Copper nitride nanoparticles supported on a superparamagnetic mesoporous microsphere for toxic-free click chemistry

Article abstract of DOI:10.1039/c001255f

Copper nitride nanoparticles supported on a mesoporous superparamagnetic silica microsphere exhibit superior activity toward the Huisgen cycloaddition of azides and alkynes. The nitride catalyst offers significant advantages over homogeneous Cu catalysts.

Full text of DOI:10.1039/c001255f

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Copper nitride nanoparticles supported on a superparamagnetic
mesoporous microsphere for toxic-free click chemistryw
Byoung Se Lee,a Mihye Yi,b So Young Chu,a Ja Young Lee,b Hye Rim Kwon,b
Kyu Reon Lee,b Donghyeon Kang,b Wan Seop Kim,c Heung Bin Lim,d Jouhahn Lee,e
Hyung-Joong Youn,e Dae Yoon Chi*b and Nam Hwi Hur*b
Received 19th January 2010, Accepted 14th April 2010
First published as an Advance Article on the web 27th April 2010
DOI: 10.1039/c001255f
Copper nitride nanoparticles supported on a mesoporous super-
paramagnetic silica microsphere exhibit superior activity toward
the Huisgen cycloaddition of azides and alkynes. The nitride
catalyst offers significant advantages over homogeneous Cu
catalysts.
Fig. 1 Reaction scheme for the synthesis of a Cu3N/Fe3N@SiO2
Click chemistry represents a group of organic reactions that
proceed rapidly under ambient conditions and give high yields
with great selectivity.1 The Huisgen 1,3-dipolar cycloaddition
(HDC) of azides and alkynes catalyzed homogeneously by
soluble copper complex has emerged as the most common
click reaction;2,3 it is a powerful tool for covalently linking
functionalized molecules.4 A major drawback of the catalyst is
that it contains the cytotoxic CuI ion, which precludes its use
in living systems and in reactions of industrial interest.5 In
order to make the HDC reaction applicable to biomolecules, it
is required to develop insoluble solid catalysts. Here we report
a newly discovered copper nitride (Cu3N) nanoparticle (NP)
supported on a superparamagnetic silica microsphere, which
exhibits superior activity toward the HDC reaction. In con-
trast to the CuI complexes,3 the heterogeneous Cu3N catalyst
is a solid and it is easily separated by applying a magnet; this
raises the prospect of toxic-free click chemistry. The nitride
catalyst can be applied to living and environmentally benign
systems.
silica microsphere. Dark blue spheres and green ovals represent Fe3N
and Cu3N nanoparticles embedded within mesoporous shells of a silica
sphere, respectively.
formed in the native oxide layer on the Cu surface.8 This
suggests that Cu-based oxides can promote the HDC reaction.
In this regard, we have screened a number of materials
containing Cu as potential heterogeneous catalysts and have
singled out Cu3N nitride as a promising candidate largely
because the formal oxidation state of Cu in Cu3N is +1, and
also because its physical properties are quite similar to those of
ionic solids. In addition, Cu3N adopts the anti-ReO3 struc-
ture with an open cubic framework of Cu6N octahedra, which
is suitable for interaction with guest organic molecules.9
Remarkably, a preliminary study with the supported Cu3N
nitride showed a high catalytic activity toward the HDC reac-
tion, and hence it can be considered as a new heterogeneous
solid catalyst that is free of cytotoxicity.
Our approach to the synthesis of Cu3N NPs supported on
superparamagnetic mesoporous silica microspheres is illustrated
in Fig. 1, which is based on our previous work concerning the
fabrication of the core/shell CoFe2O4@SiO2 microsphere.10
Using two immiscible solvents iron precursors were incorporated
into core/shell mesoporous silica microspheres, which were
subject to annealing at 700 1C in NH3. Iron nitride phases
containing Fe3N as a major component were formed within
the mesoporous shells in the silica, which are denoted as
Fe3N@SiO2. A Cu source was then infiltrated into the
Fe3N@SiO2 microsphere, and this was followed by annealing
at 250 1C in NH3. This led to the formation of Cu3N NPs
within the shells. A crucial step for embedding nitride NPs
within the mesoporous shells was the infiltration of Fe and Cu
sources into the shells, thereby enabling nitride phases to grow
exclusively within the pores. Hereafter we denote the resulting
black microsphere with an average diameter of approximately
500 nm as Cu3N/Fe3N@SiO2.
The typical procedure for carrying out the HDC reaction in
solution has involved the use of stable CuI complexes such as
CuI and CuSO4/Na-ascorbate.3 Highly efficient reactions can
also be heterogeneously catalysed by Cu and CuO particles on
various supports as well as immobilized Cu complexes.6 It has
been reported that Cu metal alone is able to catalyze this HDC
reaction in solution; the reaction proceeds slowly.7 Careful
examination of the Cu surface by various means, however,
revealed that the reaction is catalyzed by the CuI species
a Research Institute of Labeling, FutureChem Co. Ltd,
Seoul 138-736, Korea
b Department of Chemistry, Sogang University, Seoul 121-742,
Korea. E-mail: dychi@sogang.ac.kr, nhhur@sogang.ac.kr;
Fax: +82 2-701-0967
c Physical Metrology Division, Korea Research Institute of
Standards and Science, Daejeon 305-600, Korea
d Department of Chemistry, Dankook University, Yongin,
Gyeonggi-do 448-701, Korea
Transmission electron microscopy (TEM) images of
Cu3N/Fe3N@SiO2 show the core/shell structure, which has
a shell thickness of approximately 100 nm surrounding the
dense silica core (Fig. 2a and b). The surface morphology of
e Korea Basic Science Institute, Jeonju 561-756, Korea
w Electronic supplementary information (ESI) available: Synthesis
and reaction details, magnetization curves, X-ray photoemission
spectra. See DOI: 10.1039/c001255f
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data of Cu3N/Fe3N@SiO2 collected at 10 and 300 K
(see Fig. S1 in the ESIw) show that coercivity values are
negligibly small, revealing the presence of superparamagnetic
state.
Curve-fitting results of X-ray photoelectron spectroscopy
(XPS) data of Cu3N/Fe3N@SiO2 microspheres and Cu3N
powders show that the Cu 2p3/2 peak near 933 eV is composed
of two peaks at 933 and 935 eV (see Fig. S2 in the ESIw). The
former peak, shifted by approximately 0.7 eV to higher
binding energy relative to that of Cu metal, indicates the
presence of the Cu(I) species in Cu3N.12 The latter peak is
ascribed to the Cu(II) ion present in the native CuO layer on
the surface. Interestingly, a couple of satellite peaks around
942 eV associated with CuO can be distinctively observed in
the spectra of bulk Cu3N while they are buried among the
background peaks for Cu3N/Fe3N@SiO2. Although a purely
ionic model might not be appropriate to describe the bonding
nature of Cu3N, the XPS results imply that Cu3N is composed
of Cu+1 and N3À ions. Bond valence calculations, where bond
valence sums for Cu3N are +0.90 at the Cu site and À2.69 at
the N site, also support the XPS data.11 It is thus evident
that Cu3N has an ionic character although a covalent
nature should be included to accurately describe the chemical
bonding in Cu3N.
Fig. 2 High resolution TEM images of Cu3N/Fe3N@SiO2 with
low and high magnifications (a and b). Elemental distribution maps
of (c) Fe and (d) Cu present in Cu3N/Fe3N@SiO2.
the silica template was sustained during the growth of Fe3N
and Cu3N phases, suggesting that both nitride phases are
successfully incorporated into pores in the shells without
destruction of the parent core/shell shape. The scanning
TEM was employed to observe the distribution of Cu and
Fe atoms on a Cu3N/Fe3N@SiO2 microsphere. Both elements
are uniformly distributed over the silica surface (Fig. 2c and d),
thus implying that agglomeration of Cu3N and Fe3N NPs is
less likely to occur on the silica surface.
We conducted preliminary experiments on the HDC of
benzyl azide (1) with phenylacetylene (2) to yield the expected
triazole 3 with the Cu3N/Fe3N@SiO2 catalyst. A wide range
of amines and solvents were screened to optimize reaction
conditions (see Tables S1 and S2 in the ESIw). All reactions
proceeded rapidly in the presence of tertiary amines with good
yields and high selectivity. In contrast, a similar reaction in the
absence of amine yielded a small amount of 3, suggesting that
amine promotes the HDC reaction as is the case for soluble
CuI catalysts.13 Several organic solvents were tested in the
presence of triethylamine. Among them, CH3CN emerged as
the most reliable solvent for the HDC reaction, and it
was used in subsequent reactions. To confirm whether Cu3N
in Cu3N/Fe3N@SiO2 plays a major role in accelerating the
formation of triazole, the same experiment was conducted
with bulk Cu3N. Complete conversion was observed within
3 h; this clearly indicates that Cu3N is the only active com-
ponent in Cu3N/Fe3N@SiO2 for the HDC reaction.
The formation of Cu3N phase is evident from the XRD data
of Cu3N/Fe3N@SiO2 given in Fig. 3a. The first three strong
reflections can index in a cubic cell (space group: Pm3m) of
Cu3N with a lattice parameter of 3.829 A. The cell size is
slightly larger than that obtained from bulk Cu3N powders.11
In addition to the peaks ascribed to the Cu3N phase, several
broad peaks originating from iron nitrides appeared in the
XRD data (Fig. 3b). The average sizes of the Cu3N and
Fe3N particles calculated by the Scherrer equation are 22.4
and 20.5 nm, respectively. The field-dependent magnetization
For general application, the HDC reaction was performed
in the presence of Cu3N/Fe3N@SiO2 with five alkynes and
three azides, which led to triazoles in high yields (Table 1). The
reusability of the Cu3N/Fe3N@SiO2 catalyst was examined,
where the reaction of 1 with 2 in the presence of the recycled
catalyst was repeated up to 5 times. The recycled catalysts,
separated by a magnet, showed activities toward the HDC
reaction that were virtually identical to those of the virgin
catalyst, indicating that leaching of Cu3N in the catalyst is less
likely to occur during the reaction process.
The essential element for the activity of the catalyst is
believed to be the CuI species in Cu3N. It should be noted
that the solid Cu3N particle provides the active surface con-
taining CuI species, which promotes the HDC reaction.
The mechanism for the click reaction driven by the catalyst
might be analogous to that of the well-known HDC reaction
catalyzed by soluble CuI complexes although there are clear
Fig. 3 XRD patterns of (a) Cu3N/Fe3N@SiO2 and (b) Fe3N@SiO2.
Calculated XRD patterns of Fe3N and Cu3N below the experimental
data are obtained on the basis of crystallographic data. The inset is a
unit cell structure of Cu3N in which the Cu atoms occupy the edge
centres of the unit cell and the N atoms are located at the cube corners.
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Table 1 Synthesis of various 1,2,3-triazoles by Cu3N/Fe3N@SiO2
catalysta
Research Foundation funded by the Ministry of Education,
Science, and Technology in Korea. Acknowledgement is also
made to the BK 21 Program at Sogang University.
Notes and references
1 (a) H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem.,
Int. Ed., 2001, 40, 2004; (b) J. E. Moses and A. D. Moorhouse,
Chem. Soc. Rev., 2007, 36, 1249.
2 R. Huisgen, 1,3-Dipolar Cycloaddition Chemistry, ed. A. Padwa,
Wiley Interscience, New York, 1984.
12 h, 81
24 h, 89
12 h, 82
4 d, 79
12 h, 76
5 d, 72
3 (a) C. W. Tornøe, C. Christensen and M. Meldal, J. Org. Chem.,
2002, 67, 3057; (b) V. V. Rostovtsev, L. G. Green, V. V. Fokin and
K. B. Sharpless, Angew. Chem., Int. Ed., 2002, 41, 2596.
4 (a) H. C. Kolb and K. B. Sharpless, Drug Discovery Today, 2003, 8,
1128; (b) V. D. Bock, H. Hiemstra and J. H. van Maarseveen, Eur.
J. Org. Chem., 2006, 51; (c) H. He, Y. Zhang, C. Gao and J. Wu,
Chem. Commun., 2009, 1655; (d) L. Polito, D. Monti, E. Caneva,
E. Delnevo, G. Russo and D. Prosperi, Chem. Commun., 2008, 621;
(e) L. Moni, S. Rossetti, M. Scoponi, A. Marra and A. Dondoni,
Chem. Commun., 2010, 46, 475.
5 (a) F. Fazio, M. C. Bryan, O. Blixt, J. C. Paulson and C.-H. Wong,
J. Am. Chem. Soc., 2002, 124, 14397; (b) A. J. Link and
D. A. Tirrell, J. Am. Chem. Soc., 2003, 125, 11164;
(c) M. D. Best, Biochemistry, 2009, 48, 6571.
6 (a) B. H. Lipshutz and B. R. Taft, Angew. Chem., Int. Ed., 2006, 45,
8235; (b) T. Miaoa and L. Wang, Synthesis, 2008, 363;
(c) G. Molteni, C. L. Bianchi, G. Marinoni, N. Santo and
A. Ponti, New J. Chem., 2006, 30, 1137.
5 d, 79
14 d, 82
5 d, 89
2 d, 89
4 d, 60
24 h, 84
24 h, 82
24 h, 91
24 h, 90
a
All reactions were conducted with 1.0 mmol of alkyne and 1.1 mmol
of azide in 4 mL of acetonitrile using Cu3N/Fe3N@SiO2 (20 mg) and
0.3 equiv. of Et3N. The numbers in Table 1 imply ‘time, yield (%)’.
7 H. A. Orgueira, D. Fokas, Y. Isome, P. C.-M. Chan and
C. M. Baldino, Tetrahedron Lett., 2005, 46, 2911.
distinctions between individual reaction steps associated with
heterogeneous Cu3N NPs and homogeneous CuI complexes.14
In conclusion, we have found that Cu3N exhibits superior
catalytic activity toward the HDC reaction. This click reaction
catalyzed heterogeneously by Cu3N possesses kinetics compar-
able to that of the reaction catalyzed homogeneously by
soluble CuI complexes. Since Cu3N is not cytotoxic, the nitride
catalyst for the HDC reaction offers significant advantages
over soluble CuI complexes. Moreover, the Cu3N catalyst
supported on a magnetic microsphere is recyclable, thus it
can be applied to living and environmentally benign systems.
This novel nitride can be considered as the next generation
catalyst. It is expected to have considerable potential for use in
a wide range of toxic-free click chemistry applications such as
imaging and labelling of bio-molecules on live cells.
8 (a) J. Iijima, J.-W. Lim, S.-H. Hong, S. Suzuki, K. Mimura and
M. Isshiki, Appl. Surf. Sci., 2006, 253, 2825; (b) G. Molteni,
C. L. Bianchi, G. Marinoni, N. Santo and A. Ponti, New J. Chem.,
2006, 30, 1137; (c) B. H. Lipshutz and B. R. Taft, Angew. Chem.,
Int. Ed., 2006, 45, 8235; (d) S. Chassaing, A. S. S. Sido, A. Alix,
M. Kumarraja, P. Pale and J. Sommer, Chem.–Eur. J., 2008, 14,
6713.
9 F. Gulo, A. Simon, J. Kohler and R. K. Kremer, Angew. Chem.,
Int. Ed., 2004, 43, 2032.
10 K. R. Lee, S. Kim, D. H. Kang, J. I. Lee, Y. J. Lee, W. S. Kim,
D.-H. Cho, H. B. Lim, J. Kim and N. H. Hur, Chem. Mater., 2008,
20, 6738.
11 G. Paniconi, Z. Stoeva, H. Doberstein, R. I. Smith, B. L. Gallagher
and D. H. Gregory, Solid State Sci., 2007, 9, 907.
12 C. Navıo, M. J. Capitan, J. Alvarez, F. Yndurain and R. Miranda,
Phys. Rev. B, 2007, 76, 085105.
13 V. O. Rodionov, V. V. Fokin and M. G. Finn, Angew. Chem., Int.
Ed., 2005, 44, 2210.
14 F. Himo, T. Lovell, R. Hilgraf, V. V. Rostovtsev, L. Noodleman,
K. B. Sharpless and V. V. Fokin, J. Am. Chem. Soc., 2005, 127,
210.
This research was supported by the Converging Research
Center Program (No: 2009-0081952) and National Research
Lab Program (No: 2009-0083116) through the National
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3935–3937 | 3937