A new approach to the reduction of carbon dioxide: CO2 reduction to formate by transfer hydrogenation in iPrOH

Article abstract of DOI:10.1021/om900820x

A series of four Ir and Ru catalysts have been tested in the reduction of CO₂ to formate in the presence of H₂. The iridium catalysts containing the basic NHC ligands were the ones to show better catalytic activities, achieving high TON values of up to 1800 TON. The unprecedented reduction of CO₂ with iPrOH through a hydrogen transfer process is also described. TON values of up to 150 were obtained. Two other secondary alcohols, cyclohexanol and 1-phenylethanol, were also used as hydrogen sources for the reduction of carbon dioxide.

Full text of DOI:10.1021/om900820x

Organometallics 2010, 29, 275–277 275
DOI: 10.1021/om900820x
A New Approach to the Reduction of Carbon Dioxide: CO2 Reduction to
Formate by Transfer Hydrogenation in iPrOH
Sergio Sanz, Miriam Benıtez, and Eduardo Peris*
´
ꢀ
Departamento de Quımica Inorganica y Organica, Universitat Jaume I, Avenida Vicente Sos Baynat s/n,
ꢀ
Castellon, E-12071 Spain
ꢀ
Received September 21, 2009
Summary: A series of four Ir and Ru catalysts have been tested
phosphine-based catalysts has been extensively investiga-
ted,2,5 there are just a few recent reports describing the use
of NHC-containing catalysts for this type of transforma-
tion,6 but they already illustrate the great potential of this
type of ligands.
The homogeneously catalyzed hydrogenation of CO2 to
formic acid7 is a well-known reaction that was first reported
in 1976.8 The reverse reaction (formic acid to CO2) has also
been widely used in the reduction of organic molecules by the
hydrogen transfer process in which formic acid is used as the
hydrogen source.9,10 On the other hand the reversibility of
this homogeneously catalyzed process has been proposed as
a viable hydrogen storage system.11,12 With all this in mind, it
may seem clear that effective hydrogen transfer catalysts
should also be effective in the reduction of CO2, if the right
reaction conditions are used.
Some recent reports have shown that “IrIIICp*” and
“RuII(arene)” complexes constitute valuable catalysts for
the hydrogenation of CO2 to formic acid7,13,14 and that the
σ-donating power of the coligands determines the activity of
the catalysts.7,13 In the course of our investigations, we found
that “IrCp*(NHC)” fragments are effective catalysts in a
wide set of hydrogen-borrowing reactions,15,16 including an
interesting example of base-free reduction of CdO and
CdNR bonds by transfer hydrogenation.16 On the basis of
these results, we decided to see if a more challenging sub-
strate such as CO2 could be reduced to formate by a transfer
in the reduction of CO2 to formate in the presence of H2. The
iridium catalysts containing the basic NHC ligands were the
ones to show better catalytic activities, achieving high TON
values of up to 1800 TON. The unprecedented reduction of
CO2 with iPrOH through a hydrogen transfer process is also
described. TON values of up to 150 were obtained. Two other
secondary alcohols, cyclohexanol and 1-phenylethanol, were
also used as hydrogen sources for the reduction of carbon
dioxide.
Introduction
Highly efficient catalytic activation of CO2 has become an
important research area because of the position of CO2 as the
primary greenhouse gas and its great potential to become an
important feedstock and reagent for organic transforma-
tions.1,2 In the search for a catalyst suitable for the transfor-
mation of CO2, two preliminary issues have to be taken into
account: first, the catalyst has to be resistant to the harsh
reaction conditions needed for the activation of the stable
and inert CO2, and second, the catalyst requires high-energy
d-occupied orbitals to facilitate the interaction with the
electron-deficient carbon of the molecule. N-Heterocyclic
carbenes (NHCs) may provide a good choice of ligands
for the design of this type of catalysts because they are
known to be strong electron donors3 and afford highly
stable metal complexes.4 While the reduction of CO2 using
(7) Himeda, Y. Eur. J. Inorg. Chem. 2007, 3927.
(8) Inoue, Y.; Izumida, H.; Sasaki, Y.; Hashimoto, H. Chem. Lett.
1976, 863.
*Corresponding author. E-mail: eperis@qio.uji.es.
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Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell,
A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois,
D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.;
Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.;
Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.;
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Chem., Int. Ed. Engl. 1995, 34, 2207. Jessop, P. G.; Ikariya, T.; Noyori, R.
Chem. Rev. 1995, 95, 259. Jessop, P. G.; Joo, F.; Tai, C. C. Coord. Chem.
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C. R. A.; Bacsa, J.; Xiao, J. L. Chem.;Eur. J. 2008, 14, 7699. Wu, X. F.;
Li, X. H.; Zanotti-Gerosa, A.; Pettman, A.; Liu, J. K.; Mills, A. J.; Xiao, J. L.
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Commun. 2007, 2449. Carrion, M. C.; Jalon, F. A.; Manzano, B. R.;
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Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 10490. Chianese,
A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics
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Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485.
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127, 17196. Ohishi, T.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2008,
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r
2009 American Chemical Society
Published on Web 10/26/2009
pubs.acs.org/Organometallics

276 Organometallics, Vol. 29, No. 1, 2010
Sanz et al.
Scheme 1
Table 1. Comparison of Catalysts 1-4 toward the Hydrogenation
Table 2. Transfer Hydrogenation of CO2 with Alcoholsa
of CO2 to KHCOOa
entry
catalyst/mM
TON
formate/M
1
2
3
4
6
7
8
9b
1/1.75
2/1.75
3/1.75
4/1.75
1/0.17
2/0.17
4/0.17
2/0.17
67
369
60
180
291
894
516
1600
0.12
0.65
0.11
0.32
0.05
0.16
0.09
0.28
entry
alcohol
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
cat./mM
1/0.175
2/0.175
3/0.175
4/0.175
time (h)
TONb
1
2
3
4
5
6
7
8
16
16
16
16
72
16
16
72
16
16
16
26
1
1
7
5
91
116
150
6
13
16
a The reactions were carried out at 60 atm (CO2/H2, 1:1) and 80 °C
5/0.175
during 18 h in an aqueous KOH solution (1 M). b [KOH] = 2 M.
1/1.8 Â 10-2
1/1.8 Â 10-3
1/1.8 Â 10-2
1/0.175
hydrogenation procedure as an alternative to the ubiquitous
reduction with pressurized hydrogen. Here we report the
study of the catalytic activity of a set of four Ir and Ru
complexes (see Scheme 1) in the reduction of CO2 using H2
and in an unprecedented example of a hydrogen transfer
reaction using iPrOH.
9
10
11c
cyclohexanol
1-phenylethanol
iPrOH
1/0.175
1/0.175
a Reactions were carried out at 110 °C in 20 mL of a 0.5 M KOH
solution in a mixture of H2O/iPrOH (9:1). PCO2 = 50 atm. b TONs based
on the formation of formate, calculated by 1HNMR spectroscopy.
Average of two experiments. c 0.5 M of K2CO3 used instead of KOH.
Results and Discussion
with recently published mechanistic studies18 it seems clear
that for the reduction of CO2 to formic acid only one
catalytic active site is needed; thus bidentate ligands in
half-sandwich Ir and Ru catalyst are perfectly allowed in
order to achieve high catalytic performances.
We first performed a catalyst screening using the four
different catalysts depicted in Scheme 1. The reactions were
performed using 60 atm of a CO2/H2 mixture (1:1) in an
aqueous solution of KOH at 80 °C. As observed by the data
shown in Table 1, under these reaction conditions the Ir
catalyst 2 afforded the best catalytic performances.
In order to see if the reduction of CO2 could also be
achieved by a transfer hydrogenation procedure, we decided
to replace the hydrogen source by iPrOH. iPrOH is an
inexpensive and environmentally benign reagent, which
may overcome the experimental inconvenience of using H2.
A first screening of the catalytic activity of compounds 1-4
was done by using a solution of KOH in a mixture of H2O/
iPrOH (9:1) at 110 °C, at a pressure of 50 atm of CO2.
Reactions run at lower temperatures (80 °C) afforded a much
lower conversion to the desired formate, while temperatures
above 110 °C did not provide any significant improvements
on the reaction yields. From the thermodynamic point of
view, the reduction of CO2 in aqueous basic solutions was
shown to be the most favorable reaction conditions.11
Together with the formation of formate, the corresponding
amount of acetone was also detected by 1HNMR. For
comparative purposes, compound [{IrCp*Cl2}2], 5, was also
tested under these reaction conditions. Table 2 shows some
representative data for this reaction.
The hydrogenation of CO2 was affected by the initial
concentration of KOH, as can be seen from the reaction
outcomes obtained when the reactions were carried out using
1 and 2 M KOH solutions (Table 1, entries 7 and 9,
respectively, and Figure S1.2 in the Supporting Informa-
tion), with a high TON value of 1600 obtained for the
reaction performed at the higher concentration of base. A
maximum TON of 1800 was obtained when the reaction was
allowed to evolve for 64 h in the 2 M aqueous solution.
The results shown for catalysts 3 and 4 are similar to those
previously observed for other related half-sandwich species
of Ru and Ir with the bipyridine (bpy) ligand.10,13 In clear
agreement with the results previously described by Himeda
and co-workers,13 it is observed that the introduction of the
electron-donating bis-NHC ligand in catalyst 2 provides an
important improvement of the catalytic activity of the IrCp*
catalytic system, compared to that shown by 4. On the other
hand, there is a clear difference in the catalytic activity shown
by catalysts 1 and 2. In a previously reported study, we
described that the catalytic performance of 1 was far better
than that shown by 2 in the catalytic deuteration of several
organic molecules.17 In that case, we attributed this behavior
to the need of the catalyst to have two coordination sites
available to provide a good catalytic outcome. In accordance
These results show that compound 1 affords the best
catalytic outcomes from the list of compounds under study.
In order to optimize the maximum activity of 1 in terms of
TON values, we performed a series of reactions in which the
concentration of the catalyst used was gradually lowered to
1.8 Â 10-3 mM, for which the TON was 116 after a fixed time
of 16 h. For the study of the time course of formate
(17) Corberan, R.; Sanau, M.; Peris, E. J. Am. Chem. Soc. 2006, 128,
3974.
(18) Ogo, S.; Kabe, R.; Hayashi, H.; Harada, R.; Fukuzumi, S.
Dalton Trans. 2006, 4657.

Note
Organometallics, Vol. 29, No. 1, 2010 277
material,12,21 capable of producing highly pure hydrogen
suitable for all kinds of fuel cells. Given the large library of
catalysts available for transfer hydrogenation reactions, a
wide catalyst screening may afford promising results.
Further studies in the optimization of the reduction of CO2
with iPrOH using other NHC-based catalysts are currently
underway in our lab, together with detailed mechanistic
studies.
Experimental Section
General Procedures. All the catalysts used (1,17 2,22 3,23 4,24 525)
were prepared according to literature procedures. Solvents and
reagents were used as received from commercial suppliers.
NMR spectra were recorded on Varian Innova 300 and
500 MHz instruments, using CDCl3, CD3OD, and D2O as
solvents.
Catalytic Hydrogenation of CO2 with H2. The catalytic
reactions were carried out in a Hastelloy Autoclave Mini-
Reactor system equipped with a 50 mL cylinder. The catalyst
(3.5 μmol for the reactions run with a 0.175 mM concentration
of catalyst) was dissolved in a degassed aqueous KOH solution
(20 mL). The reactor was pressurized with 60 atm of CO2/H2
(1:1) at 80 °C for the appropriate time. After reducing the
pressure to 1 atm and cooling to room temperature, the solvent
was removed by evaporation, and the residue was dissolved in
D2O. The yield of HCOOK was measured by 1H NMR in D2O,
using isonicotinic acid as internal standard (100:1 reference:
catalyst molar ratio; 43 mg (0.35 mmol) for a 0.175 mM solution
of catalyst).
Catalytic Hydrogen Transfer of CO2 with Alcohol. The
reactions were carried out in a Hastelloy Autoclave Mini-
Reactor system equipped with a 50 mL cylinder. The catalyst
(3.5 μmol for the reactions run with a 0.175 mM concentration
of catalyst) and KOH or K2CO3 were dissolved in 20 mL of a
mixture of H2O/alcohol (9:1 v/v). The reactor was pressurized
with 50 atm of CO2 at 110 °C for the desired time. After
equilibrating 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
measured by 1H NMR in D2O, using isonicotinic acid as internal
standard (20:1 reference:catalyst molar ratio; 9 mg (0.07 mmol)
for a 0.175 mM solution of catalyst).
Figure 1. Time course of formate formation catalyzed by 1 (1.8
 10-2 mM). PCO2 = 50 atm. T = 110 °C.
concentration, we used a catalyst concentration of 1.8 Â
10-2 mM; this gave us the best catalytic outcomes when
considering both conversions and TONs (Figure 1). Under
these reaction conditions, a maximum TON of 150 was
obtained after 72 h.
In order to confirm the reliability of our results, several
experiments were done. When the reaction is performed in a
mixture of D2O/iPrOH-d8 (9:1), the generation of KDCO2
2
could be confirmed by DNMR spectroscopy. The general
viability of the reaction was evaluated by testing two other
secondary alcohols, such as cyclohexanol and 1-phenyletha-
nol (Table 2, entries 9 and 10). Although the reaction out-
comes were lower than those obtained with iPrOH, both
alcohols afforded the formation of formate from CO2, thus
validating the applicability of the hydrogenation transfer
process for the reduction of this inert substrate.
Although we have not performed a detailed mechanistic
study, we believe that the process starts with the formation of
a metal dihydride by reaction of the starting metal complex
with iPrOH in the presence of the base. We previously
proposed the formation of this species to justify the catalytic
dehydrogenation of alcohols promoted by 1.19 Then, inser-
tion of a molecule of CO2 into one Ir-H bond would form a
formate complex that may undergo reductive elimination of
formic acid, which is then deprotonated by the base. In
principle, this simple mechanism should not differ from the
very well-known opposite reaction implying the use of for-
mic acid (or formate) as hydrogen source in transfer hydro-
genation processes, although we need to carry out studies in
order to elucidate the detailed mechanism of interaction
between the CO2 molecule and the metal hydride. Ab initio
metadynamics have proved to be a good choice for such
studies in the reduction of CO2 by ruthenium complexes.20
In summary, we have shown that the reduction of CO2 can
be performed by a hydrogen transfer process in which iPrOH
is used as the hydrogen source. This reaction is unprece-
dented and should be of great interest if the process can be
fully optimized. The reaction is interesting not only because
it uses the inexpensive and environmentally benign iPrOH as
hydrogen source but also because it provides an easy method
for the generation of formic acid/sodium formate, a system
that has been postulated as a potential hydrogen storage
Acknowledgment. We gratefully acknowledge finan-
ꢀ
cial support from the Ministerio de Ciencia e Innovacion
of Spain (CTQ2008-04460 and CTQ2007-31175-E/BQU)
and Bancaixa (P1.1B2007-04). The authors are grateful to
ꢀ
the Serveis Centrals d’Instrumentacio Cientıfica (SCIC)
of the Universitat Jaume I for providing us with spectro-
scopic facilities.
Supporting Information Available: Selected NMR spectra are
available free of charge via the Internet at http://pubs.acs.org.
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