Mesoporous graphitic carbon nitride as a versatile, metal-free catalyst for the cyclisation of functional nitriles and alkynes

Article abstract of DOI:10.1039/b618555j

Mesoporous graphitic C₃N₄ was successfully employed as an effective catalyst for the cyclotrimerisation of various nitriles into triazine derivatives and the cyclisation of functional alkynes. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b618555j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Mesoporous graphitic carbon nitride as a versatile, metal-free catalyst
for the cyclisation of functional nitriles and alkyneswz
Frederic Goettmann, Anna Fischer, Markus Antonietti and Arne Thomas*
´ ´
Received (in Montpellier, France) 20th December 2006, Accepted 26th March 2007
First published as an Advance Article on the web 24th April 2007
DOI: 10.1039/b618555j
Mesoporous graphitic C N was successfully employed as an effective catalyst for the
3
4
cyclotrimerisation of various nitriles into triazine derivatives and the cyclisation of functional
alkynes.
Introduction
tible with numerous functionalities. Recently, de la Hoz et al.
reported on a catalytic synthesis of substituted s-triazine
11
derivatives that relies on silica-supported Lewis acids. In
Cyclotrimerisations of triple bonds are usually very exother-
mic reactions because they result in the formation of aromatic
rings. Their practical realisation, however, is hindered by very
high activation energies. This can be exemplified by the
archetypical case of ethyne, where the activation energy of
the concerted mechanism was estimated to be around 250 kJ
the case of benzene derivatives, the use of transition metals, for
which cost, availability and toxicity are main issues, is re-
2
garded as unavoidable.
Here, we report on a heterogeneous metal-free catalytic
system, which turned out to be effective for these cyclisations.
12,13
ꢀ
1 1
mol
.
Nevertheless, such reactions are of great synthetic
An ‘‘ideal’’ catalyst
should be a solid (i.e. easy to handle
significance because they permit access to functional aromatic
systems, such as benzene and triazine derivatives. Functional
and to separate); it should also be as simple as possible in
terms of chemical composition (i.e. easy to synthesise), meso-
2
benzene rings are ubiquitous chemical commodities. Tria-
porous (exhibiting high surface areas and good accessibil-
14–17
zines, formally resulting from the trimerisation of nitriles
ity
) and, of course, highly catalytically active. A ‘‘metal-
18
(
Scheme 1), are important building blocks for supramolecular
free’’ catalyst (like, for instance, graphite or the proline
19,20
systems used for homogeneous organocatalysis
3
chemistry, polymer syntheses and have been used as crop
4,5
) would
6
protecting agents.
provide additional benefits to the synthetic process, consider-
ing cost, toxicity and environmental sustainability.
Due to the increased attention paid to the environmental
7
–9
sustainability of chemical processes,
concerns about the
In previous studies, we have shown that mesoporous gra-
phitic carbon nitride, named mpg-C N , is such a potent
usual syntheses of such compounds have arisen. The main
syntheses of functional triazines rely either on the wasteful
3
4
metal-free heterogeneous catalyst, as it catalysed Frie-
21,22
3
substitution of chlorine in cyanuric chloride or on the alkali
del–Crafts type aromatic substitutions on benzene,
as well
23
1
promoted trimerisation of nitriles, the latter being incompa-
0
as the reaction of benzene and CO to form phenol and CO.
2
Herein, we report on the catalytic activity of this purely
organic material in the cyclisation of nitriles and alkynes.
3 4
The application of graphitic C N as a catalyst for activat-
ing aromatic six-membered rings was motivated by the hexa-
gonal symmetry of the tri-s-triazine units in its connectivity
pattern (the structure of the graphite-like sheets of g-C N
3
4
was initially proposed by Kroke et al. and is now widely
2
accepted, Fig. 1) and the assumption that the p orbitals or
4
z
the lone pairs of its aromatic nitrogens could interact with p*-
orbitals of aromatic rings. For the same symmetry reasons,
graphitic C
3 4
N was thought to be able to stabilise one of the
transition states of cyclotrimerisations, even though their
2
aromaticity is still a matter of debate.
Scheme 1 Formal representation of the syntheses of triazines from
nitriles, and benzene derivatives from alkynes.
5,26
Experimental
Max-Planck-Institute of Colloids and Interfaces, Department of
Colloid Chemistry, Research Campus Golm, D-14424 Potsdam,
Germany. E-mail: arne.thomas@mpikg.mpg.de; Fax: +49 331-567-
The chemicals that were used were employed as received. For
the catalytic tests, we used the previously described mesopor-
9502; Tel: +49 331-567-9509
2
1
w Electronic supplementary information (ESI) available: FT-IR spec-
tra of the used powders; computational details. See DOI: 10.1039/
b618555j.
zim Ta hg ee s H. TML version of this article has been enhanced with colour
ous graphitic C
N
4
, mpg-C
N
3 4
.
In a typical synthesis, 2.5 g
3
s
of a water dispersion of 12 nm silica spheres (Ludox HS40,
Aldrich) were added to 1.0 g of melted cyanamide (99%,
Aldrich). The mixture was subsequently heated to 550 1C in
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All computations were performed using the Gaussian 03
2
suite of programs and gradient-corrected density functional
8
2
9
theory by using the B3LYP functional. Optimisations were
carried out using the 6-311G basis set.
Results
The results of our experiments are summarised in Table 1. The
conversion rates were calculated as the molar ratio of products
and initial amount of reactant as determined from GC with an
internal standard. The turnover frequencies (TOFs) were
calculated as the number of mols of product formed per day
and per theoretical number of mols of tri-s-triazine units in the
catalyst. As only the tri-s-triazine units at the surface of the
catalyst are likely to be accessible and can thus be active, our
TOFs are thus largely underestimated. Nevertheless, these
values are a useful tool for comparing the reactivities of our
substrates as it can be assumed that the TOF is underestimated
in the same way for each test.
3 4
Fig. 1 Structure proposed by Kroke et al. for g-C N (ref. 24).
a closed ceramic crucible for 4 h. After an additional heat
treatment at 600 1C for 10 h and removal of the silica with
¨
NH HF (Riedel de Haen), the resulting powder exhibited a
4
2
2
ꢀ1
specific surface area of 140 m
g
and an average pore
diameter of 12 nm, as measured from nitrogen adsorption
(
measured with a Micromeritics Tristar 3000 system).
For a reference test, we also synthesised a heat treated mpg-
Within the series of tested cyano compounds, acetonitrile
showed the highest turnover frequency (Table 1, entry 1 with a
3 4 3 4
C N (mpg-C N -ht). For that, 500 mg of the previously
ꢀ1
described mesoporous powder was heated at 550 1C for 1 h
in a closed iron autoclave fitted with a glass vessel. The specific
surface area and the mean pore size of the resulting powder
TOF of 55 d ), while reference tests without catalyst (Table 1,
entry 3) or with adenine as a catalyst (as nitriles are known to
be able to react in basic media, Table 1, entry 4) failed to yield
any detectable reaction products. It is worth noticing that
molecules with comparable steric hindrance but different
polarity exhibit very different reactivities. In the cases of
propionitrile, chloropropionitrile and chloroacetonitrile
(Table 1, entries 6, 11, 10), which are comparable from a steric
point of view, the reactivities decrease according to the
polarity of the C–N triple bond (which can, for instance, be
3 4
were not significantly different from those of mpg-C N . FT-
IR measurements, however, showed that the amount of un-
condensed amino groups at the surface of the pores was
ꢀ
1
,
strongly decreased:w the broad band around 3130 cm
corresponding to the N–H stretching modes, is noticeably
ꢀ
1
weaker, while a strong peak at 725 cm , present in the
primary mpg-C , which could be attributed to NH bending
modes according to Wang et al., almost vanished. A strong
N
3 4
2
2
7
30
evaluated from tabulated IR data ). Carbonitriles with aro-
ꢀ
1
peak at 2162 cm , which was attributed to uncondensed
cyano moieties, is also reduced by this additional step.
matic moieties, however, behave differently; pyrazinecarboni-
trile (Table 1, entry 8), for example, is one of the most reactive
ꢀ
1
In typical catalytic experiments, 50 mg of mpg-C
3
N
4
was
nitriles (TOF = 8.9 d ), while benzonitrile (Table 1, entry 7)
does not react at all.
suspended in 5 ml of either the pure nitrile or alkyne, or a
solution of nitrile in hexane (95%, Aldrich), or a solution of
alkyne in xylenes (mixed isomers, extra pure, Merck). The
resulting suspension was then placed in a 50 ml stainless steel
autoclave fitted with a Teflon mantle (Samplatec Company)
and heated to 150 or 180 1C for the planned reaction time.
Acetonitrile (hyper grade, Merck) was used as the reactant and
solvent. For the other nitriles, 10 mmol were dissolved in
hexane, corresponding to 550 mg propionitrile (99%, Aldrich),
For alkynes, the expected cyclotrimerisation products are
obtained in modest to good yields with dimethyl acetylenedi-
ꢀ1
carboxylate (TOF = 0.2 d ) and propargyl alcohol (TOF =
ꢀ
1
ꢀ1
33 d at 5% conversion and TOF = 18 d at 70% conver-
sion). As seen from Table 1, phenyl- and diphenylacetylene
also yielded Diels–Alder products (1-phenylnaphthalene and
1,2,3-triphenylnaphthalene).
1
g benzonitrile (98%, Fluka), 1 g pyrazinecarbonitrile (99%,
Aldrich), 760 mg chloroacetonitrile (99%, Aldrich) and
90 mg 3-chloropropionitrile (98%, Aldrich). Phenylacetylene
97%, Fluka) and propargyl alcohol (98%, Alfa Aesar) could
Discussion
8
For the experiments in which alkynes are the reactants, the
formation of trimerisation products is indeed catalysed, pos-
sibly through a mechanism that relies on the same electron
transfer mechanism as the one postulated for the activation of
benzene. Nevertheless, when the reactants can also act as
dienes, Diels–Alder reactions are an alternative for the reac-
tion pathway, which is indeed also observed, in part, for
phenylacetylene and as the main reaction channel for diphe-
nylacetylene. As reference tests without the catalyst showed no
detectable conversion, these Diels–Alder reactions are appar-
ently also catalysed by mpg-C N .
(
be used as the reactant and solvent, as well. When working
with xylenes as the solvent, 1 g of diphenylacetylene (98%,
Aldrich), 500 mg of dimethyl acetylenedicarboxylate (99%,
Aldrich) or 200 mg of propargyl alcohol (98%, Alfa Aesar)
were used.
The products of the reactions were analysed using GC-MS
(
Agilent Technologies, GC 6890N, MS 5975) and, after isola-
1
13
tion of the products, using H and C NMR spectroscopy
(
Bruker DMX 400).
3
4
FT-IR analyses of the catalysts were undertaken with a
This is consistent with the results of Katritzky et al., who
BioRad FTS600 spectrometer.
456 | New J. Chem., 2007, 31, 1455–1460
showed that higher temperatures and pressures were required
1
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Table 1 Results of the cyclotrimerisation tests catalysed by mpg-C
3 4
N
b
Temperature/ Reaction Conversion TOF /
a
ꢀ1
c
Entry Reactant
Solvent
—
1C
time/h
80
rate (%)
day
55
Products
1
Acetonitrile
180
20
2
Acetonitrile
—
150
48
1
1.2
d
e
f
3
4
5
6
Acetonitrile
Acetonitrile
Acetonitrile
—
—
—
Hexane
180
180
180
180
80
48
80
48
o0.5
o0.5
o0.5
15
—
—
—
2.4
—
—
—
Propionitrile
7
8
Benzonitrile
Pyrazinecarbonitrile
Hexane
Hexane
180
180
48
48
o0.5
—
8.9
—
100
f
9
Pyrazinecarbonitrile
Hexane
180
48
45
4.0
1
1
0
1
2-Chloroacetonitrile
3-Chloropropionitrile
Hexane
Hexane
180
180
48
48
o0.5
—
1.2
—
10
12
Phenylacetylene
—
140
24
8
13
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Table 1 (continued )
b
Temperature/ Reaction Conversion TOF /
a
ꢀ1
c
Entry Reactant
Solvent
—
1C
time/h
rate (%)
day
Products
1
3
Phenylacetylene
Phenylacetylene
Phenylacetylene
180
24
75
126
f
14
—
180
24
99.5
166
d
15
6
— 180
Xylenes 150
24
60
o0.5
—
4.3
—
1
Diphenylacetylene
100
17
Dimethyl acetylenedicarboxylate Xylenes 120
48
10
0.2
18
Propargyl alcohol
—
120
24
5
33
19
Propargyl alcohol
Xylenes 120
24
70
18
a
b
The conversion rates were calculated as the molar ratio of products and initial amount of reactant as determined from GC. The turnover
frequencies (TOFs) were calculated as the number of mols of product formed per day and per theoretical number of mols of tri-s-triazine units in
c
the catalyst. The percentages indicated below each product reflect the relative amount of this species among the other reaction products, as
d
e
f
determined from GC. Reference test without catalyst. Reference test with adenine as a nitrogen rich soluble organic base. Reference test with
heat-treated mpg-C
3
4
N .
to allow the non-catalytic Diels–Alder condensation of such
3
effects. In the case of phenylacetylene, a change in temperature
allows tuning of the ratio between the two different reaction
products. 1-Phenylnaphthalene remains, however, the main
product.
1
3 4
alkynes to take place, i.e. the role of the mpg-C N would be,
in part, to generate close-packed surface states by adsorption.
Thermal [2 + 4] cyclisations are known to proceed via a
concerted mechanism involving the p-system of both reac-
The reactivity of the various nitriles can be correlated
neither to their bulkiness nor—over the whole set of examined
starting products—directly to the polarity of their triple bond.
The strong differences in the observed TOFs could thus rely on
the way these molecules interact with the catalyst. FT-IR
3 4
tants. It is thus not surprising that mpg-C N also activates
those reactions, which are similar to the trimerisation reac-
tions. In the case of diphenylacetylene, the Diels–Alder con-
densation is observed exclusively, probably because of steric
1
458 | New J. Chem., 2007, 31, 1455–1460
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some of the optimised adducts. Important features of the
calculated structures are summarised in Table 2.
Interestingly, while benzene and tri-s-triazine units in mpg-
C N are probably interacting face-to-face through p-stack-
3
4
2
1,23
ing,
stable adducts of nitriles with the melem unit were
found to occur through H-bonding with the amino groups
Fig. 2). The adduct formation is always exothermic, up to
(
ꢀ
1
about ꢀ50 kJ mol in the case of pyrazinecarbonitrile. This
molecule interacts with the melem unit through three H-bonds
(
Fig. 2b), comparable to the lock–key pattern interaction
3
between substrate and reactant found for many enzymes
3,34
3
5
and known in supramolecular chemistry. The complexation
of the nitriles does not noticeably affect the lengths of their
N–C bonds or the corresponding N–C–C angles. On the other
hand, the dipole moment of the N–C triple bond is strongly
modified by the H-bond formation. For example, while the
non-complexed acetonitrile features a N–C dipole moment of
3
.48 D, the adduct reaches 4.39 D. The observed polarisation
of the N–C triple bond originates mainly from the electrostatic
deformation of its electronic cloud by the nearby proton
involved in the H-bond and could be involved in the mechan-
ism of the trimerisation. This would be consistent with experi-
mental evidence from early reports on the use of strong acids
3
6,37
as suitable solvents for the cyclotrimerisation of nitriles.
These calculations do not allow the building of a whole
model to account for the observed activation of nitriles by
mpg-C N , but suggest that H-bonding is playing an impor-
3
4
tant role. The ability of mpg-C N to form such H-bonds is
3
4
strongly dependent on the amount of uncondensed amino
functionalities at its surface. Indeed, Schnick and Lotsch
1
recently showed, by means of solid state N NMR spectro-
5
scopy, that bulk graphitic C features such uncondensed
3
N
4
3
8
amino groups to a large extent. We showed that these amino
groups also play a predominant role in the activation of CO
2
2
3
by mpg-C
3
N
4
.
As a cross-testing experiment, we tried to
reduce the number of those amine groups by a heat treatment
step at the borderline of mpg-C N stability. Indeed, a so-
3
4
treated material showed no catalytic activity in the cyclotri-
merisation of acetonitrile (Table 1, entry 5) and a much lower
activity with pyrazinecarbonitrile (Table 1, entry 9), but it
proved even more effective in catalysing the cyclisation of
Fig. 2 Optimised structures of the adducts of a model triamino tri-s-
triazine (melem) unit and (a) acetonitrile, (b) pyrazinecarbonitrile and
(c) benzonitrile. Some typical bond lengths and NBO charges on the
ꢀ1
phenylacetylene (Table 1, entry 14, TOF = 166 d ). In our
cyano group are given.
opinion, this experiment supports the idea that activation of
nitriles, at least partly, relies on appropriate H-bonding pat-
terns involving free amino functionalities, while the cyclisation
of alkynes (also of the Diels–Alder type) is presumably due to
aromatic charge-transfer interactions, as described for the
metal-free catalysis of Friedel–Crafts reactions.
3 4
measurements on acetonitrile adsorbed on mpg-C N show a
weak bathochromic shift in the C–N asymmetric stretching
ꢀ1
vibration of the cyano moiety (from 2293 to 2290 cm ),
consistent with a weakening of this bond.w
DFT modelling was undertaken to illustrate further details
on how nitriles could interact with the active centres in mpg-
C N , here modelled by the triamino tri-s-triazine (melem)
Conclusion
3
4
molecule. These calculations are not meant to be a full
investigation of the actual reaction mechanism, which would
require a better theoretical model and expensive transition
state optimisations. We investigated the geometries, relative
thermodynamic stabilities and charge repartitions (in a natural
In this contribution, we have shown that mpg-C N is a
3
4
versatile, functionality-tolerant catalyst for the cyclotrimerisa-
tion of triple bonds. Numerical investigations also showed that
the activation of nitriles is likely to proceed via H-bonds,
whereas activation of alkynes presumably progresses via p-
stacking, as is assumed for the activation of benzene. The
simultaneous presence of different activation mechanisms in
one structure (‘‘multifunctional catalysis’’) renders the
3
2
bond orbital, NBO, population analysis ) of adducts of
various nitriles and melem. Geometric parameters of the
optimised molecules are reported in the ESI.w Fig. 2 displays
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a
Table 2 Some geometric and electronic parameters of the optimised adducts
NBO charges in the cyano group
H-bond length
between the cyano
and amino
Formation
enthalpy/
kJ mol
C–N distance
in the cyano
˚
bond/A
N–C–C angle of
the cyano
function/1
Dipole
moment/
debye
Adduct of
melem with
ꢀ
1
˚
group/A
N
C
Acetonitrile
Propionitrile
Pyrazinecarbonitrile
Benzonitrile
a
ꢀ38.9
1.164
2.063
2.060
2.139
2.013
176.9
(180.0)
177.0
(179.3)
176.8
(178.4)
178.1
(180.0)
ꢀ0.42
0.37
(0.29)
0.37
(0.25)
0.33
(0.27)
0.35
(0.28)
4.39
(3.48)
4.38
(3.48)
4.03
(2.98)
4.10
(3.26)
b
(
1.165)
1.165
1.165)
1.165
1.166)
1.167
1.168)
(ꢀ0.33)
ꢀ0.42
ꢀ38.1
ꢀ49.9
ꢀ37.8
(
(ꢀ0.33)
ꢀ0.39
(
(ꢀ0.26)
ꢀ0.38
(
(ꢀ0.30)
28
All computations were performed using the Gaussian 03 suite of programs and gradient-corrected density functional theory by using the
29
B3LYP functional. Optimisations were carried out using the 6-311G basis set. Evaluation of the charge repartition was achieved using the natural
For comparison, the corresponding lengths, angles and charges in the non-complexed nitriles are
32 b
bond orbital (NBO) population analysis.
given in brackets.
different carbonitrides a very promising class of functional
solid-state structures, particularly to explore the possibilities
of a metal-free coordination chemistry and a coupled ‘‘green’’
group-efficient heterogeneous catalysis.
20 B. List, Acc. Chem. Res., 2004, 37, 548.
2
2
2
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2
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