Imidazolium-functionalized bipyridine derivatives: A promising family of ligands for catalytical Rh(0) colloids

Article abstract of DOI:10.1016/j.tetlet.2009.09.039

A new imidazolium-monofunctionalized bipyridine ligand has been synthesized and efficiently used as a protective agent for Rh(0) nanoparticles. Preliminary results in arene hydrogenation have been obtained.

Full text of DOI:10.1016/j.tetlet.2009.09.039

Tetrahedron Letters 50 (2009) 6531–6533
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Imidazolium-functionalized bipyridine derivatives: a promising family
of ligands for catalytical Rh(0) colloids
Bastien Léger a,b, Audrey Denicourt-Nowicki a,b, Hélène Olivier-Bourbigou , Alain Roucoux
c
a,b,*
a
Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35 708 Rennes Cedex 7, France
b
Université Européenne de Bretagne, France
c
IFP Solaize, BP3, 69390 Vernaison, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2009
Revised 3 September 2009
Accepted 7 September 2009
Available online 11 September 2009
A new imidazolium-monofunctionalized bipyridine ligand has been synthesized and efficiently used as a
protective agent for Rh(0) nanoparticles. Preliminary results in arene hydrogenation have been obtained.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Bipyridine
Imidazolium
Nanoparticles
Hydrogenation
Arenes
0
In the last decades, transition-metal nanoparticles have proved
the solubility of the 2,2 -bipyridine ligand in ionic liquids and also
to induce new interactions between the protective agent and the
reaction media, we have designed an imidazolium-monofunction-
alized bipyridine 1 as precursor of a promising family of ligands
(Scheme 1). In this Letter, we report the synthesis of this new li-
1
to be highly efficient catalytic systems in various reactions and are
2
particularly known as a relevant catalyst for arene hydrogenation.
As they are thermodynamically unstable, the protective agents
should be chosen according to the nature of the metallic species
and the reaction media to avoid their aggregation.3 Among them,
ionic liquids are promising, as they can play the dual role of solvent
0
gand 1 based on 2,2 -bipyridine, which has proved to be efficient
for the stabilization of Rh(0) nanoparticles. Catalytic results in
terms of activity and selectivity in arene hydrogenation are com-
pared with 2,2 -Bipyridine-protected Rh(0) colloids.
The new bipyridine ligand bearing an imidazolium tag 1 is
synthesized starting from 4,4 -dimethyl-2,2 -bipyridine 2 by a
4
and protective agent. Rh, Ru, Ir, and Pd nanoparticles have been
0
easily prepared in various ionic liquids and have proved to be effi-
5
cient catalysts for olefin or arene hydrogenation reactions. Never-
0
0
theless, Rh(0) and Ir(0) nanocatalysts in simple imidazolium ionic
liquids tend to aggregate after reactions such as hydrogenation of
1
2,13
procedure adapted from the literature.
The route is described
6
0
0
aromatic compounds or ketones with loss of catalytic activity. In
in Scheme 2. Reaction of 4,4 -dimethyl-2,2 -bipyridine 2 with BuLi
in THF generates the monolithiated intermediate, which is
quenched with 1,7-dibromoheptane. The resulting 4-bromooc-
tyl-4 -methyl-2,2 -bipyridine 3 reacted with 1-methylimidazole
in toluene at 90 °C for 7 days to yield 4-(1-methylimidazolium-
that context, more stable catalytic systems could be obtained by
7
the addition of an extra-protective agent, such as PVP or 1,10-phe-
8
0
0
nanthroline, which has proved to play a synergistic effect on the
activity and durability of the catalyst. Moreover, as the commonly
used organic stabilizers present low solubility in ionic liquids, new
ionic copolymers containing imidazolium units, which could act as
soluble bifunctional costabilizers when dissolved in ionic liquids,
9
have been designed. Recently, our team have described the stabil-
ization in ionic liquids of Rh(0) nanoparticles by N-donor ligands,
such as 2,2 -bipyridine or triazine and pyrazine derivatives,
and their application in arene hydrogenation. In order to improve
N
8
0
10
11
N
N
N
Br
1
*
Corresponding author. Tel.: +33 (0) 2 23 23 80 37.
E-mail address: alain.roucoux@ensc-rennes.fr (A. Roucoux).
Scheme 1. Imidazolium-monofunctionalized bipyridine ligand 1.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.039

6
532
B. Léger et al. / Tetrahedron Letters 50 (2009) 6531–6533
Br
N
8
8
N
a.
b.
N
N
N
N
N
N
Br
2
3
1
Scheme 2. Synthesis of imidazolium-functionalized bipyridine ligand 1. Reagents and conditions: (a) (i) diisopropylamine, BuLi, THF, ꢀ78 °C; (ii) 1,7-dibromoheptane, THF,
78 °C and (b) methylimidazole, toluene, 90 °C, 7 days.
ꢀ
0
0
3
-yloctyl)-4 -methyl-2,2 -bipyridine bromide 1 with an overall
ethylbenzene (entries 1–6), the decrease in catalytic activity is
typical of the influence of the increasing steric hindrance as usually
14
non-optimized yield of 36%.
On the basis of the method previously developed for the synthe-
sis of Rh(0) nanoparticles stabilized by 2,2 -bipyridine,
1
9
observed with other catalysts. As the colloidal suspensions are
always stable after the catalytic reaction, we could presume that
the complete hydrogenation of aromatic rings such as toluene (en-
try 3), ethylbenzene (entry 5), and styrene (entry 7) could be
achieved for longer reaction times and that the catalytic system
could be easily recycled by simple liquid–liquid extraction with
diethylether. Thus, similar catalytic activities have been obtained
for the two catalytic systems, showing that the imidazolium cation
tagged to the bipyridine has no significant influence on the sub-
strate approach on nanoparticles surface. Moreover, in the absence
of bipyridine ligand, styrene was totally converted in ethylcyclo-
hexane in 15 h; however, the colloidal suspension is unstable after
0
10a
the col-
loidal Rh(0) suspensions have been prepared by the chemical
reduction of rhodium trichloride salt with an excess of sodium
borohydride in a monophasic tetrahydrofuran-ionic liquid med-
ia. Immediately after reduction, the protective agent 1, dissolved
in THF, was added to the pre-stabilized metallic Rh(0) nanoparti-
cles, according to the procedure described in Scheme 3. The reduc-
tion, which is characterized by a color change from red to black,
has been performed in the open air and at room temperature
occurring instantaneously. A previously optimized molar ratio
protective agent]/[M] of 0.5, which has already proved to be a
good compromise between stability and activity of the nanocata-
lyst, has been chosen and [BMI][PF
ionic liquid.
The catalytic activity of the imidazolium-functionalized bipyri-
dine ligand 1-protected Rh(0) nanoparticles in [BMI][PF ] has been
1
5
[
1
0a
catalytic reaction with formation of aggregates.
Thus, this preli-
16
6
]
has been used as a standard
minary work is promising as this new synthesized compound 1 of-
fers the opportunity to develop a new class of functionalized ionic
liquids by simple anion-exchange reactions, as recently described
2
0
6
in the literature. Moreover, according to the possibility to access
0
0
evaluated in the hydrogenation of benzene and monofunctional-
ized derivatives according to the experimental conditions previ-
3,3 - and 4,4 -bipyridines and to modify the chain length, this class
of new ligands could significantly be increased.
ously optimized (40 bar H
(
2
, 80 °C) in an arbitrarily chosen time
15 h). The conversion was determined by gas chromatography
analysis. The catalytic results are summarized in Table 1 and com-
In conclusion, a new imidazolium-monofunctionalized bipyri-
dine ligand 1 has been easily prepared in two steps starting from
commercial 4,4 -dimethyl-2,2 -bipyridine and has proved to be
an efficient protective agent for the stabilization of Rh(0) nanopar-
1
7
0
0
0
10a
pared with the results already described for 2,2 -Bipy.
The results clearly show that the catalytic performances of imi-
dazolium-functionalized bipyridine ligand 1-protected Rh(0) nano-
species in the hydrogenation of monofunctionalized aromatic
6
ticles in [BMI][PF ]. These ligand 1-stabilized Rh(0) colloids are
efficient and stable catalysts for arene hydrogenation in ionic liq-
uids. The use of ligands as nanoparticles protective agent is a
promising alternative to avoid aggregation in some hydrogenation
reactions. Finally, a new class of functionalized ionic liquids could
be easily prepared starting from this imidazolium-functionalized
bipyridine ligand 1 in order to improve, for example, the perfor-
mances of these nanocatalysts in terms of activity and selectivity
with asymmetric tags.
0
substrates are quite similar to those of 2,2 -Bipyridine-stabilized
Rh(0) colloids with turnover number (TON) up to 300. The turnover
number (TON) was established according to the amount of rho-
dium introduced, taking into account the true number of active
metal sites. Thus, we could presume that TON may be underesti-
1
8
mated. In both cases, in the series of benzene, toluene, and
THF/[BMI][PF6]
NaBH4
1
Bipyridine-functionalized imidazolium IL
THF
RhCl .3H O
Rh(0)/IL
Rh(0)/1/[BMI][PF ]
6
3
2
Scheme 3. Synthesis of Rh(0) nanoparticles stabilized by bipyridine-functionalized imidazolium IL 1 in ionic liquids.
Table 1
Hydrogenation of arene derivatives with Rh(0) nanoparticles stabilized by imidazolium-functionalized bipyridine ligand 1 or 2,2 -bipyridine
0
a
Entry
Ligand
Substrate
Product
Conversionb (%)
TONc
1
2
3
4
5
6
7
8
1
Benzene
Benzene
Toluene
Toluene
Ethylbenzene
Ethylbenzene
Styrene
Cyclohexane
Cyclohexane
Methylcyclohexane
Methylcyclohexane
Ethylcyclohexane
Ethylcyclohexane
Ethylbenzene (35)/ethylcyclohexane (65)
Ethylbenzene (40)/ethylcyclohexane (60)
100
100
85
100
60
60
100
100
300
300
255
300
180
180
300
300
0
2,2 -Bipy
1
2,2 -Bipy
1
2,2 -Bipy
1
2,2 -Bipy
0
0
0
Styrene
a
b
c
Reaction conditions: Rh (3.8 ꢁ 10 mol), ligand (1.9 ꢁ 10ꢀ5 mol), [BMI][PF
ꢀ5
6
2
] (2 mL), substrate/Rh(0) (mol/mol) = 100, 40 bar H , 80 °C, 15 h, stirred at 1500 rpm.
Determined by GC analysis.
2
Turnover number defined as the number of moles of consumed H per mole of introduced rhodium.

B. Léger et al. / Tetrahedron Letters 50 (2009) 6531–6533
6533
temperature. The resulting yellow-brown oil was purified by column
chromatography (neutral aluminum oxide, petroleum ether/ diethyl ether 9/
Acknowledgments
1
) yielding 3 as a white solid (143 mg, 36%); R
f
= 0.2 (petroleum ether/diethyl
1
We thank Institut Français du Pétrole and CNRS for financial
support.
ether 9/1); H NMR (400 MHz, CDCl , d ppm): 8.60 (s, 2H), 8.40 (d, 2H), 7.02 (d,
2H), 3.30 (t, 2H), 2.55 (t, 2H), 2.37 (s, 3H), 1.79 (st, 2H), 1.62 (st, 2H), 1.29 (m,
8
1
2
3
H). 13C NMR (100 MHz, CDCl
, d, ppm): 157.9, 157.2, 150.5, 149.8, 148.3,
47.7, 123.2, 122.6, 121.3, 120.7, 36.1, 34.1, 32.9, 32.4, 30.3, 29.9, 29.3, 28.7,
3
References and notes
0
0
1.2. Preparation of 4-(1-methylimidazolium-3-yloctyl)-4 -methyl-2,2 -bipyridine
bromide 1: 35.7 mg (0.43 mmol, 1.1 equiv) of distilled methylimidazole was
0
0
1
.
(a) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757–3778; (b)
Roucoux, A.; Nowicki, A.; Philippot, K. In Nanoparticles and Catalysis; Astruc, D.,
Ed.; Wiley-VCH: Weinheim, 2008; pp. 349–388. Chapter 11.
Roucoux, A.; Philippot, K. In Handbook of Homogeneous Hydrogenation; de Vries,
J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, 2007; pp 217–256. Chapter 9.
Duncan Pachon, L.; Rothenberg, G. Appl. Organomet. Chem. 2008, 22, 288–299.
Astruc, D.; Lu, L.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852–7872.
(a) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R.
J. Am. Chem. Soc. 2002, 124, 4228–4229; (b) Fonseca, G. S.; Umpierre, A. P.;
Fichtner, P. F. P.; Teixeira, S. R.; Dupont, J. Chem. Eur. J. 2003, 9, 3263–3269;
added to 143 mg (0,39 mmol) of 4-(bromooctyl)-4 -methyl-2,2 -bipyridine 3 in
toluene (10 mL). The reaction mixture was stirred under inert atmosphere at
90 °C for a week. Toluene was removed under reduced pressure at ambient
temperature and the final product 1 (60 mg) was obtained with a nearly
2
.
quantitative yield. 1H NMR (400 MHz, CDCl
, d, ppm): 8.,92 (s, 3H), 8.42 (d, 2H),
7.76 (d, 1H), 7.71 (d, 1H), 7.19 (d, 2H), 4.39 (s, 3H), 4.04 (t, 2H), 2.62 (t, 2H), 2.36
(s, 3H), 1.74 (m, 2H), 1.59 (m, 2H), 1.33 (m, 2H), 1.29 (m, 4H). 13C NMR
(100 MHz, CDCl , d, ppm): 156, 155.7, 152.9, 149.7, 149, 147.7, 136.2, 123.6,
3
122.2, 121.3, 120.7, 123.2, 122.6, 51.8, 36.1, 34.2, 32.4, 30.3, 30, 29.9, 28.1, 21.2,
13.5.
3
3.
4.
5.
(
c) Huang, J.; Jiang, T.; Han, B.; Gao, H.; Chang, Y.; Zhao, G.; Lu, W. Chem.
15. Synthesis of ligand-stabilized Rh(0) nanoparticles in ionic liquid: RhCl
3
ꢂ3H
2
O
ꢀ
5
Commun. 2003, 1654–1655; (d) Mévellec, V.; Léger, B.; Mauduit, M.;
Roucoux, A. Chem. Commun. 2005, 2838–2839; (e) Fonseca, G. S.;
Domingos, J. B.; Nome, F.; Dupont, J. J. Mol. Catal. A: Chem. 2006, 248, 10–
(10 mg, 3.8 ꢁ 10 mol, 2.5 equiv) was dispersed in a mixture of THF (5 mL)
ꢀ
5
and ionic liquid (2 mL). NaBH
4
(3.6 mg, 9.5 ꢁ 10 mol, 2.5 equiv) dissolved in
water (two drops) was quickly added to the mixture under vigorous stirring.
ꢀ
5
1
6; (f) Prechtl, M. H. G.; Scarlot, M.; Scholten, J. D.; Machado, G.; Teixeira, S.
Immediately, the ligand 1 (1.9 ꢁ 10 mol, 0.5 equiv/Rh) that was considered
and dissolved in 5 mL of THF was quickly added under vigorous stirring to the
mixture. Then, THF was removed under reduced pressure and the colloidal
suspension was dried under vacuum during 2 h. The reduction occurs
instantaneously and is characterized by a color change from red to black.
The obtained suspensions are stable for several weeks.
R.; Dupont, J. Inorg. Chem. 2008, 47, 8995–9001; (g) Rossi, L. M.; Machado, G.
J. Mol. Catal. A: Chem. 2009, 298, 69–73.
Migowski, P.; Dupont, J. Chem. Eur. J. 2007, 13, 32–39.
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6
7
8
.
.
.
2
003, 1654–1655.
(a) Mu, X. D.; Meng, J. Q.; Li, Z. C.; Kou, Y. J. Am. Chem. Soc. 2005, 127, 9694–
695; (b) Zhao, C.; Wang, H. Z.; Yan, N.; Xiao, C. X.; Mu, X. D.; Dyson, P. J.; Kou,
16. [BMI][PF
6
] was prepared from a procedure described in the literature (Adv.
Synth. Catal. 2006, 348, 243–248) and was dried under vigorous stirring for 6 h
9
.
9
3
at 70 °C and under vacuum. Its chloride content was checked by AgNO test
and the chloride content’s purity by 1H NMR and P NMR spectra.
31
Y. J. Catal. 2007, 250, 33–40; (c) Olivier, J. H.; Camerel, F.; Selb, J.; Retailleau, P.;
Ziessel, R. Chem. Commun. 2009, 1133–1135.
17. General procedure for hydrogenation under hydrogen pressure: the stainless steel
1
0. (a) Léger, B.; Denicourt-Nowicki, A.; Roucoux, A.; Olivier-Bourbigou, H. Adv.
Synth. Catal. 2008, 350, 153–159; (b) Léger, B.; Denicourt-Nowicki, A.; Olivier-
Bourbigou, H.; Roucoux, A. Inorg. Chem. 2008, 47, 9090–9096.
autoclave was charged with 2 mL of ligand-stabilized Rh(0) colloidal
suspension in [BMI][PF ] and a magnetic stirrer. The appropriate substrate
6
(3.8 ꢁ 10 mol, 100 equiv) was added into the autoclave and dihydrogen was
admitted to the system at constant pressure up to 40 atm. The mixture was
heated to 80 °C and stirred for 15 h. After cooling to ambient temperature, the
ꢀ
5
1
1. Léger, B.; Denicourt-Nowicki, A.; Olivier-Bourbigou, H.; Roucoux, A.
ChemSusChem 2008, 1, 984–987.
ꢀ
2
1
2. Marin, V.; Holder, E.; Schubert, U. S. J. Polym. Sci. Part A: Polym. Chem. 2004, 42,
3
mixture was dispersed into 10 mL of CH CN and centrifuged (g = 20152 m s )
3
74–385.
during 10 min for the precipitation of nanoparticles. The sample was analyzed
by gas chromatography, using Carlo Erba GC 6000 with FID detector equipped
with a Factor Four column (30 m, 0.25 mm i.d.). Parameters were as follows:
temperature, 80 °C; injector temperature, 220 °C; and detector temperature,
250 °C.
1
1
3. Wu, X. E.; Ma, L.; Ding, M. X.; Gao, L. X. Chem. Lett. 2005, 34, 312–313.
4. Experimental procedure for the synthesis of 4-(bromooctyl)-4 -methyl-2,2 -
bipyridine 3: 1.2 mL (1.3 mmol, 1.2 equiv) of BuLi was added dropwise to an
0
0
anhydrous solution of diisopropylamine (190 lL, 1.3 mmol, 1.2 equiv) in THF
(
2 mL) at ꢀ78 °C. The resulting solution was maintained under vigorous
18. (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317–341; (b)
Widegren, J. A.; Finke, R. G. Inorg. Chem. 2002, 41, 1558–1572. and 1625–1638.
19. (a) Roucoux, A.. In Topics in Organometallic Chemistry; Surface and Interfacial
Organometallic Chemistry and Catalysis; Coperet, C., Chaudret, B., Eds.; Springer:
Heidelberg, 2005; Vol. 16, pp 261–279; (b) Fonseca, G. S.; Silveira, E. T.;
Gelesky, M. A.; Dupont, J. Adv. Synth. Catal. 2005, 347, 847–853.
20. (a) Yang, X.; Yan, N.; Fei, Z.; Crespo-Quesada, R. M.; Laurenczy, G.; Kiwi-
Minsker, L.; Kou, Y.; Li, Y.; Dyson, P. J. Inorg. Chem. 2008, 47, 7444–7446; (b) Hu,
Y.; Yu, Y.; Hou, Z.; Li, H.; Zhao, W.; Feng, B. Adv. Synth. Catal. 2008, 350, 2077–
2085.
0
stirring at ꢀ78 °C for 30 min. Then, 203 mg (1.1 mmol, 1 equiv) of 4,4 -
0
dimethyl-2,2 -bipyridine in THF (6 mL) was added to the mixture dropwise at
ꢀ
78 °C. The reaction mixture was vigorously stirred at ꢀ78 °C for 1 h. 743
lL
(
4.3 mmol, 4 equiv) of 1,7-dibromoheptane in THF (2 mL) was added to the
mixture which was slowly warmed to room temperature and stirred overnight.
The reaction was quenched by slow addition of 2 mL of water. Subsequently,
8
extracted with diethyl ether. The organic layer was dried over magnesium
sulfate, and the ether was removed under reduced pressure at room
mL of phosphate buffer (pH 7) was added and the reaction mixture was