Recycling Pd colloidal catalysts using polymeric phosphine ligands and polyethylene as a solvent

Article abstract of DOI:10.1039/c3gc36932c

Polyisobutylene (PIB)-bound azo dyes were prepared from aryl amine terminated polyisobutylene oligomers and used to form palladacycle precatalysts that can be used for catalytic carbon-carbon cross coupling reactions. The catalysts so formed were recyclable using thermomorphic heptane-DMF solutions that are monophasic at 80 °C and biphasic at room temperature. Under these conditions, the Pd catalyst can be recycled but some Pd leaches into the product solution. Using a low melting polyethylene oligomer as a solvent in place of the volatile heptane solvent reduces this leaching by roughly an order of magnitude. Further modification that involves using a second polyisobutylene (PIB)-bound phosphine ligand both increases the activity of the colloidal Pd catalyst and decreases the total Pd leaching by almost two orders of magnitude with 99.88% of the Pd being recovered. In this case, the Pd content in the solution of the product was ca. 0.3 ppm. These two modifications together lead to a much more sustainable strategy for the use of Pd colloidal catalysts in catalytic cross coupling chemistry.

Full text of DOI:10.1039/c3gc36932c

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Recycling Pd colloidal catalysts using polymeric
phosphine ligands and polyethylene as a solvent†
Cite this: DOI: 10.1039/c3gc36932c
Nilusha Priyadarshani, Jakkrit Suriboot and David E. Bergbreiter*
Polyisobutylene (PIB)-bound azo dyes were prepared from aryl amine terminated polyisobutylene oligo-
mers and used to form palladacycle precatalysts that can be used for catalytic carbon–carbon cross coup-
ling reactions. The catalysts so formed were recyclable using thermomorphic heptane–DMF solutions that
are monophasic at 80 °C and biphasic at room temperature. Under these conditions, the Pd catalyst can
be recycled but some Pd leaches into the product solution. Using a low melting polyethylene oligomer as
a solvent in place of the volatile heptane solvent reduces this leaching by roughly an order of magnitude.
Further modification that involves using a second polyisobutylene (PIB)-bound phosphine ligand both
increases the activity of the colloidal Pd catalyst and decreases the total Pd leaching by almost two orders
of magnitude with 99.88% of the Pd being recovered. In this case, the Pd content in the solution of the
product was ca. 0.3 ppm. These two modifications together lead to a much more sustainable strategy for
the use of Pd colloidal catalysts in catalytic cross coupling chemistry.
Received 2nd December 2012,
Accepted 22nd March 2013
DOI: 10.1039/c3gc36932c
www.rsc.org/greenchem
show that at least part of the solvent that is necessary for sepa-
ration and recovery of these colloidal catalysts can be replaced
Introduction
Carbon–carbon cross coupling reactions are among the most by a low melting polyethylene wax. Using this polyethylene
widely studied metal-catalyzed chemical transformations. The solvent and added ligands together leads to very efficient recov-
various cross-coupling processes involving Mizoroki–Heck, ery of 99.88% of the Pd in a solid wax phase. This solid dis-
Suzuki, Miyaura and Sonogashira reactions are catalyzed both persion of the Pd can then be reused in subsequent catalytic
by molecular catalysts that contain phosphine and N-hetero- reaction cycles. These two modifications together lead to a
cyclic carbene ligands and by colloidal palladium catalysts that much more sustainable strategy for the use of Pd in these Pd
1
–8
are formed in situ.
synthesis in academic and industrial laboratories and its
importance was recognized by the 2010 Nobel Prize in chem- phine and N-heterocyclic carbenes (NHC) are highly active and
This chemistry has been widely used in colloidal catalyzed cross coupling reactions.
Homogeneous molecular complexes of Pd ligated by phos-
4
,7–15
istry to professors Heck, Negishi and Suzuki.
useful catalysts for Pd-catalyzed cross-coupling chemistry.
The work described in this paper explores alternative more However, it has become increasingly apparent that Pd colloids
sustainable approaches to Pd-catalyzed cross coupling catalysis generated in a designed fashion or inadvertently through
that proceed through Pd colloids. Two issues are addressed. decomposition or in situ reduction of a Pd(II) species are com-
First, we have prepared Pd(II) complexes of colorimetric azo parably efficient as catalysts for cross coupling chemistry. The
dyes attached to polyisobutylene (PIB) that serve as precatalysts many examples that use what are variously described as col-
that in turn form Pd colloids in situ that act as catalysts for loidal or nanoparticle Pd species show the increasing impor-
5
,7,16–22
cross coupling chemistry. We show here that the activity and tance of these types of catalysts.
Thus, some of the
recoverability of Pd in such Pd catalyzed processes can be attention previously devoted to recycling of homogeneous
influenced not just by the presence of a polymer support, but phosphine and NHC-ligated catalysts has now turned to re-
2
3–27
also by the presence of other soluble polymeric ligands that cycling of colloidal catalysts.
Diverse strategies where cata-
normally are used with molecular Pd complexes. Second, we lyst recycling and catalyst product separation are effected with
2
8–30
dendrimer encapsulated Pd catalysts,
Pd catalysts on mag-
3
1–35
netic nanoparticles, Pd nanoparticles in the presence of
conventional or hyperbranched polymers,
3
6–38
Department of Chemistry, Texas A&M University, College Station, TX 77842-3012,
USA. E-mail: Bergbreiter@chem.tamu.edu; Tel: +1 (979) 845-3437
or Pd catalysts
39,40
localized in a separable solid
or liquid phase have all been
Such catalysts are generally designed to achieve
†
Electronic supplementary information (ESI) available: Experimental procedures
4
0–43
described.
for the synthesis of the PIB-azobenzene Pd(II) complex, details of the catalytic
1
13
several goals – the formation of a highly active catalyst, an
efficient catalyst/product separation process, and catalyst
reactions and recycling experiments, and H and C NMR spectra of coupling
products are provided. See DOI: 10.1039/c3gc36932c
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Scheme 1 Synthesis of the PIB-bound azophenol dye 3 (PIB = H–(CH
2
C-
3 2
(CH ) )40–).
Fig. 1 (a) Thermomorphic liquid–liquid schemes for the use of cross-coupling
catalysts using a polar and a nonpolar solvent (e.g. heptane and DMF) with a
heptane soluble catalyst and products that separate into a polar phase after
temperature perturbation generate a biphasic mixture after a reaction; (b) the
use of polyethylene as a cosolvent in a thermomorphic catalyst separation
where the catalyst is isolated as a dispersion in solid polyethylene wax and the
products remain in solution after a hot monophasic reaction step is followed by
phase perturbation and cooling.
Scheme 2 Synthesis of the PIB-azobenzene Pd(II) complex 5.
recyclability with minimal Pd leaching into the product-
containing phase.
commercially available as a mixture of vinyl-terminated oligo-
mers. Diazotization was carried out using isoamyl nitrite in
50
Our earlier work too has included such chemistry. For
example, we along with a group at Georgia Tech showed that
soluble polymer supported pincer S–C–S Pd complexes form
highly active Pd colloidal catalysts for cross coupling chemistry
dichloromethane. The product of this reaction was a PIB-
bound diazonium salt, which was then allowed to react with
an electron-rich arene to form a PIB-bound azo dye. For
example, a diazonium salt formed from a 2,6-dimethylaniline-
terminated polyisobutylene forms an isolable electrophilic di-
azonium salt 2 that readily reacts with an excess of phenol to
4
4–46
that are easily recyclable and recovered.
This chemistry is
similar to our other chemistry involving recyclable and sepa-
rable catalysts or dye-labelled catalysts that uses a biphasic
51
form the azo dye 3. While this reaction requires excess
4
7,48
separation after a monophasic reaction.
This general strat-
phenol, the excess phenol is easily separated from the
heptane-soluble azo dye by a liquid–liquid biphasic extraction
taking advantage of the heptane phase selective solubility of 3.
egy that we have used in catalyst recovery and separation is
shown in Fig. 1a where a thermomorphic mixture of solvents
that is miscible hot and immiscible cold is used to separate a
polymer-bound catalyst and products into two liquid phases of
different polarities and densities. This strategy was recently
expanded to replace the volatile heptane solvent with a narrow
dispersity, low melting polyethylene wax (Polywax) as shown in
This reaction can be carried out with PIB oligomers PIB1000
,
PIB1300, or PIB2300 on a multigram scale. We used PIB2300 to
prepare the ligands and catalysts below.
We expected that the electron-rich aryl group of the PIB-
bound azo dye 3 would react with Pd(II) to form an ortho-
palladated intermediate. However, palladation of 3 disappoint-
4
9
Fig. 1b. The results below couple these separation strategies
with the use of additional polyisobutylene bound phosphine
ligands to separate and recycle the Pd colloidal cross coupling
catalysts derived from PIB-bound azo dye ortho-metallated
Pd(II) complexes.
1
edly led to a mixture of products based on H NMR spec-
troscopy. Fortunately, the butyl ether 4 formed from 3
underwent facile metallation with Pd(PhCN) Cl in chloroform
2
2
at room temperature (Scheme 2) to afford the red chloro-
bridged PIB-bound palladium(II) complex 5 in quantitative
yield (Scheme 2).
The formation of the PIB-palladacycle was followed by UV-
visible and H NMR spectroscopy. Ligand 4 has a λmax of ca.
Results and discussion
1
Our initial objective was to synthesize colorimetric polymer- 348 nm. On palladation the UV-visible spectrum changes and
bound Pd catalysts that could be used in thermomorphic sepa- a new absorbance peak appears at 448 nm. These spectral
rations like those in Fig. 1(a). This was accomplished using changes are comparable to those seen in the formation of
5
2 1
13
polyisobutylene (PIB) bound azophenols like 3 (Scheme 1) as other similar palladium complexes. H and C NMR spectra
the starting material for the synthesis of a colored PIB-bound of 4 (Fig. 2) were also consistent with metallation in that the
Pd(II) precatalyst. The PIB-bound aniline 1 is easily prepared aryl protons of the electron rich aryl group in the butoxyarene
on a multigram scale from polyisobutylene (PIB2300) 1 that is ring changed from a doublet of doublets to a set of three
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Scheme 3 Carbon–carbon cross coupling reactions catalyzed by 5 in a thermo-
morphic heptane–DMF system where catalysts are separated from products and
recycled using a liquid–liquid biphasic separation (cf. Fig. 1a).
1
Fig. 2 H NMR spectrum of the PIB-bound phenyl butyl ether azo dye 4
(above) and the PIB-Pd(II) palladacycle 5.
Table 1 Recycling results using precatalyst 5 in a solvent system containing
heptane and DMF where the catalyst is separated and recycled by thermo-
morphic liquid–liquid separation
a
Aryl halide
Acceptor
Product
Cycles
5
Yield (%)
9
92
1
1
0
5
5
5
90
96
Fig. 3 Visually evident phase selective solubility of the yellow phenyl butyl
ether dye 4 or the red-orange PIB(II) palladacycle 5 in the less dense heptane
phase of a biphasic thermomorphic heptane–DMF system.
16
6
5
94
89
1
7
resonances each integrating to one proton at 7.74, 6.67, and
6
.61 δ with an ABX coupling pattern typical of that of a 2,4-dis-
1
3
ubstituted phenyl ether. C NMR spectroscopic analysis con-
firmed this result. As a result of the palladation, the two aryl
The reactions were carried out with 1 mmol of the aryl halide and
.2 mmol of butyl acrylate or 1.4 mmol of phenyl boronic acid using
CO as a base and 0.01 mmol of 5 in a mixture of 2 mL of
heptane and 2 mL of DMF. The reaction was stirred under N at 100 °C
for 6–12 h. The yields are isolated average yields per cycle. H NMR
spectroscopic analyses of the product solution showed complete
1
C–H signals of the butoxyarene ring were replaced with three 2 mmol of K
2
3
2
new aryl C–H signals and a new peak at 147.2 δ that is consist-
a
1
4
4,52
ent with the formation of an aryl–Pd bond.
A DEPT exper-
iment confirmed that the peak at 147.2 δ was a quaternary conversion of the limiting aryl halide in each cycle. Yields were
obtained for each cycle in one experiment using p-iodoacetophenone
and butyl acrylate. The yields after 12 h for each cycle were 81%, 80%,
carbon and that there were four aryl methine carbons. The
PIB-bound Pd species 5 was also analyzed for Pd. For com-
71%, 78%, and 70%, in cycles 1–5, respectively. These studies used a
pound 5 containing a 2300 Mn PIB the calcd value for Pd is lower catalyst loading (0.1 mol%) and did show that the initial reaction
times with these lower loadings of Pd could be as short as 6 h.
However, in these experiments, the time for complete conversion of
starting material to product increased to 12 h by the fifth cycle. This
3
.92%. The found value by ICP-MS for Pd was 3.74%.
Our initial studies showed that the PIB-bound azo dye 4 as
well as the Pd(II) complex 5 were soluble in a hot equivolume longer reaction time is consistent with the observation of loss of Pd
into product solutions seen by ICP MS.
mixture of DMF and heptane but were phase selectively
soluble in the heptane phase of the biphasic mixture that
formed on cooling based on the absence of color in the polar
DMF phase (Fig. 3). DMF was used in these studies as a polar competent at Mizoroki–Heck and Suzuki catalysis with a
phase because it is a polar solvent that we have used previously variety of substrates of varying reactivity although aryl
as part of a thermomorphic liquid–liquid solvent mixture. Our chlorides are unreactive. In these reactions, the products were
expectation was that this PIB-bound Pd(II) complex 5 could be isolated from the DMF phase. Catalyst recycling was accom-
used as a precatalyst for C–C cross coupling reactions where plished by adding a heptane-saturated solution of DMF con-
the PIB-bound group would recycle the Pd catalyst in a taining fresh substrate to the recovered heptane catalyst-
heptane phase. To explore this, we carried out catalytic reac- containing phase.
tions at 100 °C in a 1 : 1 (vol : vol) mixture of heptane and DMF
The results of a series of reactions where the catalyst was
using 1 mol% of 5 as shown in Scheme 3. As expected, 5 was recycled through 5–6 cycles are summarized in Table 1. The
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results in this table show that the PIB-azobenzene Pd(II) pre- Table 2 Recycling results using precatalyst 5 in a solvent system consisting of
Polywax, THF, and DMF where the catalyst is isolated as a solid Polywax
catalyst 5 has an activity for cross-coupling chemistry that is
dispersion
similar to that of other low molecular weight Pd precatalysts
5
3
reported in the literature.
a
Aryl halide
Acceptor
Product
Cycles
5
Yield (%)
Visual observation of the formation of a colorless DMF
phase on cooling indicated that no significant amount of the
starting complex 5 remained in this phase. However, ICP-MS
analysis showed that ca. 7% of the starting Pd leached into the
polar product phase after the 4th cycle in a Mizoroki–Heck
coupling that formed butyl cinnamate from iodoacetophenone
and n-butylacrylate (entry 1, Table 1). Thus, while 5 is an
effective precatalyst for cross coupling, the leaching of Pd from
solutions of 5 is higher than desired. This leaching led to a
product solution containing 37 ppm Pd. While fractionation
9
90
85
90
1
1
0
7
5
5
The reactions were carried out with 1 mmol of the aryl halide and
1
2
.2 mmol of butyl acrylate or 1.4 mmol of phenyl boronic acid using
mmol of K CO as a base and 0.01 mmol of 5 in a mixture of 2 mL of
2
3
during product isolation could reduce this Pd leaching in the THF and 2 mL of DMF and 1.5 g of Polywax (Mn ∼ 400). The reactions
a
were stirred under N at 90 °C for 12 h. The yields are isolated
2
final product through standard purification procedures or
while a Pd sequestration reagent could be used to reduce the
1
average yields per cycle. H NMR spectroscopic analyses of the product
solution showed complete conversion of the limiting aryl halide in
Pd content in the product to the 1–10 ppm range some reports each cycle.
8
,54
suggest as a guideline for products,
in trying to minimize the total amount of Pd leaching to below
ppm by a simple physical phase separation of a phase that
our interests here were
1
We initially prepared a suspension of 1.5 g of polyethylene
recycled the catalyst from a phase that contained the product – oligomer (Polywax400) in 4 mL of a 1 : 1 mixture of THF–DMF
a result we achieved as discussed below. While we also showed containing substrate and 5. Polywax was used for this purpose
that a lower loading of Pd (e.g. 0.1 mol%) could effect these since NMR studies showed that Polywax could be separated
reactions, we used a higher 1 mol% Pd loading of catalysts in without contaminating the polar solutions of the product,
the remainder of these studies, as this higher Pd loading is avoiding the need for chromatography to remove traces of the
8
often used in industry for catalyst robustness and affords a polymeric solvent. At room temperature, no catalysis occurred
more rigorous platform for testing Pd recovery and the extent and no 5 visually sorbed onto the suspended Polywax powder.
of Pd leaching to the product solution.
However, on heating this mixture to 90 °C a solution formed
To address this problem of leaching as well as to make the and Mizoroki–Heck and Suzuki reactions could be effected
overall catalytic reaction more sustainable we examined the (Table 2). After the reaction was complete, the Polywax could
use of 5 as a precatalyst in a mixture of THF and DMF repla- be separated as a solid on cooling. However, this quantitative
cing the heptane solvent used in Scheme 3 with a commer- separation of Polywax from the product solution led to precipi-
cially available polyethylene oligomer (PEOlig). This Polywax tation of the Polywax without entrapment of the PIB-bound
oligomer is a solid at room temperature. We recently reported species. However, by using a THF–DMF mixture as the polar
that this low melting wax can serve as a solvent replacement solvent, the addition of 1 mL of hot water to the homogeneous
for alkanes as it dissolves on heating with THF as well as with hot solution perturbed by the hot solvent mixture to form a
4
9
a 1 : 1 (vol : vol) mixture of THF and DMF at ca. 80 °C. When liquid–liquid biphasic mixture with a colored low viscosity
these binary or ternary solvent mixtures are perturbed by the Polywax-rich phase and a polar THF–DMF rich phase. Cooling
addition of water while hot, the Polywax phase separates as a this biphasic mixture led to solidification of the Polywax to
low viscosity hydrocarbon liquid phase that PIB-bound species form a colored waxy solid that could then be physically sepa-
quantitatively partition into. This solvent system was used here rated from the colorless THF–DMF solution that contained the
as shown in Fig. 1b for the reactions shown in Scheme 4.
cross coupling product. This waxy catalyst containing Polywax
solid was then added to a fresh THF–DMF solution to carry out
subsequent reactions cycles. A total of 5 cycles of each
Mizoroki–Heck and Suzuki reaction were carried out with iso-
lated product yields of product (Table 2) that were equivalent
to those in Table 1. Again, while visual observation indicated
that there was no red color in the clear polar phase, Pd analy-
sis after a Mizoroki–Heck reaction showed that 1.2% of the Pd
leached into the polar product phase after the 4th cycle. This
lower leaching in comparison to Pd leaching, seen in the reac-
tions in Scheme 3, suggests that the formation of a solid
solvent dispersion of the catalyst lowers the loss of Pd.
Scheme 4 Carbon–carbon cross coupling reactions catalyzed by 5 using a
Polywax, THF, and DMF solvent mixture where the catalyst is recycled as a solid
dispersion in Polywax (cf. Fig. 1b).
Based on the assumption that 5 is generating a Pd(0)
species during these reactions, we examined 5 as a precatalyst
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Scheme 6 Synthesis of the PIB-bound triarylphosphine 21.
Scheme 5 Synthesis of PIB-azobenzene Pd(II) phosphine complex 18 and its
additive ligand. This reaction like that seen with the pre-
formed phosphine complex 18 was successful. Moreover, in
this scheme where the PIB-bound 5 and 21 were used in
concert, recycling of an active catalyst was feasible because 21
use in an allylic amination reaction.
for an allylic amination reaction. However, a heptane–DMF is phase selectively soluble in heptane and does not detectably
5
8
solution of 5 did not form any detectable allylic substitution leach into a polar solvent phase. In these cases, the catalysis
product from cinnamyl acetate and morpholine even after 24 h was carried out in heptane at 80 °C and catalyst recycling was
of stirring at 90 °C. To overcome this unexpected lack of reac- accomplished by cooling to room temperature and extracting
tivity of 5 in allylic substitution, we considered adding phos- the product using acetonitrile. Under these conditions, both
phine ligand since we had previously seen that phosphine the catalyst and the PIB-bound phosphine ligand remain in
ligands can enhance the reactivity of heterogeneous Pd(0) cata- the heptane phase. Additional reaction cycles were then
5
5,56
lysts in a similar chemistry.
Instead of simply adding a effected by adding fresh substrates to this heptane solution for
phosphine ligand to 5 and heating, we first sought to confirm each subsequent reaction cycle. We carried out 8 cycles
that a phosphine ligand would form a complex with the Pd(II) without any decomposition of the phosphine ligand and
center in 5 by treating the PIB-bound chloride bridged dinuc- without any diminution of the product yield. These reactions
lear complex 5 with 2 equiv. of triphenylphosphine (PPh
3
) in show that an oligomeric phosphine can be an effective ligand
dichloromethane at room temperature (Scheme 5). Gratifyingly for the presumed Pd colloidal catalyst formed from another
this afforded a PIB–azophenol Pd(II) phosphine complex 18 oligomer bound species. A further feature of this chemistry is
1
13
31
(
Scheme 5) based on H, C, and P NMR spectroscopic that ICP-MS analysis showed that Pd leaching further dimin-
analysis. Evidence for the formation of a phosphine complex ished by using 5 along with 21. A metal leaching experiment
1
from 5 includes the changes in the H NMR spectrum of 18 on the product phase of the 5th cycle using ICP-MS analysis
wherein the resonance for H3 of 18 became a doublet due to showed 0.45 ppm Pd in the product phase showing that only
3
1
1
P– H coupling. The chemical shift for this proton also 0.17% of the starting Pd was lost in this cycle.
changed from 6.5 to 5.9 δ consistent with phosphine complex
The ability of 21 to lower leaching in the allylic substitution
5
7
formation. When we examined the use of 18 in an allylic reaction described above suggested that 21 could be similarly
amination reaction using cinnamyl acetate and morpholine in useful in cross coupling chemistry. To test this hypothesis, we
heptane–DMF at 80 °C, complete conversion of the starting repeated the Mizoroki–Heck coupling reported in Table 2 that
cinnamyl acetate to a mixture of structurally isomeric allylic involved the reaction of 4-iodoacetophenone or bromobenzene
amines occurred (Scheme 5). However, attempted recycling and carried out this reaction using 1 mol% of 5 along with 2
experiments of 18 failed, giving no conversion to products in equiv. of 21 using the more sustainable THF–DMF–Polywax
the second cycle presumably due to the loss of phosphine to solvent system used in Scheme 4 in place of the heptane used
the DMF phase.
in Scheme 3. This Mizoroki–Heck reaction was carried out
Encouraged by the observation that the presumed phos- through 5 cycles under the conditions used in Table 2. The
phine complex 18 is active in allylic amination in the presence average yield of product in each cycle in this case was 89%,
of added phosphine and by our ability to recycle catalytic essentially equivalent to what was seen in Table 2 with the
species derived from precatalysts like 5, we next explored a same substrates. Catalyst recycling was accomplished by per-
more sustainable way to use colloidal Pd catalysts derived from turbing the hot THF–DMF–Polywax monophasic mixture with
5
in the presence of phosphines. In this work we examined the water as described above. Cooling generated a solid Polywax
use of a PIB-bound phosphine as a ligand to activate 5 for phase that contained a dispersion of the PIB-bound Pd
allylic substitutions. This study used a PIB-bound triarylphos- complex 5 and the PIB-bound phosphine 21 that was physi-
phine (21) which we had previously used for organocatalysis. cally separated from the polar solution of product and then
This phosphine was prepared using the chemistry shown in recycled with a fresh substrate in a subsequent cycle. ICP-MS
5
8
Scheme 6.
analysis for Pd leaching in the 4th cycle showed that the polar
We then carried out an allylic substitution reaction of cin- phase contained 0.31 ppm Pd showing that only 0.12% of the
namyl acetate by morpholine using 5 as a precatalyst in the Pd leached into the product phase. This Pd leaching is two
presence of 2 equiv. of PIB-bound aryl phosphine 21 as an orders of magnitude lower than the Pd leaching in a system
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observed and the Pd catalyst was recyclable through eight
cycles with very low Pd leaching. The latter systems that used
an added PIB-bound triarylphosphine and polyethylene oligo-
mers in place of heptane also showed the least Pd leaching
both in allylic substitutions and in cross coupling chemistry,
suggesting that the use of polymeric solvents coupled with
multiple polymeric ligands could be a generally useful way to
further enhance the sustainability of homogeneous catalysts.
Fig. 4 (a) SEM image of Pd colloidal particles entrapped in Polywax, and (b)
SEM image of Pd colloidal particles entrapped in Polywax solid from a Mizoroki–
Heck reaction that included added PIB-bound phosphine 21.
Acknowledgements
The generous gift of samples of Polywax by Dr Paul K. Hanna
of Baker-Hughes and of Glissopal (PIB) from BASF, assistance
in ICP-MS analyses by Dr Yun-Chin Yang, and support by
the National Science Foundation (CHE-0952134) and the
Robert A. Welch Foundation (Grant A-0639) are gratefully
acknowledged.
without Polywax or without a phosphine ligand. This result
where we have eliminated one of the volatile organic solvents
and shown the effectiveness of polymeric cofactor ligands
suggests the possibility that it should be possible to signifi-
cantly improve the sustainability of other homogeneous cata-
lysts – something we are currently studying.
While the catalyst recycling experiments and metal leaching
data show that we can recover the Pd catalyst in these various
reactions and that high levels of Pd separation are possible
using added ligands and a Polywax matrix as a cosolvent, the
notion that the PIB-bound palladacycle is a precatalyst that
forms colloidal palladium during the reaction was conjecture.
To confirm that Pd colloidal species formed, we carried out
scanning electron microscopy (SEM) experiments on the solid
Polywax dispersion of the recycled catalyst isolated from cycle
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