Aryl iodides as strong inhibitors in gold and gold-based bimetallic quasi-homogeneous catalysis

Article abstract of DOI:10.1039/c3cc39019e

The strong inhibitor effect of aryl iodides on the quasi-homogenous gold-catalyzed oxidation reaction was described. Aryl iodides were adsorbed on the exposed surface of Au clusters, which affected the accessibility of the nanoclusters to the reacting species and acted as strong inhibitors in catalysis.

Full text of DOI:10.1039/c3cc39019e

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Aryl iodides as strong inhibitors in gold and
gold-based bimetallic quasi-homogeneous catalysis†
Cite this: Chem. Commun., 2013,
4
9, 2542
ab
c
c
d
Raghu Nath Dhital, Choavarit Kamonsatikul, Ekasith Somsook, Yoshinori Sato
Received 18th December 2012,
Accepted 6th February 2013
ab
and Hidehiro Sakurai*
DOI: 10.1039/c3cc39019e
www.rsc.org/chemcomm
The strong inhibitor effect of aryl iodides on the quasi-homogenous
gold-catalyzed oxidation reaction was described. Aryl iodides were
adsorbed on the exposed surface of Au clusters, which affected the
accessibility of the nanoclusters to the reacting species and acted as
strong inhibitors in catalysis.
1
Since the discovery of the Ullmann coupling in 1901 and the
subsequent development of transition metal-catalyzed reactions,
it has been believed that the reactivity of aromatic halides in
oxidative addition processes is in the order iodides > bromides >
chlorides. Previously, however, we reported that the Ullmann
coupling reaction of 4-bromotoluene (1-Br) catalyzed by bimetallic
Au–Pd alloy nanoclusters (NCs) stabilized by poly(N-vinylpyrrol-
idone) (Au0.5Pd0.5:PVP, a 1 : 1 ratio of Au and Pd) is nearly three
Fig. 1 Time-dependent conversion of 1-Cl, 1-Br, and 1-I.
times slower (Fig. S1 and S2, ESI†) than that of 4-chlorotoluene
2
(
1-Cl). For example, the Au0.5Pd0.5:PVP catalyzed coupling of
bimetallic catalyst would be expected to show no catalytic activity.
The inertness of 1-I led us to speculate that the organic iodo group
might inhibit gold-based quasi-homogeneous catalysts. We there-
fore examined the Ullmann coupling reaction of 1-Cl in the
presence of 2 mol% of iodoarenes such as 1-I, 4-iodobenzoic acid
1
-Cl occurred smoothly under ambient conditions to give
0
4,4 -dimethylbiphenyl (2) in 98% yield for 8 h, whereas the
reaction of 1-Br was incomplete even after 24 h and gave 2 in
only 65% yield. The unusual reactivity of Au0.5Pd0.5:PVP toward
2
1-Cl and 1-Br prompted us to investigate the corresponding
(3-I), and (4-iodophenyl)boronic acid (4-I) as external additives in
reaction of 4-iodotoluene (1-I). To our surprise, no reaction
occurred with 4-iodotoluene (1-I) and the substrate was recovered
quantitatively. The order of reactivity in Ullmann coupling of Ar–X
catalyzed by Au0.5Pd0.5:PVP is therefore Cl > Br c I (Fig. 1).
amounts equivalent to that of the catalyst. As expected, iodoarenes
such as 1-I, 3-I, and 4-I acted as strong inhibitors and completely
stopped the reaction (Scheme 1).
A similar inhibitory influence of iodoarenes has also been
observed in oxidation reactions catalyzed by monometallic Au NCs.
It is well-known that Au NCs can catalyze the oxidative homo-
3
Synergistic participation of both metals is crucial for Ullmann
coupling of chloroarenes by bimetallic catalysts, as discussed in
our previous paper. If one of the two metals is poisoned, then the
2
4
coupling of phenylboronic acid (4-H). For example, Au:PVP cata-
a
lyzed the homocoupling of 4-H under ambient conditions to give
Institute for Molecular Science, Myodaiji, Okazaki 444 8787, Japan.
E-mail: hsakurai@ims.ac.jp; Fax: +81-564595527
b
Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
c
NANOCAST Laboratory, Center for Catalysis, Department of Chemistry and
Center for Innovation in Chemistry, Faculty of Science, Mahidol University,
2
72 Thung Phaya Thai, Rama VI Rd., Ratchathewi, Bangkok 10400, Thailand
d
Graduate School of Environmental Studies, Tohoku University, Aoba, Aramaki,
Aoba-ku, Sendai 980-8579, Japan
†
Electronic supplementary information (ESI) available: Experimental details,
UV-vis spectra, TEM images with distribution plots, and STEM/EDS spectra.
See DOI: 10.1039/c3cc39019e
Scheme 1 Effect of aryl iodides on Ullmann coupling.
This journal is c The Royal Society of Chemistry 2013
2
542 Chem. Commun., 2013, 49, 2542--2544

View Article Online
Communication
ChemComm
Table 1 Au-catalyzed oxidative homocoupling of PhB(OH)
2
Yield (%)
a
b
c
Entry
Catalyst
Additives
Conditions
5
6
1
2
3
4
Au:PVP
Au:PVP
Au:chit
Au:chit
—
3-I or 4-I
—
A
A
B
B
74
0
93
0
26
0
7
3-I or 4-I
0
a
2 2 3
Reaction conditions: A: 4 (0.25 mmol), H O (5 mL), K CO (300 mol%),
2
7 1C, 24 h, air; B: 4 (0.25 mmol), acetate buffer (pH 4.57; 5 mL), 30 1C, 9 h,
b
c
air. Yield of isolated products. Yield by NMR. 10 mL of Au catalyst
0.5 mM stock) was used.
(
74% of biphenyl (5-H) and 26% of the oxygenated side product,
phenol (6-H) (Table 1, entry 1). The presence of 2 mol% of 3-I or 4-I as Fig. 2 (a) Changes in the UV-vis spectra of Au0.5Pd0.5:PVP upon addition of 3-I
and (b) the corresponding absorption bands at 274 and 247 nm. (c) Changes in
an additive had a strong inhibitory effect on the catalytic activity of Au
the UV-vis spectra of Au:PVP upon addition of 3-I and (d) the corresponding
(entry 2). The inhibitory effect of iodides was also observed in
absorption bands at 274 and 247 nm. The dotted line in (a) represents the
the homocoupling of 4-H catalyzed by chitosan (b-1,4-linked poly
D-glucosamine)-protected nanogold (Au:chit). Like Au:PVP, Au:chit
catalyzed the homocoupling of 4-H to give the corresponding pro-
ducts 5 and 6 in superior yields (entry 3). The addition of 2 mol% of
absorption spectrum of aqueous solution of 3-I.
absorption band at 274 nm increased with an increase in the
concentration of 3-I from 2.5 to 20 mM and did not increase more
when it exceeded 20 mM (Fig. 2b). Subsequent addition of 3-I to
Au0.5Pd0.5:PVP resulted in subtle changes in the absorption of
monomeric 3-I at 247 nm. To examine these changes, the change
of absorption intensity at 247 nm was plotted as a function of the
concentration of 3-I and a linear relationship was found (Fig. 2b).
The observation of an exclusively monomeric absorption band at 3-I
concentrations of above 20 mM is probably the result of surface
saturation of the NCs and the consequent availability of free 3-I. We
also observed the new absorption band in the UV region (274 nm)
3-I or 4-I inhibited the coupling of 4-H (entry 4). More importantly, the
addition of 4-chlorobenzoic acid (3-Cl) or 4-bromobenzoic acid (3-Br)
did not alter the selectivity of 5 and 6 in the homocoupling of 4-H
catalyzed by Au NCs (Table S1, ESI†). Note that the aryl iodide
5
modified Au:PVP did not catalyze the homocoupling of 4H.
Iodoarene derived inhibition was also observed in the alcohol
6
oxidation reaction catalyzed by Au NCs. Our previous report
showed that various types of alcohols are effectively oxidized by
6
b,7
Au:PVP.
7) gave the corresponding ketone (8) quantitatively within
0 minutes (Scheme 2). The oxidation of 7 was completely inhibited
by the presence of 2 mol% of 3-I, whereas addition of 2 mol% of
-Cl did not suppress the oxidation product.
In particular, benzylic cyclic alcohols such as 1-indanol
(
3
(Fig. 2c), when 3-I was added to Au:PVP (100 mM) and it increased
markedly with increasing 3-I concentration up to 40 mM (Fig. 2d).
The intensity of the absorption band at 274 nm became constant
and that of the monomeric absorption band of 3-I (247 nm)
increased when the concentration of 3-I exceeded 40 mM (Fig. 2d).
Furthermore, the presence of an isosbestic point at 570 nm and
damping of the SPR band of Au in the presence of 3-I (Fig. S3, ESI†)
also confirm that these absorption changes arise from a complexa-
3
To elucidate the mechanism of inhibition, we performed spectro-
scopic investigations of the reactions between aryl iodides and metal
NCs. First, the interaction of 3-I with Au Pd :PVP, Au:PVP, and
0.5 0.5
Pd:PVP was performed by means of UV-vis spectroscopy. Fig. 2a
shows the changes in the UV-vis spectra of a 100 mM aqueous
solution of Au0.5Pd0.5:PVP containing 3-I at concentrations of
9
tion equilibrium between Au NCs and 3-I.
The possibility that the new absorption band at 274 nm
arose from the interaction of 3-I with Pd was excluded because
it was not observed in a control experiment with Pd:PVP
0
–30 mM. When 2.5 mM 3-I was present in 100 mM Au0.5Pd0.5:PVP,
a pronounced surface-binding effect was observed near the UV
region, as manifested by the appearance of a new absorption band
at 274 nm; this is the characteristic absorption of a complex formed
(Fig. S6a, ESI†). The possibility that the absorption band at
8,9
2
74 nm arose from the interactions between ꢀCOOH and Au
by interaction of 3-I with the metal NCs. The intensity of the
was also excluded because no absorption at 274 nm was observed
in control experiments with 4-chlorobenzoic acid (3-Cl) and
Au:PVP (Fig. S6b in ESI†). Therefore, inhibition of the catalyst
by aryl iodides must result from the adsorption of iodides on the
surface of Au, thereby destroying the Au–Pd synergy.
Fluorescence is an excellent probe for revealing electronic
interactions between surfaces of nanomaterials and adsorbed
10
molecules. Because the fluorescence of arylboronic acids is sensi-
tive to binding events, we chose 4-I for this study. Fig. 3a shows a
representative set of changes in fluorescence spectra of 4-I (100 mM)
Scheme 2 Au:PVP-catalyzed oxidation of 7 in the absence and presence of
2
mol% of additives.
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2542--2544 2543

View Article Online
ChemComm
Communication
data using Scherrer’s equation. The results of XRD and TEM indicate
an increase in crystallinity of the particles as a result of aggregation
and surface modification through adsorption of iodides on the Au
surface. A sample used in the above experiment was centrifuged
at 4000 rpm and washed with Milli-Q water and then analyzed by
SEM/EDS. The EDS results confirmed the presence of 4-I as an
adsorbed species on the surface of Au (Fig. 4b and Fig. S9, ESI†).
Indeed, some reports are available for the activation of C–I
bonds by heterogeneous Au nanoparticles in organic transfor-
Fig. 3 (a) Effect of concentration of Au on the fluorescence response of 4-I. (b) A
plot of I/I
of Au and I
em = 356 nm). (c) The absorption spectra of 4-I and the mixture of 4-I and Au.
0
vs. concentration of Au, where I = emission intensity of 4-I in the presence mation under ultra-high vacuum conditions or at temperatures
1
1
0
= emission intensity of 4-I in the absence of Au (lex = 280 nm and more than 100 1C. Nevertheless, C–I bond activation by quasi-
l
homogeneous nanogolds has not yet been reported. Instead,
formation of a charge transfer complex between the iodoper-
fluorobenzene and citrate stabilized Au NCs was recently
8
reported, which is in good agreement with our current iodide
inhibition effect.
In conclusion, we have demonstrated that aryl iodides act as
inhibitors rather than reactants in reactions catalysed by quasi-
homogenous nanogold and gold-based bimetallic catalysts.
UV-vis and fluorescence spectra showed that aryl iodides bind
to the exposed surfaces of gold NCs, thereby masking active
sites for catalysis and acting as strong inhibitors. As a result,
the catalytic activity of bimetallic Au–Pd NCs in Ullmann
coupling reaction is in the order R–Cl > R–Br c R–I. Further-
more, selective adsorption of aryl iodides on the surface of gold
might be applied to surface modification/protection, as an
alternative group to thiol derivatives.
We are grateful to Dr Yuki Morita for helpful discussions
and Ms Setsiri Haesuwannakij for TEM measurements.
Fig. 4 (a) XRD profiles of Au:PVP before and after mixing with 4-I. (b) SEM/EDS
spectrum of Au:PVP after mixing with 4-I in 1 : 1 ratio. (c) TEM image of Au:PVP.
(
d) and (e) TEM and HR-TEM images of Au:PVP after mixing with 4-I in 1 : 1 ratio.
Notes and references
1
F. Ullmann and J. Bielecki, Chem. Ber., 1901, 34, 2174.
upon addition of Au:PVP. Monomeric 4-I itself shows weak fluores-
cence, whereas a marked increase in fluorescence intensity with a
small blue shift (Fig. S7, ESI†) is observed upon the addition of
Au:PVP in amounts from 1.25 mM to 55 mM. Upon increasing the
amount of Au:PVP above 1.25 mM, the emission intensity increased
and reached a maximum of 100-fold that of initial intensity of
monomeric 4-I when the amount of Au reached 55 mM (Fig. 3b).
Similar to 3-I, upon mixing Au with 4-I, a new absorption band was
observed at 274 nm (Fig. 3c). Hence from the result of optical
properties, it is observed that Au can form a complex with 4-I/3-I,
and the thus-formed complex has its own absorption at 274 nm and
emission at 356 nm whose intensity increases linearly with an
increase in the amount of complex formed.
The aggregation of gold clusters and surface modification
occurred during the adsorption process, as evidenced by the X-ray
diffraction (XRD) measurement and transmission electron micro-
scopy (TEM) observation. The change in the peak shape as well as
the shift in the 2Y have been observed in XRD analysis (Fig. 4a).
Upon mixing Au:PVP with 4-I in a 1 : 1 ratio, the broad (111) peak
centered at 2Y = 39.081, associated with small face-centered cubic
gold crystals, became acute as compared with Au:PVP, centered at
2 R. N. Dhital, C. Kamonsatikul, E. Somsook, K. Bobuatong, M. Ehara,
S. Karanjit and H. Sakurai, J. Am. Chem. Soc., 2012, 134, 20250.
3
For bimetallic effects see: (a) G. J. Hutchings, Chem. Commun., 2008,
148; (b) M. Chen, D. Kumar, C.-W. Yi and D. W. Goodman, Science,
2005, 310, 291; (c) R. N. Dhital and H. Sakurai, Chem. Lett., 2012,
1, 630; (d) T. Balcha, J. R. Strobl, C. Fowler, P. Dash and R. W.
1
4
J. Scott, ACS Catal., 2011, 1, 425; (e) A. Murugadoss, K. Okumura and
H. Sakurai, J. Phys. Chem. C, 2012, 116, 26776.
4 (a) R. N. Dhital, A. Murugadoss and H. Sakurai, Chem.–Asian J., 2012, 7, 55;
b) L. Chaicharoenwimolkul, A. Munmai, S. Chairam, U. Tewasekson,
S. Sapudom, Y. Lakliang and E. Somsook, Tetrahedron Lett., 2008, 49, 7299;
c) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi and T. Tsukuda,
(
(
Langmuir, 2004, 20, 11293; (d) L. Wang, W. Zhang, D. S. Su, X. Meng and
F.-S. Xiao, Chem. Commun., 2012, 48, 5476.
5
6
See ESI.
For Au NC catalyzed oxidation reactions see: (a) Y. Zhang, X. Cui,
F. Shi and Y. Deng, Chem. Rev., 2012, 112, 2467; (b) T. Tsukuda,
H. Tsunoyama and H. Sakurai, Chem.–Asian J., 2011, 6, 736.
(a) A. Murugadoss and H. Sakurai, J. Mol. Catal. A: Chem., 2011,
7
8
341, 1; (b) H. Tsunoyama, T. Tsukuda and H. Sakurai, Chem. Lett.,
2007, 36, 212.
For the recent report of a charge transfer complex formed between
the iodoperfluorobenzene and the surface of 16 nm citrate-stabilized
Au NCs see: I. Blakey, Z. Merican, L. Rintoul, Y.-M. Chuang, K. S. Jack
and A. S. Micallef, Phys. Chem. Chem. Phys., 2012, 14, 3604.
P. V. Kamat, J. Phys. Chem. B, 2002, 106, 7729.
9
1
0 (a) I.-I. S. Lim, F. Goroleski, D. Mott, N. Kariuki, W. Ip, J. Luo and
C.-J. Zhong, J. Phys. Chem. B, 2006, 110, 6673; (b) S. Y. Lim, J. H. Kim,
J. S. Lee and C. B. Park, Langmuir, 2009, 25, 13302.
2Y = 38.291. TEM images of Au:PVP before and after mixing with 4-I
1
1 (a) S. K. Beaumont, G. Kyriakou and R. M. Lambert, J. Am. Chem.
Soc., 2010, 132, 12246; (b) B. Karimi and F. K. Esfahani, Chem.
Commun., 2011, 47, 10452; (c) G. Li, C. Liu, Yu Lei and R. Jin,
Chem. Commun., 2012, 48, 12005.
(Fig. 4c–e) showed changes both in the size (increased from 1.3 nm
to 2.7 nm, Fig. S8, ESI†) and the shape of gold crystals, which is in
close agreement with the average crystalline size estimated by XRD
2
544 Chem. Commun., 2013, 49, 2542--2544
This journal is c The Royal Society of Chemistry 2013