Efficient Cp?Ir Catalysts with Imidazoline Ligands for CO2 Hydrogenation

Article abstract of DOI:10.1002/ejic.201501030

We report newly developed iridium catalysts with electron-donating imidazoline moieties as ligands for the hydrogenation of CO₂ to formate in aqueous solution. Interestingly, these new complexes promote CO₂ hydrogenation much more effectively than their imidazole analogues and exhibit a turnover frequency (TOF) of 1290 h^-1 for the bisimidazoline complex compared to that of 20 h^-1 for the bisimidazole complex at 1 MPa and 50 C. In addition, the hydrogenation proceeds smoothly even under atmospheric pressure at room temperature. The TOF of 43 h^-1 for the bisimidazoline complex is comparable to that of a dinuclear complex (70 h^-1, highest TOF reported) [Nat. Chem. 2012, 4, 383], which incorporates proton-responsive ligands with pendent-OH groups in the second coordination sphere. The catalytic activity of the complex with an N-methylated imidazoline moiety is much the same as that of the corresponding pyridylimidazoline analogue. This result and the UV/Vis titrations of the imidazoline complexes indicate that the high activity is not attributable to the deprotonation of NH on the imidazoline under the reaction conditions. A novel complex having imidazoline ligands shows much higher catalytic activity for CO₂ hydrogenation than the conventional complex having imidazole ligands. The change from a double bond in imidazole to a single bond in imidazoline leads to a 60-fold increase in the catalytic activity.

Full text of DOI:10.1002/ejic.201501030

SHORT COMMUNICATION
DOI:10.1002/ejic.201501030
Efficient Cp*Ir Catalysts with Imidazoline Ligands for
CO2 Hydrogenation
Shaoan Xu,[a,b] Naoya Onishi,*[a,b] Akihiro Tsurusaki,[a]
Yuichi Manaka,[a] Wan-Hui Wang,[c] James T. Muckerman,[d]
Etsuko Fujita,[d] and Yuichiro Himeda*[a,b]
Keywords: Homogeneous catalysis / Iridium / CO2 hydrogenation
We report newly developed iridium catalysts with electron-
donating imidazoline moieties as ligands for the hydrogen-
ation of CO2 to formate in aqueous solution. Interestingly,
these new complexes promote CO2 hydrogenation much
more effectively than their imidazole analogues and exhibit
a turnover frequency (TOF) of 1290 h–1 for the bisimidazoline
complex compared to that of 20 h–1 for the bisimidazole com-
plex is comparable to that of a dinuclear complex (70 h–1
,
highest TOF reported) [Nat. Chem. 2012, 4, 383], which in-
corporates proton-responsive ligands with pendent-OH
groups in the second coordination sphere. The catalytic ac-
tivity of the complex with an N-methylated imidazoline moi-
ety is much the same as that of the corresponding pyridyl-
imidazoline analogue. This result and the UV/Vis titrations
plex at 1 MPa and 50 °C. In addition, the hydrogenation pro- of the imidazoline complexes indicate that the high activity
ceeds smoothly even under atmospheric pressure at room
temperature. The TOF of 43 h–1 for the bisimidazoline com-
is not attributable to the deprotonation of NH on the imid-
azoline under the reaction conditions.
a wide variety of ligand backbones that link to the iridium
center: phosphine-type,[9] pincer-type,[10] N-heterocyclic-
carbene-type,[11] bipyridine-type,[12] and so on.[13] In ad-
dition, the realization of a hydrogen storage system in aque-
ous solution is desirable from the viewpoint of green chem-
istry.
Introduction
For a future hydrogen-based economy, the development
of hydrogen storage and transfer technology is highly de-
sired in terms of energy and cost efficiency as well as safety.
One of the promising solutions is conversion of hydrogen
gas to a liquid chemical compound.[1] From the late 1970s,
the combination of CO2 hydrogenation and dehydrogena-
tion of formic acid (FA) has been regarded as a viable
hydrogen storage system.[2] Furthermore, CO2 hydrogen-
ation has attracted attention from the viewpoint of small
molecule activation as well as CO2 utilization.[3] A number
of transition-metal catalysts for hydrogenation of CO2 to
formate and FA have been reported for Cp*Ir complexes
(Scheme 1), as well as for complexes of Fe,[4] Ru,[5] Cu,[6]
Co,[7] and Rh.[8] There has been a marked development of
Ir catalysts for CO2 hydrogenation in recent years. There is
[a] National Institute of Advanced Industrial Science and
Technology,
Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
E-mail: n.onishi@aist.go.jp
Scheme 1. CO2 Hydrogenation catalyzed by Cp*IrIII complexes (1–
8) incorporating various bidentate ligands (N N), where N N is
Bpy, 4DHBP, 6DHBP or one of L4–L8.
∧
∧
himeda.y@aist.go.jp
[b] Japan Science and Technology Agency, ACT-C
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 Japan
[c] School of Petroleum and Chemical Engineering,
Dalian University of Technology,
Panjin 124221, China
[d] Chemistry Department, Brookhaven National Laboratory
Upton, NY 11973-5000, USA
∧
Recently, a series of water-soluble Cp*Ir(N N) com-
plexes has been found to have good catalytic efficiency for
CO2 hydrogenation.[12c,14] Our previous reports demon-
strated that the introduction of electron-donating ligands
toward the Ir center improves the catalytic activity for hyd-
rogenation of CO2 and dehydrogenation of FA. In particu-
Supporting information and ORCID(s) from the author(s) for
this article are available on the WWW under http://dx.doi.org/
10.1002/ejic.201501030.
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lar, an acidic hydroxy moiety on a ligand, which provides an Table 1. Results of CO2 hydrogenation.[a]
electron-donating oxyanion by deprotonation under basic
conditions (a so called “proton-responsive” ligand), dra-
matically accelerates catalytic CO2 hydrogenation.[12c,14a]
On the other hand, the introduction of a ligand with a five-
membered ring (e.g., azole) significantly improves the cata-
lytic efficiency relative to a complex with an unsubstituted
bipyridine reference ligand, [Cp*Ir(Bpy)(H2O)]SO4, which
was attributed to the strong electron donation of the azole
moiety.[14c,15] Very recently, we developed an Ir complex
having a pyrimidyl–imidazoline moiety that shows ex-
tremely high catalytic activity (TOF up to 322,000 h–1)
compared to those of its imidazole analogues for dehydro-
genation of FA.[16] In addition, Xiao et al. reported that the
Entry
Catalyst
T [°C]
TOF [h–1]
Ref.
[12b]
[12b]
[12b]
[15]
1
2
3
4
1
2
3
4
5
6
6
6
6
7
7
7
7
8
8
8
8
50
50
50
50
50
50
25
80
100
50
25
80
100
50
25
80
100
1
650
1400
9
20
168
4
400
593
1290
43
1520
842
181
5
[15]
5
6
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
7[b]
8
9
10
11[b]
12
13
14
phenylimidazoline analogue effectively catalyzes dehydroge- 15[b]
nation of FA by using NEt3.[17] Similar to the hydrogen-
16
ation of CO2, the catalytic efficiency of FA dehydrogenation
464
413
17
[a] Conditions: 1 h, PH2/PCO2 = 1.0 MPa (1:1), [KHCO3] = 2.0 m,
[cat] = 20 μm in aqueous solution under pH 8.3. All numbers are
the average of two runs. [b] Conditions: H2/CO2 = 1:1 (0.1 MPa)
at 25 °C by 6 and 8 (0.05 mm), and 7 (0.025 mm) in 1 m NaHCO3.
with Cp*Ir complexes was thought to be improved by an
increase in electron donation by the ligand.[18] These results
led to the hypothesis that the imidazoline moiety would be
the proper ligand backbone for CO2 hydrogenation. Herein,
we report the efficient hydrogenation of CO2 using simple
imidazoline complexes in basic water.
Next, we investigated the catalytic CO2 hydrogenation
under ambient conditions {[NaHCO3] = 1.0 m, 25 °C,
0.1 MPa of H2/CO2 (1:1), Figure 1 and Entries 7, 11, and
15 in Table 1}. Complex 7 achieves a TOF of 43 h–1 after
the first 1 h, and a TON of 298 after 1 d (Table S2). Com-
pared to complex 6, complex 7 demonstrates higher cata-
lytic activity. The catalytic activity of 7 is comparable to
those of the complexes with proton-responsive ligands with
pendent-OH groups (e.g., 4,4Ј,6,6Ј-tetrahydroxy-2,2Ј-bipyr-
Results and Discussion
As shown in Scheme 1, we synthesized the new iridium
complexes 6–8 with imidazoline derivatives (see Detailed
Procedure in the Supporting Information and Figures S1
and S2 for calculated DFT geometries of the six- and five-
coordinate complexes of 7). The results of complexes 6–8
for catalytic CO2 hydrogenation {conditions: [cat] =
0.02 mm, [KHCO3] = 2.0 m, 50 °C, 1 MPa of H2/CO2 (1:1),
1 h} are shown in Table 1 along with those of the previously
reported complexes 1–5 (see Scheme 1) for comparison.
These complexes are able to catalyze hydrogenation in
aqueous solution without any organic additives. Complex 6
shows good catalytic activity (Entry 6), which is better than
conventional complexes such as 1, 4 and 5. Since the intro-
duction of one imidazoline moiety in a ligand increases the
catalytic activity, we also examined complex 7 having two
imidazoline moieties. The TOF of 1290 h–1 of 7 is much
higher than that of complex 2 with the proton-responsive
ligand with OH groups (pKa = 5.2).[12b] To the best of our
knowledge, this is the first report that a ligand such as an
imidazoline without a proton-responsive OH group was
more effective for CO2 hydrogenation in aqueous solution
than the proton-responsive ligand 4DHBP. This is attribut-
able to the powerful electron-donation of an imidazoline
moiety. In addition, complex 7 shows slightly lower cata-
lytic activity than 3, which also has two proton-responsive
OH groups, but the synthesis of ligand 4 is easier because
of commercially available precursors. In other words, com-
plex 7 is simple yet effective. Although the TOF value of
complex 6 increases as the temperature is increased from 50
to 100 °C, that of complex 7 decreases (Table S1). This re-
sult suggests that the pyridyl moiety enhances the heat en-
durance of the complex.
imidine[12c]
and
2,2Ј,6,6Ј-tetrahydroxy-4,4Ј-bipyrimid-
ine),[14b] which achieves the highest activity for CO2 hyd-
rogenation under ambient conditions among all the com-
plexes we have studied. In addition, complex 7 is more
active than Fukuzumi’s complex (TOF = 6.8 h–1), which is
Figure 1. Time course for the hydrogenation of CO2 under H2/CO2
= 1:1 (0.1 MPa) at 25 °C by 6 and 8 (0.05 mm), and 7 (0.025 mm)
in 1 m NaHCO3.
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the only other example apart from ours that can work well The imine substituent in the amidine structure has a reso-
under these conditions.[13]
nance structure in which it is partially converted to an
An imidazoline moiety has an acidic proton on NH, amide form. The electron density of the Ir center is in-
which may deprotonate to form an N-anion under basic creased by ligation of the amide, which is more electron rich
conditions. We have already reported that the tetramethyl- than the imine. However, in aromatic imidazole, which has
bisimidazole analogue has proton-responsive NH moieties an amidine backbone the same as in imidazoline, its π-elec-
(pKa = 8.8).[15] As a follow-up investigation of the imidazole trons are delocalized over the entire ligand. By contrast, π-
analogue, we measured the UV/Vis spectrum of bisimid- electrons on non-aromatic imidazoline are delocalized
azoline complex 7 under various pH conditions (Figure 2). within the amidine moieties, which leads to an increase in
The UV/Vis spectrum for the titration of 7 exhibits a red- the electron density of the N atom coordinating the Ir cen-
shift in the region from pH 8 to 10, which is attributed ter in comparison with those of imidazole. As described in
to a change in the electronic properties of the ligand by the Introduction, the catalytic efficiency of CO2 hydrogen-
deprotonation of NH.[12b] The pKa value of 7 was deter- ation is improved by an increase in electron donation from
mined to be 8.94, and this value is close to that of the the ligand to the Ir center. Hence, bisimidazoline complex
tetramethylbisimidazole analogue.[15] However, the results 7 is an outstanding catalyst for CO2 hydrogenation.
suggest that deprotonation of complex 7 does not signifi-
Comparing the electronic ligand effects of the 4- or 6-
cantly affect its catalytic behavior because the pH used in hydroxy group in a pyridinol moiety with those of an imid-
CO2 hydrogenation was 8.3. Based on this result, we sought azoline moiety, in the former there is a moderate σ donation
to determine whether or not the deprotonation of the NH- effect at low pH, but at the higher pH at which deproton-
proton has a significant effect on the catalytic activity of 7. ation of the hydroxy group occurs, the resulting oxyanion
has a keto resonance structure that is a strong π-donor. This
effect is enhanced by the strong π-acceptor property of the
metal center in the Cp*IrIII moiety. This causes the weaken-
ing or detachment of the aqua ligand, creating a site for H2
heterolysis, the rate-determining step in CO2 hydrogenation.
In the case of the imidazoline moiety, the restricted conju-
gation of the double bonds is extended by a resonance
structure that places negative charge on the π-accepting IrIII
center through π-donation even at low pH.
Conclusions
We have reported that imidazoline ligands improve the
Ir-catalyzed hydrogenation of CO2 to formate. In particular,
bisimidazoline complex 7 achieves a high TOF value under
both pressurized and ambient conditions. There are many
∧
N N-bidentate ligands that have been linked to Cp*Ir com-
plexes for CO2 hydrogenation, but a ligand such as bisimid-
azoline that is not completely conjugated has not previously
been reported as an effective ligand for catalytic CO2
hydrogenation. This is a potentially important finding for
expanding the “toolbox” for the design of improved com-
plexes for CO2 hydrogenation.
Acknowledgments
Figure 2. (a) UV/Vis absorption spectra of complex 7 (0.2 mm) in
a pH titration and (b) absorbance change at a single wavelength as
S. X., N. O., Y. M., and Y. H. thank the Japan Science and Technol-
a function of pH. Boltzmann fits are depicted by the red lines.
ogy Agency (JST), ACT-C for financial support. W.-H. W. is grate-
ful for the financial support from Dalian University of Technology
[the Fundamental Research Funds for the Central Universities,
Grant No. DUT14RC(3)082; Grant No. 844401] and the National
Natural Science Foundation of China (Grant No. 21402019). The
The N-methylated pyridyl–imidazoline complex 8 reacts
with CO2 under both pressurized and ambient conditions.
The measured TOF value is almost the same as that of the
non-methylated complex 6 (Entries 6 vs. 14 in Table 1, and
work at BNL was carried out with the U.S. Department of Energy,
Figure 1). These results confirm that deprotonation of the
NH moiety has hardly any effect on this catalytic system.
The reason that an imidazoline ligand is effective in pro-
moting the hydrogenation of CO2 is attributable to the in-
fluence of the amidine backbone in the imidazoline moiety.
Office of Science, Office of Basic Energy Sciences under contract
DE-SC00112704.
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Received: September 9, 2015
Published Online: November 9, 2015
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