Highly efficient hydrogenation of carbon dioxide to formate catalyzed by iridium(iii) complexes of imine-diphosphine ligands

Article abstract of DOI:10.1039/c5sc00248f

A new iridium catalyst containing an imine-diphosphine ligand has been developed, which showed high efficiency for the hydrogenation of CO₂ to formate (yield up to 99%, TON up to 450000). A possible catalytic mechanism is proposed, in which the imine group of the catalyst plays a key role in the cleavage of H₂ and the activation of CO₂.

Full text of DOI:10.1039/c5sc00248f

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Highly Efficient Hydrogenation of Carbon Dioxide to
Formate Catalyzed by Iridium(III) Complexes of
Imine-diphosphine Ligands
Cite this: DOI: 10.1039/x0xx00000x
Chong Liu, JianꢀHua Xie, GuiꢀLong Tian, Wei Li and QiꢀLin Zhou*
Received 00th January 2012,
Accepted 00th January 2012
A new iridium catalyst containing an imineꢀdiphosphine ligand has been developed, which
showed high efficiency for the hydrogenation of CO2 to formate (yield up to 99%, TON up to
450,000). A possible catalytic mechanism is proposed, in which the imine group of the catalyst
plays a key role in the cleavage of H2 and the activation of CO2.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Carbon dioxide (CO2), an economical, safe, environmentally stable and efficient metalꢀligand cooperative catalysts,10 we designed
friendly, and renewable carbon source, is an ideal oneꢀcarbon a new type of iridium(III) catalysts containing imineꢀdiphosphine
building block for organic chemicals, including carbohydrates ligands, which exhibit high activity for the hydrogenation of CO2.
and fuels.1 However, its thermodynamic and kinetic stability Herein, we report the preparation and characterization of
presents a fundamental obstacle to the use of CO2 in both iridium(III)/imineꢀdiphosphine catalysts 1 and their application for
academia and industry.2 Highꢀenergy reagents, harsh reaction the hydrogenation of CO2 to formate.
conditions, and special activation mechanisms are typically
required for transforming CO2 into other chemicals. Hydrogen
is a green highꢀenergy material that can be used to convert CO2
to valuable secondary energy carriers, such as methane,
methanol, and formic acid. Formic acid is widely used in
agriculture and in the leather and dye industries, as well as in
fuel cells and synthetic chemistry.3
Homogeneous catalytic hydrogenation of CO2 into formic acid
catalyzed by transitionꢀmetal complexes based on rhodium,4
ruthenium,5 iridium,6 iron,7 and cobalt8 has been extensively
investigated since it was first reported in 1976 by Inoue4a et al., who
used Wilkinson’s catalyst, RhCl(PPh3)3, to accomplish the reaction.
In the early 1990s, Graf and Leitner4b achieved turnover numbers
(TONs) as high as 3,400 for CO2 hydrogenation reactions catalyzed
by rhodium phosphine complexes. Noyori5a,b et al. found that with
RuH2(PMe3)4 as a catalyst, the reaction was more efficient in
supercritical CO2 than in traditional organic media. Halfꢀsandwich
iridium(III) complexes containing a strong electron donor ligand
such as 4,4′ꢀdihydroxyꢀ2,2′ꢀbipyridine or 4,7ꢀdihydroxyꢀ1,10ꢀ
phenanthroline were used for the hydrogenation of CO2 by Himeda6b
et al., who reported TON of 2.2×105. Subsequently, a significant
breakthrough was made by Nozaki6c et al., who discovered that an
iridium(III) trihydride complex with a pyridineꢀbased PNPꢀpincer
Scheme 1 The preparation of iridium catalysts
ligands and X-ray structure of complex . The H atoms except hydride ligands
from the practical point of view, the reported catalysts exhibited high and disorder were omitted for clarity. Thermal ellipsoids shown at the 30%
1 containing imine-diphosphine
ligand has extraordinary catalytic activity (TON 3.5×106). However,
B
probability level. Selected bond lengths [Å] and angels [°] for complex B: Ir1–P1a
activity only at harsh conditions such as high temperature and high
pressure. Hence the research focus in the field of hydrogenation of
CO2 is still the search of efficient catalysts, especially those with
new activation model.
2.319 (2), Ir1–N1 2.115(7), Ir1–Cl1 2.391(3), Ir1–Cl2 2.579(3); P1a–Ir1–N1
91.59(4), P1a–Ir1–P1 174.90(9), N1–Ir1–Cl2 82.5(2), N1–Ir1–Cl1 177.3(2).
Catalysts 1 were synthesized in high yields by the complexation
of tridentate amineꢀdiphosphine ligands 211 with [Ir(COE)2]2 under
hydrogen and subsequent treatment with base (Scheme 1).
Complexation in THF at 100 °C produced complexes 3 (85–90%
yield), which were stable solids that could be kept for several months
under a nitrogen atmosphere. Catalysts 1 were obtained in 75–80%
Lateꢀtransitionꢀmetal–amine complexes, which have a nitrogen
atom with strong nucleophilicity, basicity, and donor ability, have
been used as metalꢀligand cooperative catalysis.9 A noteworthy
example is the iridium hydride catalyst containing an amineꢀ
diphosphine ligand developed by Hazari6e et al., which shows high
activity for the hydrogenation of CO2. As part of our exploration of
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DOI: 10.1039/C5SC00248F
yield from the reaction of 3 with KOtBu in THF. Interestingly, when
The activity of iridium catalysts 1 for the hydrogenation of CO2
we monitored the reaction of 3b by NMR at –45 °C, we observed a to formate was evaluated. The initial experiments were carried out
new species characterized by a singlet at δ = 54.84 ppm in the 31P
under 60 atm of 1:1: CO2/H2 in aqueous KOH in the presence of 0.1
mol% iridium catalyst at 140 °C for 20 h. Catalysts 1a and 1b gave
93% and 99% yields of formate, respectively (Table 1, entries 1 and
2). When the catalyst loading was reduced to 0.001 mol%, 1b, with
four tertꢀbutyl groups on the phosphorus atoms, still afforded an
81% yield (entry 4); however, 1a, which has four phenyl groups on
the phosphorus atoms, gave only a 23% yield (entry 3). Increasing
reaction temperature and extending reaction time have a negligible
improvement to the yield of the reaction. The use of K3PO4 and
other weaker base resulted in a lower yield. We were delighted to
find that 1b could be generated in situ from 3b under the reaction
conditions, and the yield (78%) with the in situꢀgenerated catalyst
was similar to that obtained from the isolated catalyst (entries 4 and
6). The use of stable catalyst precursor 3b, instead of the active
catalyst 1b itself, offered an additional advantage for the reaction
performance.
Using the most active catalyst (1b), we optimized the
conditions for the hydrogenation of CO2 to formate (Table 2).
First, we investigated the influence of the initial pressures of H2
and CO2 and found that when the total initial pressure of a 1:1
mixture of H2 and CO2 was lowered from 60 to 40 atm, the
yield of formate decreased from 81% to 35% (entries 1 and 2).
Raising the H2/CO2 ratio from 1:1 to 2:1 or to 5:1, at the same
total initial pressure, markedly increased the yield (to 99%,
entries 3 and 4). Increasing the initial concentration of KOH
from 1 M to 2 M or 5 M at the catalyst loading of 0.0001 mol%
(relative to KOH) dramatically increased the yield and TON of
the reaction (entries 6 and 7 vs 5).
NMR spectrum and a doubleꢀdoublet at δ = –16.87 ppm (J = 21.8,
1
13.1 Hz) in the H NMR spectrum, neither of which belonged to 3b
[31P NMR: δ = 40.74 ppm (t, J = 10.4 Hz); 1H NMR: δ = –21.87 ppm
(td, J = 15.8, 7.4 Hz, 1H), –26.04 ppm (td, J = 16.9, 7.8 Hz, 1H)] or
to 1b [31P NMR: δ = 52.98 ppm (dd, J =13.0, 346.8 Hz), 59.83 ppm
1
(dd, J = 7.3, 346.8 Hz); H NMR: δ = –10.77 ppm (t, J = 17.4 Hz,
2H), –19.39 ppm (t, J = 15.2 Hz, 1H)] (see Supporting Information).
We speculated that this species was an 16ꢀelectron iridium dihydride
complex, IrH2[(tBu2PC6H4CH2)2N] (A),12 generated by elimination
of HCl from 3b upon treatment with KOtBu. Complex A was
unstable and rapidly converted to an 18ꢀelectron iridium trihydride
complex of imineꢀdiphosphine complex 1b by means of a shift of
hydride from CH2 to iridium.13
Attempts to grow crystals of 3 and 1 were unsuccessful.
Fortunately, 3a slowly underwent an exchange of hydride to chloride
in 1,1,2,2ꢀtetrachloroethane, affording dichloride complex B as a
crystal (Scheme 1).14 Xꢀray crystallographic analysis of B confirmed
the structure of 3a as having one nitrogen atom and two phosphorus
atoms coordinated to iridium, as well as two sixꢀmembered
heterometallic rings. In the crystal of B, two phosphines were
coordinated to iridium in
a trans arrangement (P1a–Ir–P1
174.90(9)°), and two chlorides were in a cis arrangement (Cl1–Ir–
Cl2 100.24(8)°). One chloride (Cl2) was cis to the nitrogen (Cl2–Ir–
N 82.5(2)°), and the other chloride (Cl1) was trans to the nitrogen
(Cl1–Ir–N 177.3(2)°). The bite angles of the ligands in the
complexes 3 and 1 are larger than those in the conventional PNP
liangds.6c,6e The flexibility of the ligands in the complexes 3 and 1 is
important because a proton is released from the ligand during the
catalytic process, which changes the conformation of ligand.
Table 2 Optimization of reaction conditions a
Table 1 The hydrogenation of CO2 to formate a
Entry
P(H2)/P(CO2)
KOH (M)
Yield (%)b
TON (× 103)c
(atm)
1
30/30
20/20
40/20
50/10
40/20
40/20
40/20
1
1
1
1
1
2
5
81
35
99
99
20
34
45
81
Entry
Catalyst (mol%)
Yield (%)b
TON (×103)c
2
35
1
1a (0.1)
93
99
23
81
22
78
5
0.9
1
3
99
2
1b (0.1)
4
99
5d
6d
7d
200
340
450
3
1a (0.001)
1b (0.001)
3a (0.001)
3b (0.001)
4 (0.001)
5a (0.001)
5b (0.001)
6a (0.001)
6b (0.001)
7a (0.001)
7b (0.001)
7c (0.001)
23
81
22
78
5
4
5
6
a
General reaction conditions: 5 mL aqueous KOH solution, 0.001 mol%
iridium catalyst 1b relative to KOH in 100 ꢁL THF, under the desired CO2:H2
7
8
10
14
2
10
14
2
b
pressure, 140 °C, 20 h. Yield, which represents conversion of the added
KOH, based on 1H NMR analysis with sodium 3ꢀ(trimethylsilyl)ꢀ1ꢀ
9
c
propanesulfonate as an internal standard. TON = turnover number; number
of moles of product formed per mole of catalyst. Catalyst loading was
0.0001 mol% relative to KOH.
10
11
12
13
14
d
8
8
6
6
1
1
Under the optimal reaction conditions (P(H2)/P(CO2) 40/20 atm,
5 M KOH, 140 °C, 0.0001 mol% 1b), the formate was obtained in
45% yield (TON = 450,000, turnover frequency = 22,500 h–1, entry
7). This result showed that 1b was a very efficient catalyst for the
conversion of CO2 to formate under relatively mild reaction
2
2
a General reaction conditions: 5 mL aqueous 1 M KOH solution, 0.1 mol% or
0.001 mol% iridium catalyst relative to KOH in 100 ꢁL THF, 60 atm of
H2:CO2 (1:1, initial), 140 °C, 20 h. Yield, which represents conversion of
b
the added KOH, based on 1H NMR analysis with sodium 3ꢀ(trimethylsilyl)ꢀ1ꢀ conditions.
c
propanesulfonate as an internal standard. TON = turnover number; number
of moles of product formed per mole of catalyst.
For a comparison, the iridium complexes with tridentate PNP,
PNN,15 and PNO16 ligands (4–7, Figure 1) were also synthesized (for
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5SC00248F
Scheme 2 Proposed mechanism for the hydrogenation of CO2 catalyzed by 1b
.
details, see Supporting Information), and their catalytic activity for
the hydrogenation of CO2 to formate were studied. The very low
activity of the complex 4 containing Nꢀmethyl (entry 7, Table 1)
indicates that the NH moiety in the catalyst 3 was essential for the
formation of iridium(III)/imineꢀdiphosphine catalysts 1. The iridium
complexes with PNN ligands (5 and 6) and PNO ligands (7), which
can also form iridium(III)/imineꢀdiphosphine catalysts, showed
extremely low activity for the hydrogenation of CO2 (Table 1,
entries 8–14,).
In summary, we developed a new type of iridium catalyst
containing an imineꢀdiphosphine ligand, which showed high activity
for the hydrogenation of CO2 to formate (yields up to 99%, TONs up
to 450,000). The Irꢀimineꢀdiphosphineꢀcatalyzed hydrogenation of
CO2 conducted through a metalꢀmine biꢀfunctional mechanism. This
activation model should be applicable for other transformations.
Acknowledgements
We thank the National Natural Science Foundation of China,
the National Basic Research Program of China
(2012CB821600), and the “111” project (B06005) of the
Ministry of Education of China for financial support.
Notes and references
State Key Laboratory and Institute of Elementoꢀorganic Chemistry,
Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), Nankai University, Tianjin 300071, China. Eꢀmail:
qlzhou@nankai.edu.cn; Fax: +86ꢀ22ꢀ2350ꢀ6177
Figure 1 Other iridium complexes containing tridentate PNP, PNN, and PNO
ligands.
†
Electronic Supplementary Information (ESI) available: experimental
procedures; spectral data for all new compounds. See DOI:
10.1039/b000000x/
On the basis of the aboveꢀdescribed experimental results for the
hydrogenation of CO2 catalyzed by iridium(III)/imineꢀdiphosphine,
we proposed a mechanism involving metalꢀimine cooperative
catalysis (Scheme 2).17 First, a molecule of H2 adds to the C=N
double bond of 1b to generate iridiumꢀtrihydride complex C, which
was isolated in 75% yield and was identified (see Supporting
Information). Baseꢀmediated addition of the N–H proton and the Ir–
H hydride of C to the carbonyl group of CO2 via a sixꢀmemberedꢀ
ring transition state (TS) generates formic acid and complex A,
which was detected by NMR (vide ante). A rapid shift of hydride
from αꢀCH2 to iridium regenerated the catalyst 1b and finished a
catalytic cycle. An experiment with C and CO2 showed the
formation of intermediate A, which was quickly converted to
complex 1b in 69% yield. This metalꢀimine biꢀfunctional catalysis
provides a highly efficient hydrogenation of CO2.
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