A R T I C L E S
Crudden et al.
In several cases, it has been demonstrated that the metal can
leach from the surface by interacting with the aryl halide
substrate.19b-d,20 Depending on the affinity of the product for
Pd, reprecipitation of Pd on the surface generally occurs at the
end of the reaction after the substrate has been consumed. Even
ligand-functionalized surfaces are prone to leaching, which may
be attributed to instability of the Pd-ligand bond or ligand-
surface bond.19 Ultimately the inherently higher catalytic activity
of the trace amounts of soluble Pd, combined with weak ligation,
is a major difficulty to be overcome in the area of supported
catalysis.21
In the absence of truly effective supported catalysts, a variety
of scavengers for Pd have been developed to treat the solution
or substrate after the reaction. These are summarized in a recent
comprehensive review by Prasad et al.6a and include three basic
approaches: precipitation from solution after complexation by
a ligand, adsorption onto a polymeric support which can be
removed by filtration, or removal of the product while keeping
Pd dissolved in solution. For example, palladium can be
precipitated from solution using 2,4,6-trimercapto-S-triazine
(TMT).22 Treatment with unfunctionalized adsorbents such as
charcoal is sometimes effective, but can also lead to significant
losses of product which are also absorbed. Organic polymers
functionalized with thiols such as MPTMT (mesoporous poly-
styrene bound TMT) and related species are commercially
available, but require long reaction times in excess of 24 h and
larges excesses of reagent to effectively remove Pd from
solution.23b Interestingly, functionalized silica resins are reported
to require decreased time for effective scavenging of Pd.23c
Inspired by the work of Fryxell et al.24a and Pinnavaia and
Mercier,24b who demonstrated that thiol-modified mesoporous
materials are remarkable scavengers for mercury, and by Kang
et al.,24c,d who showed that SBA-15-SH has a higher affinity
for Pd and Pt compared to other metals, we began a study of
mercaptopropyl-modified SBA-15. We examined both the ability
of this material to remove various forms of Pd from aqueous
and organic solutions and, most importantly, the ability of the
resulting material to actually catalyze coupling reactions.25 Our
hope was that the functionalized material would act both as a
support and as a scavenger for any small amounts of residual
Pd that escape from the surface.
We have found that not only is mercaptopropyl-modified
SBA-15 a superb scavenger for Pd, but the resulting Pd-
encapsulated material catalyzes the Mizoroki-Heck and Su-
zuki-Miyaura reactions, without leaching Pd into solution. At
the end of the reaction, even using loadings as high as 2%, as
little as 3 parts per billion Pd are observed in solution,
accounting for only 0.001% of the initially added catalyst. Most
remarkably, heterogeneity tests including hot filtration tests and
three-phase tests have demonstrated that the catalysis occurs
on the surface or in the pores of the silicate.
Results and Discussion
Synthesis of Functionalized Silicates. We chose to employ
the mesoporous silicate SBA-1526 as a support for the thiol
ligands because it has high porosity, surface area, and high
surface concentration of silanols. In addition, studies have shown
that the thiol groups in large pore mesoporous materials are
more accessible than those on amorphous silica,27 although this
may depend more on surface area and porosity than long-range
order because disordered but highly porous organic inorganic
composites also have high capacity for mercury scavenging.28
Two methods were employed to introduce the thiol groups.
In the first approach, (CH3O)3Si(CH2)3SH was reacted with a
preformed silicate (SiO2 or SBA-15). Functionalization of the
surface occurred by condensation with surface silanols and loss
of methanol leading to a loading of 2.2 mmol/g determined by
elemental analysis. Additionally, a sol-gel synthesis in which
the thiol ligand is incorporated directly into the material during
its synthesis was also carried out according to Stucky’s
method.29 Thus, Si(OEt)4 and (CH3O)3Si(CH2)3SH were added
in a 94:6 molar ratio to a preformed solution of the polymeric
surfactant in aqueous HCl.29 After an appropriate reaction time
(see Experimental Section), the surfactant was removed by
extraction revealing the porous structure. In the sol-gel
materials, a thiol loading of 6% or 1.0 mmol/g was generally
employed, although materials with 8% and 10% loading were
also prepared.30 As a comparison, silica functionalized with
mercaptopropyl groups was prepared by grafting mercaptopropyl
trimethoxysilane on the surface of porous, amorphous silica.31
Scavenging Experiments. In an initial screen of silica-based
adsorbents, we examined the ability of SBA-15-SH (prepared
by grafting and sol-gel methods), amorphous silica (SiO2-SH,
grafted), SBA-15 itself, and Montmorillonite clay to remove
Pd(OAc)2 from THF (Figure 1) and PdCl2 from water (Table
1). Montmorillonite clay and unfunctionalized SBA-15 were
(19) (a) Corma, A.; Das, D.; Garc´ıa, H.; Leyva, A. J. Catal. 2005, 229, 322.
(b) Rocaboy, C.; Gladysz, J. A. Org. Lett. 2002, 4, 1993. (c) Bergbreiter,
D. E.; Osburn, P. L.; Frels, J. D. AdV. Synth. Catal. 2005, 347, 172. (d)
Yu, K.; Sommer, W.; Weck, M.; Jones, C. W. AdV. Synth. Catal. 2005,
347, 161. (e) Cassol, C. C.; Umpierre, A. P.; Machado, G.; Wolke, S. I.;
Dupont, J. J. Am. Chem. Soc. 2005, 127, 3298. (f) Rosner, T.; Pfaltz, A.;
Blackmond, D. G. J. Am. Chem. Soc. 2001, 123, 4621. (g) Nowotny, M.;
Hanefeld, U.; van Koningsveld, H.; Maschmeyer, T. Chem. Commun. 2000,
1877. (h) Bedford, R. B.; Cazin, C. S. J.; Hursthouse, M. B.; Light, M. E.;
Pike, K. J.; Wimperis, S. J. Organomet. Chem. 2001, 633, 173.
(20) (a) Ko¨hler, K.; Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J. Chem.-
Eur. J. 2002, 8, 622. (b) Zhao, F.; Bhanage, B. M.; Shirai, M.; Arai, M.
Chem.-Eur. J. 2000, 6, 843. (c) Zhao, F.; Shirai, M.; Ikushima, Y.; Arai,
M. J. Mol. Catal. A: Chem. 2002, 180, 211. (d) Lipshutz, B. H.; Tasler, S.;
Chrisman, W.; Spliethoff, B.; Tesche, B. J. Org. Chem. 2003, 68, 1177.
(e) Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J.; Ko¨hler, K. J. Mol.
Catal. A 2002, 182-183, 499.
(21) Bradley et al. have shown that colloidal Pd can be orders of magnitude
more active than heterogeneous Pd in the Heck reaction: Le Bars, J.; Specht,
U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999, 15, 7621.
(22) Rosso, V. W.; Lust, D. A.; Bernot, P. J.; Grosso, J. A.; Modi, S. P.;
Rusowicz, A.; Sedergran, T. C.; Simpson, J. H.; Srivastava, S. K.; Humora,
M. J.; Anderson, N. G. Org. Process Res. DeV. 1997, 1, 311.
(23) (a) Ishihara, K.; Nakayama, M.; Kurihara, H.; Itoh, A.; Haraguchi, H. Chem.
Lett. 2000, 10, 1218. (b) Argonaut technical note 515, 2003. (c) www-
.silicycle.com/html/English/products/produit_detail.php?pro_id)148.
(24) (a) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K.
M. Science 1997, 276, 923. (b) Mercier, L.; Pinnavaia, T. J. AdV. Mater.
1997, 9, 500. (c) Kang, T.; Park, Y.; Park, J. C.; Cho, Y. S., Yi, J. Stud.
Surf. Sci. Catal. 2003, 146, 527. (d) Kang, T.; Park, Y.; Yi, J. Ind. Eng.
Chem. Res. 2004, 43, 1478.
(27) Mercier, L.; Pinnavaia, T. J. AdV. Mater. 1997, 9, 500.
(28) (a) Shea, K. J.; Hobson, S. T.; Tran, J. United States Patent Application
20030176396, September 18, 2003. WO 03/055452; PCT/US02/09856. (b)
Hobson, S. T.; Braue, E. H.; Shea, K. J. U.S. Patent No. 6,417,236, July
9, 2002.
(29) Melero, J. A.; Stucky, G. D.; van Grieken, R.; Morales, G. J. Mater. Chem.
2002, 12, 1664.
(25) Note Added in Proof. During review of this manuscript, a publication by
Shimizu et al. appeared documenting the ability of Pd-treated thiol-modified
FSM-15 to act as an efficient catalyst for coupling reactions: Shimizu,
K.-I.; Koizumi, S.; Hatamachi, T.; Yoshida, H.; Komai, S.; Kodama, T.;
Kitayama, Y. J. Catal. 2004, 228, 141.
(26) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B.
F.; Stucky, G. D. Science 1998, 279, 548.
(30) The materials prepared with higher loadings of thiol were only tested for
catalysis (next section), rather than scavenging studies, and gave materials
that were active catalysts with no discernible difference in activity thus
far.
(31) The silica employed had a BET surface area of 297 m2/g.
9
10046 J. AM. CHEM. SOC. VOL. 127, NO. 28, 2005