Disperse amphiphilic submicron particles as non-covalent supports for cationic homogeneous catalysts

Article abstract of DOI:10.1002/adsc.200404284

A simple method for the effective immobilization of homogeneous catalysts on polystyrene colloids via non-covalent binding is demonstrated. Stable latices with sufficiently high loading of accessible borate anions are prepared via emulsion polymerization. Incorporation of cationic rhodium complexes, supported via their borate counter-anion is efficient, and these supported homogeneous catalysts maintain constant catalytic activity for C=C hydrogenation during several recycles, with very low metal leaching.

Full text of DOI:10.1002/adsc.200404284

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Disperse Amphiphilic Submicron Particles as Non-Covalent
Supports for Cationic Homogeneous Catalysts
a
a
b
c,
¨
Rafael Sablong, Jarl Ivar van der Vlugt, Ralf Thomann, Stefan Mecking, *
Dieter Vogta,*
a
Schuit Institute of Catalysis, Laboratory of Homogeneous Catalysis, Eindhoven University of Technology, PO Box 513,
5600 MB Eindhoven, The Netherlands
Fax: (þ31)-40-245-5054, e-mail: d.vogt@tue.nl
b
Institute for Macromolecular Chemistry and Freiburg, Materials Research Center, Albert-Ludwigs-University Freiburg,
Stefan-Meier-Str. 31, 79104 Freiburg, Germany
Department of Chemistry, University of Konstanz, Universitꢀtsstr. 10, 8457 Konstanz, Germany
c
Fax: (þ49)-7531-88-5152, e-mail: stefan.mecking@uni-konstanz.de
Received: September 8, 2004; Accepted: January 24, 2005
Abstract: A simple method for the effective immobi-
lization of homogeneous catalysts on polystyrene
colloids via non-covalent binding is demonstrated.
Stable latices with sufficiently high loading of acces-
sible borate anions are prepared via emulsion poly-
merization. Incorporation of cationic rhodium com-
plexes, supported via their borate counter-anion is
efficient, and these supported homogeneous cata-
potential immobilization site during the emulsion poly-
merization. The synthesis and copolymerization behav-
ior in emulsions of styryl-functionalized anions and
ꢀ
¼
PEO macromonomers such as CH2 CHC6H4SO3 and
¼
CH3O-(CH2CH2O)45CH2C6H4CH CH2 (C1-PEO-VB-
45)[7] with monomers like styrene have been described
in detail by others.[8,9]
The air-stable sodium salt NaBS was prepared in a
straightforward two-step method, according to modified
literature procedures (Scheme 1), by generation of the
Grignard reagent (p-styryl)MgBr and subsequent reac-
tion with one equivalent of triphenylborane.[10]
¼
lysts maintain constant catalytic activity for C C hy-
drogenation during several recycles, with very low
metal leaching.
Keywords: catalyst immobilization; counter-anion;
emulsion polymerization; polymerizable borate; re-
cycling
Scheme 1. Synthesis of the sodium salt of the polymerizable
A variety of approaches has been demonstrated for the borate anion.
immobilization of homogeneous catalysts, relying near-
ly exclusively on covalent binding of ligands (such as
phosphines) to a particular support.[1] In contrast, an-
ꢁSurfactant-freeꢂ emulsion polymerizations of styrene
choring of transition metal compounds via non-covalent in the presence of various amounts of NaBS, cross-linker
binding, e.g., by electrostatic interactions of appropri- divinylbenzene (DVB), and macromonomer (C1-PEO-
ately substituted mono- or multidentate ligands has at- VB-45), and with the water-soluble initiator 2,2’-
tracted relatively little attention to date.[2,3] To avoid azobis[2-methyl-N-(2-hydroxyethyl)propionamide][11]
even the synthetic challenges associated with the intro- (VA-086) yielded stable and redispersable latices with
duction of ionic substituents in (phosphine) ligands, we complete monomer conversion. A purification se-
have investigated the concept of non-covalent binding quence of filtration, dialysis with cellulose membranes
of a transition metal complex carrying a cationic charge and centrifugation yielded the desired products as white
on the metal itself. precipitates. Redispersion in water or methanol was
We decided to investigate the immobilization, via the possible by ultrasound sonication.
counter-anion, of the cationic complexes [Rh(diphos-
Dynamic light scattering on the dispersions revealed
phine)(diene)]BF4, well known precursors for active average particle sizes in the range of 90 to 150 nm, which
[4]
¼
C C hydrogenation catalysts, on polystyrene-based also do not change significantly during centrifugation/
latices, obtained by emulsion polymerization.[5,6] For redispersion steps. Particle sizes were confirmed by
this purpose, we introduced the polymerizable triphe- transmission electron microscopy (TEM), which also
nylstyrylborate anion, [p-(BPh3)styreneꢀ, (BS)] as the demonstrated a reasonably narrow particlesize distribu-
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DOI: 10.1002/adsc.200404284
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633

¨
Rafael Sablong et al.
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tion (Figure 1). Various anion loadings were used to es- 9 ppm, similar to that of [Rh(dppp)(cod)]BPh4 (sharp
tablish the optimal conditions for cation exchange and signal at 10.7 ppm). The polymer sample was too inho-
stability of latices. As expected, the incorporation of mogeneous to obtain the same resolution as for the mi-
the borate-substituted comonomer (BS) was a prerequi- cro-crystalline benchmark complex. Stirring the precipi-
site for obtaining colloidal stable lattices in the ꢁsurfac- tated orange Rh-impregnated polymer for 2 h in metha-
tant–freeꢂ emulsion polymerization. When replacing nol under 1.3 bar of H2, followed by filtration, washing
the BS for NaBPh4, only a coagulate was formed. Re- and drying under vacuum yielded a yellow solid. The
markably, stable lattices were also produced in the ab- CP/MAS 31P NMR spectrum showed a broad resonance
sence of additional PEO macromonomer. However, at 24 ppm, attributed to the catalyst precursor
[14]
for the introduction of the cationic Rh complex by cati- [Rh(dppp)(h6-BS)]coag
,
along with a weak signal due
on exchange, the additional steric stabilization provided to traces of unreacted [Rh(dppp)(cod)]BScoag
.
by the polyethylene glycol moieties proved to be crucial
to avoid coagulation.
While the use of latices as such as catalyst supports is
also of interest, in the present work only the supporting
In order to elucidate the metal immobilization quality of the catalyst was performed in emulsion. We chose a-
by this approach, first catalyst recycling studies were car- acetamidocinnamic acid (ACA) as substrate for a first
ried out with a non-cross-linked polymer consisting sole- evaluation of the catalytic performance of the immobi-
ly of the sodium triphenylstyrylborate (NaBS) and styr- lized rhodium precursor [Rh(dppp)(cod)]BScoag in terms
ene in a ratio of 1:20. The reaction between the selected of initial activity and recycling stability (Scheme 2).
catalyst precursor [Rh(dppp)(cod)]BF4 [dppp¼bis(di-
phenylphosphino)propane,
cod¼cyclooctadiene][12]
and the emulsion was performed in methanol or in a
methanol/water mixture.[13] Latices of high borate con-
tents (styrene/NaBS of 20:1) were required to avoid co-
agulation on incorporation of the Rh complex. In a typ-
Scheme 2. Rh-catalyzed hydrogenation of a-acetamidocin-
ical experiment, a methanol solution of 8.9ꢁ10ꢀ5 mol namic acid.
[Rh(dppp)(cod)]BF4/g latex (dry solid) was reacted
with the emulsion (particle size 135 nm). A molar ratio
of 1:4.5 of Rh complex and borate groups of the poly-
The hydrogenation reactions were carried out in a fil-
mer was chosen in order to ensure an excess of tetraphe- tration cell in the repetitive batch mode, in methanol,
nylborate for efficient ion exchange. After impregna- with an upward hydrogen flow, similar to a floating
tion and coagulation of the emulsion by removal of sol- bed reactor. The separation of the catalyst from the
vent, the material was extracted three times with meth- product was achieved by a simple filtration, before the
anol. The combined liquids were analyzed for rhodium cell was recharged with a new batch of substrate.
by inductively coupled plasma (ICP) analysis showing
The average initial turnover frequency, based on the
only 2% of the Rh remaining in solution. Further extrac- initial amount of rhodium added, measured after 15 mi-
tion did not show any Rh leaching within the detection nutes, was about 30 hꢀ1, at which point conversion
limits. In this way 98% of the initial amount of rhodium reached 7%. This activity is about 5 times lower than ob-
had been immobilized, indicating a loading of ca. 25% of
the theoretical latex capacity.
The CP/MAS 31P NMR spectrum of [Rh(dppp)
(cod)]BScoag consisted of a broad signal centered around
Figure 2. Hydrogenation of a-acetamidocinnamic acid
(ACA), catalyzed by [Rh(dppp)(cod)]BScoag: activity as a
Figure 1. TEM micrograph and histogram of a typical poly-
styrene latex with both borate- and PEO-substituted como-
nomers incorporated (S/NaBS/PEO: 120/1/1).
function of catalytic runs. Reaction conditions: Rh¼
0.019 mmol; p(H2)¼1.3 bar; solvent¼MeOH (20 mL); sub-
strate/Rh: 100/1; T¼238C; reaction times¼15 min.
634
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Adv. Synth. Catal. 2005, 347, 633–636

Amphiphilic Submicron Particles
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(20 mL). A ratio of borate-containing latex and Rh complex
of 4.5 was chosen to ensure an excess of borate for efficient ex-
change. The reaction mixture was stirred for 30 min and the vol-
atiles removed under vacuum, affording a yellow/orange solid,
which was dried azeotropically with toluene (twice 10 mL)
and washed with methanol (3ꢁ10 mL). CP/MAS 31P NMR
(109.4 MHz): d¼8.5 ppm (s br, w1/2 ¼965 Hz). The combined
methanolextracts were analyzedfor rhodium byICP-AAS indi-
cating that 98% of the rhodium had been immobilized, corre-
sponding to 25% loading of the borate in the polymer (based
on the initial amount of BS in the copolymerization).
served with non-immobilized [Rh(dppp)(cod)]BPh4
(160 hꢀ1) in a corresponding batch experiment in homo-
geneous solution. Six cycles were performed without sig-
nificant loss of activity as shown in Figure 2. The lower
TOF observed during the first cycle (20 hꢀ1) was proba-
bly due to incomplete activation of the precursor. ICP-
AAS analysis of the combined product filtrates revealed
a total loss of 0.4% of the total amount of rhodium, cor-
responding to <0.07% average loss per cycle. This ini-
tial recycling experiment was repeated twice giving the
same results within a margin of 10%.
A batch experiment in a Schlenk tube showed 94%
conversion of the a-acetamidocinnamic acid within Hydrogenation Reaction
one hour.
The polystyrene-supported catalyst precursor (0.019 mmol of
In summary, we have developed a simple and efficient
method for the immobilization of catalytically active
cationic complexes through non-covalent electrostatic
interactions. The catalyst carriers, based on a polymeriz-
able borate anion, are easily obtained by aqueous emul-
sion polymerization as stable and redispersable poly-
styrene latices. As a first proof of principle in terms of
applicability, we have shown that a rhodium complex
supported on coagulated latex has constant catalytic ac-
tivity in the hydrogenation of a model substrate during
several recycles, with very low metal leaching. Future
work will focus on asymmetric hydrogenation, exploring
the full scope of this approach and on the use of func-
tionalized lattices in continuously operated membrane
reactors.[15]
Rh) was placed in a glass custom-made filtration cell equipped
with a G4 frit, in methanol suspension (20 mL). Hydrogen
(1.3 bar) was bubbled through the suspension for 2 h. The sol-
vent was expelled from the cell and replaced by a solution of
substrate a-acetamidocinnamic acid (1.9 mmol) in methanol.
After 15 minutes, the reaction mixture was filtered through
the frit and the filtrate analyzed by GC. The solid catalyst left
in the cell was re-used for several runs. The filtrates were com-
bined, dried under vacuum, and the residue analyzed for Rh
content by ICP-AAS.
Acknowledgements
R. S. thanks the National Research School Combination – Cat-
alysis (NRSCC) for a fellowship. We thank Dr. Magusin
(TU/e) for CP-MAS NMR measurements. A sample of C1-
PEO-H-45 by Dr F. Reichel (Goldschmidt AG, Essen) as well
as a generous loan of noble metals by Umicore AG & Co. KG
are gratefully acknowledged.
Experimental Section
Typical Latex Preparation
Sodium triphenylstyrylborate (NaBS; 0.36 g, 0.98 mmol) was
dissolved in degassed deionized water (80 mL). The solution
was transferred into a four-neck 250-mL flask equipped with
a mechanical stirrer and heated to 808C, with a stirring rate
of 100 rpm. Styrene (14 mL, 122 mmol) and a solution of
VA086 (0.135 g, 0.5 mmol) in water (14 mL) were then added.
The stirring speed was increased to 300 rpm. After 15 minutes,
the reaction mixture became milky, indicative of the latex for-
mation. The reaction mixture was stirred for 4 hours, during
which complete conversion of the monomers occurred as evi-
denced by analysis of the solids content of periodically with-
drawn samples. The latex was cooled to room temperature
and purified by dialysis using a benzoylated cellulose mem-
brane during 3–4 days and then centrifuged at 15,000 g. The
precipitate was carefully separated from the supernatant and
then redispersed in water or methanol by sonication. This op-
eration was repeated twice (particle size 135 nm).
References and Notes
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[Rh(dppp)(cod)]BScoag
4 mL of an aqueous NaBS emulsion (3.19ꢁ10ꢀ2 M, 1.28ꢁ10ꢀ1
mmol borate, 4.5 equivs.) were added to a solution of
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´
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