Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure

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Cite this: DOI: 10.1039/c2ee03315a

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Catalytic interconversion between hydrogen and formic acid at ambienttemperature and pressure†

Yuta Maenaka,a Tomoyoshi Suenobua and Shunichi Fukuzumi*ab

Received 29th November 2011, Accepted 4th January 2012

DOI: 10.1039/c2ee03315a

Interconversion between hydrogen and formic acid in water at ambient temperature and pressure has

beenmadepossible byusinga [C,N] cyclometalatedorganoiridiumcomplex, [IrIII(Cp*)(4-(1H-pyrazol-1-

yl-kN2)benzoic acid-kC3)(H2O)]2SO4 [1]2$SO4, as an efficient catalyst for both directions depending on

pH.Hydrogenationof carbondioxide byhydrogenoccurs in thepresence of a catalytic amount of 1under

an atmospheric pressure of H2 and CO2 in weakly basic water (pH 7.5) at room temperature, whereas

formic acid efficiently decomposes to afford H2 and CO2 in the presence of 1 in acidic water (pH 2.8).

1. Introduction

Hydrogen (H2) is regarded as an environmentally benign and

renewable energy source because the requested energy is

produced with water as the sole product when it reacts with

oxygen.1–8 Despite the potential availability of hydrogen as the

most promising energy source, it requires either high pressure to

decrease the volume of gaseous hydrogen or high energy to keep

hydrogen as a liquid under cryogenic conditions. Extensive

efforts have so far been devoted towards the storage of hydrogen

using metal hydrides,9–12 metal–organic frameworks,13–18 and

other chemical hydrogen sources.19–21

In contrast to these materials, the utility of formic acid

(HCOOH) has merited significant attention, because HCOOH

aDepartment of Material and Life Science, Division of Advanced Scienceand Biotechnology, Graduate School of Engineering, Osaka University,ALCA, Japan Science and Technology Agency (JST), Suita, Osaka,565-0871, Japan. E-mail: [email protected]; Fax: +81-6-6879-7370; Tel: +81-6-6879-7368bDepartment of Bioinspired Science, EwhaWomans University, Seoul, 120-750, Korea

† Electronic supplementary information (ESI) available: X-Raycrystallographic data, pKa titration, ORTEP drawing, time course ofTON for the formate formation, gas chromatogram, time course ofTON for the decomposition of formic acid and formic acid-d, ESImass spectrum and FTIR spectrum. See DOI: 10.1039/c2ee03315a

Broader context

The difficulty of storing and transporting gaseous hydrogen at amb

of hydrogen as a clean energy source. We report a convenient hydro

formic acid (liquid) by catalytic fixation of carbon dioxide with h

catalytic decomposition of formic acid. Cyclometalated organoiridi

between hydrogen and formic acid in water at ambient temperature

using the same catalyst in water.

This journal is ª The Royal Society of Chemistry 2012

is a liquid at room temperature with relatively high volumetric

density (d ¼ 1.22 g cm�3) and can be formed by reduction of

carbon dioxide (CO2) with H2 [eqn (1)].22–27 From the viewpoint

of safety and cost-cutting, the liquid form is suitable for

transportation, handling and storage as compared to the

gaseous form. In addition, HCOOH is a valuable raw material

in organic syntheses and also an important intermediate in the

water–gas-shift reaction.28–30 Thus, the combination of H2

storage with the aid of CO2 as a carrier, i.e., hydrogenation of

CO2 with H2 to produce formic acid, with H2 evolution in the

decomposition of formic acid to produce CO2 as the sole

byproduct, is an ideal carbon-neutral process. Each of the

reactions, i.e., eqn (1) or (2), is usually investigated separately

with the use of a different catalyst appropriate for each reac-

tion. The use of water as a solvent is preferred for intercon-

version between H2 and HCOOH, because the standard free

energy change is slightly negative (�4 kJ mol�1 at 298 K) in

water under the conditions that all the reactants and the

products are soluble in water, whereas the reaction between

gaseous H2 and CO2 yielding liquid HCOOH is accompanied

by a free energy change of +33 kJ mol�1.29,31

CO2 + H2 # HCOOH (1)

HCO3� + H2 # HCOO� + H2O (2)

ient temperature and pressure has precluded the convenient use

gen-on-demand system in which hydrogen (gas) can be stored as

ydrogen and, whenever needed, hydrogen is produced by the

um aqua complexes act as efficient catalysts for interconversion

and pressure. The direction of the reaction is controlled by pH

Energy Environ. Sci.

http://dx.doi.org/10.1039/c2ee03315a






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Fig. 1 Acid–base equilibria of iridium aqua complexes.

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Hydrogenations of CO2 (or bicarbonate) to produce HCOOH

(or formate: HCOO�) in aqueous systems have so far been studied

with homogeneous catalysts such as rhodium,32–34 ruthenium35–39

and iridium complexes.32,34,39–44 In most cases, basic conditions

and/or high pressure are required for the catalytic hydrogenation

of CO2. A much more reactive catalyst under ambient tempera-

ture and pressure is certainly required for a carbon-neutral and

environmentally benign hydrogen storage system.

On the other hand, the catalytic decomposition of HCOOH to

H2 and CO2 (the reverse reaction of hydrogenation of CO2) in

water has also been investigated with homogeneous23,45–51 and

heterogeneous52–58 catalysts. We have reported that a hetero-

dinuclear iridium–ruthenium complex [IrIII(Cp*)(H2O)(bpm)

RuII(bpy)2](SO4)2 {Cp* ¼ h5-pentamethylcyclopentadienyl, bpm

¼ 2,20-bipyrimidine, bpy ¼ 2,20-bipyridine} acts as the most

effective catalyst for selective production of hydrogen from for-

mic acid in an aqueous solution at room temperature among

catalysts reported so far.59 Therefore, an efficient catalyst effec-

tive for both directions of the interconversion between H2 and

HCOOH had certainly been desired.23–26,45 Since then, efforts

have been devoted to develop the catalyst for interconversion

between H2 and HCOOH with either complexes of Ru and Rh27

or composites comprising organoruthenium salt and substituted

diphenylmethylphospines;60 however, an efficient catalytic

system operating at ambient temperature and pressure with the

use of a catalyst effective for both directions of the intercon-

version has yet to be reported.61

We report herein that a [C,N] cyclometalated water-soluble

iridium aqua complex acts as an efficient catalyst for intercon-

version between H2 and HCOOH at ambient temperature and

pressure in water by controlling pH. Hydrogenation of CO2 by

H2 has successfully been achieved in the presence of a catalytic

amount of [IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-

kC3)(H2O)]2SO4 [1]2$SO4 under an atmospheric pressure of H2

and CO2 in weakly basic water (pH 7.5) at ambient temperature.

On the other hand, in acidic water (pH 2.8), hydrogen is effi-

ciently generated by the catalytic decomposition of HCOOH at

298 K. The overall catalytic mechanism was examined for the

interconversion between H2 and HCOOH with 1. The catalytic

efficiency for hydrogenation of carbon dioxide as well as

decomposition of HCOOH has been tuned by choosing an

appropriate pH. The catalytic mechanisms of these reactions

were revealed.

Fig. 2 Time course of the concentration of formate and TON for the

formate formation in the hydrogenation of bicarbonate by H2 catalysed

under an atmospheric pressure of H2 (50 mL min�1) and CO2 (50 mL

min�1) by 2 (0.26 mM) in deaerated H2O at 303 K and pH 7.5.

2. Results and discussion

A water-soluble iridium aqua complex 1 was synthesized by the

reaction of a triaqua complex [IrIII(Cp*)(H2O)3]SO4 with 4-(1H-

pyrazol-1-yl)benzoic acid in H2O under reflux conditions. The

aqua complex 1 can release protons from the carboxyl group and

the aqua ligand to form the corresponding benzoate complex 2

and hydroxo complex 3, respectively (Fig. 1). The pKa values of

complexes 1 and 2 were determined from the spectral titration to

be pKa1 ¼ 4.0 and pKa2 ¼ 9.5, respectively; see Fig. S1 in the

ESI†. These values are consistent with those for benzoic acid62

(pKa ¼ 4.19) and [IrIII(Cp*)(4,40-OMe-bpy)(OH2)](SO4)39 (pKa ¼

9.2). The benzoate complex 2 is less soluble in water because of its

neutral charge. The structure of 2 was successfully determined by

X-ray single crystal structure analysis (Fig. S2 in the ESI†).


2.1. Catalytic hydrogenation of bicarbonate

An aqueous solution of complex 2 was bubbled with an atmo-

spheric pressure of CO2 in the presence of K2CO3 (0.1 M) for 1 h

at 303 K. Additional bubbling with H2 and CO2 in 1 : 1 volu-

metric ratio under atmospheric pressure at pH 7.5 and 303 K

resulted in formation of formate with a high concentration,

which was detected by 1H NMR at 8.47 ppm. The time course of

the formate formation is shown in Fig. 2 where the turnover

number (TON) increases linearly with time to exceed over 100.

The turnover frequency (TOF) was determined from the slope of

the linear plot as 6.8 h�1 which is the highest TOF value ever

reported under otherwise the same experimental conditions in

H2O.43 In the same manner, TOF at 333 K was also determined

to be 22.1 h�1 from the slope of the linear plot (Fig. S3 in the

ESI†). The time course of TON at 333 K was examined at

different pH in a bicarbonate/carbonate (KHCO3/K2CO3) mixed

water solution with the sum of concentrations of KHCO3 and

K2CO3 ([KHCO3] + [K2CO3]) kept constant at 2.0 M under an

atmospheric pressure of H2 (Fig. S4 in the ESI†). Turnover

frequency (TOF) increased with a decrease in pH to afford the

highest value at pH 8.8 (Fig. 3). Further increase in pH resulted

in a decrease in TOF to reach 0.0 at pH 10.4. Judging from the

similar pH dependence of TOF (black line in Fig. 3) to that of the

amount ratios of 2 and HCO3� (red line and red dashed line in

Fig. 3, respectively), which were determined based on the acid-

dissociation equilibrium between 2 and 3 as well as that between

HCO3� and CO3

2– [eqn (3)],63 hydrogenation of bicarbonate

(HCO3�) rather than carbonate (CO3

2�) is catalysed by 2 (not by

3) to produce formate at pH 8.8. Under slightly acidic conditions,

an aqueous solution of complex 2 was bubbled with H2 and CO2

CO2þH2O *)pKa1¼6:35

HCO�3 þHþ *)

pKa2¼10:33CO2�

3 þ2Hþ (3)



Fig. 3 pH-dependence of the formation rate (TOF) of formate in the

catalytic generation of formate from H2, HCO3�, and CO3

2� ([HCO3�] +

[CO32�] ¼ 2.0 M) catalysed by 2 and 3 ([2] + [3] ¼ 0.18 mM) in deaerated

H2O at 333 K (black line). TOF values were determined based on the

progress of the reaction for initial 2 h (pH 8.8), 3 h (pH 9.4) and 7 h (pH

9.9), respectively. No formate was detected at pH 10.4 after 12 h. Red and

green solid lines show the amount ratios of complex 2 and complex 3,

respectively, to the total amount of these complexes. Red and green

dashed lines show the ratios of HCO3� and CO3

2�, respectively.

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in 1 : 1 volumetric ratio under atmospheric pressure at 333 K,

and no formation of formate was confirmed at pH 6.0. This is

consistent with a previous report that supports the easier

hydrogenation of bicarbonate than carbon dioxide based on the

energetics derived from theoretical calculations.31 Indeed, TOF

linearly increased with concentrations of HCO3� and 2 at pH 8.8

(Fig. 4).64 These results indicate that both catalyst 2 and the

bicarbonate anion (HCO3�) are involved in the rate-determining

step of the catalytic hydrogenation reaction. The TOF for

hydrogenation of HCO3� at pH 8.8 increases with increasing

temperature. The Arrhenius plot (Fig. S5 in the ESI†) afforded

the activation energy to be 11.3 kcal mol�1, which is much smaller

than that of the hydrogenation of CO2 without catalysts (79 kcal

mol�1).65 Thus, catalyst 2 remarkably lowers the activation

energy of the hydrogenation of bicarbonate. Each step of the

hydrogenation reaction of HCO3� by H2 with 2 was followed by

the UV-vis spectral changes (Fig. 5 and the right-hand catalytic

Fig. 4 (a) Plot of TOF versus the concentration of KHCO3 and K2CO3

mixture (KHCO3/K2CO3), i.e., [KHCO3] + [K2CO3] in the hydrogena-

tion reaction of KHCO3/K2CO3 with H2 catalysed by 2 (0.18 mM) under

an atmospheric pressure of H2 in deaerated H2O at pH 8.8 and 333 K. (b)

Plot of the formation rate (r) of formate versus the concentration of 2 in

the hydrogenation reaction of KHCO3/K2CO3 ([KHCO3] + [K2CO3] ¼2.0 M) catalysed by 2 under an atmospheric pressure of H2 in deaerated

H2O (10 mL) at pH 8.8 and 333 K.


cycle in Scheme 1). The hydride complex 6 (lmax ¼ 340 nm) can

be generated upon bubbling an aqueous solution of the aqua

complex 2 with H2 (Fig. 5a and b). Even under strongly basic

conditions (pD 13.4), formation of the hydride complex 6 was

confirmed by the 1H NMR spectrum by flowing H2 at atmo-

spheric pressure into a deaerated aqueous solution of 3 in D2O

for 5 h which showed a typical hydride peak in negative region at

d ¼ �14.33 ppm (see Experimental section). A diluted aqueous

solution of the solution containing 6 (pH 13.7) shows a similar

spectrum with the same lmax (¼340 nm) as shown in Fig. 5c. In

the presence of HCO3�, the hydride complex 6 partially reacts

with HCO3� to form a formate complex 7 (lmax ¼ 430 nm) under

an atmospheric pressure of H2. The formation of the formate

complex 7 observed in Fig. 5a was also confirmed by 13C NMR

(see Experimental section) to give rise to a singlet signal at 173.89

ppm42,66 in the reaction of 13C-enriched sodium bicarbonate

(NaH13CO3,13C 99%) with the catalyst 2 under an atmospheric

pressure of H2 at pD 9.0 at room temperature. After the H2 gas in

the headspace was removed and the solution was kept for 1 h, the

remaining 6 may react with HCO3� to be converted to the

formate complex 7 (Fig. 5a). Without HCO3�, the spectra of

the hydride complex 6 remain unchanged (Fig. 5b). Thus, the

Fig. 5 (a) UV-vis spectral changes of 2 (0.16 mM, black line) in the

presence of KHCO3 (2.0 M) in deaerated H2O at 298 K and pH 8.8 (1 cm

light-path length). The blue line was recorded under an atmospheric

pressure of H2 for 5 min. The red line exhibits the spectrum when the

solution was kept for 1 h after removal of H2 gas in the headspace by

introducing Ar gas. (b) UV-vis spectral changes of 2 (0.16 mM) in

a phosphate buffer solution (50 mM) at 298 K and pH 8.0 (black line).

The reaction of 2 with an atmospheric pressure of H2 for 5 min generates

the hydride complex 6 (blue line). The final spectrum remains unchanged

for 1 hour. (c) UV-vis spectrum of the diluted aqueous solution con-

taining the hydride complex 6 (0.11 mM) which was initially detected by1H NMR at 298 K (pH 13.7). (d) UV-vis spectral changes of 2 (0.16 mM,

black line) by addition of HCOOK (25 mM) in a phosphate buffer

solution (50 mM) at 298 K and pH 8.0 (1 cm light-path length) after 10 s

(red line) and 10 min (blue line).



Scheme 1 Catalytic cycles of interconversion between hydrogen and formic acid.

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formation rate of the hydride complex 6 may be faster than the

formation rate of the formate complex 7 in water at pH 8.8 and

298 K. This result is in good agreement with the linear plot of

TOF versus [HCO3�].

On the other hand, the formation rate of the hydride complex

6 is relatively slow under basic conditions because it took 5 h to

convert 3 into 6 by flowing H2 at atmospheric pressure into the

D2O solution at 298 K. These results are consistent with pH-

dependence of the formation rate (TOF) of formate (Fig. 3),

which shows lower catalytic efficiency under strongly basic

conditions.

Fig. 6 pH-dependence of the H2 evolution rate (TOF) in the catalytic

hydrogen generation from formic acid and formate ([HCOOH] +

[HCOOK]¼ 3.3 M) catalysed by 1 (0.20 mM) in deaerated H2O at 298 K

(black line). TOF values were determined based on the progress of the

reaction for initial 10 minutes. Blue, red and green lines show the amount

ratios of complex 1, complex 2 and complex 3, respectively, to the total

amount of these complexes.

Fig. 7 (a) Plot of TOF versus concentration of HCOOH and HCOOK

mixture (HCOOH/HCOOK), i.e., [HCOOH] + [HCOOK] in the

decomposition reaction of HCOOH/HCOOK catalysed by 1 (0.20 mM)

in deaerated H2O at pH 2.8 and 298 K. (b) Plot of H2 evolution rate (r)

versus concentration of 1 in the decomposition of HCOOH/HCOOK

([HCOOH] + [HCOOK]¼ 3.3 M) catalysed by 1 in deaerated H2O at pH

2.8 and 298 K.

2.2. Catalytic hydrogen evolution from formate

In contrast to the catalytic formation of formate by the fixation

of CO2 by H2 with 2 in slightly basic water (Fig. 3), the reverse

reaction, i.e., the catalytic decomposition of HCOOH, occurs

with 1 in acidic water to produce H2 and CO2 in 1 : 1 molar ratio,

which was detected by GC (Fig. S6a in the ESI†). It was also

confirmed that no CO was produced during the reaction

(Fig. S6b in the ESI†). The time course of TON at 298 K was

examined at different pH in a formic acid/potassium formate

(HCOOH/HCOOK) mixed water solution with the sum of

concentrations of HCOOH and HCOOK ([HCOOH] +

[HCOOK]) kept constant at 3.3 M. The pH dependence of TOF

is shown in Fig. 6 in which the maximum TOF value 1880 h�1 is

obtained at pH 2.8 and 298 K. The black line in Fig. 6, which

represents an increase in TOF with a decrease in pH in the region

between 2.8 and 9.0, well overlaps with the curve of the ratio of 1

to 2 (blue line in Fig. 6). Further decrease in pH less than 2.8 may

result in decomposition of the complex 1. This indicates that the

complex 1 exhibits significantly higher catalytic reactivity than

the benzoate complex 2. Indeed, no hydrogen was evolved at pH

9.0 where the complex 1 is completely converted to complexes 2

and 3 at 298 K. The saturation behaviour of TOF with varying

concentrations of [HCOOH] + [HCOOK] at pH 2.8 indicates

that hydrogen is produced via the formate complex 4 (Fig. 7a).59

At lower pH, formation of a hydride complex 5 via b-hydrogen

elimination from 4 may be the rate-determining step in the

Energy Environ. Sci. This journal is ª The Royal Society of Chemistry 2012


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catalytic cycle in Scheme 1 (left-hand catalytic cycle) because of

the relatively high concentration of protons. The rate-deter-

mining b-hydrogen elimination of the formate complex 4 to form

the hydride complex 5was independently confirmed by observing

the kinetic deuterium isotope effect (KIE) for the catalytic

hydrogen evolution from formic acid-d (DCOOH). From the

slope of the time course of TON for HCOOH as compared with

that for DCOOH (Fig. S7 in the ESI†), KIE turned out to be 4.0

at pH 2.8 and 298 K. The value (4.0) is consistent with the results

reported previously for the catalytic decomposition of formic

acid in acidic water at room temperature.59 Even under slightly

basic conditions (pH 8.0) formate reacts with complex 2 to form

the hydride complex 6 via the formate complex 7 as detected by

UV-vis absorption spectral changes (Fig. 5d) which are in good

agreement with those independently observed during hydroge-

nation of bicarbonate with formate in the presence of 2 (Fig. 5a).

Under highly basic conditions, the hydride complex 6, which was

formed in the reaction of 2 with HCOOH, cannot react with

a proton to produce H2 because of less concentration of protons,

so that the hydride complex 6 can be identified spectroscopically

(see Experimental section). The TOF of the catalytic decompo-

sition of formate with 1 linearly increased with increasing

concentration of the catalyst 1 (Fig. 7b). This result indicates that

catalyst 1 is involved in the rate-determining step of the catalytic

hydrogenation reaction. The temperature dependence of TOF of

the catalytic decomposition of formate with 1 at pH 2.8 was also

examined, and the Arrhenius plot (Fig. S8 in the ESI†) afforded

the activation energy to be 18.9 kcal mol�1, which is much smaller

than the activation energy of the decomposition of formic acid

without catalysts (78 kcal mol�1).65 Thus, the catalyst 1

remarkably lowers the activation energy of the formate

decomposition.

2.3. Catalytic interconversion between hydrogen and formate

The catalytic mechanism of the interconversion between H2 and

HCOOH is summarized in Scheme 1. First, the reaction of 2with

H2 affords the hydride complex 6 in slightly basic water (right-

hand catalytic cycle in Scheme 1). The formation of the hydride

complex 6 was confirmed by the 1H NMR spectrum (see

Experimental section), UV-vis absorption spectra (Fig. 5) and

ESI mass spectrum (Fig. S9 in the ESI†). Second, the reaction of

6 with HCO3� affords the formate complex 7, which may be the

rate-determining step because TOF increased linearly with

increasing concentration of HCO3� (vide supra). The formate

complex 7 might be converted to regenerate the corresponding

aqua complex 2 by releasing HCOO� in competition with the

back reaction to form the hydride complex 6 and CO2.

Under acidic conditions (e.g., pH 2.8), 2 is converted to 1 and

the hydride complex 5 reacts with H3O+ to produce H2, accom-

panied by regeneration of 1 (left-hand catalytic cycle in Scheme

1). Formation of the hydride complex 5 was confirmed by the 1H

NMR spectrum of the isolated hydride complex in DMSO-d6,

which showed a typical hydride peak at d ¼ �14.74 ppm

(Fig. S10 in the ESI†). Because the iridium hydride complex 5 is

a neutrally charged complex, the solubility of 5 in water is too

low to be detected by 1HNMR in D2O. Thus, the direction of the

reaction is reversed under acidic conditions. The reaction of

cationic 1 with HCOO� is favoured as compared with neutral 2


to afford the formate complex 4. Second, b-hydrogen elimination

from 4 affords the hydride complex 5. Finally, the reaction of the

hydride complex 5 with a proton yields H2.

3. Conclusions

In summary, a phenylpyrazolyl organoiridium complex 1

exhibited high catalytic reactivity for hydrogenation of bicar-

bonate in slightly basic water at ambient temperature and pres-

sure, whereas the reverse reaction, that is, the decomposition of

formic acid to H2 and CO2, was also catalysed by 1 in acidic

water. Furthermore, complex 1 acts as the most effective catalyst

for both reactions under atmospheric pressure at room temper-

ature. The catalytic interconversion between hydrogen and for-

mic acid in this study provides a convenient hydrogen-on-

demand system in which hydrogen (gas) can be stored as formic

acid (liquid) and whenever needed hydrogen is produced by the

catalytic decomposition of formic acid.

4. Experimental section

4.1. General

All experiments were carried out under an Ar or N2 atmosphere

by using standard Schlenk techniques unless otherwise noted.

Purification of water (18.2MU cm) was performed with aMilli-Q

system (Millipore; Direct-Q 3 UV). The 1HNMR spectra and 13C

NMR spectra were recorded on a JEOL JNM-AL300 spec-

trometer and a Varian UNITY INOVA600. The UV-vis

absorption spectra were observed using a Hewlett Packard 8453

diode array spectrophotometer with a quartz cuvette (light-path

length ¼ 1 cm) at 298 K. Infrared spectra of solid samples were

recorded on a Thermo Nicolet NEXUS 870 FT-IR instrument

with 0.12 cm�1 resolution at ambient temperature. The electro-

spray ionization mass spectrometry (ESI-MS) data were

obtained by an API 150EX quadrupole mass spectrometer (PE-

Sciex), equipped with an ion spray interface. The sprayer was

held at a potential of +5.0 kV or �4.4 kV for positive or negative

ion detection modes, respectively, and compressed N2 was

employed to assist liquid nebulization. The orifice potential was

maintained at +30.0 V or�40.0 V for positive or negative modes,

respectively. The pH values were determined by a pH meter

(TOA, HM-20J) equipped with a pH combination electrode

(TOA, GST-5725C). The pH of the solution was adjusted by

using 1.00 M H2SO4/H2O and 1.00–10.0 M NaOH/H2O without

buffer unless otherwise noted. The pD of the solution was

adjusted by using 40 wt% NaOD without buffer. Values of pD

were corrected by adding 0.4 to the observed values (pD ¼ pH

meter reading + 0.4).67

4.2. Chemicals and reagents

Commercially available reagents: hydrogen hexachloroiridate,

H2IrCl6 (4N grade, Furuya Metal Co., Ltd.), 1,2,3,4,5-penta-

methylcyclopentadiene (>90%, Kanto Chemical Co., Inc.), 4-

(1H-pyrazol-1-yl)benzoic acid (90% Aldrich Chemicals Co.),

3-(trimethylsilyl)propionic-2,20,3,30-d4 acid sodium salt (>98%,

Aldrich Chemicals Co.), MeCN, potassium hydrogen carbonate,

potassium carbonate, potassium formate, potassium dihy-

drogenphosphate, potassium hydroxide, formic acid, sodium



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hydroxide, diluted sulfuric acid (0.5 M) (Wako Pure Chemical

Industries), KBr (Nacalai Tesque Inc.), sodium hydroxide-d in

D2O (40 wt% NaOD, 99.5% D; Aldrich Chemical Co.), 13C-

enriched NaHCO3 (99%13C, Cambridge Isotope Laboratories),

formic acid-d (DCOOH, >99.5%, 98% D, Cambridge Isotope

Laboratories), D2O (99.9% D; Cambridge Isotope Laborato-

ries), dimethylsulfoxide-d6 (99.9% D; Cambridge Isotope Labo-

ratories), H2 (99.99%; Japan Air Gases Co.), D2 (99.5%;

Sumitomo Seika Chemicals Co., Ltd.), CO2 (99.99%; Ekika

Carbon Dioxide Co. Ltd.), and standard gas (H2 1.07%, CO2

1.07%, CO 1.06%, and N2 96.8%; GL Sciences Co., Ltd.) were of

the best available purity and used without further purification

unless otherwise noted. A phosphate buffer solution was

prepared by mixing a 0.10 M potassium dihydrogenphosphate

solution, a 0.10 M sodium hydroxide solution and H2O in

appropriate ratio. [IrIII(Cp*)(H2O)3]SO4 was synthesized

according to the reported procedure.66

4.3. Synthesis

[IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-kC3)(H2O)]2SO4

{[1]2$SO4}. [IrIII(Cp*)(H2O)3]SO4 (0.20 g, 0.423 mmol) and 4-

(1H-pyrazol-1-yl)benzoic acid (0.085 g, 0.454 mmol) in H2O

(50 mL) were stirred under reflux for 12 h, and then the solution

was filtered with a membrane filter (Toyo Roshi Kaisha, Ltd.,

H100A025A, pore diameter, 1 mm). The solvent of the filtrate

was evaporated under reduced pressure to yield a yellow

powder of [IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-

kC3)(H2O)]2SO4, which was dried in vacuo {yield: 94% based on

[IrIII(Cp*)(H2O)3]SO4}.1H NMR (600 MHz, in D2O, reference

to TSP in D2O, 298 K, pD 2.0): d (ppm) 1.70 (s, h5-C5(CH3)5,

15H), 6.80 (dd, J ¼ 1.8 Hz, J ¼ 3.0 Hz, 1H), 7.54 (d, J ¼ 8.4 Hz,

1H), 7.80 (dd, J ¼ 8.4 Hz, J ¼ 1.8 Hz, 1H), 8.15 (d, J ¼ 1.8 Hz,

1H), 8.35 (d, J¼ 3.0 Hz, 1H), 8.57 (d, J¼ 1.8 Hz, 1H). 13C NMR

(600 MHz, in D2O, reference to TSP in D2O, 298 K, pD 2.0):

d (ppm) 173.49, 150.86, 147.92, 144.24, 140.69, 131.26, 130.33,

130.03, 114.19, 112.75, 91.17, 11.10. Anal. calcd For

[IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-kC3)(H2O)](H-

SO4)(H2SO4)0.3: C20S1.3H25.6O8.20IrN2: C, 36.44%; H, 3.91%; N,

4.25%. Found: C, 36.46%; H, 3.74%; N, 4.24%. FTIR (KBr,

cm�1): 1683 n(C]O) (Fig. S11 in the ESI†). ESI-MS in MeOH:

m/z 515 [1 � H2O]+.

IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoate-kC3)(H2O) {2}.

Addition of KHCO3 (530 mg, 5.3 mmol) to an aqueous solution

(12.5 mL) of [1]2$SO4 (37.9 mg, 33 mmol) yields a yellow powder

precipitate of IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoate-

kC3)(H2O) because of its neutral charge {isolated yield: 65%,

43 mmol based on [1]2$SO4}1H NMR (300 MHz, in DMSO-d6,

298 K): d (ppm) 1.75 (s, h5–C5(CH3)5, 15H), 6.99 (dd, J¼ 2.2 Hz,

J ¼ 2.7 Hz, 1H), 7.84 (dd, J ¼ 8.4 Hz, J ¼ 1.5 Hz, 1H), 7.91 (d,

J ¼ 8.4 Hz, 1H), 8.19 (d, J ¼ 1.5 Hz, 1H), 8.33 (d, J ¼ 2.2 Hz,

1H), 9.03 (d, J¼ 2.7 Hz, 1H). 13C NMR (400MHz, in DMSO-d6,

298 K): d (ppm) 167.50, 145.73, 144.09, 137.59, 135.36, 131.10,

129.76, 127.58, 112.96, 111.37, 96.56, 9.06. Anal. calcd for

IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoate-kC3)(H2O): C20H23-

O3IrN2: C, 45.18%; H, 4.36%; N, 5.27%. Found: C, 44.90%; H,

4.30%; N, 5.11%. FTIR (KBr, cm�1): 1602 n(COO) (Fig. S12 in

the ESI†).


[IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoate-kC3)(OH)]Na

{3$Na}.Under strongly basic conditions, 1 can release one proton

from the carboxyl group and the other proton from the aqua

ligand to form a hydroxo complex 3. pD was adjusted by adding

NaOD aqueous (D2O) solution. 1H NMR (600 MHz, in D2O,

reference to TSP in D2O, 298 K, pD 14.0): d (ppm) 1.68 (s, h5-

C5(CH3)5, 15H), 6.71 (dd, J¼ 1.8Hz, J¼ 3.0Hz, 1H), 7.50 (d, J¼8.4 Hz, 1H), 7.65 (dd, J¼ 8.4 Hz, J¼ 1.8 Hz, 1H), 7.94 (d, J¼ 1.8

Hz, 1H), 8.26 (d, J ¼ 3.0 Hz, 1H), 8.40 (d, J ¼ 1.8 Hz, 1H). 13C

NMR (600 MHz, in D2O, reference to TSP in D2O, 298 K, pD

14.0): d (ppm) 178.71, 149.36, 148.79, 142.08, 140.04, 136.62,

129.78, 127.73, 113.62, 111.73, 89.42, 10.92. ESI-MS in NaOH

aqueous solution (1.5 mM) and MeCN [1 : 1 (v/v)]: m/z 531 [3]�.

4.4. Catalytic hydrogenation of bicarbonate under atmospheric

pressure at 303 K and 333 K

An aqueous K2CO3 (0.10 M) solution (20 mL) of [1]2$SO4 (0.13

mM) was saturated with CO2 at ambient pressure by bubbling

CO2 for 1 hour in the presence of 3-(trimethylsilyl)propionic-

2,20,3,30-d4 acid sodium salt (TSP, 10 mM). No reaction takes

place between TSP and the iridium catalyst, as the catalytic

reactivity remains unchanged irrespective of the presence or

absence of TSP in the reaction solution. Then the solution was

vigorously stirred under bubbling with H2 (50 mL min�1) and

CO2 (50 mL min�1) at 303 K or 333 K. 0.5 mL of the reaction

solution was injected by a syringe with a needle, and pH values

and yield were measured at one time. The yield of formate was

determined by 1H NMR measurements of the product solution

with TSP as an internal standard using a sealed capillary tube

(i.d. ¼ 1.5 mm) filled with D2O for deuterium lock. TOF and

TON for 303 K were determined from the yield of formate to be

6.8 h�1 and 100 (15 h), respectively.

4.5. Catalytic hydrogenation of bicarbonate under H2

atmosphere

Typically, an aqueous solution (1.0 mL) of [1]2$SO4 (1.1 mg,

0.90 mmol) was added to 9.0 mL of a KHCO3/K2CO3 aqueous

solution ([KHCO3] + [K2CO3] ¼ 2.2 M) in the presence of TSP

(17 mg, 0.10 mmol). The solution was vigorously stirred under

the hydrogen atmosphere at the desired temperature. For the

measurements of the dependence of TOF on the concentration of

an aqueous KHCO3/K2CO3 solution, [KHCO3] + [K2CO3] was

changed from 0.75 mM to 2.0 M. For the measurements of the

formation rate of formate depending on [2], the concentration of

2 was changed from 0.13 mM to 0.34 mM. The formation rate of

formate was determined at various temperatures ranging from

303 K to 333 K. 0.5 mL of the reaction solution was injected by

a syringe with a needle, and pH values and yield were measured

at one time. The yield of formate was determined by 1H-NMR

measurements of the product solution with TSP as an internal

standard using a sealed capillary tube (i.d. ¼ 1.5 mm) filled with

D2O for deuterium lock.

4.6. Synthesis of 6 from 3 under an atmospheric pressure of

hydrogen

By flowing H2 at atmospheric pressure (50 mL min�1) into

a deaerated aqueous solution of 3 (0.11 M) at pD 13.4 and 298



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K for 5 h, the formation of the hydride complex 6 was

confirmed by 1H NMR and 13C NMR. pD was adjusted by

adding NaOD aqueous solution. The mixture was permitted to

stand at 298 K for 12 h. 1H NMR (600 MHz, in D2O, reference

to TSP in D2O, 298 K, pD 13.4): d (ppm) �14.33 (s, 1H), 1.91

(s, h5-C5(CH3)5, 15H), 6.40 (dd, J ¼ 2.4 Hz, J ¼ 2.4 Hz, 1H),

7.41 (d, J ¼ 8.4 Hz, 1H), 7.57 (dd, J ¼ 8.4 Hz, J ¼ 1.8 Hz, 1H),

7.77 (d, J ¼ 2.4 Hz, 1H), 8.22 (d, J ¼ 1.8 Hz, 1H), 8.25 (d, J ¼2.4 Hz, 1H). 13C NMR (600 MHz, in D2O, reference to TSP in

D2O, 298 K, pD 13.4): d (ppm) 178.98, 147.56, 143.78, 142.00,

140.39, 135.81, 129.06, 126.01, 113.13, 111.01, 92.99, 11.68. 1H

NMR spectroscopic analysis indicated >98% conversion to 6

by using TSP as an internal integration standard. ESI-MS in

NaOH aqueous solution (1.5 mM) and MeCN [1 : 1 (v/v)]: m/z

515 [6]� (Fig. S10 in the ESI†).

4.7. Detection of the formate complex 7

An aqueous 99% 13C-enriched KHCO3 (1.0 M) solution (0.5 mL)

of [1]2$SO4 (0.75 mM) was saturated with H2 under hydrogen

atmosphere at room temperature and kept for 2 hours. 13C NMR

spectra were recorded at room temperature before and after

reaction.

4.8. Detection of the hydride complex 5

By flowing H2 at atmospheric pressure (50 mL min�1) into

a deaerated aqueous solution of [1]2$SO4 (0.28 M) for 5 min, the

hydride complex 5 was obtained as an orange precipitate because

of the neutral charge. After centrifugation and decantation, the

precipitate was dried under reduced pressure to remove water. 1H

NMR of complex 5 in deaerated DMSO-d6 is shown in Fig. S11

in the ESI†. Complex 5 as well as 6 were too unstable to observe

their UV-vis spectral changes for the determination of pKa values

by titration with acids.

4.9. Catalytic hydrogen evolution

Typically, 0.50 mL of an aqueous solution of [1]2$SO4 (0.30 mM)

was added to 1.0 mL of a HCOOH/HCOOK aqueous solution

([HCOOH] + [HCOOK] ¼ 5.0 M) at pH 2.8–9.0 and 298 K. The

amount of evolved hydrogen gas was determined by measuring

the volume of the evolved gas collected in a 5.0 mL measuring

cylinder by water replacement. The content of the evolved gas

was analysed by a gas chromatograph. In order to remove CO2

from the evolved gas, a 5.0 M NaOH solution was used as a CO2

trap. H2 and CO2 gases were analysed by a Shimadzu GC-14B

gas chromatograph {N2 carrier, active carbon with a particle size

of 60–80 mesh at 353 K} equipped with a thermal conductivity

detector. No CO gas was detected by a Shimadzu GC-17A gas

chromatograph {Ar carrier, a column with molecular sieves

(Agilent Technologies, 19095P-MS0) at 313 K} equipped with

a thermal conductivity detector. TOF values were determined by

measuring the amounts of H2 and CO2 for initial 10 minutes. For

the measurements of the dependence of TOF on the concentra-

tion of an aqueous HCOOH/HCOOK solution, [HCOOH] +

[HCOOK] was changed from 0.35 M to 5.3 M. For the

measurements of the evolution rate of hydrogen depending on

[1], the concentration of 1 was changed from 0.10 mM to


0.40 mM. The rate of hydrogen evolution was determined at

various temperatures from 288 K to 303 K.

4.10. Catalytic hydrogen evolution from DCOOH

An aqueous solution (0.50 mL) of [1]2$SO4 (0.30 mM) was added

to 1.0 mL of an aqueous DCOOH/DCOOK solution ([DCOOH]

+ [DCOOK] ¼ 5.0 M in H2O) at pH 2.8 and 298 K. The amount

of evolved hydrogen gas was measured by measuring the volume

of the evolved gas collected in a 5.0 mL measuring cylinder by

water replacement. pH was adjusted by using 4.0 M KOH

aqueous solution. The content of the evolved gas was analysed by

a gas chromatograph.

Acknowledgements

This work was partially supported by a Grant-in-Aid (no.

20108010 to S.F. and no. 21550061 to T.S.) and a Global COE

program, ‘‘the Global Education and Research Centre for Bio-

Environmental Chemistry’’ (to S.F.) from the MEXT, Japan,

and NRF/MEST of Korea throughWCU (R31-2008-000-10010-

0) and GRL (2010-00353) Programs (to S.F.).

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Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure

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Transcript of Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure