Catalytic hydrogenation of carbon dioxide using Ir(III)-pincer complexes

Article abstract of DOI:10.1021/ja903574e

(Chemical Equation Presented) Catalytic hydrogenation of carbon dioxide in aqueous potassium hydroxide was performed using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as a catalyst. Potassium formate was obtained with t

Full text of DOI:10.1021/ja903574e

Published on Web 09/23/2009
Catalytic Hydrogenation of Carbon Dioxide Using Ir(III)-Pincer Complexes
Ryo Tanaka, Makoto Yamashita, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received May 2, 2009; E-mail: nozaki@chembio.t.u-tokyo.ac.jp
Formic acid is an important chemical product used as a
preservative and an insecticide as well as for tanning leathers. In
addition, formic acid plays a major role in synthetic chemistry as
an acid, a reductant, and a carbon source. In industrial processes,
formic acid is mainly synthesized by carbonylation of alcohols using
carbon monoxide followed by hydrolysis, which requires a high
pressure of toxic carbon monoxide gas.1 Therefore, development
of an alternative route to formic acid or its salt is highly desired.
Hydrogenation of carbon dioxide to generate formic acid is a
promising candidate because carbon dioxide is abundant, inexpen-
sive, and a less toxic C1 source. Hydrogenation of carbon dioxide
has been widely investigated using transition-metal complexes,2
especially with rhodium,3 ruthenium,4-6 and iridium7,8 catalysts.
Noyori and co-workers carried out the reaction in supercritical
carbon dioxide using a Ru(II) catalyst and obtained ammonium
formate, alkyl formate, and formamide in high yields.4 For example,
the highest turnover frequency (TOF) (95 000 h-1) has been
achieved using RuCl(OAc)(PMe3)4, which is soluble in supercritical
CO2.5 Himeda reported a highly active cationic Cp*Ir(III) catalyst
containing phenanthroline derivatives as ligands. The maximum
turnover number (TON) of the hydrogenation reached 222 000 (the
highest TON reported to date) in a basic aqueous solution.8 They
stated that electron-donating ligands accelerated the reaction.
Pincer ligands are known as multidentate ligands that strongly
bind to a metal center to prevent dissociation of the ligand from
the metal.9 The idea that an alkylphosphine-based pincer ligand
would be an efficient electron donor prompted us to use a transition-
metal-pincer complex as a catalyst for hydrogenation of CO2.
Although insertion of CO2 into PCP-ligated rhodium dihydrogen
complexes such as 1 and 2 to form formato complexes were
intensively studied,10 catalytic hydrogenation of CO2 with these
complexes has never been reported.
hydride. Thus, two hydrides are suggested to occupy positions cis
and trans to the nitrogen atom, forming an octahedral structure.
Scheme 1. Synthesis of PNP-Ligated Chloroiridium(III) Dihydride
Complexes 4a and 4b and PNP-Ligated Iridium(III) Trihydride Complex
5 and ORTEP Drawing of 5 (50% Thermal Ellipsoids, Hydrogen Atoms
except Hydrido Ligands Have Been Omitted for Clarity)
PNP-ligated iridium(III) trihydride complex 5 was synthesized from
4b in 77% yield by addition of an excess amount of NaH. X-ray
crystallographic analysis of 5 revealed a meridional coordination mode
of the PNP pincer ligand, with three peaks in the Fourier map that could
be assigned as hydrides to form the octahedral geometry. The IR spectrum
of 5 showed a trans iridium dihydride stretch at 1678 cm-1, which is a
much lower wavenumber than that previously reported for mer-IrH3(PPh3)3
(1745 cm-1),11 indicating that the Ir-H bonds in 5 are weakened by the
electron-rich PNP pincer ligand. Structural optimization of 5 using
B3LYP//6-31G(d)/LANL2DZ calculations reproduced the crystal structure
as well as the IR spectrum.
The hydrogenation of carbon dioxide in aqueous KOH was
investigated using Ir(III)-PNP pincer complex 4a, 4b, or 5 or
tBu-PCP-ligated iridium(III) complex 6a9a or 6b9c as the catalyst
(Table 1). At a reaction temperature of 200 °C, potassium formate
was formed even in the absence of catalyst12 (entry 1), and thus,
background corrections to the catalyst TONs based on the data from
entry 1 were made. PNP complex 4a showed higher activity than
t
PCP complex 6a (entries 2, 3). Changing the Bu groups on the
phosphorus atoms in 4a to iPr groups in 4b afforded higher activity,
probably because of the higher solubility of 4b (entries 3, 4).
Addition of THF as a cosolvent was necessary because of the poor
solubility of 4b in water (entry 5).
Herein we describe the synthesis of new PNP-Ir(III) hydride
complexes and their application to the catalytic hydrogenation of CO2
in aqueous base. The PNP-Ir(III) trihydride complex 5 bearing
isopropyl groups on the phosphorus atoms shows higher TON and
TOF than reported for any other catalyst to date. Deprotonation of the
PNP ligand is proposed to play an essential role in the catalytic cycle.
PNP-ligated chloroiridium(III) dihydride complexes 4a and 4b
were synthesized by the reaction of an Ir(I) source with an excess
amount of alkyl-substituted PNP pincer ligands 3a and 3b,
respectively, under H2 pressure (Scheme 1). The structures were
Iridium trihydride complex 5 showed much higher catalytic
activity than its chloroiridium dihydride analogue 4b (entries 4,
7). PCP-ligated iridium dihydride complex 6b also showed higher
activity than its chloroiridium hydride analogue 6a (entries 2, 6).
These results indicate that chloride may suppress the catalytic
reaction. Using 5 increased the TON to 150 000 h-1 at 200 °C (entry
7),13 which is 1.5 times as high as the best TOF value in the
literature.5 At 120 °C and lower catalyst loading (100 ppb) (entry
9), the TON of 3 500 000 was 15 times higher and the TOF almost
1
determined by NMR spectroscopy. In the H NMR spectra of 4a
and 4b, each of the two hydrides is coupled with the two
magnetically equivalent phosphorus atoms and with the other
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10.1021/ja903574e CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
doubled in comparison with the best result at 120 °C reported to
date.8 It should be noted that the use of a weaker base (K3PO4
instead of KOH) resulted in a lower yield (entry 10), suggesting
that the base plays an essential role in the catalytic cycle.
8 to regenerate the trihydride complex 5 completes a cycle. The
unprecedentedly high catalytic activity performed by catalyst 5 may
be attributed to the acceleration of formate dissociation from 7
mediated by the deprotonation of the ligand.
In conclusion, a highly active catalyst, 5, for hydrogenation of
carbon dioxide was developed. The best TOF and TON using 5
reached 150 000 h-1 and 3 500 000, respectively. Further mecha-
nistic studies are now in progress.
Table 1. Hydrogenation of Carbon Dioxide Catalyzed by
Ir(III)-Pincer Complexes
Acknowledgment. This work was supported in part by the
Global COE Program “Chemistry Innovation through Cooperation
of Science and Engineering”, MEXT, Japan.
entry Ir cat. T (°C) P (MPa)a time (h) yield (%)b TON (×103) TOF (102 h-1
)
1
2
3
none 200
5.0
5.0
5.0
5.0
5.0
5.0
5.0
6.0
6.0
5.0
13
13
13
13
13
2
4
16
20
24
9
12
60
94
70
8
-
Supporting Information Available: Experimental procedures,
characterization data, and crystallographic data for 5 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
6a
4a
4b
4b
6b
5
200
200
200
200
200
200
120
120
200
64
49
68
77
89
100
34
4
5c
6
26
59
300
1500
98
730
200
References
7
2
300
470
3500
40
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a Total pressure at room temperature. Yield based on H NMR analysis
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1
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Scheme 2. Plausible Mechanism for Hydrogenation of CO2 Using
Iridium Trihydride Complex 5
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A catalytic cycle consisting of iridium trihydride 5, iridium
formate 7, and amidoiridium dihydride 8 (Scheme 2) is proposed
on the basis of the following experiments. Treatment of complex
5 with aqueous KOH resulted in no reaction at room temperature
in 30 min.14 On the other hand, exposure of trihydride complex 5
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15
to 1 atm CO2 led to an immediate equilibrium between 5 and
1
dihydridoiridium(III) formate 7 at 25 °C, as observed by H and
13C NMR spectroscopy. Characteristic signals for a formato ligand
were observed in the 1H and 13C NMR spectra (δH ) 7.88 ppm, δC
) 168 ppm), although isolation of 7 failed. The two distinct hydride
signals of 7 indicated that the formato ligand is located cis to the
nitrogen atom of the pyridine moiety. As the next step, elimination
of formate anion from 7, accompanied by deprotonation from
Ar-CH2-P and subsequent dearomatization of the pyridine ring,
would give amidoiridium dihydride complex 8.16 We prepared
complex 8 by an alternative method, namely, the addition of
CsOH·H2O to chloroiridium dihydride complex 4b. Treatment of
8 with hydrogen resulted in the production of 5. Thus, as shown in
Scheme 2, insertion of CO2 into 5, deprotonative dearomatization
with dissociation of the formato ligand in 7, and hydrogenation of
(14) Treatment of 5 with aqueous KOH at 200 °C for 2 h gave a new signal at
46.6 ppm in the 31P NMR spectrum. Hydride signals were observed at-
19.77 and-23.33 ppm in the 1H NMR (CD2Cl2) spectrum.
(15) In the 13C NMR spectrum of CO2 (2.5 MPa) in a KOH/THF-d8/D2O
solution, a sole peak was detected at 125.8 ppm, which was identical to
the result for CO2 in THF-d8. This suggested that CO2 rather than
bicarbonate or carbonate is a major species under the reaction conditions.
(16) Deprotonation from the pyridylmethyl group to dearomatize the pyridine
ring has been observed previously. See: (a) Ben-Ari, E.; Leitus, G.; Shimon,
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D. Organometallics 2008, 27, 2627. (f) Gunanathan, C.; Shimon, L. J. W.;
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