Enhanced catalytic activity and unexpected products from the oxidation of cyclohexene by organic nanoparticles of 5,10,15,20-tetrakis-(2,3,4,5,6- pentafluorophenyl)porphyrinatoiron(III) in water by using O2

Article abstract of DOI:10.1002/chem.200901086

The catalytic oxidation of alkenes by most iron porphyrins using a variety of oxygen sources, but generally not dioxygen, yields the epoxide with minor quantities of other products. The turnover numbers for these catalysts are modest, ranging from a few hundred to a few thousand depending on the porphyrin structure, axial ligands, and other reaction conditions. Halogenation of substituents increases the activity of the metalloporphyrin catalyst and/or makes it more robust to oxidative degradation. Oxidation of cyclohexene by 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinato iron-(III), ([Fe^III(tppf₂₀)]) and H₂O₂ is typical of the latter: the epoxide is 99% of the product and turnover numbers are about 350.^[1-4] Herein, we report that dynamic organic nanoparticles (ONPs) of [Fe^III(tppf₂₀)] with a diameter of 10 nm, formed by host-guest solvent methods, catalytically oxidize cyclohexene with O₂ to yield only 2-cyclohexene-1-one and 2-cyclohexene-1-ol with approximately 10-fold greater turnover numbers compared to the non-aggregated metalloporphyrin in acetonitrile/methanol. These ONPs facilitate a greener reaction because the reaction solvent is 89% water and O₂ is the oxidant in place of synthetic oxygen sources. This reactivity is unexpected because the metalloporphyrins are in close proximity and oxidative degradation of the catalyst should be enhanced, thus causing a significant decrease in catalytic turnovers. The allylic products suggest a different oxidative mechanism compared to that of the solvated metalloporphyrins. These results illustrate the unique properties of some ONPs relative to the component molecules or those attached to supports.

Full text of DOI:10.1002/chem.200901086

FULL PAPER
DOI: 10.1002/chem.200901086
Enhanced Catalytic Activity and Unexpected Products from the Oxidation of
Cyclohexene by Organic Nanoparticles of 5,10,15,20-Tetrakis-(2,3,4,5,6-
pentafluorophenyl)porphyrinatoironACTHNUTRGNE(UNG III) in Water by Using O2
Gabriela Smeureanu,[b] Amit Aggarwal,[b] Clifford E. Soll,[b] Julius Arijeloye,[b]
Erik Malave,[b] and Charles Michael Drain*[a, b]
Abstract: The catalytic oxidation of al-
kenes by most iron porphyrins using a
variety of oxygen sources, but generally
not dioxygen, yields the epoxide with
minor quantities of other products. The
turnover numbers for these catalysts
are modest, ranging from a few hun-
dred to a few thousand depending on
the porphyrin structure, axial ligands,
and other reaction conditions. Halo-
genation of substituents increases the
activity of the metalloporphyrin cata-
lyst and/or makes it more robust to oxi-
dative degradation. Oxidation of cyclo-
hexene by 5,10,15,20-tetrakis-(2,3,4,5,6-
pentafluorophenyl)porphyrinato iron-
cal of the latter: the epoxide is 99% of
the product and turnover numbers are
about 350.[1–4] Herein, we report that
dynamic organic nanoparticles (ONPs)
methanol. These ONPs facilitate a
greener reaction because the reaction
solvent is 89% water and O2 is the oxi-
dant in place of synthetic oxygen sour-
ces. This reactivity is unexpected be-
cause the metalloporphyrins are in
close proximity and oxidative degrada-
tion of the catalyst should be en-
hanced, thus causing a significant de-
crease in catalytic turnovers. The allylic
products suggest a different oxidative
mechanism compared to that of the
solvated metalloporphyrins. These re-
sults illustrate the unique properties of
some ONPs relative to the component
molecules or those attached to sup-
ports.
of [FeIII
ACTHNUTRGEN(UNG tppf20)] with a diameter of
10 nm, formed by host–guest solvent
methods, catalytically oxidize cyclohex-
ene with O2 to yield only 2-cyclohex-
ene-1-one and 2-cyclohexene-1-ol with
approximately 10-fold greater turnover
numbers compared to the non-aggre-
gated metalloporphyrin in acetonitrile/
Keywords: alkenes
·
catalytic
oxidation · nanoparticles · oxygen ·
porphyrinoids
ACHTUNGTRENNUNG ACHTUNGTRENNUNG(tppf20)]) and H2O2 is typi-
(III), ([FeIII
Introduction
lytic applications.[5,6] The inorganic cores of these conven-
tional nanoparticles are robust and structurally static. In
contrast, the structures, properties, and functions of aggre-
gates of organic molecules organized into nanoparticles, or-
ganic nanoparticles (ONPs), by weak intermolecular interac-
tions are much less understood because the organization of
the molecules in the ONPs can be dynamic.[7]
Organic nanoparticles: Inorganic nanoparticles with various
capping groups or imbedded into polymers or other matrix-
es are widely used, or proposed, for a diverse array of cata-
[a] Prof. Dr. C. M. Drain
Most methods to make nanoaggregates of small organic
molecules have their historical roots in the formation of col-
loidal dispersions of organic systems.[8] The methods to
make nanoscaled aggregates of dyes such as porphyri-
noids[7,9–17] include: a) the rapid exchange of solvent,
b) host–guest solvents whereby aggregation occurs by
mixing of solutions containing the chromophoric molecules
with miscible solvents in which they are not soluble (e.g.
THF/H2O) and by stabilizing with surfactants or amphipath-
ic molecules, c) interfacial precipitation, and d) the rapid ex-
pansion of supercritical solvents. The former two methods
result in dispersions in solution and the latter two methods
The Rockefeller University
1230 York Avenue, New York, NY 10065 (USA)
[b] Dr. G. Smeureanu, A. Aggarwal, Dr. C. E. Soll, J. Arijeloye,
E. Malave, Prof. Dr. C. M. Drain
Department of Chemistry and Biochemistry
Hunter College of the City University of New York
695 Park Avenue, New York, NY 10065 (USA)
Fax : (+1)212-772-5333
E-mail: cdrain@hunter.cuny.edu
Supporting information for this article (results from control reactions,
characterization of the ONPs, standardizations, and representative
GC of the products from a catalytic reaction with O2) is available on
the WWW under http://dx.doi.org/10.1002/chem.200901086.
Chem. Eur. J. 2009, 15, 12133 – 12140
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12133

result in many types of nanostructures that are kinetically
trapped from further aggregation by deposition on surfaces.
There is considerable interest in understanding the intermo-
lecular processes governing formation of organic colloids
and ONPs.[7,9,18] Hierarchical organization of molecules into
nanostructured aggregates can have a profound effect on
their photonic and catalytic properties. For example, forma-
tion of ONPs of porphyrinoids and other chromophoric sys-
tems offers the potential to enhance or modulate the pho-
tonic properties of the molecules through quantum mechani-
cal effects.[19,20] Additionally, understanding the spontaneous
formation of suspensions of molecular aggregates is becom-
ing an important topic for the formulation of hydrophobic
drugs.[21] The formation of narrowly dispersed ONPs may
arise from a thermodynamic limitation due to nanoparticle
surface energies, and a kinetic limitation whereby the rates
of formation of the individual aggregates influence the avail-
ability of material;[9,13,22] the recent work of van Keuren
et al. indicats the importance of transient formation of un-
stable clusters.[15]
tion of cyclooctene by [FeIII
ACTHNGUTRE(UNNG tppf20)] in acetonitrile/methanol,
which yields over 98% of the epoxide and traces of the 2-cy-
clooctene-1-one and the 2-cyclooctene-1-ol.[1–4] Under the
same conditions we find similar reactivity for cyclohexene
(Scheme 1).
Scheme 1. Oxidation of cyclohexene.
Given the enhanced catalytic activity of [FeIII
ACHTUNGTRENNUNG(tppf20)],
there are a considerable number of reports on other halo-
genated metalloporphyrins. Gray et al. reported significantly
different oxidation chemistry for a derivative of [FeIII-
AHCTUNTGREGN(NNU tppf20)] wherein the eight b-pyrrole positions are also halo-
genated.[48–50] The differences in the catalytic oxidation of
perhalogenated porphyrins arise from the distortions in the
otherwise planar macrocycle and the electronic effects.
When the b positions are chlorinated the activity of ethyl-
benzene oxidation by using oxygen at 1008C is increased
but not the stability to oxidative degradation, but the stabili-
Porphyrin catalysts: The discovery by Groves and co-work-
ers[23–29] that iron porphyrins in organic solvents with oxygen
sources such as iodosylbenzene can mimic the oxidative cat-
alysis observed for cytochrome P450[30–35] led to a huge
amount of research on the reactivity and mechanism of this
reaction. Different metalloporphyrins exhibit different cata-
lytic reactivities, which include different products or product
ratios.[26,36–41] Other major findings include: a) appropriate
modification of the porphyrin macrocycle alters the reactivi-
ty in terms of site selectivity,[28,42–44] b) halogenation general-
ly makes the metalloporphyrins more efficient cata-
lysts,[1–4,45–55] c) axial ligands can alter the reactivity,[56–59]
d) the solvent can affect the reactivity,[1–4] and e) other
oxygen sources such as H2O2, and O2, can be used with
some systems.[59–62] Various metalloporphyrins are now used
in laboratory scale reactions. Heterogeneous porphyrin sys-
tems include those in lipids, micelles, zeolites, or on supports
such as silica, or Montmorillonite clay.[37,42,51,63–65] Several re-
action types are catalyzed by metalloporphyrins, but perhaps
the best studied are oxidation reactions. The catalytic oxida-
tion of cyclohexene by iron tetraarylporphyrins is a standard
reaction that has been investigated thoroughly over the last
few decades, and the epoxide is the major product under a
range of experimental conditions. The elegant work by
Hupp et al. on self-assembled metalloporphyrin catalysts
show that rigid arrays can yield remarkably robust systems,
but require macrocycles that are difficult to obtain in high
yields.[18,40,66]
ACTHNUTRGNEUNG
ty increases when [FeIII(tppf20)] is linked to polystyrene.[61]
Organic nanoparticles of porphyrins: The formation of the
all-organic nanoparticles of porphyrins and metalloporphyr-
ins by host–guest solvent methods depends on the complex
interplay between intermolecular forces and kinetics.[9,10,15,22]
The size and structure of the ONPs depends on: a) intermo-
lecular forces between the porphyrins, host solvent, guest
solvent, and the polyethylene glycol stabilizer, b) the ratio
of host to guest solvents, and c) the vigorousness of mixing.
Therefore, the structure of the porphyrin and the specific
metal ion influence the size and the organization of the
chromophores within the nanoaggregates. For [FeIII
ACHTUNGTRENNUNG(tppf20)]
in THF/water as host–guest solvents, large ONPs with a di-
ameter of about 80 nm are formed by magnetically stirring
and are likely composed of smaller subdomains of 5–20 nm.
Sonication while adding the guest solvent results in the for-
mation of ONPs with a diameter of approximately 10 nm,
which are less prone to reorganization or disaggregation
(Figure 1). Since the ONPs are self-organized solely by in-
termolecular forces, these are dynamic systems in that they
can reorganize or disaggregate in response to environmental
changes.[7,9]
In general, the turnover number for iron porphyrin cata-
lysts in solution is a few hundred because of the degradation
of the catalyst, but this increases to about 350 when
5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinato
We report herein that aqueous suspensions of organic
nanoparticles of commercially available [FeIII
ACHTUNGTRENNUNG(tppf20)],
formed by the methods described above, have significantly
enhanced catalytic properties compared to the component
molecules and those on supports, and lead to different prod-
uct distributions. This also allows the study of the fundamen-
tal differences in the chemistry of nanoscaled aggregates
versus solvated molecules or solid-state materials.
ironACHTUNGTRENNUNG ACHTUNGTRENNUNG(tppf20)]) is used because this is a more active
(III) ([FeIII
catalyst and may be somewhat more resistant to oxidative
degradation. There are numerous studies on the catalytic ac-
tivity of this porphyrin,[61,67,68] including the catalytic oxida-
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Oxidation of Cyclohexene by Organic Nanoparticles
FULL PAPER
the metalloporphyrins are in close proximity in the ONPs,
which should enhance oxidative degradation of the catalyst
and cause a significant decrease in catalytic turnovers. There
have been significant efforts to isolate porphyrinoid catalysts
(see above). Furthermore, the allylic oxidation products sug-
gest a different mechanism compared to that of the corre-
sponding solvated metalloporphyrin.[1–4] Control reactions in
the absence of an oxygen source or metalloporphyrin result
in no product formation. Adding 3% each of water and
polyethylene glycol (PEG) to the homogeneous reaction
mixture has no effect on the product ratios or TON, that is,
only the epoxide is formed, H2O2 is required, and a TON of
350. The UV/Vis spectra of the exhausted reaction mixtures
reveals that eventually the metalloporphyrin in solution or
as an ONP decomposes[69] (see the Supporting Information).
Unlike the solution phase reactions, the slow addition of
H2O2 to the ONP suspension through a syringe pump results
in modest yields of the allylic products, whereas addition of
a 30% H2O2 solution in one aliquot degrades the porphyrin
within a few minutes as observed by UV/Vis spectra. These
observations indicate that the hierarchical organization of
the metalloporphyrins in the ONPs is key to the observed
activity.
Figure 1. Preparation of ONPs. Water (5.0 mL) is added to a solution of
[FeIIICl
ACHTUNGTRENNUNG(tppf20)] in THF (0.4 mL, 1.0 mm) and PEG (0.2 mL) while stirring
or sonicating to yield ONPs with a diameter of 80 nm and 10 nm, respec-
tively.
Results and Discussion
A solution of [FeIII
ACTHNUTRGEN(UGN tppf20)] in acetonitrile/methanol (3:1)
catalytically oxidizes cyclohexene to the epoxide by using
H2O2 with a turnover number
Table 1. ACHTNUTRGENNUG ACHTUNGTRENN(GUN tppf20)] ONP catalysis of cyclohexene oxidation.
[FeIII
(TON) of about 350. Previous
reports describing the use of
cyclooctene as a substrate par-
allel these results in the exclu-
sive formation of the epoxide,
and in the fact that only H2O2
can be used.[1–4] In contrast,
10 nm diameter ONPs of [FeIII-
Solution[a] or Conditions
ONPs[b]
Yield Yield Yield
(oxide) ACTHNGUTER(NNUG ene- (ene-1-
TON
Comments[d,e]
G
[%]
1-ol)
[%]
one)
[%]
solution
solution
CH3CN/
CH3OH
H2O2
CH3CN/
CH3OH
H2O2
98
<1
<1
350
15 min
95Æ5 5Æ1
<1
not re-
ported
cyclooctene, 15 min,
ref. [1-4]
ACHTUNGTRENNUNG(tppf20)] catalytically oxidize
cyclohexene by using O2 to
yield exclusively 2-cyclohex-
ene-1-one and 2-cyclohexene-
1-ol with about 10-fold greater
TON than the completely sol-
10 nm ONPs
10 nm ONPs
10 nm ONPs
10 nm ONPs
H2O2
<1
<1
<1
<1
30
26
28
20
70
74
72
80
175
500
3500
ca. 5 min
O2 limiting reagent
16 h
6.5 mL O2
125 mL O2
99.6% D2O
125 mL O2
10% H218O
125 mL O2
no D in products other than the ex-
changeable alcohol proton
18O in 10% of ketone and <1% in alco-
hol; via acetal
10 nm ONPs
10 nm ONPs
10 nm ONPs
<1
34
23
28
66
76
72
vated
metalloporphyrin,
though at a much slower rate
(Table 1).
H2O, 125 mL <1
18O in <8% of ketone and >90% of al-
cohol
98% 18O2
125 mL O2
0.5 mL cy-
clohexene
125 mL O2
no PEG
<1
<1
3500
430
large excess of substrate; 16 h
The TON is defined as the
total amount of products
(ketone/alcohol 3:1) formed
per porphyrin, and since the
porphyrin slowly decomposes,
these reactions are run until
[metalloporphyrin]<0.2 mm.
This represents a greener alter-
native to effect these organic
transformations since dioxygen
is efficiently used as oxidant in
place of H2O2 or other synthet-
ic oxygen sources, and the re-
action solvent is 89% water.
The increased TON is con-
trary to expectations because
10 nm ONP
30 nm ONPs
25
75
8 h
125 mL O2
1
70
29
11
70
19
3100
16500
6 h
8 h
35 nm ONPs[c] H2O/DMF
C6H5IO
120 nm
H2O/DMF
C6H5IO
85
6
10
12000
8 h
ONPs[c]
[a] Solution reactions: 0.1 mm catalyst in acetonitrile/methanol (3:1, 2.75 mL); porphyrin/cyclohexene/H2O2 =
1:2000:3000. This is similar to the cyclooctene oxidations reported previously.[1-4] [b] ONP reactions: ONP sus-
pension (2.5 mL, 70 mm, 1.75ꢁ10À7 mol of porphyrin); porphyrin/substrate/H2O2 =1:2000:3000. Alternatively,
the porphyrin–ONP suspension (2.5 mL) was mixed with cyclohexene (200 mL) and O2 (125 mL, 1 atm); por-
phyrin/substrate/O2 1:16000:40000. [c] ONPs made from DMF as host solvent under conditions used to obtain
this size nanoparticle.[10] [d] t1/2 for total products formed. [e] All reactions were run exhaustively. TON=
molproducts/molporphyrin has an error of Æ5%. Products were extracted into CH2Cl2 and analyzed by using an Agi-
lent 5975 series GC-MS. Control reactions: neither H2O2 nor O2 react directly with cyclohexene under these
conditions. (See Experimental Section and the Supporting Information.).
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C. M. Drain et al.
Nanoparticle structure: The detailed structural arrangement
of the [FeIII
(tppf20)] within the ONPs is not known because
positions). Since the addition of about 3% each of water
and PEG to the solution-phase reaction in acetonitrile/meth-
anol has no effect on the product ratio or the TON, the or-
ganization of the metalloporphyrins inside the ONPs is the
primary cause of the observed differences in the catalytic
ACHTUNGTRENNUNG
the intermolecular forces used to self-organize the molecules
into nanoparticles are weak, non-specific, and reversible.[9,10]
As discussed above, the nanoarchitecture of porphyrin mol-
ecules within the ONPs strongly depends on the component
molecules, solvents, and mode of preparation.[9–12,70,71]
process. The rapid decomposition of [FeIII
ACHTUNTRGENUNG(tppf20)] in the
ONPs in the H2O2 reactions is consistent with the close
proximity of the metalloporphyrins, and the allylic products
indicate that the oxidative mechanism with this oxygen
source is similar to the O2 reactions. The PEG used to stabi-
lize the suspension from precipitation plays a role in the re-
activity of the ONPs, since about 10 nm diameter ONPs
formed without PEG (which do not precipitate for about
3 days) result in the same 3:1 ketone/alcohol formation by
using O2, but with a TON of only around 430. Thus, in addi-
tion to a different catalytic mechanism, the observed activity
is the result of the slower rate of self-oxidation for the O2
reactions, the organization of the metalloporphyrins in the
ONPs, and the partition of the hydrophobic substrate from
the aqueous solvent into the ONPs.
Densely packed, 10 nm diameter ONPs of [FeIII
ACHTUNTRGENUNG(tppf20)] (ca.
1.75 nmꢁ1.75 nmꢁ1.0 nmꢀ3 nm3) can contain up to about
200 porphyrins, but this represents an upper limit. Experi-
ments that combine ONPs composed of different porphyrins
indicate that in solution under ambient conditions, the por-
phyrins do not exchange between the ONPs. It is also likely
that some PEGs and host–guest solvents may be present
inside the ONPs. Neither AFM nor XRD indicate crystal-
line material.
Note that for all of the ONPs of iron porphyrins we have
studied to date, the rates of the oxidation reactions are
about 60 times slower than those for the corresponding met-
alloporphyrin in solution. The data in Table 1 show that cat-
alytic activity depends on particle size and host solvent,
which likely indicates differences in the nanoarchitectures of
the porphyrins in the ONPs. For example, the oxidation of
cyclohexene by large ONPs (ca. 120 nm diameter) of [FeIII-
Isotope experiments: Note that under a variety of conditions
the ketone/alcohol product ratio is about 3:1. Experiments
probing the source of the incorporated oxygen were used to
garner insights into the mechanism of ONP catalysis. These
preliminary investigations are consistent with previously re-
ported radical mechanisms for reactions that result in allylic
oxidations, for example, Mn, Sn, or Ru porphyrins, and are
akin to the P450 reactions.[30,32,74] The oxygen in the prod-
ucts may originate from the water and/or the O2. GC-MS
analysis of reactions run with nanoparticles that were prepa-
rated with water containing 10% H218O indicates that about
10% 18O in the ketone and no additional 18O in the alcohol
is observed within the error of the experiment. When 98%
18O2 is used in the reaction <8% of the ketone and around
90% of the alcohol products contain 18O. These results indi-
cate that the oxygen in the alcohol comes primarily from
O2, but oxygen in the ketone also originates from water.
The most likely explanation for the observed incorporation
of oxygen from water into the ketone is the reversible for-
mation of the acetal or hydrate. Reactions run in 99.6%
D2O result in no incorporation of deuterium into the prod-
ucts, but some exchange with the alcohol proton. The role
of water in the mechanism is supported by the narrow pH
window in which the ONP catalyst is active, see below.
ACHTUNGTRENNUNG(tppf20)] by using DMF as a host solvent requires H2O2 or
iodosylbenzene and results in epoxidation similar to the ho-
mogeneous system, yet has an approximately 30-fold greater
TON than the solution phase reaction.
Catalytic insights: There is still considerable discussion over
the mechanisms of hydrocarbon oxidation by iron porphyr-
ins and other metalloporphyrins,[1–4,29,34,48,72,73] wherein the
two dominant proposed mechanisms are: a) a radical hydro-
gen-abstraction–oxygen-rebound mechanism and b) an
oxygen- (or hydroxyl-)insertion reaction that proceeds
+
through a cationic ROH2 species. It appears that different
mechanisms are operative with different iron porphyrin sys-
tems depending on factors such as macrocycle ligand field,
axial ligands, and solvent. Because the ONPs are dynamic in
that the porphyrins can reorganize upon changing the envi-
ronment, probing the mechanism of catalysis is particularly
challenging, and detailed mechanistic studies that parallel
several decades of metalloporphyrin research is beyond the
scope of one manuscript.
For the ONP systems, the intermolecular interactions such
as p stacking have significant effects on electronic structure
of the macrocycle as shown by the substantially broadened
UV/Vis spectra. Substrate accessibility and orientation to
the reactive centers may be different compared to the sol-
vated metalloporphyrin. Considering the close proximity of
The presence of an alcohol to form an active axially
bound [(tppf20)FeACTHNUTRGNEUNG
(HOCH3)]+ adduct is reported to be im-
portant for cyclooctene-epoxidation reactions by this com-
plex in solution,[1–4] and in the case of the ONPs the alcohol
moiety on the PEG can initially serve in this capacity.
During the course of the reaction, the 2-cyclohexene-1-ol
may become an axial ligand and be activated towards fur-
ther oxidation to yield the major product, the ketone, and
water may be involved in this second step. Hydroxide or
water may serve as axial ligands, but since no products are
found outside the range of pH 6.5–7.0 hydroxide may
quench the reaction by irreversibly binding to the iron
the [FeIII
ACHTUNGTRENNUNG(tppf20)] in the ONPs, the formation of m-oxo and/
or dioxo dimers may also play an important role, as the
former species is known to have increased catalytic activity
in benzylic oxidations at elevated temperatures.[61] In the ab-
À
sence of steric effects on substrate binding, the C H-bond
energies should correlate with expected oxidation rates (i.e.,
allylic oxidations should be more facile than those at the 28
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Oxidation of Cyclohexene by Organic Nanoparticles
FULL PAPER
centre or by changing the redox potential.[73] The lack of D
or 18O incorporation from water into the alcohol argues
against free radical reactions, wherein the intermediates
escape from the ONP cage. The cyclohexene epoxide is
cleanly converted by these ONPs to the gem diol, thus the
epoxide is not an intermediate of the alcohol and ketone
formation. Only traces of the ketone are observed when the
2-cyclohexene-1-ol is used as a substrate because its octanol/
water partition coefficient is 40-fold less than the on for cy-
clohexene (k2-cyclohexene-1-ol =18 vs. kcyclohexene =730) and thus, it
does not partition into the ONPs as well.
only the epoxide, axial coordination by these solvents does
not explain the ONP results. The role of protic solvents on
the reaction mechanism of the reaction in solution has been
discussed in terms of proton transfer before the heterolytic
À
cleavage of the O O bond in the adduct hydrogen perox-
ide.[4] These results indicate that the hierarchical structure of
the macrocycles in the ONPs is the dominant factor in the
mechanistic differences. The products and greater TONs
may be consistent with a radical-initiated reaction mecha-
nism as suggested by Labinger et al.,[48–50] or shown by the
ACTHNUTRGNEUNG
use of [FeIII(tppf20)] in super critical CO2 and 20 atm O2.[67,81]
When O2 is the limiting reagent in the ONP system the
TON is reduced significantly, indicating the importance of
oxygen in the catalysis and that the extent of radical-chain
reactions is limited.
Iron centre: The nearly 20 ppm shift for the b-pyrrole reso-
nances (d=80 ppm to d=62 ppm; 10 mm in CD3CN/MeOH
3:1) in the NMR spectra reported[4] indicates that the para-
magnetism of [FeIII
tion of methanol to form [(tppf20)Fe
studies on [FeIII
(tppf20)] in acetonitrile/methanol indicate the
G
Deformation of the otherwise planar macrocycle by steric
crowding of the peripheral substituents has been proposed
as a major source of reactivity differences in the perhalogen-
ated metalloporphyrins relative to metal complexes of tppf20
and other arylporphyrins.[50] Nonplanar metalloporphyrins
are known to have significantly different photonic properties
including the dynamics of axial ligand binding.[82–84] The por-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
likely presence of more than one paramagnetic species in
solution under aerobic conditions since we cannot accurately
calculate the effective magnetic moment, meff, from paramag-
netic shifts of the solvent resonances,[75] which also slowly
change with time. This may be an indication of a shift from
a 5/2 to a 3/2 system and/or for the formation of oxygenated
species. Similarly, the meff of the iron porphyrin in the ONPs
under aerobic conditions and during catalysis has been diffi-
cult to ascertain by the Evans method NMR experiments.[76]
A diminished paramagnetic shift for the b-pyrrole H is ob-
served relative to the solution-phase complex under identi-
cal conditions (d=42 ppm, 7 mm in CD3CN/MeOH 3:1),
and a smaller difference in the residual solvent resonances is
observed. Though it is clear that at least some metallopor-
phyrins remain paramagnetic, the reduction in the paramag-
netic shifts may be an indicator of antiferomagnetically cou-
pled oxo-bridged dimers.[77] In principle these oxidation re-
actions can be influenced by a magnetic field if there are
paramagnetic species, porphyrin or organic intermediate, in
the rate-determining step in the reaction mechanism,[78,79]
but we observed no differences in the product ratios be-
tween reactions stirred magnetically (ca. 800 G) or mechani-
cally by a shaker.
ACTHNUTRGNEUNG
phyrin in [FeIII(tppf20)] is not distorted[75] and these macrocy-
cles likely adopt a nearly planar conformation in the ONPs
because the intermolecular forces between the nanoparticle
components are too weak to force the macrocycles into en-
ergetically unfavorable conformations. In addition, nonpla-
nar porphyrins are characterized by broad red-shifted Soret
bands, and the optical spectra of the ONPs are broad but
contain several underlying peaks shifted to the red and the
blue of those of the parent complex consistent with porphy-
rin J and H aggregation.
ONP mechanism: One plausible explanation for the in-
creased TON is that the metalloporphyrins on the exterior
of the ONPs are rigidly held in a structure that diminishes,
but does not eliminate, the oxidation of one macrocycle by
another. When the porphyrins on the exterior of the ONPs
eventually decompose they fall off because of the greatly in-
creased polarity of the oxidized product, thereby exposing
the next layer of catalytically active molecules. This onion-
type mechanism may account for the slow rates of catalysis
by the present ONPs since fewer catalytic sites are available
at a given time. Since cyclohexene is hydrophobic and the
reaction solvent is mostly water, the substrate rapidly parti-
tions into the ONPs as indicated by UV/Vis spectral shifts.
This significantly increases the concentration of the sub-
strate proximal to the catalytic sites.
Our working hypothesis is that the close proximity of the
iron porphyrins in the ONPs facilitates the formation of m-
dioxo-bridged dimers and/or m-oxo-bridged dimers that have
known enhanced catalytic activity in terms of alkane hy-
droxylation.[38,61] Under the reaction conditions, the FeIV–
oxo monomers may be in equilibrium.[29] Coordination of
water, hydroxide, and the alcohol moiety of PEG may affect
the stability or reactivity of the iron–oxo complexes. The im-
portance of axial ligands is consistent with the observation
that imidazole axial ligands block the incorporation of the
There are two consequences of this concentrator
effect:[85,86] the presence of more substrate diminishes the
probability of inter-porphyrin oxidation. Since the ketone is
the dominant product, an initially formed alcohol may be
further oxidized before it escapes the ONP cage (see
above). This partitioning of the substrate is supported by
ACTHNUTRGNEUNG
oxygen from water in epoxidations by [FeIII(tppf20)].[80] The
absence of cyclohexene oxide as a significant product in
nanoparticle catalysis argues against the presence of signifi-
cant amounts of solvated [FeIII
N
the observation that ONPs of ironACTHNGUTERNNU(G III) tetra(4-phenylsulfo-
catalysis. However, since homogeneous reactions in acetoni-
trile/methanol (3:1) with a few percent water and PEG yield
nate)porphyrin, made by adding DMF or acetonitrile/metha-
nol to an aqueous solution of the porphyrin, are inactive in
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12137

C. M. Drain et al.
this reaction, and suggests that the cyclohexene substrate
does not partition into these water containing ONPs. The re-
duced TONs but similar product ratios of suspensions of
ONPs without PEG, see above, may indicate that PEG also
serves as a phase-transfer agent. The last observation further
emphazises the importance of the organization and composi-
tion of the metalloporphyrins in the ONPs.
grees of H versus J aggregates depending on preparative
methods.[9] The spectroscopic signatures and particle sizes
are quite different for the same metalloporphyrin, in this
case [FeIII
ACHTNUGTRENUNG(tppf20)], prepared from different solvent systems,
different solvent ratios, different mixing conditions, and dif-
ferent temperatures. Previously reported catalysis with the
[FeIII
ACHTNUGTRENUNG(tppf20)] nanoparticles used DMF as the host solvent
and iodosylbenzene as the oxygen source to form the epox-
ide as the major product.[10] We hypothesize that: a) there
are differences in the axial coordination, b) that the iodosyl-
benzene partitions rapidly into the ONPs, and c) that the re-
sulting Fe–oxo species is different than what is formed upon
dioxygen binding.
Given the great variety of porphyrinoids and their metal
complexes, the full potential of self-organized organic nano-
particles is yet uncharted.
Conclusions
These results illustrate that ONP materials composed of por-
phyrins can display unique properties, or in this case, unex-
pected catalytic activities relative to the component mole-
cules. These functionalities arise from the self-organized ar-
chitecture inside the ONPs. Other nanoscaled materials of
porphyrins, such as tubes, rods, and crystals, can be formed
from simple commercially available compounds or from
more complex molecular designs.[18,87] Self-assembled por-
phyrinic materials generally require specifically designed
recognition motifs in predefined geometries to affect specif-
ic architectures. Conversely, the construction of self-organ-
ized materials, such as ONPs, does not require complex exo-
cyclic moieties.[12,71,88,89] Thus, the self-organization strategy
obviates the need for macrocycles that are synthetically
challenging and the result of low-yield procedures. Porphyr-
ins bearing the same substituent at the four meso positions
are easy to prepare in large scales, and can be prepared in
Experimental Section
Materials and instrumentation: [FeIII
ACTHNUTRGEN(NUG tppf20)], cyclohexene oxide, 2-cyclo-
hexene-1-ol, 2-cyclohexene-1-one, and polyethylene glycol monomethyl
ether (PEG164) were purchased from Aldrich Chemical Co. The solvents
(tetrahydrofuran, toluene, 99.9% acetonitrile, 99.9%, methanol, and
HPLC-grade dichloromethane), cyclohexene and 30% H2O2 were pur-
chased from Fisher Scientific Co. Nanopure water was obtained by using
Barnstead Nanopure water system. D2O (99.6%), was obtained from
Cambridge Isotope laboratories Inc. H218O (10%) and 18O2 (98%) were
obtained from Sigma–Aldrich.
green, solventless reactions.[90] The [FeIII
ACHTNUGTRENUNG(tppf20)] ONP cata-
Product analyses were performed by using GC-MS Agilent 5975 series
system with HP-5 column (HP-5MS 30 mꢁ0.250 mm, 0.25 micron nomi-
nal, 5% phenylmethyl siloxane). Electronic spectra were recorded on
lyst system represents an advance in green chemistry since
despite numerous efforts in catalyst discovery and design
there are still few molecular-based catalysts that can per-
form oxidation reactions under mild conditions by activation
of O2 and in water. The metalloporphyrins in the ONP cata-
lysts reported herein are organized by weak intermolecular
interactions, so the structure is dynamic.
Cary Bio-3 UV/Vis spectrophotometer.
A
Precision Detector
PD2000DLS Cool-Batch dynamic light scattering instrument was used in
batch mode at 258C to determine the particle size. A Veeco Nanoscope
III Multi-mode AFM was used to examine the ONPs on surfaces. A
Fisher SF15 sonicator was used for nanoparticle preparations.
Reactions: Reactions were performed at ambient temperature. All reac-
tions were run a minimum of five times except the isotope experiments,
which were repeated three times, and the reported data represent the
average of these reactions. All reactions were agitated by using a magnet-
ic stirring bar unless otherwise noted. Although purchased as the chlo-
ride, since the counter ion on the metalloporphyrin is unknown in the
ONP solution and in equilibrium in the solution-phase reactions, it is not
specified. For the homogeneous, protic solvent reactions, a 1.0 mm stock
solution of the iron porphyrin complex in acetonitrile/methanol (3:1) was
used. The reaction was initiated in a 9 mL screw-capped vial by mixing of
the 1.0 mm porphyrin stock solution (250 mL) with acetonitrile/methanol
The dynamic organization of the molecules may enable
the ONPs to adapt to a variety of substrates with different
topologies. Preliminary work shows that the allylic ketones
and alcohols of R(+)-limonene are formed under the same
conditions. Although other inorganic and metallic systems
can be superior alkene oxidation catalysts than the present
metalloporphyrin ONPs in terms of TONs, and the epoxide
is a versatile intermediate, there are numerous organic
transformations requiring mild allylic oxidations. Because al-
lylic oxidations are widely used in small scale reactions and
in commercial organic synthesis, more efficient and greener
methods to accomplish this transformation are of inter-
est.[91–93] The allylic oxidation of alkenes by SeO2 (to yield
the alcohol) and other reagents (to yield the ketone) have
been used in organic synthesis for many decades, and the
mechanisms of allylic reactions proceed through an array of
complex mechanisms.[94] Our chemistry reported here is
greener in that it is less toxic than SeO2 reactions.
(2.5 mL, 3:1 for
a final concentration of 0.1 mm) and cyclohexene
(50 mL). Whereupon 30% H2O2 (80 mL) was slowly added to the reaction
through a Teflon cannula securely fitted through a hole in the cap by
using a syringe pump over the course of 80 min (1 mLminÀ1). The reac-
tion was stirred for four hours (UV/Vis spectra analysis indicated that
most of the porphyrin has decomposed by ca. 30 min). Ratio of porphy-
rin/substrate/H2O2 =1:2000:3000 equivalents. An aliquot of the reaction
was analyzed by GC-MS and product yields were determined relative to
an added internal standard (toluene). Of the three possible products, cy-
clohexene oxide was obtained in greater than 99% (see the Supporting
Information). The results are shown in Table 1.
For ONP reactions, 5.6 mL batches of the [FeIII
pared in 10 mL vials (or test tubes) by adding nanopure water (5.0 mL)
to a mixture of PEG (0.2 mL) and a 1.0 mm solution of [FeIII
(tppf20)] in
ACHTUNGTREN(NUNG tppf20)] ONPs were pre-
As stated in the previous reports and here, the exact or-
ganization of the porphyrins in the aggregates are unknown,
but differences in the electronic spectra indicate varying de-
ACHTUNGTRENNUNG
THF (0.4 mL) while sonicating (the ONP suspension is 70 mm, 4.0ꢁ
10À7 mol of porphyrin). The solution was further sonicated for 1 min. The
12138
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Oxidation of Cyclohexene by Organic Nanoparticles
FULL PAPER
prepared nanoparticles are stable for more than four weeks and were
stored in a refrigerator at about 48C. Each batch of nanoparticles was
checked by dynamic light scattering (DLS) and UV/Vis absorption spec-
troscopy. The solutions appear slightly cloudy. The ONP solution had a
pH 6.5–7.0. The porphyrin–ONP stock solution (2.5 mL, 70 mm, 1.75ꢁ
10À7 mol of porphyrin) was mixed in a 9.5 mL screw-capped vial with cy-
clohexene (25 mL) and 30% H2O2 (40 mL), which was slowly added to
the reaction through a teflon cannula securely fitted through a hole in
the cap over the course of 40 min. Ratio of porphyrin/substrate/H2O2 =
1:2000:3000 equivalents. The reaction mixture was stirred for about 24 h.
The reaction mixture (2.6 mL) was extracted thoroughly once with di-
chloromethane (2.8 mL) and the layers were allowed to separate. The
water fraction and some of the organic fraction was removed to leave a
total volume of 2.0 mL (this assures the same volume for every assay).
Toluene (20 mL, 1.88ꢁ10À4 mol) was added to the 2.0 mL extract as an in-
ternal standard, whereupon 4.0 mL were diluted with dichloromethane
(1.0 mL); 2.0 mL of this last solution was injected into the GC-MS.
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