Covalently functionalized carbon nanoparticles with a chiral Mn-Salen: A new nanocatalyst for enantioselective epoxidation of alkenes

Article abstract of DOI:10.1039/c9cc01825e

A new protocol to obtain carbon nanoparticles (CNPs) covalently functionalized with a chiral Mn-Salen catalyst is described here. The new nanocatalyst (CNPs-Mn-Salen) was tested in the enantioselective epoxidation of some representative alkenes (CN-chromene, 1,2-dihydronaphthalene and cis-β-ethyl styrene), obtaining better enantiomeric excess values than that of the catalyst single molecule, highlighting the role of the nanostructure in the enantioselectivity.

Full text of DOI:10.1039/c9cc01825e

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Sfrazzetto, A. Pappalardo, N. Tuccitto, A. Zammataro, C. M. A. Gangemi, R. Puglisi, R. M. Toscano, M. E.
Fragala and G. Nicotra, Chem. Commun., 2019, DOI: 10.1039/C9CC01825E.
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DOI: 10.1039/C9CC01825E
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Covalently functionalized carbon nanoparticles by a chiral Mn-
Salen: a new nanocatalyst for enantioselective epoxidation of
alkenes
Received 00th January 20xx,
Accepted 00th January 20xx
Agatino Zammataro,a Chiara Maria Antonietta Gangemi,a Andrea Pappalardo,a,b Rosa Maria
Toscano,a Roberta Puglisi,a Giuseppe Nicotra,c Maria Elena Fragalà,a Nunzio Tuccittoa,d and
Giuseppe Trusso Sfrazzetto*a,b
DOI: 10.1039/x0xx00000x
A new protocol to obtain Carbon Nanoparticles (CNPs) covalently wide range of application fields, such as analytical,16,17
functionalized with a chiral Mn-salen catalyst is here described. biosensing,18,19 bioimaging,20,21 theranostic22,23 molecular
The new nanocatalyst (CNPs-Mn-Salen) was tested in the communication24,25 and photocatalytic energy conversion.26-28
enantioselective epoxidation of some representative alkenes (CN- However, very few examples of CNPs used as catalysts have
chromene, 1,2-dihydronapthalene and cis--ethyl styrene), been reported.29-37 Among these, no example of catalyst for
obtaining better enantiomeric excess value respect to the catalyst epoxidation has been described.
single molecule, highlighting the role of the nanostructure in the Although the one pot synthesis of CNPs is green, simple and
enantioselectivity.
versatile, the obtained CNPs derivatives do not display yet the
properties reached by covalent functionalization of the
external shell. Attractive hybrid approaches that combine the
one-pot synthesis followed by covalent functionalization of the
functional groups of the external shell may lead to a versatile
and efficient preparation of new CNPs having different surface
functionalities. Thus, decorating CNPs with a functional
substrate/catalyst is a promising option to enhance their
properties.
Here, the first example of covalent functionalization of CNPs
with a chiral Mn-salen catalyst (Mn-Salen-OH, Scheme 1) is
reported. The salen ligand has been functionalized in order to
covalently react with carboxylic groups on the surface of the
native CNPs, leading to an ester bond. The new nanostructure
was tested as catalyst for the enantioselective epoxidation of
alkenes, by using 1,2-dihydronaphtalene, CN-chromene and
Olefin epoxidation is one of the most important and useful
reaction due to the possibility to obtain a wide range of
organic derivatives. This reaction found application in several
technological fields.1 In this context, chiral Mn(III)-salen
derivatives play a central role due to the huge employment in
the synthesis of chiral epoxides, which represent an important
target for the pharmaceuticals and industries.2-4 However, the
exposure to transition metal ions, in particular manganese, at
concentrations more than 5 mg/m-3, can lead to important
neurological disorders.5 Therefore, many efforts have been
devoted to reduce the amount of this metal ion into the
catalytic systems. One of the most used strategy is to
heterogenize the Jacobsen catalyst onto a solid surface.6-13
Another strategy is to obtain nanocatalysts,14 in which the
amount of metal atoms is significantly reduced respect to the
standard catalysts.
cis-
synthetic pathway to obtain catalyst Mn-Salen-OH
Salicylaldehyde was chloromethylated by using aqueous
formaldehyde in hydrochloric acid, obtaining aldehyde
which, after reaction with CuSO4 in DMSO/H2O, was converted
into 5-hydroxymethyl-2-hydroxy-benzaldehyde 2 38
The chiral
salen ligand was obtained by reaction of with (1R,2R)-
diphenyl-ethylendiamino-monochloride derivative 3 39-40
in the
-ethyl styrene as model substrates. Scheme 1 shows the
.
Carbon Nanoparticles (CNPs) are a new class of carbon
nanomaterial, which display interesting photo-chemical and
redox properties.15 CNPs are nanoparticles about 10 nm of
1
diameters, with
a quasi-spherical shape. Due to their
.
properties, including high optical and chemical stability, good
water solubility, photobleaching resistance, biocompatibility
and low cost, nowadays CNPs are successfully applied in a
4
2
,
presence of triethylamine. Then, manganese was introduced
into the salen ligand obtaining Mn-Salen-OH in almost
quantitative yield.41,42 All new compounds have been
characterized by NMR and ESI-MS (see ESI†). The presence of
hydroxylic group on the salen backbone allows to anchor this
catalyst on the surface of native CNPs. These, in fact, contain
several free carboxylic groups on their surface which, after
activation, can react with Mn-Salen-OH leading to a stable
a.Department of Chemical Sciences, University of Catania, Viale Andrea Doria 6,
95125, Catania, Italy. E-mail: giuseppe.trusso@unict.it
b.INSTM Udr of Catania, Viale Andrea Doria 6, 95125, Catania, Italy.
c. CNR-IMM, Zona industriale strada VIII, 5, 95121 Catania, Italy
d.Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN), Department
of Chemical Sciences, University of Catania and CSGI, Italy.
† Electronic Supplementary Information (ESI) available: Synthetic procedures,
characterization of compounds, NMR, ESI-MS, XPS spectra, SEM and TEM images.
See DOI: 10.1039/x0xx00000x
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covalent bond. Native CNPs were synthesized by hydrothermal
decomposition.24
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DOI: 10.1039/C9CC01825E
Figure 1. Al K

excited XPS of the CNPs sample, measured in the a) Mn 2p, b) O
1s and c) N 1s binding energy region. Structure due to satellite radiation has
been subtracted from the spectra.
Figure 1a shows the XPS Mn 2p3/2,½ spin-orbit components at
642.3 and 654.1 eV, partially overlapped with the high energy
shake-up satellites typical of Mn(III) species,12,13,43,44 while the
XPS spectrum of the O 1s states for the CNPs-Mn-Salen sample
shows two evident signals at 532.5 and 530.2 eV (Figure 1b).
The first peak is largely due to the SiO2 substrate, the shoulder
at lower binding energy is assigned to chemical shift related to
O-Mn and –COOH groups. Moreover, the XPS of CNPs-Mn-
Salen sample in the N 1s binding energy region exhibits a
signal at 400.0 eV (Figure 1c), since the Mn-salen complex
possesses two imine groups.12,13 Although XPS is a surface
sensible analytical technique, since the diameter of the CNPs is
smaller24 than the sampling depth of the XPS, we assumed to
probe the whole CNPs particle. Considering that the area of a
Mn-Salen molecule is 1.4 nm2,13 we expect, in the case of
complete surface coverage, 150 atoms of Mn per CNP having
diameter of 7 nm. The XPS atomic concentration ratio C/Mn
found is 195, corresponding to 55 atoms of Mn per CNP
matching with a coverage of 33%. Taking into account the
well-known adventitious carbon contamination, omnipresent
in all air-exposed materials, the coverage is satisfactory in view
of the catalytic purpose of such organic nanostructure.
Scheme 1. Synthesis of Mn-Salen-OH. Reagents and conditions: a) formaldehyde
(37% aqueous solution), concentrated HCl, 90 °C, 16h, 85%; b) CuSO4,
H2O/DMSO (1/2), 110 °C, 2 h, 87%; c) Et3N, EtOH, RT, 16h, 65%; d) Mn(OAc)3,
EtOH, RT, 16h, 95%.
The covalent functionalization of their surface is shown in
Scheme 2. Carboxylic groups of CNPs (CNPs-COOH) were
activated by using pentafluorophenol and EDAC x HCl (N-(3-
Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride), in
solvolysis at 50°C overnight. The functionalized CNPs (CNPs-
PFF) were purified by extraction in water/dichloromethane,
due to the higher solubility in organic solvent respect to the
starting CNPs-COOH. Then, reaction of CNPs-PFF with a large
excess of Mn-Salen-OH leads to the nanocatalyst CNPs-Mn-
Salen, which was purified by dialysis. CNPs-Mn-Salen has been
1
1
characterized by H NMR, XPS, TEM and SEM. In particular, H
NMR spectrum shows broad signals, according to the presence
of manganese metal ion, in the region relative to the salen
complex (see ESI†).
-
Figure 2. TEM BF-TEM acquired at 200KeV in low e dose, show the presence of small
CNPs with an average size of 8nm.
TEM analysis reveals the presence of nanoparticles having an
average diameter of 8 nm (Figure 2). Moreover, images taken
at high resolution (HR-TEM, see ESI†, Fig. S11) reveal the
presence of graphite-like carbon crystal structure in the
nanoparticles core. The external shell around the CNPs is even
visible as a slightly dark contrast around the core of the CNPs
(inset of Figure 2). This is compatible with the presence of the
functionalized complex anchored onto the CNPs' surface. SEM
analysis is in good agreement with TEM results. SEM images
Scheme 2. Functionalization of CNPs with the chiral Mn-Salen-OH.
The XPS spectrum of the as prepared CNPs-Mn-Salen sample
shows a C 1s band, at 285.0 eV (Figure S8, see ESI†), partially
asymmetric due to chemical shift of C 1s related to carboxylate
species having at higher binding energy.
2 | J. Name., 2012, 00, 1-3
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confirm the formation of carbon nanoparticles with dimension excess values are 95-96%, higher respect to those obtained
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ranging from few nanometers (see ESI†, Fig. S10 red arrows) with Mn-Salen-OH.‡ These results suggest that the presence of
DOI: 10.1039/C9CC01825E
up to 20-40 nm. However, particles aggregation on substrate CNPs increase the enantiomeric excess, in the case of CN-
cannot be excluded during solvent evaporation.
chromene, although the conversion is low. The origin of the
Preliminary epoxidation results by using CNPs-Mn-Salen are high enantioselectivity in the Jacobsen’s epoxidation has been
shown in Table 1.
extensively studied,46 but until now, not fully elucidated. The
most common rationalization is attributed to the directions of
approach of the alkene to the manganese active site
(manganese-oxo). Cavallo and co-workers reported theoretical
calculations that support this hypothesis.47 More recently, a
new justification about the origin of the enantioselectivity was
proposed by Corey and coworkers.48 In this hypothesis, the
transition state assembly is slightly similar to a [3 + 2]
cycloaddition between alkene and the catalyst. A possible
explanation of the high enantioselectivity with CN-chromene
can be ascribe to the chemical structure of this alkene. In
particular, due to the presence of an oxygen atom in the fused
ring system, some possible interactions can occur with the
surface of the CNPs (e. g. H-bonding with some
unfunctionalized groups) that reduce the reactivity, leading to
low conversion values but, at the same time, increase the
geometric constrains, thus obtaining high enantiomeric excess
values.
Table 1. Enantioselective epoxidation of 1,2-dihydronaphthalene, 6-CN-2,2-
dimethylchromene and cis--ethyl styrene with NaClO catalyzed by CNPs-Mn-Salen in
CH2Cl2 at 25 °C.a
[Nanocat.] Time
e.e.
(%)c
Conv.
(%)c
TONf
(TOF)g
8264
(8264)
9917
(4959)
12397
(1033)
2893
(2893)
4050
(337)
4132
(172)
1488
(1488)
2397
(200)
2479
C.B.
(%)h
Alkene
Entry
(mg/mL)b
(h)
1
2
3
4
5
6
7
8
9
0.05
1
80d
81d
80d
79d
10
12
15
35
97
95
94
95
93
0.05
0.05
0.5
12
24
1
0.5
12
24
1
80d
80d
49
50
92
0.5
(82)‡ (100)‡
(97)‡
Epoxidation reactions with cis--ethyl styrene show good
0.5
96e
18
96
94
conversion values after 1 hour (59%, Table 1, entry 10), higher
respect to the other examined alkenes. This higher reactivity
leaded to lower enantiomeric excess values (~71%, Table 2,
entries 10-12) respect to the other substrates, as well as
respect to the Mn-Salen-OH (Table 1, entry 13).‡ Furthermore,
the [cis]/[trans] ratio (~7, with ~50% of e.e. values) suggests a
short lifetime of the radical intermediate, invoked by Jacobsen
and co-workers,49 leading to the partial isomerization of the
radical intermediate.
Recycling tests were performed by using 1,2-
dihydronaphtalene as selected substrate (see ESI†, Fig. S9).
CNPs-Mn-Salen can be recovered from the reaction media by
extraction with CH2Cl2 (to remove the aqueous phase), and
vacuum treatment at 100 °C for two hours (to remove the
organic products/byproducts). After 4 cycles, the enantiomeric
excesses remained almost unaltered, while conversion values
decrease from 50% to 25%, suggesting a partial degradation of
the nanocatalyst.
In summary, a new synthetic protocol for the covalent
functionalization of CNPs was here reported. In particular, we
anchored a chiral Mn-salen catalyst onto CNPs, leading to a
new nanocatalyst able to catalyze the enantioselective
epoxidation of some selected alkenes. This new catalytic
system, using CN-chromene as substrate, permits to obtain
chiral epoxides, with higher enantiomeric excess values
respect to the molecular catalyst. We are still testing this
nanocatalyst with other substrates in order to shed light on
the origin of this enantioselectivity. Preliminary results
revealed that the role of the nanostructure is essential in the
increasing of enantioselectivity. Respect to the commercial
catalysts used for the oxidation of organic substrates,50 chiral
Mn-Salen allows to control the enantioselectivity of the
reaction. Furthermore, the functionalization of carbon
0.5
12
24
96e
95e
29
30
94
0.5
(75)‡ (100)‡
(96)‡
(103)
e.e.cis
Time
(h)
Conv.
TONf
C.B.
(%)h
Alkene Entry
e.e.trans cis/trans
(%)c
59
(TOF)g
(%)c
71d
4876
(4876)
5372
(448)
5455
(227)
1176
(49)
10i
11i
12i
13‡
1
7
7
7
5
97
95
95
97
52d
71d
12
24
24
65
66
50d
72d
50d
85d
100
59d
a In all experiments [alkene] = [NaClO] =1.17 x 10-2 M, buffered with 1 mL of 0.05
M Na2HPO4 at pH 11.2 in a total volume of 2 mL; stock solution of CNPs-Mn-
b
Salen was prepared dissolving 5 mg of the nanocatalyst in 5 mL of CH2Cl2;
cdetermined by GC analysis using a chiral column (see ESI
†) and n-dodecane as
internal standard; d config. (1R,2R) determined by measuring the optical rotation;
e
f
config. (3R,4R) determined by measuring the optical rotation; TON = [overall
products]/[ catalyst]; g TOF = TON/reaction time (h); h Carbon Balance C.B. = (total
carbon detected/total carbon feed);45 i [CNPs-Mn-Salen] = 0.5 mg/mL.
1,2-dihydronaphtalene was chosen as substrate to setup the
epoxidation conditions. In particular, by using a nanocatalyst
concentration of 0.05 mg/mL, good enantiomeric excess but
low conversion values were obtained (Table 1, entries 1-3).
Increasing the concentration of CNPs-Mn-Salen (0.5 mg/mL)
the conversion values have been increased (Table 1, entries 4-
6). This setup was used with CN-chromene (Table 1, entries 7-
9) and cis--ethyl styrene (Table 1, entries 10-12). In particular,
with CN-chromene, conversion values are lower respect to
those obtained with 1,2-dihydronaphtalene, but enantiomeric
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J. Name., 2013, 00, 1-3 | 3
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20 S. T. Yang, L. Cao, P. J. G. Luo, F. S. Lu, X. Wang, H. F. Wang,
materials with Mn-Salen catalysts permits to reduce the
amount of Mn in the reaction media and, in addition, to
improve the stability of these nanocatalysts in alkaline
conditions, required by the presence of NaClO as oxidant.
Basic conditions, in fact, are harmful for the silica supports,
which are largely used for the heterogenization of Jacobsen’s
catalysts.
The authors thank Prof. Antonino Gulino and Dr. Luca Spitaleri
for XPS measurements. This work was supported by the
University of Catania, Department of Chemical Sciences (Piano
per la Ricerca 2016-2018– Linea Intervento 2).
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‡ Control reaction was performed by using [Mn-Salen-OH] =
8.50 x 10-4 M, [alkene] = [NaClO] = 1.17 x 10-2 M, buffered with 1
mL of 0.05 M Na2HPO4 at pH 11.2 in a total volume of 2 mL. With
1,2-dihydronaphtalene as substrate, after 24h, total conversion
and 82% of enantiomeric excess were obtained. With CN-
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enantiomeric excess were obtained. With cis--ethyl styrene as
substrate, after 24h, total conversion and 85% of enantiomeric
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Covalently functionalized carbon nanoparticles by a chiral Mn-Salen: a new
DOI: 10.1039/C9CC01825E
nanocatalyst for enantioselective epoxidation of alkenes
Agatino Zammataro, Chiara Maria Antonietta Gangemi, Andrea Pappalardo, Rosa Maria Toscano, Roberta
Puglisi, Nunzio Tuccitto, Giuseppe Trusso Sfrazzetto
The first nanocatalyst, obtained via “step-by-step” functionalization of CNPs, for enantioselective
epoxidation of non-functionalized alkenes is here reported