Easy and One-Step Synthesis of Ir Single Atom Doped PPy Nanoparticles for Highly Active N-Alkylation Reaction

Article abstract of DOI:10.1002/ejic.202000971

Single atoms are nowadays considered new catalysts offering a lot of advantages such as reduced cost and use of noble metals. Here, we report the catalytic activity towards N-alkylation reactions of single atoms (SACs) of iridium doped polypyrrole (Ir?PPy nanoparticles). The new nanocatalyst exhibits excellent behavior due to the combination of highly active SACs catalyst and the organic conductive support providing strong binding of single atoms and thus improved reusability and easiness to handle. In the presence of a low amount of Ir and of a suitable base, excellent activity (yield >99 %) and selectivity (>99 %) were obtained. In the optimized Ir condition (3 wt.% of Ir) an aniline conversion of about 99 %, with excellent selectivity, just after 600 min, was obtained. Recyclability tests show that the catalyst can be successfully recycled six times without significant catalyst activity loss.

Full text of DOI:10.1002/ejic.202000971

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Easy and One-Step Synthesis of Ir Single Atom Doped PPy
Nanoparticles for Highly Active N-Alkylation Reaction
Mariagrazia Iuliano,[a] Maria Sarno,*[b, c] Claudia Cirillo,[a] Eleonora Ponticorvo,[a, c] and
Salvatore De Pasquale[b, c]
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Single atoms are nowadays considered new catalysts offering a
lot of advantages such as reduced cost and use of noble metals.
Here, we report the catalytic activity towards N-alkylation
reactions of single atoms (SACs) of iridium doped polypyrrole
(IrÀ PPy nanoparticles). The new nanocatalyst exhibits excellent
behavior due to the combination of highly active SACs catalyst
and the organic conductive support providing strong binding
of single atoms and thus improved reusability and easiness to
handle. In the presence of a low amount of Ir and of a suitable
base, excellent activity (yield >99%) and selectivity (>99%)
were obtained. In the optimized Ir condition (3 wt.% of Ir) an
aniline conversion of about 99%, with excellent selectivity, just
after 600 min, was obtained. Recyclability tests show that the
catalyst can be successfully recycled six times without signifi-
cant catalyst activity loss.
Introduction
catalysts such as ruthenium (Ru), iridium (Ir), platinum (Pt), silver
(Ag), and palladium (Pd) complexes.[4] However, these catalysts
are characterized by some problems, such as the recovery of
expensive metals and the use of metal complexes. The develop-
ment of recoverable and effective catalysts to overtake these
difficulties is now still challenging. Recently, heterogeneous
inorganic-supported transition metal catalysts have been
studied, such as Ru,[5a] Au,[5b] Pd.[6] On the other hand, typically,
harsh conditions and/or poor selectivity was observed. Atomi-
cally dispersed catalysts on supports, as mononuclear metal
complexes or single metal atoms, named as single-atom
catalysts (SACs), are nowadays considered as new catalysts.
They are characterized by full metal dispersion and utilization,
SACs offer the best strategy for generating cheaper catalysts
and utilization of noble metals.[7] Single-atom catalysts exhibit
superior performance to be considered as the next-generation
catalysts.[8] This is due to the lack of coordination, which allows
easy anchoring to substrates, forming tough connections that
favor charge-transfer processes. They don’t suffer from particle
size effects and they lack a surface, avoiding the adsorption
step and allowing reaction in solution or gas phases. Although
single atoms are a successful approach to minimize catalyst
loading, a fine dispersion on supports avoiding aggregation is
required. Intrinsic organic supports functionalities can provide
stronger binding of single atoms and thus improved reusability
and are easier to handle.[9a] In particular, polypyrrole (PPy) is
increasingly studied[9] for many applications due to its good
electrical conductivity, environmental and thermal stability, and
ease of synthesis.[10] Conductive polymers,[11] such as PPy, are
characterized by easy exchanges of electrons and are partic-
ularly useful where these exchanges take place. Here we report,
for the first time, the catalytic activity towards N-alkylation
reactions by using single atoms of iridium doped polypyrrole
(IrÀ PPy nanoparticles). These nanoparticles were obtained
through a one-step microemulsion approach at room temper-
ature. SACs of iridium can be dispersed and stabilized in the
polypyrrole matrix. In particular, the N-alkylation of aniline with
Thanks to huge applications in the synthesis of organic
compounds, amines have been the object of a lot of studies.[1]
Indeed, amines, especially aromatic, are intermediates for the
production of pharmaceuticals, colorants, polymers, and
agrochemicals.[1] N-alkylated amines can be prepared by a lot of
methods.[2] These methods often suffer from environmental
problems, the use of expensive starting materials, and low
selectivity for the desired product. The N-alkylation of primary
amines with alcohols, through “borrowing hydrogen”, to
produce secondary amines is an alternative and atom efficient
approach.[3] This method has been documented as a “green”
and concrete approach for the formation of carbon-carbon or
carbon-nitrogen bonds. It does not involve pre-modification of
the substrates or oxidants/reductants and provides water as the
only by-product. In particular, it consists of: (i) alcohol
dehydrogenation to give a carbonyl compound; (ii) reaction of
the carbonyl group with an amine to restitute a condensation
product; (iii) transfer of hydrogen atoms to the condensation
product to form the final product. N-alkylation reactions have
been studied in the presence of homogeneous transition metal
[a] M. Iuliano, C. Cirillo, E. Ponticorvo
Department of Industrial Engineering,
University of Salerno
Via Giovanni Paolo II, 132 – 84084 Fisciano (SA), Italy
[b] Prof. M. Sarno, Prof. S. De Pasquale
Department of Physics “E.R. Caianiello”
University of Salerno
Via Giovanni Paolo II, 132 – 84084 Fisciano (SA), Italy
[c] Prof. M. Sarno, E. Ponticorvo, Prof. S. De Pasquale
NANO_MATES Research Centre
University of Salerno
Via Giovanni Paolo II, 132 – 84084 Fisciano (SA), Italy
E-mail: msarno@unisa.it
http://docenti.unisa.it/maria.sarno
https://www.nanomates.unisa.it
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202000971
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Figure 1. Schematic synthesis of IrÀ PPy nanoparticles.
benzyl alcohol as an alkylating agent was investigated with
different bases and different alkylating agents, showing high
conversion and selectivity, in the presence of a low amount of
iridium, which is in the form of single atoms, in short times and
mild conditions.
Results and Discussion
Characterization of the IrÀ PPy NPs
The schematic representation of the IrÀ PPy synthesis is shown
in Figure 1. Homogeneous IrÀ PPy NPs were obtained through a
polymerization performed in a one-step micro-emulsion and by
using Fe3+ as the oxidizing agent. Polyvinyl alcohol (PVA) was
introduced to improve nanoparticle water solubility and
biocompatibility. Iridium complexes were wrapped in the PPy
nanoparticles during their formation by p À p interactions or
non-covalent interaction. By centrifugation, the removal of
excess free PVA was performed, and IrÀ PPy NPs were obtained.
Figure 2. XRD spectra of PPy, IrÀ PPy NPs and IrCl3 precursor.
The wide-angle powder XRD pattern of IrÀ PPy is shown in
[12]
°
°
°
°
Figure 2. The four peaks at 2Θ~11.5 , 22.8 , 29.4 , and 41.4
are due to PPy chains scattering. In particular, the broad peak at
difference between the redox potentials of iridium chloride and
iron chloride, which decomposes much more easily. The Ir
weight on the respect of PPy is equal to 1.95%, which is
consistent with the amount of Ir loaded and indicating that Ir is
preferentially localized on the surface of PPy NPs. Figure 3f
shows the aberration-corrected HAADF/STEM image of the as-
synthesized material.
In the image, contrast points coherent with single iridium
atoms can be seen. Finally, Figure 3g illustrated the SEM image
of IrÀ PPy NPs, showing a spherical granular morphology for the
sample consistent with that shown in the TEM image. The
higher size exhibited by the nanoparticles can be ascribed to
the gold coating used during the sample preparation for SEM.
Ir, in the catalyst, was examined by CO-DRIFTS. CO symmetrical
and asymmetrical stretching due to Ir(CO)2 species, with
mononuclear Ir, can be seen at 2069 cmÀ 1 and 1989 cmÀ 1,[13a]
see Figure 4a.
°
about 29 is related to the scattering of the interplanar spacing
of amorphous PPy.[12c] The amorphous nature of the hydrated
precursor is also visible in Figure 2. The more resolved
diffraction peaks can be attributed to PPy regular arrays
producing short crystallinity. The spacing between the planes
of benzene rings in the adjacent PPy chains determines the
[12d]
°
diffraction peak at 2θ=19 .
The spectrum of IrÀ PPy shows
more resolved peaks and increased crystallinity for the support,
probably induced by Ir. No peaks from Ir can be detected.
Transmission electron microscopy (TEM) images of PPy and
IrÀ PPy NPs are shown in Figure 3. In particular, Figure 3a shows
a TEM image of PPy nanoparticle that has a size in the range
60–80 nm. Figure 3b, Figure 3c, and Figure 3e show, at different
magnifications, images of IrÀ PPy NPs. The images exhibit quasi-
spherical nanoparticles with a size slightly smaller than those of
PPy nanoparticles alone (in the range 50–70 nm with a peak at
60 nm, see the histogram in Figure 3c). EDX spectrum in
Figure 3d shows the peaks from Ir and Cu of the TEM grid. No
peaks from Fe can be detected, likely due to the significant
The spectrum after 200 cycles of use is also reported in
Figure 4a, showing the stability of the sample after usage. X-ray
absorption fine extended structure (EXAFS) and X-ray absorp-
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Figure 3. TEM images of: PPy NPs (a); IrÀ PPy NPs (b, c, e). IrÀ PPy EDX analysis (d). Aberration-corrected HAADF/STEM image of IrÀ PPy (f). SEM image of IrÀ PPy
NPs (g).
tion energy near edge structure (XANES) spectroscopy, Fig-
ure 4b, were used to analyze the dispersion of Ir-SACs and their
coordination environment. Ir/C was used as a benchmark. Ir
single atom in IrÀ PPy exhibits the main peak at ca. 1.5 c. On the
other hand, Ir/C profile is dominated by a peak at about 2.35 c,
which can be assigned to IrÀ Ir scattering. It is worth noting that
the IrÀ Ir scattering is absent in the spectrum of Ir-SACs.The
XANES spectra shown in Figure 4c, evidence a shift for Ir-SACs,
green profile, indicating the Irδ+ form for Ir. In Figure 5, the
thermogravimetric analysis profiles of PPy and IrÀ PPy NPs are
Catalytic N-alkylation
N-alkylation of aniline with benzyl alcohol was, initially, chosen
as a model reaction (Figure 7), to examine the catalytic activity
of IrÀ PPy NPs.The results of the tests carried out by using the
catalyst in the presence of different bases, and with bases or
PPy alone, are reported for comparison in Table 1 and Table 2,
Figures 8 and 10 and Figures S1–S11. The data show that the
kinetic of imine formation is faster than that of hydrogenation
to the secondary amine.[13d] In particular, in Table 1 and Table 2
the results of the tests performed with benzyl alcohol (1 mmol),
aniline (1 mmol), catalyst (25 mg at 2 wt.% Ir), base (2 mmol),
°
shown. It can be observed that the temperature range ~200 C
°
and ~300 C is characterized by a sharp thermal degradation,
°
accounting for a weight loss of up to 40%.
toluene (5 mL), at 100 C and under N2 were shown after 12 and
IrÀ PPy NPs showed a slight up-shift of this first weight loss
suggesting higher thermal stability for the Ir containing
composite materials. FT-IR profiles of IrÀ PPy NPs and PPy are
shown in Figure 6. The FT-IR spectrum of polypyrrole alone
shows vibrational bands at 1690 cmÀ 1 and 1540 cmÀ 1 corre-
sponding to CÀ N and CÀ C stretching. The peak at 1455 cmÀ 1
can be assigned to pyrrole ring vibrations. The peak at about
1270 cmÀ 1 is due to the breathing vibrations of the pyrrole ring,
the peak at 1190 cmÀ 1 to CÀ N vibrations, and the peaks at
1049 cmÀ 1 and 907 cmÀ 1 to CÀ H in and out plane vibrations.[13b,c]
What can be seen in the IrÀ PPy spectrum is the appearance of
the band at 1562 cmÀ 1 due to CÀ C vibration and the downshift
of the band at 1190 cmÀ 1 occuring for CÀ N bonds. This is due
to the coordination of iridium with the polymer, indicating the
formation of a strong interaction between PPy and Ir single
atom in the composite material.
24 hours, respectively. Compounds 1 and 2 together with a
number of other molecules were formed, depending on the
operating conditions, catalyst and base chosen. The results
shown in entry 1 and entry 2 of Table 2, in comparison with
entry 0 (Figure S1), highlights the role of the bases allowing
reagents conversion. Moreover, poor selectivity vs. compound
2, compare Figure S1, Figure S2, and Figure S3, was observed.
The comparison between the two bases evidence that KOtBu
allows a higher selectivity vs. compound 2, although azoben-
zene and other by-products were produced in a higher amount.
In all three cases, the formation of benzaldehyde is observed,
indicating borrowing hydrogen occurring in the system; the
quantities are higher in the presence of the two bases. The
reduction of the benzyl alcohol amount during the experiments
is probably due to the conversion into benzaldehyde, which is
more volatile. In the case of entry 2, the formation of
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Figure 5. TGA profiles of PPy and IrÀ PPy.
Figure 4. CO-DRIFT spectra of IrÀ PPy before and after 200 cycles of usage (a).
Fourier transforms of k3-weighted Ir L3-edge EXAFS data for IrÀ PPy and Ir/C
(b). XANES of Ir L3-edge for IrÀ PPy and Ir/C (c).
benzaldehyde appears more controlled over time, leading to
the formation of component 1 with high selectivity. The
presence of PPy, entry 3 and entry 4, Figure S4 and Figure S5,
increases conversion and contributes to the formation of the
amine. This suggests that PPy supports the base during the
catalytic borrowing hydrogen process, likely with its ability to
exchange electrons acting as a nucleophile and electrophile, as
well. On the other hand, the selectivity towards the desired
Figure 6. FT-IR spectra of PPy and IrÀ PPy (a) and enlargement (b).
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see Figure 8b, the formation of benzaldehyde supports the
1
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occurrence of hydrogen borrowing. It is to be underlined that
the key role of the base in promoting catalysis is further
evidenced by analyzing entry 7; see Figure S7, too. Low
conversion, lower yield, and selectivity to compound 2 were
observed. Other bases that, generally, give worse results have
also been tested. In particular, the use of weaker bases, entry 8
and entry 9, show less activity. The potassium-containing base
is able, in terms of selectivity, to provide better results, see
Figure S8 and S9, also in this case. The yield further decreases
by using even weaker bases, entry 10 (Figure S10).To better
analyze the catalyst performance in the presence of KOtBu and
aiming to reduce process times, further experiments were
performed; see Figure 9 and Figure 10. As the content of Ir,
Figure 9b, increases, the amount of benzyl alcohol available was
reduced for each time, probably due to a faster formation of
benzaldehyde, which is more volatile. Moreover, at 3 wt.% of Ir,
the catalyst allows a quite total aniline conversion with
excellent selectivity just after 600 min, see Figure 10.
Figure 7. Schematic N-alkylation reactions of aniline with benzyl alcohol.
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product is still low. In the case of entries 5 and 6, Figure 8 and
Figure S6, the activity and selectivity towards products 1 and 2,
due to the Ir presence, increase further; see the yield column in
Table 2. The excellent behavior of the catalyst suggests that the
formation of the imine is a surface limited step occurring on the
active sites of Ir, where the reaction proceeds fastly. It is worth
noticing that the differences in the selectivity vs. compound 2
obtained in the presence of the two bases are very large. Where
KOtBu allows a very exciting result with a selectivity of 99%. In
particular, in Figure 8a the evolution of conversion and
selectivity to amine formation are reported during reaction time
increase, in the case of KOtBu used as the base. Selectivity to
secondary amine was higher than 75%, yet after 5 h. Moreover,
The spectrum of IrÀ PPy after 4 h of usage is dominated by
new vibrational bands; see Figure 11. In particular, the bands at
694 cmÀ 1, 762 cmÀ 1, 1316 cmÀ 1, 1377 cmÀ 1, 1452 cmÀ 1,
1587 cmÀ 1, 1591 cmÀ 1 indicated the adsorption of benzylidene
aniline. Additionally, the vibrational bands from benzylaniline
Table 1. N-alkylation of various amines with alcohol over IrÀ PPy nanocatalyst.[a]
Entry
Catalyst
Base
Solvent
Conversion[b,f] [%]
Selectivity[b] [%]
1
Yield[b,f] [%]
2
2
3
0
1
2
3
4
5
6
7
8
9
10
PPy
–
–
PPy
-
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
5[e] �0.5
32�0.8
8�0.4
43�0.8
11�0.6
<2
-
<3
<8
Trace
<2
12�0.9
70�0.5
Trace
<3
KOtBu
NaOtBu
KOtBu
NaOtBu
KOtBu
NaOtBu
–
KOH
NaOH
CsCO3
30[c,e] �0.9
49[c] �0.4
31[c,d,e] �0.5
55[c] �0.8
75�0.8
–
–
–
–
–
–
–
–
–
–
98�0.4
5�0.2
44�0.4
24�1.0
94�0.4
1�0.2
17�0.3
45�0.6
25�0.7
<1
PPy
65�0.5
6�0.7
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
59[c] �1.1
10[c] �1.4
65[c] �0.7
32[c] �0.7
45�0.8
98�0.8
69�0.5
55�0.6
74�0.1
99�0.2
<29
8�0.5
Trace
°
[a] Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst (25 mg at 2 wt.% Ir), base (2 mmol), toluene (5 mL), 100 C and under N2. [b] After
12 h. [c] Benzyl ether formation. [d] Bibenzyl formation. [e] Azobenzene formation. [f] Conversion and yield were determined by GC analysis using
hexadecane as an internal standard.
Table 2. N-alkylation of various amines with alcohol over IrÀ PPy nanocatalyst.[a]
Entry
Catalyst
Base
Solvent
Conversion[b,f] [%]
Selectivity[b] [%]
1
Yield[b,f] [%]
2
2
3
0
1
2
3
4
5
6
7
8
9
10
PPy
–
–
PPy
–
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
23[e] �0.7
64[c,e] �2.1
50[c] �0.4
97[c,d,e] �0.9
77[c] �0.6
>99
23�0.5
10�2.2
98�0.6
<1
47�0.4
12�1.0
<1
64�0.9
21�1.1
>99
1.5�0.2
18�1.3
72�0.9
27�0.8
6�0.5
–
–
–
–
–
–
–
–
–
–
–
10�0.7
10�1.6
Trace
60�0.6
16�0.8
>99
Trace
<10
68�0.3
24�0.7
5�0.9
KOtBu
NaOtBu
KOtBu
NaOtBu
KOtBu
NaOtBu
–
KOH
NaOH
CsCO3
PPy
60�1.0
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
IrÀ PPy
<0.7
97[c] �0.4
54[c] �1.5
95[c] �0.9
97[c] �0.4
94�0.7
97�0.3
77�0.5
24�1.2
72�0.3
94�0.4
°
[a] Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst (25 mg at 2 wt.% Ir), base (2 mmol), toluene (5 mL), 100 C and under N2. [b] After
24 h. [c] Benzyl ether formation. [d] Bibenzyl formation. [e] Azobenzene formation. [f] Conversion and yield were determined by GC analysis using
hexadecane as an internal standard.
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Figure 8. Effect of the time on conversion and selectivity of N-alkylation reaction in the presence of IrÀ PPy NPs. Reaction conditions: benzyl alcohol (1 mmol),
°
aniline (1 mmol), catalyst (25 mg 2 wt.% Ir)), KOtBu (2 mmol), toluene (5 mL), 100 C and under N2. Entry 5 Table 2.
Figure 10. Effect of the time on conversion and selectivity of N-alkylation
reaction in the presence of IrÀ PPy NPs. Reaction conditions: benzyl alcohol
Figure 9. Effect of the time on conversion and selectivity of N-alkylation
(1 mmol), aniline (1 mmol), catalyst (25 mg 3 wt.% Ir), KOtBu (2 mmol),
reaction in the presence of IrÀ PPy NPs. Reaction conditions: benzyl alcohol
°
toluene (5 mL), 100 C and under N2.
(1 mmol), aniline (1 mmol), catalyst (25 mg 4 wt.% Ir)), KOtBu (2 mmol),
°
toluene (5 mL), 100 C and under N2.
bands are hidden by higher benzylidene aniline presence
°
are also visible (values in green in the spectrum). More
anchored benzaldehyde, probably to Ir sites, which vibrational
before the thermal treatment up to 180 C for 4 h in nitrogen
flow, becomes visible at 1455 cmÀ 1 and 1591 cmÀ 1 after the
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heating. Moreover, the FTIR spectrum in the range 2400 cmÀ 1–
1
2
3
2457 cmÀ 1, see the light blue box in Figure 11, suggests the
intermediate formation of IrÀ H species.[13e]
On the base of the experimental observation, a mechanism
for the N-alkylation on IrÀ PPy was proposed in Figure 12, where
the two different steps of imine formation and hydrogenation
were shown. In particular, first of all, the bases are responsible
for the alcohol deprotonation, since catalysis proceeds very
slowly without the bases. The first step consists of forming an
alkoxide on Ir active sites, followed by a condensation to form
the imine on the same sites. Hydrogenation occurs more slowly,
thanks to the hydride formed on the metal surface. Moreover,
since it proceeds very differently with different bases and better
with potassium based-bases, we have to consider a role for the
base also in this process phase. The differences exhibited in the
presence of NaOtBu and KOtBu can be ascribed to the higher
electronegativity of K, which results in the ability to act as a
bridge for hydrogen towards the formation of the secondary
amine. Finally, it is not possible to exclude a role for the PPy
surface, probably in supporting catalysis as an H2 reservoir.
Other examples of N-alkylation of various amines with
alcohols were reported in Table 3. The presence of N in the
amine ring (2-amino pyridine) determines a reduction of
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
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53
54
55
56
57
Figure 11. FT-IR spectra of IrÀ PPy after 4 h of usage and after the treatment
°
for 4 h at 180 C in nitrogen flow.
Figure 12. The proposed possible mechanism for IrÀ PPy nanocatalyst for the N-alkylation reaction of aniline.
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Table 3. N-alkylation of various amines with alcohol over IrÀ PPy nanocatalyst.[a]
1
2
Entry
Amine
Alcohol
Targeted products
Conversion[b] [%]
Selectivity[c] [%]
Yield[b] [%]
3
4
5
1
>99
>99
>99
6
7
8
9
2
3
>98
>98
>99
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
97
96
94
4
5
6
7
96
95
92
94
91
94
80
93
88
>99
>99
>99
>99
8
9
98
98
98
>99
>99
°
[a] Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst (25 mg at 2 wt.% Ir), KOtBu (2 mmol), toluene (5 mL), 100 C under N2, and 24 h of
synthesis. [b] Conversion and yield were determined by GC analysis using hexadecane as an internal standard. [c] Selectivity of targeted products.
conversion, selectivity, and yield. Moreover, the substrate benzyl
alcohol with electron-withdrawing Cl groups produced 80%
and 88% yields of products.
Recyclability of the nanocatalysts
We have studied the recyclability of the nanocatalyst; see
Figure 13. The results highlight that the catalyst can be
successfully recycled six times without any significant loss in
catalytic activity. The slight decrease in product yield could be
attributed to a small loss of the catalyst during the recycling
process and probably to a feeble activity reduction due to the
use.
Figure 13. Recycling efficiency of IrÀ PPy NPs in the N-alkylation of aniline.
Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), catalyst
°
(25 mg 2 wt.% Ir)), KOtBu (2 mmol), toluene (5 mL), 100 C and under N2.
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Conclusion
Characterization techniques of IrÀ PPy NPs
1
2
For IrÀ PPy characterization: TEM (Transmission electron microscopy)
(FEI-Tecnai; 200 KV) equipped with an Energy Dispersive X-ray
Analysis (EDX) probe; High-resolution analytical TEM (F20 FEI
Tecnai-200 kV) for high-angle annular dark-field (HAADF) analysis,
with HAADF detector employed in STEM mode; SEM (Scanning
electron microscope) (FESEM LEO1525), UV-Visible spectroscopic
analysis (EVOLUTION S60, Thermo Fischer Scientific), XRD measure-
ments (Bruker D8 X-ray diffractometer using CuKα radiation),
thermogravimetric analysis (TG-DTG, SDTQ 600 Analyzer TA Instru-
ments) at 10 K/min heating rate in airflow and FT-IR analysis (Vertex
70apparatus (Bruker Corporation)), were performed. Thermo-Scien-
tific Nicolet (i550 FTIR) was used for Diffuse Reflectance Infrared
Fourier Transform spectroscopy (DRIFTs) measurements. CO flux,
25 mL/min, was provided for the reaction chamber, which was
successively purged by a N2 flow. XANES and EXAFS were obtained
at beamline of the Elettra Synchroton (x.ray beam 16–20 eV). The
measurements were performed at room temperature. For the
measurement a homogenous layer of the sample was prepared.
Typically, 5 different scans were performed to reduce the signal-to-
noise ratio. Data analysis was performed in agreement with
ATHENA program pack.
Morphological and structural characterization shows the for-
mation of PPy nanoparticles covered by iridium single atoms
anchored with the support. N-alkylation reactions with aniline
and benzyl alcohol were examined in the presence of different
bases and with and without PPy and IrÀ PPy. The data show that
the kinetic of imine is faster than that of hydrogenation to the
secondary amine. The formation of benzaldehyde is observed,
indicating that borrowing hydrogen occurs in the system.
Activity, and selectivity towards products 1 and 2 increase, in
the presence of Ir. The excellent behavior of the catalyst
suggests that the formation of the imine is a surface limited
step occurring on the active sites of Ir. KOtBu, in comparison
with NaOtBu allows higher selectivity vs. compound 2. This is
likely due to the higher electronegativity of K acting as a bridge
for hydrogen towards secondary amine formation. Moreover, at
3 wt.% of Ir, the catalyst allows a very high aniline conversion,
with excellent selectivity, just after 600 min, likely due to a large
amount of Ir active sites, probably favoring benzaldehyde
anchoring and evolution vs. the products. Recyclability tests
show the catalyst stability without any significant loss in catalyst
activity.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
General procedure for the N-alkylation reactions over IrÀ PPy
The N-alkylation reactions were carried out in a round-bottomed
°
flask (capacity, 50 mL) at 100 C under N2 atmosphere. In a typical
experiment, in 5 ml of toluene, aniline (1 mmol), benzyl alcohol
(1 mmol), base (2 mmol), and IrÀ PPy nanoparticles (25 mg) were
Experimental Section
°
added and heated up to 100 C for a specific time. The progress of
the reaction was monitored at different time intervals and
quantified by gas chromatography-mass spectrometry (GC-MS,
Thermo Fischer Scientific) analysis. After the reaction time, using a
simple filtration, the nanocatalyst was removed. The mixture was
dried over Na2SO4, and the residue was purified by flash column
chromatography on silica gel. GC-MS was used for identified
desired products. The IrÀ PPy after filtration was washed with water
Materials
Pyrrole, iridium(III) chloride hydrate (IrCl3), iron(III) chloride hexahy-
drate (FeCl3 ·6H2O), polyvinyl alcohol (PVA), ethanol, distilled water,
aniline, benzyl alcohol, toluene, potassium hydroxide (KOH), sodium
hydroxide (NaOH), cesium carbonate (CsCO3), sodium tert-butoxide
(NaOtBu), potassium tert-butoxide (KOtBu), dodecane (internal
standard) and all other reagents and substrates were purchased
from Sigma Aldrich.
°
several times and dried at 60 C for further use.
Product analysis
Synthesis of IrÀ PPy nanoparticles
The products obtained by the N-alkylation of amines with alcohols
were analyzed through
a GC-MS (Thermo Fischer Scientific)
First of all, PVA (500 mg) and FeCl3·6H2O (500 mg) were mixed in
deionized water (10 mL). Then, IrCl3 was dissolved in 2 ml of
ethanol under stirring for 30 min and added to the previously
prepared solution. After that, pyrrole (70 μL) was added to the
reaction mixture and stirred for 24 h. Iridium precursor was added
to give an Ir loading of 2 wt.% pyrrole weight-based. After
polymerization, the solution was washed and centrifuged. During
the synthesis, lone pairs electrons in nitrogen atoms (existing in
pyrrole monomer) can reduce Ir precursor. In this process, the role
of PVA cannot be neglected, too. The Ir NPs generated and
attached on the PPy matrix via π-π, or non-covalent interaction
among N in PPy matrix and Pd occurs. PPy nanoparticles were also
prepared with the same procedure but the absence of Ir precursor.
equipped with a TG-POLAR capillary column (0.25 μm×0.25 mm×
60 m). All reaction products were identified by matching the GC
retention time with authentic samples. The column temperature
°
°
°
was kept at 160 C for 1 min, then raised to 270 C at 3 C/min and
maintained at this temperature for 10 min. The temperatures of the
°
injector and detector were set at 250 C. Helium with a flow of
1.2 ml/min was used as the carrier gas. The qualitative analysis was
compared with the chromatographic standard and mass spectrom-
etry of the product. The obtained results were in good agreement
with parallel evaluation performed by using hexadecane as an
internal standard. The experiments were performed in triplicate.
The texts, along with the results, are expressed with a standard
deviation, to take into account the different results. The conversion
of amine was calculated as follows:[14a]
Ir/C consisting of Ir atoms and Ir nanoparticles dispersed on carbon
was obtained through pyrolysis of and Ir precursor, too. In
particular, Ir(acac)3 was mixed under ultrasounds with Vulcan XC-
72. After drying in vacuum conditions, the mixture was pyrolyzed
(1)
°
under Ar/H2 atmosphere (15 vol.% H2) at 320 C for 3 h.
The product selectivity was calculated as:[14b]
(2)
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doi.org/10.1002/ejic.202000971
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The Benzyl alcohol mol % was calculated as
The Benzaldehyde mol % was calculated as
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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57
FT-IR spectra of the samples were obtained by using Vertex 70
apparatus (Bruker Corporation). Spectra were recorded in the
scanning range from 4000 to 400 cmÀ 1. 1H NMR analysis was
performed using Bruker Advance 400 (Operating at 400 MHz)..
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Borrowing hydrogen · N-alkylation · Nanocatalysis ·
Nanoparticles · Reaction mechanisms
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Manuscript received: October 20, 2020
Revised manuscript received: December 7, 2020
Accepted manuscript online: December 9, 2020
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