Arene Hydrogenation with a Stabilised Aqueous Rhodium(0) Suspension: A Major Effect of the Surfactant Counter-Anion

Article abstract of DOI:10.1002/adsc.200390016

A reduced aqueous colloidal suspension of rhodium shows an efficient activity in the catalytic hydrogenation of various benzene derivatives under biphasic conditions at room temperature and under atmospheric hydrogen pressure. The rhodium nanoparticles in the size range of 2-2.5 nm have been synthesised by reducing RhCl₃ · 3 H₂O with sodium borohydride and were stabilised by highly water-soluble N,N-dimethyl-N-cetyl-N-(2- hydroxyethyl)ammonium salts (HEA16X, X = Br, Cl, I, CH₃SO ₃, BF₄). The major influence of the counter-ion of these surfactants on catalytic activity and recycling is described. The best results have been obtained with chloride ammonium salts HEA16Cl.

Full text of DOI:10.1002/adsc.200390016

FULL PAPERS
Arene Hydrogenation with a Stabilised Aqueous Rhodium(0)
Suspension: A Major Effect of the Surfactant Counter-Anion
Alain Roucoux,* J¸rgen Schulz, Henri Patin
Ecole Nationale Sup e¬ rieure de Chimie de Rennes, UMR CNRS 6052 ™Synth e¡ ses et Activations de Biomol e¬ cules,∫ Institut
de Chimie de Rennes, Avenue du Gal Leclerc, 35700 Rennes, France
Fax: ( ‡ 33)-2-2323-8199, e-mail: Alain.Roucoux@ensc-rennes.fr
Received: June 12, 2002; Accepted: October 10, 2002
Abstract: A reduced aqueous colloidal suspension of monium salts (HEA16X, X ˆ Br, Cl, I, CH SO , BF ).
3
3
4
rhodium shows an efficient activity in the catalytic The major influence of the counter-ion of these
hydrogenation of various benzene derivatives under surfactants on catalytic activity and recycling is
biphasic conditions at room temperature and under described. The best results have been obtained with
atmospheric hydrogen pressure. The rhodium nano- chloride ammonium salts HEA16Cl.
particles in the size range of 2 ± 2.5 nm have been
synthesised by reducing RhCl ¥ 3 H O with sodium
3
2
borohydride and were stabilised by highly water- Keywords: biphasic catalysis; colloids; counter-ions;
soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)am- hydrogenation; rhodium; surfactants
Introduction
and to facilitate recycling, particles must be stabilised by
a highly water-soluble protecting agent. We have chosen
The complete hydrogenation of benzene derivatives an ionic surfactant to prepare and to protect the aqueous
represents an important industrial catalytic transforma- colloidal suspension of metallic particles. Previously we
tion due to the increasing industrial demand for low- have described the easy synthesis of N,N-dimethyl-N-
[
1]
aromatic diesel fuels. Moreover, the conversion of alkyl-N-(2-hydroxyethyl)ammonium bromide salts
benzene to cyclohexane still represents the most im- (HEAC Br). The best system has been defined as
1
2±1ꢀ
[
1±8]
portant industrial hydrogenation reaction.
In some Rˆ2ˆHEAC Br/Rh which gives sufficiently hydrophil-
cases, the partial arene hydrogenation to cyclohexenes is ic behaviour to maintain catalytic species within the
1
6
[
9±13]
[44]
desired and in others observed during the reduction.
aqueous phase and to prevent aggregation. In this
Undoubtedly, in the future, this reaction will be an active paper, we describe the effect of the counter-ion to
area of research. Traditionally, monocyclic arene hydro- ammonium on the catalytic system. In fact, its influence
[
14±23]
genation is performed with heterogeneous catalysts
on the rhodium nanoparticle size and on the hydro-
such as Rh/Al O and Raney Nickel or metal sulfides. genation of various arene derivatives was investigated.
2
3
Nevertheless, some pure homogeneous systems have
been reported.[
7,24±28]
These systems require in many if
not most cases drastic conditions (high pressure and/or Results and Discussion
temperature). In a few cases, these homogeneous
catalysts have been shown to be microheterogeneous The catalytically active aqueous suspension was made of
catalysts.[
29±31]
Consequently, the use of nanoparticles as metallic rhodium(0) particles prepared by reducing
catalysts has received increased attention and now some rhodium trichloride with sodium borohydride in dilute
colloidal catalysts give satisfactory results for benzene/ aqueous solutions of hydroxyethylammonium salts
arene reduction in organic mixtures. The use of rhodium HEA16X (X ˆ Br, Cl, I, CH SO , BF ). These com-
3
3
4
and ruthenium nanoparticles for arene derivative hy- pounds are easily synthesised by quaternisation of N,N-
[
4,5,31±39]
drogenation has been largely reported.
dimethylethanolamine with the appropriate functional-
Our original approach is to use colloidal metallic ised cetylalkanes or by ion exchange (Scheme 1).
[
40]
particles finely dispersed in water.
The colloidal
Surface tension measurements demonstrate that
suspension must be stabilised by protective agents. HEA16X self aggregates into micelles above the critical
À3
À3
Several stabilisers are used in the hydrogenation of micellar concentration (cmc) of 1.1 Â 10 , 1.4 Â 10 ,
À4
À5
À4
À1
arene derivatives to prevent aggregation such as tet- 9.3  10 , 1.3  10 and 2.8  10 mol ¥ L for X ˆ
[
4,5,31,34,38,39,41±43]
[35]
raalkylammonium salts,
polymers, sur- Br, Cl, Ms, I, BF , respectively. The decrease of the
4
factants[
31,40,44,45]
or polyoxoanions.[12±13,46] In this context surface tension of aqueous solutions of these surfactants
222
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/03/3451+2-222 ± 229 $ 17.50-.50/0
Adv. Synth. Catal. 2003, 345, No. 1+2

Arene Hydrogenation with a Stabilised Aqueous Rhodium(0) Suspension
FULL PAPERS
X
EtOH, reflux
HO
+
C16H33X
HO
NMe2
N
Me2
Yield 75−95%
13
X = Br, Cl, I, CH3SO3
HEA16X
Cl
NaBF4
acetone, reflux
BF4
HO
HO
N
Me2
13
N
Yield 83%
13
Me2
HEA16Cl
HEA16BF4
Scheme 1. Synthesis of N,N-dimethyl-N-alkyl-N-(2-hydroxy-
ethyl)ammonium salts HEA16X.
Scheme 3. Schematic representation of the double-layer
around particles.
À1
below the cmc (from 72 mN ¥ m for pure water to about ly, these nearly monodispersed colloidal suspensions
À1
3
8 mN ¥ m ) is related to their adsorption at the water/ were highly stable and can be used under our mild
[
47]
air interface by the Gibb×s law.
The cmc values catalytic conditions. The zeta potential measurements
decreased from Cl > Br > BF > Ms > I. The larger have been performed to find the apparent charge of a
4
counter-ions are more polarisable and contribute to nanoparticle in solution. The surfactants form with their
decreased electrostatic repulsions between head groups counter-ions an electrical double-layer around the
and consequently promote the micelle formation.
rhodium(0) particles (Scheme 3). The subshell (Stern
Based on this study, catalytically active aqueous layer) characterises the ionic species adsorbed on nano-
suspensions are made of metallic rhodium(0) colloids particles surface and the outershell (Gouy À Chapman
prepared at room temperature by reducing rhodium layer) is a disperse layer where salt concentration
trichloride with sodium borohydride in dilute aqueous decreases with increasing distance.
solutions of HEA16X salts. The particle size of the Rh-
These values characterise the difference of potential
HEA16X systems have been determined by transmis- between the solution far from the metal/solution inter-
sion electron cryomicroscopic observations. The histo- face and the mobile part of the double-layer. The results
grams (Scheme 2) of the nanoparticles size distribution of zetametric studies of catalytic suspensions are
were estimated once the original negative had been summarised in Table 1. In all cases, rhodium(0) nano-
digitally scanned for more accurate resolution. Meas- particles give an apparently positive charge in solution
urement of about 300 particles was made with an from 40 to 100 mV. This phenomenon has already been
[
49]
automatic counting objects program based on shape observed with cationic stabilisers.
The measured
recognition.[ The average particles size was 2.1 to values show the important role of the electrostatic
.4 nm. repulsion (coulombic interaction) for the stabilisation of
These comparative TEM studies show that the nanoclusters.
counter-ion of the surfactant does not seem to present
48]
2
In preliminary catalytic studies, we compared the
a major influence on the average particle size of colloidal rhodium systems Rh-HEA16X (X ˆ Br, Cl, I,
colloidal suspensions. Finally, no evolution of distribu- Ms, BF ) in the reduction of anisole under atmospheric
4
tion was observed after catalysis as we have previously pressure and at room temperature (Scheme 4). All
reported with the Rh-HEA16Br system.[44] Consequent- aqueous catalytic suspensions were sufficiently stable
Scheme 2. Size distribution histograms of rhodium(0) suspensions.
Adv. Synth. Catal. 2003, 345, 222 ± 229
223

FULL PAPERS
Alain Roucoux et al.
À
Table 1. Zeta potential measurements versus X stabiliser.
Surfactant
z [mV][a]
HEA16Cl
HEA16Ms
HEA16BF4
HEA16Br
‡ 40.0
‡ 100.0
‡ 55.5
‡ 50.0
[
a]
Measurements performed in catalytic conditions: rhodium
±5
±5
(
3.8 Â 10 mol), surfactant (7.6 Â 10 ), water (10 mL),
temperature (20 8C).
Table 2. Influence of hydrogen pressure on anisole reduc-
tion[
a]
Surfactant
10 bar/15 min
30 bar/15 min
st
[b] nd
run[b]
st
[b] nd
run[b]
1
run
2
1 run
2
HEA16Cl
HEA16Ms
HEA16BF4
HEA16Br
HEA16I
58
44
44
30
±
56
-
42
27
±
100
100
100
80
100
-
100
77
±
[c]
[c]
±
[
a]
Conditions: reaction time (15 min), catalyst (3.8
Â
À5
À5
À3
1
1
0
mol), anisole (3.8 Â 10 mol), surfactant (7.6 Â
0 ), water (10 mL), hydrogen pressure (1 atm), tem-
À1
perature (20 8C), stirred at 1500 min .
Percent of methoxycyclohexane after 15 min determined
by GLC analysis.
[
[
b]
c]
Emulsion, cannot be recycled by decantation.
during the hydrogenation to allow their recycling after a
meticulous decantation of the biphasic mixture. Each
reaction and its recycling were performed under iden-
tical conditions.
À
The results show that the counter-ion (X stabiliser)
significantly influences the catalytic activities. The
rhodium suspensions stabilised by HEA16Cl,
HEA16BF and HEA16Ms give analogous activities
4
during the first run. The reaction time varies between 3.6
and 4.3 h. Nevertheless, the CH SO anion gives an
Scheme 4. Influence of counter-ion on activity and recycling.
3
3
emulsion generating a small metal loss. In this case, the
aqueous phase cannot be easily recycled justifying an
No hydrogen consumption was detected indicating
increasing of the reaction time during the recycling in that I had completely killed the catalytic system. This
2
contrast to the use of Rh-HEA16Br and Rh-HEA16Cl experiment confirms that iodine is a poison of rhodium
which give better results. Surprisingly, no anisole hydro- nanoparticles and justifies the absence of reactivity
genation was observed with the use of HEA16I as when HEA16I was used.
surfactant. This phenomenon can be explained by a
The efficiency of the catalytic system modified by
3‡
redox reaction between the Rh /Rh (E ˆ 0.758 V) and different surfactants was tested under hydrogen pres-
À
I /I (E ˆ 0.536 V) couples giving iodine during the sure. The results obtained in anisole hydrogenation after
2
preparation of the rhodium suspension. We tested the 15 min of reaction time under 10 and 30 bars are
behaviour of the more efficient Rh-HEA16Cl system summarised in Table 2.
after addition of iodine (Scheme 5). To a traditional
catalytically active solution containing anisole, 760 mL gave rise to an activation of the catalytic suspension and
7.6 mmol) of I were added after 2 hours of reaction the conversion determined by GLC analysis was usually
In most of cases the increasing hydrogen pressure
(
2
time (about 65%) and the solution was reconnected to complete after 15 min under 30 bar. We also observed a
the hydrogenation apparatus.
good conservation of the conversion rate during the
recycling. Finally, the relative activities of different
224
Adv. Synth. Catal. 2003, 345, 222 ± 229

Arene Hydrogenation with a Stabilised Aqueous Rhodium(0) Suspension
FULL PAPERS
Table 3. Hydrogenation of benzene and monosubstituted derivatives[a]
Substrate
Surfactant
Products (yields %)[b]
1st
t [h]
run
TOF [h ]
2nd
t [h]
run
TOF [h ]
À1 [c]
À1 [c]
Benzene
Benzene
Benzene
Benzene
Cumene
Cumene
Cumene
Cumene
Toluene
Toluene
Ethylbenzene
Ethylbenzene
Propylbenzene
Propylbenzene
HEA16Cl
HEA16Br
HEA16Ms
HEA16BF4
HEA16Cl
HEA16Br
HEA16Ms
HEA16BF4
HEA16Cl
HEA16Br
HEA16Cl
HEA16Br
HEA16Cl
HEA16Br
Cyclohexane (100)
Cyclohexane (100)
Cyclohexane (100)
Cyclohexane (100)
Isopropylcyclohexane (100)
Isopropylcyclohexane (100)
Isopropylcyclohexane (100)
Isopropylcyclohexane (100)
Methylcyclohexane (100)
Methylcyclohexane (100)
Ethylcyclohexane (100)
Ethylcyclohexane (100)
Propylcyclohexane (100)
Propylcyclohexane (100)
3.6
5.3
3.7
3.7
5.2
7.9
6.4
5.2
3.6
5.7
3.7
6.9
3.9
7.1
83
57
81
81
58
38
47
58
83
53
81
43
77
42
3.9
5.9
4.1
4.0
5.7
8.6
± [
77
51
73
75
53
35
±
d]
± [
d]
±
4.0
6.3
3.9
7.4
4.3
7.6
75
48
77
40
70
39
[
a]
À5
À3
À5
Conditions: catalyst (3.8 Â 10 mol), substrate (3.8 Â 10 mol), surfactant (7.6 Â 10 ), water (10 mL), hydrogen
À1
pressure (1 atm), temperature (20 8C), stirred at 1500 min .
[
[
[
b]
c]
d]
Determined by GC analysis.
Turnover frequency defined as mol of H per mol of rhodium per h.
Emulsion, cannot be recycled by decantation.
2
Scheme 5. I poisoning experiment.
2
rhodium suspensions did not change with hydrogen
In all cases, the colloidal suspension of rhodium(0)
pressure, in fact the Rh-HEA16I system stays inactive stabilised by HEA16Cl gave the better turnover frequen-
whereas the Rh-HEA16Cl system reacts more quickly. cies compared with HEA16Br, Ms and BF . Moreover,
4
We also hydrogenated benzene and some of its HEA16Ms and BF compounds lead at the end of the
4
classical monoalkyl-substituted derivatives under bi- reactiontoanemulsionwhich prevents, insomecases, the
phasic conditions at room temperature and under recycling of the aqueous phase. As previously described
[
44]
atmospheric hydrogen pressure (Table 3). This compa- with bromide stabiliser, whatever the counter-ion of
rative study shows the major influence of the counter- HEA16 we observed i) no cyclohexene or cyclohexa-
ion of the surfactant on the conversion rate. The reaction dienederivativesasintermediates, andii) asteric effectof
was monitored by the volume of hydrogen consumed the arene substituents on the reaction time.
and a gas chromatographic analysis. The turnover
We also investigated the hydrogenation of function-
frequency, defined as mol of H per mol of rhodium alised arenes (Table 4). The reaction is influenced by the
2
per h, has been determined for the first run and for electronic effects of the arene substituents. The arenes
recycling after separation of the aqueous phase.
substituted by electron-withdrawing groups react slowly
Adv. Synth. Catal. 2003, 345, 222 ± 229
225

FULL PAPERS
Alain Roucoux et al.
Table 4. Hydrogenation of functionalised arene derivatives.[a]
Substrate
Surfactant
Products (yields %)[b]
1st
t [h]
run
TOF [h ]
2nd
t [h]
run
TOF [h ]
À1 [c]
À1 [c]
Anisole
Anisole
Anisole
Anisole
Phenol
Phenol
Ethyl benzoate
Ethyl benzoate
Styrene
HEA16Cl
HEA16Br
HEA16Ms
HEA16BF4
HEA16Br
HEA16Cl
HEA16Br
HEA16Cl
HEA16Br
HEA16Cl
HEA16Br
HEA16Cl
HEA16Br
HEA16Cl
HEA16Cl
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Cyclohexanol (100)
3.6
5
83
60
71
67
54
58
32
64
55
100
43
74
39
80
36
3.9
5.3
77
57
55
64
36
53
25
57
48
95
40
70
36
74
35
5.5[
4.7
d]
4.2
4.5
5.6
5.2
9.5
4.7
7.3
4.0
9.2
5.4
10.3
5.0
8.8
8.4
5.7
11.9
5.3
8.4
4.2
10.1
5.7
11.2
5.4
9.2
Cyclohexanol (100)
Ethyl cyclohexanoate (100)
Ethyl cyclohexanoate (100)
Ethylcyclohexane (100)
Ethylcyclohexane (100)
Isopropylcyclohexane (100)
Isopropylcyclohexane (100)
Propylcyclohexane (100)
Propylcyclohexane (100)
Cyclohexylethan-1-ol (80)
Cyclohexyl methyl ketone (20)
Styrene
a-Methylstyrene
a-Methylstyrene
Allylbenzene
Allylbenzene
Acetophenone
[
a]
À5
À3
À5
Conditions: catalyst (3.8 Â 10 mol), substrate (3.8 Â 10 mol), surfactant (7.6 Â 10 ), water (10 mL), hydrogen
À1
pressure (1 atm), temperature (20 8C), stirred at 1500 min .
[
[
[
b]
c]
d]
Determined by GC analysis.
Turnover frequency defined as mol of H per mol of rhodium per h.
Recycling after centrifugation of the emulsion.
2
Table 5. Hydrogenation of disubstituted benzene derivatives.[a]
Substrate
Surfactant Products (yields %)[b]
1st
t [h] TOF [h ]
run
2nd
run
À1 [c]
À1 [c]
t [h] TOF [h ]
o-xylene
o-xylene
m-xylene
m-xylene
p-xylene
p-xylene
HEA16Br 1,3-dimethylcyclohexane cis (95), trans (5)
HEA16Cl 1,3-dimethylcyclohexane cis (97), trans (3)
HEA16Br 1,3-dimethylcyclohexane cis (87), trans (13)
HEA16Cl 1,3-dimethylcyclohexane cis (90), trans (10)
HEA16Br 1,4-dimethylcyclohexane cis (67), trans (33)
HEA16Cl 1,4-dimethylcyclohexane cis (70), trans (30)
7.5
5.3
7.3
4.3
7.1
4.0
40
57
41
70
42
75
32
57
33
63
37
38
8.9 34
6.1 49
8.5 35
5.1 59
8.2 37
4.6 65
10.3 29
6.2 48
o-methylanisole HEA16Br 1-methoxy-2-methylcyclohexane cis (97), trans (3) 9.5
o-methylanisole HEA16Cl 1-methoxy-2-methylcyclohexane cis (98), trans (2) 5.3
p-methylanisole HEA16Br 1-methoxy-4-methylcyclohexane cis (92), trans (8) 9.2
p-methylanisole HEA16Cl 1-methoxy-4-methylcyclohexane cis (94), trans (6) 4.8
m-cresol
m-cresol
10
30
5.2 58
9.1 33
8.6 35
HEA16Br 3-methylcyclohexanol cis (99), trans (1)
HEA16Cl 3-methylcyclohexanol cis (99), trans (1)
8.2
8.0
[
a]
À5
À3
À5
Conditions: catalyst (3.8 Â 10 mol), substrate (3.8 Â 10 mol), surfactant (7.6 Â 10 ), water (10 mL), hydrogen
À1
pressure (1 atm), temperature (20 8C), stirred at 1500 min .
[
[
b]
c]
Determined by GC analysis.
Turnover frequency defined as mol of H per mol of rhodium per h.
2
in contrast with those with electron-donating groups. In xylene, methylanisole or cresol compounds. Based on
all cases we observed the reduction of the double bond our preliminary results, we essentially compared the two
prior to the hydrogenation of the aromatic ring.
halogenated systems HEA16Cl and HEA16Br. In all
The catalytic activity was most influenced by the cases, a complete conversion was obtained (Table 5).
choice of counter-ion (X ˆ Br, Cl, CH SO , BF ). In The cis diastereomers are largely the major products (up
3
3
4
numerous cases, the reaction is twice as active when the to 99%) as is usually observed in heterogeneous catalytic
[
42,50,51]
Rh(0) nanoparticles are stabilised by HEA16Cl than the systems.
other standard surfactants HEA16Br, Ms or BF4. The cis/trans ratio was not modified by the choice of
In a third series of studies, we examined the hydro- the stabiliser, however the chloride anion gives better
genation of disubstituted benzene derivatives such as reaction times and turnover frequencies. The reaction
226
Adv. Synth. Catal. 2003, 345, 222 ± 229

Arene Hydrogenation with a Stabilised Aqueous Rhodium(0) Suspension
FULL PAPERS
time is affected by the nature and the position of monium chloride salt. As previously reported,[45] the
the substituents. Thus, the reactivity decreases in the control of the surfactant concentration gives rise to high
series para > meta > ortho positions. Complementary catalytic activities (up to 5-fold more active), therefore
laboratory studies have shown that the increasing the combination of three parameters i) the use of the
hydrogen pressure contributes to activating the catalytic ammonium chloride salt, ii) a surfactant concentration
process but increases also the cis/trans ratio.
around the critical micellar concentration and iii) the
The results described in Tables 2 ± 5 demonstrate that recycling of the aqueous phase containing the catalyst
the stabilised aqueous dispersions of rhodium particles and its reuse for further runs, should give the best
can efficiently catalyse various monoalkyl-substituted, catalytic lifetime.
disubstituted, and functionalised benzene derivatives.
We have studied the influence of the anion stabiliser
(
X ˆ Br, Cl, CH SO , BF ) on the catalytic activity
3
3
4
Experimental Section
characterised by the turnover frequency (TOF) defined
as number of moles of consumed H2 per mol of
introduced rhodium per hour. Here, this parameter Starting Materials
was sufficient to compare results but, as recently
reported by Finke, the TOF should, ideally, be corrected
Rhodium chloride hydrate was obtained from Strem Chem-
icals. Sodium borohydride and fluoride, various functionalised
cetylalkanes, N,N-dimethylethanolamine, and all aromatic
for the true number of actives site. Except for some
works,[
53,54]
determining the number of actives sites for a substrates were purchased from Aldrich or Fluka and were
colloid is not frequent and in a first approximation can used without further purification. Water was distilled twice by
be estimated by the number of surface atoms on a conventional methods before use.
particle. For example, a nanocluster with an average
diameter of 2.1 nm is expected to have about 50% of the
atoms on the surface. In this condition it is necessary to Analytical Procedures
divide by 0.5 to give a corrected TOF.
The surface tension measurements were performed at 20 8C
using the ring method with a Du Nouy tensiometer (Kr¸ss
K10T).
Our work is essentially focused on the influence of the
counter-anion in the catalytic reaction. We have shown
that, in all cases, the chloride stabiliser gives the best
result. The usual literature of colloid stabilisation suspensions of rhodium(0) under catalytic conditions: catalyst
described that nanoparticles can be in part stabilised
Zeta potential measurements were performed on catalytic
À5
À5
(
3.8 Â 10 mol), surfactant (7.6 Â 10 ), water (10 mL),
temperature (20 8C) with Z e√ tasizer 3000 Malvern.
by the adsorption of anions to the electron-deficient
metal surface.[
55±58]
The transmission electronic cryomicroscopic studies were
conducted using a Philips CM 12 transmission electron
microscope at 100 keV. Samples were prepared by a dropwise
addition of the stabilised colloid in water onto a copper sample
mesh covered with carbon. The colloidal dispersion was
removed after 1 min using cellulose, the samples were then
quickly frozen in liquid ethane before transfer to the micro-
The zeta potential measurements of
the catalytic system (z ˆ‡ 40 to 100 mV, Table 1) only
show that the sum of the partial positive charge and
ammonium salt exceeds the amount of surface-bound
anion present. The difference between catalytic activ-
ities cannot be correlated with the particle size because
the various histograms of the size distribution show a scope.
similar average. However, we can suggest that the
weakly basic, small and non-chelating Cl stabiliser
binds less strongly to colloid surface, affording a higher
TOF. Probably, stabilised nanoparticles present a more
Gas chromatography was performed using a Carlo Erba GC
000 with an FID detector equipped with an Alltech AT1
column (30 m, 0.25 mm i.d.). Parameters were as follows:
initial temperature, 40 8C; initial time, 3 min; ramp, 8 8C/min;
final temperature, 140 8C; final time, 5 min; injector temper-
ature, 220 8C; detector temperature, 250 8C; injection volume,
À
6
™
naked surface∫increasing the catalytic activities. In this
context the research on clean-surface colloids should be
a focused goal. To our knowledge, the first ™anion
0
.3 mL.
series∫ approach for stabilising abilities recently descri-
bed by Finke[
59,60]
for Ir(0) nanoclusters showed the basic
Synthesis of Surfactants HEA16X
polyoxoanions as the best stabiliser.
The surfactants HEA16X were prepared as previously des-
cribed in the literature or after adapted modification and were
fully characterised.[
52]
Conclusion
N,N-Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium
Bromide HEA16Br: yield: 92%; white powder; M : 394.47 g ¥
R
The results described herein confirm that Rh(0) nano-
particles are a very efficient catalyst for the reduction of
arenes under mild conditions. The best activities are
À1
mol ; C H BrNO; R (MeOH): ˆ 0.15; mp 207 ± 208 C;
2
0
44
f
1
H NMR: d ˆ 0.85 (t, J ˆ 7 Hz, 3H, H-16'), 1.23 (m, 24H, H-
1
5' ±H-4'), 1.32 (m, 2H, H-3'), 1.73(m, 2H, H-2'), 3.36 (s, 6H, H-
obtained with particles stabilised by the highly water- 3), 3.56 (m, 2H, H-1'), 3.74 (m, 2H, H-2); 4.10 (m, 2H, H-1); 5.0
1
3
soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)am- (t, J ˆ 5.3 Hz, 1H, OH); C NMR: d ˆ 14.1 (C-16'), 22.6 ± 31.9
Adv. Synth. Catal. 2003, 345, 222 ± 229
227

FULL PAPERS
Alain Roucoux et al.
(
(
C-15 ± C-2'), 52.1 (C-3), 55.8 (C-1), 65.6 (C-2), 66.0 (C-1'); IR
KBr): n ˆ 3245 cm (C-OH); Anal. calcd.: C 60.89, H 11.24%;
bottomed flask, charged with the chosen aqueous suspension
of Rh(0) (10 mL) and a magnetic stirrer, was connected with a
gas burette (500 mL) with a flask to balance the pressure. The
flask was closed by a septum, and the system was filled with
À1
found: C 60.56, H 11.34%.
N,N-Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium
Chloride HEA16Cl: yield: 92%; white powder; M : 350.02 g ¥
hydrogen. The appropriate aromatic substrate (3.8
Â
R
À1
À3
mol ; C H ClNO; R (MeOH): 0.13; mp 203 ± 205 8C;
10 mol) was injected through the septum, and the mixture
2
0
44
f
1
À1
H NMR: d ˆ 0.84 (t, J ˆ 7.1 Hz, 3H, H-16'), 1.17 (m, 24H, H-
was stirred at 1500 min . The reaction was monitored by the
1
3
(
2
3
6
5' ±H-4'), 1.30 (m, 2H, H-3'), 1.71(m, 2H, H-2'), 3.34 (s, 6H, H-
), 3.51 (m, 2H, H-1'), 3.68 (m, 2H, H-2), 4.06 (m, 2H, H-1), 5.8
volume of gas consumed and by gas chromatography. At the
end of the reaction, the two phases were separated by
meticulous and slow decantation and the aqueous phase was
re-used in a second run. The turnover frequencies (TOF) were
determined for 100% conversion.
1
3
m, 1H, OH); C NMR: d ˆ 14.2 (C-16'), 22.7 ± 31.9 (C-15' ±C-
') 52.0 (C-3), 55.9 (C-1), 65.7 (C-2), 66.0 (C-1'); IR (KBr): n ˆ
,
À1
291 cm (C-OH); Anal. calcd.: C 68.63, H 12.67%; found: C
8.59, H 12.69%.
N,N-Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium Me-
sylate HEA16Ms: yield: 84%; white powder; M : 409,66 g ¥
R
À1
1
mol ; C H NO S; R (MeOH): 0.11; mp 68±72 8C; H NMR: Acknowledgements
21
47
4
f
d ˆ0.84 (t, Jˆ7 Hz, 3H, H-16'), 1.21 (m, 24H, H-15' ±H-4'), 1.30
(
3
(
2
(
6
m, 2H, H-3'),1.69(m,2H, H-2'),2.68(s, 3H, H-4),3.21(s, 6H, H-
We thank CNRS and the Region Bretagne for financial support
and Dr. Rolland (University of Rennes 1) for TEM experi-
ments.
), 3.38 (m, 2H, H-1'), 3.59 (m, 2H, H-2), 4.03 (m, 2H, H-1), 4.27
m, 1H, OH); 13C NMR: d ˆ14.1 (C-16'), 22.7±31.9 (C-15' ±C-
'), 39.6 (C-4), 51.6 (C-3), 56.1 (C-1), 65.6 (C-2), 65.7 (C-1'); IR
À1
KBr): n ˆ3358 (C-OH), 1190 cm (CH -S); anal. calcd.: C
3
1.57, H 11.56%; found: C 61.74, H 11.78%.
N,N-Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium Io- References
À1
dide HEA16I: yield: 84%; white powder; M : 441.47 g ¥mol ;
R
1
C H INO; R (MeOH): 0.13; mp 176±178 8C; H NMR: d ˆ
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20
44
f
0
(
.85 (t, J ˆ 7.1 Hz, 3H, H-16'), 1.23 (m, 24H, H-15' ±H-4'), 1.34
m, 2H, H-3'), 1.75 (m, 2H, H-2'), 3.36 (s, 6H, H-3), 3.55 (m, 2H,
1
3
H-1'), 3.76 (m, 2H, H-2), 4.14 (m, 3H, H -OH); C NMR: d ˆ
1
1
(
4.2 (C-16'), 22.7±31.9 (C-15' - C-2'), 52.5 (C-3), 55.8 (C-1), 65.6
À1
C-2), 66.4 (C-1'); IR (KBr): n ˆ3276 cm (C-OH); Anal.
calcd.: C 54.41, H 10.04%; found: C 54.27, H 10.23%.
1
992.
N,N-Dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium Tet-
rafluoroborate HEA16BF4: yield: 83%; white powder; M :
[
[
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R
À1
4
1
01.37 g ¥ mol ; C H BF NO; R (MeOH): 0.12; mp ˆ 113 ±
2
0
44
4
f
1
15 8C; H NMR: d ˆ 0.87 (t, Jˆ7 Hz, 3H, H-16'), 1.24 (m, 24H,
1
995, 36, 885.
H-15' ±H-4'), 1,32 (m, 2H, H-3'), 1.71 (m, 2H, H-2'), 3.13 (s, 6H,
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3
4
.04 (m, 2H, H-1); C NMR: d ˆ 14.2 (C-16'), 22.8 ± 32.0 (C-
5' ±C-2'), 51.4 (C-3), 56.3 (C-1), 65.5 (C-2), 66.1 (C-1);
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1
19
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4
4
À1
(
5
KBr): n ˆ 3327 (C-OH); 1074 cm (BF ); Anal. calcd.: C
4
[
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[
Synthesis of the Aqueous Rh(0) Colloidal Suspensions
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À1
10
0
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À4
3
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2
colloidal suspension (100 mL). The reduction occurs instanta-
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228
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