Monolayer protected Au cluster (MPC)-bound Ru-carbene complex: Synthesis and its catalytic activity in ring-closing olefin metathesis

Article abstract of DOI:10.1016/j.tetlet.2005.04.123

Monolayer-protected gold cluster (Au-MPC)-bound Ru-carbene complex (MPC-7) has been synthesized and examined its catalytic activity in ring-closing olefin metathesis. The MPC-7 showed almost the same catalytic activity with that of simple molecules (Grubb

Full text of DOI:10.1016/j.tetlet.2005.04.123

Tetrahedron
Letters
Tetrahedron Letters 46(2005) 4501–4503
Monolayer protected Au cluster (MPC)-bound
Ru–carbene complex: synthesis and its catalytic activity in
ring-closing olefin metathesis
Bang Sook Lee,a,b Sung Keon Namgoongb and Sang-gi Leea,*
aLife Sciences Division, Korea Institute of Science and Technology (KIST), PO Box 131, Cheongryang, Seoul 130-650, South Korea
bDepartment of Chemistry, Seoul Women’s University, Seoul 139-774, South Korea
Received 4 March 2005; revised 6April 2005; accepted 15 April 2005
Abstract—Monolayer-protected gold cluster (Au-MPC)-bound Ru–carbene complex (MPC-7) has been synthesized and examined
its catalytic activity in ring-closing olefin metathesis. The MPC-7 showed almost the same catalytic activity with that of simple mole-
cules (Grubbs Ru-complex 1b and the thioacetate-functionalized Ru-complex 8) and reused several times.
Ó 2005 Elsevier Ltd. All rights reserved.
The modification of monolayer-protected nanosize
metal clusters, especially Au clusters (Au-MPCs) by
the incorporation of a functionalized thiol or disulfide
is of potential significance for the expansion of chemical
and biological applications of these materials.1 Based on
the reactive x-functional groups, a variety of organic
reactions such as nucleophilic substitution and addition
reactions, coupling, and polymerizations etc. have been
extensively studied.2 However, reports concerning
MPCs coated with thiols or disulfides bearing cata-
lytically active species, especially metal-complexes at
terminal position are quite limited.3 Utilization of
nanoparticle-supported catalysts may provide a number
of advantages: (i) nanometer-sized MPC (usually in the
2–25 nm size range) behaves like homogeneous, but can
be precipitated by varying solvents allowing an easy sep-
aration of the catalyst; (ii) The catalysts are readily ex-
posed on the surface of the metal clusters, and
expected to show high catalyst activity, which would
be comparable to the parent homogeneous catalysts;
(iii) Moreover, in contrast to polymer-supported cata-
lysts, the MPC-supported catalysts are amenable to
characterization by 1H NMR in solution. Herein, we re-
port synthesis of a novel Au-MPC-bound Ru–carbene
complex and its catalytic activity in ring-closing olefin
metathesis.
The ring-closing olefin metathesis (RCM) catalyzed by
Ru-complexes, most notably the Grubbs-type Ru alkyl-
idenes 1a and b and HoveydaÕs 2a and b (Fig. 1), has tre-
mendous impact in synthetic organic chemistry.4 Hence,
various kinds of support materials including polymers,5
dendrimers,6 and most recently ionic liquids7 and fluori-
nated polymer8 have already been utilized for the devel-
opment of robust and recyclable Ru-catalysts for RCM.
However, no attempts have been made to immobilize
the Ru–carbene complex onto the Au-MPCs.
Scheme 1 shows the synthesis of Au-MPC-bound Ru–
carbene complex. The required styrene-functionalized
thiol 4b could be obtained by O-alkylation of the 5-hy-
droxy-2-isopropoxystyrene 35f with 1-bromo-12-thioace-
tyldodecane9 followed by the hydrolysis of the resulting
thioacetate 4a. Subsequently, we accomplished an asso-
ciative place-exchange reaction by dissolving the octane-
thiol-passivated Au-MPC 5 (average core diameter:
ꢀ2.5 nm), which was prepared according to the
detailed protocol reported by Murray and co-workers,10
L
L
Cl
L
a: PCy3
Ru
Cl
Cl
Ru
Cl
O
Ph
N
N
Mes
Mes
b:
PCy3
..
Cy: Cyclohexyl
Mes: 2,4,6-Trimethylphenyl
2
1
Keywords: Monolayer; Gold cluster; Ru–carbene complex; Metathesis.
*
Corresponding author. Tel.: +82 2 958 5163; fax: +82 2 958 5189;
e-mail: sanggi@kist.re.kr
Figure 1. Typical Ru–carbene complexes used for the RCM of olefins.
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.123

4502
B. S. Lee et al. / Tetrahedron Letters 46 (2005) 4501–4503
H
H
H
O
O
H
O
O
O
C8
Mes
i
H
O
H
O
N
H
N
S
S S
S
S
S
S
Cl
O
S
O
S
S
S
S
AuSS
S
S
N
C8
Ru
S
S
S
S
C8
O
O
C12
SS
Mes
SR
12
C12
OH
3
Cl
S
S
Ha
S
S
5
iii
iv
S
S
4a: R = Ac
4b: R = H
S
Au S
iv
ii
S
S
Au
Mes
N
Mes
S
S
S
S
O
O
Cl
S
S
S
S
S
S
Ru
O
O
O
Cl
Hb
H
O
O
H
H
Hc
O
SCOCH3
12
O
O
8
H
H
MPC-7
MPC-6
Scheme 1. Synthesis of Au-MPC-bound Ru–carbene complex (MPC-7). Reagents and conditions: (i) NaH/THF/Br(CH2)12SCOCH3, rt, 30 min,
87%; (ii) aqueous NaOH, EtOH, rt, 1 h, 98%; (iii) compound 5, CH2Cl2, rt, 20 h; (iv) compound 1b, CuCl, CH2Cl2, reflux, 2 h, 8 (85%).
and styrene-functionalized thiol 4b in dichloromethane.
After removing the solvent, the resulting black solid
was washed several times with anhydrous, degassed eth-
anol to give the MPC-6. The ligand exchange could be
1
confirmed by H NMR spectrum analysis, in which the
resonance signals of the vinyl (d 5.70 and 5.23 ppm)
and phenyl ring protons (d 6.75 and 7.03 ppm) of the
styrene moiety appeared, and revealed that the obtained
MPC-6 comprises a 3:1 mixture of octane- and styrene-
functionalized thiolates.11 Treatment of MPC-6 with sec-
ond generation Grubbs Ru-complex 1b in the presence of
CuCl4d resulted in the exchange of the styrene groups to
Figure 2. TEM images of (a) MPC-7 as prepared and (b) after 7th
cycle (entry 9 in Table 1).
deliver the black, nano-size MPC-7, which was washed
thoroughly with ethanol to remove the residual 1b.
Under the same reaction condition, the parent homoge-
neous Ru complex 8 could also be synthesized by reac-
tion of the thioacetate 4a with 1b in 85% yield.
Formation of Ru-complex in MPC-7 and 8 can clearly
ized Ru-complex 8 (entry 2), and all reactions afforded
the ring-closed product 10a in quantitative yields. To as-
sess the reusability of the MPC-bound Ru–carbene com-
plex MPC-7, the catalyst was recovered from the first
run (entry 3 in Table 1),12 and used then in subsequent
reactions. It has been found that the catalytic activity
of the recovered MPC-7 was retained during the first
four runs (entries 3–6), but slightly dropped in 5th (entry
7) and 6th cycles (entry 8). After, 6th cycle, the catalytic
activity decreased dramatically, and the reaction in 7th
cycle was not completed even in extended reaction time
(12 h) (entry 9). Noteworthy observation is that the sol-
uble MPC-7 in dichloromethane became insoluble dur-
ing the reaction in 7th run (entry 10), and precipitated
out. The TEM analysis of the insoluble particles ob-
tained from the 7th run (Fig. 2b) clearly indicate that
the size of the particles has been increased (average
diameter: 8.0 nm).13 Based on these considerations, it
could be assumed that the coated monolayers on the
gold surfaces are destroyed, and once they start desorp-
tion from the gold surface, the coated monolayers could
destruct dramatically leading to the aggregation of the
gold clusters, and the catalytically active Ru–carbene
complex could be decomposed, and lose their catalytic
1
1
be confirmed by the H NMR spectra analyses. In H
NMR spectrum of MPC-7, the characteristic Ru@CH
proton (Ha) resonated at 16.44 ppm. The ratio of the
integration area between the Ha and Hb (or Hc) indi-
cates that one-fourth of the vinyl moiety presented in
MPC-6 formed Ru complex. The MPC-supported Ru–
carbene complex is soluble in dichloromethane and tolu-
ene, but insoluble in methanol, ethanol, and diethyl
ether. Elemental analysis indicated that 0.18 mmol/g of
Ru incorporated in the MPC-7. The transmission elec-
tron microscopy (TEM) measurements show that the size
of Au core (average 2.4 nm) retained during the place-
exchange and incorporation of Ru species (Fig. 2a).
To examine the catalytic activity of the Au-MPC-bound
Ru–carbene complex (MPC-7), the ring-closing meta-
thesis was carried with 9a as model substrate using
5 mol% of catalyst in dichloromethane at 40 °C. As
shown in Table 1, the catalytic activity of the MPC-7
(entry 3) is quite comparable with those of the Grubbs
Ru-complex 1b (entry 1) and the thioacetate-functional-

B. S. Lee et al. / Tetrahedron Letters 46 (2005) 4501–4503
4503
Table 1. Ring-closing metathesis of dienes 9a–c
J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003,
15, 81; (c) Katz, E.; Willner, I. Angew. Chem., Int. Ed.
2004, 43, 6042.
2. Shon, Y.-S.; Choo, H. D.K.P.Ng CR. Chimie 2003, 6,
1009, and references cited therein.
Catalyst (5 mol%)
CH2Cl2, 40 o
C
n
n
n
n
N
N
N
N
3. (a) Bartz, M.; Kuther, J.; Seshadri, R.; Tremel, W. Angew.
¨
Ts
Ts
Ts
Ts
Chem., Int. Ed. 1998, 37, 2466; (b) Li, H.; Luk, Y.-Y.;
Mrksich, M. Langmuir 1999, 15, 4957; (c) Marubayashi,
K.; Takizawa, S.; Kawakusu, T.; Arai, T.; Sasai, H. Org.
Lett. 2003, 5, 4409.
9a: n = 1
10a: n = 1
10v n = 2
9c
10c
9b: n = 2
Entry
Cycle
Cat.
9
Conv (%)a
1b
2c
1st
1st
1b
8
9a
9a
>98
>98
4. (a) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2039; (b) Schwab,
P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 100; (c) Kingsbury, J. S.; Harrity, J. P. A.; Bonitate-
bus, P. J.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121,
791; (d) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.;
Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168.
5. (a) Nguyen, S. T.; Grubbs, R. H. J. Organomet. Chem.
1995, 497, 195; (b) Ahmed, M.; Barrett, A. G. M.;
Braddock, D. C.; Cramp, S. M.; Procopiou, P. A.
3d
4
1st
MPC-7
MPC-7
MPC-7
MPC-7
MPC-7
MthPC-7
MPC-7
9a
9a
9a
9a
9a
9a
9a
>98
>98
>98
98
2nd
3rd
5
64th
7
5th
6
85
80
8
9e
7th
20
10
11
1st
1st
MPC-7
MPC-7
9b
9c
>98
>98
Tetrahedron Lett. 1999, 40, 8657; (c) Schurer, S. C.;
¨
Gessler, S.; Buschmann, N.; Blechert, S. Angew. Chem.,
Int. Ed. 2000, 39, 3898; (d) Jafarpour, L.; Nolan, S. P. Org.
Lett. 2000, 2, 4075; (e) Kingsbury, J. S.; Garber, S. B.;
Giftox, J. M.; Gray, B. L.; Okamoto, M. M.; Farrer, R.
A.; Fourkas, J. T.; Hoveyda, A. H. Angew. Chem., Int. Ed.
2001, 40, 4251; (f) Yao, Q. Angew. Chem., Int. Ed. 2000,
39, 3896; (g) Connon, S. J.; Dunne, A. M.; Blechert, S.
Angew. Chem., Int. Ed. 2002, 41, 3835; (h) Yao, Q.; Motta,
A. R. Tetrahedron Lett. 2004, 45, 2447.
a Determined by 1H NMR and reactions carried out for 1.5 h otherwise
noted.
b Reaction carried out for 3 h.
c Reaction carried for 30 min.
d For entries 3–8, the catalyst was recovered for the next run.12
e Reaction carried for 12 h.
activity. Finally, we examined the catalytic activity of
the MPC-7 for other substrates 9b and c, and afforded
the corresponding products 10b (entry 10) and c (entry
11) in quantitative yields.
6. (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda,
A. H. J. Am. Chem. Soc. 2000, 122, 8168; (b) Gatard, S.;
Nlate, S.; Cloutet, E.; Bravic, G.; Blais, J.-C.; Astruc, D.
Angew. Chem., Int. Ed. 2003, 125, 452.
7. (a) Yao, Q.; Zhang, Y. Angew. Chem., Int. Ed. 2003, 42,
3395; (b) Audic, N.; Clavier, H.; Mauduit, M.; Guillemin,
J.-C. J. Am. Chem. Soc. 2003, 125, 452.
8. Yao, Q.; Zhang, Y. J. Am. Chem. Soc. 2004, 126, 74.
9. Yokokawa, S.; Tamada, K.; Ito, E.; Hara, M. J. Phys.
Chem. B 2003, 107, 3544.
In conclusion, we have synthesized a novel monolayer-
protected gold nanoparticles-bound Ru–carbene com-
plex MPC-7, which showed high reactivity and high
levels of reusability in ring-closing olefin metathesis
suggesting that the monolayer-protected nanoparticles
could have a high potential as support-materials for
the recyclable catalysts. Studies on MPC-supported cat-
alysts for other catalytic reactions are under progress.
10. Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J.
E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green,
S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M.
D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
11. The ratio of 3:1 was determined by comparing the
integrated areas for the vinyl protons of 4b and the
poly(methylene) chains for both 4b and octanethiolate.
12. The dichloromethane solvent was removed, and the
product 10a was dissolved in methanol, and the precip-
itated MPC-7 was isolated, dried, and used for the next
run.
13. In UV–vis absorption spectra (1 mg sample in 10 mL of
CH2Cl2) of the MPC-7 recovered until 6th cycle, the kmax
(ꢀ520 nm) and absorbance (0.60) were almost retained.
However, no kmax could be observed from the MPC-7
after 7th cycle, which may be largely due to the poorer
solubility by the formation of aggregates as they demon-
strated in TEM.
Acknowledgements
The authors thank financial support from National Re-
search Laboratory and Future Core Technology Pro-
gram at KIST, Center for Molecular Design and
Synthesis at KAIST.
References and notes
1. Reviews, see: (a) Templeton, A. C.; Wuelfing, W. P.;
Murray, R. W. Acc. Chem. Res. 2000, 33, 27; (b) Gooding,