A covalently anchored homogeneous gold complex on carbon nanotubes: A reusable catalyst

Article abstract of DOI:10.1039/c3cc46097e

The gold complex [Tf₂NAuPPh₃CH₂NH ₂] was synthesized and covalently anchored on modified carbon nanotubes in order to obtain a supported gold homogeneous catalyst. Simple 1,6 enynes were chosen as benchmark substrates to assess its behaviour in cyclization catalysis as well as study its recycling.

Full text of DOI:10.1039/c3cc46097e

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Covalently anchored homogeneous gold complex on carbon nanotubes:
a reusable catalyst
Charles Vriamont, a Michel Devillersa , Olivier Riant*a and Sophie Hermans*a
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
50
The present strategy was based on the use of the bifunctional
ligand 1 (Scheme 1) able to interact both with a gold complex
thanks to the phosphine group and with modified MWNTs using
the amine group to obtain a homogeneous gold complex
supported onto MWNTs. The bifunctional ligand 1 can be
The gold complex [Tf2NAuPPh3CH2NH2] was synthesized and
covalently anchored on modified carbon nanotubes in order
to obtain a supported gold homogeneous catalyst. Simple 1,6
enynes were chosen as benchmark substrates to assess its
10 behaviour in cyclization catalysis as well as study its
recycling.
55 obtained on a multi-gram scale 6. A ligand exchange between the
gold complex ClAuSMe2 2 and bifunctional ligand 1 provided the
modified gold complex 3 7. The chlorine atom was then replaced
by the bis(trifluoromethanesulfonyl) imidate moiety (NTf2) by
adding 1 equivalent of AgNTf2 leading to the gold complex 4
60 (Scheme 1). This gold complex tailored for immobilization was
obtained in 80% yield starting from 1 and was extensively
Homogeneous catalysts display often high activities and
selectivities towards the targeted product but suffer from various
drawbacks: they are often expensive and most of the time single
15 use meaning that a potential recycling of these catalysts is of
paramount importance. The concept of supported homogeneous
catalysis combining advantages of both homogeneous and
heterogeneous catalysis opens promising perspectives. A highly
active catalyst coupled with ease of separation from products
20 allows reusability. Many strategies have been carried out for the
covalent and non-covalent immobilization of various metal
complexes on different materials. Amorphous and ordered silica,
1
characterized by TGA, HR-MS, H NMR, 31P NMR, 19F NMR
and 13C NMR (see ESI for experimental section and
characterizations). This synthesis affords a stable and non-
65 hygroscopic gold (I) complex which is isolable and easy to
handle, thanks to the NTf2 moiety. When other counter-anions are
used (TfO-, SbF6 , BF4 ), silver salts are added to a solution
containing the gold pre-catalyst to form the catalytic species in
situ 4. In those cases, the active complex is not isolated and
70 residual Ag might also play an active role in catalysis. In
addition, complex 4 is able to interact in one single step with
modified MWNTs to cause its covalent anchoring.
-
-
clays and highly cross-linked polymers are the standard supports
1
to create a heterogeneous catalyst from a homogeneous one
.
25 Carbon nanotubes as supports for the immobilization of
homogeneous catalysts are surprisingly not developed in relation
with the rich chemistry dealing with the functionalization of these
tubes 2. Among the different possible functionalization strategies,
the most common is the covalent one 3 and to our knowledge only
30 a few papers deal with the covalent anchoring of homogeneous
catalysts on carbon nanotubes 4. In addition, gold complexes have
been increasingly used in homogeneous catalysis in the past
decade thanks to their ability to promote a myriad of chemicals
Pristine MWNTs (Thin MWNTs, 95+ % C purity, Nanocyl
S.A.) were firstly modified by the most common method to
75 generate carboxylic acids at the surface of carbon nanotubes, i.e.
refluxing in concentrated HNO3 according to a general procedure
aiming at oxidizing these nanotubes without degrading them8.
The oxidized MWNTs were submitted to chlorination using
SOCl2 in toluene under reflux to obtain acyl chloride functions at
80 the surface of the tubes (see ESI) 9. These modified MWNTs (m-
MWNTs) were dispersed in dichloromethane under ultrasound
before addition of the gold complex 4 (Scheme 1). The reaction
that should occur would lead to an amide bond between the
ligand and surface acyl chlorides. Various periods of time were
85 tested (from 2h to 24h). The resulting functionalized MWNTs (f-
MWNTs) were filtrated in order to separate them from the
solution containing the unreacted gold complex 4. The solids
were analysed by X-ray photoelectron spectroscopy (XPS) to
check if incorporation indeed occurred at the surface of the tubes.
90 These XPS results (see ESI, Table S1) show that a non-negligible
amount of gold was present on the support after reaction and that
the ratios between the different elements (N, P, S, F, Au) were in
agreement with the theoretical numbers. The incorporation
transformations with impressive turnover numbers, particularly
5
35 C-H activation followed by C-C bond formation
. This
communication aims at reporting a simple methodology to
transform a homogeneous gold catalyst into a heterogeneous one
using modified Multi-Walled Carbon Nanotubes (MWNTs) as
support and at showing the potential of this supported
40 homogeneous catalyst in terms of recyclability using enynes as
benchmark substrates.
a Institute of Condensed Matter and Nanosciences, Molecules, Solids and
Reactivity (IMCN/MOST) Université catholique de Louvain, Place Louis
Pasteur 1, 1348 Louvain-la-Neuve, Belgium. Fax: +32 10-472330; Tel:
45 +32 10-472810; E-mail: sophie.hermans@uclouvain.be;
olivier.riant@uclouvain.be
†Electronic Supplementary Information (ESI) available: [synthesis of
gold complex, XPS analysis, NMR of cyclized products and recycling].
See DOI: 10.1039/b000000x/
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duration was not found to be a decisive factor. The incorporated
heterogenized catalyst is active in other cyclization reactions
gold amount was nearly the same irrespective of the time. A 60 (Scheme 2). Similar results were obtained considering the
similar observation can be made concerning the atomic ratios
between N, P, S, F and Au. A duration of 7h was selected and
reproducibility assays produced similar results (see ESI, Table
S1). Furthermore, the oxidation state of gold was analysed by
XPS and remained AuI after incorporation (see ESI). These
results prove that catalyst 5 was covalently anchored and
remained unaffected by this anchoring process. ICP analyses
quantitative conversion of 9 into 10 and 11 (ratio 55/45) and of
12 into 13. Nevertheless, longer time and higher catalytic
loadings were required (i.e. 18 h hours and 5 mol%, Scheme 2)
than with enyne 6.
The covalently anchored gold catalyst 5 was finally tested in
terms of recyclability in the cyclization of enyne 6. After 6 hours,
the solvent was removed in order to separate the filtrate
containing the cyclized products from 5. These f-MWNTs were
5
65
10 were realized to determine the bulk Au amount on MWNTs: the
numbers found (between 3.5 and 4.1 weight %) fall within the
vacuum dried before being used in a new catalytic run and the
1
range of covalently supported homogeneous catalysts on carbon 70 collected solutions were analysed by H NMR. The covalently
nanotubes10.
anchored gold catalyst was found to be reusable according to the
results shown in Scheme 2. Indeed, a quantitative conversion of
enyne 6 into the cyclized products 7 and 8 in a constant ratio
88/12 was found for the first four catalytic runs before a slight
75 decrease appeared during the last three runs. Conversions of 94
%, 81% and finally 43% were observed respectively for runs 5 to
7. When quantitative conversion occurred, an isolated yield of 85
% was obtained (see ESI).
15
20
25
80
85
90
95
Scheme 1. Synthesis and covalent grafting of the gold catalyst.
The homogeneous gold catalyst 4 and its supported counterpart
30 5 were both tested in the cyclization of the model substrate enyne
6 (scheme 2). The gold complex 4 was catalytically effective and
gave complete conversion of 6 using 2 mol% of gold catalyst in
dichloromethane at room temperature in 28h. The cyclization
reaction afforded predominantly the 5-exo-dig product 7 and a
35 minor amount of 6-endo-dig product 8 in a ratio 88/12 as
outcome of the reaction. Compared to the structurally close gold
catalyst Tf2NAuPPh3, the catalytic activity of 4 is much lower
(i.e. 28 hours instead of 10 min for complete conversion) (see
ESI). This drop in catalytic activity can only be explained by the
40 presence of the dangling amine group. An inhibition of the
cyclization reaction could have been explained if the amine group
played an active role in the enyne cyclization mechanism but this
possibility can be ruled out. One postulated hypothesis consists in 100 Scheme 2. Cyclization of enynes 6, 9, 12 with 5 (top). Recycling
a head-to-toe complex dimerization meaning that the amine group
45 could act as ligand taking up the position of NTf2 as ligand and
leading to a loss of the complex activity.
of catalyst 5 (2 mol%) in the cyclization of enyne 6 (bottom).
XPS was performed after run 3 and run 7 to check the gold
oxidation state and the obtained results were in agreement with a
Cyclization of enyne 6 was next performed similarly using the
supported homogeneous catalyst 5 (Scheme 2). This supported 105 gold(I) oxidation state proving that it remained unchanged during
homogeneous catalyst (2 mol % based on ICP results) was first
50 dispersed under ultrasound in distilled dichloromethane before
addition of enyne 6. A complete conversion was obtained after 6
hours giving cyclized products. Compared to its homogeneous
the recycling tests. Although the XPS atomic percents of gold(I)
decreased after each run, the ratios between the constitutive
elements of the catalysts (F,N,P,S,Au) remained globally constant
(see ESI, Table S3). Furthermore, solutions were analysed by ICP
counterpart, a decrease in the reaction time was observed here. 110 after each run in order to quantify the gold leaching. These results
Indeed, due to the formation of an amide group for incorporation
55 onto MWNTs, the above-mentioned effect about the deleterious
role of the dangling amine group does not take place in this case.
Based on these results, enynes 9 and 12 were tested with the
are given in Table 1 and show that gold losses along the runs
were quite constant and really weak. These results prove the
strength of the covalent bond between the gold complex and the
carbon support. Leaching explains more than half of the activity
supported homogeneous catalyst 5 in order to verify if the 115 loss observed after the sixth run. The rest must be due to Au
2
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complex degradation/rearrangment.
Table 1. Solutions analyzed by ICP after each catalytic run.
References
45 1 D. E. De Vos, M. Dams, B. F. Sels, P. A. Jacobs, Chem. Rev.,
2002, 102, 3615; C. E. Song, S. Lee, Chem. Rev., 2002, 102, 3495;
N. E. Leadbeater, M. Marco, Chem. Rev., 2002, 102, 3217; A.
Corma, H. Garcia, Top Catal., 2008, 48, 8; A. Choplin, F. Quignard,
Coord. Chem. Rev., 1998, 98, 1679; M. H. Valkenberg, W. F.
Run
1
Au leaching (ppm)[a]
0.95
Gold lost (%) [b]
1.1
50
55
60
65
70
Hölderich, Catal. Rev., 2002, 44, 321. A. F. Trindade, P. M. P. Gois,
C. A. M. Afonso, Chem. Rev., 2009, 109, 418; M. Heitbaum, F.
Glorius, I. Escher, Angew. Chem. Int. Ed., 2006, 45, 4732.
A. Schaetz, M. Zeltner, W. J. Stark, ACS Catal., 2012, 2, 1267.
P. Singh, S. Campidelli, S. Giordani, D. Bonifazi, A. Bianco, M.
Prato, Chem. Soc. Rev., 2009, 38, 2214; A. Hirsch, Angew. Chem. Int.
Ed., 2002, 41, 1853; D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato,
Chem. Rev., 2006, 106, 1105.
R. Giordano, P. Serp, P. Kalck, Y. Khin, J. Schreiber, C. Marhic, J.-
L. Duvail, Eur. J. Inorg. Chem., 2003, 610; S. Banerjee, S. S. Wong,
J. Am. Chem. Soc., 2002, 124, 8940; Y. Zhang, H.-B. Zhang, G.-D.
Lin, P. Chen, Y.-Z. Yuan, K. R. Tsai, Appl. Catal. A., 1999, 187, 213;
C. Baleizão, B. Gigante, H. Garcia, A. Corma, J. Catal., 2004, 221,
77; C. Baleizão, B. Gigante, H. Garcia, A. Corma, Tetrahedron,
2004, 60, 10461; M. Moghadam, S. Tangestaninejad, V. Mirkhani, I.
Mohammadpoor-Baltork, N. S. Mirbagheri, J. Organomet. Chem.,
2010, 695, 2014; M. Moghadam, S. Tangestaninejad, V. Mirkhani, I.
Mohammadpoor-Baltork, N. S. Mirbagheri, J. Mol. Catal. A: Chem.,
2010, 329, 44; M. Nooraeipour, M. Moghadam, S. Tangestaninejad,
V. Mirkhani, I. Mohammadpoor-Baltork, N. Iravani, J. Coord.
Chem., 2012, 65, 226; M. Zakeri, M. Moghadam, I. Mohammadpoor-
Baltork, S. Tangestaninejad, V. Mirkhani, A. R. Khosropour, J.
Coord. Chem., 2012, 65, 1144; M. Moghadam, I. Mohammadpoor-
Baltork, S. Tangestaninejad, V. Mirkhani, H. Kargar, N Zeini-
Isfahani, Polyhedron, 2009, 28, 3816.
2
0.85
3.3
1.0
4.0
2.8
1.6
1.1
0.7
12.3
3
2
3
4
1.35
0.95
0.62
1.56
9.58
5
4
6
7
Total 1-7
Gold loaded in f-MWNTs 5 before catalysis: 82.4 ppm. [a] Leaching of gold in ppm
5 run by run. [b] Percentage of gold lost run by run (Au leaching (ppm) divided by the
initial quantity: 82.4 ppm)
Heterogenization of Gold (I) complexes is rare irrespective of
the support: it is still at an early stage. MCM-41, polystyrene, P-
10 functionalized coordination polymers and delaminated zeolite
were selected so far as supports to covalently heterogenize and
recycle gold(I) catalysts 11. These supported homogeneous
catalysts were typically recycled four or five times and
accompanied by a slight decrease of the catalytic activity
15 throughout the catalytic cycles, which falls in the range of the
supported gold catalyst 5 anchored onto MWNTs. Compared to
the only other supported gold catalyst onto MWNTs immobilized
by a non-covalent strategy 12, the required time to reach a
complete conversion of enyne 6 is longer using the same
20 conditions (i.e. 2 mol% gold catalyst, dichloromethane), most
probably due to the reaction occurring at the solid/liquid
interface. In the non-covalent case, a boomerang effect was
unravelled which allows the gold complex to act as soluble
species. Nevertheless, the covalent anchoring realized in the
25 present work enabled to obtain significant advantages.
Considering the same enyne 6 cyclization reaction, an increase in
the number of runs realized before deactivation as well as a
higher conversion along the runs were obtained. The gold
leaching along the runs was much lower and reached barely 11%
30 after 5 consecutive runs, which is lower than the gold boomerang
catalyst (29% after 5 runs) and undoubtedly due to the covalent
linkage between catalyst 4 and m-MWNTs.
75 5 A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180; Z. Li, C. Brouwer, C.
He, Chem. Rev., 2008, 108, 3239; N. Krause, C. Winter, Chem. Rev.,
2011, 111, 1994; A. Corma, A. Leyva-Pérez, and M.J. Sabater,
Chem. Rev., 2011, 111 -
Chem. Rev., 2008, 108, 3326; A. Arcadi, Chem. Rev., 2008, 108,
80
3266; C.Aubert, L. Fensterbank, P. Garcia, M. Malacria, A.
Simonneau, Chem. Rev., 2011, 111, 1954; L. Lesseure, C.-M. Chao,
T. Seki, E. Genin, P. Y. Toullec, J.-P. Genêt, V. Michelet,
Tetrahedron, 2009, 65, 1911; M. C. B. Jaimes, C. R. N. Böhling, J.
M. Serrano-Becerra, A. S. K. Hashmi, Angew. Chem. Int. Ed., 2013,
52, 7963.
M. Nishimura, M. Ueda, N. Miyaura, Tetrahedron, 2002, 58, 5779.
N. Dupré, C. Brazel, L. Fensterbank, M. Malacria, S. Thorimbert, B.
Hasenknopf, E. Lacôte, Chem. Eur. J., 2012, 18, 12962.
S. Hermans, V. Bruyr, M. Devillers, J. Mater. Chem., 2012, 22,
14479.
85
6
7
8
9
90
D. Vidick, M. Herlitschke, C. Poleunis, A. Delcorte, R. P. Hermann,
M. Devillers, S. Hermans, J. Mater. Chem. A, 2013, 1, 2050.
10 C. Baleizão, B. Gigante, H. Garcia, A. Corma, Tetrahedron, 2004,
60, 10461; M. Moghadam, S. Tangestaninejad, V. Mirkhani, I.
Mohammadpoor-Baltork, N. S. Mirbagheri, J. Organomet. Chem.,
2010, 695, 2014; M. Nooraeipour, M. Moghadam, S.
Tangestaninejad, V. Mirkhani, I. Mohammadpoor- Baltork, N.
Iravani, J. Coord. Chem., 2012, 65, 226; M. Zakeri, M. Moghadam, I.
Mohammadpoor-Baltork, S. Tangestaninejad, V. Mirkhani, A. R.
Khosropour, J. Coord. Chem., 2012, 65, 1144; M. Moghadam, I.
Mohammadpoor-Baltork, S. Tangestaninejad, V. Mirkhani, H.
Kargar, N Zeini-Isfahani, Polyhedron, 2009, 28, 3816.
95
100
105
110
Conclusions
A gold complex with a pendant amine group was designed for
35 its covalent anchoring onto modified MWNTs in order to give a
supported homogeneous catalyst. Compared to its homogeneous
counterpart, the selectivity to the cyclized products was
unchanged but the activity was considerably improved. Recycling
is conceivable and is combined with a fairly low gold leaching.
40 To the best of our knowledge, this is only the second case of
reported supported gold homogeneous catalyst onto carbon
nanotubes and the first example of supported homogeneous gold
catalyst onto carbon nanotubes via covalent anchoring.
11 M. Raducan, C. Rodriguez-Escrich, X. C. Cambeiro, E. C. Escudero-
Adan, M. A. Pericàs, A. M. Echavarren, Chem. Commun., 2011, 47,
4893; A. Corma, C. Gonzalez-Arellano, M. Iglesias, S. Pérez-
Ferreras, F. Sanchez, Synlett, 2007, 1771; W. Cao, B. Yu,
Synth. Catal., 2011, 353, 1903; G. Villaverde, A. Corma,
Adv.
M.
Iglesias, F. Sanchez, ChemCatChem, 2011, 3, 1320; G. Villaverde,
A. Corma, M. Iglesias, F. Sanchez, ACS Catal., 2012, 2, 399; J.
Vaclavik, M. Servalli, C. Lothschütz, J. Szlachetko, M. Ranocchiari,
J. A. van Bokhoven, ChemCatChem, 2013, 5, 692.
12 C. Vriamont, M. Devillers, O. Riant, S. Hermans, Chem. Eur. J.,
2013, 19, 12009.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3