Dendrimer-Stabilized Metal Nanoparticles as Efficient Catalysts for Reversible Dehydrogenation/Hydrogenation of N-Heterocycles

Article abstract of DOI:10.1021/jacs.7b10768

Nanoparticles (Pd, Pt, Rh) stabilized by G4OH PAMAM dendrimers and supported in SBA-15 (MNPs/SBA-15 with M = Pd, Pt, Rh) were efficiently used as catalysts in the acceptorless dehydrogenation of tetrahydroquinoline/indoline derivatives in toluene (release of H₂) at 130 °C. These catalysts are air stable, very active, robust, and recyclable during the process. The reverse hydrogenation reaction of quinoline derivatives (H₂ storage) was also optimized and successfully performed in the presence of the same catalysts in toluene at 60 °C under only 1 atm of hydrogen gas. Such catalysts may be essential for the adoption of organic hydrogen-storage materials as an alternative to petroleum-derived fuels. Hot filtration test confirmed that the reaction follows a heterogeneous pathway. Moreover, PdNPs/SBA-15 was an excellent catalyst for the direct arylation at the C-2 position (via C-H activation) of indole in water in the presense of a hypervalent iodine oxidant. Thus, a one-pot dehydrogenation/direct arylation cascade reaction between indoline and an arylated agent was efficaciously performed in water, demonstrating the potential of the system to catalyze tandem heterogeneous/homogeneous processes by choice of the appropriate oxidant/reductant.

Full text of DOI:10.1021/jacs.7b10768

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Article
Dendrimer-Stabilized Metal Nanoparticles as Efficient Catalysts
for Reversible Dehydrogenation/Hydrogenation of N-Heterocycles
Christophe V. Deraedt, Rong Ye, Walter Ralston, F. Dean Toste, and Gabor A. Somorjai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10768 • Publication Date (Web): 16 Nov 2017
Downloaded from http://pubs.acs.org on November 16, 2017
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Dendrimer-Stabilized Metal Nanoparticles as Efficient
Catalysts for Reversible Dehydrogenation/Hydrogenation
of N-Heterocycles
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Christophe Deraedt, ‡,† Rong Ye, ‡,† Walter T. Ralston, ‡,† F. Dean Toste *,
‡,†
Gabor A. Somorjai*,
‡,†
‡
Department of Chemistry, University of California, Berkeley, California 94720, United States.
†
Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United
States
ABSTRACT
Nanoparticles (Pd, Pt, Rh) stabilized by G4OH PAMAM dendrimers and supported in SBAꢀ
15 (MNPs/SBA-15 with M= Pd, Pt, Rh) were efficiently used as catalysts in the acceptorless
dehydrogenation of tetrahydroquinoline/indoline derivatives in toluene (release of H2) at 130
°C. These catalysts are air stable, very active, robust, and recyclable during the process. The
reverse hydrogenation reaction of quinoline derivatives (H2 storage) was also optimized and
successfully performed in presence of the same catalysts in toluene at 60 °C under only 1
atmosphere of hydrogen gas. Such catalysts may be essential for the adoption of organic
hydrogenꢀstorage materials as an alternative to petroleumꢀderived fuels. Hot filtration test
confirmed that the reaction follows a heterogeneous pathway. Moreover, PdNPs/SBA-15 was
an excellent catalyst for the direct arylation of the Cꢀ2 position (via CꢀH activation) indole in
water in the presense of an hypervalent iodine oxidant.
Thus, a oneꢀpot
dehydrogenation/direct arylation cascade reaction between indoline and an arylated agent was
efficaciously performed in water, demonstrating the potential of the system to catalyze
tandem heterogeneous/homogeneous processes by choice of the appropriate
oxidant/reductant.
INTRODUCTION
Catalytic dehydrogenation of alkanes has received significant attention because it converts
lowꢀvalue hydrocarbon feedstocks into valuable alkenes and arenes that are important in
organic synthesis, polymer synthesis, and widely used in many industry processes.1 Nꢀ
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containing compounds has been experimentally and computationally shown to undergo
dehydrogenation more readily than cyclic alkanes, because the nitrogen atom decreases the
endothermicity of the reaction Moreover, acceptorless dehydrogenation of saturated Nꢀ
heterocycles provides one of the most atomꢀefficient processes to produce heterocycles, such
as indole and quinolone, that are found in numerous pharmaceutical and bioactive
compounds. Therefore, tremendous effort has been aimed at identifying and improving
catalytic methods for the dehydrogenation of Nꢀheterocycles. Homogeneous catalysts (Ir,3
Rh,4 Ru,5 and Co6 pincer complexes, airꢀsensitive in most cases), heterogeneous catalysts
(Au, Rh, Pt, Pd, Cu nanoparticles)7 have been applied in the catalytic dehydrogenation
processes; but, these methods may require additional oxidizers (O2, TEMPO), bases,
hydrogen acceptors (ethylene or tꢀbutylethylene (TBE)), and/or required harsh conditions.8 In
addition to the formation of valuable dehydrogenated Nꢀheterocycles, the catalytic
dehydrogenation is also applicable to the field of liquid organic hydrogen storage materials
and commercial fuel cells.9 However, examples of catalysts describing activity for both the
dehydrogenation of Nꢀheterocycles (release of hydrogen H2) and the reverse hydrogenation
(storage of H2) of the dehydrogenated Nꢀheterocycles, are exceedingly rare. In 2009 Fujita et
al. reported a homogeneous Irꢀpincer complex capable of catalyzing the dehydrogenation of
1,2,3,4ꢀtetrahydroquinoline to quinoline in pꢀxylene under reflux for 20 hours. The reverse
reaction was possible in the presence of the same catalyst (4 mol%) at 110 °C for 20h under 1
atm of hydrogen gas.10 The same dehydrogenation/hydrogenation sequence was described by
Kaneda et al. using Cu nanoparticles supported on TiO2 as catalysts. In this case, the
dehydrogenation step was carried out at 150 °C in mesitylene for 16 hours, while the
hydrogenation reaction required 6 hours under 1 atm of hydrogen .11 More recently Jones et
al. described the use of an homogeneous cobalt pincer catalyst in the reverse
dehydrogenation/hydrogenation of 1,2,3,4ꢀtetrahydroquinoline/quinolone and derivatives.6
The dehydrogenation reaction was only efficient in the presence of 10 mol% of catalyst at 150
°C in xylene for 4 days (no reaction was detected at 120°C in toluene). The same catalyst (5
mol%) allowed the hydrogenation of the quinoline derivatives at 120 °C in THF in the
presence of 10 atm of hydrogen. Toyao et al. synthesized different metallic nanoparticles
supported on carbon. They found that only 0.1 mol% of their Pt/C was necessary to catalyze
the dehydrogenation of 2ꢀmethylꢀ1,2,3,4ꢀtetrahydroquinoline; but under these catalyst
loading, the reaction required refluxing xylene for 60h. The reverse reaction was promoted by
the same catalyst, but only at 160 °C with 3 bar of hydrogen.12
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We have developed the synthesis of heterogeneous metallic nanoparticles stabilized by
PolyAMido AMine (PAMAM) dendrimers supported on SBAꢀ15 (MNPs/SBA-15).13 While
the unsupported MNPs have been reported as competent catalysts for CꢀC cross coupling
reactions (Suzuki, Heck, Sonogashira, Stille), hydrogenation of double bonds, selective
hydrogenation of triple bonds, nitrophenol/methyl blue/methyl orange hydrogenation,14 our
MNPs/SBA-15 catalysts were also efficient in promoting other reaction classes,13 such as
cyclization reactions via πꢀbond activation,13aꢀ13d ring opening reaction of strained 3ꢀ
membered rings,13e and aldol reactions of methyl isocyanoacetate.13f In this article, we report
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on
our
studies
of
the
dehydrogenation/hydrogenation
reaction
of
tetrahydroquinoline/quinoline derivatives in presence of MNPs/SBA-15 (M= Pd, Pt, Rh).
PdNPs/SBA-15, PtNPs/SBA-15, and RhNPs/SBA-15 show activities in the dehydrogenation
of these Nꢀheterocycles in toluene (or without solvent) at 130 °C that are higher than their
unsupported MNP homologues, or their metallic salt precursors. These catalysts also operated
under mild conditions (20ꢀ60°C, 1 atm of H2) for the hydrogenation of quinolines derivatives.
Moroever, PdNPs/SBA-15 showed remarkable activity toward the CꢀH arylation at the Cꢀ2
position of indole. The mild conditions allowed for the PdNPs/SBA-15ꢀcatalyzed to direct
conversion indoline to 2ꢀphenylindole through a oneꢀpot cascade involving indoline
dehydrogenation step followed by direct arylation.
RESULT AND DISCUSSION
Synthesis of the catalysts
The catalysts studied in the reverse dehydrogenation/hydrogenation were prepared in two
steps. The first step consisted of synthesizing the homogeneous MNPs (M= Pd, Pt, Rh)
stabilized by the fourth generation G4OH PAMAM dendrimer in water as described by
Crooks.15 This step is composed of two parts (see Scheme 1): complexation of the metallic
ions to the tertiary amines of the dendrimer and subsequent reduction of these metallic ions to
the NPs. Briefly, PdNPs were prepared by addition of an aqueous solution of H2PdCl6 (0.1
mmol in 10 mL of deionized water) to an aqueous solution of PAMAM G4OH (2.5 ꢀmol of
in 10 mL of deionized water) in order to achieve a ratio 40/1 Pd atoms/dendrimer. The
resulting solution was diluted by four fold, which is necessary to avoid aggregation during the
reducing step (no dilution is required for the synthesis of PtNPs or RhNPs). The solution was
degassed with argon for 30 minutes, allowing for complete complexation of Pd(II) by the
tertiary amines of the dendrimer (complexation for the formation of RhNPs and PtNPs
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required one night or three days, respectively). Fresh sodium borohydride NaBH4 (10
equivalents per Pd) dissolved in 4 mL of deionized water at 5 ºC, was added dropwise to the
Pd(II)/PAMAMꢀG4OH solution with rapid stirring, leading to an instantaneous change of
solution color from yellow to dark black. The solution was stirred for an additional 3h prior to
the purification/washing step (i.e. dialysis).
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Scheme 1. Synthesis of MNPs stabilized by PAMAMꢀG4OH dendrimer13a
The final supported catalyst (second step, heterogenization of the homogeneous catalyst) was
obtained by mixing a mesoporous SBAꢀ15 support with the nanoparticle water solution in an
ultrasonic bath for 3h at room temperature, in order to achieve 1.0 wt% Pd loading. The
catalyst solution was centifuged and then dried in an oven at 80 °C for 2ꢀ3 days. The success
of the impregnation was evaluated using ICPꢀOES, and TEM: ICPꢀOES revealed that the
loading was 100% successful and TEM images showed that the MNP size remained
unchanged after loading (1.5 ± 0.3 nm). Using this technique, we also produced supported
MNPs of 1.0 wt% Rh and 2.0 wt% Pt (1.6 ± 0.4 nm, 1.8 ± 0.4 nm respectively), see Figure 1.
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Figure 1. NPs loading. TEM images of PdNPs/SBA-15 (average size = 1.5 ± 0.3 nm) a),
RhNPs/SBA-15 (average size = 1.6 ± 0.4 nm) b), PtNPs/SBA-15 (average size = 1.8 ± 0.4
nm) c).
Dehydrogenation process
2ꢀMethylꢀ1,2,3,4ꢀtetrahydroquinoline 1a was chosen as a model substrate for the investigation
of the catalytic dehydrogenation (eq 1). As the reaction typically requires high temperature
and long time reaction times, 110°C and 15 hours were chosen accordingly (see Table 1).
Under these conditions, approximately 10% of 1a was converted into 2ꢀmethylꢀquinoline 2a
when the reaction was carried out using 0.5 mol% PdNPs/SBA-15 as catalyst without solvent
and or additives (entry 1). No significant enhancement was observed by adding an excess of tꢀ
butylethylene (TBE) as a hydrogen acceptor (entries 2, 3). The use of dichloro iodobenzene
(PhICl2), an oxidizer,13d resulted in a decrease in the conversion to the desired product (entry
4). No trace of 2ꢀmethylꢀquinoline 2a was detected by 1H NMR or by GCꢀMS when SBAꢀ15
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alone was used as catalyst or when no catalyst was used during the reaction. By increasing the
temperature to 130 °C and the time of reaction to 23h, a significant improvement was
observed. A catalyst loading of 1 mol% or 5 mol% was necessary to induce the complete
conversion of 1a into 2a using PdNPs/SBA-15, PtNPs/SBA-15 and RhNPs/SBA-15, at
130°C during 23 hours (entries 8, 9, 10, 11). Only PtNPs/SBA-15 quantitatively catalyzed the
dehydrogenation of 1a into 2a with 1 mol% catalyst loading (entry 11, only 59.9 % and 24%
of conversion were observed with RhNPs/SBA-15 and PdNPs/SBA-15 respectively). No
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trace of partial dehydrogenated product was detected by GCꢀMS or H NMR with any of the
conditions tested.
Table 1. Selective dehydrogenation of 2ꢀmethylꢀ1,2,3,4ꢀtetrahydroquinoline into 2ꢀmethyl
b
c
quinoline.a 0.5 mol % catalysts. 5 mol %. 1 mol%. Conversion (formation of the desired
product) was calculated by 1HꢀNMR.
entry
catalysta
solvent
additive
temperature
110°C
110°C
110°C
110°C
110°C
130°C
130°C
130°C
130°C
130°C
130°C
time
15 h
15 h
15 h
15 h
15 h
23 h
23 h
23 h
23 h
23 h
23 h
conversion
10.3%
9.6%
1
2
PdNPs/SBA-15
PdNPs/SBA-15
PdNPs/SBA-15
PdNPs/SBA-15
PdNPs/SBA-15
SBAꢀ15
TBE (300%)
TBE (600%)
PhICl2
15%
3
5.6%
4
Tolꢀd8
Tolꢀd8
Tolꢀd8
7.8%
5
< 0.2%
< 0.2%
>99.9%
>99.9%
>99.9%
>99.9%
6
7
8
PdNPs/SBA-15b
RhNPs/SBA-15b
PtNPs/SBA-15b
PtNPs/SBA-15c
9
10
11
The dehydrogenation of 2ꢀmethylꢀ1,2,3,4ꢀtetrahydroquinoline 1a was also evaluated using Pt,
Pd, Rh ionic salts (0.5ꢀ5 mol%) as catalysts at 130°C for 23 hours in tolueneꢀd8 or without
solvent; however, low conversions were observed in all cases (Table 2). The low efficiency
was attributed to reduction of the ionic species to the corresponding metallic form during the
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reaction (black metallic particles were observed inside the solution and on the wall of the
vessel).
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a
Table 2. All the reactions were carried out with 1 mmol of reactant, at 130°C, for 23 hours
9
b
without solvent. Conversion to product when carried out in tolueneꢀd8. The conversion
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(formation of desired product) was calculated by 1HꢀNMR.
entry
12
catalyst
RhCl3
amount
0.5 mol %
5 mol %
conversiona (tol-d8)b
13.3% (8.9%)
26.3%
RhCl3
13
Rh(acac)3
K2PtCl4
Pt/C
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
5 mol %
3.9% (0.5%)
6.1% (0.8%)
6.1% (8.1%)
8.4% (3.8%)
15.4% (11%)
7.7% (4%)
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15
16
K2PdCl4
K2PdCl4
PdCl2
17
18
0.5 mol %
19
The scope of the reaction was studied (Table 3). As expected, the three catalysts were also
efficient for dehydrogenation of similar ꢀheterocycles in toluene. 1,2,3,4ꢀ
N
Tetrahydroquinoline 1b, 2ꢀmethylꢀ1,2,3,4ꢀtetrahydroquinoxaline 1c and 1,2,3,4ꢀ
tetrahydroquinoxaline 1d were quantitatively dehydrogenated using PdNPs/SBA-15 and
RhNPs/SBA-15 as catalysts. The dehydrogenation of 1c and 1d in the presence of
PtNPs/SBA-15 reached only 77% and 72% of conversion, respectively, in toluene. However,
performing the reaction under neat conditions allowed for almost complete transformation of
the starting materials in both cases. The lower activity observed in this case is likely a result
of the increase of absorption strength of the reactants (presence of an additional nitrogen
atom) onto the PtNPs/SBA-15 catalyst. The dehydrogenation of cycloalkane is more
endothermic than
Nꢀheterocycles; therefore, as expected, 1,2,3,4ꢀtetrahydronaphthalene 1e
underwent slower dehydrogenation to afford naphthalene 2e in 60% conversion under neat
conditions (only 10% were observed in toluene). The dehydrogenation of indoline derivatives
(1f and 1g) under the optimized conditions with only 2% mol of any of the three catalysts,
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gave quantitative conversion into indole derivatives (2f and 2g) as detected by H NMR and
GCꢀMS. Finally, dehydrogenation of 2,3ꢀdihydrobenzofuran 1h was successful only with
PdNPs/SBA-15.
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Table 3. aAll the reactions were carried out with 0.1 mmol of reactant, 5 mol% catalyst, 1 mL
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b
of tolueneꢀd8 at 130 °C for 23 h. Conversion to product in reaction conducted without
c
1
d
solvent. The yield was calculated by HꢀNMR using of mesitylene as an internal standard.
e
catalyst loading lowered from 5 mol% to 2 mol%. The reaction was also carried out in H2O,
and identical conversion of the reaction was observed.
Reactanta
product
catalyst
Conversion
(neat)b
Yieldc
PdNPs/SBA-15e
RhNPs/SBA-15
PtNPs/SBA-15
PdNPs/SBA-15
>99.9%
>99.9%
>99.9%
>99.9%
98%
98%
98%
92%
2a
1a
>99.9%
>99.9%
>99.9%
>99.9%
91%
93%
99%
99%
RhNPs/SBA-15
PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
2b
1b
2c
1c
77% (95%)
>99.9%
77%
99%
PtNPs/SBA-15
PdNPs/SBA-15
H
N
N
N
>99.9%
99%
RhNPs/SBA-15
2d
N
H
1d
72% (>93%)
10% (60%)
<2% (<2%)
<2% (<2%)
>99.9%
72%
(50%)
ꢀ
PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
1e
1f
2e
2f
ꢀ
96%
97%
96%
>99.9%
>99.9%
>99.9%
>99.9%
96%
94%
1g
2g
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>99.9%
96%
3
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PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
PtNPs/SBA-15b
5% (>99.9%)
0% (0%)
(89%)
ꢀ
ꢀ
1h
2h
0% (0%)
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In order to investigate whether the catalyst leaching occurred during the reaction, the catalyst
was filtered after reaction (dehydrogenation of 1a into 2a) and the supernatant analyzed by
ICPꢀAES. The metal content of the solution was below the limit of detection of the apparatus
(0.1 ppm), consistent with a minimal or no metal leaching under reaction conditions.
Moreover, spent catalysts analyzed by TEM showed roughly no change in particle size (1.7 ±
0.4 nm instead of 1.5 ± 0.3 nm for PdNPs/SBA-15, Figure 2a). Additionally, the
PdNPs/SBA-15 catalyst was used an additional two times without a significant loss in
activity (95% of conversion was detected after 3 runs). PtNPs/SBA-15 and RhNPs/SBA-15
were also analyzed by TEM after reaction, and no change in size for the nanoparticles (1.8 ±
0.3 nm for PtNPs/SBA-15) was observed (Figure 2b and S.I.). Finally, the catalysts were
analyzed before and after reaction using Xꢀray photoelectron spectroscopy (XPS). Analysis of
the XPS data is consistent with the three catalysts becoming reduced (more metallic) at the
end of the reaction, as hydrogen gas is forming from the substrates.
Figure 2. TEM images of PdNPs/SBA-15 (a) and PtNPs/SBA-15 (b) after reaction
The mechanism of the dehydrogenation reaction likely follows that of previous reported
palladiumꢀcatalyzed processes.16 Consistent with these reports, release of hydrogen gas was
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detected with gas burette connected to the reactor. In the case of tetrahydroquinoline
derivatives, two successive dehydrogenation reactions are required; as no intermediate was
1
observed by HꢀNMR and GCꢀMS, it is likely that the second dehydrogenation reaction,
which forms an aromatic molecule, is the faster of the two steps.
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Hydrogenation process
The hydrogenation reaction (i.e. “hydrogen storage” step) of 2ꢀmethyl quinoline 2a into 1a
was investigated in order examine whether the catalyst systems could be applied in a
reversible dehydrogenation/hydrogenation system (eq 3). The hydrogenation reaction can be
challenging to perform under mild conditions; typically the hydrogenation of Nꢀheterocycles
requires between 10 to 70 atm of hydrogen or elevated temperature; relatively few examples
of this transformation operating under low temperatures and only 1 atm of hydrogen have
been disclosed.7f,10,11,17 Gong et al reported that Pd nanoparticles supported on graphitic
carbon nitrides catalyzed the hydrogenation of quinoline derivatives under 1 atm of H2 in
ethanol at 60°C.17a More recently, Bai et al. described that 1.2 nm PtNPs supported on silica
spheres presented the highest reported activity for the hydrogenation of quinoline derivatives.
The hydrogenation occurred at 30°C with 1 atm of hydrogen and 5% mol catalyst loading.17d
As mentioned in the introduction Fujita et al.10a and Kaneda et al.11 described the synthesis of
catalysts (homogeneous and heterogeneous, respectively) capable of catalyzing both the
dehydrogenation and hydrogenation. In these studies, the hydrogenation step was performed
under only 1 atm of hydrogen, but required elevated temperature of 110°Cꢀ150°C.
The catalytic hydrogenation of 2ꢀmethylquinoline was examined in various solvents (see
Table 4) at 60 °C for 24 h under 1 atm of hydrogen (via a balloon). The reaction in toluene,
catalyzed by PtNPs/SBA-15 (entry 24) or PdNPs/SBA-15 (entry 25), provided the optimal
1
results, furnishing complete conversion to 2ꢀmethylꢀ1,2,3,4ꢀquinoline 1a as detected by H
NMR and GCꢀMS). The unsupported PdNPs catalyst was not efficient for the hydrogenation
reaction affording only 16% of conversion to the tetrahydroquinoline (entry 27)
Table 4. Hydrogenation of 2ꢀmethylquinoline. The reactions were carried out with 0.1 mmol
1
of 2ꢀmethylquinoline. The yield was calculated by HꢀNMR using mesitylene as an internal
standard.
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entry
20
catalyst
solvent
yield
87%
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PtNPs/SBA-15
PtNPs/SBA-15
PtNPs/SBA-15
PtNPs/SBA-15
PtNPs/SBA-15
PdNPs/SBA-15
RhNPs/SBA-15
PdNPs solution
CD3OD
D2O
65%
21
98.4%
86%
22
THF
23
Tolꢀd8
Tolꢀd8
Tolꢀd8
H2O
>99.9%
>99.9%
48%
24
25
26
16%
27
Further reaction optimization revealed that 2a to 1a underwent quantitatively hydrogenation
catalyzed by PdNPs/SBA-15 or Pt/SBA-15 after only 6h under reaction conditions of entries
24 and 25. On the other hand, decreasing the catalyst leading to 1 mol%, produced 2a was
92% and 73% conversion at 60°C in 24 hours in the presence of PdNPs/SBA-15 or
PtNPs/SBA-15, respectively; however, the hydrogenation of 2a with 2.5 mol% of
PtNPs/SBA-15 was quantitative at 40°C after 24h and almost quantitative at 20°C (91% of
conversion) for the same time of reaction.
The scope of the hydrogenation reaction was also studied in presence of PdNPs/SBA-15 or
PtNPs/SBA-15 as their activity for the reaction was higher than RhNPs/SBA-15. The
hydrogenation of quinolone derivatives (2a-2d) was quantitative under optimized conditions
with both catalysts. However, only traces of indoline 1f or 2ꢀmethyl indoline 1g were detected
when the hydrogenation was tested on indole 2f or 2ꢀmethyl indole 2g. Notably, low
hydrogen pressure was not sufficient to hydrogenate indole derivatives, even at elevated
temperatures (110°C) where indole was cracked/destroyed.
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5 mol % catalyst
Tol-d8
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2H2
23 h, 130°C
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X
X
N
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N
H
R
R
H2 (1 atm)
X = C, N
2.5 mol%catalyst
Tol-d8
R = H, CH3
6 h, 60 °C
Scheme 2. Reversible system for hydrogen storage/release.
The three catalysts were recovered after the hydrogenation of 2ꢀmethylꢀquinoline 2a, and
analyzed by TEM. No change of particle size was observed (see Figure 3). As before, ICP
analysis of the supernatant (< 0.1 ppm), after filtration of the catalyst, suggested that
significant leaching of the catalysts during the reaction was absent.
Figure 3. TEM images of PdNPs/SBA-15 (average size 1.5 ± 0.6 nm) a), RhNPs/SBA-15
(average size 1.6 ± 0.6 nm) b) and PtNPs/SBA-15 (1.8 ± 0.4 nm) c) after reaction.
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To confirm that the reaction takes place at the interface between the solid catalyst and the
liquid medium, a hot filtration test was performed during hydrogenation process. After one
hour of hydrogenation of 2ꢀmethylquinoline 2a at 60°C in toluene, the catalyst (PdNPs/SBA-
15 or PtNPs/SBA-15) was quickly filtered, and the reaction solution was allowed to stir for
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an additional 5h at 60°C. The conversion of the reaction was analyzed by HꢀNMR after 3h
and 6h of reaction (Figure 4). After filtration of the catalyst, the hydrogenation seems to stop.
Indeed, after 1 hour of reaction with PdNPs/SBA-15, extra 5 more hours of stirring of the
filtrate under H2 after filtration, the conversion of the reaction remains the same as after one
hour of reaction (Figure 4a). The same phenomenon has been observed for the hot filtration
test in the presence of PtNPs/SBA-15 (Figure 4b). The reaction is stopped after filtration,
confirming the heterogeneous role of the catalyst during the reaction. The catalysts were
reused three times without apparent loss of activity. Indeed, only 88% conversion of 2a into
1a was detected by 1H NMR in the presence of PdNPs/SBA-15 after 3 reuses and 6 hours of
reaction, instead of >99.9%. Two more hours were necessary to achieve a quantitative
conversion. This “loss” of activity was attributed to a diffusion problem linked to the higher
concentration of 2a and 1a after the 4th reaction as the catalyst was used 4 times in a row. The
filtration of the catalyst plus the drying under vacuum allows the total recovery of the catalyst.
Figure 4. Kinetics of the reaction and hot filtration test results with PdNPs/SBA-15 a) and
PtNPs/SBA-15 b) (see experimental section).
The product selectivity for the hydrogenation of quinoline derivatives depends on how
quinoline absorbs to the metal surface.18 As only one product was detected by HꢀNMR and
1
GCꢀMS during the hydrogenation, this observed selectivity implies exclusive absorption of
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the quinoline derivatives to metal through the
N
ꢀheterocycle. While at this stage it is difficult
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to conclusively explain the absorption behavior of the quinoline on the metal, it is reasonable
to posit that the absorption is impacted by specific microꢀenvironment of the metal in these
catalysts. Previous studies have reported that the Nꢀheterocycle of quinoline can interact with
8
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hydroxylated supports through hydrogen bonding.17d,17e,19 In the MNPs/SBA-15 catalyst
system, the amine, amide, hydroxyl groups of the dendrimer capping agent, as well as the
hydroxyl group of the support SBAꢀ15, all have the potential to engage in this type of
interaction. Accordingly, while NP catalysts supported on SBAꢀ15 were not effective
catalysts, the combination of metal/dendrimer/SBAꢀ15 provided very active and stable
catalysts.
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C-H activation of indole
Given that indole could be effectively produced with this catalyst system, we further
investigated the potential activity of PdNPs/SBA-15 in the CꢀH functionalization of indole.
While there have been numerous reports of palladium salts as catalysts for CꢀH bond arylation
at position Cꢀ2 or Cꢀ3 of indole 2f,20 examples of nanoparticleꢀcatalyzed CꢀH
funcationalization have only recently been described.20d We have previously reported the
MNP/SBAꢀ15 can be oxidized using hypervalentiodine reagents. Moreover, while in nonꢀ
polar solvents the oxidized catalysts retain their heterogeneous nature, in polar solvents they
are best considered homogeneous molecular catalysts.21 Therefore, we envisioned that the
heterogeneous catalyst employed in the dehydrogenation reaction could be subsequently
leveraged to catalyze a homogeneous CꢀH functionalization reaction by treatment with the
appropriate oxidant in a polar solvent. In the event, reaction of PdNPs/SBA-15 with Ph2IBF4
allowed for a highly efficient direct arylation of indole 2f leading to the exclusive formation
of the 2ꢀphenylꢀindole 3f with 100% of conversion at 60°C in water for 22 hours (entry 28).
Two equivalents of Ph2IBF4 were necessary to transform quantitatively the indole 2f into the
desire product (entries 28, 29). The amount of catalyst was reduced to 0.75 mol % and a very
high activity was still observed. Consistent with the hypothesis that a homogeneous catalyst
was being formed, the catalyst lacking the support (water solution of PdNPs stabilized by
dendrimers) was also tested with the same conditions as for PdNPs/SBA-15, and revealed to
only be slightly less efficient (entry 31). Moreover, Pd(OAc)2 is a classic homogeneous
catalyst for this reaction, provided similar activity to that observed with the oxidized
nanoparticles (entry 33).
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a
Table 5. CꢀH activation of indole. The reactions were conducted with 1 mmol of indole 2f
and 2 mmol of Ph2IBF4 in water at 60°C. bThe reaction was conducted with 1 mmol of 2f and
c
1
1 mmol of Ph2IBF4. The yield was calculated by H NMR using mesitylene as an internal
standard.
entrya
Catalyst (mol %)
Time Selectivity
Conversion
Yieldc
PdNPs/SBA-15 (3.8)
22 h
>99.9 %
>99.9 %
>99.9 %
>99.9 %
>99.9 %
>99.9 %
>99.9 %
73.9 %
>99.9 %
90 %
98%
70%
97%
89%
80%
92%
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33
PdNPs/SBA-15 (3.8)b 22 h
PdNPs/SBA-15 (0.75) 24 h
PdNPs (0.75)c
K2PdCl4 (0.75)
Pd(OAc)2 (0.75)
24 h
24 h
24 h
84.7 %
93.9 %
Finally, the oneꢀpot reaction sequential dehydrogenation/CꢀH arylation reaction of the
indoline 1f and Ph2IBF4 in presence of 5 mol % PdNPs/SBA-15 was examined. The
dehydrogenation/CꢀH activation/CꢀC cross coupling reaction was successful as only the final
product 2ꢀphenylꢀindole 3f was observed at the end of the reaction (plus byproducts coming
1
from Ph2IBF4). No starting material indoline 1f or intermediate indole 2f was detected by H
NMR or GCꢀMS.
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CONCLUSION
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Metallic nanoparticles stabilized by G4OH PAMAM dendrimer are very active catalysts for
the dehydrogenation of tetrahydroꢀquinoline/indoline derivatives, combining efficiency, air
compatibility, and robustness/recyclability/reuse of the catalyst. The hydrogenation of
quinoline derivatives catalyzed by PdNPs/SBA-15 and PtNPs/SBA-15, under one hydrogen
atmosphere, was also found to be efficient. The catalysts tested here showed higher activity
for this hydrogenation/dehydrogenation sequence than the tested homogeneous homologues
and reported catalysts. As a consequence, the combination of these catalysts for quinoline
derivatives raises the possibility that this system might be applied to hydrogen storage
through reversible dehydrogenation/hydrogenation processes. Finally, we found that the
heterogeneous catalyst, PdNPs/SBA-15, can readily be oxidized with Ph2IBF4 to generate an
efficient catalyst for the Cꢀ2 arylation of indole. This observation suggests a strategy in
which a single catalyst precursor can be employed to promote sequential heteroꢀ and
homogeneous process by the choice of the appropriate oxidant/reductant.
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EXPERIMENTAL SECTION
Chemicals. Information about chemicals, including synthesis of supported NPs and
characterization methods, can be found in the Supporting Information.
Representative Procedure for Catalytic Reactions.
Dehydrogenation reactions
To 1 mmol or 0.1 mmol of
Nꢀheterocycle, in a Schlenk flask or closed vial, was added the
corresponding amount of a catalyst in 1 mL of tolueneꢀd8. The system was closed in order to
avoid the vaporization and. allowed to stir for 24 hours. At the end of the reaction, the
1
solution was filtered and used directly for H NMR or GCꢀMS analysis. Mesitylene was
introduced in the NMR tube as an external standard for calculation of the reported
conversions.
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Hydrogenation reactions
To 1 mmol or 0.1 mmol of
N
ꢀheterocycle, in a Schlenk flask or closed vial, was added the
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corresponding amount of a catalyst in 1 mL of tolueneꢀd8. The system was closed, degassed,
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and then placed under 1 atm of hydrogen gas. The reaction was allowed to stir for the
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necessary time. At the end of the reaction, the solution was filtered and used for H NMR or
GCꢀMS analysis. Mesitylene was introduced in the NMR tube as an external standard for
calculation of the reported conversions.
.
Direct arylation of indole
To 0.1 mmol of indole and 0.2 mmol of Ph2IBF4, in a Schlenk flask or closed vial, was added
the corresponding amount of a catalyst in 1 mL of water. . The reaction was allowed to stir
1
for the necessary time. At the end of the reaction, the solution was filtered and used for H
NMR or GCꢀMS analysis. Mesitylene was introduced in the NMR tube as an external
standard for calculation of the reported conversions.
Recyclability test
The catalyst was recycled by simple centrifugation at 3900 rpm for 6 minutes.
Hot filtration test
In the case of Figure 5b: 3 reactions were performed simultaneously. The first reaction was
1
stopped after 1 hour and the conversion was checked by HꢀNMR (32%). The second
reaction was filtered to remove the catalyst after 1 hour. The supernatant was allowed to stir
for an additional under reaction condition and the conversion by 1HꢀNMR after and additional
2h and 5h (conversion 41% in both cases). The final reaction was check every hours during 6
hours without filtration the catalyst. The reaction reached 100% conversion after 6 hours.
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ASSOCIATED CONTENT
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*
Supporting
Information
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The Supporting Information is available free of charge on the ACS Publications website at
DOI: Detailed information on synthesis of catalysts, materials characterization, and catalysis
(PDF)
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: fdtoste@berkeley.edu
*Eꢀmail: somorjai@berkeley.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge support from the Director, Office of Science, Office of Basic Energy
Sciences, Division of Chemical Sciences, Geological and Biosciences of the US Department
of Energy under contract DEꢀ AC02ꢀ05CH11231. We thank Prof. A. Paul Alivisatos for the
use of his TEM equipment and Dr. Gerome Melaet for useful discussions.
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