Water-soluble IrIII N-heterocyclic carbene based catalysts for the reduction of CO2 to formate by transfer hydrogenation and the deuteration of aryl amines in water

Article abstract of DOI:10.1002/chem.201002907

Two new water-soluble [IrI₂(AcO)(bis-NHC)] complexes (NHC=N-heterocyclic carbene) incorporating a sulfonate functionality have been synthesized. The two complexes have been tested in the reduction of CO ₂ with H₂ and iPrOH

Full text of DOI:10.1002/chem.201002907

FULL PAPER
DOI: 10.1002/chem.201002907
Water-Soluble IrIII N-Heterocyclic Carbene Based Catalysts for the
Reduction of CO2 to Formate by Transfer Hydrogenation and the
Deuteration of Aryl Amines in Water
Arturo Azua, Sergio Sanz, and Eduardo Peris*[a]
Abstract: Two new water-soluble [IrI2-
(AcO)(bis-NHC)] complexes (NHC=
N-heterocyclic carbene) incorporating
sulfonate functionality have been
synthesized. The two complexes have
been tested in the reduction of CO2
with H2 and iPrOH, and their activity
has been compared with similar species
without the sulfonate moiety. In both
reactions, the complex with the two ab-
normally bound NHCs shows the best
catalytic efficiencies, due to the higher
s-electron-donor character of the
ligand. Remarkably, the activities ob-
tained for the reduction of CO2 under
the transfer hydrogenation conditions
are the best reported to date in terms
of TON value (max. TON=2700). The
two new complexes have also shown
very good activity in the selective deut-
eration of arylamines, a process that is
known to proceed through a chelate as-
sisted N-directed process.
G
ACHTUNGTRENNUNG
a
Keywords: deuteration · iridium ·
N-heterocyclic carbenes · reduction ·
water chemistry
Introduction
catalysts, our initial hypothesis was that the high electron-
donor character of NHCs, together with their ability to form
stable metal complexes, would constitute a very useful type
of ligand for the preparation of catalysts for CO2 activation,
as previously proposed by other authors.[8] In our studies, we
proposed for the first time that iPrOH could be used as the
hydrogen source for the reduction of CO2 by a transfer-hy-
drogenation process,[6,7] a reaction that may constitute a
valid alternative over the use of hydrogen gas. Following the
same idea, we now decided to modify well-known transfer-
hydrogenation catalysts to make them soluble in water, so
that they can be applied to the effective reduction of CO2 to
formate in the presence of iPrOH/H2O. Among all the iridi-
um-based transfer hydrogena-
During the last two decades, there has been an increasing in-
terest in the use of water as the solvent for many homogene-
ously catalyzed reactions.[1] Cost, environmental benefits,
and safety, are among the advantages of water over organic
solvents. However, organic solvents have a series of attrac-
tive features, such as their abilities to dissolve organic sub-
strates, their (often) volatile character that makes them easy
to remove, and their wide range of properties (polarity, co-
ordination capabilities, etc.). Although many organic sub-
strates require organic solvents for their transformations
through homogeneously catalyzed processes, some other re-
actions are more easily performed in water, for which water-
soluble catalysts are then required. Among this latter type
of reaction, those involving CO2 reduction may represent an
important class,[2–4] because the solubility of CO2 in water
represents one of the key points to consider in the design of
the reaction protocols. For these reactions, one of the limita-
tions is that a water-soluble catalyst that is also able to stand
the harsh reaction conditions must be used,[3–5] otherwise
mixtures of organic solvents and water have to be utilized to
facilitate the solubility of the catalyst.
tion catalysts reported to date,
those described by Crabtree
and co-workers of the type [Ir-
A
E
(1,
Scheme 1) are particularly inter-
esting because they gather an
extraordinary high stability
toward air and moisture, and a
very high catalytic activity
Scheme 1. R=Me, nBu, iPr,
neopentyl, benzyl (Bn), or tBu.
We have recently described a series of N-heterocyclic car-
bene (NHC) based iridium and ruthenium catalysts for the
reduction of CO2 to formate.[6,7] In the design of these new
(TOF>50000 hÀ1).[9]
this into account, we now de-
scribe the preparation of a series of [IrCAHTUNGTREN(UNNG bis-NHC)ACHTUNGTRENNUNG(AcO)I2]
Taking
complexes in which the NHC ligand contains a sulfonate
functionality to make the complex soluble in water. We
have used the complexes obtained in the reduction of
carbon dioxide to formate with H2 and with iPrOH. In a
parallel study, we have also used these complexes in the se-
lective deuteration of aryl amines with D2O.
[a] A. Azua, S. Sanz, Prof. E. Peris
Departamento de Quꢀmica Inorgꢁnica y Orgꢁnica
Universitat Jaume I, 12071 Castellꢂn (Spain)
Fax : (+34)964-728-214
E-mail: eperis@qio.uji.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002907.
Chem. Eur. J. 2011, 17, 3963 – 3967
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3963

Results and Discussion
the same region where other abnormally bound bis-
AHCTUNGTRENNUNG
(aNHC)IrIII appear.[14]
Synthesis and characterization of new compounds: To
obtain water-soluble IrIII
(bis-NHC) complexes, we used the
Catalytic reduction of CO2 to formate: Both 2 and 3 were
tested in the catalytic reduction of CO2 with H2. The reac-
tions were carried out in a 1m solution of KOH in water at
80 and 2008C, using 60 atm of a CO2/H2 mixture (1:1). For
comparative purposes, catalyst 1 (with R=nBu) was also
tested in this reaction. The catalytic results are listed in
Table 1.
two sulfonate-functionalized bis-imidazolium salts, which
were prepared according to the general strategy used by
us[10] and others,[11] consisting of the reaction of the corre-
sponding neutral bisimidazoles with propanesultone. The
IrIII complexes 2 and 3 were obtained by reaction of the cor-
responding sulfonate-functionalized bisimidazolim salts with
[IrCl
KI and NaOAc in refluxing MeOH. For the preparation of
the bis(aNHC) (aNHC=abnormal NHC) complex 3, we fol-
ACHTUNGTRENNUNG(cod)]2 (COD=1,5-cyclooctadiene) in the presence of
Table 1. Reduction of CO2 with H2.[a]
ACHTUNGTRENNUNG
lowed the general strategy of blocking the C2-position of
the azolium salt with a methyl group, to force the coordina-
tion of the NHC by the C4/C5 carbons.[12–14] We thought that
the preparation of the bis-abnormally bound bis-NHC com-
plex may introduce some improvements in the catalytic per-
formance of 3 compared with 2, due to the higher electron-
donor character of the bis-aNHC ligand,[13] which may favor
the interaction of the metal with the electrophilic CO2 mole-
cule.
Entry Catalyst t [h] T [8C]
[Cat.] [mm] nHCOOK [mmol] TON[b]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
none
none
none
20
20
75
20
20
75
20
20
20
75
20
75
20
20
20
75
200
80
200
80
80
80
–
–
0.7
0
–
–
–
1.08
0.1
–
243
1
2
2
2
2
2
2
3
3
3
3
3
3
2ꢄ10À2
2ꢄ10À2
2ꢄ10À2
2ꢄ10À3
2ꢄ10À2
2ꢄ10À3
2ꢄ10À3
2ꢄ10À2
2ꢄ10À2
2ꢄ10À3
2ꢄ10À2
2ꢄ10À3
2ꢄ10À3
0.25
0.43
0.17
1.90
1.30
1.65
0.33
0.59
0.19
3.05
3.03
3.8
1247
2153
8480
9500
65000
82300
1663
2930
9340
15240
151300
190000
80
Compounds 2 and 3 were obtained in high yield (69 and
75%, respectively; Scheme 2) as the corresponding potassi-
um salts, and were characterized by NMR spectroscopy and
elemental analysis. Both complexes were very soluble in
water and DMSO, sparingly soluble in MeOH, and insoluble
in most organic solvents such as THF, CH2Cl2, Et2O, and so
forth. The NMR spectra of both 2 and 3, confirms the two-
fold symmetries of the molecules. The 13C NMR spectrum of
2 in [D6]DMSO, shows the signal due to the carbene carbon
at d=123.7 ppm, in the same region as for other similar pre-
200
200
200
80
80
80
200
200
200
9
10
11
12
13
14
15
16
[a] Reactions carried out at 60 atm (30CO2:30H2) in 1m KOH (10 mL) in
H2O. nHCOOK (number of mmols of HCOOK) and TONs corrected ac-
cording to data obtained without catalysts, entries 1–3. [b] TONs based
1
viously reported IrIII analogues.[9] The H NMR spectrum of
1
on the formation of formate, calculated by H NMR spectroscopy.
3 shows a singlet due to the equivalent protons of the C2
Me groups at d=2.8 ppm. The 13C NMR spectrum shows
the signal due to the carbene carbons at d=146.4 ppm, in
The introduction of the sulfo-
nate functionality results in a
very important improvement of
the catalytic performance, as
can be confirmed by comparing
the
catalytic
performances
shown by 1 (Table 1, entry 4,
TON=243) and 2 under the
same
reaction
conditions
(Table 1, entry 5, TON=1247).
This, in fact, validates our first
hypothesis that the solubility of
the catalyst is crucial in the effi-
ciency of this reaction. If we
compare the data for the reac-
tions carried out under the
same conditions using 2 and 3,
we can see that catalyst
3
always provides the best per-
formances, which actually vali-
Scheme 2.
dates our hypothesis that a
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Chem. Eur. J. 2011, 17, 3963 – 3967

Water-Soluble IrIII N-Heterocyclic Carbene Based Catalysts
FULL PAPER
stronger electron-donor ligand should provide a better cata-
lytic outcome in this reaction.
reduction of CO2 to formate using iPrOH was 874 for a
[RuACTHNUGRTNENUG ACHUTNGTRENNUGN
(h6-arene)(bis-NHC)] complex reported by us,[6] under
For the reactions carried out at 2008C, potassium formate
is formed even in the absence of catalyst,[6,15] and thus back-
ground corrections to the catalyst TONs based on the data
without catalyst were made (Table 1, entries 1 and 3). Under
these reaction conditions, catalyst 2 provided a high TON
value of 82300 (Table 1, entry 10), whereas 3 afforded an ex-
traordinary high activity with a TON value of 190000. These
data are far better than those previously reported by us,[6,7]
and lie among the highest outcomes reported to date, to-
gether with the Ir pincer complex reported by Nozaki,[15]
similar reactions conditions.
Regioselective deuteration of arylated heterocycles: Hydro-
gen/deuterium exchange processes are powerful methods to
evaluate the potential of a catalyst for the cleavage and for-
[16]
À
mation of C H bonds. In general, D2O is preferred as a
deuterating agent over other sources, due to its low cost and
low toxicity. We recently reported the N-directed regioselec-
À
tive deuteration of C H bonds of several arylated N-hetero-
cycles with a [Ru
G
N
and the Ir
Himeda.[4]
(bpy) (bpy=bipyridine) catalyst reported by
case our results showed the extraordinarily high activity of
our catalyst in this unprecedented selective deuteration, al-
though we were unable to make the catalyst effective in
D2O ([D4]MeOH was the deuterium source). Also, it was re-
cently described that a dirhodium(II) complex was able to
selectively deuterate benzo[h]quinoline at the C10-position,
although in this case the reaction needed a stoichiometric
amount of the dimetallic species (the reaction is not catalyt-
ic) and base.[18] Aiming to study the same process with our
new water-soluble catalysts 2 and 3, and having proved its
Because the activity of 1 was reported to be extraordinari-
ly high in transfer hydrogenation processes,[9] we thought
that complexes 1–3 may be good catalysts for the reduction
of CO2 to formate using iPrOH as the hydrogen source. The
reactions were carried out in a mixture of H2O/iPrOH (9:1,
20 mL) so again, for this process, the solubility of the cata-
lyst in water should play an important role. As can be seen
from the data shown in Table 2, complexes 2 and 3 are more
À
activity in C H activation processes such as those shown in
Table 2 for the reduction of CO2 with iPrOH, we thought
that these two catalysts should have a high chance of activity
in this reaction. The results that we obtained are listed in
Table 3.
Table 2. Transfer hydrogenation of CO2 with iPrOH.[a]
Entry Catalyst t [h] T [8C]
[Cat.] [mm] nHCOOK [10À6 mol] TON[b]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
none
none
16
16
16
16
16
16
75
16
16
16
75
200
110
110
110
200
200
200
110
200
200
200
–
–
90
0
–
–
60
Table 3. Deuteration of pyridines in D2O using 2 and 3.[a]
Entry Substrate
Product
Cat. Conv. [%][b]
1
2
2
2
2
3
3
3
3
0.018
0.018
0.018
0.0018
0.0018
0.018
0.018
0.0018
0.0018
21.6
39.2
64.1
32.7
62.3
48.6
90
109
178
910
1730
135
250
1320
2700
1
2
2
3
45
100
2-phenylpyridine
3
4
2
3
74
96
9
10
11
benzo[h]quinoline
47.5
97.2
5
6
2
3
70
95
[ꢅ] Reactions carried out at 1108C, 50 atm CO2, in the presence of 0.5m
KOH in H2O/iPrOH (9:1, 20 mL). nHCOOK (number of mols of HCOOK
generated) and TONs corrected according to data obtained without cata-
lysts, entries 1 and 2. [b] TONs based on the formation of formate, calcu-
1,2-bis(2-pyridyl)ethylene
N-phenylpyrazole
7
8
2
3
100
100
1
lated by H NMR spectroscopy.
[a] Reactions were carried out at 1208C with catalyst (0.0125 mmol), sub-
active than 1 (compare entries 3, 4, and 8), so again, the
presence of the sulfonate functionalities improves the cata-
lytic activities of 2 and 3 due to the increase of their solubili-
ty in water. As seen for the reactions with H2, the activity of
3 is higher than that shown by 2. For the reactions carried
out at 2008C, again we had to make background corrections
comparing with the data obtained for the reactions without
catalyst. Compound 2 provided a highest TON value of
1730 for the reaction carried out at 2008C with a catalyst
loading of 1.8ꢄ10À3 mol% after 75 h. Under the same con-
ditions, compound 3 provided a maximum TON value of
2700, which is the highest reported so far for this type of
process. Notably, the maximum TON value obtained for the
strate (0.25 mmol), and D2O (2 mL) for 12 h. [b] Conversions determined
1
by H NMR spectroscopy.
The reactions were carried out in D2O at 1208C with a
catalyst loading of 5 mol% over 12 h. As can be seen from
the data shown in Table 3, both catalysts showed a good ac-
tivity in the deuteration of a variety of aryl amines, although
the activity of 3 was higher than that shown by 2. Remarka-
bly, the reactions proceeded in the absence of a base or any
other additive. In general, the search of catalysts capable to
selectively deuterate organic molecules is a matter of contin-
uous interest because deuterium-labeled compounds can be
Chem. Eur. J. 2011, 17, 3963 – 3967
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www.chemeurj.org
3965

E. Peris et al.
Cimid), 121.3 (2C; Cimid), 57.3 (1C; NCH2N), 47.4 (2C; CH2SO3), 46.9
(2C; NCH2), 24.5 (2C; CH2CH2SO3), 9.7 ppm (CH3imid).
used in a wide range of applications such as the study of bio-
logically active systems, solvents for NMR spectroscopy, and
the study of reaction mechanisms.[19] In particular, our re-
sults offer the possibility of selectively deuterating a series
of aryl amines through a N-directed process, using the most
convenient deuterating agent, D2O.
Synthesis of 2: A mixture of methylenebis
ACHTUNGTRNE[NUNG N,N’-(propanesulfonate)imid-
azolium] (78 mg, 0.2 mmol), [Ir(cod)Cl]2 (67 mg, 0.1 mmol), KI (100 mg,
AHCTUNGTRENNUNG
0.6 mmol), and NaOAc (65 mg, 0.8 mmol) was stirred in MeOH at reflux
temperature for 16 h. The suspension was filtered through Celite, and
after drying under vacuum, the solid was washed with CH2Cl2 (40 mL)
and acetone (40 mL). The solid was purified by chromatography. Elution
with MeOH/acetone (1:1, 40 mL) afforded the separation of a yellow
band that contained the compound. The complex was obtained as a
yellow solid by precipitation from MeOH/iPrOH (134.37 mg, 69%).
Conclusion
1H NMR (CD3OD, 300 MHz): d=7.41 (s, 2H; Himid), 7.31 (s, 2H; Himid),
3
6.26 (s, 2H; NCH2N), 4.56 (t, J
G
Herein, we have prepared two new [IrI2ACTHNUGRTENNU(G AcO)ACHUTTGNREN(NUNG bis-NHC)]
(t, 3J
(HÀH)=7.27 Hz, 4H; NCH2), 2.42–2.24 (m, 4H; CH2CH2SO3),
complexes in which the NHC ligands incorporate a sulfo-
nate substituent. The presence of the sulfonate makes the
two IrIII complexes water soluble, so the two species are
very good candidates for the study of their catalytic activity
in aqueous solvents.
1.93 ppm (s, 3H; CH3COO); 13C{1H} NMR (DMSO, 500 MHz): d=176.4
(1C; CH3COO), 123.7 (2C; IrC), 122.1 (1C; Cimid), 120.5 (1C; Cimid),
48.3 (2C; CH2SO3), 47.8 (2C; NCH2), 26.9 (2C; CH2CH2SO3), 24.5 (1C;
CH3COO); electrospray MS (15 V): m/z: 448 [M]2À; elemental analysis
calcd (%) for C15H21N4O8S2I2IrK2 (973.70): C 18.50, H 2.17, N 5.75;
found: C 18.37, H 2.50, N 5.61.
Both complexes have been tested in the reduction of CO2
to formate using H2 and iPrOH. For comparative purposes a
similar complex without the sulfonate functionality was also
tested, and for all the reactions studied, both catalysts with
the sulfonate group showed better catalytic performances.
In both processes the complex with the bis-abnormal coordi-
nation of the NHC ligand (3), shows the best catalytic out-
comes. In the reduction with H2, compound 3 has an activity
that is comparable to the best catalyst reported for this reac-
tion.[4,15] In the reduction with iPrOH through a transfer hy-
drogenation process, compound 3 provides the best catalytic
results reported to date.
Both 2 and 3 have also shown to be very active catalysts
in the selective deuteration of aryl amines through an N-di-
rected process. The high activity of the complexes is compa-
rable to data previously reported by us,[17] but has the ad-
vantage that the reaction can be carried out using D2O as
deuterating agent. This new process suggests that both cata-
lysts and, in particular the one with the bis-abnormal coordi-
nation (3), may be a promising catalyst for the study of fur-
Synthesis of 3: A mixture of 1,1’-methylenebis[(2,2’-methyl)(3,3’-propane-
sulfonate)imidazolium (84 mg, 0.2 mmol), [IrACTHNUTRGNEN(UG cod)Cl]2 (67 mg, 0.1 mmol),
KI (100 mg, 0.6 mmol), and NaOAc (65 mg, 0.8 mmol) was stirred in
MeOH at reflux for 16 h. The suspension was filtered through Celite, and
after drying under vacuum, the solid was washed with CH2Cl2 (40 mL)
and acetone (40 mL). The complex was obtained as a brown solid by pre-
cipitation from MeOH/iPrOH (150.26 mg, 75%). 1H NMR (D2O,
300 MHz): d=7.72 (brs, 2H; Himid), 6.63 (s, 2H; NCH2N), 4.42 (t, 3J-
A
U
NCH2), 2.8 (s, 6H; CH3), 2.40–2.28 (m, 4H; CH2CH2SO3), 1.94 ppm (s,
3H; CH3COO); 13C{1H} NMR (D2O, 300 MHz): d=181.7 (1C;
CH3COO), 146.4 (2C; IrC), 122.9 (2C; CCH3imid), 121.5 (2C; Cimid),
47.5 (2C; CH2SO3), 47.3 (2C; NCH2), 24.5 (2C; CH2CH2SO3), 23.6 (1C;
CH3COO), 10.0 ppm (CH3imid); elemental analysis calcd (%) for
C17H25N4O8S2I2IrK2 (1001.75): C 20.38, H 2.52, N 5.59; found: C 20.51, H
2.38, N 5.23.
Catalytic hydrogenation of CO2 with H2: Catalytic reactions were carried
out in a Hastelloy Autoclave Mini-Reactor system equipped with a
50 mL cylinder. The catalyst was dissolved in a degassed aqueous KOH
solution (10 mL). The reactor was pressurized with 60 bar of CO2/H2
(1:1) and heated at 80–2008C for the appropriate time. After reducing
the pressure to 1 bar and cooling to room temperature, the solvent was
removed by evaporation, and the residue was dissolved in D2O. The yield
of HCOOK was determined by 1H NMR in D2O, using isonicotinic acid
as internal standard.
À
ther C H activation processes in water. Further investiga-
tions in this direction are underway.
Catalytic hydrogen transfer of CO2 with 2-propanol: The reactions were
carried out in a Hastelloy Autoclave Minireactor system equipped with a
100 mL cylinder. The catalyst and KOH were dissolved in 20 mL of a
mixture of H2O/alcohol (9:1 v/v). The reactor was pressurized with
50 bar of CO2 and heated at 110–2008C under 1100 r.p.m. stirring for the
experiment time. After equilibration to atmospheric pressure and cooling
to room temperature, the solvent was removed by evaporation, and the
residue was dissolved in D2O. The yield of HCOOK was determined by
1H NMR spectroscopy in D2O, using isonicotinic acid as internal stan-
dard.
Experimental Section
General procedures: NMR spectra were recorded on Varian Innova 300
and 500 MHz spectrometers, using CD3OD, DMSO and D2O as solvents.
Elemental analyses were carried out in an EA 1108 CHNS-O Carlo Erba
analyzer. Electrospray mass spectra (ESI-MS) were recorded on a Micro-
mass Quatro LC instrument, and nitrogen was employed as drying and
nebulizing gas. Solvents and reagents were used as received from the
commercial suppliers. Complex 1 was prepared according to literature
methods.[9]
Deuteration of N-heterocycles in D2O:
A mixture of the substrate
(0.25 mmol) and catalyst 1 and 2 (0.0125 mmol) in D2O (2 mL) was
heated at 1208C in a thick-walled glass tube fitted with Teflon cap. At
the desired reaction times, aliquots were extracted from the reaction
vessel with CDCl3 and added to an NMR tube.
Synthesis of 1,1’-methylenebis[(2,2’-methyl)(3,3’-propanosulfonate)]imi-
dazolium:
A
mixture of 1,1’-methylenebis[(2,2’-methyl)]imidazole
(828 mg, 4.7 mmol) and 1,3-propanosultone (2.8 g, 23.5 mmol) was stirred
in CH3CN at reflux for 14 h. The suspension was filtered and the solid
was washed with CH2Cl2, affording a white pure product (1.5 g, 78%).
H1 NMR (D2O, 300 MHz): d=7.73 (s, 4H; Himid), 6.65 (s, 2H; NCH2N),
Acknowledgements
3
3
4.45
(t, J
N
T
4H; NCH2), 2.86 (s, 6H; CH3), 2.40 ppm (m, 4H; CH2CH2SO3);
We gratefully acknowledge financial support from the Ministerio de
Ciencia e Innovaciꢂn of Spain (CTQ2008–04460 and CTQ2007–31175-E/
13C{1H} NMR (D2O, 500 MHz): d=146.2 (2C; CCH3imid), 122.4 (2C;
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Chem. Eur. J. 2011, 17, 3963 – 3967

Water-Soluble IrIII N-Heterocyclic Carbene Based Catalysts
FULL PAPER
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Jaume I for providing us with spectroscopic facilities.
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Published online: March 1, 2011
Chem. Eur. J. 2011, 17, 3963 – 3967
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