Insights on bimetallic micellar nanocatalysis for buchwald-hartwig aminations

Article abstract of DOI:10.1021/acscatal.9b02622

A nanocatalyst for micellar Buchwald-Hartwig aminations is developed, thoroughly characterized, and applied on a variety of substrates. The catalyst is stable under ambient conditions for at least six months. The catalyst retained its activity after several cycles, and its structure remained intact as confirmed by NMR spectroscopy. Association of Pd nanoparticles with Cu by a phosphine ligand is revealed by ³¹P NMR spectroscopy, and their linkage with the activated carbon surface is revealed by XAS analysis. Control NMR experiments revealed the binding of the ligand with both Cu and Pd, and all phosphine molecules are under the same environment. In addition to NMR and XAS analysis, the catalyst is characterized by SEM, HRTEM, XPS, and TGA. Reactions are highly reproducible at variable scales. Environmentally benign, proline-based amphiphile PS-750-M is critical for catalytic activity, which is achieved under mild conditions in water as the reaction medium. The inherent sustainability of these conditions coupled with a low E factor achievable through robust recycling of catalyst and reaction medium demonstrates the significant utility of this technology.

Full text of DOI:10.1021/acscatal.9b02622

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Article
Insights on Bimetallic Micellar Nanocatalysis
for Buchwald-Hartwig Aminations
Tharique N. Ansari, Armand Taussat, Adam H Clark, Maarten
Nachtegaal, Scott Plummer, Fabrice Gallou, and Sachin Handa
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02622 • Publication Date (Web): 17 Sep 2019
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ACS Catalysis
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Insights on Bimetallic Micellar Nanocatalysis for Buchwald-
Hartwig Aminations
,
Tharique N. Ansari,† ‡ Armand Taussat,†,‡ Adam H. Clark, ⊥ Maarten Nachtegaal,⊥ Scott Plummer,ǁ
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Fabrice Gallou,# Sachin Handa*†
†Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States
⊥Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen, Switzerland
ǁNovartis Institutes for Biomedical Research, 250 Massachusetts Ave, Cambridge, MA 02139, United States
#Novartis Pharma AG, Basel, 4056, Switzerland
ABSTRACT: A nanocatalyst for micellar Buchwald-Hartwig aminations is developed, thoroughly characterized, and applied on a
variety of substrates. The catalyst is stable under ambient conditions for at least six months. The catalyst retained its activity after
several cycles, and its structure remained intact as confirmed by NMR spectroscopy. Association of Pd nanoparticles with Cu by a
phosphine ligand is revealed by 31P NMR spectroscopy and their linkage with the activated carbon surface is revealed by XAS
analysis. Control NMR experiments revealed the binding of the ligand with both the Cu and Pd, and all phosphine molecules are
under the same environment. In addition to NMR and XAS analysis, the catalyst is characterized by SEM, HRTEM, XPS, and TGA.
Reactions are highly reproducible at variable scales. Environmentally benign, proline-based amphiphile PS-750-M is critical for
catalytic activity, which is achieved under mild conditions in water as the reaction medium. The inherent sustainability of these
conditions coupled with a low E factor achievable through robust recycling of catalyst and reaction medium demonstrates the
significant utility of this technology.
KEYWORDS. micellar catalysis, chemistry in water, cross-couplings, heterogeneous catalysis, E factor
Introduction. Aiming to replace highly toxic organic solvents,
Kobayashi,1 Lipshutz,2 Uozomi,3 Krause,4 and our group5,6 have
recently developed technologies that use water as a recyclable
reaction medium. This aqueous chemistry works by harnessing
the hydrophobic effect either “on water”7,8 or inside
nanomicelles. Excellent work on the use of nanomicelles for C–
H activation has very recently been reported by Ackermann and
coworkers.9 Despite these advances, nano-organometallic
catalysis in water is still at its infancy.10-12 NP-mediated micellar
Suzuki-Miyaura11,12 and Sonogashira13 couplings have been
very well documented. The NP catalysts in these cases are more
efficient and robust than their corresponding organometallic
molecular complexes.14 Despite the high catalytic efficiency,
better recyclability, broader scope, and potential benefit of the
hydrophobic effect in these reactions, the NP-mediated micellar
Buchwald-Hartwig amination reaction15,16 remains challenging.
This challenge may be due to the association of the amine
coupling partner with the NP surface, which interferes in the
desired catalytic cycle and shuts down catalysis. Micelle-
enabled aminations using a molecular complex as the catalyst
have been reported.17,18 These protocols suffer from poor
catalyst recyclability and share a common requirement for
(allyl)palladium (Pd). Recently, our group documented the role
of allyl species in the catalytic cycle when a molecular
organometallic complex was used as a catalyst for convenient
sp2–sp3 couplings of nitroalkanes with aryl bromides.5 The role
of the allyl group was as an ancillary ligand to tune the
electronics for achieving the active catalytic species.19
However, translating the (allyl)Pd molecular complex to a NP
catalyst may cause problematic side reactions with the allyl
group. On the catalytically active NP, allyl fragments may
participate in Heck coupling and polymerization side reactions
due to the higher surface area and change in surface free energy.
Therefore, (allyl)Pd may not be the right choice for the
development of NP catalysts for C–N couplings. The
requirement of (allyl)Pd for micellar aminations poses a
difficulty to design a recyclable nanocatalyst that can work well
in the aqueous micellar medium.
Figure 1. Synergy between Cu-Pd-C-micelle in water for
sustainable C–N couplings.
A nanocatalyst that harnesses the synergy between Pd and
another earth-abundant metal could potentially solve the issues
mentioned above. Such synergy could exist when both the
metals are in close proximity by sharing the same ligand while
immobilized on a heterogeneous surface (Fig. 1). The
heterogeneous surface or solid support must bind with the
surfaces of both metals NPs to hold two metals nearby for
optimal catalytic activity. The resulting catalyst must also have
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an association with the amphiphile to prevent catalyst nanocatalyst that displays a strong synergy between each
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aggregation, enhance activity, and harness the micellar effect
for catalysis in water. With the development of such type of
nanocatalyst, answers of some critical questions in the domain
of micellar catalysis can be obtained, including insight into how
the chemistry works, where the catalyst resides, whether the
transformation happens on water or in the micellar interior,
whether there is any real interaction between the different
metals in the NPs, and whether the NP catalyst activity and
relationship to the inexpensive nano support remains
unchanged once subjected to the micellar media.
component of the catalyst and with micelles of our engineered
amphiphile PS-750-M. Optimization study revealed the
dependence of nanocatalyst activity on the presence of Pd, Cu,
cBRIDP as a ligand, PS-750-M as an amphiphile, and activated
carbon as support (Table 1, also see Supporting Information,
Tables S1-S3, S7). Admixing of 1 mol % Pd(OAc)2, 2 mol %
Cu(CH3CN)4PF6, and 3 mol % cBRIDP in THF, followed by
addition of 1.0 equiv. activated carbon particles and subsequent
addition of an aqueous solution of PS-750-M spontaneously
affords the NP catalyst, which can be used as such or isolated
as solid material after removal of volatiles in vacuo. The
isolated catalyst is highly stable for at least six months and
retains the catalytic activity (see SI, page S10). Although the
Cu-free NP catalyst was moderately active, no catalytic activity
was observed in the absence of Pd in the NP catalyst (entries 2,
3). Poor catalytic activity was observed in the absence of
amphiphile PS-750-M (entry 20). Conversion in neat water was
only 20%, which is indicative of the importance of the
amphiphile in catalytic activity. Replacing Cu from the standard
nanocatalyst with other earth-abundant metals, such as Fe, Ni,
and Co afford catalytically inactive NP (entries 4–6). Chitin,
chitosan, and Fe2O3 were ineffective as supports, presumably
due to the incompatible electronic environment (entries 11–13).
Notably, Cu(CH3CN)4PF6 was the most efficient Cu source
(entries 14, 15). KOH was found to be the optimal base (entries
16–19). Optimal stoichiometry of aryl halide and amine
coupling partners was 1:1.2 (see SI, Table S9). Reaction in neat
water and THF was not as efficient as in aqueous PS-750-M
(entries 20, 21).
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Table 1. Optimization study aiming to develop the desired
supported NP catalysta
entry deviations from standard conditions
3 (%)b
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nonec
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no Pd
no Cu
traces
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5 mol % FeCl3 instead of Cu
Ni(OAc)2 instead Cu
CoCl2 instead Cu
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Table 2. Substrate scopea
traces
traces
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95
0.5 mol % Pd(OAc)2 instead 1 mol %
t-BuXPhos instead cBRIDP
XPhos instead cBRIDP
SPhos instead cBRIDP
chitin instead carbon
chitosan instead carbon
Fe2O3 instead carbon
Cu(OTf)2 instead Cu(CH3CN)4PF6
CuI instead Cu(CH3CN)4PF6
K3PO4 instead KOH
Et3N instead KOH
i-PrNEt2 instead KOH
3.0 equiv KOHd
water instead PS-750-M, 3.0 equiv KOHd 20
THF instead PS-750-M, 3.0 equiv KOHd
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aAll reactions were performed on 0.25 mmol scale; bGCMS yields;
cstandard conditions: [0.25 mmol 1, 0.3 mmol 2, 1 mol %
Pd(OAc)2, 2 mol % Cu(CH3CN)4PF6, 3 mol % cBRIDP, 1 equiv.
carbon, 4 equiv. KOH, 0.5 mL 3 wt % PS-750-M in H2O, 60 oC, 12
d
h; 3.0 equiv KOH instead 4.0 equiv. For details, see Supporting
Information, Section 2.
Results and Discussion. Aiming at the development of highly
recyclable supported nanocatalysts that address many questions
related to micellar catalysis and are general, convenient to
synthesize, suitable for mild conditions, recyclable, and use
water as the reaction medium, we report a supported Cu-Pd
aConditions: ArBr (0.3 mmol), aryl amine (0.36 mmol), nanocatalyst
(9 mg, 1 mol % based on Pd), KOH (0.9 mmol, 3.0 equiv), 3 wt % PS-
750-M in H2O (0.6 mL), 60 oC. All yields are isolated.
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After obtaining the desired nanocatalyst and establishing the Figure 2A, the catalyst has a clear association with the long
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optimal reaction conditions, substrate scope was then explored
to find out whether the nanocatalyst is generally applicable on
a variety of substrates or not. Many representative cases
afforded good-to-excellent yields (Table 2). Both aryl amines
and alkyl amines were well tolerated. Electron-deficient (5, 6,
14, 15, 19, 22, 24) as well as rich (7, 8, 10, 11-12, 19, 23) aryl
bromides displayed excellent reactivity. Amine coupling
partners where reactive nitrogen is either inside the aromatic
ring (7, 14, 15, 18, 19, 22, 23, 25) or a substituent on the ring
(3, 5, 8-10, 12, 13, 16, 17, 20) displayed good-to-excellent
reactivity. A range of aromatic and heteroaromatic arrays in
either coupling partner, containing both electron-rich as well as
electron-deficient rings were well tolerated. Functionalities
such as benzyl (17, 25), Boc (16), chloro (9, 14), fluoro (24),
nitrile (15), and trifluoromethyl (5, 6, 17, 22, 24) can be found
among these examples. Transformations were clean in the cases
where the resulting product’s structure is ligand-like (18, 19);
there was no issue with these products, not strongly binding
with the surface of the nanocatalyst to adversely affect the
catalysis. Notably, with this technology, potential
photocatalyst-like molecules (22, 23) can be conveniently
obtained.20
chains of amphiphile PS-750-M, which caused a nest-like
morphology of the material. HRTEM of the finely-ground
material enabled detection of metal NPs that were 2.5 nm in
diameter on average (Fig. 2B-D). XPS analysis revealed that the
material is composed of significant amounts of carbon (82%),
oxygen (16.6%), phosphorus (0.6%), Cu (0.5%), and Pd
(0.15%) (see SI, Table S17). The high levels of oxygen are
associated with residual THF and amphiphile integrated within
these clusters; the C–O appears as a shoulder in the C1s
spectrum (284.5 eV; see SI, Fig. S13.a). Remarkably, only 0.5%
Cu (Cu(I) and Cu(II), 2p3/2 peak at 932.8 eV and 935.8,
respectively; see SI, Fig. S13b) and 0.15% Pd (Pd 3d5/2, 3d3/2,
337.2 and 334.7 eV; Fig. 2E, also see SI, Fig. S13e ) were found
on the surface of the nanocatalyst, which was obtained from 2
mol % Cu(II) and ca. 1 mol % Pd(OAc)2; this translates into
only 110 ppm Pd and 340 ppm Cu present on the surface based
on total weight. The oxidation states of Cu as +1 and +2 while
Pd as in zero and +2 states were detected. Most likely, under
catalytic reaction conditions, the +2 states of Cu and Pd are
reduced to +1 and zero, respectively. TGA analysis of the
nanocatalyst revealed that the catalyst is thermally stable up to
200 oC.
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Next, isolated nanoparticle catalyst was analyzed by SEM,
TEM, XPS, and TGA analysis (Fig. 2 A–F). As illustrated in
Figure 2. Catalyst characterization. (A) SEM analysis; (B, C) HRTEM analysis; (D) particle size distribution; (E) XPS analysis; (F)
TGA analysis of nanocatalyst.
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phosphorus. Whether this type of association stays in the
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presence of aryl amine or not was still questionable. Therefore,
control 31P NMR experiments of the nanocatalyst were
conducted in the presence of stoichiometric as well as excess p-
toluidine. A clear association of p-toluidine with the catalyst
was observed when stochiometric p-toluidine (1:1 with respect
to catalyst) was added to the catalyst. This association is likely
to facilitate the transmetallation step of the catalytic cycle. As
illustrated in Figure 4, the 31P NMR signal of a catalyst shifted
from 69.6 to 59.1. This upfield shift was due to the binding of
p-toluidine with the catalyst. Upon addition of excess of p-
toluidine to the catalyst, no significant change in the NMR
suggested the stability of a catalyst even in the presence of
excess amine.
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Cu K-edge and Pd K-edge XAS analyses were performed to
further confirm the Cu-Pd interactions, the phosphine ligand,
and carbon support. The results of the fitting of the Cu and Pd
K-edge XAS spectra are summarized in Table 3.
Table 3. Best fitting parameters for the Cu K edge and Pd
K edge EXAFS spectra
Scattering
path
Figure 3. 31P NMR suggesting the association of phosphine with
nanoparticle.
σ2(Å2)
Edge
Cu K
N
R(Å)
Cu-C/O/N
Cu-P
3.8
0.4
0.7
0.3
4.4
2.00
2.31
1.99
2.31
2.75
0.004
0.005
0.007
0.006
0.006
NMR experiments were performed to confirm the association
of the phosphine ligand with both metals (Figure 3). Pd-bound
cBRIDP displayed a strong deshielding, and its 31P signal was
detected at 102.8 ppm. Cu-bound cBRIDP was found at 49.5
ppm in 31P NMR. A sample containing Pd, Cu, and cBRIDP in
1:2:3 ratio displayed two signals at 102.8 and 49.5 were
indicative of ligation of both the Pd and Cu by cBRIDP. Mixing
the activated carbon powder with the analyte containing Pd, Cu,
and cBRIDP under the conditions where the nanocatalyst was
formed, provided a new 31P signal at 69.6 ppm. The presence of
a new signal was indicative of ligation of a P-atom to both the
Pd and Cu.
Pd-C/O/N
Pd-P
Pd K
Pd-Pd
* Refinement uncertainties are estimated as: N = ± 10%,49 R =
±0.01Å;21 σ2 was held constant due to the high correlation with N.
The value of σ2 for Cu-C/N/O was iteratively refined and is in
close agreement with previously reported Cu-O.22 σ2 for Cu-P was
set from the average refined value reported for various
complexes.22 The value of Pd-C/N/O was iteratively refined and
is in close agreement with that of reported Pd-O.23,24 σ2 for Pd-P
was set from the average reported for various complexes.25,26
In the Cu K edge X-ray absorption near edge structure
(XANES) spectra (Fig. 5A), the presence of a pre-edge peak at
approximately 8982 eV suggested the presence of copper as
Cu(I) with potentially a degree of Cu(II). The shifted nature of
Cu(I) compared to a bulk Cu2O has previously been noted in
Cu-zeolite materials with a peak position at approximately 8982
eV.27 These oxidation states were similar to that detected by
XPS analysis (see SI, Fig. S13). Notably, due to the absence of
suitable references for this system, linear combination fitting
was not possible to determine the exact Cu(I):Cu(II) ratio.
EXAFS analysis revealed Cu-C and Cu-P interactions, the
corresponding fitting to the |χ(R)| and the R(χ(R)) are illustrated
in Figure 5B. The coordination sphere around Cu was found to
mainly consist of C/O/N with a refined coordination number of
3.8. A minor component of P was also found a low, i.e., 0.4.
The coordination number suggested that not all Cu atoms bind
to the phosphine ligand in a solid phase. The Cu-P contribution
Figure 4. 31P NMR predicting the association of amine with
nanoparticle.
From the NMR study illustrated in Figure 3, it was clear that
cBRIDP ligand binds with both the Cu and Pd via a lone pair of
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to the fitting was considered significant from inspection of the associated to a light scattering near neighbour around the Pd
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Q space (Fig. 5C). The absence of any Cu-Cu contributions was
indicative of the nature of Cu as isolated ions. Furthermore, no
significant direct interaction of Pd and Cu was detected from
the Cu K edge EXAFS analysis.
absorber (Fig. 5E). The large contribution between 2-3.5 Å was
fitted with a Pd-Pd coordination number of 4.4 suggests the Pd
as small nanoparticles. EXAFS analysis also revealed the Pd-
C/O/N and Pd-P interactions. Although the refined coordination
numbers were low, 0.7 and 0.3 respectively, their contribution
was significant by inspection of the Q-space (Fig. 5F). The
presence of a Pd-P interaction was also confirmed by NMR
spectroscopy. No significant direct interaction of Pd and Cu was
detected from the Pd K edge EXAFS analysis. Based and these
results and NMR analysis, Cu and Pd may share the same
ligand.
Pd K edge XANES suggested a mixture of oxidation states (Fig.
5D). A comparison between PdO and metallic Pd. The sample
suggested a partial metallic structure and a small oxidized
second component visible from the small shift in the Pd K edge
position and increase in the white line intensity compared to
metallic Pd. The FT of the EXAFS region was consistent with
this interpretation with a small contribution between 1-2 Å
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Figure 5. XAS analysis of nanocatalyst: (A) Cu XANES; (B) Cu EXAFS fitting to the FT EXAFS spectra; (C) Cu EXAFS fitting; (D) Pd
XANES; (E) Pd EXAFS fitting to the FT EXAFS spectra; (F) Pd EXAFS fitting.
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with reaction medium, and 3, 9, and 13 were obtained in
excellent yields in each cycle.
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Recycling of the nanocatalyst was also performed on gram-
scale reactions (Scheme 2A). A coupling between 26 and 27
affords clean adduct 6. Separation of 6 from the reaction
mixture leaves the nanocatalyst in the aqueous phase. The
recycled catalyst was as active as the fresh catalyst, as
evidenced by a coupling between 1 and 2 mediated by the
recycled catalyst. Notably, after the first reaction, the recycled
catalyst was analyzed by 31P NMR spectroscopy before
applying to the next reaction. The chemical shift of the
recovered catalyst was exactly the same as for the fresh catalyst
(Scheme 2B). No additional phosphorus signal was detected,
which suggests no disintegration of catalyst. In each recycle,
only coupling partners and KOH were added. In-flask recycling
of catalyst and reaction medium was also performed to access
the recyclability at small scale reactions; the catalyst displayed
high recyclability as illustrated in Scheme 2C. However,
reaction yield dropped in the 3rd and 4th recycles, which may be
due to the loss of catalyst during work-up. Addition of extra
copper in the recycled catalyst did not improve the efficiency of
recycled catalysts (see SI, Section 10). A low E factor of 5.6,
use of environmentally benign proline-based amphiphile PS-
750-M in water as reaction medium, and high catalyst
recyclability demonstrate the greenness of this technology.
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Scheme 1. Comparative recycle study: molecular catalyst versus
nanocatalyst.
Another aim behind the development of this nanotechnology
was to attain high in-flask recyclability of catalyst as well as
reaction medium, which has not been viable with molecular
catalysts. Therefore,
a comparative recycle study was
performed under micellar conditions (Scheme 1). Notably, only
a fraction of catalyst was recycled when the molecular catalyst
was employed in the reactions as illustrated in Scheme 1. On
the other hand, the nanocatalyst was highly recyclable along
Scheme 2. Recycle study, E factor, and leaching study.
To rule out the role of leached Pd or Cu in catalysis, an aqueous
solution of PS-750-M containing base and the catalyst was
stirred under reaction conditions. The aqueous solution was
filtered and subjected to the catalytic reaction between 26 and
27. However, no desired product 6 was detected, which was
indicative of no leaching of metal (Scheme 2D). Neither
dehalogenation nor homocoupling side products were detected.
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It was still questionable whether the reaction rates with the indicative of accommodation of both the coupling partners and
recycle catalyst were the same as with fresh.28 With micellar
catalysis and the use of heterogenous catalyst, it is nearly
impossible to accurately study the reaction kinetics, especially
due to the quasi-homogeneous nature of the reaction mixture
and the presence of a wide range of different sized micelles.
However, we attempted to obtain the reaction kinetics by
following a plan as described in the Scheme S10. As illustrated
in Figure 6, reaction rate slightly decreased in each cycle, which
may be either due to slight deactivation of catalyst or loss of
catalyst during work-up in each cycle.
the catalyst within the micelle at elevated temperature.
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Figure 7. Dynamic light scattering (DLS) experiments.
The activity of the nanocatalyst was also accessed over Suzuki
and Sonogashira couplings (Scheme 3). The nanocatalyst
displayed excellent activity for both coupling types.
Sonogashira coupling products from electron-rich (28, 29) and
deficient (30) aryl halides were obtained in excellent yields.
Activity for Suzuki couplings was accessed over challenging
substrates, and excellent reactivity was observed with good
isolated yields of 31 and 32. In these reactions, the nanocatalyst
was also recyclable (see SI, Tables S13, S14).
Figure 6. Valid assessment of catalyst stability: rate constant with
recycled catalyst.
Control experiments were also conducted to verify the role of
micelles. PS-750-M likely serves to keep the NPs intact and to
accommodate the nanocatalyst and substrate in the optimal
hydrophobic micellar interior. Notably, micelles of our PS-750-
M amphiphile are designed to mimic DMF, DMSO, and 1,4-
dioxane. To further confirm the accommodation of substrate in
the micellar core, dynamic light scattering (DLS) experiments
were conducted at room temperature (Figure 7). The average
diameter was increased to 535 nm when nanocatalyst was
dispersed in the aqueous solution of PS-750-M, indicating the
binding of the nanocatalyst with either interface or inner core of
the micelles. On the other hand, the increase in diameter was
more significant when bromonaphthalene was dissolved, i.e.,
683 nm, which clearly indicates its accommodation inside the
micellar core. The increase in size was larger in the case where
bromonaphthalene and nanocatalyst were dissolved in
surfactant solution, i.e., 1220 nm, clearly indicating the higher
solubility of the coupling partner inside the micellar core and
nanocatalyst at the interface. Thus, during the exchange
process, the delivery of nanoparticle catalysts to the
nanomicelles results in effective catalysis via the ‘nano-to-
nano’ effect.
Similar trends in the DLS study was observed when
experiments were carried at 60 oC (see SI, Fig. S4). The particle
size was generally larger at the elevated temperature, which
may be due to the fast exchange process and merging of smaller
particles. The average diameter of micelle increased to 1839 nm
upon dissolution of 1-bromonaphthalene. The particle size
further increased to 2466 nm upon dissolution of catalyst and
3628 nm upon dissolution of both the catalyst and 1-
bromonaphtalene. This increase in the micelle size was
Scheme 3. Broader application of nanocatalyst.
Since high recyclability of the catalyst had been demonstrated,
the probability of carrying over the metal content to product was
expected to be minimal. This assumption was verified by ICP-
MS analysis of products for trace amounts of Pd and Cu
(Scheme 3C, also see SI, page S32) in crude product 18.
Virtually no Pd and Cu were detected in the product, which
demonstrates the sustainability this nanotechnology.
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Conclusions. In conclusion, an aqueous technology has been product was further purified by flash chromatography over
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developed by harnessing the synergy between Pd and Cu on a
suitable donor surface along with the hydrophobic effect
exerted by the micelles of PS-750-M. The catalyst is recyclable,
enables Buchwald-Hartwig aminations catalyzed by
nanoparticles associated with the micelles of proline-based
amphiphile. A broad substrate scope and diverse recycle study
proved the effectiveness of the technology. Phosphine is bound
with both the Cu and Pd, presumably bridged with both metals
as suggested by 31P NMR and XAS analysis. Coordination of
the carbon support with both Cu and Pd is responsible for high
recyclability and optimal catalytic activity. Catalyst stability
over an extended period is another excellent feature of this
technology for its utility by broad users.
silica gel using EtOAc and hexanes as eluent.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Supporting Materials include materials and methods, optimization
details, figures, tables, analytical data, and references (file type, i.e.,
PDF).
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AUTHOR INFORMATION
Corresponding Author
*sachin.handa@louisville.edu
Experimental Method.
Author Contributions
XAS Analysis. Time-resolved XAS measurements at the Cu K
edge (8979 eV) and Pd K edge (24350 eV) were performed at
the Super XAS beamline of the SLS using the QEXAFS setup
using a channel-cut Si(111) crystal monochromator.29 The
polychromatic X-ray beam from the 2.9 Tesla bending magnet
was collimated by a Si-coated collimating mirror (for Cu) or a
Pt-coated mirror (for Pd) at 2.5 mrad. Focusing was achieved
by a toroidal mirror located after the monochromator yielding a
beam size of 1mm by 0.2 mm at the sample position. A Pt
coating of the toroidal mirror Pt was used for Pd K-edge
measurements and a Rh coating for the Cu K-edge.
Transmission geometry XAS spectra were collected using 15
cm ion chambers filled with 2 bar N for the Cu K edge and 2
bar N, 1 bar Ar at the Pd K edge. The data collection was
performed with 1Hz repetition rate and averaged over a 10-
minute data acquisition period. Energy calibration was
performed by consideration of the maximum derivative of Cu
and Pd metal foils. The spectra were processed using
Butterworth filtering for high frequency noise reduction and
interpolation onto a constant energy grid by application of radial
basis functions using an in-house developed software. Analysis
of the EXAFS was undertaken using the Demeter software
package.30
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript. ‡These authors contributed equally.
ACKNOWLEDGMENT
Financial support provided by the ORAU and Novartis Institutes
for Biomedical Research is warmly acknowledged.
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