Nanoaggregates of simple Mn porphyrin complexes as catalysts for the selective oxidation of hydrocarbons

Article abstract of DOI:10.1002/ejic.201201437

A stable suspension of simple manganese(III) meso-tetraphenylporphyrin nanoaggregates was prepared by a host-guest procedure, in which ethanol and water are used as "green" solvents. The organic nanoparticles were studied by dynamic light scattering (DLS), AFM, and UV/Vis and fluorescence spectroscopy. The results indicate that the nanoaggregates have a spherical morphology with sizes between 15-40 nm. The epoxidation of olefins and oxygenation of saturated hydrocarbons by PhI(OAc)₂ were efficiently enhanced with excellent selectivity under the influence of simple Mn(TPP)OAc (TPP = meso-tetraphenylporphyrin) nanoparticles. Enhanced stabilities and activities were achieved with nanostructured Mn catalysts compared to those of the individual counterparts in solution according to turnover numbers and UV/Vis studies. The title nanocatalyst facilitates a greener reaction because the reaction solvent is a water/ethanol mixture and PhI(OAc)₂ is safe to use. The efficiency of the oxidation system depends critically upon the steric hindrances and electronic structures of both nitrogen donor ligands and porphyrin nanoparticles. Copyright

Full text of DOI:10.1002/ejic.201201437

FULL PAPER
DOI:10.1002/ejic.201201437
Nanoaggregates of Simple Mn Porphyrin Complexes as
Catalysts for the Selective Oxidation of Hydrocarbons
Abdolreza Rezaeifard,*[a] Vahideh Soltani,[a] and
Maasoumeh Jafarpour*[a]
Keywords: Self-organization / Nanoaggregates / Nanoparticles / Oxygenation / Epoxidation / Porphyrinoids
A stable suspension of simple manganese(III) meso-tet-
raphenylporphyrin nanoaggregates was prepared by a host–
guest procedure, in which ethanol and water are used as
“green” solvents. The organic nanoparticles were studied by
dynamic light scattering (DLS), AFM, and UV/Vis and fluo-
rescence spectroscopy. The results indicate that the nanoag-
gregates have a spherical morphology with sizes between
15–40 nm. The epoxidation of olefins and oxygenation of sat-
urated hydrocarbons by PhI(OAc)2 were efficiently enhanced
with excellent selectivity under the influence of simple
Mn(TPP)OAc (TPP = meso-tetraphenylporphyrin) nanopar-
ticles. Enhanced stabilities and activities were achieved with
nanostructured Mn catalysts compared to those of the indi-
vidual counterparts in solution according to turnover num-
bers and UV/Vis studies. The title nanocatalyst facilitates a
greener reaction because the reaction solvent is a water/eth-
anol mixture and PhI(OAc)2 is safe to use. The efficiency of
the oxidation system depends critically upon the steric hin-
drances and electronic structures of both nitrogen donor li-
gands and porphyrin nanoparticles.
high under mild conditions because of their very large sur-
face area.[7] Nanoscaled particles composed of porphyrins
are expected to have chemical activities significantly dif-
ferent from those of the free porphyrins or of those immo-
bilized onto/into supports. Porphyrin nanoparticles are
promising components of advanced materials because of
their rich photochemistry, stability, and catalytic rates ow-
ing to the structures of the aggregates and their greater sur-
face area and proven catalytic activity.[8]
The extensive use of toxic and volatile organic solvents
in many oxidation systems mediated by metalloporphyrins
is another problem that has considerably reduced the im-
portance of biologically inspired oxidation catalysis.[3]
Furthermore, during recent years, increasingly demanding
legislation as well as public and economic pressure has led
to increased interest in new “clean” methods for the pro-
duction of chemical compounds.[9]
Accordingly, in line with our ongoing research on the
development of new oxidation methods,[10] we recently
turned our attention to sustainable procedures by using
nanocatalysts in aqueous media.[11]
In the present work, we have prepared colloidal
Mn(TPP)OAc (TPP = meso-tetraphenylporphyrin) nano-
particles with an average size of 24 nm through a modified
host–guest procedure with H2O and EtOH as green sol-
vents. The presence of simple Mn(TPP)OAc nanoparticles
enhanced the efficiency of the epoxidation of olefins and
the selective oxidation of saturated hydrocarbons to the re-
lated ketones in aqueous media by phenyliodonium diacet-
ate [PhI(OAc)2], which is as an environmentally friendly
Introduction
The controlled and selective oxidation of hydrocarbons
is one of the most important technologies for the conver-
sion of petroleum products to valuable commodity chemi-
cals.[1] Transition-metal complexes are often used as cata-
lysts for industrial-scale oxidation reactions.[2] In the past
two decades, the catalysis of alkane hydroxylation and ole-
fin epoxidation by metalloporphyrins has received consider-
able attention.[3] The oxidative degradation of the ligand,
difficulties in separating the products, and contamination
by residual catalyst are problems often encountered in oxi-
dation reactions catalyzed by metalloporphyrins.[4] One ap-
proach to increase the catalyst lifetime is to immobilize it
on an adequate support, which provides site isolation of the
metal center and, thus, minimizes catalyst self-destruction
and dimerization of unhindered metalloporphyrins.[5] How-
ever, owing to the diffusion of substrates and products
through the pores of the support materials, a substantial
decrease in the reaction rate is frequently observed com-
pared with the rates of homogeneous systems.[6] Recently,
nanoparticles have been used (NPs; sometimes called giant
clusters, nanoclusters, or colloids) as their activity is very
[a] Catalysis Research Laboratory, Department of Chemistry,
Faculty of Science, University of Birjand,
Birjand, 97179-414 Iran
Fax: +98-561-2502515
E-mail: rrezaeifard@birjand.ac.ir
rrezaeifard@gmail.com
mjafarpour@birjand.ac.ir
Homepage:www.birjand.ac.ir
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and commonly used oxidant (Scheme 1).[12] The influence creased in comparison to with those of the ethanol-solvated
of axial nitrogen donor ligands and the porphyrin structure porphyrin. The existence of a small redshift could be indica-
on the activity of the nanocatalysts was also investigated.
tive of the presence of a J-type aggregate.[8j]
Scheme 1. Oxygenation of hydrocarbons with PhI(OAc)2 catalyzed
by Mn–porphyrin nanoparticles in aqueous media.
Figure 2. UV/Vis spectra of Mn(TPP)OAc in ethanol (blue line)
and its nanoparticle in water/ethanol (red line) with c = 5.3ϫ10–5
m
in both cases. The inset intensity scale is magnified five times.
The fluorescence spectra of the nanoaggregates and eth-
anol-dissolved Mn(TPP)OAc are compared in Figure 3. To
attain such a comparison, the same concentration and sim-
ilar instrumental conditions were used. The fluorescence in-
tensity of colloidal porphyrin nanoparticles decreases com-
pared to that of the dissolved compound. Porphyrin
nanoaggregates usually present a quenching of the fluores-
cence for several reasons, such as nonradiative pathways
and photoscreening of chromophores. Particles with sizes
lower than 150 nm are reported to present anomalous and
depressed fluorescence.[13]
Result and Discussions
Characterization of Colloidal Mn(TPP)OAc Nanoparticles
The use of high boiling point, harmful organic host sol-
vents such as N,N-dimethylformamide (DMF) and tetra-
hydrofuran (THF) in the preparation of porphyrin nano-
particles[8g–8i] causes difficulty in product isolation as well
as environmental problems. We used ethanol as a safe host
solvent for the preparation of porphyrin nanoaggregates
and found that it enhanced catalytic activity.
Figure 1 shows the TEM images of a colloidal solution
of Mn(TPP)OAc nanoparticles and its bulk counterpart;
their average sizes are 30 and 350 nm, respectively.
Figure 1. TEM images of Mn(TPP)OAc dispersed in water (A) and
colloidal nanoparticles formed in water/ethanol (B).
Figure 3. Fluorescence spectra of Mn(TPP)OAc in ethanol (blue
line) and its nanoparticles (red line) with c = 5.3ϫ10–5 m; excited
at 270 nm in both cases. The inset intensity scale is magnified eight
times.
The UV/Vis spectrum of a suspension of Mn(TPP)OAc
nanoaggregate in water/ethanol was recorded and com-
pared with that obtained with the ethanol-dissolved coun-
terpart (Figure 2). To establish such a comparison, both
Dynamic light scattering (DLS) shows that the size dis-
samples were adjusted to the same concentration value. In tribution of the nanoparticles is remarkably narrow (16–
the spectrum of the dispersed porphyrin nanoparticles, the 51 nm) and the average particle diameter is 24 nm (Fig-
Soret and Q bands broadened and the absorbance de- ure 4), which is in excellent agreement with the TEM re-
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sults. Particles in the 10–40 nm size range can also easily be
Our investigation showed that the efficiency of the epox-
detected by AFM of Mn(TPP)OAc nanoparticles deposited idation reaction was affected significantly by increased cata-
on the glass surfaces (Figure 5).
lyst loading and reaction temperature (Table 2). A slight in-
crease in the reaction temperature to 40 °C and an in-
creased catalyst loading of 0.40 mol-%, resulted in 65 and
100% conversion of cyclooctene within 7 and 12 h, respec-
tively, with 2 equiv. of oxidant (Table 2, Entry 2). Neverthe-
less, changes in temperature (Table 2, Entries 1–5), catalyst
loading (Table 2, Entries 6–11), and oxidant amount
(Table 2, Entries 12–15) did not improve the conversion
rate.
Table 2. Screening of temperature and catalyst and oxidant concen-
tration in the oxidation of cyclooctene by PhI(OAc)2 catalyzed by
Mn(TPP)OAc nanoparticles.[a]
Entry
T
[°C]
Catalyst
[mol-%]
Oxidant/olefin
molar ratio
Yield
[%][b]
1
2
3
4
5
6
7
8
25
40
50
60
70
40
40
40
40
40
40
40
40
40
40
0.40
0.40
0.40
0.40
0.40
0.10
0.20
0.25
0.30
0.50
1.00
0.40
0.40
0.40
0.40
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
0.50
1.00
2.50
3.00
27
65
67
69
75
31
34
39
45
65
55
50
54
61
52
Figure 4. DLS characterization of Mn(TPP)OAc nanoparticles
with an average diameter of 24 nm.
9
Preliminary Catalytic Studies
10
11
12
13
14
15
The oxidation of cyclooctene with iodobenzene in neat
water, which does not proceed in the absence of catalyst,
was used as a model reaction. Preliminary catalytic experi-
ments were conducted with cyclooctene (0.1 mmol) and
PhI(OAc)2 (0.2 mmol) in a colloidal solution of Mn(TPP)-
OAc nanoparticles (0.40 mol-%) in water/ethanol at 25 °C
under air; cyclooctene oxide formed in 27% yield after 7 h.
Other oxidants, namely, H2O2, tBuOOH, and NaIO4, were
less reactive than PhI(OAc)2 under the same conditions at
different temperatures (Table 1).
[a] The reactions were run under air for 7 h. [b] Yield of cyclooctene
oxide determined by GC.
Cocatalytic Effect of Nitrogen Donor Ligands
The influence of axial ligands as cocatalysts for the epox-
idation of cyclooctene was also explored. Initially, the effect
of the concentration of imidazole (Im) as a model nitrogen
donor ligand was investigated (Figure 6). An increase in the
Im/Mn–porphyrin nanoparticles molar ratio to 20 remark-
ably improved the rate of the epoxidation reaction, and cy-
clooctene oxide was obtained quantitatively within 4 h (17,
54, 80, and 93% epoxide yields after 1, 2, 3, and 4 h, respec-
tively). However, a further increase in the molar ratio de-
creased the olefin conversion, which confirmed the forma-
tion of the catalytically inactive six-coordinate species
Mn(TPP)(Im)2.[10f,10g,14]
Table 1. The effect of different oxidants in the oxidation of cyclo-
octene catalyzed by Mn(TPP)OAc nanoparticles.[a]
Oxidant
Yield at 25 °C [%][b]
Yield at 70 °C [%][b]
H2O2
0
15
27
3
0
tBuOOH
PhI(OAc)2
NaIO4
29
75
22
[a] The reactions were run under air for 7 h with a molar ratio of
263:525:1 for cyclooctene/PhI(OAc)2/Mn(TPP)OAc nanoparticles.
[b] Yields of cyclooctene oxide determined by GC.
Figure 5. AFM topographic images of Mn(TPP)OAc nanoparticles on glass. (A) 3D, (B) 2D, and (C) voltage profile image.
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Table 3. Aqueous oxygenation of hydrocarbons by PhI(OAc)2 cata-
lyzed by Mn(TPP)OAc nanoparticles in the presence of imidazole
as cocatalyst.[a]
Figure 6. The effect of various Im/Mn(TPP)OAc molar ratios on
the epoxidation of cyclooctene by PhI(OAc)2 at 40 °C after 4 h.
The efficiency of the reaction was dramatically depend-
ent upon the nature of the cocatalyst (Figure 7). The results
for the oxidation of cyclooctene by PhI(OAc)2 catalyzed by
Mn(TPP)OAc nanoparticles in association with three
classes of nitrogen donor ligands as cocatalysts, including
the strong pure σ-donor amines (pKa = 10.75–11.12), the
strong π-donor imidazoles (pKa = 5.53–7.86), and weak π-
donor pyridines (pKa = 1.86–6.65), are given in Figure 7.
Our results demonstrate that the general order of the co-
catalytic activities of the various classes of nitrogen donor
ligands was amines Ͻ pyridines ϽNH imidazoles, which
indicates the importance of π-donating effects, and imid-
azole is by far the best cocatalyst. Significant decreases in
the conversion rates with bulky 4-tBuPy and Et3N indicate
that steric effects are quite pronounced. The much lower
cocatalytic activity of 1-MeIm, which lacks the N–H···B hy-
drogen bond (B = nitrogen donor or anionic species), and
also the weak base 4-CNPy (pKa = 1.86), which contains
an electron-withdrawing CN group, demonstrate the crucial
importance of electronic factors upon their donor abilities
toward the Mn–porphyrin catalyst.[10f,10g,14]
[a] The reactions were run under air for 4 h at 40 °C, and the molar
ratio of substrate/ PhI(OAc)2/imidazole/Mn(TPP)OAc nanopar-
ticles was 263:525:20:1. [b] Yield of isolated products. [c] The yields
in parenthesis are after 20 h. [d] The products were identified by
1H NMR spectroscopy or by comparison with the GC retention
times of authentic samples. The product selectivities were Ͼ99%.
[e] δ [ppm] of C–H of oxirane ring.
Figure 7. Cocatalytic activity of different nitrogen donor ligands
on the oxidation of cyclooctene by PhI(OAc)2 in the presence of
Mn(TPP)OAc nanoparticles after 4 h. The catalyst/axial ligand
molar ratio was 1:10.
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Catalytic Oxidation of Olefins and Saturated
Hydrocarbons
To establish the general applicability of the method, vari-
ous olefins were subjected to the oxidation protocol by
using PhI(OAc)2 under the catalytic influence of Mn(TPP)-
OAc nanoparticles in the presence of imidazole as cocata-
lyst at 40 °C (Table 3).
As summarized in Table 3, various olefins are generally
excellent substrates for this nanocatalyst. High conversions
of cyclohexene, cyclooctene, pinene, norbornene, indene,
and trans-β-methylstyrene were obtained with the forma-
tion of the corresponding epoxides as sole products
(Table 3, Entries 1–6). However, the oxidation of a terminal
C–C double bond conjugated to a phenyl ring gave exclu-
sively the related carbonyl compounds, which indicates that
the epoxides undergo ring-opening reactions (Table 3, En-
tries 7–9).[15]
We then applied this oxidation system to the oxygenation
of saturated C–H bonds under the same conditions as those
used for olefin epoxidation. The oxidation of the C–H benz-
ylic hydrocarbons indane and teraline gave low yields, but
excellent selectivities of the corresponding ketones were
achieved within 6 h at 40 °C (33 and 38% yields of in-
danone and α-teralone, respectively). Nevertheless, longer
reaction times improved the yields of the oxidation prod-
ucts so that 70–85% yields of the related ketones were
achieved after 20 h. Under these conditions, inactive cyclo-
octane was oxidized selectively to cyclooctanone in 25%
yield, which is a notable indicator of the stability of title
catalyst.
Electronic and Steric Effects of the Porphyrin Ligand
Figure 8. The effect of the electronic and steric requirements of
the porphyrin ligands on the catalytic activity of the Mn catalyst
nanoparticles prepared in water/DMF for the epoxidation of cyclo-
octene. The reactions were run under the conditions given in
Table 3 for 4 h.
The catalytic activities of Mn–porphyrin nanoparticles
with different electronic and structural properties in the oxi-
dation of cyclooctene in combination with Im have also
been compared (Figure 8). Owing to the low solubility of
some substituted Mn–porphyrins in ethanol, we used DMF
as the host solvent for the preparation of all nanoparticles,
including Mn(TPP)OAc, to obtain the same conditions for
this study. The lower activity of Mn[T(4-NO2P)P]OAc,
which contains an electron-withdrawing NO2 group, com-
pared to those of the simple Mn(TPP)OAc and electron-
rich Mn[T(4-OMeP)P]OAc with very similar steric environ-
Stability and Reusability of Catalyst
The impressive results obtained in the epoxidation of ole-
ments reflects the reduced influence of the electron-deficient fins and the oxygenation of hydrocarbons confirm the high
substituent upon the catalytic activity of the Mn catalyst. activity and stability of Mn(TPP)OAc nanoparticles in oxi-
Nevertheless, as the steric factors became dominant at the dation reactions. To establish this claim, the turnover num-
Mn center of the catalyst, the oxidation activity and sta- bers (TONs) of simple Mn(TPP)OAc nanoparticles were
bility were enhanced as can be seen for hindered compared with those obtained in solution (Table 4). Our
Mn(TPFPP)OAc and Mn(TDCPP)OAc, which are highly results demonstrated that 3-fold to 10-fold greater TONs
electrophilic. A more important result is the lower epoxide for the standard catalytic epoxidation of cyclooctene were
yield obtained with the Mn(TPP)OAc nanoparticles pre- obtained for the nanoparticles than for the individual por-
pared in water/DMF (42%) than those prepared in water/ phyrin in solution. Notably, water-dispersed Mn(TPP)OAc
ethanol (93%, Figure 6) under the same conditions; this displayed a moderate TON under the same conditions
highlights the superior effect of ethanol as a green solvent (Table 4, Entry 3), which demonstrates the effect of particle
on the efficiency of the nanocatalyst.
size on the catalytic efficiency (Figure 1).
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Table 4. TONs of Mn(TPP)OAc in the oxidation of cyclooctene by
PhI(OAc)2 under different conditions.[a]
The enhanced TONs and the stability of the Mn–por-
phyrin nanoaggregates can be explained by assuming that
both the metalloporphyrins on the surface of the nanopar-
ticles and those on the surface of the subdomains are the
active catalytic entities and that the substrate diffuses into
the voids between the subdomains of the larger particles.
The inner macrocycles then serve as a structural support to
reduce self-oxidation and destruction of the catalyst. When
the porphyrins on the exterior of the nanoparticle eventu-
ally decompose, they fall off because of the greatly in-
creased polarity of the oxidized product, which thereby ex-
poses the next layer of catalytically active molecules.[8h,1]
A literature review of the catalytic activity of porphyrin
nanoparticles in the oxidation of hydrocarbons demon-
strated that in most cases allylic oxidation products were
formed rather than epoxide (Ͻ1%) in the oxidation of cy-
clohexene with molecular oxygen or H2O2 as oxygen
sources.[8d,8h–8j,16] If PhIO was used as oxidant, 70% epox-
ide selectivity was obtained in the oxidation of cyclohex-
ene.[8h] Notably, in almost all reports perfluoro aryl or alkyl
porphyrins, which stabilize the porphyrin nanoparticles to-
ward self-oxidative degradation, have been used. However,
in some conditions they decomposed completely even in the
presence of O2.[8d,16] Moreover, invaluable oxyfunctionali-
zation of less reactive saturated hydrocarbons has not been
reported in these studies.
Entry Mn(TPP)OAc
Yield [%][b] TON/24 h
1
2
3
4
EtOH-dissolved
CH3CN-dissolved
water-dispersed
21
7
41
66
166
55
324
521
nanoparticles (water/ethanol)
[a] The reactions were run under air at 40 °C, and the molar ratio of
cyclooctene/ PhI(OAc)2/imidazole/Mn(TPP)OAc nanoparticles was
780:1320:20:1. [b] Yield of cyclooctene oxide determined by GC.
More evidence for the oxidative stability of the Mn cata-
lyst was achieved by comparing the UV/Vis spectral
changes of colloidal Mn(TPP)OAc nanoparticles in water/
ethanol with those of their solution counterpart (Figure 9).
Although the Soret band of ethanol-solvated Mn(TPP)OAc
disappeared completely in less than 30 min, the colloidal
nanocatalysts demonstrated significant durability under the
reaction conditions.
Thus, excellent selectivity towards the epoxidation of ole-
fins as well as the carbonylation of saturated hydrocarbons
under sustainable conditions by using simple Mn(TPP)OAc
nanoparticles with enhanced stability and activity highlight
the advantages of the title methodology over previous work.
Conclusions
A simple procedure for the sustainable preparation of
hydrophobic Mn–porphyrin nanoaggregates as aqueous
suspensions with excellent colloidal stability has been devel-
oped. DLS and AFM measurements showed that the aggre-
gates exhibit spherical morphology and are nanostructured
with an average size of 24 nm. UV/Vis and fluorescence
spectroscopic characterization has permitted us to discern
that the porphyrin particles are dispersed in the suspension
as J-type nanoaggregates. The epoxidation of olefins and
the oxidation of saturated hydrocarbons to ketones by using
PhI(OAc)2 as a safe oxidant in aqueous media were ef-
ficiently enhanced in the presence of a low catalyst loading
of the title nanocatalyst with excellent selectivity. No toxic
reagents, solvents, or byproducts were involved and no la-
borious purifications were necessary. This, together with the
use of water and ethanol as standard “green” solvents as
well as the easy and safe work-up procedure, wide substrate
range, and the reusability of the catalyst and byproduct
makes this cost-effective, environmentally benign methodol-
ogy suitable for applied goals.
Figure 9. The UV/Vis spectral changes of Mn(TPP)OAc (0.4 mm)
in the presence of Im (8.0 mm) during the epoxidation of cyclo-
octene by PhI(OAc)2 (2 equiv.) at 40 °C. Ethanol-solvated catalyst
after 30 min (A) and colloidal nanoparticles in H2O/EtOH after
1 h (B) and 5 h (C).
These promising results encouraged us to evaluate the
reusability of the nanostructured catalyst. The removal of
the ethanol from the aqueous mixture followed by cooling
of the flask precipitated the reduced form of the oxidant.
The colloidal Mn(TPP)OAc nanoparticles were separated
from the aqueous solution by centrifugation and were re-
used for subsequent oxidations under similar reaction con-
ditions. The catalyst could be reused at least four times in
the oxidation of cyclooctene without noticeable loss of ac-
tivity. The average GC yields of cyclooctane oxide for four
runs were 90%, and no Mn ions were detected in the reac-
tion solutions according to atomic absorption analysis. Af-
ter that, the water-insoluble products were isolated from the
aqueous solution by using ethyl acetate as an environmen-
tally benign solvent; this gave the desired products in high
purity (Ͼ99%, GC analysis) with no need for any laborious
purifications.
Experimental Section
Materials: Purity determinations of the products were ac-
complished by GC with a Shimadzu GC-16A instrument equipped
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4, Biochemistry and Binding: Activation of Small Molecules, Ac-
ademic Press, 2000.
with a 25 m CBP1–S25 (0.32 mm i.d., 0.5 m coating) capillary col-
umn. UV/Vis spectra were recorded with a 160 Shimadzu spectro-
photometer. All chemicals were purchased from Merck or Fluka.
The free base porphyrins: TPPH2,[17] T(4-OMeP)PH2,[17] T(4-
NO2P)PH2,[18] TDCPPH2,[19] and TPFPP[20] were prepared and
purified by the methods reported previously. The Mn(porphyrin)-
OAc complexes were obtained by using Mn(OAc)2·4H2O according
to the procedure of Adler et al.[21] Mn(TPFPP)OAc was synthe-
sized in a manner similar to that described by Kadish et al.[22]
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Synthesis of Colloidal Mn(TPP)OAc Nanoparticles: The colloidal
Mn(TPP)OAc nanoparticles were prepared according to the host–
guest procedure reported by Drain et al.[8h,1,1j] but with the modifi-
cation of the host solvent. We replaced DMF and THF with eth-
anol as a safe host solvent. Mn(TPP)OAc nanoparticles was pre-
pared in 10 mL vials by adding deionized water (5 mL, pH 6.5–7)
to a mixture of triethylene glycol monomethyl ether (0.20 mL) as
stabilizer and a stock solution of Mn(TPP)OAc in ethanol (1.8 mm,
0.40 mL). The solution was sonicated during the addition and was
sonicated for an additional 15 min. The prepared nanoparticles
were stable for more than one month and were refrigerated at ca.
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[7]
General Procedure for the Oxygenation of Hydrocarbons by
PhI(OAc)2 Catalyzed by Mn (TPP)OAc Nanoparticles: To a mix-
ture of olefin or saturated hydrocarbon (0.1 mmol) in H2O/EtOH
(13:1, 3 mL) containing colloidal Mn(TPP)OAc nanoparticles
(4.0ϫ10–4 mmol) was added PhI(OAc)2 (0.20 mmol, 0.065 g) in the
presence of imidazole (7.6ϫ10–3 mmol). The reaction mixture was
stirred at 40 °C under air. After completion of the reaction, the
ethanol was removed from the aqueous mixture. The reduced form
of the oxidant was precipitated by cooling the mixture, and the
colloidal Mn(TPP)OAc was isolated by centrifugation. The aque-
ous mixture was then washed with ethyl acetate (3ϫ2.0 mL), the
organic phase was separated, and the solvent was evaporated to
give the desired products with high purity. For incomplete reac-
tions, the pure product was obtained after chromatographic purifi-
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[8]
General Procedure for the Determination of TONs of Mn(TPP)OAc
Nanoparticles: To a mixture of cyclooctene (0.3 mmol) and imid-
azole (7.6ϫ10–3 mmol) in a colloidal solution of Mn(TPP)OAc
nanoparticles (4.0ϫ10–4 mmol, 3 mL) was added PhI(OAc)2
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24 h under air.
General Procedure for the Determination of TONs of Solvent-Dis-
solved Mn(TPP)OAc: PhI(OAc)2 (0.5 mmol) was added to a solu-
tion of ethanol or acetonitrile (3 mL) containing cyclooctene
(0.3 mmol), imidazole (7.60ϫ10–3 mmol), and a stock solution of
Mn(TPP)OAc (1.8 mm, 0.20 mL). The reaction mixture was stirred
at 40 °C for 24 h under air.
[9]
Acknowledgments
Support for this work by the Research Council of the University
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Published Online: March 28, 2013
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