Sulfur-stabilised copper nanoparticles for the aerobic oxidation of amines to imines under ambient conditions

Article abstract of DOI:10.1039/d0ta12621g

The stabilisation of metal nanoparticles and control of their oxidation state are crucial factors in nanocatalysis. Elemental sulfur has been found to be a cheap and effective stabilising agent for copper nanoparticles in the form of copper(i) oxide. The

Full text of DOI:10.1039/d0ta12621g

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Materials Chemistry A
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Sulfur-stabilised copper nanoparticles for the
aerobic oxidation of amines to imines under
ambient conditions†
Cite this: J. Mater. Chem. A, 2021, 9,
11312
Iris Martın-Garcıa, Gloria Dıaz-Reyes,
George Sloan,‡ Yanina Moglie
§
´
´
´
*
and Francisco Alonso
The stabilisation of metal nanoparticles and control of their oxidation state are crucial factors in
nanocatalysis. Elemental sulfur has been found to be a cheap and effective stabilising agent for copper
nanoparticles in the form of copper(I) oxide. The Cu2ONPs/S8 system has been characterised by ICP-
OES, EDX, XRD, XPS, FE-SEM, SEM, TEM and Cryo-EM. Astonishingly, in organic medium, the copper
nanoparticles are organised as concentric rings within nanodroplets of sulfur of ca. 20–70 nm. In
synthetic organic chemistry, imines can be directly obtained by the less studied and practiced oxidation
of primary amines; however, the reaction conditions utilised are usually harsh and far from meeting the
principles of Green Chemistry. Cu2ONPs/S8 has been successfully applied to the solvent-free aerobic
oxidation of primary amines to imines under ambient conditions, using air as a terminal oxidant. The
catalyst is effective in the homo- and heterocoupling of benzylic amines at very low copper loading
(0.3 mol%), being catalytically superior to a range of commercial copper catalysts. A reaction mechanism
has been proposed based on experimental evidence, which clarifies the major uncertainty regarding the
key intermediate. The results of this study suggest a number of new avenues for research in nanocatalysis.
Received 31st December 2020
Accepted 13th April 2021
DOI: 10.1039/d0ta12621g
rsc.li/materials-a
stabilising agents, which achieve a larger inter-particle separa-
tion by electrostatic, steric or electrosteric interactions.1b
Introduction
Although the application of stabilising agents for NPs and their
role is quite well established, research on novel effective and
inexpensive anti-agglomeration agents is welcome.
Nanocatalysis has experienced a vertiginous growth in the last
two decades.1 In particular, the development of selective and
efficient transition metal nanoparticles (NPs) represents a key
area in catalyst development.2 These NPs oen exhibit superior
catalytic activity to their bulk metal counterparts, derived from
their high specic surface area and abundance of active cata-
lytic sites.3 Therefore, research into their potential applications
can lead to the discovery of new catalytic routes or the
improvement of existing catalytic activities and selectivities.
However, NPs are susceptible to agglomeration into larger
clusters during their synthesis, use as catalysts and/or when
stored over time. This phenomenon normally curtails the
catalytic activity of the NPs but can be prevented by the use of
Copper occupies a privileged position within the realm of
transition metals because copper is relatively earth-abundant,
cheap, has low toxicity and is catalytically active in its
different oxidation states. That is why, in the context of nano-
catalysis, copper nanoparticles (CuNPs) are a competitive
alternative to the more expensive precious transition metal
nanoparticles and have gained ground, for instance, on PdNPs
in multiple organic transformations.4 Although bulk copper-
catalysed organic reactions have been widely practiced under
aerobic conditions,5 the application of CuNPs in air is more
limited because of their tendency to undergo uncontrolled
oxidation.6 Therefore, it is challenging to develop catalytic
systems based on CuNPs, where Cu is in a neat lower oxidation
state [i.e., Cu(0) or Cu(I)] and resistant to oxidation over time.7
Elemental sulfur is produced as a by-product during the
hydrodesulfurisation of crude oil and natural gas. It has limited
applications as a reactant, beyond the production of sulphuric
acid, though is an economic, simple and versatile reagent and
promoter in multiple organic transformations.8 Its application
in materials science has received comparatively little attention,
with most contributions published during the last decade.9 The
introduction of elemental sulfur as a tool to develop metal
´
´
´
´
Instituto de Sıntesis Organica and Departamento de Quımica Organica, Facultad de
Ciencias, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain. E-mail:
falonso@ua.es
† Electronic supplementary information (ESI) available: General, synthetic
procedures, compound characterisation, NMR, XPS, Auger and XRD spectra, EM
micrographs, physisorption and droplet-size distribution graphics. See DOI:
10.1039/d0ta12621g
‡ Visiting Erasmus student at the University of Alicante. On leave from the
University of Edinburg, United Kingdom.
´
´
§ Present address: Departamento de Quımica Organica, INQUISUR, Universidad
´
Nacional del Sur (UNS)-CONICET, Avenida Alem 1253, Bahıa Blanca, 8000,
Argentina.
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nanoparticles with novel or improved properties is barely
Owing to our interest in developing efficient synthetic
documented, noteworthy exception being its use as a novel methodologies based on transition-metal nanoparticles,28 we
medium to synthesise isolated ligand-capped AuNPs or present herein the unprecedented stabilising effect of elemental
a vulcanised sulfur-AuNPs nanocomposite.10 Although copper sulfur on CuNPs and their application in the aerobic oxidation
sulde nanoparticles have been widely studied and applied,11 to of primary amines to imines under ambient conditions.
the best of our knowledge, the potential use of elemental sulfur
to stabilise CuNPs remains unexplored.
Imines (Schiff bases) are versatile organic compounds used
as intermediates in the synthesis of nitrogen-containing
heterocyclic compounds, as well as building blocks for the
Results and discussion
Optimisation of the catalyst and reaction conditions
synthesis of ne chemicals and pharmaceuticals.12 Imines can
undergo organic transformations through the carbon–nitrogen
double bond leading, among others, to the very useful amines.13
Many Schiff-based complexes have demonstrated an excellent
activity in homogeneous and heterogeneous catalysis.14 Imines
have traditionally been prepared through the condensation of
carbonyl compounds and amines (Scheme 1a).15 In recent years,
signicant progress has been made in the synthesis of imines
with alternative starting materials, using molecular oxygen or
air as green terminal oxidants. These new approaches include
the oxidative cross dehydrogenation of alcohols and amines
(Scheme 1b),16 the oxidative dehydrogenation of secondary
amines (Scheme 1c)17 and the direct oxidation of primary
amines (Scheme 1d).18
The transformation of primary amines into imines has lately
attracted a great deal of attention,18 achieved variously through
transition-metal catalysis using precious (Au,19 Ru,20 Ir,21 etc.)
and non-precious metals (Cu,22 Co,23 Mn,24 Fe,25 etc.), metal-
free26 and photochemical protocols.27 Even though these
procedures have proven to be generally effective in this trans-
formation, most of the catalytic systems are sub-optimal from
the Green Chemistry viewpoint. For instance: (a) the common
harsh conditions ($100 ꢀC) applied under transition-metal
catalysis should be replaced by ambient conditions, preferably
using low-loading cheap metals; (b) the use of organic solvents
(e.g., PhMe, MeCN, THF, MeOH, CHCl3) should be minimised
in favour of solvent-free systems; (c) the catalytic systems should
be simplied whenever possible, avoiding the inclusion of
bases, ligands, co-oxidants, etc.; and (d) aerobic oxidations
should be prioritised over the deployment of stoichiometric
oxidants. Although the utilisation of molecular oxygen has
become widespread in these reactions, the development of
catalytic systems based on air that meet the aforementioned
guidelines is more challenging.
In our commitment to search for the optimum catalyst, an
initial study was conducted by screening CuNPs on different
supports and using the oxidation of benzylamine (1a) to (E)-N-
(benzylidene)benzylamine (2a) as a model reaction in air as
a terminal oxidant. The catalysts were prepared by reduction of
anhydrous CuCl2 with metal lithium in THF under argon, in the
presence of 4,40-di-tert-butylbiphenyl as an electron carrier,29
followed by the addition of the support.30 The generation of
Cu(0)NPs was evidenced by the formation of black or deep-grey
suspensions. All suspensions were subjected to ltration,
ltrate washing and drying in air, that is why the resulting
materials were obtained in the form of copper oxides.30 In the
case of elemental sulfur, the catalyst was obtained by preparing
a homogeneous mixture of CuCl2 in ethanol, followed by the
addition of S8 and reduction with NaBH4 in air (Scheme 2).
Similarly as above, Cu(0)NPs were rapidly formed and the
copper–sulfur stuff was subjected to the same work-up protocol
in air. In this case, we have noticed that the order of addition of
the reagents is important: substantial metal leaching occurs
during the washing step if NaBH4 is added before sulfur, as
a sign of the loss of the stabilisation effect apparently exerted by
sulfur.
Preliminary experiments with different solvents and reaction
temperatures allowed us to settle on room temperature and
absence of solvents as the best conditions for comparative
purposes (Table 1).
Control assays in the absence of catalyst conrmed the
necessity of a catalyst to trigger the reaction (Table 1, entries 1
and 2). Amongst the different catalysts tested, those based on
CuNPs on activated carbon, zeolite Y and elemental sulfur gave
the best results, with the latter two reaching quantitative
conversion (Table 1, entries 3, 6 and 13). It is noteworthy that
elemental sulfur alone promoted this transformation, albeit to
a lesser extent (Table 1, entries 15–17).31 In order to more
accurately discern on the suitability of CuNPs/ZY or Cu2ONPs/
S8, other amines were subjected to the action of both catalysts,
with Cu2ONPs/S8 showing higher conversions. Therefore, the
catalytic system of choice was that consisting of Cu2ONPs/S8,
Scheme 1 Different approaches to the synthesis of imines.
Scheme 2 Preparation scheme and picture of Cu2ONPs/S8.
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Table 1 Screening of different Cu catalysts in the oxidation of ben-
zylamine (1a)a
Entry
Catalystb (wt% Cu)
t (h)
Conversionc (%)
1
2
3
4
5
6
7
8
—
—
24
48d
24
24
24
24
24
24
24
24
24
3
6
24
3
6
24
0
4
89
7
6
>99
50
43
17
30
3
68
>99
>99
15
15
20
CuNPs/Ce (1.4)
CuNPs/MK-10f (1.8)
CuNPs/MgO (1.5)
CuNPs/ZYg (3.0)
CuNPs/Al2O3 (2.4)
CuNPs/K2S2O8 (2.5)
CuNPs/CeO2 (3.7)
CuNPs/ZnO (3.0)
CuNPs/TiO2 (3.0)
Cu2ONPs/S8 (1.3)h
Cu2ONPs/S8 (1.3)h
Cu2ONPs/S8 (1.3)h
Fig. 1 EDX spectrum of Cu2ONPs/S8.
9
10
11
12
13
14
15
16
17
h
S8
S8
S8
h
h
a
Reaction conditions: benzylamine (1a, 1 mmol), CuNPs/support (50
b
mg), neat, rt, air, unless otherwise stated. CuNPs refers to a mixture
c
of Cu2O and CuO. Conversion into 2a determined by GLC.
Fig. 2 XRD spectrum of Cu2ONPs/S8
(
denotes orthorhombic S).
d
g
e
f
Reaction at 70 ꢀC. Activated carbon. Montmorillonite K-10.
Zeolite Y. 20 mg.
h
EDX, Cu (32.3%), O (18.1%) and S (49.6%), gives a Cu/O ratio
(1.8) close to that expected for copper in the form of Cu2O. The
examination of Cu2ONPs/S8 by powder X-ray Diffraction (XRD)
(Fig. 2 and S2†) showed diffraction peaks corresponding to
orthorhombic sulfur; no signicant peak for copper was
observed due to the small crystal domains, low copper loading
or high dispersion.
The oxidation state of copper was ascertained by X-ray
Photoelectron Spectroscopy (XPS). For a comparative purpose,
we also analysed by XPS a sample of recently purchased Cu2S.
The XPS spectrum of Cu2ONPs/S8 and Cu2S at the Cu 2p3/2 level
look very similar, with peaks at 932.4 and 933.3 eV (Cu2ONPs/S8)
applied at room temperature under solvent-free conditions in
air.
Catalyst characterisation
The catalyst Cu2ONPs/S8 was characterised by different analyt-
ical and spectroscopic techniques, such as ICP-OES, EDX, XRD,
XPS, FE-SEM, SEM, TEM and Cryo-EM. The copper loading on
Cu2ONPs/S8 was determined to be 1.30 wt%, as analysed by
Inductively-Coupled Plasma-Optical Emission Spectroscopy
(ICP-OES). The obtained adsorption isotherm (N2 at 77 K) shows
that Cu2ONPs/S8 is a material with very low adsorption capacity
(Fig. S1†). The isotherm can be classied as a type II or pseudo-
type II, with a type-H3 hysteresis loop. This isotherm shape
indicates that the sample contains mesopores but not micro-
pores. The shape of the H3 loop is usually linked with the non-
rigid nature of the adsorbent.32 In agreement with the low
adsorption capacity, the calculated BET surface area is 22.7 m2
gꢁ1. A pore volume of 0.085 cm3 gꢁ1 has been estimated by
applying the BJH model to the desorption branch data, with an
average size of 5.5 nm (although the BJH shows other small
relative maxima, they are not signicant considering the low
pore volume).32 Energy-Dispersive X-ray (EDX) analysis on
various regions conrmed the presence of sulfur (K line, 2.31
keV), oxygen (K line, 0.53) and copper, the latter with energy
bands of 8.04, 8.90 keV (K lines) and 0.93 keV (L line) (Fig. 1); the
presence of carbon is due to the use of a carbon support during
the analysis. The estimated supercial atomic distribution by Fig. 3 XPS spectrum at the Cu 2p3/2 level of Cu2ONPs/S8.
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Auger spectroscopy on Cu2ONPs/S8 and Cu2S samples (Cu LMM
lines) brought into view very close kinetic energies at 917.7 and
917.5 eV, respectively (Fig. 4 and S3†).35 In contrast, the main
binding energies noted at the S 2p3/2 and S 2p1/2 levels are very
different: 164.4 and 165.6 eV for Cu2ONPs/S8 (typical of
elemental sulfur),33 and 161.7 and 163.1 eV for Cu2S35 (Fig. S3†).
These data point to the copper in our catalyst being primarily
composed of Cu2O, whereas the formation of Cu2S during the
catalyst preparation can be practically ruled out. Surprisingly,
a catalyst sample kept in air for some years barely changed its
composition, though the presence of some CuO was visible
through its XPS Cu 2p3/2 peak at 934.9 eV and its two charac-
teristic satellite peaks at 941.4 and 944.3 eV (ESI, Fig. S4†),30a
proving the strong stabilising effect of sulfur to prevent further
oxidation.
Fig. 4 Auger spectrum of Cu2ONPs/S8 (Cu LMM line).
The characterisation of Cu2ONPs/S8 by standard electron
microscopy techniques was troublesome. Due to the low
ꢀ
melting point of sulfur (115.2 C), the application of a higher
energy incident beam to better observe the contrast between the
Cu2ONPs and sulfur just melt the latter. The morphology of
Cu2ONPs/S8 showing the surface texture of sulfur could be
observed by FE-SEM and SEM, whereas the supercial analysis
by TEM was inconclusive (Fig. S5†). Element mapping by SEM
displayed the presence of Cu and S, with a layer of the latter
seemingly having a protective effect on Cu (Fig. 5). Cryogenic
Electron Microscopy (Cryo-EM)36 was found to be an appro-
priate technique to observe Cu2ONPs/S8 without sulfur melting
(Fig. 6). When Cu2ONPs/S8 was dispersed in an organic medium
(e.g., ethanol), circular droplets were brought into view with
a size range of ca. 20–70 nm (median ¼ 47 nm) (ESI, Fig. S6†).
The droplets contained a nearly concentric ring distribution of
the Cu2ONPs with respect to the droplet surfaces, which were
themselves surrounded by sulfur (Fig. 6c–f). The general insol-
ubility of sulfur could account for this particular arrangement
of the nanoparticles inside the droplet. Apparently, sulfur acts
Fig. 5 SEM micrographs and element mapping of Cu2ONPs/S8.
(Fig. 3), and 932.5 and 933.2 eV (Cu2S) (Fig. S3†).33 The absence
of the satellite peaks in the region 940–945 eV suggests that the
catalyst is practically free of CuII species in the form of CuO.34
Fig. 6 Cryo-EM images of Cu2ONPs/S8 on carbon grids in ethanol at (a) 17 500ꢂ, (b) 45 000ꢂ, and (c)–(f) 150 000ꢂ.
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Table 2 Aerobic oxidation of amines 1 to imines 2a
as a stabilising agent, maintaining a small nanoparticle size
and preventing their agglomeration.
Substrate scope
The applicability of Cu2ONPs/S8 to a variety of primary amines
was then explored at room temperature under solvent-free
conditions in air (Table 2). Both benzylamine (1a) and the
three regiosomeric (tolylmethyl)amines (1b–1d) were trans-
formed into the corresponding imines in >95% conversion and
good-to-excellent isolated yields. Very high isolated yields were
recorded for the methoxy-, dimethoxy- and methoxycarbonyl-
substituted imines 2e, 2f, and 2g, respectively. The standard
conditions applied to the chlorinated (1h and 1i) and bromi-
nated benzylamines (1j) led to the expected imines (2h–2j) in
nearly quantitative conversion (>96%) and moderate-to-good
yields aer recrystallisation. The method was equally efficient
for the uorinated benzylamines 1k and 1l. It is known that
heterocoupled imines by this approach are much less accessible
due to the intrinsic self-coupling properties of the substrate.
Interestingly, the oxidative heterocondensation of amines cat-
alysed by Cu2ONPs/S8 was plausible by using an excess of one of
the amines (1 : 3 molar ratio). For instance, benzylamine (1a)
was successfully coupled with 4-methoxyaniline (1m); a gentle
warming was helpful to obtain the benzylideneaniline 2am in
high isolated yield. In another example, the reaction of ben-
zylamine (1a) with a-methylbenzylamine (1n) gave the imine
2an in moderate isolated yield as a single isomer. The exclusive
formation of the C]N double bond on the unsubstituted ben-
zylamine moiety suggests that the unsubstituted benzylamine
1a is more easily oxidised than the substituted benzylamine 1n,
something that could be due to the more steric hindrance of the
latter when interacting with the catalyst.
It is worthy of note that, given the excellent conversions
generally attained, most of the reactions required a simple
ltration and solvent evaporation work-up or crystallisation of
the product in hexane. This procedural simplication is
particularly advantageous when taking into account the sensi-
tivity of imines towards hydrolysis. In a few cases (Table 2, 2c,
2am and 2an), column chromatography on basic alumina is
recommended to purify the product from small amounts of
aldehyde and/or starting material. Purication of 2f was trou-
blesome, given its susceptibility to hydrolysis; the product could
be successfully puried by bulb-to-bulb distillation. In addition,
although primary amines may also lead to nitrile, amide, or azo
compounds under oxidising conditions, these by-products were
not detected because of the very mild reaction conditions
applied, leading to a high selectivity towards the imines.
Reaction mechanism
a
Detailed studies on the reaction mechanism of the catalytic
oxidation of primary amines to imines are scarce.18 A major
challenge is to ascertain whether the reaction involves an
aldehyde intermediate, formed by the dehydrogenation of the
primary amine and further hydrolysis of the resulting imine
(Scheme 3, path A), or only the primary amine (Scheme 3, path
B).
Reaction conditions: amine (1, 1.0 mmol), Cu2ONPs/S8 (0.3 mol%),
neat, rt, air, 24 h; isolated yield in parentheses aer ltration and
solvent evaporation or recrystallisation from hexane, unless otherwise
stated. Isolated yield aer column chromatography (basic alumina,
b
c
hexane-EtOAc). From 1j$HCl in the presence of Et3N (1.0 mmol).
d
1 : 3 molar ratio 1a/1m,n. e Reaction at 50 ꢀC.
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the control experiment in Table 1 (entry 17) where, apparently,
the reaction is promoted to some extent by the solely action of
S8, which likely dehydrogenates the substrate to give species of
the type H2Sx. This hypothesis is supported by the fact that
a red-coloured stuff (typical of polysuldes or polymeric
sulfur)37 was observed at the end of the reactions, accompanied
by a smell resembling that of H2S. In order to rule out any
possible formation of copper suldes (e.g., Cu2S or Cu poly-
suldes) that could catalyse the reaction, an experiment with
commercial copper(I) sulde (10 mol% Cu2S) was conducted
under the standard conditions (Scheme 4b). The conversion
into the expected product was found to be 9%. Therefore, we
can conclude that the probability of formation of copper
suldes in the reaction medium that might catalyse the reaction
is very low.
Scheme 3 Generally accepted possible routes in the catalytic aerobic
oxidation of primary amines (1) to imines (2).
The addition of an excess of the radical trap TEMPO to the
reaction mixture did not substantially inhibit the reaction and
no TEMPO-radical adducts were detected, practically ruling out
the participation of free-radical species (Scheme 4c). We also
monitored a reaction by in situ FTIR analysis in the search for
benzaldehyde as a potential reaction intermediate (Scheme 4d);
however, this compound was not detected. In another experi-
˚
ment, 4 A molecular sieves were added to the reaction medium
Fig. 7 Reaction profile in the aerobic oxidation of benzylamine (1a) to
the imine 2a.
with the aim to trap any water released from the amine oxida-
tion step (Scheme 4e); under these conditions, low product yield
would be expected if the aldehyde were formed by the primary
imine hydrolysis and further condensed with another equiva-
lent of the amine (1a) to give the imine 2a. Quite the opposite,
the imine 2a was obtained in good yield (Scheme 4e), though
not quantitative because the solid molecular sieves could
interfere somewhat in the reaction due to the neat conditions
applied. Therefore, from these experiments we can conclude
that (a) the presence of Cu and oxygen (from air) are mandatory
to attain high conversions, though a background reaction with
S8 might be occurring at the same time to a much lower extent;
(b) the reaction takes place through an ionic pathway; and (c)
aldehydes do not participate as reaction intermediates.
We carried out a series of experiments in order to shed light
on the reaction mechanism. However, rst, the kinetic prole
for the aerobic oxidation of benzylamine (1a) to benzylidene-
benzylamine (2a) was recorded. As depicted in Fig. 7, nearly half
conversion is reached in the rst hour, with the reaction being
complete aer 6 h. The intermediate aldehyde was not detected
in any of the samples analysed by GLC or GC-MS.
The standard reaction of benzylamine, when conducted
under anaerobic and anhydrous conditions led to only 25% of
the expected imine (Scheme 4a); this result is in agreement with
It is worthwhile mentioning that, when recovered aer use,
the catalyst showed a similar XPS spectrum to that of the fresh
catalyst, with two peaks at binding energies of 932.1 and
933.7 eV (ESI, Fig. S7†). This fact, together with the absence of
the satellite peaks typical of CuO, points to a mechanism
operating through Cu2O and Cu0 species. Indeed, the groups of
Adimurthy,38 and Cao and Gu22c independently reported the use
of Cu0 in the title reaction, where in situ formed CuI is the
catalytically active species. In particular, the group of Adimur-
0
0
ꢀ
thy used Cu powder at 90 C in air for 20 h; Cu powder can
contain Cu2O on the surface if not subjected to any previous
treatment, but even if pure Cu0 would be used, the formation of
Cu2O in the reaction medium would be expected aer heating
to such temperature and time in air. Similarly, Cao and Gu
utilised red Cu at 100 ꢀC under 1 atm of O2 for 24 h, conditions
that must readily promote the oxidation of Cu0 to Cu2O.
On the basis of the aforementioned information, a reaction
mechanism was proposed (Scheme 5), involving: (a) the oxida-
tion of the starting amine to the corresponding primary imine
catalysed by Cu2ONPs/S8, with the concomitant formation of
Scheme 4 Experiments to support the reaction mechanism.
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Cu(I) catalysts generally perform much better than the Cu(II)
catalysts [CuBr$SMe (24%), Cu2Br (2%); CuCl (30%), CuCl2
(2%); CuOAc (5%), Cu(OAc)2 (3%); Cu2O (82%), CuO (6%)]; and
(c) the commercial Cu(I) oxide performs much better than any of
the Cu(I) salts. Therefore, the oxidation state (I) and the pres-
ence of oxygen in the catalyst seem to be very important factors
for successful catalytic activity. The highest conversion attained
with Cu2O (10 mol%) is in concordance with the performance
and composition of our catalyst (i.e., Cu2ONPs/S8) (Table 3,
compare entries 12 and 17). However, a substantial decrease in
the conversion was noticed when the Cu2O loading was reduced
to 5 and 1 mol% (Table 3, entries 13 and 14, respectively). A
possible co-catalytic effect between Cu2O and sulfur was ruled
out as when added together (10 mol% each) the conversion was
lower than with Cu2O alone (Table 3, compare entries 12 and
15). Commercial Cu2S proved inefficient, practically discarding
any possible formation of Cu2S during the catalyst preparation
or during the amine reaction (Table 3, entry 16). Therefore, the
nanostructured character of copper, combined with the stabil-
ising effect of sulfur, seem to be crucial for the outstanding
catalytic activity of Cu2ONPs/S8, reaching quantitative conver-
sion with low metal loading (Table 3, entry 17).
Scheme 5 Proposed reaction mechanism.
water and Cu(0)NPs/S8; (b) the addition of a second molecule of
amine to the C]N bond of the primary imine to give an aminal;
(c) the release of ammonia from the aminal to give the imine
product; and (d) the regeneration of Cu2ONPs/S8 under the
action of O2. It is worth noting that the release of ammonia from
the reaction tubes was very evident by olfactory detection.
Comparison with commercial catalysts
Finally, we compared the catalytic activity of Cu2ONPs/S8 with
that of a range of commercial copper catalysts. For this purpose,
0.3 mol% Cu2ONPs/S8 and 1–10 mol% Cu loading for the rest of
the commercial catalyst were applied at room temperature,
under air and solvent-free conditions.
Conclusions
It has been demonstrated that very abundant and inexpensive
elemental sulfur is an adequate stabilisation agent for CuNPs,
in the form of copper(I) oxide, preventing their agglomeration
and over-oxidation for years. It is remarkable that, in organic
medium, nanodroplets of Cu2ONPs/S8 are formed, where the
Cu2ONPs are assembled as concentric nanorings inside
a spherical matrix of sulfur. Cu2ONPs/S8 has been shown to
effectively catalyse the oxidation of benzylic primary amines at
very low loading (0.3 mol%) under solvent-free conditions, at
room temperature and using the molecular oxygen of air as
a terminal oxidant. The method is applicable to both the homo-
and heterocoupling of amines, furnishing the imines in
moderate-to-excellent yields (60–98%). A reaction mechanism
has been proposed, based on experimentation, where the
participation of intermediate aldehydes has been ruled out and
where the Cu0–CuI pair seems to be operating in the catalytic
cycle. Moreover, Cu2ONPs/S8 exhibits a salient performance
when compared with a range of commercial Cu catalysts. The
simplicity of the catalytic system provides a cheaper, less toxic
and more effective alternative to processes that have previously
required expensive catalysts, including those based on iridium,
ruthenium, palladium and gold. Furthermore, the ndings
described herein can pave the way for research on the stabili-
sation and catalytic applications of other metal nanoparticles.
It can be concluded from Table 3 that (a) the commercial
catalysts Cu, CuI, CuBr$SMe2, CuBr2, CuCl, CuCl2, CuOAc,
Cu(OAc)2, CuOTf and Cu(OTf)2 at 10 mol% led to the imine 2a
in conversions #30% (Table 3, entries 1–10); (b) the commercial
Table 3 Screening of different Cu catalysts in the oxidation of ben-
zylamine (1a)a
Entry
Catalyst
mol%
Conversionb (%)
1
2
3
4
5
6
7
8
Cu(0)
CuI
CuBr$SMe2
CuBr2
CuCl
10
10
10
10
10
10
10
10
10
10
10
10
5
13
15
24
2
30
2
5
3
9
15
6
82
32
5
71
9
CuCl2
CuOAc
Cu(OAc)2
CuOTf
Cu(OTf)2
CuO
Cu2O
Cu2O
Cu2O
Cu2O + S8
Cu2S
9
10
11
12
13
14
15
16
17
Experimental
General procedure for the preparation of CuNPs/support
1
10c
10
0.3
Dry THF (2 mL) was added dropwise to a mixture of lithium
powder (14 mg, 2.0 mmol) and 4,40-di-tert-butylbiphenyl (DTBB,
27 mg, 0.1 mmol), under an argon atmosphere. Anhydrous
CuCl2 (134 mg, 1.0 mmol) was then added to the green
Cu2ONPs/S8
>99
a
Reaction conditions: 1a (1 mmol), Cu catalyst, neat, air, rt, 24 h.
Conversion into 2a determined by GLC. Cu2O (10 mol%) + S8
b
c
(10 mol%).
11318 | J. Mater. Chem. A, 2021, 9, 11312–11322
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Journal of Materials Chemistry A
suspension, which rapidly turned black upon the formation of
CuNPs. This suspension was diluted with THF (18 mL), followed
by the addition of the corresponding support (1.28 g). The
resulting mixture was stirred for 1 h at room temperature,
ltered, and the solid was successively washed with THF (20
mL) and MeOH (20 mL), and dried under vacuum.
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Anhydrous CuCl2 (134 mg, 1.0 mmol) was dissolved in absolute
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General procedure for the aerobic oxidation of amines
The amine 1 (1.0 mmol) was added to a reactor tube containing
Cu2ONPs/S8 (20 mg, 0.3 mol%) and the mixture was stirred at
room temperature for 24 h without any solvent. The resulting
mixture was diluted with EtOAc, ltered through a pad of
neutral alumina, celite and MgSO4, and the ltrate was analysed
by GLC and GC-MS and concentrated under vacuum. The
imines 2 were generally puried by recrystallisation from
hexane. Imines 2a, 2g and 2l were obtained in quantitative
conversion and did not require any further purication; imines
2c, 2am and 2an were puried by column chromatography
(basic alumina, hexane-EtOAc); the very sensitive imine 2f was
puried by bulb-to-bulb distillation under vacuum.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
4 M. B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X. Huang,
R. Silva, X. Zou, R. Zboril and R. S. Varma, Cu and Cu-based
nanoparticles: synthesis and applications in catalysis, Chem.
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This work was generously supported by the Spanish Ministerio
´
de Ciencia, Innovacion y Universidades (MICIU; grant no.
CTQ2017-88171-P), the Generalitat Valenciana (GV; grant no.
´
´
´
AICO/2017/007), and the Instituto de Sıntesis Organica (ISO).
5 S. E. Allen, R. R. Walvoord, R. Padilla-Salinas and
M. C. Kozlowski, Aerobic copper-catalyzed organic
reactions, Chem. Rev., 2013, 113, 6234–6458.
I. M.-G. thanks the Vicerrectorado de Investigacion y Trans-
ferencia del Conocimiento of the Universidad de Alicante for
a pre-doctoral grant (no. UAFPU2016-034). G. S. is grateful to the
European Union for an Erasmus grant. Y. M. thanks the ISO for
a postdoctoral grant. We also acknowledge M. C. Roman-
Martınez for her advice on the nitrogen isotherm and the CNB-
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