Water-soluble platinum and palladium nanoparticles modified with thiolated β-cyclodextrin

Article abstract of DOI:10.1039/b002423f

Pt and Pd nanoparticles can be modified with surface-attached cyclodextrin receptors leading to water-soluble materials that exhibit catalytic activity for the hydrogenation of allylamine.

Full text of DOI:10.1039/b002423f

Water-soluble platinum and palladium nanoparticles modified with thiolated
b-cyclodextrin†
Julio Alvarez, Jian Liu, Esteban Roma´n and Angel E. Kaifer*
Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables,
FL 33124-0431, USA. E-mail: akaifer@miami.edu
Received (in Columbia, MO, USA) 23rd March 2000, Accepted 17th May 2000
Pt and Pd nanoparticles can be modified with surface-
attached cyclodextrin receptors leading to water-soluble
materials that exhibit catalytic activity for the hydro-
genation of allylamine.
proximity to the metal particles.4,7 These spectroscopic features
must result from the presence of HS-b-CD molecules attached
to the metal nanoparticles, since any free HS-b-CD molecules
were washed away before spectroscopic analysis.‡
TEM measurements recorded on the precipitates collected
from the reduction of the Pt (or Pd) complex in the presence of
HS-b-CD verified that these solids materials are composed of
metal particles with dimensions in the nanometer range. Typical
TEM images are shown in Fig. 1 and histograms showing the
particle size distributions (obtained from individual measure-
ments on at least 100 particles) are given as ESI (Fig. S2).† The
average diameters were 14.1 ± 2.2 and 15.6 ± 1.3 nm for the Pt
and Pd particles, respectively. Both the Pt and Pd nanoparticles
exhibit limited polydispersity and their sizes fall clearly into the
colloidal particle domain.
All these results clearly indicate that the derivatization of the
Pt and Pd nanoparticles with surface-attached, thiolated b-CD
receptors proceeds smoothly, yielding reasonably monodis-
perse, CD-modified Pt (or Pd) colloidal particles. Unlike
modification with alkanethiols,5 which passivates the particle
surfaces and renders them catalytically inactive, we anticipated
that these CD-covered Pt (and Pd) nanoparticles will exhibit
catalytic activity. Therefore, in order to substantiate this
hypothesis, we selected the hydrogenation of allylamine as a
The modification of metal1 and semiconductor2 nanoparticles
with organic monolayers has become a very fruitful and active
field of research in modern chemistry. An attractive aspect of
recent developments in this area is that the final materials may
exhibit combined properties from their inorganic nuclei and
their organic surfaces. Our group is particularly interested in the
modification of metal nanoparticles (metal clusters or colloids)
with organic monolayers prepared with suitable, synthetic
molecular hosts. We have recently reported several examples
illustrating the use of cyclodextrins (CDs) for the modification
of gold colloidal3 or cluster4 particles. In this report, we
demonstrate the surface attachment of CDs to platinum and
palladium nanoparticles that catalyze the hydrogenation of
allylamine. As opposed to classical Pt and Pd catalytic
materials, these novel nanocomposites are soluble in aqueous
media and can be easily recovered by precipitation with
ethanol.
Ulman and coworkers have recently reported the preparation
of platinum nanoparticles protected with alkanethiol mono-
layers.5 Inspired by this report, we decided to adapt our
published procedure for the preparation of CD-modified gold
clusters4 to the synthesis of Pt and Pd nanoparticles derivatized
with chemisorbed CD hosts. To this end, solutions of PtCl422 or
PdCl422 sodium salts in DMSO–H2O were readily reduced with
BH42 in the presence of per-6-thio-b-cyclodextrin6 (HS-b-CD,
see structure below), leading to the isolation of dark precip-
itates.‡ These materials show FTIR spectra which are very
similar to that exhibited by free HS-b-CD, indicating the
presence of the thiolated CD in the dark solids (ESI, Fig. S1†).
Furthermore, close inspection of these IR spectra reveals that
the weak S–H stretching peak at ca. 2560 cm21, which is clearly
visible in the spectrum of free HS-b-CD, disappears in the
spectrum of the dark precipitates collected from the reduced Pt
and Pd complex solutions. This is consistent with the antici-
pated conversion of the H–S bonds in HS-b-CD to metal–
thiolate bonds as the CD host chemisorbs on the surface of the
metal nanoparticles.3b 1H NMR spectroscopy also provides
experimental evidence that supports the presence of HS-b-CD
in the dark precipitates. However, the proton resonances from
the CD receptors appear as broad peaks, a result of their relative
† Electronic supplementary information (ESI) available: Fig. S1 (FTIR
spectra) and S2 (size distribution histograms). See http://www.rsc.org/
suppdata/cc/b0/b002423f/
Fig. 1 TEM images of (a) CD-modified Pt nanoparticles and (b) CD-
modified Pd nanoparticles.
DOI: 10.1039/b002423f
Chem. Commun., 2000, 1151–1152
This journal is © The Royal Society of Chemistry 2000
1151

View Article Online
Table 1 Percentage conversion from allylamine (1.8 mmol) to propylamine
under 1.0 atm H2(g) at room temperature in D2O solution (2.0 mL)
solution. These novel materials may find applications in ‘green
chemistry’ and, perhaps, lead to ‘selective’ catalytic activity,9
modulated by the molecular recognition properties of the
surface-attached hosts. We are currently working towards this
important goal.
Catalyst
Amount/mg
t/h
Conversion (%)
CD-mod. Pt
CD-mod. Pd
None
CD-mod. Pt
CD-mod. Pd
10
10
—
5
6
6
6
1
1
> 95
100
0
10
30
We are grateful to the National Science Foundation for the
support of this research work (to A. E. K., CHE-9982014). E. R.
thanks the University of Miami for a graduate Maytag
fellowship.
5
Notes and references
‡ Preparation of modified metal colloids: a DMSO–H2O (1+4 v/v) solvent
mixture was used to dissolve the metal complex. A sample of 0.125 mmol
of Na2PtCl4 (or Na2PdCl4) was dissolved in 20 mL of the solvent mixture
and placed in a 100 mL round bottom flask under vigorous stirring. Another
20 mL aliquot of solvent containing 12.5 mg (0.0096 mmol) of HS-b-CD
and 75.5 mg (40 mmol) of NaBH4 was prepared and carefully homogenized.
The latter solution was added through an addition funnel to the metal
complex solution, while maintaining the stirring rate. The reaction mixture
was allowed to run its course overnight. After this, 10–15 mL of absolute
ethanol was added to precipitate the metal particles. The dark precipitate
was collected by centrifugation and washed several times, first with DMSO
to remove free HS-b-CD and then with ethanol to remove the DMSO.
Complete removal of the thiolated cyclodextrin was verified by thin layer
chromatography (TLC). Once this was achieved, the sample was dried
under vacuum at 60 °C during 24 h. The resulting dried powder was
submitted for characterization by spectroscopic analysis or transmission
electron microscopy (TEM) and used in the catalytic experiments.
Catalytic experiments: the CD-modified Pd and Pt colloids prepared in
this work were tested for catalytic activity under identical reaction
conditions. A solution of a measured amount of the catalyst in 2 mL of D2O
was stirred in a 5 mL peak-shaped flask and saturated with hydrogen gas for
10 min. Allylamine (136 mL, 1.8 mmol) was added with a microsyringe and
a 1H NMR spectrum was recorded just prior to the hydrogenation reaction.
The flask was then sealed with a rubber septum under a hydrogen
atmosphere (760 mm Hg) and left for the selected reaction time. At the end
of this time, another 1H NMR spectrum was recorded to monitor the extent
of conversion from allylamine to propylamine.
1 M. J. Hostetler and R. W. Murray, Curr. Opin. Colloid Interface Sci.,
1997, 2, 42; C. R. Martin, Anal. Chem., 1998, 70, A322.
2 M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss and A. P. Alivisatos,
Science, 1998, 281, 2013; W. C. W. Chan and S. Nie, Science, 1998, 281,
2016.
Fig. 2 1H NMR spectra of a 0.9 M solution of allylamine in D2O containing
10 mg of CD-modified Pd colloids before (top) and after (bottom) 6 h of
exposure to an H2(g) atmosphere at 760 mm Hg.
straightforward test reaction. Our experimental results clearly
verified the activity of these nanoparticles as ‘homogeneous’
catalysts8 (Table 1). Under the surveyed reaction conditions,‡
both CD-modified Pt and Pd colloids catalyzed the hydro-
genation of allylamine to propylamine. In a set of experiments,
using 10 mg of each catalyst and 6 h reaction time, the surface
modified Pt and Pd colloids achieved full conversion. In a
second series of experiments, using 5 mg of each catalyst and a
shorter reaction time of 1 h, the CD-modified Pd nanoparticles
were more efficient than the CD-modified Pt nanoparticles. As
an illustrative example, Fig. 2 shows the 1H NMR spectra
recorded before and after 6 h of hydrogenation catalyzed by 10
mg of the CD-modified Pd nanoparticles. We should note here
that both types of CD-modified metal nanoparticles were
soluble in the reaction medium and could be easily recovered at
the end of the reaction by precipitation with ethanol.
3 (a) J. Liu, R. Xu and A. E. Kaifer, Langmuir, 1998, 14, 7337; (b) J. Liu,
S. Mendoza, E. Roma´n, M. J. Lynn, R. Xu and A. E. Kaifer, J. Am. Chem.
Soc., 1999, 121, 4304.
4 J. Liu, W. Ong, E. Roman, M. J. Lynn and A. E. Kaifer, Langmuir, 2000,
16, 3000.
5 C. Yee, M. Scotti, A. Ulman, H. White, R. Rafailovich and J. Sokolov,
Langmuir, 1999, 15, 4314.
6 M. T. Rojas, R. Ko¨niger, J. F. Stoddart and A. E. Kaifer, J. Am. Chem.
Soc., 1995, 117, 336.
7 R. H. Terrill, T. A. Postlethwaite, C.-H. Chen, C.-D. Poon, A. Terzis, A.
Chen, J. E. Hutchison, M. R. Clark, G. Wignall, J. D. Londono, R.
Superfine, M. Falvo, C. S. Johnson Jr., E. T. Samulski and R. W. Murray,
J. Am. Chem. Soc., 1995, 117, 12537.
8 The term homogeneous catalyst is usually reserved for molecular species.
We use this term liberally in order to emphasize the soluble character of
these Pt and Pd nanoparticles. These systems lie along the borderline
between homogeneous and heterogeneous catalysts as a natural reflection
of their colloidal character.
In conclusion, we have demonstrated that the surface
modified Pt and Pd colloidal particles reported in this work are
active catalysts for the hydrogenation of allylamine in aqueous
9 For a recent example of novel Pt- and Pd-based catalysts that exhibit
some degree of selectivity, see: M. Zhao and R. M. Crooks, Angew.
Chem., Int. Ed., 1999, 38, 364.
1152
Chem. Commun., 2000, 1151–1152