Highly reactive heterogeneous Heck and hydrogenation catalysts constructed through 'bottom-up' nanoparticle self-assembly

Article abstract of DOI:10.1039/b200334a

Polymer-mediated self-assembly of functionalized Pd and SiO₂ nanoparticles provides highly active hydrogenation and Heck coupling catalysts.

Full text of DOI:10.1039/b200334a

Highly reactive heterogeneous Heck and hydrogenation catalysts
constructed through ‘bottom-up’ nanoparticle self-assembly†
Trent H. Galow, Ulf Drechsler, Jarrod A. Hanson and Vincent M. Rotello*
Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA.
E-mail: rotello@chem.umass.edu
Received (in Columbia, MO, USA) 8th January 2002, Accepted 25th March 2002
First published as an Advance Article on the web 16th April 2002
Polymer-mediated self-assembly of functionalized Pd and
SiO2 nanoparticles provides highly active hydrogenation
and Heck coupling catalysts.
Active catalysts were prepared through calcination of the
nanocomposite materials. Thermal Gravimetric Analyses
(TGA) of 1 and the precalcinated 1+1+3 and 1+1+5 1·2·3 ternary
aggregates show that a slow ramp temperature (20 to 500 °C; 9
h) and with a ceiling temperature of 500 °C (3 h) provided total
removal of organics. Significantly, after calcination, the highly
porous nature of the systems remained intact (Fig. 2b).
The high surface area to volume ratio of noble metal
nanoparticles makes them highly attractive tools for catalysis.
One key challenge in the application of these particles is
agglomeration. While aggregation can be overcome through
surface functionalization,1 this introduces mass transport issues
that limit the catalytic efficiency of the catalytic system.
Heterogeneous noble metal catalysts, on the other hand, are
typically prepared by deposition of metal onto preformed
supports,2 resulting in limited control over metal particle size
and dispersion. The use of directed self-assembly to construct
heterogeneous catalysts in a ‘bottom-up’ fashion presents a
promising alternative, preventing agglomeration while provid-
ing the inherent advantages of heterogeneous catalysts, such as
ease of product separation and catalyst recycling.
Mixed Monolayer Protected Clusters (MMPCs)3 are in-
herently versatile ‘building blocks’ for self-organization.
MMPCs bearing recognition moieties in the monolayer can be
assembled into macroscopic aggregates through the use of
suitable mediators.4 In recent studies, we have shown that acid–
base complementarity can be used to assemble Au and SiO2
nanoparticles into binary and ternary nanoparticle–polymer
composites.5 These assembly processes were highly modular
and tailorable, with control of aggregate structure provided by
stoichiometry, composition of polymer and nanoparticle, and
order of addition of components. Using both binary and ternary
assembly strategies, large, highly porous extended super-
structures were provided featuring extensive exposure of the
more expensive noble metal component of the system. We
report here the extension of this electrostatically-mediated
assembly process to the creation of highly reactive, recyclable,
heterogeneous catalysts in which palladium MMPCs are
simultaneously used as building blocks and active catalytic
units.
Fig. 1 Chemical structures of carboxylic acid-derived Pd–MMPC 1,
carboxylic acid-derived silica colloids 2, and amine-functionalized polymer
3.
The required SiO2 building block 2 and polymer 3 (Fig. 1)
were synthesized using established procedures.4 Pd nano-
particle 1 was fabricated by place exchange of 11-mercaptoun-
decanoic acid onto 1 nm octanethiol-covered particles6 (see ESI
for all syntheses and preparations).†
MMPC 1 was dissolved in MeOH to 2.4% w/v, silica
particles 2 and polymer 3 were dissolved in DMF to 12 and 6%
w/v, respectively. Mixing polymer 3 with carboxylic acid–silica
colloids 2 followed by addition of 1 afforded the ternary
systems via acid–base reaction followed by immediate charge
pairing (Scheme 1).7 The order of addition provides control
over the distribution of the metal component; addition of the
palladium colloid to a preformed silica colloid–polymer
aggregate should lead to high concentrations of the metal near
the surface of the final aggregates. Transmission Electron
Microscopy (TEM) images of precalcinated aggregates (Fig.
2a) reveal highly open micron-scale superstructures.
† Electronic supplementary information (ESI) available: experimental
methods. See http://www.rsc.org/suppdata/cc/b2/b200334a/
Scheme 1 Formation of catalysts.
1076
CHEM. COMMUN., 2002, 1076–1077
This journal is © The Royal Society of Chemistry 2002

Table 2 Coupling of bromoarenes with alkenes in the presence of calcinated
1+1+5 1·2·3a
R1
R2
Time/d
% Yield
–NO2
–NO2
–NO2
–NO2
–H
–C6H5
–C6H5
–C6H5
–CO2CH3
–C6H5
1
88b
94b
89bc
96d
30d
25d
1.5
1.5
1
2
2
Fig. 2 Representative TEM images of (a) precalcinated 1+1+1 1·2·3
aggregate; (b) calcinated 1+1+1 1·2·3 catalyst.
–H
–CO6CH3
a 5 mmol bromoarene, 7.5 mmol alkene, 5.5 mmol NBu3, 0.045 mol% Pd
relative to bromoarene, PhMe, sealed tube. b Isolated yield. c Yield using
recycled catalyst. d Yield determined by NMR.
To assess the catalytic activity of these systems, a series of
hydrogenation reactions were carried out. In these studies, the
calcinated 1+1+1, 1+1+3 and 1+1+5 1·2·3 systems (1.6% Pd,
determined by elemental analysis) were employed as catalysts
for the hydrogenation of 9-decen-1-ol under zero order
conditions (excess substrate over catalyst, see Table 1 and Fig.
3). Rapid stirring of the reaction mixtures at different rates
showed no effect on the reaction rate, demonstrating the
absence of a mass transport limit of hydrogen or substrate. The
1+1+1 and 1+1+3 systems exhibited extremely high effi-
ciencies, with turnover frequencies (TOFs) of 10,100 and 9400
h21, respectively, after correction for catalyst loading. These
values are substantially higher than the 7200 h21 found under
the same conditions with commercial 1% Pd/C, a highly
evolved industrially important catalyst. Increasing the polymer
content to the 1+1+5 system resulted in a reduced, albeit still
high TOF of 7600 h21, consistent with the denser structure
observed with this system.5b
with styrene and methyl acrylate. As expected, the nitroarene
coupled most efficiently, requiring only 0.045 mol% of Pd, a
dramatic improvement over their commercial counterparts, Pd/
C and Pd/SiO2. Most importantly, the catalyst required no
activation, no toxic ligand, and could be recycled with only a
small decrease in activity. While this system was less efficient
in catalyzing the coupling of non-activated substrates, it is still
significantly more active than previously reported polymer-
stabilized nanoparticle systems.8b
In summary, we have demonstrated that directed self-
assembly provides highly reactive, recyclable heterogeneous
catalysts for both hydrogenation and carbon–carbon bond
formation reactions. Extension of the method to the creation of
additional catalytic systems is currently being explored and will
be reported in due course.
This research was supported by the National Science
Foundation (CHE-9905492 and MRSEC instrumentation). V.
M. R. acknowledges support from the Alfred P. Sloan
Foundation, Research Corporation, and the Camille and Henry
Dreyfus Foundation. The authors thank Professors Tsapatsis
and Venkataraman for use of equipment, and Professor Kaifer
(Miami) for helpful conversations.
Carbon–carbon bond formation reactions provide a further
test for the utility of our nanocomposite catalysts. Previous
investigations have shown that commercial Pd/C and Pd/SiO2
supports are not effective catalysts in Heck coupling reactions.8
Therefore, we investigated the ability of the 1+1+5 1·2·3 system
to undergo Heck reactions. Table 2 shows the results obtained
for coupling activated and electronically neutral bromoarenes
Table 1 All turnover frequencies of catalysts
Notes and references
Catalyst
TOF/h21a
1 L. N. Lewis, Chem. Rev., 1993, 93, 2693–2730; J. Alvarez, J. Liu, E.
Roman and A. E. Kaifer, Chem. Commun., 2000, 1151–1152; R. M.
Crooks, M. Q. Zhao, L. Sun, V. Chechik and L. K. Yeung, Acc. Chem.
Res., 2001, 34, 181–190; R. S. Underhill and G. J. Liu, Chem. Mat., 2000,
12, 3633–3641.
2 J. H. Clark, Catalysis of Organic Reactions by Supported Inorganic
Reagents, VCH, 1994; J. M. Thomas and W. J. Thomas, Principles and
Practice of Heterogeneous Catalysis, VCH, 1997; H. U. Blaser, A.
Indolese, A. Schnyder, H. Steiner and M. Studer, J. Mol. Catal. A-Chem.,
2001, 173, 3–18; M. L. Toebes, J. A. van Dillen and Y. P. de Jong, J. Mol.
Catal. A: Chem., 2001, 173, 75–98; P. T. Anastas, L. B. Bartlett, M. M.
Kirchhoff and T. C. Williamson, Catal. Today, 2000, 55, 11–22; J. H.
Clark, Pure Appl. Chem., 2001, 73, 103–111.
1+1+1 1·2·3
1+1+3 1·2·3
1+1+5 1·2·3
1% Pd/C
10154 32
9419 337
7622 67
7202 36
a Under above conditions, calculated as molproduct molPd21 h21
.
3 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, J.
Chem. Soc., Chem. Commun., 1994, 801–802.
4 A. K. Boal, F. Ilhan, J. E. DeRouchey, T. Thurn-Albrecht, T. P. Russell
and V. M. Rotello, Nature, 2000, 404, 746–748; C. A. Mirkin, R. L.
Letsinger, R. C. Mucic and J. J. Storhoff, Nature, 1996, 382, 607–609.
5 (a) T. H. Galow, A. K. Boal and V. M. Rotello, Adv. Mater., 2000, 12,
576–579; (b) A. K. Boal, T. H. Galow, F. Ilhan and V. M. Rotello, Adv.
Funct. Mater., 2001, 11, 461–465.
6 R. S. Ingram, M. J. Hostetler and R. W. Murray, J. Am. Chem. Soc., 1997,
119, 9175–9178.
7 Component ratios refer to volume equivalents of stock solutions.
8 (a) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100,
3009–3066; (b) S. Klingelhofer, W. Heitz, A. Greiner, S. Oestreich, S.
Forster and M. Antonietti, J. Am. Chem. Soc., 1997, 119, 10116–10120;
(c) J. Kiviaho, T. Hanaoka, Y. Kubota and Y. Sugi, J. Mol. Catal. A:
Chem., 1995, 101, 25–31; (d) L. Djakovitch and K. Koehler, J. Am.
Chem. Soc., 2001, 123, 5990–5999; A. Biffis, M. Zecca and M. Basato,
J. Mol. Catal. A: Chem., 2001, 173, 249–274.
Fig. 3 Graphs of initial product formation versus time under conditions for
zero order kinetics for substrate (5.6 mmol 9-decen-1-ol, 0.013 mol% Pd
(0.008 for Pd/C), MeOH, 1 atm H2) monitored by GC. Reduced substrate to
98% yield.
CHEM. COMMUN., 2002, 1076–1077
1077