Hydrogen generation from formic acid decomposition on a highly efficient iridium catalyst bearing a diaminoglyoxime ligand

Article abstract of DOI:10.1039/c8gc00495a

A new iridium catalyst bearing a dioxime derived ligand has been developed for aqueous formic acid (FA) dehydrogenation. This catalyst features high stability and high efficiency for dehydrogenation of FA in aqueous solution without any additives. With th

Full text of DOI:10.1039/c8gc00495a

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Hydrogen generation from formic acid decomposition on a highly
efficient iridium catalyst bearing diaminoglyoxime ligand
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Sheng-Mei Lua, Zhijun Wanga, Jijie Wanga, Jun Lia, Can Lia*
www.rsc.org/
A new iridium catalyst bearing dioxime derived ligand has been developed for aqueous formic acid (FA) dehydrogenation.
This catalyst features the high stability and high efficiency for dehydrogenation of FA in aqueous solution without any
additives. With the in situ formed catalysts, TONs of up to 3,900,000 (average rate: 65000 h-1) at 90 oC was achieved. At 70
oC, an even higher TON of 5,020,000 was obtained, which are the highest TON ever reported. More interestingly, this
catalyst can also give a TON of 400,000 and an average rate of 1053 h-1 even at room temperature. Electron-rich amine
substitutes and the dioxime structure of the ligand are beneficial to the high stability and high efficiency of the catalyst.
achieved a TON of 2,050,000 by using imidazoline derived
o
iridium catalyst at 60 C, but as long as 580 hours of reaction
time was needed.8k Later, they used an iridium complex of
Introduction
Formic acid (FA), one of the promising hydrogen storage
materials,1 has attracted a considerable attention because of
its stability, transportability and biodegradability under normal
conditions. From a view of practical application, an ideal
catalyst system for large amount of hydrogen generation from
FA decomposition should be easily available and can work with
high efficiency, high selectivity and long-term stability in any
concentration of aqueous FA solution in the absence of any
additives. For this purpose, significant efforts have been
devoted toward developing new catalysts for the production
of hydrogen from FA decomposition.2
pyridine-imidazoline N,N-ligand for the reaction and shortened
the reaction time to 363 hours for a TON of 2,000,000 at 50
oC.8l More recently, Kawanami group reported an extremely
high TON of up to 5,000,000 in water at 60 oC by using an
iridium complex containing 1,10-phenanthroline-4,7-diol as a
chelating ligand for FA dehydrogenation.8j However, 2600
hours were needed to complete the decomposition. In
addition, Williams group decomposed pure FA in the presence
of sodium formate with an iridium catalyst bearing a N,P-
ligand, achieving a TON of 2,160,000 at 87 oC by repeatedly
charging FA into the reaction system.9 Although the highly
desirable reaction of decomposing pure FA is feasible here, it
takes over four months to obtain this TON.
In homogeneous catalysis, complexes of non-noble metals
of iron,3 aluminum4 and nickel5 have been developed for FA
dehydrogenation. Though the metals are cheap and abundant,
most of these reactions were carried out in organic solvents.
Some complexes of rhodium6 or ruthenium7 have also been
applied for FA dehydrogenation but often in the presence of
bases. Compared with other metal catalysts, iridium
complexes are capable of dehydrogenating FA to hydrogen in
water without any additives, which makes them more
applicable. In recent years, a series of iridium complexes
containing pyrimidine, imidazoline, pyridine and other type of
N,N-ligands have been found highly effective for upscaled
hydrogen generation from FA with high selectivity.8 For
example, Himeda, Fujita, Muckerman and their coworkers
In the absence of any additives, several iridium catalysts
have shown to be quite stable for long-term continuous
upscaled hydrogen generation from FA. However, the
efficiencies of these catalysts are needed to be improved
because most of them provided an average rate of no higher
than 6000 h-1 at a TON of > 2,000,000 (ESI, Scheme 1), which is
insufficient for the applications of fast and large amount of
hydrogen supply in situ. Raising the reaction temperature is
one of the most effective methods to enhance the rate of FA
dehydrogenation, but the prerequisite is the catalysts must be
stable at high temperature. Thus, developing a catalyst to
make it workable under high temperature with high catalytic
activity and long-term stability is challenging and desirable. On
the other hand, most of the reported catalysts were used
under protection and the reactions were performed in
deaerated FA solution, which brings inconvenience for the
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practical manipulation. Recently, we reported a stable and type of dioxime ligands with the diimine structure being
efficient iridium catalyst generated in situ from 2,2'- outside of the cycles were prepared and applied for FA
tetrahydrobipyrimidine and [IrCp*Cl2]2 (Ir/L = 1/5), which can dehydrogenation. The similar reaction procedure as our
catalyze the FA dehydrogenation in water without any previous reported work was followed where no protection and
additives.8m Through repeatedly charging FA into the reaction degas were necessary during the manipulation. The catalysts
system, a TON of 2,400,000 was obtained at 80 oC (average were prepared in situ from [IrCp*Cl2]2 and L1-8 in water and
rate: 171,000h-1). As part of our continuing work and to their catalytic performance were investigated in aqueous FA
develop more practical catalyst systems, here we present solution (1.0 M, 10.0 mL) at 60 oC. As shown in Table 1, most of
another new type of efficient catalyst systems, which are the in situ prepared catalysts were active and the
highly active for FA dehydrogenation in one batch. diaminoglyoximes derived catalysts have higher activity. Using
Furthermore, all the manipulations can be conducted without [IrCp*Cl2]2 as the metal precursor and L1 as the ligand, the
any protection and degas.
formed catalyst gave too low initial reaction rate that no
measurable amount of gas can be collected in the first 3
minutes (Table 1, entry 1). When L2 with two chloride
substitutes on the glyoxime structure was used as the ligand,
no activity can be found (Table 1, entry 2). However, when L3
with two methyl groups on the glyoxime structure was
applied, the reaction gave a TOF of 3125 h-1 (Table 1, entry 3),
which indicated that the substitutes on the glyoxime are
important for the reactivity and electron donating groups are
beneficial for the reaction. This is further demonstrated by the
catalytic performance of using L4 as the ligand which bears
two amine groups on the glyoxime structure. Due to the
electron-rich properties of amine group, a much higher TOF of
23750 h-1 was obtained (Table 1, entry 4). However, L5 with
two iso-propyl amine groups gave a lower TOF (Table 1, entry
5), which is possibly due to the difficult coordinating of iridium
center to L5 caused by the bulky iso-propyl group. Other
catalysts derived from substituted diaminoglyoximes ligands
L6-8 all gave the similar initial TOF of over 26000 h-1 (Table 1,
entries 6-8) and the catalyst from L6 gave the highest initial
TOF of 28750 h-1 (Table 1, entry 6).
Results and Discussion
Table 1 Catalysts screening for FA dehydrogenationa
TOF (h-1)
625b
0c
3125b
23750
5625
TON (20 min)
208
H2 (mL)
5.5
Entry Cat.
1
2
3
4
5
6
7
8
9
10
Ir/L1
Ir/L2
Ir/L3
Ir/L4
Ir/L5
Ir/L6
Ir/L7
Ir/L8
Ir-L7
Ir-L8
The iridium complexes Ir-L7 and Ir-L8 were prepared and
/
/
obtained in good yield from [IrCp*Cl2]2 and L7/L8, respectively.
25
1042
6542
1583
7750
7417
7125
8333
8167
The complexes are air-stable and water-soluble. The crystal
structures of Ir-L8 were determined by X-ray diffraction and a
distorted octahedral coordination geometry was formed (see
ESI). Under the identical conditions, the performance of
complexes Ir-L7 and Ir-L8 were evaluated and slightly higher
TOF than that of the in situ formed corresponding catalysts
were obtained (Table 1, entries 9-10 vs 7-8). To further
understand the decomposition behavior of the reaction with
these catalysts, other reaction conditions were investigated
using complex Ir-L7 as the catalyst.
157
38
186
178
171
200
196
28750
27500
26250
33750
31250
a: General reaction conditions: [IrCp*Cl2]2 (0.5 μmol), Ir/L = 1/1.2, FA (1.0 M, 10.0
mL), 60 oC, 20 minutes; TOF was calculated based on the conversion in the first 3
minutes unless otherwise noted; b: The reaction rate was very slow and TOF was
The pH-dependence of dehydrogenation with Ir-L7 was
firstly examined in FA and sodium formate (SF) mixture
solution with a total FA-SF concentration fixed at 1.0 M. As
shown in Fig. S1 and Table S1 in electronic supporting
information (ESI), in the pH range between 1.78 and 8.01, the
maximum initial TOF of 33750 h-1 was found in pure FA
solution (pH = 1.75). It is different from the previous reported
2,2’-biimidizoline derived catalyst, whose maximum activity
was achieved in FA-SF solution with a pH value of 2.8. To check
the effect of more acidic FA solution on the reaction, we
carried out the reaction in the mixture of methanesulfonic acid
and FA solution (pH value from 0.71 to 1.59). Nevertheless, no
more even higher TOF than 33750 h-1 was found, which
c:
calculated on the conversion in 20 minutes; No readable gas volume was
collected in 20 minutes.
In our previous work, iridium catalysts bearing 2,2’-
biimidazoline and 2,2’-bi-tetrahydropyrimidine diimine ligands
are proved to be effective for aqueous FA dehydrogenation8m
and CO2 hydrogenation reactions,10 but the complexes are not
stable when the reactions were carried out at high
temperature.11 The reason is maybe due to the easily
hydrolyzing of imine motif in such a cyclic structure under the
reaction conditions.12 To avoid such a problem, a different
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demonstrates the importance of the concentration of both hours. While for L7 and L8, finishing the FA decomposition
hydronium ion (H3O+) and HCO2- species in this reaction.
needed a much shorter time of 13.5 hours (ESI, Table S5,
The temperature dependence of the dehydrogenation entries 4, 7, 10). The complexes of Ir-L7 and Ir-L8 have the
reaction with Ir-L7 was examined in FA solution and the initial similar activities as the corresponding in situ formed catalysts,
reaction rate increased with the temperature increasing and a finishing the decomposition in 15 and 16 hours (ESI, Table S5,
TOF of more than 210,000 h-1 was obtained at 90 oC (ESI, Table entries 13 and 16 ), respectively.
S2). The apparent activation energy (Ea) was calculated to be
66.9 kJ mol-1 (ESI, Fig. S2), which is slightly lower than the mL), the reaction proceeded to the end by using either in situ
reported value obtained with 2,2’-biimidizoline derived formed catalysts from ligands of L6 L7 and L8 (ESI, Table S5,
When FA amount was increased to 2.2 mole (10.0 M, 220
,
catalyst (72.1 kJ mol-1). The rate dependence on entries 5, 8, 11) or the complexes of Ir-L7 and Ir-L8 (ESI, Table
concentrations of complex Ir-L7 and FA were examined. The S5, entries 14, 17). With this FA amount, the catalyst using L8
double logarithmic plots of the initial rate against the as the ligand completed the decomposition in 17.5 hours,
concentration of complex Ir-L7 in the range of 25 to 200 μM giving an average reaction rate of 126,000 h-1, higher than that
indicate a linear dependence (ESI, Fig. S3 and Table S3) and of the other four catalysts which performed with the similar
show a similar behavior as our previous reported results of activity.
2,2’-biimidizoline derived catalyst. The initial TOF dependence
After comparing the results of in situ formed catalyst of L8
on FA concentration from 0.5 to 20.0 M indicated that at 60 oC with that of the complex Ir-L8 for the decomposition of 0.9, 1.8
and with 1.0 μmol of Ir-L7 in 10.0 mL of FA solution, the and 2.2 moles of FA (ESI, Fig. S4), we found that the former
reaction rate increased firstly with the increasing of FA one always has a slightly higher efficiency than the latter one.
concentration then decreased when it further increased, For the catalyst of L7 and the complex Ir-L7, it shows a same
showing a highest activity at 4.0 M of FA concentration (ESI, trend except for the result obtained from the decomposition
Table S4). Obviously, high concentrated FA solution is of 2.2 mole of FA. It seems that the little excess of ligand (
detrimental for the reaction, so 20.0 M of FA solution could 1.2) in the in situ formed catalyst has some good effects on the
not be decomposed completely. reaction. To verify this, we investigated the effect of ligand
L/Ir =
Then the stabilities and efficiencies of the iridium catalysts amount on the reaction by using the situ formed catalyst. As
were evaluated at high temperature in high concentrated and shown in Figure 1a and Table S6 in ESI, under the other
large amount of FA solution, still without any protection and identical conditions, with the increasing L/Ir ratio, the average
deaeration. Using 1.0 μmol of the iridium catalysts, either the rate of reaction with L6 as the ligands firstly increased and
in situ formed catalysts from L4 and L6-8 or the complexes Ir- then decreased slightly. At the ratio of 4.4/1, it achieved a
L7 and Ir-L8, can all decomposed 0.9 mole of FA smoothly at highest average rate of 176,000 h-1(the reaction finished in
90 oC, producing hydrogen and CO2 without CO contaminated. 12.5 hours). As the L6/Ir ratio increased to 7/1, the efficiency
As shown in Table S5 in ESI, all these catalysts dehydrogenated of the reaction was not enhanced further, which indicates that
FA with high conversion of over 99.9%. With the same FA too more ligand cannot improve the efficiency of the reaction
amount, the time to finish the decomposition is different and further. For the reaction with L8 as the ligand, the similar
the stabilities and efficiencies of these catalyst systems have results were observed. Obviously, the appropriate excess
been revealed initially. The in situ formed catalyst from L4 has amount of ligand plays an important role of possibly stabilizing
the lowest efficiency and over 9 hours of reaction time are the active iridium center thus improving the efficiency of the
needed to finish the reaction (ESI, Table S5, entry 1). The other catalyst.
three of the in situ formed catalysts have the similar activity,
120
10
20
which finished the decomposition reaction in less than 6 hours
(ESI, Table S5, entries 3, 6, 9). The catalyst with L7 as the ligand
has the highest average reaction rate of 171,000 h-1 and the
reaction finished in a much shorter reaction time of 5.25
hours. The complexes Ir-L7 and Ir-L8 have the similar activities
as those of the in situ formed catalysts, finishing the reaction
in 6.0 and 5.8 hours (ESI, Table S5, entries 12 and 15),
respectively.
b
L6
L8
a
18
16
14
12
10
8
6
4
2
0
100
80
60
40
20
0
8
6
4
2
0
0
200 400 600 800 1000
Time (min)
1.2
2
4.4
7
Ratio of L/Ir
To further evaluate these catalysts durability, we increased
the FA amount to 1.8 mole and kept the other reaction
conditions unchanged. The reaction with L4 as ligand was
deactivated slowly after the reaction started and low FA
dehydrogenating conversion was found (ESI, Table S5, entry 2).
Obviously, the catalyst from L4 cannot tolerate the high
Figure 1 (a) The effect of the L/Ir ratio on FA dehydrogenation; (b) The time courses of
the released gases volumes/evolution gas rate; Reaction conditions: FA (10.0 M, 220
mL), 90 oC, in situ formed catalyst from [IrCp*Cl2]2 (0.5 μmol) and L6/L8 for (a); using L6
(L6/Ir = 2) as the ligand for (b).
The time courses of the released gases volumes by using L6
as the ligand (L6/Ir = 2/1) for dehydrogenation of 2.2 mole of
FA were recorded and shown in Fig. 1b. After the reaction
started, the reaction rate increased rapidly from 4.32 L•h-1 for
the first 5 minutes to 7.99 L•h-1 at 80 minutes. In the time
o
temperature of 90 C with large amount of FA. Nevertheless,
under such conditions, other three of the in situ formed
iridium catalysts from L6-8 can all complete the decomposition
smoothly. With L6 as the ligand, the reaction finished in 18
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range of 1.5 to 8.0 hours, the reaction proceeded at a rate of reaction in 220 mL of FA solution (10.0 M) at 70 oC, the
over 8.0 L•h-1. After that, the reaction rate gradually slowed reaction can still proceed to the end with almost full
down until the gas evolution stopped. The final TON value was conversion in 45 hours when L6 was used as the ligand (Table
calculated around 2,200,000 (the residual FA was measured to 3, entry 1). The similar results were obtained when L7 and L8
be 0.115 mmol by ion chromatograph). It means totally around were used as the ligands and the reactions proceeded to the
102 liter of H2 and CO2 gas released during the whole process; end in 46 and 47 hours, respectively (Table 3, entries 2 and 3).
and 3.4 liter of H2 was produced per hour with 1.0 μmol It is noted that the catalyst of L4 was unable to complete 1.8
catalyst loading.
To check the best performance of these catalysts, we smoothly at 70 oC in 93 hours (Table 3, entry 4). Obviously,
further increased the FA amount. At the /Ir ratio of 2/1, when under lower reaction temperature, the catalyst stability is
mole of FA at 90 oC, but it can decompose 2.2 mole FA
L
2.64 mole of FA (12.0 M, 220 mL) was submitted, the reaction higher. As expected, 5.02 mole of FA can be decomposed at 70
with L6 as the ligand could not proceed to the end and the oC with the catalyst from L6 in 175 hours (Table 3, entry 5),
reaction stopped after 28 hours with large amount of FA left. achieving a TON of 5,020,000, the highest values ever reported
However,when we increase the ratio of L6/Ir to 4.4/1, the in (ESI, Scheme S1). With L8 as the ligand, a TON of 4,470,000
situ formed catalyst system can finish the decomposition was obtained.
reaction in 17.3 hours (Table 2, entry 1), giving an average
At room temperature, the efficiency of the catalyst was
reaction rate of 152,000 h-1, which demonstrates the obvious also checked. Using the in situ formed catalyst from [IrCp*Cl2]2
and beneficial effect of excess ligand on the reaction again. and L6, the effect of FA concentration on the activity was
When the FA amount further increased to 2.82 mole (12.0 M, investigated. The initial TOF dependence on FA concentration
235 mL), the reaction still proceeded to the end in 19 hours from 0.5 to 12.0 M indicated that at 25~26 oC and with 1.0
(Table 2, entry 2). When to 3.0 mole (12.0 M, 250 mL), the μmol of catalyst in 10.0 mL of FA solution, the reaction rate
reaction proceeded smoothly and completed after 26 hours increased firstly with the increasing of FA concentration then
(Table 2, entry 3). When L7 was used as the ligand for the decreased when it further increased, showing a highest TOF of
decomposition of 2.82 mole FA, as long as 29 hours was 1417 h-1 at 2.0 M of FA concentration (ESI, Table S7). When
needed to finish the reaction (Table 2, entry 4), showing a large amount of FA (10.0 M, 200 mL) was submitted to
much lower catalytic efficiency than that of L6. Under the decomposition, an average rate of 1053 h-1 was obtained using
same conditions, the catalyst from L8 decomposed 2.82 mole a catalyst loading of 5 μmol. All the FA was decomposed
and 3.0 mole of FA completely in 18.8 and 29.3 hours (Table 2, completely in 380 h, giving a TON of 400,000 (Table 3, entry 7).
entries 5 and 6), showing a similar catalytic efficiency as the
catalyst from L6. From these results, we can conclude that the
catalysts from L6 and L8 are more stable and efficient than the
catalyst from L7 for large amount of FA decomposition. When
4.0 mole of FA was added in the reaction, the catalysts from L6
(Ir/L6 = 1/6) could not finish the decomposition and stopped
after 34 hours (Table 2, entry 7), achieving a TON of around
3,390,000. For L8, a TON of 3,900,000 and an average rate of
65000 h-1 was achieved (Table 2, entry 8).
Table 3 Large amount of FA decomposition with iridium catalysts at 70 oCa
Time
(h)
TON (
×
104 ) Average
104 h-1)
rate
Entry Cat. Ir/L
(×
45
220
220
220
220
502
447
40
4.89
4.78
4.68
2.37
2.87
2.48
0.105
1
2
3
4
5
6
7
Ir/L6
Ir/L7
Ir/L8
Ir/L4
Ir/L6b
Ir/L8c
Ir/L6d
46
47
93
175
180
380
Table 2 Large amount of FA decomposition with iridium catalysts at 90 oCa
Entry Cat.
FA
Time TON
Average
(×
104 h-1)
rate
(mol)
2.64
(h)
(×
104 )
a: General reaction conditions: [IrCp*Cl2]2 ( 0.5 μmol), Ir/L = 1/2.5; FA (10.0 M, 220
mL), 70 oC; b: Ir/L6 = 1/6, FA (10.0 M, 505 mL); c: Ir/L8 = 1/6, FA (10.0 M, 500 mL);
d: 5 μmol catalyst, Ir/L6 = 1/1.2, FA (10.0 M, 200 mL), 25~26 oC.
17.3
19.0
26.0
29.0
18.8
29.3
34.0
60.0
15.2
14.3
11.5
9.72
15.0
10.2
9.97
6.50
1
2
3
4
5
6
7
8
Ir/L6
Ir/L6
Ir/L6
Ir/L7
Ir/L8
Ir/L8
Ir/L6b
Ir/L8b
264
282
300
282
282
300
339
390
2.82
3.00
2.82
2.82
3.00
4.0
Conclusions
In conclusion, we have developed
a new type of
diaminoglyoxime derived iridium catalyst systems for the
highly efficient hydrogen generation from FA decomposition in
water without any additives. These catalysts can be produced
in situ from [IrCp*Cl2]2 and dioxime ligands, and used directly
without the isolation of the complexes. Amine substitutes
make the dioxime ligand more electron-rich, leading to the
catalyst system with high stability. It can work efficiently at
high or low temperatures according to the practical
4.0
a: General reaction conditions: [IrCp*Cl2]2 ( 0.5 μmol), Ir/L = 1/4.4; 90 oC, FA (12.0
M); b: Ir/L = 1/6，FA (10.0 M, 400 mL).
The reaction was also carried out at low temperature, such
as 70 oC, to control the gas release in a relatively low rate and
to satisfy different requirements. When we carried out the
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requirement, giving a TON of up to 3,900,000 at 90 oC (average
water bath until the required temperature was attained. Then,
freshly prepared aqueous catalyst stock solution (0.2 mL, 1.0
µmol) was injected into the reaction through the septa, and
the timing started immediately. The side branch was
connected to the gas collection apparatus (standard water
o
rate: 65000 h-1), 5,020,000 at 70 C (average rate: ~28700 h-1)
and 400,000 at 25~26 oC (average rate: 1053 h-1), respectively.
The easily prepared catalysts and their high efficiency and
long-term stability, free of protection and degas of the
experimental manipulation make them more practical for FA
dehydrogenation.
displacement apparatus, using
a graduated cylinder to
determine volume). The volume of gas generated was
recorded.
General procedure for catalytic dehydrogenation of HCO2H in a
Experimental section
large scale:
Synthesis and characterization of complex Ir-L7 and Ir-L8
All of the solvents and reagents were used directly without
any manipulations unless otherwise specified. The reactions
were carried out without protection.
The aqueous in situ formed catalyst stock solution was
prepared freshly before use. First, aqueous formic acid
solution (10.0 M, 220 mL) and a stir bar were placed in a two
necked flask (250 mL) equipped with rubber septa and reflux
condenser. The upper of the reflux condenser was connected
to the gas collection apparatus (standard water displacement
apparatus, using a graduated cylinder to determine volume).
Afterwards, the flask was preheated at given temperature (e.g.
90 °C) in a water bath for 30-90 minutes until the required
temperature was attained and stable at 90 oC. Then, freshly
prepared aqueous catalyst stock solution (0.2 mL, 1.0 µmol)
was added into the reaction, and the timing started
immediately. The volume of gas generated was recorded if
necessary. The residue FA was measured by SHINEHA CIC-100
ion chromatograph.
Under an argon atmosphere, dry methanol (20 mL) was
added to the mixture of [IrCp*Cl2]2 (0.1 mmol) and L7 (0.2
mmol) in a flask bottle and then stirred at room temperature.
After approximately 5 minutes, the mixture became a yellow
clear solution, and stirring was maintained for another half an
hour. Then, the methanol was removed under reduced
pressure. The residue was purified by flash column
chromatography using CH2Cl2/MeOH (30:1-15:1) as the eluent
to get complex 80.2 mg, Yield: 72%. 1H NMR (400 MHz, D2O): δ
(ppm) = 3.83-3.74 (m, 1H), 3.47 (dd, J = 12.4, 4.0 Hz, 1H), 3.18
(dd, J = 12.4, 8.4 Hz, 1H), 1.69 (s, 15H), 1.21 (d, J = 6.4 Hz, 3H);
13C NMR (100 MHz, D2O) δ 151.6, 89.1, 45.9, 44.7, 17.3, 8.3.
HRMS: calcd. for C15H24IrN4O2 [M-2Cl]+, 485.1529, Found
485.1553
Conflicts of interest
There are no conflicts to declare.
Complex Ir-L8 was obtained from [IrCp*Cl2]2 (0.1 mmol)
and L8 (0.2 mmol) using the similar procedure as complex Ir-L7.
Yellow solid, 80.5 mg, yield: 68%. Crystal that was suitable for
X-ray diffraction was obtained from a mixture of methanol and
Acknowledgements
We thank the National Natural Science Foundation of China,
(NSFC Grant No. 21321002). This is also supported by DICP
Fundamental Research Program for Clean Energy (DICP
M201302).
1
ether. H NMR (400 MHz, D2O): δ (ppm) = 3.30 (s, 2H), 2.08-
2.06 (m, 2H), 1.78-1.76 (m, 10H), 1.75 (s, 15 H) 1.37-1, 38 (m,
4H); 13C NMR (100 MHz, D2O) δ 152.1, 89.3, 55.2, 28.9, 23.3,
8.4. HRMS: calcd. For C18H28IrN4O2 [M-2Cl]+, 525.1842, Found
525.1877.
General procedure for catalytic dehydrogenation of HCO2H in a
small scale:
Notes and references
1
For reviews: see a) A. F. Dalebrook, W. Gan, M. Grasemann,
S. Moret, G. Laurenczy, Chem. Commun. 2013, 8735; b) M.
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Energy Environ. Sci. 2009, 2, 480.
All of the solvents and reagents were used directly without
any manipulations unless otherwise specified. The reactions
were carried out without protection.
For the in situ formed catalysts from [IrCp*Cl2]2 and
ligands, we get the catalysts stock solution (5 mM) with the
help of ultrasound for 5-10 minutes until the solution get clear.
If the complex is used as the catalyst, we also submit the
solution for ultrasonic as the in situ formed catalyst to get the
catalyst stock solution.
2
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For the reaction, aqueous formic acid solution (0.38 mL FA,
9.42 mL H2O) and a stir bar were placed in a Schlenk tube
equipped with a side branch and rubber septa. Afterwards, the
tube was preheated at given temperature (e.g. 60 °C) in a
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Green Chemistry
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DOI: 10.1039/C8GC00495A
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Green Chemistry
View Article Online
DOI: 10.1039/C8GC00495A
Graphical Abstract:
A new iridium catalyst bearing dioxime derived ligand has been developed for aqueous formic acid (FA)
dehydrogenation in the absence of any additives. These catalysts can work at high temperature or room
temperature with high efficiency and stability.
1