Mechanistic insights into the co-catalyst effect of Au clusters in Mn-porphyrin-catalyzed olefin oxidation

Article abstract of DOI:10.1039/b810360g

Styrene oxidation with iodosylbenzene catalyzed by a Mn-porphyrin/Au- cluster binary system was investigated from several aspects to disclose the mechanism of unique and prominent positive effects of Au clusters. In the catalytic system utilizing a fixed amount of Mn(TPP)Cl (TPP = tetraphenylporphinato) and a varying amount of dodecanethiolate-capped Au cluster (Au:SC12, 1), the reaction rate was increased sigmoidally with the increase of the charged amount of 1. From these profiles together with the results of absorption spectral studies, a possible mechanism involving two cluster molecules is proposed. Comparative studies utilizing Mn(TPP)Cl and Mn(TMP)Cl (TMP = tetramesitylporphinato) showed that the latter is superior in terms of the robustness towards oxidative degradation and catalytic activity. Furthermore, at a certain condition, a characteristic induction period was observed only for the reaction with conformationally rigid and sterically hindered Mn(TMP)Cl. Analogous Au clusters protected by non-thiolate ligands such as triphenylphosphine and poly(N-vinyl-2-pyrrolidone) also promoted the catalytic oxidation by Mn(TMP)Cl. Based on these observations, we discuss the critical paths and factors that govern the catalytic activity of the present binary catalyst system. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b810360g

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www.rsc.org/njc | New Journal of Chemistry
Mechanistic insights into the co-catalyst effect of Au clusters in
Mn-porphyrin-catalyzed olefin oxidationw
Yoshitaka Murakami and Katsuaki Konishi*
Received (in Montpellier, France) 18th June 2008, Accepted 4th September 2008
First published as an Advance Article on the web 26th September 2008
DOI: 10.1039/b810360g
Styrene oxidation with iodosylbenzene catalyzed by a Mn-porphyrin/Au-cluster binary system
was investigated from several aspects to disclose the mechanism of unique and prominent
positive effects of Au clusters. In the catalytic system utilizing a fixed amount of Mn(TPP)Cl
(TPP = tetraphenylporphinato) and a varying amount of dodecanethiolate-capped Au cluster
(Au:SC12, 1), the reaction rate was increased sigmoidally with the increase of the charged amount
of 1. From these profiles together with the results of absorption spectral studies, a possible
mechanism involving two cluster molecules is proposed. Comparative studies utilizing Mn(TPP)Cl
and Mn(TMP)Cl (TMP = tetramesitylporphinato) showed that the latter is superior in terms of
the robustness towards oxidative degradation and catalytic activity. Furthermore, at a certain
condition, a characteristic induction period was observed only for the reaction with
conformationally rigid and sterically hindered Mn(TMP)Cl. Analogous Au clusters protected by
non-thiolate ligands such as triphenylphosphine and poly(N-vinyl-2-pyrrolidone) also promoted
the catalytic oxidation by Mn(TMP)Cl. Based on these observations, we discuss the critical paths
and factors that govern the catalytic activity of the present binary catalyst system.
Introduction
The prominent capability of Au nanoclusters to mediate
various oxidative processes has received considerable attention
Scheme 1
not only based on their fundamental aspects but also in
relation to the development of practical oxidation catalysts.1,2
Further studies combined with 1H NMR have implied that
We have recently provided an example of the unique aspects in
the notable positive effects on styrene oxidation with iodosyl-
benzene (PhIO) over Mn(TPP)Cl (TPP: tetraphenylporphinato)
(Scheme 1).3,4 This reaction,5,6 which is rather slow with the
Mn catalyst alone under catalytic conditions, proceeded
smoothly in the presence of simple dodecanethiolate-capped
Au clusters7 (Au:SC12, average core diameter (dav) = 1.4 or
4.0 nm) to give a high catalytic turnover number (TON).
Several control experiments have revealed that the Au cluster
itself has no ability to catalyze the styrene oxidation by PhIO
but serves as an efficient promoter to accelerate the
Mn-porphyrin-mediated reaction.3 The uniqueness of the Au
element constituting the cluster has been demonstrated in
comparison with analogous Pd, Ag, and Pt clusters. Absorp-
tion spectroscopic studies have indicated that a major role of
the Au cluster is to regenerate the active catalytic path invol-
ving MnIII/MnV from the catalytically ineffective MnIV species
two important processes are involved in the reactivation
process: (1) the formation of exposed Au surfaces as a result
of the irreversible elimination of some dodecyl groups of
Au:SC12 by the reaction with PhIO (Fig. 1(b), step 1), and
(2) the regeneration of the active forms of the Mn catalyst
(MnIII/MnV) from the MnIV species on the exposed Au surface
(step 2).3 We have also suggested that the catalytic cycle turns
over Mn porphyrin attached to the Au cluster (step 3), based
on the results of spectroscopic investigations and product
distribution ratio (epoxide/aldehyde).3
In the present paper, we report the results of studies on
several factors that affect the activity of the above binary
catalyst system, and provide further insights into the role of
the Au clusters in the co-catalyst effect. The reaction rates and
the oxidation states of the Mn catalyst in styrene oxidation
with Mn(TPP)Cl were investigated as a function of the initial
amount of Au:SC12 (dav B1.4 nm, 1), from which a plausible
catalyst reactivation mechanism involving two cluster mole-
cules is proposed. We also demonstrate that rigid and steri-
cally hindered Mn(TMP)Cl (TMP: tetramesitylporphinato)
catalyst is superior in terms of the reaction rates (and also
acceleration factors) and stability towards oxidative degrada-
tion, and discuss the critical step in the reactivation process
based on the difference of Mn(TPP)Cl and Mn(TMP)Cl in the
time-course profile. Through studies utilizing analogous Au
clusters protected by non-thiolate ligands such as phosphine
that is inevitably formed in the catalytic cycles (Fig. 1(a)).8,9
z
Division of Environmental Materials Science, Graduate School of
Environmental Science, Hokkaido University, North 10 West 5,
Sapporo, 060-0810, Japan. E-mail: konishi@ees.hokudai.ac.jp
w Electronic supplementary information (ESI) available: Absorption
spectra of the reaction mixtures under conditions in Fig. 2 and 7. See
DOI: 10.1039/b810360g
z The true active species under catalytic condition is still elusive (see,
ref. 5f). Under our reaction/measurement conditions, the Mn(V)QO
species was not detected because of its high reactivity.
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Table 1 Catalytic Styrene oxidation by 1/Mn(TPP)Cl systema
Relative rate
[1]0/
Rateb
Entry [Mn(TPP)Cl]0 (TON/h) factor)
(acceleration Yieldcd Valence
(%)
of Mne
1
2
3
4
5
6
7
8
9
0
17
22
39
44
69
90
117
179
188
195
223
216
1.0
1.3
2.3
2.6
4.1
5.3
6.9
10.5
11.1
11.5
13.1
12.7
17
25
36
40
59
70
83
IV
0.05
0.07
0.10
0.15
0.17
0.20
0.30
0.50
0.70
1.00
1.50
IV
IV
IV
IV
IV
III/IV
III
III
III
III
III
86
78f
71g
63g
53g
10
11
12
a
[Mn(TPP)Cl]0/[styrene]0/[PhIO]0 = 0.09/450/90 mM in CH2Cl2 at
b
25 1C under argon. Estimated from the slope of the linear part in the
+ [phenylacetaldehyde])/[PhIO]0.
c
time course. ([styrene oxide]
d
e
After 7 h of reaction unless otherwise noted. From absorption
f
spectrum of the reaction mixture at 2 h. After 4 h. After 3 h.
g
the reaction in the presence of a sufficient amount of 1
proceeded smoothly to give oxidation products (styrene oxide
and phenylacetaldehyde) in high yield (Scheme 1). For
example, when the reaction was conducted with 0.054 mmol
of 1 (0.3 molar equiv. to Mn(TPP)Cl), the TON ([oxidation
products]/[Mn(TPP)Cl]0) after 4 h reached B860 (86% yield)
(Table 1, entry 8) (Fig. 2, K), whereas the reaction in the
absence of the cluster gave only 10% yield under otherwise
identical conditions (entry 1, ꢁ). As revealed from the time
courses of the reactions at [1]0/[Mn(TPP)Cl]0 = 0, 0.10, 0.15,
0.20, 0.30 and 1.0 (Fig. 2), the acceleration effect drastically
depended on the charged amount of 1. The reaction rates
(TON/h) were estimated from the slope of the linear part in
the time course (Table 1) and are plotted against [1]0/
[Mn(TPP)Cl]0 (Fig. 3). As seen in the inset of Fig. 3, the plot
showed a sigmoidal feature. Thus, only modest accelerations
were observed when the initial ratios were less than 0.1 ([1]0/
[Mn(TPP)Cl]0 = 0.05, 0.07), but a steep rise of the reaction rates
was observed in the region 0.1 r [1]0/[Mn(TPP)Cl]0 r 0.3. The
rate increase reached saturation at [1]0/[Mn(TPP)Cl]0 = 0.3,
Fig. 1 Outlines of (a) overall catalytic mechanism in the Au cluster/
Mn porphyrin system and (b) Mn-catalyst reactivation paths on the
Au cluster surface.
(PPh3) and polymer (PVP), we show that the co-catalyst effect
essentially arises from the intrinsic nature of the Au cluster.
Results and discussion
Effects of initial cluster concentration on the catalytic activity
Styrene oxidation with PhIO catalyzed by Mn(TPP)Cl in the
presence of a varying amount of 1 was carried out in a similar
manner to that reported previously.3 Thus, a designated
amount (0.009–0.18 mmol) of 1 and PhIO (180 mmol) were
pretreated in CH2Cl2 (2 mL) at 25 1C under Ar, and then the
catalytic reaction was started by the addition of Mn(TPP)Cl
(0.18 mmol) and styrene (900 mmol). As reported previously,
Fig. 2 Time courses of styrene oxidation with PhIO catalyzed by
Mn(TPP)Cl in CH2Cl2 under argon at 25 1C in the presence of a
varying amount of 1 ([Mn(TPP)Cl]0/[PhIO]0/[styrene]0 = 0.09/90/
450 mM). [1]0/[Mn(TPP)Cl]0 = 0 (ꢁ), 0.1 (.), 0.15 (E), 0.2 (’), 0.3
(K), and 1.0 (m). TON: ([styrene oxide] + [phenylacetaldehyde])/
[Mn(TPP)Cl]0.
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Scheme 2 (a) Possible pathways of the first reactivation process
involving free MnIV(O) porphyrin. (b) A possible reactivation path
of the MnIV species on the Au cluster.
Fig. 3 Correlation of the reaction rate and acceleration factor with
the initial cluster/Mn molar ratio [1]0/[Mn(TPP)Cl]0. The reaction rate
was estimated from the linear part in the time course.
(Fig. 1(a)). Although the axial ligand of the MnIV species
generated under catalytic conditions has not been well identi-
fied to date,7 Newcomb and co-workers recently reported that
the substrate-free comproportionation reaction between
(TPFPP)MnIIICl and (TPFPP)MnVQO (TPFPP: tetrakis-
(pentafluorophenyl)porphyrinato) gives (TPFPP)MnIVQO.10
Supposing that such an oxo species was produced in the
present catalytic system, two reactivation mechanisms shown
in Scheme 2(a) are anticipated: (i) the reaction with the
oxidant (PhIO) activated on the Au surface, and (ii) the direct
oxidative attack of the oxo group to the Au surface. With
either of the mechanisms, the redox process may accompany a
linkage formation between the Au surface and the Mn
porphyrin. In connection with this, we previously reported
definite blue shifts of the absorption bands of MnIII porphyrin
in the reaction mixture and distribution ratio of the oxidation
products (epoxide/aldehyde) when the catalytic oxidation was
conducted at [1]0/[Mn(TPP)Cl]0 = 0.17.3 Therefore, it is likely
that the Mn catalyst is bound to the Au surface as a con-
sequence of the first reactivation process, and thereafter the
subsequent catalytic cycle, which is responsible for the forma-
tion of the majority of the oxidation products, occurs over
cluster-linked Mn catalyst.
where the catalytic reaction was accelerated by a factor of B12
over the background reaction.
Absorption spectra
We also measured absorption spectra of the reaction mixture
after 2 h of reaction to investigate the state of the Mn catalyst
under reactive conditions. In oxidation reactions catalyzed by
Mn porphyrin, MnIII and/or MnIV complexes are detectable
species under conventional reaction/measurement conditions.z
The MnIII complex (lmax = B480 nm) is ‘‘active,’’ since it can
be transformed into the reactive MnVQO species that is
responsible for the substrate oxidation, whereas the MnIV
complex (lmax = B430 nm) is much inferior in terms of the
catalytic activity (Fig. 1(a)). When the reaction was conducted
in the absence of the cluster or with a low charge of the cluster
([1]0/[Mn(TPP)Cl]0 r0.17), less active MnIV species were
predominantly observed in the absorption spectra (430, 525,
666 nm) (Table 1, entries 1–6) (S-Fig. 1a, ESIw). Accordingly,
the color of the reaction mixture was orange, which is
characteristic of MnIV porphyrin. As the initial cluster
concentration increased, the absorption bands due to MnIII
complex appeared and the color of the reaction mixture
became greenish. At [1]0/[Mn(TPP)Cl]0 = 0.2, the mixture
showed two Soret bands, indicating the presence of both MnIII
and MnIV species (entry 7) (S-Fig. 1b, ESIw). When the ratio
reached 0.3, MnIII porphyrin was observed as the dominant
porphyrin species (entries 8–12) (S-Fig. 1c, ESIw). Thus, the
rate increase in Fig. 3 is approximately in parallel to the
amount of MnIII species present in the reaction system. This
observation supports our previous assumption that the major
role of the Au cluster is to regenerate the original active path
involving MnIII/MnV, rather than directly activate the catalyst
in the form of the MnIV complex.3
Along with the above mechanism, the promoting effect of
the Au cluster appears to arise from its perturbation effect on
the reactivity of the linked Mn catalyst to suppress the
deactivation (comproportionation). If so, the oxidation rate
and the content of the MnIII complex should increase con-
stantly with the increase of [1]0, since the Mn-catalyst once
bound to the Au cluster would retain the active form to show
high catalytic activity. However, the plot of the reaction rate
vs. [1]0/[Mn(TPP)Cl]0 gave a somewhat peculiar sigmoidal
shape (Fig. 3), and the MnIV species was solely detected in
the reaction mixture at the low cluster concentration region
(Table 1, entries 1–6) (S-Fig. 1a, ESIw). Accordingly, the
Mn-porphyrin catalyst on the Au cluster should also suffer
deactivation into the MnIV species during the catalytic cycle,
which must be reactivated to acquire high catalytic activity. At
a low feed of the cluster ([1]0/[Mn(TPP)Cl]0 r0.1), the
exposed Au surfaces that serve as the reactivation sites may
be already occupied by Mn porphyrins (Fig. 4(a)), so the
regeneration process should be inhibited. On the other
Reactivation mechanism
In our previous paper, we proposed that the reactivation of the
catalytically ineffective MnIV species into the active forms
(MnIII/MnV) is associated with the redox events on the
exposed Au surface.3 The first reactivation process should
occur with the MnIV species that is initially formed by
comproportionation without the involvement of the Au cluster
hand, when
a large amount of 1 is initially charged
([1]0/[Mn(TPP)Cl]0 40.3), unoccupied surfaces may be
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Fig. 4 Schematic illustration of possible molecular events on the Au
cluster surface at (a) low cluster concentration and (b) high cluster
concentration in the 1/Mn(TPP)Cl catalyst system.
available to allow effective reactivation of MnIV species
(Fig. 4(b)). Thus, two Au cluster molecules appear to be
associated with high catalytic efficiency in the high cluster-
concentration condition, where one serves as a scaffold for the
Mn porphyrin and the other provides a Mn-catalyst reactiva-
tion site. Scheme 2(b) shows a possible reactivation path on
the Au cluster. At the present stage it is difficult to provide a
clear-cut mechanism because of the ambiguity of the axial
ligand of the MnIV complex. In this respect, more intensive
studies including the identification of the Mn-porphyrin/
Au-cluster adduct are required to disclose the overall mechanism.
Fig. 6 Time courses of styrene oxidation with PhIO in CH2Cl2
under argon at 25 1C catalyzed by 1/Mn porphyrin ([Mn(TPP)Cl
or Mn(TMP)Cl]0/[PhIO]0/[styrene]0
= 0.09/90/450 mM): [1]0/
[Mn(TPP)Cl]0 = 0 (ꢁ), 0.17 (E) and 0.5 (K); [1]0/[Mn(TMP)Cl]0
0 (+), 0.17 (’) and 0.5 (m).
=
suppressed at low cluster concentration. When the reac-
tion was conducted at [1]0/[Mn(TPP)Cl]0 = 0.2 under
otherwise identical conditions, the absorption due to the Mn
porphyrin clearly survived.
We also examined the catalytic reaction utilizing
Mn(TMP)Cl (TMP: tetramesitylporphinato), which shows
excellent stability towards oxidative degradation.11 Also in
the present catalytic system coupled with the Au cluster,
Mn(TMP)Cl was much more robust than Mn(TPP)Cl under
the catalytic condition. For example, when the reaction was
conducted with Mn(TMP)Cl and 0.5 molar equiv. of 1 for 7 h,
no significant decrease of the Mn-porphyrin absorptions was
found in the reaction mixture (Fig. 5(b)).y It should also be
noted that Mn(TMP)Cl is superior to Mn(TPP)Cl in terms of
the reaction rates and acceleration effects by the Au cluster.
The reaction catalyzed by Mn(TMP)Cl coupled with 0.5 molar
equiv. of 1 (Fig. 6, m) proceeded more than three times faster
than that with Mn(TPP)Cl (K) (647 and 188 TON/h, respec-
tively). The reaction rate was 76 times larger than that in the
cluster-free condition (7.8 TON/h) (ꢁ), and the yield reached
B 91% within only 3 h (m). Thus, Mn(TMP)Cl is an excellent
catalyst that shows high reactivity and stability in the presence
of the Au cluster.
Mn(TPP) vs. Mn(TMP): differences in the catalyst lifetime and
activity
As shown in Fig. 2 and 3, the rise of the reaction rate upon
increasing the initial cluster concentration is almost saturated
at [1]0/[Mn(TPP)Cl]0 = 0.3. However, when the initial cluster
concentration is further increased, the final yield drops. For
example, when the reaction was conducted with 1.0 equiv. of 1
(Fig. 2, m), the oxidation reaction proceeded with a compar-
able rate to that observed with 0.3 equiv. of 1 (K) but was
almost terminated at 2 h. As a result, the final yield remained
at B60%, which is evidently lower than that observed with
0.3 equiv. of 1 (B85%). Metalloporphyrin-based oxidation
catalysts occasionally suffer self-degradation by the oxidative
attack of highly reactive oxo-metal species,5 so the loss of the
catalytic activity can be attributed to the decomposition of the
Mn porphyrin catalyst under the catalytic condition. Actually,
the reaction mixture after 7 h showed no absorption bands due
to porphyrin species (Fig. 5(a)). Such a degradation was
Another marked difference between Mn(TMP)Cl and
Mn(TPP)Cl was found in the time courses when the initial
molar equiv. of 1 to the Mn catalyst was decreased to 0.17
(Fig. 6, E, ’ and associated lines). The catalytic reac-
tion utilizing Mn(TMP)Cl occurred sluggishly for the initial
B0.5 h but thereafter proceeded very rapidly (’), whereas
such an induction period was hardly observed in the reaction
with Mn(TPP)Cl (E). This difference suggests that the first
reactivation process, which occurs with the free MnIV(TMP)
species to allow the linking with the Au surface (Scheme 2(a)),
is relatively slow due to the conformational rigidity and
sterically hindered metal center of the TMP complex.
y Similarly to the TPP complex (ref. 3), the TMP complex under
the catalytic conditions showed obvious blue shifts (B11 nm) of the
absorption bands around 600 nm (S-Fig. 2, ESIw), suggesting the
linking of Mn catalyst with Au cluster.
Fig. 5 Absorption spectra in CH2Cl2 at 25 1C of the filtered reaction
mixtures of (a) 1/Mn(TPP)Cl and (b) 1/Mn(TMP)Cl catalyst systems
after 7 h of reaction under the conditions given in Fig. 2.
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However, once attached to the Au surface, the MnIV species
generated in the subsequent catalytic cycle may be smoothly
reactivated (Fig. 4(b) and Scheme 2(b)) to give high catalytic
efficiency, as seen in the fast reaction between 1–2 h in the time
course (Fig. 6, ’). Accordingly, the Au cluster scaffold is
considered to affect the reactivity of the linked Mn catalyst to
assist the reactivation process.
the presence of the induction period suggests that the initial
reactivation step involving free MnIV porphyrin and the Au
surface is rather slow due to steric hindrance of the phosphine
ligand. A poly(N-vinyl-2-pyrrolidone)-protected Au cluster
(Au:PVP, 4) also promoted the reaction, but the rate was
significantly slower than those with the thiolate- or phosphine-
protected Au clusters (Fig. 7(a) (E) and (b)). This result agrees
well with the reactivation mechanism involving the two clus-
ters illustrated in Fig. 4(b), since the polymer matrix that
densely encapsulates the Au core would hamper the bicluster
molecular events to lower the reactivation efficiency. Actually,
MnIV porphyrin was observed as the sole porphyrin species in
the absorption spectrum of the reaction mixture, whereas
active MnIII species was dominant when the other Au clusters
protected by low-molecular weight ligands (1–3) were
employed (S-Fig. 2, ESIw).y However, all the Au clusters
examined here exhibited definite positive effects on the
catalytic reaction. Thus, the reactivation effect essentially
originates from the inherent activity of the Au cluster. The
high co-catalyst activities of the thiolate- (1, 2) and phosphine-
(3) protected Au clusters suggest that the residual ligands
provide cavities suitable for the accommodation of the
porphyrin catalyst.
Effects of surface ligands on Au cluster on the catalytic activity
In the experiments described above, we investigated the co-catalyst
effect of the Au cluster by using a dodecanethiolate-protected Au
cluster (1). We also found that Au clusters of a similar size
(d B1.4 nm) bearing different protecting ligands cause similar
positive effects. For this experiment, we utilized robust
Mn(TMP)Cl as the catalyst, and 0.5 molar equiv. of Au cluster
was charged. Under this condition, no bleaching of the
Mn-porphyrin catalyst (decrease/disappearance of the
porphyrin absorptions) was observed at any time during the
reaction. Typically, the reaction coupled with a benzylthiolate-
protected Au cluster (Au:SBn, 2) proceeded with a comparable
rate to that with 1 (Fig. 7(a),K vs. m) to give a TON of B810
after 2 h. When a triphenylphosphine-protected Au cluster
(Au:PPh3, 3) was used instead, the time course of the reaction
showed an induction period, but thereafter the oxidation
reaction occurred very rapidly (’). Based on the above
discussion concerning the steric effects on the time course,
Conclusion
In the present study, we investigated the Au-cluster/
Mn-porphyrin binary catalyst system in styrene oxidation
from several aspects, and we provide mechanistic information
on the nature of the unique positive effect of Au clusters.
Based on the correlation of the reaction rates with the initially
charged amount of Au cluster, we proposed a bicluster
mechanism for the facile regeneration of the active
Mn-porphyrin species, where one cluster serves as a scaffold
and the other serves as a reactivation site for the Mn-catalyst.
We have also suggested that the first reactivation process,
which occurs with the free MnIV(TMP) species to allow the
linking with the Au surface (Scheme 2(a)), is relatively slow,
but, once attached to the cluster, the reactivation of the MnIV
species is facilitated by the linked Au cluster moiety. One of
the interesting points that emerged in this study is the unique
ability of Au clusters to affect the properties of the linked
metalloporphyrin complex. In this respect, studies on the
isolation, characterization, and reactivity of the Au-cluster/
metalloporphyrin hybrid entity are worthy of further
investigations.
Experimental
General
Gas chromatographic (GC) analyses were performed on a
Shimadzu Type GC-14B instrument equipped with an FID
detector and a RESTEC Stabilwax Rtx-1 dimethylpoly-
siloxane capillary column (30.0 m ꢁ 0.25 mm ꢁ 0.25 mm)
using naphthalene as an internal standard. Electronic absorp-
tion spectra were measured on a JASCO Type V-550 UV/VIS
spectrophotometer by using a quartz cell with a 1-cm path
length.
Fig.
7 (a) Time courses of styrene oxidation catalyzed by
Mn(TMP)Cl in CH2Cl2 under argon at 25 1C ([Mn(TMP)Cl]0/
[PhIO]0/[styrene]0 = 0.09/90/450 mM) in the absence (ꢁ) and in the
presence of
1 (K), 2 (m), 3 (’) and 4 (E) ([Au cluster]0/
[Mn(TMP)Cl]0 = 0.5). (b) Relative rates of styrene oxidation cata-
lyzed by Mn(TMP)Cl and Mn(TMP)Cl/Au cluster system. The rate
was estimated from the linear part in the time course.
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Materials
K. Tanaka and M. Haruta, J. Catal., 1998, 178, 566;
(c) M. Valden, X. Lai and D. W. Goodman, Science, 1998, 281,
1647; (d) M. Comotti, C. D. Pina, R. Matarrese and M. Rossi,
Angew. Chem., Int. Ed., 2004, 43, 5812; (e) H. Tsunoyama,
H. Sakurai, Y. Negishi and T. Tsukuda, J. Am. Chem. Soc.,
All solvents including dry CH2Cl2 were from Kanto Chemical
and were used without further purification. Iodosylbenzene
(PhIO), styrene, and poly(N-vinyl-2-pyrrolidone) (PVP-K15,
MW B10000) were purchased from TCI. The chloromanga-
nese(III) complexes of tetraphenylporphyrin [Mn(TPP)Cl] and
tetramesitylporphyrin12 [Mn(TMP)Cl] were synthesized accor-
ding to previously described procedures.13 Au55(PPh3)12Cl6
(3, denoted here as Au:PPh3) was prepared by diborane
reduction of Au(PPh3)Cl.14 Au:SC12 (1) and Au:SBn (2) were
synthesized by ligand exchange reaction of 3 with dodeca-
nethiol and benzylthiol, respectively.3 Au:PVP (4) was pre-
pared by NaBH4 reduction of HAuCl4 in the presence of PVP
according to a previously described procedure.15 All clusters
have a diameter of B1.5 nm from TEM and/or SAXS
analyses.
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W. J. Kruper and R. C. Haushalter, J. Am. Chem. Soc., 1980, 102,
6375; (d) J. P. Collman, L. Zeng and R. A. Decreau, Chem.
Commun., 2003, 2974; (e) M. L. Merlau, S. Cho, S. Sun,
S. T. Nguyen and J. T. Hupp, Inorg. Chem., 2005, 44, 5523;
(f) W. J. Song, M. S. Seo, S. D. George, T. Ohta, R. Song,
M. Kang, T. Tosha, T. Kitagawa, E. I. Solomon and W. Nam,
J. Am. Chem. Soc., 2007, 129, 1268.
6 Examples of related Mn complexes: (a) Z. Gross, G. Golublov and
L. Simkhovich, Angew. Chem., Int. Ed., 2000, 39, 4045;
(b) S. H. Wang, B. S. Mandimutsira, R. Todd, B. Ramdhanie,
J. P. Fox and D. P. Goldberg, J. Am. Chem. Soc., 2004, 126, 18.
7 (a) Colloids and Colloid Assemblies, ed. F. Caruso, Wiley-VCH,
Weinheim, 2004; (b) M. Brust, J. Fink, D. Bethell, D. J. Schiffrin
and C. Kiely, J. Chem. Soc., Chem. Commun., 1995, 1655;
R. L. Whetten, M. N. Shafigullin, J. T. Khoury, T. G. Schaaff,
I. Vezmar, M. M. Alvarez and A. Wilkinson, Acc. Chem. Res.,
1999, 32, 397; (c) A. C. Templeton, W. P. Wuelfing and
R. W. Murray, Acc. Chem. Res., 2000, 33, 27.
8 R. W. Lee, P. C. Nakagaki and T. C. Bruice, J. Am. Chem. Soc.,
1989, 111, 1368.
9 (a) J. T. Groves and M. K. Stern, J. Am. Chem. Soc., 1988, 110,
8628; (b) R. D. Arasasingham, G. X. He and T. C. Bruice, J. Am.
Chem. Soc., 1989, 115, 7985.
Oxidation reactions
Typically, solid PhIO (39.6 mg, 180 mmol) was added to a
20 ml Schlenk tube containing a dry CH2Cl2 solution of 1
(0.03 mmol in 1.8 mL) and naphthalene (B20 mg, internal
standard), and the mixture was stirred at 25 1C under argon.
After 3 h, a CH2Cl2 solution of Mn(TPP)Cl (0.18 mmol/
0.2 mL) and styrene (105 mL, 900 mmol) were successively
added. Aliquots of the reaction mixture were taken at appro-
priate intervals and subjected to GC analysis to determine the
yields of the oxidation products and were then subjected to
absorption spectroscopy after centrifugation followed by
dilution with CH2Cl2. All kinetic runs were conducted in
duplicate or triplicate to check the reproducibility.
10 R. Zhang, J. H. Horner and M. Newcomb, J. Am. Chem. Soc.,
2005, 127, 6573.
11 J. P. Collman, J. I. Brauman, P. D. Hampton, H. Tanaka,
D. S. Bohle and R. T. Hembre, J. Am. Chem. Soc., 1990, 112, 7980.
12 J. S. Lindsey and R. W. Wagner, J. Org. Chem., 1989, 54,
828.
References
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15 H. Tsunoyama, Y. Negishi and T. Tsukuda, J. Am. Chem. Soc.,
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1 For recent reviews, see: (a) P. Burks, Chem. Eng. News, 2007,
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Ed., 2007, 46, 7154; (c) A. S. K. Hashmi and G. J. Hutchings,
Angew. Chem., Int. Ed., 2006, 45, 7896.
2 For selected examples, see: (a) M. Haruta, T. Kobayashi, H. Sano
and N. Yamada, Chem. Lett., 1987, 16, 405; (b) T. Hayashi,
ꢀc
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