Alternative energy input for transfer hydrogenation using iridium NHC based catalysts in glycerol as hydrogen donor and solvent

Article abstract of DOI:10.1021/om300109e

Four Ir(I) and Ir(III) N-heterocyclic carbene (NHC) based complexes with 3,4,5-trimethoxybenzyl N-substituents have been obtained and fully characterized. The new complexes have been used as catalysts in the reduction of aldehydes and ketones with glycerol, and their activities have been compared to those shown by other Ir(III) NHC-complexes previously reported. The reactions were carried out under oil bath heating, and a detailed comparative study has been carried out using microwave (MW) and ultrasound (US) activation. The new Ir(III) complexes proved to be most efficient in the reduction of ketones, while the Ir(I) complexes are more active in the reduction of aldehydes. The use of ultrasound has a tremendous impact in shortening reaction times, and good results have been obtained in the reduction of aldehydes. In some experiments transmision electron microscopy (TEM) and UV-vis analysis showed the presence of iridium-containing nanoparticles after MW or US activation.

Full text of DOI:10.1021/om300109e

Article
pubs.acs.org/Organometallics
Alternative Energy Input for Transfer Hydrogenation using Iridium
NHC Based Catalysts in Glycerol as Hydrogen Donor and Solvent
Arturo Azua,† Jose A. Mata,*,† Eduardo Peris,† Frederic Lamaty,‡ Jean Martinez,‡ and Evelina Colacino*,‡
†
́
Departamento de Quımica Inorgan
́
ica y Organ
́
ica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071 Castellon
́
, Spain
‡Institut des Biomolec
34095 Montpellier Cedex 5, France
́
ules Max Mousseron (IBMM) UMR 5247, CNRS-UM I-UM II Universite
́
de Montpellier II, Place E. Bataillon,
S
* Supporting Information
ABSTRACT: Four Ir(I) and Ir(III) N-heterocyclic carbene
(NHC) based complexes with 3,4,5-trimethoxybenzyl N-substitu-
ents have been obtained and fully characterized. The new
complexes have been used as catalysts in the reduction of aldehydes
and ketones with glycerol, and their activities have been compared
to those shown by other Ir(III) NHC-complexes previously
reported. The reactions were carried out under oil bath heating,
and a detailed comparative study has been carried out using
microwave (MW) and ultrasound (US) activation. The new Ir(III)
complexes proved to be most efficient in the reduction of ketones,
while the Ir(I) complexes are more active in the reduction of
aldehydes. The use of ultrasound has a tremendous impact in
shortening reaction times, and good results have been obtained in
the reduction of aldehydes. In some experiments transmision electron microscopy (TEM) and UV−vis analysis showed the
presence of iridium-containing nanoparticles after MW or US activation.
which the alcohol acts as both the reaction solvent and the
source of hydrogen.
INTRODUCTION
■
The current search for energy-saving and more selective
protocols has turned ultrasound and microwave into useful
alternatives to prolonged heating in metal-catalyzed reactions.1
In particular, these two nonconventional activation tools have
been widely used in many palladium-catalyzed reactions,1b,c,2
for which substantial benefits may be found, including reduced
reaction times and improved yields and selectivity. The science
of green chemistry was developed to meet the increasing
demand for environmentally benign chemical processes. In this
regard, the combination of efficient catalytic protocols and
environmentally friendly solvents are of importance in the
search for laboratory-scale syntheses. Although many solvents
meet the green chemistry criteria, glycerol has recently caught
the attention of many researchers, due to its unique physical
and chemical properties and its extraordinarily low cost and
ready availability.3 Apart from its use as a solvent, new and
innovative catalytic processes based on the use of glycerol have
recently been developed,4 including its use as an environ-
mentally friendly hydrogen donor for transfer hydrogenation
(TH) reactions.5
In a number of recent examples, glycerol has replaced 2-
propanol as a hydrogen source in ruthenium-catalyzed TH
reactions.5b−d,6 The experiments were carried out in an oil bath
(OB) or using a domestic microwave (MW) oven. Generally,
the efficiency of all processes was low, and high catalyst
loadings and long reaction times (6−24 h) were required for
achieving acceptable substrate conversion. Better results were
obtained when oil bath (OB) or microwave (MW) heating
were used in combination with ultrasound (US) activation.6b
However, TH reactions in glycerol catalyzed by iridium-based
catalysts have been scarcely investigated.5a,e,7 The efficiency of
these organoiridium derivatives functionalized with NHCs,5e
N−N ligands,5a or P−N7 was evaluated by heating the reaction
mixture in an oil bath, and in some cases the use of a cosolvent
was required.7
In our search for efficient catalysts for hydrogen-borrowing
processes,8 we recently reported the use of a series of Ir NHC
based catalysts (1−3, Figure 1) for the reduction of organic
carbonyl compounds, using glycerol as both solvent and
hydrogen donor.5e On the basis of previous findings, we now
report the preparation of new Ir(I) and Ir(III) NHC based
catalysts (4 and 5, Figure 1), which have been used in the
transfer hydrogenation of different carbonyl compounds, using
Transfer hydrogenation (TH) is a metal-catalyzed process
during which hydrogen is transferred from an alcohol (2-
propanol or cyclopentanol) to an unsaturated bond. With
respect to the conventional hydrogenation reaction using the
highly flammable molecular hydrogen, TH is a safer and more
valuable atom-efficient, environmentally benign method, in
Received: February 10, 2012
Published: May 7, 2012
© 2012 American Chemical Society
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Scheme 2. Synthesis of Iridium NHC Based Catalysts
Figure 1. Ir(III) and Ir(I) NHC based catalysts for transfer
hydrogenation reactions.
precursor, complexes 5A,B were obtained (75% 5A and 55%
5B) (Scheme 2).
Positive ion ESI-MS analysis of the isolated products 4 and 5
in MeCN showed an intense peak for [M − Cl]+ (m/z 729 for
4A, m/z 605 for 4B, m/z 791 for 5A, and m/z 667 for 5B). The
second highest peak in intensity for complexes 4 corresponds
to the fragment which coordinates a molecule of acetonitrile
[M − Cl + MeCN]+ (m/z 770 for 4A and m/z 646 for 4B).
This fragment was not observed in the IrIII complexes 5. HRMS
analysis of complexes 4 and 5 showed a good agreement
between the simulated and theoretical spectra, with relative
errors of less than 2 ppm, confirming the proposed nature of
these complexes (see the Supporting Information for details).
X-ray Diffraction Studies. Crystals of 4A,B and 5A
suitable for X-ray diffraction were obtained by slow evaporation
from corresponding concentrated dichloromethane−hexanes
solutions. Figures 2−4 show the molecular diagrams of the
iridium complexes with the atom-numbering scheme and the
selected bond lengths (Å) and angles (deg). The geometry at
iridium of 4A (Figure 2) is square planar. This iridium
coordination plane is almost perpendicular to the plane angle
defined by the azole ring (α = 85.8°). The Ir−C(carbene) bond
length is 2.015(8) Å, as expected for NHCs. The trimethox-
ybenzyl groups point away from the metal center, just avoiding
steric repulsion with the cyclooctadiene (COD) and chlorine
ligands. The molecular structure of 4B is similar to that of 4A
(Figure 3). The Ir−C(carbene) bond length is 2.031(3) Å, and
the α angle is 86.0°.
The molecular structure of compound 5A (Figure 4) has a
symmetrically substituted NHC ligand coordinated to the metal
center. Two chlorides and a Cp* ligand complete the
coordination sphere about the Ir(III) center. The Ir−
C(carbene) distance is 2.061 Å. The two bulky 3,4,5-
trimethoxybenzyl N substituents point out of the coordination
sphere, minimizing the steric interaction.
Catalytic Studies. In our search of new methodologies
using glycerol activation, we have observed previously that
complexes 1−3 are very efficient in transfer hydrogenation
processes under oil bath heating.5e The Ir(III) complexes 1 and
2, with a chelating bis-NHC ligand and sulfonate groups, were
the most active, probably due to their high solubility in the
reaction media (see Figure 1 in the Supporting Information)
and to the strong electron-donor properties of the bis-carbene
glycerol as reducing agent and solvent (Scheme 1). The
catalytic properties of these new complexes have been
Scheme 1. Iridium-Catalyzed Transfer Hydrogenation
Reactions of Carbonyl Compounds
compared with those previously obtained.9 In order to establish
the greenest and most efficient catalytic protocol, we have
performed the reactions under microwave and ultrasound
conditions, and the results were compared with those obtained
by the conventional oil-bath heating procedure.
RESULTS AND DISCUSSION
■
Iridium(III) complexes of general formula [IrOAcI2(bis-
NHC)] (1−3, Figure 1) were prepared as previously
described.9 Imidazolium ligand precursors were readily
accessible by alkylation of commercial available N-alkylimida-
zoles or by following previously described procedures.10
Imidazolium salt A (Scheme 2) is a symmetrical salt with two
trimethoxybenzyl groups,11 while imidazolium salt B is
asymmetrical with one trimethoxybenzyl group and an n-
butyl group. The products are colorless and hygroscopic with
spectral properties similar to those of other reported
imidazolium salts. The iridium NHC based complexes were
obtained by transmetalation from the corresponding silver
carbene derivatives by a two-step process (Scheme 2). The first
step involves the deprotonation of the imidazolium salt with
silver oxide to form the silver-NHC complex. We used this
complex in situ without isolation. In the second step, after the
addition of [IrCl(COD)]2 immediately a white precipitate of
silver chloride was formed, indicating the formation of the
iridium NHC complexes in good yields (80% 4A and 78% 4B).
Following the same procedure but using [IrCp*Cl2]2 as metal
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Figure 4. Molecular diagram of complex 5A. Ellipsoids are given at the
50% probability level. Hydrogens are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ir(1)−C(3) = 2.061(11), Ir(1)−Cl(2) =
2.428(2), Ir(1)−Cp(cent) = 1.979, C(3)−N(4) = 1.356(9), N(4)−
C(5) = 1.384(10); C(3)−Ir(1)−Cl(2) = 90.8(2), C(3)−N(4)−C(5)
= 111.5(7), C(3)−N(4)−C(6) = 124.5(7).
Figure 2. Molecular diagram of complex 4A. Ellipsoids are given at the
50% probability level. Hydrogens are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ir(1)−C(3) = 2.015(8), Ir(1)−Cl(2) =
2.372(2), Ir(1)−C(35) = 2.094(9), C(3)−N(4) = 1.366(11), N(4)−
C(8) = 1.462(11); C(3)−Ir(1)−Cl(2) = 89.4(2), C(3)−N(4)−C(8)
= 124.1(8), C(11)−O(15)−C(16) = 117.1(7). α = 85.8° (α = angle
between the iridium coordination plane and the azole ring plane).
Ir(III) (5A, 5B) catalysts, bearing nonionic polar groups, in TH
reactions in an oil-bath, using benzaldehyde and acetophenone
as substrates. Their efficiency was compared with that of
catalysts 1−3, having ionic polar sulfonate functionalities. Table
1 summarizes the data that we obtained.
ligands, especially for catalyst 2 with the abnormally bound bis-
NHC ligand.
As a starting point of our investigation, we explored the
catalytic performances of the novel prepared Ir(I) (4B) and
Table 1. Selected Data for Transfer Hydrogenation in Oil
Bath (OB)
a
b
entry
cat.
R
yield (%)
1
1
H
66
99
71
98
91
45
69
25
80
40
35
2
2
H
3
4B
5A
5B
1
H
4
H
5
H
6
Me
Me
Me
Me
Me
Me
7
2
8
3
9
4B
5A
5B
10
11
a
Reactions were carried out with 0.5 mmol of substrate, KOH (0.5
mmol), and 0.8 mL of glycerol at 120 °C for 7 h. Anisole (0.5 mmol)
b
was used as internal standard. Yields determined by GC on the basis
of 1-phenylethanol or benzyl alcohol production.
In a typical experiment acetophenone (or benzaldehyde),
potassium hydroxide, the catalyst (2.5 mol %), and glycerol
were heated at 120 °C for 7 h (Table 1). Acetophenone is
reduced to 1-phenylethanol and benzaldehyde to benzyl
alcohol, while glycerol is dehydrogenated to dihydroxyacetone
(DHA), a compound which is extensively used in the cosmetics
industry as a sunless tanning compound. Under the reaction
conditions used, DHA partially decomposes, as previously
demonstrated.5b,7
Figure 3. Molecular diagram of complex 4B. Ellipsoids are given at the
50% probability level. Hydrogens are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ir(1)−C(3) = 2.031(3), Ir(1)−Cl(2) =
2.3581(9), Ir(1)−C(25) = 2.166(3), C(3)−N(4) = 1.357(4), N(4)−
C(8) = 1.460(5); C(3)−Ir(1)−Cl(2) = 88.22(9), C(3)−N(4)−C(8)
= 124.2(3), C(12)−O(16)−C(19) = 114.0(3). α = 86.0° (α = angle
between the iridium coordination plane and the azole ring plane).
As seen from the data shown in Table 1, all the Ir(I) and
Ir(III) catalysts were active in the reduction of carbonyl groups
with moderate to excellent yields, depending on the catalyst
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used, but with the advantage that no additive or cosolvent was
required, differently than similar iridium-catalyzed trans-
formations.5a Catalysts 2 and 5A afforded excellent catalytic
performances in the reduction of benzaldehyde (Table 1,
entries 2 and 4), while catalysts 2 and 4B afforded the best
catalytic performances in the reduction of acetophenone (Table
1, entries 7 and 9). The very low activity displayed by catalyst 3
in the reduction of acetophenone (Table 1, entry 8) may be
due to its poor solubility, in comparison to that shown by the
sulfonate-substituted complex 1 (Table 1, entry 6).
Scheme 3. Plausible Mechanism for Iridium-Catalyzed
Transfer Hydrogenation Reactions in Glycerol
A yield vs time plot for benzaldehyde reduction in glycerol
was evaluated using catalysts 4B and 5A, 5B (Figure 5). The
alcohol. Introduction of electron donor NHC ligands may
produce the enhancement of the catalytic activity through the
increase of the nucleophilicity of the iridium hydride
intermediate, leading to a more facile interaction with the
electrophilic carbonyl substrate.
The use of glycerol as the reaction solvent presents some
drawbacks such as the low solubility of highly hydrophobic
molecules and gases, such as hydrogen and oxygen. Its high
viscosity does not facilitate the diffusion of substrates and
catalyst. These limitations may be overcome by performing the
reactions under microwave (MW)1i,14 or high-intensity ultra-
sound (US)6b,15 activation.
Because of its intrinsic characteristics such as high boiling
point (T = 290 °C), low vapor pressure (0.0025 mm at 50 °C),
high dielectric constant (ε = 42.48 at 25 °C), and a polarity
similar to that of other organic solvents such as DMSO and
DMF, glycerol can be used as a suitable solvent for microwave
irradiation,1i where the heating characteristics of the solvent
play a crucial role. Moreover, the recovery of the final products
is simplified by simple decantation of the crude reaction
mixture with glycerol-immiscible solvents (e.g., Et2O or
cyclohexane).
In an unprecedented study, the iridium-catalyzed TH
reaction was investigated under microwave irradiation in a
chemistry-dedicated microwave apparatus, in closed vessels.
Selected data and conditions are summarized in Table 2.
Reduction of the carbonyl group (benzaldehyde or acetophe-
none) into the corresponding alcohols could be achieved in
short reaction times (1−2 h) and relatively low temperatures
(80−120 °C) in good yields (60−95%) under microwave
activation. Catalyst 2 displayed the best activity in the reduction
of benzaldehyde (Table 2, entries 1−5) in glycerol as well as in
a comparative experiment using 1,2-propanediol as hydrogen
donor (Table 2, entry 2). Very low conversion of substrate
(30%) and poor yield of benzyl alcohols (35%) were observed
on heating the mixture in poly(ethylene glycol) (average
molecular weight ca. 300 Da). At 120 °C, no improvement was
possible by the presence of a cosolvent (DMSO) or by
doubling either the reaction times or the catalyst loading (5 mol
%). The use of AgOTf as additive was detrimental, leading to
the formation of many byproducts.
Figure 5. Time course of benzaldehyde reduction in glycerol.
Conditions: 2.5 mol % catalyst, 0.5 mmol of benzaldehyde, KOH
(0.5 mmol), 0.8 mL of glycerol at 120 °C. Anisole (0.5 mmol) was
used as internal standard. Yields were determined by GC.
reaction was very clean, and we did not observe any other side
products apart from benzyl alcohol. All catalysts employed were
very active at short times, reaching 50% yield after 1 h. The best
catalyst is 5A, reaching almost quantitative yields (98%) within
7 h. The absence of an induction period before initiation of the
catalytic process is in agreement with the higher initial activity
expected for catalysts with electron donor NHC ligands.
However, rationalization of these data is not obvious, taking
into account only the catalyst solubility in glycerol. Electron-
releasing ligands were expected to produce more active
catalysts, while COD complexes were poorly soluble in
glycerol, and several other factors affect the TH reaction: (i)
the ligand steric hindrance on the coordination of the reactants
and the products, (ii) the kinetics of catalyst deactivation, (iii)
poor mass and heat transport in glycerol, and (iv) the solubility
of all reactants.
The iridium-catalyzed transfer hydrogenation most probably
goes through the “hydridic route”, involving a monohydri-
de.9a,12 In the first step, an iridium alkoxide is formed after the
deprotonation of glycerol in basic medium (Scheme 3). This
intermediate evolved to the formation of an iridium hydride
intermediate with a β-hydrogen elimination of dihydroxyace-
tone (DHA), followed by the insertion of the carbonyl
substrate into the iridium−hydride bond to give an alkoxide.
In the final step, the product is obtained after an alkoxide
exchange step in which a second molecule of glycerol enters the
metal coordination sphere, regenerating the catalytic species.
The base plays a double role: to activate the metal complex by
abstracting the acidic proton of the hydrogen donor13 and to
assist in proton dissociation from the hydroxyl group of the
The heating mode of the sample and the magnetron power
set to reach the selected instruction (T = 120 °C) seemed to be
important parameters (see Figure 3 in the Supporting
Information). When the initial starting heating power was off
(Table 2, entry 3), the yield was moderate. The results were
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Table 2. Selected Data for Transfer Hydrogenation
Processes under Microwave Irradiation (MW)
Table 3. Selected Data for Transfer Hydrogenation
Processes under Ultrasound Irradiation (US)
a
a
b
entry
cat.
R
T (°C)
t (min)
yield (%)
ab
,
entry
1
cat.
R
T (°C)
t (h)
yield (%)
1
2
3
1
H
98
60
98
98
98
98
80
98
98
98
80
80
40
40
80
80
80
5
49
35
55
25
20
30
64
2
H
80
80
2
2
1
1
1
1
1
2
1
2
2
60
2
H
60
5
c
2
2
H
70
32
68
2
H
d
3
2
H
120
120
120
120
120
120
120
200
120
c
4
2
H
5
e
4
2
H
d
5
2
H
5
f
5
2
H
95
g
e
6
2
H
5
6
7
8
4B
5A
5B
2
H
13 (54)
f
7
2
H
20
g
g
H
20 (49)
22 (54)
g
g
8
4A
4B
5B
1
H
5 (30)
73 (11)
H
g
g
g
g
g
9
H
5 (900)
10 (10)
25(900)
60(900)
40 (7)
f
9
Me
Me
Me
83
77
0
g
10
11
H
45(25)
h
10
11
2
g
Me
Me
Me
Me
Me
Me
Me
34(40)
f
g
12
2
60(68)
a
Reactions were carried out with 0.5 mmol of substrate, KOH (0.5
13
2
30
40
30
20
30
56
21
20
20
22
mmol), and 0.8 mL of glycerol. Anisole (0.5 mmol) was used as
h
14
2
b
internal standard. Yields were determined by GC on the basis of 1-
15
16
17
4A
4B
5A
c
phenylethanol or benzyl alcohol produced. Using 0.8 mL of 1,2-
d
propanediol instead of glycerol. Starting heating power off.
e
Simultaneous cooling system, with starting heating power off.
a
f
g
Reactions were carried out with 0.5 mmol of substrate, KOH (0.5
Starting heating power set at its maximum level (400 W). Yields
mmol), 0.8 mL of glycerol, and an amplitude of 40%. Anisole (0.5
in parentheses are given for experiments carried out in an oil bath at
120 °C. 1 mol % of catalyst 2.
b
h
mmol) was used as internal standard. Yields determined by GC based
c
on 1-phenylethanol or benzyl alcohol produced. Using 0.4 mL of
d
glycerol and 0.4 mL of DMSO. Using 0.4 mL of glycerol and 0.4 mL
e
f
improved by heating the mixture with the technique of
simultaneous cooling (Table 2, entry 4), introducing higher
levels of MW energy into the reaction vessel16 even if the initial
starting heating power was set to off (see Figure 4 in the
Supporting Information). When the starting heating power was
set at its maximum level (400 W) (Table 2, entry 5), high yields
of reduced products were obtained.
Disappointingly, catalysts 4B, 5A, and 5B were ineffective
under microwave heating, giving better results under oil bath
heating, most probably due to some degree of catalyst
decomposition under microwaves (Table 2, entries 6−8).
When acetophenone was the substrate, catalyst 2 was the most
efficient (Table 2, entries 9 and 10). Despite the fact that
comparable yields of 1-phenylethanol were obtained, it is not
possible to determine if the reaction was driven by thermal or
nonthermal microwave effects.17
of H2O. Using 0.4 mL of glycerol and 0.4 mL of PEG300. 2.5 mol %
g
cat. Data in parentheses are given for experiments carried out in an oil
bath at 90 °C. The amplitude was 20%.
h
very short time (5 min) and with low catalyst loading (Table 3,
entry 8). This is maybe due to an optimal dispersion of the base
in glycerol, as well as excellent cavitation, producing a
microenvironment (microbubbles) where the reaction takes
place.
As a general trend, when the catalyst loading was increased
(2.5 mol %, Table 3 entry 7) or the reaction time extended to
up to 1 h (Table 3, entry 2), no pronounced decrease of the
substrate could be observed but other byproducts were
observed in the crude mixture together with benzyl alcohol,
perhaps due to the formation of cyclic acetals between glycerol
and benzaldehyde.
MW results can be analyzed on the basis of different factors,
related not only to the relative solubility of substrate and
catalyst in glycerol phase, the viscosity of the medium, and the
diffusion properties of the compounds but also the microwave
absorption of the medium. This is also a function of the
hydration layer thickness surrounding the glycerol and/or
molecules,18 glycerol being able to absorb moisture from the
atmosphere, with unpredictable consequences on organo-
metallic catalysis.
Since this reaction with NHC-based iridium catalysts was
unexplored under ultrasound activation,6b complexes 1−5 were
then evaluated in transfer hydrogenation with the same model
substrates using ultrasound activation. Parameters such as
reaction time, catalyst loading, temperature, and wave
amplitude were screened for each catalyst, and the selection
of the best data is reported in Table 3. Catalyst 4A displayed
the best results in the reduction of benzaldehyde: the reaction is
fast and selective at the beginning, and the yield is good in a
The addition of a cosolvent such as DMSO, H2O, and
PEG300 did not improve the catalytic outcomes (Table 3,
entries 4−6). Because the transfer hydrogenation is an
equilibrium reaction, we also tried to use a larger excess of
hydrogen donor (glycerol), to displace the equilibrium toward
the product side, but the results were not improved.
The bis-NHC abnormally bound 2 was the most efficient for
the reduction of acetophenone (Table 3, entry 12). However,
the amplitude of the ultrasound wave seems to have an effect
on the yield (Table 3, entries 13 and 14). The ultrasound beam
experienced a loss of energy when traveling through the
medium. The input signal amplitude can be adjusted to
compensate this loss of energy, modifying the energy content,
or “strength” of the ultrasound pulse. The optimum could be
obtained with an amplitude of 40%, allowing a much greater
energy to be delivered. The low yields obtained in the other
cases could be rationalized in terms of a partial deactivation of
the catalyst, probably due to an ultrasound-induced mod-
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ification of the structure of the catalytic site or by the presence
of the coordinating chloride counteranion slowing the
formation of the active [Ir]-H intermediate under these
conditions. It is worth noting that comparative experiments
using an oil bath at the same temperature (Table 3, entries 8−
12) proved to be ineffective even after prolonged heating. This
is not so surprising, as the effect of sonication is explained in
terms of “hot spots” theory, and hundreds of atmospheres are
generated by the collapse of cavitation bubbles responsible for
the chemical reactions.19 No similar bubble formation is
possible under conventional oil bath heating or during
microwave irradiation.
During the reaction the solution turned into a homogeneous
dark brown suspension. Glycerol is a strong reducing agent of
the metallic center, leading to nanoparticles (polyol meth-
od).20,21 The glycerol phase recovered after microwave or
ultrasound irradiation was analyzed by transmission electron
microscopy (TEM), showing the formation of iridium-
containing nanoparticles (Figure 6). It is important to point
out that the formation of these nanoparticles resulted in the
deactivation of the catalytic process; this is not unexpected,22
especially under ultrasound activation where strong cavitation
phenomena occur in a viscous medium such as glycerol.
The TEM analysis of the glycerol phase obtained in the
experiment carried out under microwave irradiation with the
starting heating power set at its maximum level (400 W) (Table
2, entry 5) showed the formation of a dispersion of spherical
iridium-containing nanoparticles with uniform and narrow size
distribution (ca. 2−3 nm), free of sintering, throughout the grid
(Figure 6a). The TEM analysis of glycerol phase obtained after
microwave irradiation with the technique of simultaneous
cooling, while keeping the starting heating power off (Table 2,
entry 4), showed again the formation of similar shaped iridium-
containing nanoparticles (ca. 2−3 nm), homogeneously
distributed (sintering-free) but in a lower concentration (Figure
6b). This observation may be explained by the different heating
profile, under microwave irradiation, since at the early
beginning of the reaction (see Figure 3 in the Supporting
Information), before that the set value (T = 120 °C) is reached.
This may influence the kinetics of nucleation, that would be
probably retarded in the case of a simultaneous cooling
method. To our knowledge, iridium nanoparticles have never
been previously synthesized using glycerol under microwave
activation. We have observed that glycerol/ultrasound synergy
allowed an effective and fast nucleation of iridium-containing
nanoparticles in only 5 min (Figure 6c and Table 3, entry 8).
The particles are homogeneously spread throughout the
support with a narrow size distribution of ca. 3−5 nm.
Iridium-containing nanoparticles were examined with ultra-
violet−visible (UV−vis) spectroscopy (Figure 7). The iridium
nanospheres showed a small intense band at 270 nm similarly
to previous reports23 for metallic iridium formed under
reducing conditions, which in our case is provided by the use
of glycerol. It is out the scope of the present study to investigate
the physical mechanism leading to nanoparticle formation and
growth.
Figure 6. TEM micrographs for iridium-containing nanoparticles
synthesized (a) under MW irradiation using catalyst 2 with starting
heating power set at its maximum level (400 W), (Table 2, entry 5),
(b) under MW irradiation using catalyst 2 with simultaneous cooling
system and starting heating power off (Table 2, entry 4), and (c)
under US irradiation using catalyst 4A (Table 3, entry 15).
complexes in the transfer hydrogenation reaction using glycerol
as solvent and hydrogen donor under microwave, ultrasound
and oil bath conditions. In particular, the ultrasound gains a
special place as a promising heating technique for developing
new catalytic processes in glycerol. The formation of spherically
shaped Ir(0)-containing nanoparticles in glycerol was demon-
strated, using microwave or ultrasound heating. Glycerol is a
cheap, nontoxic, biodegradable, and easily available byproduct
in biodiesel fuel production, obtained from the saponification of
triglycerides of all natural fats and oils. Due to the its industrial
CONCLUSIONS
■
Novel iridium NHC based complexes have been synthesized
and fully characterized. The easy modulation of the NHC
ligands leads to physical differences in the catalyst properties
such as solubility. The catalytic properties have been evaluated
and compared with those of previously reported iridium
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Organometallics
Article
spectra were recorded using a Jenvay 7315 scanning spectropho-
tometer with quartz cells of 1 cm path length.
X-ray studies. Diffraction data were collected on a Agilent
SuperNova diffractometer equipped with an Atlas CCD detector using
Mo Kα radiation (λ = 0.710 73 Å). Single crystals were mounted on a
MicroMount polymer tip (MiteGen) in a random orientation.
Absorption corrections based on the multiscan method were applied.24
Structures were solved by direct methods in SHELXS-97 and refined
by the full-matrix method based on F2 with the program SHELXL-97
using the OLEX software package.25
Catalytic Studies. General Method for TH in an Oil Bath. In a
typical experiment of transfer hydrogenation using glycerol as
hydrogen donor, a capped vessel containing a stirrer bar was charged
with the substrate (0.5 mmol), potassium hydroxide (0.5 mmol),
anisole as internal reference (0.5 mmol), and catalyst (2.5%) in 0.8 mL
of glycerol. The solution was heated to 80−120 °C for the appropriate
time. Yields and conversions were determined by GC chromatography
during the reaction course. Products and intermediates were
characterized by GC/MS. Isolated products were characterized by
1H NMR and 13C NMR after column chromatography purification
using n-hexane/ethyl acetate mixtures.
Figure 7. UV−vis absorption spectra of iridium-containing nano-
particles: (A) catalyst 2 under MW irradiation with starting heating
power set at its maximum level (400 W); (B) catalyst 2 under MW
irradiation with simultaneous cooling system and starting heating
power off; (C) catalyst 4A under US irradiation.
General Method for TH under Microwave Irradiation. In a typical
experiment, a mixture of substrate (0.5 mmol), catalyst (2.5 mol %),
and finely powdered KOH (0.028 g, 0.5 mmol) in glycerol (0.4 mL)
was heated under microwave irradiation at 120 °C for 1 h, in a sealed
reactor. After cooling, a 2/1 v/v mixture of Et2O and CH2Cl2 (3 mL)
was added to the crude product and this mixture was stirred at room
temperature for 5 min. The supernatant was recovered, and the
operation was repeated three times. The organic phase was dried on
MgSO4, filtered, and evaporated under reduced pressure to afford the
reduced product as a pure compound. Product conversion was
determined by HPLC; the yield was determined by GC/MS.
General Method for TH under Sonication. In a typical experiment,
a mixture of substrate (0.5 mmol), catalyst (2.5 mol %), and finely
powdered KOH (0.028 g, 0.5 mmol) in glycerol (0.4 mL) was placed
in a Pyrex glass reactor and clamped to a vertical support on a
magnetic stirrer. The vessel was hold in place such that the tip of the
horn was immersed into the reaction mixture to a depth of 1.0 cm and
the glass part did not touch the sonochemical probe. The reaction
mixture was sonicated at 98 °C continuously for the indicated time
(Table 3), with 40% amplitude, while vigorous magnetic stirring was
maintained. At the end of the reaction, a 2/1 v/v mixture of Et2O and
CH2Cl2 (3 mL) was added to the crude product and this mixture was
stirred at room temperature for 5 min. The supernatant was recovered,
and the operation was repeated three times. The organic phase was
dried on Mg2SO4, filtered, and evaporated under reduced pressure to
afford the reduced product as a pure compound. Product conversion
was determined by HPLC; the yield was determined by GC/MS.
Synthesis of Imidazolium Salt B. A mixture of 1-n-
butylimidazole (248 mg, 2 mmol) and 3,4,5-trimethoxybenzyl chloride
(476 mg, 2.2 mmol) was stirred in a Pyrex tube for 48 h at 95 °C. The
brown oil obtained was washed twice with 5 mL of ether, affording an
analytically pure product (yield 655 mg, 96%). 1H NMR (CDCl3, 500
MHz): δ 10.31 (s, 1H, Himid), 7.49 (d, 3JH−H = 1.4 Hz, 1H, Himid), 7.24
importance as the intermediate of many valued fine chemicals,
as well as the solvent of choice for many industrial and
pharmaceutical preparations (foods, cosmetics, liquid deter-
gents, antifreeze), new methodologies are being developed in
our laboratories for a green and sustainable chemistry in
glycerol.
EXPERIMENTAL SECTION
■
General Procedures. Compounds 1−3 and imidazolium salt A
were prepared according to literature procedures.9a−c,11 All experi-
ments were carried out under nitrogen using standard Schlenk
techniques and high vacuum, unless otherwise stated. Anhydrous
solvents were dried using a solvent purification system (SPS MBraun).
All other reagents were used as received from commercial suppliers.
Glycerol was used as received from Sigma-Aldrich (ref. no. G9012,
>99%). The products were identified by a GCMS-QP2010
(Shimadzu) gas chromatograph/mass spectrometer equipped with a
Teknokroma (TRB-5MS, 30 m × 0.25 mm × 0.25 mm) column, and
the spectra obtained were compared with the standard spectra. Yields,
conversion, and product selectivity were determined by a GC-2010
(Shimadzu) gas chromatograph equipped with an FID and a
Teknokroma (TRB-5MS, 30 m0.25 mm 0.25 mm) column. NMR
spectra were recorded on Varian spectrometers operating at 300 or
500 MHz (1H NMR) and 75 and 125 MHz (13C NMR), respectively,
and referenced to SiMe4 (δ in ppm and J in hertz). NMR spectra were
recorded at room temperature with the appropriate deuterated solvent
(CDCl3, CD3OD, or d6-DMSO). The identity of analytically pure
samples of the saturated alcohols was assessed by comparison of their
1H NMR data previously described in the literature and by their
fragmentation in GC/MS. Microwave-assisted reactions were
performed in sealed vessels with a Biotage Initiator 60 EXP
instrument. The temperature was measured with an IR sensor on
the outer surface of the reaction vial. Open vessel sonochemical
reactions were performed in probe systems (VCX-400 Sonics
Materials Vibracell) equipped with an immersion horn made from
titanium alloy. The working frequency was 20 kHz, using 40%
amplitude. Analytical high-performance liquid chromatography
(HPLC) was performed on a Waters Millenium 717 equipped with
an Autosampler, with a variable-wavelength diode detector using a
Chromolith RP18 column (50 × 4.6 mm), flow 5 mL/min, linear
gradient CH3CN in water 0−100% (+ 0.1% TFA) in 4.5 min.
Transmission electron microscopy (TEM) micrographs were recorded
on a JEOL 1200EX2 (Tokyo, Japan, 1990) at an operating voltage of
100 kV. Particles were dispersed in ethanol (microwave experiments)
or in water (ultrasound experiments) by ultrasonication for 30 min,
loaded on carbon-coated copper grids (300 mesh), and allowed to dry
at room temperature before recording the micrographs. UV−vis
3
(d, JH−H = 1.5 Hz, 1H, Himid), 6.63 (s, 2H, Ar), 5.21 (s, 2H, NCH2),
3
3.97 (t, JH−H = 7.4 Hz, 2H, nBu), 3.53 (s, 6H, OCH3), 3.46 (s, 3H,
OCH3), 1.60−1.48 (m, 2H, nBu), 1.10−0.94 (m, 2H, nBu), 0.59 (t,
3JH−H = 7.5 Hz, nBu). 13C{1H} NMR (CDCl3, 125 MHz): δ 138.1
(Cimid) 136.5 (Ar), 129.1 (Ar), 122.4 (Ar), 122.1 (Cimid), 121.9 (Cimid),
106.3 (Ar), 60.4 (OMe), 56.3 (OMe), 52.9 (NCH2), 49.5 (nBu), 31.7
(nBu), 19.1 (nBu), 13.1 (nBu). Electrospray MS (cone 15 V; m/z,
fragment): 305 [M − Cl]+. HRMS ESI-TOF-MS (positive mode): [M
− Cl]+ monoisotopic peak 305.1863, calcd 305.1865, εr = 0.7 ppm.
Synthesis of 4A. A suspension of A (206 mg, 0.446 mmol) and
silver oxide (206 mg, 0.892 mmol) in 1,2-dichloroethane was stirred at
60 °C for 4 h under Ar. After the reaction mixture was cooled to room
temperature, [IrCl(COD)]2 (150 mg, 0.223 mmol) was added. The
resulting mixture was stirred at room temperature for 48 h under Ar.
After solvent removal, CH2Cl2 was added to the residue. The resulting
suspension was filtered through Celite, and the filtrate was
3917
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Organometallics
Article
concentrated to dryness. The product was recrystallized from CH2Cl2/
hexanes and dried under vacuum, affording a dark orange solid (yield
270 mg, 80%). 1H NMR (CDCl3, 500 MHz): δ 6.72 (s, 4H, Ar), 6.70
(s, 2H, Himid), 6.12 (d, 2JH−H = 14.3 Hz, 2H, NCH2Ar), 5.14 (d, 2JH−H
= 14.3 Hz, 2H, NCH2Ar), 4.67−4.65 (m, 2H, COD), 3.84 (s, 12H,
OCH3), 3.82 (s, 6H, OCH3), 3.2−2.9 (m, 2H, COD), 2.21, 2.20 (m,
4H, COD), 1.76−1.74 (m, 4H, COD). 13C{1H} NMR (CDCl3, 75
MHz): δ 180.5 (Ir−Ccarbene), 153.5 (Ar), 138.0 (Ar), 131.9 (Ar), 120.5
(CHimid), 105.7 (Ar), 85.2 (COD), 60.8 (OMe), 56.5 (OMe), 54.5
(NCH2Ar), 51.9 (COD), 33.6 (COD), 29.52 (COD). Anal. Calcd for
C31H40N2O6IrCl·C6H14 (850.51): C, 52.25; H, 6.40; N, 3.29. Found:
C, 52.14; H, 6.78; N, 3.35. Electrospray MS (cone 15 V; m/z,
fragment): 729 [M − Cl]+. HRMS ESI-TOF-MS (positive mode): [M
− Cl]+ monoisotopic peak 729.2531, calcd 729.2518, εr = 1.7 ppm.
Synthesis of 4B. A suspension of B (151 mg, 0.446 mmol) and
silver oxide (206 mg, 0.892 mmol) in 1,2-dichloroethane was stirred at
60 °C for 4 h under Ar. After the mixture was cooled to room
temperature, [IrCl(COD)]2 (150 mg, 0.223 mmol) was added, and
the resulting mixture was stirred at room temperature for 48 h under
Ar. After solvent removal, the product was extracted with CH2Cl2,
filtered through Celite, and concentrated to dryness. The residue was
recrystallized from CH2Cl2/ hexanes, affording a light yellow solid
(s, 3H, OCH3), 1.98−1.92 (m, 2H, nBu), 1.63 (s, 15H, Cp*), 1.44−
3
1.39 (m, 2H, nBu), 1.0 (t, JH−H = 7.3 Hz, 3H, nBu). 13C{1H} NMR
(CDCl3, 75 MHz): δ 156.5 (Ir−Ccarbene), 153.5 (Ar), 138.7 (Ar), 132.0
(Ar), 121.7 (CHimid), 120.9 (CHimid), 106.3 (Ar), 88.8 (Cp), 60.8
(OMe), 56.4 (OMe), 54.7 (NCH2Ar), 50.8 (nBu), 33.7 (nBu), 20.2
(nBu), 13.9 (nBu), 9.2 (Cp). Anal. Calcd for C27H39N2O3IrCl2·C6H14
(788.91): C, 50.24; H, 6.77; N, 3.55. Found: C, 49.93; H, 6.78; N,
3.65. Electrospray MS (cone 15 V; m/z, fragment): 667 [M − Cl]+.
HRMS ESI-TOF-MS (positive mode): [M − Cl]+ monoisotopic peak
667.2273, calcd 667.2272, εr = 0.2 ppm.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving general procedures,
details of the catalytic studies, and ESIMS, HRMS, and X-ray
diffraction data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
(yield 227 mg, 78%), 1H NMR (CDCl3, 500 MHz): δ 6.92 (d, 3JH−H
=
*J.A.M.: e-mail, jmata@qio.uji.es; fax, (+34) 964728214; tel,
(+34) 964729166. E.C.: e-mail, evelina.colacino@univ-montp2.
fr; fax, +33 (0)467144866; tel, +33 (0)467144285.
3
2.0 Hz, 1H, Himid), 6.71 (d, JH−H = 2.0 Hz, 1H, Himid), 6.69 (s, 2H,
2
2
Ar), 6.06 (d, JH−H = 14.4 Hz, 1H, NCH2-Ar), 5.13 (d, JH−H = 14.4
Hz, 1H, NCH2-Ar), 4.59−4.42 (m, 2H, COD), 4.44−4.37 (m, 2H,
nBu), 3.85 (s, 6H, OCH3), 3.83 (s, 3H, OCH3), 2.96 (br, 2H, COD),
2.21−2.18 (m, 4H, COD), 1.99−1.93 (m, 2H, nBu), 1.82−1.78 (m,
Notes
The authors declare no competing financial interest.
3
4H, COD), 1.47−1.45 (m, 2H, nBu), 1.02 (t, JH−H = 7.4 Hz, 3H,
nBu). 13C{1H} NMR (CDCl3, 75 MHz): δ 180.0 (Ir−Ccarbene), 153.5
(Ar), 137.7 (Ar), 132.0 (Ar), 120.5 (CHimid), 119.8 (CHimid), 105.6
(Ar), 84.5 (COD), 84.3 (COD), 60.8 (OMe), 56.4 (OMe), 54.4
(NCH2Ar), 51.9 (COD), 51.4 (COD), 50.2 (nBu), 33.7 (COD), 33.4
(COD), 32.9 (nBu), 29.7 (COD), 29.4 (COD), 20.0 (nBu), 13.8
(nBu). Anal. Calcd for C25H36N2O3IrCl (640.24): C, 46.90; H, 5.67;
N, 4.38. Found: C, 46.64; H, 5.73; N, 4.04. Electrospray MS (cone 15
V; m/z, fragment): 605 [M − Cl]+. HRMS ESI-TOF-MS (positive
mode): [M − Cl]+ monoisotopic peak 605.2348, calcd 605.2357, ε =
1.0 ppm.
ACKNOWLEDGMENTS
We thank the financial support from the Ministerio de Ciencia
■
e Innovacio
n of Spain (CTQ2011-24055) and Bancaixa
(P1.1B2010-02 and P1.1B2008-16). We also thank the
“Generalitat Valenciana” for a fellowship (A.A.). We are
́
grateful to the Serveis Centrals d’Instrumentacio Cientifica
́
(SCIC) of the Universitat Jaume I for providing us with
spectroscopic facilities and to M. Franck Godiard of the
“Service Commun de Microscopie Electronique et Analytique”
of the University of Montpellier II (France) for TEM analysis
and fruitful discussions. We also acknowledge the CNRS and
MESR (France) for financial support.
Synthesis of 5A. A suspension of A (116 mg, 0.25 mmol) and
silver oxide (116 mg, 0.5 mmol) in 1,2-dichloroethane was stirred at
60 °C for 4 h under Ar. After the mixture was cooled to room
temperature, [Cp*IrCl2]2 (100 mg, 0.125 mmol) was added and the
reaction mixture was stirred at room temperature for 48 h under Ar.
After solvent removal, the product was extracted with CH2Cl2, filtered
through Celite, and concentrated to dryness. The complex was
obtained as a brown solid by recrystallization from CH2Cl2/hexanes
REFERENCES
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