-
up, molecules with specific properties are selected as building
blocks. Once the molecules are assembled, their properties
complement each other to yield an integrated functional material.
In this context, polyoxometalates (POMs) have attracted increas-
ing attention due to their remarkable chemical, structural, and
electronic versatility in order to prepare new compounds with
pared by electrostatic interactions and/or coordination and used
5
6
as oxidation catalysts or for electrocatalytic reductions.
Although such assemblies are easy to prepare and rather robust,
the relative orientation of the components and their stoichio-
metry are not under control. The covalent linkage of POM to
porphyrin is a way to overcome this problem. Ruhlmann and co-
workers reported the preparation of a metallosupramolecular
system consisting of
a POM sandwiched between two
ruthenium(II) or zinc(II) porphyrins via axial ligation with a pyri-
1
4
a
interesting optical, magnetic or electric properties. In this way,
it becomes possible to utilize the high electron affinity of the
POM for catalytic purposes.
dinyl ligand. Attanasio and Bachechi reported the preparation
and charge transfer properties of tetraphenylporphyrinato zinc(II)
4
b
complex and Keggin heteropolyanions.
2
−
Besides, metalloporphyrins are attractive components in
materials chemistry because of their photochemical excitations
and rich redox chemistry. Therefore, porphyrins bearing appro-
priate metal ions can be used as electron donors, and they are
often used as building blocks for supramolecules or artificial
The Lindqvist anions, M O
(M = Mo, W) (Fig. 1), are
among the smallest members of the POM family. These anions,
6
19
7
which exhibit high point group symmetry (O ), are highly stable
h
8
in condensed phase and even in the gas phase. In the case of
2
−
the hexamolybdate ion, Mo O
, a sizable class of derivatives
6
19
2
n−
dyad systems.
Mo O (L) has been prepared in which an exogenous ligand L
replaces a terminal oxo ligand within the parent structure.
6
18
9
Compounds composed of polyoxometalates and metallopor-
phyrins have been rarely considered. These compounds can be
envisaged as potential new bi-functional catalysts and can be
expected to originate systems in which the catalytic abilities of
these two species reinforce each other. On the other hand, such
The protection of a hydroxy functional group is often necess-
ary during the course of various transformations in a synthetic
a
Department of Chemistry, Saveh Branch, Islamic Azad University,
Saveh, Iran
b
Department of Chemistry, Catalysis Division, University of Isfahan,
Isfahan 81746-73441, Iran. E-mail: moghadamm@sci.ui.ac.ir,
mirkhani@sci.ui.ac.ir; Fax: +98-311-6689732; Tel: +98-311-7932712
†
Electronic supplementary information (ESI) available: UV-Vis and FT
6
O
192−.
IR spectra and voltammograms. See DOI: 10.1039/c2dt31553j
Fig. 1 The structure of hexamolybdate ion Mo
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sequence, especially in the construction of polyfunctional mo-
lecules such as nucleosides, carbohydrates, steroids, and natural
absorption analysis was carried out on a Shimadzu 120 spectro-
photometer. The electronic spectra of the compounds in the
UV-Vis region were recorded in DMF solution using a Shimadzu
160 UV-Vis or a Varian Cary NIR spectrophotometer. FT-IR
spectra were obtained as potassium bromide pellets in the range
400–4000 cm− with a Nicolet-Impact 400D instrument. Gas
chromatography experiments (GC) were performed on a Shi-
madzu GC 16A instrument using a 2 m column packed with
silicon DC-200 or Carbowax 20 m. In GC experiments,
n-decane was used as internal standard. Redox potentials of the
10
products. Many heterogeneous and homogeneous catalysts or
reagents have been reported for acetylation of alcohols and
11
phenols. However, some of the reported methods for the
acetylation and formylation of alcohols suffer from one or more
of disadvantages, such as high temperature and drastic reaction
conditions, formation of undesirable or toxic by-products,
expensive reagents, hygroscopicity and thermal instability of the
reagents, long reaction times, low yields of the desired products
and bulk requirement of solid bed. Therefore, introduction of
new methods and catalysts for the preparation of esters is still in
demand.
1
compounds (10− mol L ) in DMF containing 0.1 mol L
3
−1
−1
tetrabutylammonium bromide (TBAB) as a supporting electro-
lyte were determined at room temperature by cyclic voltammetry
with a PGSTAT30 (Eco Chemie, Utrecht, The Netherlands) elec-
trochemical analyzer. A three-electrode cell was used, including
glassy carbon electrode as working electrode, together with an
Ag/AgCl (saturated KCl) and a platinum wire as reference and
counter electrode, respectively. Conversions and yields were
obtained by GC experiments and the products were identified
after isolation and purification.
The successful applications of metalloporphyrins as mild
12
Lewis acid catalysts prompted us to explore the potential of
these complexes as catalysts for the conversion of alcohols and
phenols to their corresponding esters. Recently, we reported the
11t
use of tetraphenylporphyrinatotin(IV) perchlorate, tetraphenylpor-
1
1u,x
phyrinatotin(IV) trifluoromethanesulfonate,
tetraphenylpor-
11v
phyrinatotin(IV) tetrafluoroborate
and tetraphenylporphyrinato-
11w
vanadium(IV) trifluoromethanesulfonate
in the acetylation of
alcohols and phenols.
Recently, we reported the synthesis of manganese(III) por-
Synthesis of hybrid compound 1a
phyrin–polyoxometalate hybrid compound and investigated its
The 0.1 g (0.14 mmol) dry 1, 0.81 g (0.59 mmol, 4.2 eq) [(n-
C H ) N] [Mo O ] and 0.17 g (0.84 mmol, 6 eq) N,N′-dicyclo-
hexylcarbodiimide (DCC) were dissolved in 10 mL anhydrous
DMSO and heated to 100 °C under a dry atmosphere. The reac-
tion was cooled to room temperature and filtered under vacuum
13
catalytic activity in the alkene epoxidation with NaIO4. In an
attempt to expand this field, two new hybrid compounds, in
which a metalloporphyrin as an electron donor is covalently
attached to a Lindqvist-type polyoxometalate as acceptor mo-
lecule in a supramolecular assembly were prepared. A combined
study on the preparation, characterization and catalytic appli-
cation of these new polyoxometalate/metalloporphyrin associ-
ations is described here. The new compounds were prepared by
4
9 4
2
6 19
(to remove the dicyclohexylcarbodiurea) into a flask containing
1
00 mL dry ether and 20 mL dry methanol. A precipitate was
formed and washed several times with acetone and ether. The
dried product (0.63 g, 73% yield) was stored in a desiccator. IR
2−
reaction of Mo O
to metalloporphyrins carrying four 4-
6
19
−1
(
cm ): 433 (m), 599 (m), 781 (s), 944 (s), 975 (sh), 1030 (m),
amino groups in the meso positions. The hexamolybdate–tin(IV)
porphyrin, HMTP, hybrid catalyst was used as catalyst in the
acylation of alcohols and phenols with acetic anhydride at room
temperature.
1
180 (m), 1381 (m), 1459 (s), 1608 (s), 1664 (s), 2873 (s), 2963
1
(s). H NMR at 400 MHz (d -DMSO): δ = 0.94 [t (J = 7.2 Hz),
6
9
6H, (CH CH CH CH N)], 1.31 [q (J = 7.2 Hz), 64H,
3 2 2 2
(
CH CH CH CH N)], 1.57 [m, 64H, (CH CH CH CH N)],
3 2 2 2 3 2 2 2
3
.17 [t (J = 8.4 Hz), 64H, (CH CH CH CH N)], 7.67 [d (J =
3 2 2 2
Experimental
8.4 Hz), 8H, protons 2,6-AminoPh], 8.14 [d (J = 8.4 Hz), 8H,
protons 3,5-AminoPh], 8.93 [s, 8H, protons β-pyrrole]. UV-Vis
spectra (in DMF): (λmax = 377 nm). Anal. calcd for
C172H312N O Mo Zn: C, 33.73; H, 5.13; N, 3.66; Mo,
Reagents and synthetic procedures
16
72
24
All chemicals were used as received from the suppliers. Solvents
were used as received or distilled and dried using the
standard procedures. Tetrabutylammonium hexamolybdate,
3
7.60%. Found: C, 33.85; H, 5.04; N, 3.50. Mo, 37.41%.
[
(n-C H ) N] [Mo O ] was prepared according to the literature
4 9 4 2 6 19
14
Synthesis of hybrid compound 2a
method. 5,10,15,20-Tetrakis(4-aminophenyl)porphyrin, 5,10,-
1
5
5,20-tetrakis(4-aminophenyl)porphyrinatozinc(II)
,10,15,20-tetrakis(4-aminophenyl)porphyrinatotin(IV) chloride
(1)
and
A 0.1 g (0.12 mmol) dry 2, 0.69 g (0.50 mmol, 4.2 eq)
[
(n-C H ) N] [Mo O ] and 0.15 g (0.72 mmol, 6 eq) N,N′-
4 9 4 2 6 19
1
5
(
2) were prepared according to known procedures. All the syn-
dicyclohexylcarbodiimide (DCC) were dissolved in 10 mL anhy-
drous DMSO and heated to 100 °C under a dry atmosphere. The
reaction was cooled to room temperature and filtered under
vacuum (to remove the dicyclohexylcarbodiurea) into a flask
containing 100 mL dry ether and 20 mL dry methanol. A pre-
cipitate was formed and washed several times with acetone and
ether. The dried product (0.52 g, 70% yield) was stored in a
desiccator. IR (cm− ): 432 (m), 599 (w), 739 (sh), 780 (s),
943 (s), 975 (sh), 1034 (w), 1181 (m), 1381 (m), 1469 (s),
1604 (s), 1624 (s), 2872 (s) 2963 (s). Anal. calcd for
thetic procedures were performed protected from light.
Instruments
1
H NMR spectra were recorded on a Bruker-Avance AQS 400
(MHz) NMR spectrometer using DMSO as solvent and TMS as
1
internal standard. Elemental analyses were measured by a Perkin
Elmer 2400 instrument. The amount of Mo was measured by a
Leaman inductively coupled plasma (ICP) spectrometer. Atomic
11746 | Dalton Trans., 2012, 41, 11745–11752
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Scheme 1 The preparation route for hybrid compounds.
C172H312N O Mo SnCl : C, 33.06; H, 5.03; N, 3.59; Mo,
Mo O192−, to porphyrins was successfully carried out using
6
16
72
24
2
3
6.85%. Found: C, 33.15; H, 4.92; N, 3.48. Mo, 36.67%.
dicyclohexylcarbodiimide (DCC) as
a
coupling agent
(Scheme 1). These reactions were carried out according to the
procedure reported for the synthesis of manganese(III) counter-
part and for the reaction of hexamolybdate with aromatic
General procedure for acetylation of alcohol and phenols with
13,16
2−
Ac O catalyzed by 2a
amines.
Direct coupling of Mo O
6 19
with metallopor-
2
phyrins 1 and 2 in the presence of triethylamine was unsuccess-
ful. However, good results were obtained using DCC as a
To a solution of alcohol or phenol (1 mmol) and Ac O (2 mmol
17
2
per OH group) in CH CN (1 mL) was added 2a (65 mg,
3
coupling reagent. DCC can dramatically facilitate the reaction of
0
.010 mmol) and stirred at room temperature for appropriate
2−
16
Mo O
with aromatic amines. The hybrid compound 1a is
6
19
time. The progress of the reaction was monitored by GC. After
soluble in polar, aprotic solvents such as DMSO and DMF but
hybrid compound 2a is insoluble in aprotic solvents.
completion of the reaction, Et O (10 mL) was added and the
2
catalyst was filtered. The filtrates were washed with brine, dried
Direct metallation of free tetrakis(4-aminophenyl)porphyrin–
polyoxometalate with ZnCl and SnCl for preparation of hybrid
over Na SO and concentrated under reduced pressure to afford
2
4
2
2
the crude product.
compounds 1a and 2a was not successful and did not give good
yield. Therefore, the best route for preparation of these hybrid
compounds is hybridization of Zn(TNH PP) and Sn(TNH PP)-
2
2
Results and discussion
Cl with hexamolybdate.
2
Synthesis and characterization of the hybrid complexes
Several attempts for preparation of single crystals of these
hybrid compounds failed. Therefore, these new hybrid materials
were characterized by FT IR, UV-Vis and H NMR spectro-
scopic methods, and cyclic voltammetric studies. All these tech-
niques provide more details for characterization of this new class
of polyoxometalate–porphyrin hybrid materials.
Usually, the association of a molecular unit to a POM is attained
1
4a–c
by electrostatic interaction.
However, in order to increase the
stability of the hybrid material, especially in polar solvents; a
covalent linkage was established between POM and porphyrin.
Initially, we developed a mild and efficient synthetic route for
covalent attachment of 5,10,15,20-tetrakis(4-amino-phenyl)-
porphyrinatozinc(II) (1) and 5,10,15,20-tetrakis(4-aminophenyl)-
porphyrinatotin(IV) chloride (2) to a Lindqvist polyoxometalate.
The covalent attachment of the hexamolybdate polyoxoanion,
Characterization of the prepared compounds
Polyoxometalates show characteristic infrared bands in the
700–1000 cm− region assigned to M–O and M–O–M stretching
1
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1
Fig. 2 H NMR spectra of (A) 5,10,15,20-tetrakis(4-aminophenyl)porphyrinatozinc(II) (1) and (B) hybrid compound 1a in DMSO.
11748 | Dalton Trans., 2012, 41, 11745–11752
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1
8
vibrations. These bands were observed in the IR spectra of the
Table 1 Optimization of catalyst amount in the acetylation of
a
4
2
-chlorobenzyl alcohol with Ac O
prepared hybrid compounds with significant changes. The bands
with lower intensities at 1100–1600 cm and near 3000 cm
confirmed the presence of the organic moieties in these com-
pounds (see Fig. S1†). In the hybrid compounds, the new Mo–N
stretching vibration presents at 975 cm . Compared to the
parent hexamolybdate anion, Mo–O stretching and asymmetric
Mo–O –Mo stretching vibrations are bathochromically shifted
respectively from 957 to 944 cm and from 799 to 781 cm
−1
−1
b
Entry
Time [min]
Catalyst amount [mg, mmol]
0
Yield [%]
1
2
3
4
5
6
7
8
4
4
4
4
4
4
4
4
15
20
16
52
74
95
100
100
c
−1
0.01 mmol
0.01 mmol
d
40 mg (0.0064 mmol)
50 mg (0.0080 mmol)
60 mg (0.0096 mmol)
65 mg (0.0104 mmol)
70 mg (0.0112 mmol)
b
−1
−1
−1
for hybrid compound 1a and from 957 to 943 cm and from
1
7
99 to 780 cm− for hybrid compound 2a. These observations
a
Reaction conditions: alcohol (1 mmol), Ac
2
O (2 mmol), CH
PP)Cl ] (2) was used as
19] was used as catalyst.
3
CN
imply that these bonds are weakened to some extent due to the
stronger Mo–N bonding interaction than that of Mo–O.
The H NMR spectra of compounds 1 and 1a are shown in
b
c
IV
(1 mL), catalyst. GC yield. [Sn (TNH
2
2
d
catalyst. [(n-Bu
4
N)]
2
[Mo
6
O
1
Fig. 2. These spectra clearly show resolved signals that can be
unambiguously assigned. The signals correspond to the aromatic
protons are present at 6.97–8.84 ppm for compound 1. The
Table 2 Optimization of Ac O amount in the acetylation of 4-
chlorobenzyl alcohol
2
a
–
NH proton signal is observed at 5.47 ppm for compound 1. In
2
compound 1, the singlet at 8.84 ppm is assigned to the eight β-H
protons and the two doublets (AB system) at 6.97 and 7.81 ppm
b
Entry
Time [min]
2
Ac O [mmol]
Yield [%]
(
8H, J = 8.0 Hz) correspond to the phenyl protons. Upon hybrid-
1
2
3
4
4
4
4
4
0.5
1.0
1.5
2.0
36
55
82
100
ization of compound 1 with hexamolybdate, all these signals
exhibited downfield chemical shifts compared to the compound
1
molybdate moiety. In the hybrid compound 1a, the aromatic
proton signals are present at 7.67–8.92 ppm and the signal corre-
. This is due to the electron-withdrawing nature of the hexa-
a
Reaction conditions: alcohol (1 mmol), Ac
2
O, CH
3
CN (1 mL), catalyst
b
(0.01 mmol). GC yield.
sponding to the –NH protons was absent in the hybrid com-
2
pound 1a. The protons ortho to the amine group showed the
largest shift, from 6.97 to 7.67 ppm for hybrid compound 1a.
The electronic property of the hybrid compound 1a was also
studied by UV-Vis spectrophotometry in DMF (see Fig. S2,
ESI†). In the hybrid compound 1a, the molybdenum–oxygen
potentials upon attachement of polyoxometalate. This can be
attributed to the relatively facile reduction of the porphyrin
moiety due to the very strong electron-withdrawing effect of the
polyoxometalate. All these observations confirmed that the POM
cluster interacts and electronically communicates with the por-
phyrin moiety.
2
−
charge-transfer absorption peak of Mo O
(λmax = 326 nm)
6
19
was significantly shifted to the red (and λmax = 377 nm). Also,
the reflectance of the compound 2a showed a bathochromic shift
(
λmax = 369) (see Fig. S3, ESI†), which implies that the for-
mation of the Mo–N π bonds increases the delocalization of the
aromatic π electrons. It is noted previously that the strong elec-
tronic interaction between the polyoxometalate cluster and the
Catalytic studies
HMTP was used as catalyst in the acetylation of alcohols and
phenols with acetic anhydride. First, the amount of catalyst was
optimized in the acetylation of 4-chlorobenzyl alcohol with
1
9
organic moiety depends on the conjugated system.
The electrochemical properties of the hybrid compound 1a
was also evaluated by cyclic voltammetry (CV). The CV of 1
and 1a in DMF, containing tetrabutylammonium bromide
Ac O. The results, which are summarized in Table 1, showed
2
that in the presence of 0.01 mmol (65 mg) of 2a, the highest
yield was obtained. In order to show the effect of hexamolybdate
groups on the catalytic activity of tin(IV) porphyrin, the catalytic
−
1
(
TBAB) (0.1 mol L ) as supporting electrolyte are shown in
Fig. S4 (ESI†). All potentials are quoted relative to an Ag/AgCl
2
0
IV
(
saturated KCl) electrode. The Zn(TNH PP) is reduced at
activity of [Sn (TNH PP)Cl ] (2) (0.01 mmol) was also investi-
2
2
2
−
1.188 and −1.631 V. The first reduction corresponds to the for-
gated in the acetylation of 4-chlorobenzyl alcohol with Ac O. In
2
mation of π anion radical and the second reduction is due to the
formation of porphyrin dianion. The CVof the hybrid compound
showed a more complex picture. The CV of the hybrid com-
pound 1a showed a reduction wave at −1.78 V. This remarkable
cathodic shift in the reduction potential of this compound may
be associated to the more difficult reduction of the polyoxometa-
late due to the electron-donating properties of the porphyrin
toward the polyoxometalate. This is a similar behaviour which
has been reported in the attachment of 1,10-phenanthroline to
this case, only 20% of the corresponding acetate was obtained
(Table 1, entry 2) while in the presence of catalyst HMTP (2a),
the reaction was completed after 4 min. The increase of catalytic
activity of HMTP can be attributed to the electron-withdrawing
nature of hexamolybdate which increases the electron deficiency
of the tin(IV) porphyrin that is responsible for the catalytic pro-
perty. These results are in consistent with the data obtained by
electrochemical studies. As mentioned above, upon attachment
of electron-withdrawing hexamolybdate substituents to the por-
phyrin ring, the electron density of the porphyrin ring decreased
and therefore reduced more easily. As we reported in our pre-
vious works on the catalytic activity of tin(IV) porphyrins, the
2
1
hexamolybdate by Neumann and co-workers. This observation
is consistent with the UV-Vis data. Also, all waves shown in the
CV of the parent compound 1 showed a shift to positive
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a
2
Table 3 Acetylation of alcohols with Ac O catalyzed by hybrid compound 2a at room temperature
b
Entry
1
Alcohol
Acetate
Time [min]
4
Yield [%]
100
100
100
2
3
4
4
4
7
100
5
6
4
4
100
100
7
8
4
4
100
100
9
4
4
100
100
1
0
1
1
4
4
5
100
100
100
1
1
2
3
1
1
4
5
5
5
100
100
1
1
6
7
5
6
100
95
1
8
7
95
a
b
Reaction conditions: alcohol (1 mmol), Ac
2
O (2 mmol), catalyst (1 mol%), CH
3
CN (1 mL). GC yield.
−
−
introduction of electron deficient ligands such as OTf , BF
acetate was the as same as blank experiments (Table 1, entry 3).
These results clearly indicated that the tin(IV) porphyrin is the
4
11t–v
and ClO4− increases the electron-deficiency of the tin center.
In the HMTP, the same role was observed by using hexamoly-
bdate as an elecron-withdrawing substituent. On the other hand,
the catalytic activity of hexamolybdate (0.01 mmol) was also
investigated in the acetylation of 4-chlorobenzyl alcohol with
catalyst active site. Then, the amount of Ac O was also opti-
mized. The results showed that in the presence of 2 mmol of
2
Ac O, the reaction was completed (Table 2).
2
The optimized conditions which were obtained for acetylation
Ac O. The results showed that the amount of corresponding
of 4-chlorobenzyl alcohol were alcohol, Ac O and catalyst in a
2
2
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Table 4 Acetylation of phenols with Ac
compound 2a at room temperature
2
O catalyzed by hybrid
Table 5 Reusability of hybrid compound 2a in the acetylation of
4-chlorobenzyl alcohol with Ac O
2
a
a
b
b
c
Time
[min]
Yield
[%]
Run
Acetate (%)
Time [min]
Sn leached [%]
Entry
1
Phenol
Acetate
1
2
3
4
5
6
7
8
9
100
100
100
100
100
100
100
100
100
4
4
4
4
4
4
4
4
4
0
0
0
0
0
0
0
0
0
6
100
2
3
4
6
6
6
6
6
100
100
100
100
100
a
2
Reaction conditions: 4-chlorobenzyl alcohol (1 mmol), Ac O
b
c
(2 mmol), catalyst (1 mol%), CH
3
CN (1 mL). GC yield. Determined
by ICP.
c
5
c
6
respectively, and dried before using with fresh 4-chlorobenzyl
alcohol and Ac O. The catalyst was consecutively reused nine
2
times without detectable catalyst leaching or significant loss of
its activity (Table 5). The nature of the recovered catalyst was
monitored using FT-IR and UV-Vis spectra of recycled catalyst
c
7
6
100
(
(
2a), which showed no changes in comparison with fresh catalyst
Fig. S1 G and S2 C, ESI†).
c
8
6
8
100
100
Conclusion
9
A new hybrid porphyrin–POM complex was prepared via
covalent interaction of Zn(TNH PP) and Sn(TNH PP)Cl with
2
2
2
2−
Mo O
Lindqvist type polyoxometalate. These new por-
6
19
a
phyrin–polyoxometalate hybrid materials were characterized by
Reaction conditions: phenol (1 mmol), Ac
2
O (2 mmol), catalyst
CN (1 mL). GC yield. Reaction was performed with
2
O per OH group.
b
c
1
(1 mol%), CH
3
H NMR, FT IR and UV-Vis spectroscopic methods and cyclic
2
mmol of Ac
voltammetry. The influence of the polyoxometalate on the por-
phyrin ring was studied. The hexamolybdate–tin(IV) porphyrin
hybrid material was used as catalyst for efficient acetylation of
primary, secondary and tertiary alcohols and phenols with Ac O.
2
molar ratio of 100 : 200 : 1. Under these conditions, a wide
Short reaction times, excellent yields, easy work-up and reusabil-
ity and stability of the catalyst are noteworthy advantages of this
method.
variety of alcohols were subjected to acetylation with Ac O. The
2
obtained results showed that different primary, secondary
(including aliphatic and aromatic alcohols) and tertiary alcohols
were acetylated successfully at room temperature (Table 3). As
can be seen, in benzylic alcohols the nature of substituents (elec-
tron-withdrawing or electron-releasing) has no significant effect
on the product yield. In the case of tertiary alcohols such as
Acknowledgements
We are thankful to the Research Council of the University of
Isfahan for financial support of this work.
2
-methyl-1-phenyl-2-propanol (entry 18) no elimination product
was observed.
Under the optimized conditions, which were described for
Notes and references
acetylation of alcohols, the acetylation of phenols with Ac O
2
1
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