Hydrogenation of Carbon Dioxide with Organic Base by PCIIP-Ir Catalysts

Article abstract of DOI:10.1021/acs.organomet.8b00377

Novel PC^IIP-Ir^I monochloride complexes (1-Cl and 2-Cl) bearing a phosphine-carbene-phosphine pincer type ligand were synthesized. Reactions of 1-Cl with hexachloroethane, hydrogen chloride, and lithium triethylborohydride under a dihydrogen atmosphere afforded PC^IIP-Ir^III trichloride (1-Cl₃), hydride dichloride (1-HCl₂), and trihydride (1-H₃) complexes, respectively. The strong electron-donating ability of carbene in PC^IIP-Ir complexes was confirmed by X-ray crystallography and DFT calculations. Moreover, in complex 1-Cl, strong π back-donation from the iridium center to the carbene carbon was observed. Hydrogenation of CO₂ with triethanolamine catalyzed by PC^IIP-Ir complexes was investigated. The novel PC^IIP-Ir complex 1-Cl exhibited a longer lifetime in comparison to the PNP-Ir^III complex 3-H₃: the turnover number of 1-Cl is significantly higher than that of 3-H₃ (in 46 h, 1-Cl 230000 and 3-H₃ 54000).

Full text of DOI:10.1021/acs.organomet.8b00377

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
II
Hydrogenation of Carbon Dioxide with Organic Base by PC P‑Ir
Catalysts
Satoko Takaoka,† Aya Eizawa, Shuhei Kusumoto, Kazunari Nakajima,‡
‡
†
,‡
,†
Yoshiaki Nishibayashi,* 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
‡
Department of Systems Innovation, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
1
13-8656, Japan
*
S Supporting Information
II
I
ABSTRACT: Novel PC P-Ir monochloride complexes (1-Cl
and 2-Cl) bearing a phosphine-carbene-phosphine pincer type
ligand were synthesized. Reactions of 1-Cl with hexachloroethane,
hydrogen chloride, and lithium triethylborohydride under a
II
III
dihydrogen atmosphere afforded PC P-Ir trichloride (1-Cl3),
hydride dichloride (1-HCl ), and trihydride (1-H ) complexes,
2
3
respectively. The strong electron-donating ability of carbene in
II
PC P-Ir complexes was confirmed by X-ray crystallography and
DFT calculations. Moreover, in complex 1-Cl, strong π back-
donation from the iridium center to the carbene carbon was
observed. Hydrogenation of CO with triethanolamine catalyzed
2
II
II
by PC P-Ir complexes was investigated. The novel PC P-Ir
complex 1-Cl exhibited a longer lifetime in comparison to the
III
PNP-Ir complex 3-H : the turnover number of 1-Cl is
3
significantly higher than that of 3-H (in 46 h, 1-Cl 230000 and 3-H 54000).
3
3
In 2009, we reported that a PNP-IrIII pincer-type complex
showed the highest catalytic turnover number (TON),
INTRODUCTION
■
Formic acid is one of the most fundamental chemical products,
which is used as a preservative agent in livestock feed, for
leather tanning, and for dyeing textiles. The global demand of
11
3500000, in the presence of potassium hydroxide. From
1
the viewpoint of industrial application, weak organic bases have
an advantage in the purification process. Namely, formic acid
can be isolated via simple distillation from the produced salts
with a weak organic base, while with strong inorganic base,
formic acid was 579000 tons/year in 2013 and average annual
growth of consumption in the world will continue at 3.7% until
2
,3
2
020. Most of the industrial production relies upon the
22
1
neutralization is inevitable for formic acid production.
carbonylation of methanol with CO and successive hydrolysis.
Moreover, the amine after separation from formic acid salt
by distillation can be recycled. The PNP-Ir catalyst, however,
showed a much lower turnover frequency (TOF 14000 h− in
2 h) and turnover number (29000) with trialkylamine instead
Hydrogenation of CO has attracted great attention and has
2
been developed in industry as an alternative route for the
production of formic acid because of the abundance and
lessened toxicity of CO in comparison to CO. In addition
to the high demand of formic acid as a basic chemical, its high
potential as a hydrogen-storage material has also motivated the
increasing importance of catalyst development. Along with
one hundred years of history for catalytic CO hydrogenation,
especially since the first report with a well-defined complex
catalyst by Inoue in 1976, numerous researchers have focused
1
3
,4
2
23
of hydroxide base. Other representative examples where
catalytic hydrogenation of CO was conducted under organic
2
1
base conditions are summarized in Scheme 1A. Jessop et al.
−
1
5
reported a ruthenium-based catalyst (TOF 95000 h , 0.3 h)
14b
2
with triethylamine.
Pidko and co-workers reported a
6
remarkably high TOF by PNP-Ru catalyst in the presence of
−
1
15a
DBU (1100000 h , ∼2 min). However, there still remains a
their attention on the development of active and robust
homogeneous and heterogeneous catalysts for hydrogenation
strong demand for the development of a highly active and
7
−9
robust catalyst in CO hydrogenation with organic bases.
2
of CO2.
Until now, various transition-metal homogeneous
10−12 13 14,15 16 17 18 19
In 2016, we found that an electron-rich ligand improved the
catalysts, such as Ir,
Rh, Ru,
Fe, Ni, Co, etc.,
24
catalytic activity under organic base conditions (Scheme 1B).
have been developed for highly active CO hydrogenation. In
2
almost all the precedents, bases have been employed in order
to accelerate the reaction and make it thermodynamically
favorable.
Received: June 4, 2018
7
,20,22a
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. (A) Representative Examples for Hydrogenation
Scheme 2. Synthesis of (a) 1-Cl, (b) 2-Cl, (c) 1-HCl , 1-Cl ,
2
3
of CO with Organic Bases, (B) Electron-Donating Effect
and 1-H , and (d) 2-HCl2
2
3
for Hydrogenation of CO Catalyzed by PNP-pincer-Ir
2
Complexes and Cationic Rate-Determining Transition State
in Hydrogenation of CO by PNP-pincer-Ir Complex, and
2
II
(
(
C) Hydrogenation of CO by PC P-pincer-Ir Complexes
2
This Work)
The TON was 1.5 times higher with a 4-methoxy-substituted
PNP iridium complex than with unsubstituted PNP-Ir (TOF
−
1
1
2000 and 8500 h , respectively, in 2 h), presumably because
the cationic key transition state suggested by theoretical
calculations was stabilized by an electron-donating methoxy
2
1,23
group (Scheme 1B).
In the last four decades, on the other
butylphosphinomethyl)benzimidazolium hexafluorophosphate
+
hand, many variations of PXP pincer-type tridentate ligands
have been developed, aiming at stable coordination of a
(1-H ) and 1,3-bis(2-(di-tert-butylphosphino)ethyl)-
+
imidazolium chloride (2-H ) with [IrCl(coe) ] and [IrCl-
2
2
−
25
33
strongly electron-donating X moiety such as anionic C ,
(C H ) ] proceeded cleanly to afford the tetracoordinated
2 4 2 2
36
N , B , and Si− and neutral P, N, C(II),
−
26
−
27
28
29
30
31,32
Si(II),
iridium(I) chlorides 1-Cl and 2-Cl, respectively. The
formation of a stable iridium(I) chloride complex is in sharp
contrast to a PNP-Ir monochloride complex, which could not
be isolated because of the immediate release of hydrogen
3
4
II
etc. Among them, we focused our attention on a PC P
skeleton bearing carbene central donor. A PC P pincer ligand
was shown in 2017 to exhibit higher electron donation
stemming from its higher σ-donating ability in dinitrogen
reduction catalyzed by PC P molybdenum complexes. Thus,
we envisioned that the introduction of a more electron-
donating moiety, a carbene donor, into a tridentate pincer
skeleton would lead to a dramatic improvement in the catalytic
performance in CO2 hydrogenation. In this report, we
developed PC P-Ir complexes bearing a carbene as a neutral
donor at the center of a pincer-type tridentate ligand. In the
II
23
chloride to afford a dearomatized PN(amide)P Ir complex.
II
35
The structures of 1-Cl and 2-Cl were unambiguously
characterized by X-ray single-crystal diffraction analyses
(Figure 1). Complexes 1-Cl and 2-Cl both have square-planar
coordination geometries. Complex 1-Cl has almost the same
Ir−Cl bond length as complex 2-Cl. In contrast, complex 1-Cl
showed a significantly shorter Ir−carbene bond length than 2-
Cl by 0.06 Å (1-Cl, 1.889(5) Å; 2-Cl, 1.959(4)/1.943(5)/
II
37
investigation of hydrogenation of CO with triethanolamine
catalyzed by PC P-Ir complexes, an improved turnover
number was achieved by PC P-Ir complexes in comparison
to a pyridine-based PNP pincer iridium complex.
1.957(4) Å ). This shorter Ir−C bond in 1-Cl than in 2-Cl is
probably because of the stronger π back-donation from Ir to
the carbene carbon in 1-Cl. In 1-Cl, filled d orbital on iridium
is effectively overlapped with a vacant p orbital on carbene,
which can be confirmed in the X-ray structure as a smaller
torsion angle between the NHC plane and Ir-coordination
plane in 1-Cl than in 2-Cl (1-Cl, 2.55°; 2-Cl, 44.2°; see also
the NBO analysis section).
2
II
II
RESULTS AND DISCUSSION
Synthesis and Characterization of PC P-Ir and -Ir
Complexes. The novel PC P-Ir monochloride complexes 1-
Cl and 2-Cl were prepared according to Scheme 2a,b.
Complexation of the PCP ligand precursors 1,3-bis(di-tert-
■
II
I
III
II
Previously, we reported that the number of chloride ligands
and hydride ligands on the iridium(III) catalyst precursor
B
DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
addition of HCl, the tilt angle between the NHC plane and
Ir coordination plane was only slightly changed from 44.2° in
2
-Cl to 42.0° in 2-HCl , 1-HCl exhibited a significantly larger
2 2
tilt angle in comparison to 1-Cl (2.55° in 1-Cl to 16.1° in 1-
HCl ). This is presumably because the strong π back-bonding
2
interaction in 1-Cl is greatly weakened on oxidation of Ir(I) to
Ir(III) (see also the NBO analysis section). The trichloride
complex 1-Cl and iridium trihydride complex 1-H were also
3
3
successfully synthesized by treatment of 1-Cl with hexachloro-
ethane or with LiHBEt under 2.5 MPa of dihydrogen,
3
38
respectively. In contrast to the clean formation of all these
iridium(III) complexes, 1-HCl , 1-Cl , and 1-H , the
2
3
3
attempted oxidative addition of dihydrogen to 1-Cl resulted
in a low conversion of 3.3% (see the Supporting Information
for details).
Theoretical Studies on Comparison of PCP and PNP
Complexes. We employed theoretical calculations using DFT
and NBO analysis for complexes 1-Cl, 1-H Cl, 3-Cl, 3-H Cl,
2
2
and 2-Cl in order to estimate the strength of σ donation, π
back-donation, and electron density on the iridium in PCP-Ir
and PNP-Ir complexes. Calculations were performed with the
Gaussian 09 package at the M06L level using SDD basis sets
Figure 1. Molecular structures and structural parameters of complexes
1
-Cl (left) and 2-Cl (right). Thermal ellipsoids are drawn with 50%
probability. Hydrogens and cocrystallized solvents are omitted for
clarity.
(
and associated effective core potentials) for transition metals
III
and 6-311+G** basis sets for all light elements in the gas phase
at 298.15 K. First, geometry optimization and free energy
calculations revealed that oxidative addition of dihydrogen to
strongly affected the turnover number in PNP-Ir -catalyzed
2
4
III
reactions. Therefore, we synthesized PCP-Ir complexes
with different numbers of chloride ligand or hydride ligand on
iridium: complexes 1-HCl , 1-Cl , 1-H , and 2-HCl were
synthesized from PCP-Ir complexes 1-Cl and 2-Cl as shown in
Scheme 2c,d. Oxidative addition of HCl to complexes 1-Cl and
-Cl afforded Ir(III) hydride dichloride complexes 1-HCl and
-HCl as the sole reaction products. 1-HCl was stable
1
-Cl is endergonic by 13 kcal/mol while that to PNP-Ir-Cl (3-
2
3
3
2
I
Cl) is exergonic by −17 kcal/mol (Scheme 3), which is in
Scheme 3. Theoretical Calculation for Oxidative Addition of
H to an Ir(I) Complex
2
2
2
2
2
2
enough to be fully characterized, while 2-HCl2 gradually
converted to a hardly soluble unknown complex upon standing
in THF solution. Both 1-HCl and 2-HCl were successfully
2
2
characterized by X-ray crystallography. X-ray structures of
complexes 1-HCl and 2-HCl and the previously reported
2
2
PNP-Ir hydride dichloride complex 3-HCl are shown in
2
Figure 2 with their structural parameters. All of these three
complexes have a hydride ligand at the coordination site cis to
the carbene or pyridine donor. Complexes 1-HCl and 2-HCl2
2
show significantly longer Ir−Cl bond lengths in comparison to
3
-HCl , reflecting the stronger σ donation of carbene ligands
2
in comparison to pyridine (2.4602(9) , 2.4408(16)/
3
7
37
2
.4451(17), and 2.300(3)/2.327(2) Å for 1-HCl , 2-
2
HCl , and 3-HCl respectively). While, upon oxidative
2
2,
Figure 2. X-ray structures and structural comparison among 1-HCl , 2-HCl , and 3-HCl . Thermal ellipsoids are drawn with 50% probability.
2
2
2
Cocrystallized solvents are omitted for clarity.
C
DOI: 10.1021/acs.organomet.8b00377
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Article
good agreement with the observed low conversion of oxidative
addition of dihydrogen to 1-Cl (see Schemes S1 and S2 in the
Supporting Information). This strong preference of low
oxidation state with PCP ligand could be attributed to the
Catalytic Hydrogenation of Carbon Dioxide. The
hydrogenation of carbon dioxide catalyzed by PCP-Ir
complexes 1-Cl, 2-Cl, 1-HCl , and 1-Cl was examined by
2
3
36
using triethanolamine as a base in water at 120 °C (Table 2).
2
stronger stabilization of occupied Ir dyz and dz orbitals by
back-donation to the vacant p orbital on the carbene carbon in
the PC P complex (1-Cl), which is suggested by NBO second-
Table 2. Catalytic Hydrogenation of CO by Ir Complexes
2
II
order perturbation analyses (the results are summarized in
Table 1). Both PCP-Ir complexes show much stronger π back-
Table 1. Results of Theoretical Calculations
1
-Cl
1-H
2
Cl
3-Cl
12.2
3-H
2
Cl
2-Cl
95.3
a
back-donation
kcal/mol)
torsion angle (deg) 3.37
NCN and PCP,
137
39.0
2.13
(
10.6
15.4
12.9
40.8
(
CNC and PNP)
Ir−carbene or −N
1.89685
2.46882
2.02702
1.66154
2.03328
2.39841
2.20021
1.59877
1.95029
2.48858
bond length (Å)
Ir−Cl or −H
(trans to carbene
or N) bond
length (Å)
a
1
TON based on H NMR analysis using sodium 3-(trimethylsilyl)-1-
NBO charge
−0.597
−1.09
−0.532
−0.957
−0.619
b
propanesulfonate as an internal standard. Trioctylamine (5.0 mmol),
THF (2.5 mL), and H O (2.5 mL) were used.
a
Electron transfer from the d orbital on the metal to the empty p
orbital on the ligand:
2
Each entry (other than entry 10) shows the average value of
more than two trials, and standard errors are given after the
turnover numbers. The catalytic reaction was performed with
complete exclusion of any metal impurities or carbon
monoxide for better reproducibility (see section 3 in the
Supporting Information for experimental details). Complex 1-
Cl showed a higher TON in comparison to complex 2-Cl in 2
h (entries 1 and 2, TON: 1-Cl 8700, 2-Cl 3500). While further
investigation is required to clarify the reason for the lower
activity of 2-Cl, the decomposition to an unknown insoluble
complex, which was observed in the oxidative addition of HCl
to 2-Cl, could have suppressed the catalytic activity. When
2
donation from iridium d and d orbitals to a carbene p-type
yz
z
orbital in comparison to that of a PNP-Ir complex to a pyridine
π* orbital (stabilization energy: 137 kcal/mol in 1-Cl, 95.3
kcal/mol in 2-Cl, and 12.2 kcal/mol in 3-Cl). In comparison to
imidazolylidene complex 2-Cl, benzo-fused imidazolylidene
complex 1-Cl displayed a much stronger π back-bonding
interaction, probably because of the larger overlap of iridium
d and d orbitals and the carbene p orbital accompanied by
smaller torsion between the NHC plane and Ir coordination
plane. Upon oxidation to Ir(III) (1-H Cl), the torsion angle
in 1-Cl was significantly increased (3.37° to 10.6° in the
calculated structure) along with a decrease in π back-donation
from the iridium center, which is the same trend as oxidative
addition of HCl to 1-Cl observed experimentally (torsion angle
.55° in 1-Cl to 16.1° in 1-HCl ) (Figures 1 and 2). The bond
distance between Ir and carbene in 1-Cl is shorter than that in
-Cl (1-Cl, 1.89685 Å; 2-Cl, 1.95029 Å). These calculated
II
complexes bearing a PC P ligand (1) were employed, the
increase of chloride ligands on iridium resulted in a higher
TON to achieve 30000 turnovers in 2 h with 1-Cl
(compare
3
entries 1, 3, and 4). The effect of the chloride ligand on
24
2
z
catalytic activity was also observed in our previous research,
yz
although the reason is still unclear at this moment. On
3
5
comparison of these results with PNP-Ir trihydride complex 3-
2
II
H
, PC P complexes 1-HCl
and 1-Cl
achieved significantly
3
2
3
higher TON than 3-H
(entries 1 and 3−5). In 46 h, the
3
highest turnover number of 230000 was accomplished by 1-Cl
among the examined complexes (entries 6−9). Although
significant deceleration was observed, 2-Cl and 1-Cl still
2
3
2
achieved higher TONs (62000 and 130000, respectively, in
2
entries 7 and 8) than 3-H in 46 h (entry 9, TON 54000). This
3
bond lengths are consistent with experimental data (Figure 1
and Table 1). PC P-Ir complexes 1-Cl and 2-Cl have longer
Ir−Cl bonds in comparison to the corresponding PNP-Ir
complex 3-Cl (1-Cl, 2.46882 Å; 2-Cl, 2.48858 Å; 3-Cl,
result indicates that PCP-Ir complexes are more robust than
PNP-Ir complexes and, among them, complex 1-Cl is the most
robust, enough under these reaction conditions to keep its
activity over 46 h. The significant increase in the reaction rate
along with the reaction progress using 1-Cl as a catalyst (TOF
4400 in the initial 2 h to 5000 in 46 h) suggests that the active
Ir(III) species was gradually generated in situ, presumably via
the slow oxidative addition of dihydrogen. In addition to
triethanolamine, trioctylamine was also examined as a base to
II
2
.39841 Å). This implies that the carbene in the PCP-Ir
complex has strong electron-donating ability. Significantly
larger negative NBO charges on the iridium centers in 1-Cl
and 1-H Cl in comparison to 3-Cl and 3-H Cl (1-Cl, −0.597;
2
2
1
-H Cl, −1.09; 3-Cl, −0.532; 3-H Cl, −0.957) also support
2
2
the stronger electron donation from the carbene carbon to the
iridium center in comparison to that from pyridine nitrogen in
both Ir(I) and Ir(III) states.
show that complex 1-Cl is also active in CO hydrogenation
2
with trioctylamine, albeit with lower activity (entry 10).
Although the initial turnover frequency is not comparable to
D
DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
that of the ruthenium-catalyzed system reported by Pidko and
co-workers, complex 1-Cl exhibited one of the highest catalytic
turnover numbers (230000) in the reported hydrogenation of
time. The yield of ammonium formate was determined by H NMR in
2
standard.
D O with sodium 3-(trimethylsilyl)-1-propanesulfonate as an internal
Preparation of [1,3-Bis(di-tert-butylphosphinomethyl-
κP,P′)benzimidazol-2-ylidene-κC]chloridoiridium (1-Cl). To a
mixed solid of 1,3-bis(di-tert-butylphosphinomethyl)benzimidazolium
hexafluorophosphate (425 μmol, 247 mg) and potassium bis-
CO with organic bases (e.g., Pidko’s PNP-ruthenium catalyst
2
−
1
15a
(
TOF 1100000 h in 2 min and TON 200000) or Jessop’s
RuCl(OAc)(PMe ) (TOF 95000 h in 0.3 h and TON
2000) ).
−
1
3
4
1
4b
3
(
trimethylsilyl)amide (553 μmol, 110 mg) was added toluene (5.0
mL), and the resulting suspension was stirred for 2 h at room
temperature. The insoluble salt that formed was filtered off through a
pad of Celite, and the solvent was removed under reduced pressure.
To a solution of the residual solid in THF (5.0 mL) was added
bis(cyclooctene)chloridoiridium dimer (212 μmol, 190 mg) in THF
CONCLUSION
■
II
In conclusion, PC P-Ir complexes bearing a tridentate P-
C(carbene)-P ligand were synthesized and used for catalytic
hydrogenation of carbon dioxide with organic bases. The
(
5.0 mL). The reaction mixture changed from dark yellow to yellow
II
stronger electron-donating ability of the PC P ligand in
during the following 12 h with vigorous stirring at room temperature.
After the reaction, the solvent was removed under vacuum. The
residue was recrystallized from THF/pentane to afford the desired
comparison to the PNP ligand was confirmed by X-ray
crystallography and theoretical calculations. DFT calculations
and NBO analysis revealed that strong π back-donation as well
as σ donation exists in PCP-iridium complexes and made the
complex 1-Cl as yellow crystals (238 mg, 85% yield).
1
H NMR (THF-d , 400 MHz): δ 1.45 (vt, apparent J = 6.6 Hz,
8
II
I
catalyst robust. By use of the newly synthesized PC P-Ir
36H), 3.83 (vt, apparent J = 2.3 Hz, 4H), 6.99 (dd, J = 5.8 Hz, 3.1 Hz,
3
1
1
2
H), 7.12 (dd, J = 5.7 Hz, 3.2 Hz, 2H). P{ H} NMR (THF-d , 162
monochloride complex 1-Cl, a catalytic turnover of 230000
was achieved. The improved reactivity in comparison to the
PNP-Ir system could be attributed to the stabilization of the
cationic key transition state by the higher electron-donating
8
1
3
MHz): δ 77.9. C NMR (THF-d , 126 MHz): δ 29.9 (vt, apparent
8
JCP = 3.2 Hz, CH ), 36.9 (vt, apparent J = 9.1 Hz, C), 41.8 (vt,
3
CP
apparent JCP = 13.2 Hz, CH ), 109.1 (s, CH), 122.0 (s, CH), 136.3
2
(
vt, apparent JCP = 4.5 Hz, C), 187.2 (t, JCP = 5.9 Hz, carbene). Anal.
II
nature of the PC P ligand.
Calcd for C H N P ClIr: C, 45.34; H, 6.70; N, 4.23. Found: C,
2
5
44
2 2
4
5.58; H, 6.58; N, 4.26.
Preparation of [1,3-Bis(di-tert-butylphosphinoethyl-κP,P′)-
EXPERIMENTAL SECTION
■
imidazolylidene-κC]chloridoiridium (2-Cl). To a mixed solid of
1
General Procedures. All manipulations involving air- and
moisture-sensitive compounds were carried out in an argon-filled
glovebox or using standard Schlenk techniques under purified argon
or nitrogen by passing through a hot column packed with BASF
catalyst R3-11. All of the organic solvents used for reactions were
purified by the method of Pangborn et al. Most of the reagents were
used without further purification unless otherwise specified. NMR
spectra were recorded on 400 and 500 MHz spectrometers (JEOL
JNM-ECS400 or BRUKER Ascend500). Chemical shifts are reported
,3-bis(2-(di-tert-butylphosphino)ethyl)imidazolium chloride (103
μmol, 46.3 mg) and potassium bis(trimethylsilyl)amide (124 μmol,
24.7 mg) was added toluene (3.0 mL), and the resulting suspension
was stirred for 2 h at room temperature. The insoluble salt that
formed was filtered off through Celite. The solvent was removed
under reduced pressure, and to a solution of the residual solid in THF
(2.0 mL) was added bis(ethylene)chloridoiridium dimer (28.9 μmol,
16.4 mg) in THF (5.0 mL). The reaction mixture changed from
colorless to orange during the following 24 h with vigorous stirring.
After the reaction, the solvent was removed under vacuum. The
residue was recrystallized from diethyl ether at −35 °C to afford the
desired complex 2-Cl as yellow crystals (53.7 mg, 82% yield).
39
1
in ppm relative to the residual protiated solvent for H (C D , 7.16
6
6
ppm; CDCl , 7.26 ppm; tetrahydrofuran-d , 1.72 ppm), deuterated
3
8
1
3
solvent for C (C D , 128.06 ppm; CDCl , 77.16 ppm;
6
6
3
3
1
tetrahydrofuran-d , 25.31 ppm), and external 85% H PO for
P
8
3
4
1
nuclei. Data are presented as follows: chemical shift, multiplicity (s =
singlet, d = doublet, t = triplet, vt = virtual triplet, q = quartet, m =
multiplet, br = broad), coupling constant in hertz (Hz), signal area
H NMR (C D , 400 MHz): δ 1.29 (br, 4H), 1.48 (vt, apparent J =
6
6
3
1
1
6.1 Hz, 36H), 3.42 (br, 4H), 5.92 (s, 2H). P{ H} NMR (C D , 162
MHz): δ 35.2. C NMR (C D , 126 MHz): δ 17.9 (vt, apparent J
6
6
1
3
6
6
CP
integration in natural numbers and class of carbon (CH = primary
= 3.2 Hz, CH ), 31.0 (s, CH ), 36.5 (vt, apparent J = 10 Hz, C),
3
2 3 CP
carbon, CH2 = secondary carbon, CH = tertiary carbon, C =
quaternary carbon, carbene). IR spectra were recorded on a Shimadzu
FTIR-8400 instrument. Elemental analyses were performed by the
Microanalytical Laboratory, Department of Chemistry, Graduate
School of Science, The University of Tokyo, or by the One-stop
Sharing Facility Center for Future Drug Discoveries, Graduate School
of Pharmaceutical Sciences, The University of Tokyo. X-ray
crystallographic analyses were performed on a Rigaku Varimax with
a Saturn diffractometer. The following compounds were used as
received: triethanolamine (Kanto Chemical Co. Inc. (Kanto)),
potassium bis(trimethylsilyl)amide (Aldrich), 4.0 M HCl in dioxane
49.2 (vt, apparent JCP = 3.2 Hz, CH ), 116.6 (s, CH), 157.9 (t, J
=
CP
2
10 Hz, carbene). Anal. Calcd for C H N P ClIr: C, 43.15; H, 7.24;
2
3
46
2 2
N, 4.38. Found: C, 43.23; H, 6.98; N, 4.12.
Preparation of [1,3-Bis(di-tert-butylphosphinomethyl-
κP,P′)benzimidazol-2-ylidene-κC]hydridodichloridoiridium
(1-HCl ). To a solution of 1-Cl (102 μmol, 67.6 mg) in THF (1.0
2
mL) was added 4.0 M HCl in dioxane (4.0 mmol, 1.0 mL), and the
reaction mixture was allowed to stand for 10 s at room temperature.
The desired complex 1-HCl
as a colorless solid (34.6 mg, 49% yield).
was reprecipitated from THF/pentane
2
1
H NMR (CDCl , 400 MHz): δ −20.0 (t, JHP = 14 Hz, 1H), 1.35
3
(
(
Aldrich), hexachloroethane (Tokyo Chemical Industry Co. Ltd.
TCI)), lithium triethylborohydride (Kanto). The following com-
(vt, apparent J = 6.7 Hz, 18H), 1.66 (vt, apparent J = 7.1 Hz, 18H),
4.08 (dvt, JHH = 13 Hz, apparent JPH = 2.5 Hz, 2H), 4.37 (d, JHH = 13
Hz, 2H), 7.24 (dd, J = 5.9 Hz, 3.2 Hz, 2H), 7.31 (dd, J = 5.7 Hz, 3.1
pounds were synthesized according to the literature procedures:
bis(cyclooctene)chloridoiridium dimer,
chloridoiridium dimer,
benzimidazolium hexafluorophosphate (1-H ), 135,3-bis(2-(di-tert-
4
0a
31
1
13
bis(ethylene)-
Hz, 2H). P{ H} NMR (C D , 162 MHz): δ 53.0. C NMR
6 6
4
0b
1,3-bis(di-tert-butylphosphinomethyl)-
(CDCl , 126 MHz): δ 29.4 (s, CH ), 30.6 (s, CH ), 37.2 (vt,
3 3 3
apparent JCP = 12 Hz, C), 39.9 (vt, apparent JCP = 7.7 Hz, C), 43.0
+
35
+
butylphosphino)ethyl)imidazolium chloride (2-H ).
(vt, apparent JCP = 11.4 Hz, CH ), 110.7 (s, CH), 123.5 (s, CH),
2
Standard Procedure for Hydrogenation of Carbon Dioxide.
The catalyst (20 μmol) was dissolved in THF (10.0 mL) and diluted
to 0.10 μmol/mL. In the 50 mL stainless autoclave equipped with a
glass liner, solutions of the catalyst (100 μL, 0.010 μmol) and
degassed aqueous triethanolamine (1.00 M, 5.00 mL) were charged,
and the autoclave was pressurized with CO (2.5 MPa) and H (2.5
134.1 (vt, apparent J = 3.2 Hz, C), 179.2 (s, carbene). IR (ATR,
CP
−
1
cm ) ν
2303. Anal. Calcd for C H N OP Cl Ir (1-HCl ·THF):
Ir−H
29 53 2 2 2 2
C, 45.19; H, 6.93; N, 3.63. Found: C, 44.96; H, 6.94; N, 3.54.
Preparation of [1,3-Bis(di-tert-butylphosphinomethyl-
κP,P′)benzimidazol-2-ylidene-κC]trichloridoiridium (1-Cl ).
3
To a solution of 1-Cl (106 μmol, 70.2 mg) in THF (5 mL) was
added hexachloroethane (162 μmol, 38.4 mg). After the mixture was
2
2
MPa). The reaction mixture was stirred at 120 °C for an appropriate
E
DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
stirred at room temperature for 24 h, the solvent was removed under
Yoshiaki Nishibayashi: 0000-0001-9739-9588
Kyoko Nozaki: 0000-0002-0321-5299
reduced pressure. The desired complex 1-Cl was reprecipitated from
3
THF/pentane as a yellow solid (46.9 mg, 60% yield).
1
H NMR (CDCl , 400 MHz): δ 1.53 (vt, apparent J = 6.9 Hz,
Notes
3
3
5
6H), 4.57 (s, 4H), 7.32 (dd, J = 5.7 Hz, 3.1 Hz, 2H), 7.43 (dd, J =
The authors declare no competing financial interest.
3
1
1
13
.9 Hz, 3.2 Hz, 2H). P{ H} NMR (CDCl , 162 MHz): δ 27.4.
C
=
3
NMR (THF-d , 126 MHz): δ 31.5 (s, CH ), 40.6 (vt, apparent J
8
3
CP
ACKNOWLEDGMENTS
6
1
.8 Hz, C), 44.2 (vt, apparent JCP = 11.8 Hz, CH ), 111.6 (s, CH),
■
2
24.0 (s, CH), 135.2 (vt, apparent JCP = 2.7 Hz, C), 172.4 (s,
A part of this work was conducted in the Research Hub for
Advanced Nano Characterization, The University of Tokyo,
supported by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT; Japan). The computations
were performed using the Research Center for Computational
Science, Okazaki, Japan. A part of this work was supported by
grants-in-aid for Research on Innovative Areas “Precise
Formation of a Catalyst Having a Specified Field for Use in
Extremely Difficult Substrate Conversion Reactions” (Nos.
JP15H05796 and JP15H05798) from MEXT. A part of this
work was supported by a grant from the Sumitomo foundation.
carbene). Anal. Calcd for C H N OP Cl Ir (1-Cl ·THF): C, 43.26;
H, 6.51; N, 3.48. Found: C, 43.36; H, 6.54; N, 3.40.
2
9
52
2
2
3
3
Preparation of [1,3-Bis(di-tert-butylphosphinomethyl-
κP,P′)benzimidazol-2-ylidene-κC]trihydridoiridium (1-H ). To
3
a solution of 1-Cl (73.1 μmol, 48.5 mg) in THF (5 mL) was added
lithium triethylborohydride (0.99 M THF solution, 1.43 mmol, 1.44
mL), and the reaction mixture was pressurized with 2.5 MPa of H in
a 50 mL stainless autoclave. After the mixture was stirred at 60 °C for
2
2
.5 days, the solvent was removed under vacuum. The product was
extracted with Et O from the residual solid and filtered through a pad
2
of Celite. After the solvent was removed under reduced pressure, the
residue was dissolved in THF and passed through a pad of silica gel.
Recrystallization from diethyl ether at −35 °C afforded the desired
complex 1-H as red crystasl (20.0 mg). In spite of the repetitive
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AUTHOR INFORMATION
Corresponding Authors
E-mail for Y.N.: ynishiba@sys.t.u-tokyo.ac.jp.
E-mail for K.N.: nozaki@chembio.t.u-tokyo.ac.jp.
ORCID
Satoko Takaoka: 0000-0001-6207-8151
Aya Eizawa: 0000-0001-6713-1871
Shuhei Kusumoto: 0000-0002-4743-4784
Kazunari Nakajima: 0000-0001-9892-5877
■
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2
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DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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i
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i
hydrogenation of CO by PNP-Ir complexes, the complex that had Pr
2
groups on phosphorus atoms exhibited a higher turnover number in
t
comparison to the complex that had Bu groups on phosphorus atoms.
i
However, unfortunately, we were unable to isolate Pr-PCP-Ir
t
complex. Instead, we evaluated the catalytic activity by using Bu-
PCP-Ir complexes.
(
37) When one crystal lattice consists of multiple inequivalent
complexes, every bond distance is shown.
38) The trihydride complex decomposed to an unknown complex
(
in a solution state although no reductive elimination of dihydrogen
was observed, preventing further investigation in catalytic reaction.
Details are given in the Supporting Information.
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2
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2
I
DOI: 10.1021/acs.organomet.8b00377
Organometallics XXXX, XXX, XXX−XXX