Multilayer graphene functionalized through thermal 1,3-dipolar cycloadditions with imino esters: A versatile platform for supported ligands in catalysis

Article abstract of DOI:10.1039/c9cc00939f

Multilayer graphene (MLG), obtained by mild sonication of graphite in NMP or pyridine, was fully characterized via atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoemission spectroscopy (XPS). Then, it was functionalized via 1,3-dipolar cycloaddition with azomethine ylides generated by thermal 1,2-prototropy of various imino esters. The resulting MLG, containing substituted proline-based amine functions, was characterized by XPS and it showed high nitrogen loading, ranging from 0.6 to 4.2 at% depending on the imino ester used. Among these functionalized MLGs a probe sample was subjected to ester hydrolysis and used as a heterogeneous N,O-chelating ligand to coordinate iridium atomic centers. This supported complex was also characterized by XPS and its catalytic activity was tested in the hydrogen transfer reduction of acetophenone, obtaining up to 85% yield. Furthermore, this catalyst could be recycled up to four times.

Full text of DOI:10.1039/c9cc00939f

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ARTICLE
Multilayer graphene functionalized through thermal 1,3-dipolar
cycloadditions with imino esters. A versatile platform for
supported ligands in catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Marcos Ferrándiz-Saperas,a,b‡ Alessio Ghisolfi,c‡ Diego Cazorla-Amorós,c* Carmen Nájerab and José
M. Sansano.a,b*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Multilayer graphene (MLG), obtained by mild sonication of graphite in NMP or pyridine, was fully characterized via atomic
force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoemission
spectroscopy (XPS). Then, it was functionalized via 1,3-dipolar cycloaddition with azomethine ylides generated by thermal
1,2-prototropy of various imino esters. The resulting MLG, containing substituted proline-based amine functions were
characterized by XPS and presented a high nitrogen loading, ranging from 0.6 to 4.2 at.% depending on the imino ester used.
Among these functionalized MLG a probe sample was subjected to the ester hydrolysis and used as a heterogeneous N,O-
chelating ligand to coordinate iridium atomic centers. This supported complex was also characterized by XPS and its catalytic
activity was tested in the hydrogen transfer reduction of acetophenone obtaining up to 85% yield. Furthermore, this catalyst
could be recycled up to four times.
Introduction
mediated dispersion (liquid exfoliation) of graphene in suitable
[24]
organic solvents as pyridine
or N-methylpyrrolidone
Due to electrical, optical and mechanical properties of
graphene,[1][2][3][4] the functionalization of these few layer
planar structures can produce many attractive synergies.[5]
Although these features make graphene a potentially useful and
robust atomic-scale scaffold for the design of new
nanomaterials its covalent functionalization still remain a
difficult task because of its low reactivity. As consequence,
harsh reaction conditions (pressure, temperature, etc.) are
commonly needed and this often limits the exploitation of the
full potential of this type of chemistry. Moreover, the reactivity
also depends on the type of graphene support,[6][7][8] and on the
number of graphene layers.[9][10][11]
Typically, multilayer graphene (MLG)[12] can be used as 4 or
2 component of the [4+2] cycloadditions[13][14] in the presence
of very reactive dienophiles or dienes, respectively. Besides,
only [3+2] cycloadditions involving azomethine ylides,
generated through the decarboxylation route,[15][16][17] allowed
(NMP)[25][26] is appropriate to react with large excesses of N-
alkylglycines and the corresponding aldehyde in
a
multicomponent sequence, at 130-180 ºC during 3-5 days, to
give the desired prolinate functionalized graphene. With the
aim to investigate new milder and versatile strategies that
permit the structural modification of graphene nanomaterials,
in this work we present a novel and convenient synthetic route
to anchor stabilized azomethine ylides to MLG. Such method
implies lower temperature than the usual (e.g. 85-95 ºC against
the 130-180 ºC). Furthermore, this type of prolinate derivatized
graphene nanomaterial can be used as ligand for the generation
of a graphene supported metal complexes. As a proof of
concept iridium complex immobilization is presented, which
was evaluated as heterogeneous recyclable catalyst in the
hydrogenation of acetophenone.
the introduction of a secondary amine as functional Results and Discussion
group.[18][19][20][21][22][23][24] Furthermore in this way, an extra
The starting graphite was first heated at 930 ºC in inert
functionality to the system was added, with a total atom
economy in the process. This type of functionalization is known
to mostly take place on the face of the sheets and not on the
edges, thus allowing a more complete exploitation of the
support surface.[15] Hence dipolar cycloaddition is a potentially
useful approach for the preparation of MLG-based functional
materials. Generally, in these type of reactions an ultrasound
atmosphere in order to remove most of the oxygen functional
groups, which could lead to undesired byproducts during the
1,3-DC. Exfoliation of graphene was performed in NMP and in
pyridine, which are known to be good solvents for liquid
exfoliation and are also compatible with the following chemical
reaction. The exfoliated graphene was characterized by TEM,
TG, AFM, and Raman spectroscopy (see SI). The isolated
graphene nanomaterials consisted of few layers (MLG), what is
in agreement with the literature.[15] The analysis also revealed
that under this sonication method negligible functionalization
by oxidizing radicals occurred.[27] 2D Region of the Raman
a
Departamento de Química Orgánica e Instituto de Síntesis Orgánica (ISO),
University of Alicante, E-03080 Alicante, Spain.
b Centro de Innovación en Química Avanzada (ORFEO-CINQA).
c Departamento de Química Inorgánica and Instituto Universitario de Materiales,
University of Alicante, E-03080 Alicante, Spain.
*Corresponding Author.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C9CC00939F
spectrum for the MLG (sample G1) has a typical profile of few Further studies confirmed that the incorporation of the
layers graphene materials (see SI).[28,29] Whereas 2D region for nitrogen occurred during the ultrasound-promoted exfoliation
graphite is composed by 2D1 and 2D2 components with a peak of the graphene in pyridine. Thus, we decided to avoid the use
intensity ratio 2D2/2D1 close to 2, the 2D region for the MLG is of pyridine and continued the study with G2 and 1a. These NMP
much broader and it has a higher contribution from the lower dispersed samples displayed an incorporation of pure
frequency 2D1 peaks.[28,29,30] Raman measurements have been heterocyclic nitrogen of 1 at.%, after five days and
done at different positions of the material to get useful independently of the temperature used (Table 1, entries 6 and
information of 2D region.[31] The obtained data are in 7). The evolution of the reaction course versus time was also
agreement with a MLG material with less than ten sheets (and analyzed, taking samples at different days during the reaction.
more than four sheets) due to the higher contribution of the As it can be deduced from Table 2 (entries 7-12) the increase of
region below 2700 cm-1 to the 2D peak.[29] AFM images are also the percentage of nitrogen was negligible between the 5th and
in agreement with Raman results (see SI). Interestingly, the the 6th day, whilst more reaction time resulted in decomposition
Raman spectrum for sample G2a is very similar to that for G1 of G2a.
although it is blue-shifted, probably due to the functionalization
reaction (the G band is shifted from 1586 cm-1 to 1608 cm-1) (see
Table 1 Optimization of 1,3-DC between G1 or G2 and 1a.a
SI). This shift blue of the G band has been reported for graphite
oxide formation from graphite. It can be explained considering
H
N
the existence of sp3 carbon atoms on the edges that may create
CO2Me
Solvent
T (ºC)
single-double bond alternation within extended sp2 carbon
areas.[32] This is in agreement with the functionalization
achieved in this study.
Ph
N
CO2Me
+
time
1a
G1 G2
or
G1a G2a
or
Concerning the organic functionalization, the dispersed
MLG G1 (in pyridine) and G2 (in NMP) were independently
submitted to several conditions and solvents. In order to
optimize the 1,3-DC with 1a. Usually, 30 mL of this suspension
were added to xylenes (50 mL) containing the target imino ester
1a. The mixture of xylenes was employed as co-solvent because
of its low cost and low toxicity, large temperature range and
good dispersion achieved during the reaction. G1 and 1a were
used as probe and were firstly studied (Table 1, entries 1-3) and
the reaction resulted in the incorporation of 0.92 at.% of
nitrogen (XPS evidence) or 0.34at.% (TG evidence) (Table 1,
entry 1). Lower functionalization as determined by TG analyses
is reasonable because the thermal treatment of covalently
bonded groups on carbon materials, may produce the thermal
decomposition of the lateral groups thus remaining the N
heterocycle that can even be incorporated into the graphene
layers. In fact, the thermal treatment of a carbon material
modified with N-containing precursors is a method often used
for N-doping in carbon materials.[33] Many reactive fleeting
radical species could be formed, which reacted with MLG (Table
Ent.
1
Gx T (ºC)
t (d) Gxa N (%)b
N (%)c
0.34
0.36
0.35
0.11
0.02
0.37
0.37
0.12
0.21
0.30
0.37
0.10
G1 150-160
G1 150-160
G1 150-160
G1 150-160
G2 150-160
G2 150-160
G2 85-95
G2 85-95
G2 85-95
G2 85-95
G2 85-95
G2 85-95
5
5
5
5
5
5
5
1
2
4
6
10
G1a 0.92
G1a 0.98
G1a 0.95
---
---
G2a 1.00
G2a 1.00
G2a 0.33
G2a 0.57
G2a 0.80
G2a 1.00
G2a 0.26
2d
3e
4f
5f
6
7
8
9
0.30
0.05
10
11
12
a 0.3 mg of graphene (in 30 mL of pyridine or in NMP), xylene (50
mL), addition of imino ester (100-120 mg) each day. b Determined
c
d
by XPS analysis. Determined by TG analysis. The concentration
of the graphene suspension was twice higher. e Double amount of
f
the imino ester 1a (200 mg) was added per day. These
experiments were run in the absence of imino ester.
With the optimized reaction conditions in hand, we
1). Higher functionalization (0.98 at.%) was observed (Table 1, examined the scope of this thermal 1,3-DC employing the [1,2]-
entry 2) when double concentration of MLG was employed. The prototropy route for the generation of the reactive azomethine
daily addition of a double amount of imino ester 1a did not ylide 1 (Table 2). This study revealed that the reaction is
ameliorate the functionalization of G1 (Table 1, entry 3). In all compatible with a large library of structural motifs. In this
these previous experiments three different types of regard, the presence of a cyano group in o-, m- and p-
nitrogenated species were identified by XPS analysis, instead of substituents was introduced to analyze the functionalization by
the only one expected from the prolinate (see SI). Indeed, the subtractive-FTIR-ATR but this technique did not give the desired
expected pyrrolidinic nitrogen signal was flanked by two smaller information. Furthermore the presence of heterocycles in the
peaks at 399.2 eV and 401.2 eV, related to amine and imino moiety was also tolerated as revealed the reaction
quaternary nitrogen groups, respectively.[34] So, blank performed with imino ester 1e, obtained from 2-
experiments of both G1 and G2 were performed in order to pyridylcarboxaldehyde, to afford G2e with a high nitrogen
investigate the possibility of some nitrogen contribution from loading of 3.2 at.% (by XPS, Table 2). On the other hand, also the
the solvent. These two tests showed that G2 was nitrogen-free presence of hindered moieties in the imino ester, such as the
(0.05 at.% approx.) but a notable residual 0.30 at.% of nitrogen benzylic group in α-position, or a tert-butyl ester afforded very
was observed in G1 case (Table 1, entries 4 and 5 and SI).
good results in terms of incorporation of nitrogen with 1.6 at.%
2 | J. Name., 2012, 00, 1-3
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and 1.3 at.% for G2f and G2g respectively (Table 2). All
functionalized MLG G2a-G2g were performed twice with a
complete reproducibility.
View Article Online
Na
Cp*
DOI: 10.10C3l9/C9CC00939F
H
H
N
O
CO2Me
H
Ir
Ph
Ph
N
O
H
O
NaOH
(1M)
Ph
N
O
[Cp*IrCl2
]
2
All the G2a-g derivatives display a common peak comprised
between 400.1 to 400.7 eV, respectively, corresponding to the
prolinate nitrogen (see SI). A second peak appears in the case of
G2b-g samples where another nitrogen is present in the
cycloadduct, and its binding energy, respectively ranging from
399.4 and 399.8 eV, depends on the nature of the
corresponding nitrogen. In every case, the area of the signals is
coherent with the stoichiometry of the derivative (see SI). All
the spectra present an almost negligible signal at around 401.0
eV due to some NMP adsorption.
Concerning the stereoselectivity of the reaction, the relative
configuration of C2 and C5 in the proline cycloadduct was not
confirmed (Table 2). The normal trend of this mechanism
predicts a 2,5-cis arrangement as a consequence of the
existence of the intermediate W-shape 1,3-dipole,[35] but in
many other examples, performed at higher temperatures (e.g.
150-160 ºC), the presence of 2,5-trans arrangement was also
favored.[35][36]
Me2CO
rt, 19 h
DCM
rt, 19 h
G2a
Non-isolated intermediate
G3a
, 0.8% content of Ir
Scheme 1 Synthesis of covalently bonded iridium-graphene
complexes G3a.
N1s XPS of G3a displayed a principal peak at 402.2 eV,
corresponding to a highly oxidized nitrogen, which is in
agreement with a prolinate nitrogen coordinated to Ir centers.
This signal is flanked by a second one, less intense, at 400.10 eV,
that corresponds to some unreacted prolinate nitrogen. The
ratio between the area of these two peaks allowed to estimate
the yield of the coordination reaction to about 75%. An almost
negligible signal at 401.2 eV due to some NMP adsorption on
the MLG edges is also found. The presence in the sample of
traces of NaCl, coming from the ester hydrolysis, prevents the
iridium XPS characterization, as sodium shares the same
spectral window of the highest intensity signal from iridium.
G3a was then evaluated in the Meerwein–Ponndorf–Verley
(MPV) reduction of acetophenone through a hydrogen transfer
from isopropyl alcohol, IPA, (Table 3).[39] The amount of iridium
involved was about 0.12 mol%, corresponding to 40 mg of Ir-
MLG. The amount of isopropyl was crucial because more diluted
reaction mixture (using 3 mL of IPA per 1.5 mmol of
acetophenone) only afforded a 22% yield after 1.5 d (Table 3,
entry 2). However, the reaction performed with 1 mL of IPA led
to a good conversion and yield (82%, Table 3, entry 3). In all the
examples, the average of the recovery of the graphene hybrid
material was ≥90% after careful centrifugation. Functionalized
graphene G2a did not catalyze this reaction (Table 3, entry 1).
Several cycles (up to four) with the same iridium catalyst G3a
were performed (Table 3, entries 4-6). The maximum chemical
yield of 1-phenylethanol was achieved diluting the reaction with
1.5 mL of IPA with 1.5 mmol of acetophenone and refluxing for
1.5 d (Table 3, entry 7). The recovered iridium-graphene
complex was analyzed via ICP-Mass and no significant iridium
leaching was detected, so 0.12 mol% of Ir can be considered the
limit for the reaction completion and lower amounts afforded
lower chemical yields. Entry 8 of Table 3 confirmed this fact.
Table 2 Substrate scope.a
H
CO2R2
R1
Ar
N
5
2
NMP/Xy
85-90 ºC
Ar
N
CO2R2
+
5 d
R1
1a
G2
G2a g
-
H
H
H
H
CO2Me
H
CO2Me
H
CO2Me
H
CO2Me
3-(CN)C6H4
N
N
N
2-(CN)C6H4
N
4-(CN)C6H4
Ph
H
G2c
G2d
G2a
G2b
(0.57% N content, XPS)
(0.36% N content, TG)
(1.20% N content, XPS)
(0.50% N content, TG)
(1.00% N content, XPS)
(0.37% N content, TG)
(4.20% N content, XPS)
(1.46% N content, TG)
H
H
H
CO2Me
CO2tBu
N
CO2Me
2-Pyridyl
N
N
4-(CN)C6H4
4-(CN)C6H4
H
H
Bn
G2g
G2e
G2f
(1.60% N content, XPS)
(0.61% N content, TG)
(3.20% N content, XPS)
(1.51% N content, TG)
(1.30% N content, XPS)
(0.81% N content, TG)
a 0.3 mg of graphene (in 30 mL of NMP), xylene (50 mL), addition of
imino ester (100-120 mg) each day, 100 ºC, 5 d. Nitrogen content
was determined by XPS analysis and represents the overall content.
These results pushed us to investigate the possibility of Here, the amount of the ketone was adjusted to the amount of
using this new type of functionalized MLG as metal centers functionalized MLG recovered from the reaction shown in entry
support in the preparation of fine tunable heterogeneous 7 of Table 3. The experiments described in entries 3, 7 and 8 of
catalysts. To this purpose G2a was used as heterogeneous Table 3 were performed up to three times demonstrating a
ligand to prepare the iridium complex G3a. In this synthesis, reliable reproducibility for this process.[40][41]
G2a was firstly treated with sodium hydroxide (1 M, 1 mL/mg of
G2a) in acetone (1 mL/mg of G2a) in order to hydrolyze the
Table 3 Hydrogen transfer reduction of acetophenone.a
ester moiety and the resulting sodium salt was directly allowed
G3a
(0.12 mol% Ir)
to react with [Cp*IrCl2]2 (0.5 mg/mg graphene) affording the
desired complex G3a. In this new material, the prolinate
cycloadduct behave as a N,O-chelating ligand and forms with
the iridium center a stable five member ring (Scheme 1).[37] This
new metallated MLG presented an Ir loading of 0.8% in mass
(ICP- mass evidence).[38] This procedure was repeated three
times obtaining identical G3a samples.
O
K2CO3 (0.5 mol%)
OH
Ph
IPA (reflux)
1.5 d
Ph
1.5 mmol
Entry
G
G2ac
G3ad
Cycle
---
---
G3 recov. (%)
90
90
Yield (%)b
---
22
1
2
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3
4
5
6
7
8
G3a
1st
92
90
91
89
92
90
82
73
52
32
85
85
G3ae
G3af
G3ag
G3ah
G3ah,i
2nd
3rd
4th
1st
5
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a See experimental section for details. b Determined 1H NMR. c Pure
8
9
d
e
G2a was used. 3 mL of IPA were added instead of 0.5 mL. 0.12
mol% of Ir/1.5 mmol of acetophenone was used. f 0.12 mol% of Ir/1.5
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Conclusions
The 1,3-DC of dispersed MLG and stabilized azomethine ylides
originated from thermal 1,2-prototropy imino esters occurred
in unexpected milder conditions than the reported ones for the
analogous 1,3-DC with azomethine ylides generated via
decarboxylative route. To the best of our knowledge, this is the
first example of a graphene material functionalized via imino
esters-based (1,3-DC). This is originated by the stability of the
azomethine ylide. In addition, this ester moiety could be
hydrolyzed and used as oxygen donor site to coordinate iridium
centers, together with the prolinate nitrogen. This Ir-MLG was
very efficient (more than the [Cp*IrCl2]2 itself and iridium
complex anchored to fullerene[42]) in heterogeneous catalyst of
the MPV reduction of acetophenone. G3a proved to be enough
robust (no metal leaching was observed) to be recycled up to
four times. All these set of transformations are highly 26 U. Khan, A. O’Neill, M. Loyta, S. De, J. N. Coleman, Small, 2010, 6, 864.
27 M. Quintana, E. Vázquez, M. Prato, Acc. Chem. Res., 2013, 46, 138.
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29 D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano
reproducible. Considering the wide library of imines available
for this type of reaction, this approach combines the possibility
of a fine tune of the coordination sphere of the metal with the
advantages of a graphene type heterogeneous support.
Lett., 2007, 7, 238.
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Conflicts of interest
51, 11727.
32 K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud’homme, I. A. Aksay, R. Car,
There are no conflicts to declare.
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33 (a) Y. Deng, Y. Xie, K. Zoua, X.. Ji J. Mater Chem. A, 2016, 4, 1144. (b) D. Salinas-
Author information
Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós. Carbon 2015, 82, 205. (c) M.
J. Mostazo-López, D. Salinas-Torres, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Materials 2019, 12, 1346.
Corresponding Author: * cazorla@ua.es, * jmsansano@ua.es
Author Contributions: 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 in the experimental work.
34 E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J. Find, U. Wild, R.
Schlögl, Carbon, 2002, 40, 597.
35 V. Selva, E. Selva, P. Merino, C. Nájera, J. M. Sansano, Org. Lett., 2018, 20, 3522.
36 This trans-arrangement was observed during the reaction of C60-fullerene at rt using
a chiral copper(II) complex: M. Suárez, A. Ruíz, L. Almagro, J. Coro, E. E. Maroto,
S. Filippone, D. Molero, R. Martínez-Álvarez, N. Martín, J. Org. Chem., 2017, 82,
4654.
Acknowledgements
We gratefully acknowledge financial support from the Spanish
Ministerio de Economía y Competitividad (MINECO) (projects CTQ2013-
43446-P and CTQ2014-51912-REDC), the Spanish Ministerio de
Economía, Industria y Competitividad, Agencia Estatal de Investigación
37 J. Marco-Martínez, S. Vidal, I. Fernández, S. Filippone, N. Martín, Angew. Chem.
Int. Ed. Engl., 2017, 56, 2136.
(AEI) and Fondo Europeo de Desarrollo Regional (FEDER, EU) (projects 38 When tert-butyl ester G2g was allowed to react with trifluoroacetic acid in DCM
(see ref. 34) and treated with [Cp*IrCl2]2 (0.5 mg/mg graphene) no incorporation of
iridium was detected after XPS analysis of G3g.
39 (a) O. Saidi, J. M. J. Williams, Top. Organomet. Chem., 2011 34, 77. (b) U.
CTQ2015-66080-R, CTQ2016-76782-P and CTQ2016-81797-REDC), the
Generalitat Valenciana (PROMETEOII/2014/017) and the University of
Alicante.
Hintermair, J. Campos, T. P. Brewster, L. M. Pratt, N. D. Schley, R. H. Crabtree.
ACS Catal., 2014, 4, 99.
40 To get a 90% yield in identical conditions, in the presence of [Cp*IrCl2]2, a 1 mol%
of this catalyst was required.
41 This process was not so fast as the reported using N-Heterocyclic Carbenes by
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4 | J. Name., 2012, 00, 1-3
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