Selected paper development of highly active IrPNP catalysts for hydrogenation of carbon dioxide with organic bases

Selected Paper

Development of Highly Active Ir-PNP Catalysts for Hydrogenation of

Carbon Dioxide with Organic Bases

Wataru Aoki, Natdanai Wattanavinin, Shuhei Kusumoto, 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

E-mail: nozaki@chembio.t.u-tokyo.ac.jp

Received: September 11, 2015; Accepted: October 2, 2015; Web Released: January 15, 2016

Kyoko Nozaki

Kyoko Nozaki, born in Osaka received her B.Sc. in 1986 and her Ph.D. in 1991 from Kyoto University under

the guidance of Professor Kiitiro Utimoto. During her Ph.D. study, she joined Professor Clayton H.

Heathcock’s group at the University of California at Berkeley as an exchange student. In 1991, she started

her research career as an instructor at Kyoto University, became an associate professor in 1999, and then

moved to the University of Tokyo as an associate professor in 2002. Since 2003, she is a full professor at the

University of Tokyo. Her research interest is focused on development of homogeneous catalysts for polymer

synthesis and organic synthesis.

Abstract

exoergonic, by 4 kJ mol^¹1, in aqueous solution (eq 2).^2-6Bases

such as KOH, NaHCO₃, and NR₃are therefore used to make

the reaction thermodynamically more favourable (eq 3)^2-6and

to accelerate it kinetically.^7-11Recently, homogeneous hydro-

genation of carbon dioxide with various transition-metal cata-

lysts (Ir,^2-6,8,12-24Rh,^2-6,14,15,25Ru,^{2-6,8-11,14-16,26-38}Co,^39-43

Fe,^{2-6,38,41,44-46}and others^2-6,46) has been extensively studied.

Among the catalysts developed for CO₂hydrogenation, the

trihydridoiridium-PNP pincer complex A-H₃showed the high-

est turnover number (TON) for formic acid production using

KOH as the base (Scheme 2).¹²

The use of strong bases, however, requires an additional

process, neutralization of the resulting formate salt with strong

acid, to isolate formic acid; this is accompanied by forma-

tion of undesired inorganic salts (Scheme 1, top).¹¹Formic

acid can be isolated by simple distillation from ammonium

formate, therefore the development of a highly active cata-

lyst for hydrogenation of carbon dioxide in the presence of

less volatile amine bases is desirable for industrial applica-

tions (Scheme 1, bottom).²⁹After several decades of intensive

research on catalysts, a signiﬁcant improvement was made by

Methoxy-substituted PNP-iridium(III) complexes and pyr-

azine-based PNP-iridium(III) complexes were developed and

used to hydrogenate carbon dioxide in the presence of

triethanolamine as a base. The methoxy-substituted PNP-

hydridodichloridoiridium complex (C-HCl₂) showed the high-

est turnover number, 160000; this is the highest value ever

reported with an organic base in an aqueous medium. The

reactivities of these complexes, derived from their ligand

modiﬁcation, were further studied. The results were as follows.

(i) The pyrazine-based PNP-trihydridoiridium complex under-

goes release of dihydrogen to aﬀord dihydridoamido complex,

possibly because of easy dearomatization of the pyrazine ring.

This process was reversible, i.e., B-H₂amido can readily be

converted back to B-H₃on exposure to dihydrogen. (ii) The

p-methoxy-substituted dihydridochlorido complex showed fac-

ile disproportionation of the chloride anion on the iridium

center; this is attributed to the electron-donating nature of the

methoxypyridine backbone.

Carbon dioxide is a cheap, abundant, nontoxic, but less

reactive carbon resource, compared with carbon monoxide, one

of the most widely used C1 carbon sources. Catalytic hydro-

genation to produce formic acid is a promising approach to

CO₂use, and has been intensively studied as an alternative to

the reaction of carbon monoxide with water, which is generally

used in current industrial processes.¹The reaction of carbon

dioxide with dihydrogen to produce formic acid is endoergonic

CO₂

+

H₂

HCOOK neutralization

+H₂O

KOH

HX

KX

HCOOH

CO₂

+

H₂

[HCOO] distillation

[HNR₃]

NR₃

¹1

by 33 kJ mol in the gas phase (eq 1) and only slightly

Scheme 1. Production and isolation of formic acid.

Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

1.0 M Base aq. (5.0 mL)

CO₂(2.5 MPa)

+

H₂(2.5 MPa)

cat. A-H₃(0.010 mmol),

HCOO·HBase

THF (0.1 mL), 200°C, 2 h

H

Ir

N

H

ⁱPr₂P

PⁱPr₂

TOF

150000 h^–1(Base: KOH)

14000 h^–1(Base: triethanolamine)

H

A-H₃

Scheme 2. CO₂hydrogenation catalyzed by A-H₃.

Y

Ir

Z

ⁱPr₂P

PⁱPr₂ⁱPr₂P

PⁱPr₂

ⁱPr₂P

PⁱPr₂

X

N

A~C-H₃:

X = Y = Z = H

A~C-H₂Cl:

A~C-HCl₂:

C-Cl₃:

OMe

C

X = Y = H, Z = Cl

X = H, Y = Z = Cl

X = Y = Z = Cl

A

B

Figure 1. Ligands and iridium complexes used in this study for catalytic CO₂hydrogenation.

Jessop et al.; they developed [RuCl(OAc)(PMe₃)₄], which dis-

phinomethyl)pyridine) trihydrido complex A-H₃and di-

¹1

played a high initial turnover frequency (TOF, 95000 h in

hydridochlorido complex A-H₂Cl were synthesized using a

previously reported method.¹²One hydride in A-H₂Cl can be

replaced by a chloride anion by reaction with excess chloro-

form at room temperature, to aﬀord the hydridodichlorido

complex A-HCl₂. All these complexes bearing pyridine-based

ligands (complexes A-H₃-A-HCl₂) were highly stable against

the loss of dihydrogen and ligand disproportionation (discussed

in the following chapter) and no changes were observed after

several days in tetrahydrofuran (THF) under argon at room

temperature. The pyrazine-based dihydridochlorido complex

B-H₂Cl was synthesized in four steps from commercially

available 2,6-dimethylpyrazine (2; analogous to the procedure

for A-H₂Cl, Schemes 3 and 4). Treatment of B-H₂Cl with

chloroform resulted in formation of dichlorido complex B-

HCl₂. Although nuclear magnetic resonance (NMR) spectros-

copy conﬁrmed the formation of trihydrido complex B-H₃

when B-H₂Cl was treated with excess sodium hydride, B-H₃

was not stable and the reaction always resulted in formation of

a mixture of B-H₃and B-H₂amido dihydrido complexes after

release of one dihydrogen molecule. Complex C-H₂Cl, bear-

ing a PNP ligand with a methoxy group, was also synthesized

from the methoxy-substituted ligand using a similar procedure

for A-H₂Cl and B-H₂Cl shown in Schemes 3 and 4. A-H₂Cl

and B-H₂Cl were stable in solution and could be isolated by

single recrystallization, but the crystals of C-H₂Cl obtained

by recrystallization from THF/hexane were always contami-

nated by a small amount of dichlorido complex C-HCl₂. The

formation of C-HCl₂resulted from disproportionation of the

dihydridochlorido complex (C-H₂Cl) to aﬀord the mono-

hydridodichlorido complex (C-HCl₂) and trihydrido complex

(C-H₃) on standing in solution (vide infra). The trihydrido com-

plex C-H₃was prepared by treatment of C-H₂Cl with sodium

hydride. C-H₃, in contrast to B-H₃, is stable against dihydro-

20 min) with triethylamine in supercritical carbon dioxide.³⁵

Recently, [Ru(PNP)(H)(Cl)CO] was reported to show a high

¹1

initial TOF (1100000 h in the initial stage, TON 200000) in

the presence of DBU as a base.³⁰Further improvement of the

catalytic activity, however, remains a challenge, because the

activities recorded with organic bases are much lower than

those achieved using inorganic strong bases. For example,

¹1

the high TOF (150000 h in 2 h) we recently reported with

the trihydridoiridium complex A-H₃in the presence of KOH

decreased signiﬁcantly when triethanolamine was used as the

base (Scheme 2).^12,13

In this study, we developed pyrazine-based PNP and

methoxy-substituted PNP ligands B and C, and their iridium

complexes (Figure 1). Iridium complexes containing ligands

A-C and various numbers of hydride and chloride ligands

were examined in hydrogenation of carbon dioxide. The

methoxy-substituted PNP-hydridodichloridoiridium (C-HCl₂)

showed the highest activity under the examined reaction con-

ditions using triethanolamine as the base.

CO₂ðgÞ þ H₂ðgÞ ! HCOOHðlÞ

ꢀGð298KÞ ¼ 33 kJ mol^ꢀ1

ð1Þ

ð2Þ

ð3Þ

CO₂ðaqÞ þ H₂ðaqÞ ! HCOOHðaqÞ

ꢀGð298KÞ ¼ ꢀ4 kJ mol^ꢀ1

CO₂ðaqÞ þ H₂ðaqÞ þ NH₃ðaqÞ ! HCOO=H₄NðaqÞ

ꢀGð298KÞ ¼ ꢀ35 kJ mol^ꢀ1

Results and Discussion

Synthesis of Complexes.

The synthetic schemes for

the PNP pincer-type iridium complexes A-C-H₃, A-C-H₂Cl,

A-C-HCl₂, and C-Cl₃are summarized in Schemes 3 and 4.

The pyridine-based PNP ligand 1 (2,6-bis(diisopropylphos-

114 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

BH₃

ⁱPr₂P

BH₃

PⁱPr₂

LiPⁱPr₂

Cl

ⁱPr₂P

PⁱPr₂

BH₃

(2.3 equiv)

morpholine

(neat)

NCS (2.3 equiv)

BPO (0.03 equiv)

N

THF,12 h

–78 °C to rt

CCl₄, reflux, 1 d

N

90 °C, 15 h

N

5

2

3

4

n

1) BuLi (2.1 equiv)

TMEDA (2.2 equiv)

2) ClPⁱPr₂(2.2 equiv)

ⁱPr₂P

PⁱPr₂

(MeO)₂SO₂(1.0 equiv)

K₂CO₃(2.9 equiv)

N

acetone, reflux, 2 h

THF/hexane, 21 h

–78 °C to rt

OH

OMe

6

7

8

Scheme 3. Synthesis of ligands (BPO; benzoyl peroxide. NCS; N-chlorosuccinimide. TMEDA; N,N,N¤,N¤-tetramethylethylene-

diamine).

H

Cl

H₂(2.5–3.0 MPa)

ⁱPr₂P

PⁱPr₂

ⁱPr₂P

Ir

PⁱPr₂

[IrCl(coe)₂]₂

(0.35–0.45 equiv)

H

N

X

N

THF, 80 °C, 7–12 h

X

1 (X = CH)

A-H₂Cl (X = CH)

B-H Cl

C-H₂Cl (X = C–OMe)

5

(X = N)

8 (X = C–OMe)

(X = N)

2

H

Ir

H

Ir

H

Ir

N

ⁱPr₂P

PⁱPr₂

ⁱPr₂P

PⁱPr₂

ⁱPr₂P

PⁱPr₂

NaH (excess)

THF, rt, 1 d

H

N

or

+

X

N

A-H₃(X = CH)

C-H₃(X = C–OMe)

B-H₃

B-H₂amido

(37 : 63)

(77 : 23 [H₂, 5.0 MPa, rt, 16 h])

Cl

Ir

ⁱPr₂P

Ir

PⁱPr₂

ⁱPr₂P

PⁱPr₂

C₂Cl₆(1.1 equiv)

Cl

A-H₂Cl

B-H₂Cl

N

H

[IrCl(coe)₂]₂

(0.5 equiv)

CHCl₃

N

C-H₂Cl

8

THF, 65 °C, 1 d

X

OMe

A-HCl₂

(X = CH)

C-Cl₃

B-HCl₂(X = N)

C-HCl₂

(X = C–OMe)

Scheme 4. Synthesis of PNP-Ir(III) complexes.

gen release, even when it is exposed to reduced pressure in THF

solution. C-H₂Cl can be cleanly chlorinated by chloroform to

aﬀord the dichlorido complex C-HCl₂. The trichlorido complex

C-Cl₃was also synthesized by the reaction of ligand 8 and an

iridium precursor in the presence of a chlorine source, hexa-

chloroethane. The structures of all these complexes were deter-

the chloride anion in B-H₂Cl and hydride in A-HCl₂, B-HCl₂,

and C-HCl₂occupy the coordination cite cis to the nitrogen.

Comparison of Iridium Complexes. The use of diﬀerent

ligands, i.e., pyridine, pyrazine, and methoxypyridine, resulted

in signiﬁcant changes in the structures and spectroscopic param-

eters of the iridium complexes. The X-ray structures, spectro-

scopic parameters, and calculated natural atomic charges for

complexes A-HCl₂, B-HCl₂, and C-HCl₂are shown in Table 1.

Regardless of the number of chloride anions on the iridium

centre, the hydride signals of the pyrazine-based complexes B-

H₃(¹10.08 and ¹18.95 ppm), B-H₂Cl (¹17.77 and ¹21.37

1

mined using H, ³¹P, and ¹³C NMR, spectroscopies, infrared

(IR) spectroscopy, and electron-spray ionization mass spec-

trometry (See the next section and Supplementary Information).

The structures of B-H₂Cl, A-HCl₂, B-HCl₂, and C-HCl₂were

elucidated by X-ray single-crystal analysis, which revealed that

Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

Table 1. Crystal structures, observed parameters, calculated data, and catalytic activity for A-HCl₂,

B-HCl₂, and C-HCl₂

A-HCl₂

B-HCl₂

C-HCl₂

Crystal structure^a)

1

Hydride in H NMR/ppm

¹21.81

2204

¹0.71690

¹20.44

2214

¹0.71607

¹22.50

2197

¹0.71709

¹1

Ir-H vibration in IR/cm

Natural atomic

charge of Ir^b)

Natural atomic

0.12037

12000

0.12496

8100

0.11946

38000

charge of hydride^b)

TON for hydrogenation

c)

of CO₂

a) Thermal ellipsoids are drawn with 50% probability. Hydrogens on carbon atoms and solvents are

omitted for clarity. Detailed information is available in the Supporting Information. b) Hybrid functional

B3LYP¹¹and Lanl2dz¹²on iridium, and 6-31++G(d,p) on other atoms were used as basis sets for the

calculations. c) Turnover number for hydrogenation of CO₂in one hour (corresponding to the data in

Figure 4, blue bars).

ppm), and B-HCl₂(¹20.44 ppm) were observed at signiﬁcantly

presence of two hydrides on the iridium. The signals were

assigned to B-H₂amido, by comparison with the reported data

for the dihydridoamido complex A-H₂amido.¹²After standing

in a dihydrogen atmosphere at room temperature for 12 h, the

intensities of the ³¹P NMR signal at 41 ppm and the hydride

signals at ¹16.7 and ¹22.5 ppm decreased, accompanied by the

appearance of signals corresponding to B-H₃(Figure 2(ii)), and

the intensities of the signals for B-H₂amido decreased to 20%

of the original level after further 11 h at 55 °C (Figure 2(iii)).

This dihydrogen addition was reversible since B-H₃was cleanly

converted back to B-H₂amido at 80 °C after 28 h in an argon

atmosphere in THF solution (Figures 2(iii)-2(v)). Similar di-

hydrogen addition was also observed with A-H₂amido, but the

resulting trihydrido complex A-H₃was stable against dihydro-

gen release even at reduced pressure.¹²This facile dihydrogen

release from trihydrido complex B-H₃, which sharply con-

trasts with the stability of the corresponding A-H₃and C-H₃

complexes, even under vacuum in solution, is attributed to the

lower aromaticity of pyrazine compared to those of pyridine

and methoxypyridine.⁵³Although the dihydridochlorido com-

plexes A-H₂Cl and B-H₂Cl were isolated in their pure forms by

recrystallization, C-H₂Cl could only be isolated by repeated,

usually three times, recrystallization, resulting in a low yield. To

elucidate the diﬀerent behaviour of C-H₂Cl, the precipitates and

lower ﬁeld compared with those for the corresponding pyridine-

based complexes A-H₃(¹10.31 and ¹20.22 ppm), A-H₂Cl

(¹18.85 and ¹22.18 ppm), and A-HCl₂(¹21.81 ppm). In con-

trast, the hydride signals for the p-methoxy-substituted PNP

complexes C-H₃(¹10.47 and ¹20.53 ppm), C-H₂Cl (¹19.05

and ¹22.57 ppm), and C-HCl₂(¹22.50 ppm) were observed at

slightly higher ﬁeld than those for A-H₃(¹10.31and ¹20.22

ppm), A-H₂Cl (¹18.85 and ¹22.18 ppm), and A-HCl₂(¹21.81

ppm). The IR absorbance of the Ir-H vibration in A-HCl₂, B-

HCl₂, and C-HCl₂showed the same trend as the hydride

signals, i.e., the wavenumber for C-HCl₂(2197 cm^¹1) was

lower than those for A-HCl₂(2204 cm^¹1) and B-HCl₂(2214

cm^¹1); this indicates that the Ir-H bonding energy in C-HCl₂

was the lowest. Diﬀerences among the electronic characters

were also investigated theoretically, using density functional

theory (DFT) calculations.^47-52In natural population analysis,

the natural atomic charge on the iridium centre in B-HCl₂

(¹0.71607) was less negative than that in A-HCl₂(¹0.71690).

The higher electron population on the iridium and larger

negative charge on the hydride in C-HCl₂(¹0.71709 and

0.11946, respectively) are consistent with the electron-donating

ability of methoxypyridine.

Characteristic Behaviours of Complexes B-H₂amido and

C-H₂Cl in Solution. During the preparation of the complexes,

we observed two characteristic behaviours, as brieﬂy described

in the previous section. Representative results are summarized

in Figures 2 and 3. Complex B-H₂amido underwent facile

dihydrogen addition on exposure to a dihydrogen atmos-

1

mother liquids were carefully analysed using H and ³¹P NMR

spectroscopies (Figure 3). Recrystallization from THF/pentane

solution gave crystals of C-H₂Cl contaminated with a small

amount of hydridodichlorido complex C-HCl₂(Figure 3(ii)).

In addition to the original dihydridochlorido complex C-H₂Cl,

the hydridodichlorido complex C-HCl₂and trihydrido complex

C-H₃were observed in the mother liquid (Figure 3(iii)). This

facile disproportionation can be attributed to the strong electron-

donating ability of the methoxypyridine ligand,^54-56that is, the

1

phere in C₆D₆. The ³¹P and H NMR spectra and the reaction

are shown in Figure 2. The signals at 41 ppm in the ³¹P NMR

1

spectrum, and ¹16.7 and ¹22.5 ppm in the H NMR spectrum

(Figure 2(i)) indicate an unsymmetrical PNP structure and the

116 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

H

Ir

H

Ir

H

ⁱPr₂P

PⁱPr₂

ⁱPr₂P

PⁱPr₂

H

1 atm H₂

rt, 12 h

1 atm H₂

55 °C, 11 h

1

THF

N

³¹P, ¹H NMR ³¹P, H NMR ³¹P, ¹H NMR

N

B-H₂amido

B-H₃

(i)

(ii)

(iii)

H

Ir

ⁱPr₂P

PⁱPr₂

degassed H₂

55 °C, 12 h

degassed H₂

80 °C, 28 h

H

N

1

³¹P, H NMR

N

(iv)

(v)

B-H₂amido

Figure 2. Interconversion of B-H₂amido and B-H₃in C₆D₆, monitored by ¹H NMR (hydride region, left) and ³¹P NMR

(right) spectroscopies.

methoxypyridine moiety facilitates the formation of a cationic

intermediate, with release of one anionic ligand, providing an

intermediate for disproportionation of anions on the iridium

centre.

catalytic activity of this catalytic hydrogenation system. This

would be the result of stabilization of the cationic intermediates

C-b and C-d in the proposed catalytic cycle (Scheme 5), as

observed in disproportionation of the chloride anion. Although

the eﬀect of the chloride anion and amines requires further

investigation, the stabilization of cationic intermediate C-d

would lower the TS for the hydrogen splitting (C-d to C-e) in

energy and eventually, the overall energy requirement (¦G^‡)

for this catalysis.⁵⁷Further investigation of the counter anion

showed that use of iodide or acetate resulted in a small drop in

the TON for this catalytic system (see Supporting Information).

The TON of 160000 recorded by C-HCl₂is the second highest

activity ever reported in hydrogenation of carbon dioxide using

amines as a base. The highest reported TON was by a PNP-

Ru complex, Ru-^tBuPNP (TON ca. 200000),³⁰with DBU as a

Hydrogenation of CO₂. These PNP pincer-type iridium

catalysts (A-H₃-C-Cl₃) were used in the hydrogenation of

carbon dioxide in aqueous triethanolamine solution; the high-

est TON, 160000, was achieved with C-HCl₂. Representative

results are summarized in Figure 4. The homogeneity of this

catalytic system was conﬁrmed by a mercury poisoning test

using A-H₃as the catalyst (see the Supporting Informa-

tion). When the three trihydrido complexes A-H₃, [B-H₃+

B-H₂amido (77:23)], and C-H₃were compared, A-H₃gave

the highest TON after 13 h (green bars). The chloride anion

lowered the catalytic activities of the pyridine-based PNP com-

plexes (compare A-H₃with A-H₂Cl and A-HCl₂), and the

TONs for B-H₂Cl and C-H₂Cl were higher than those for the

corresponding trihydrido complexes [B-H₃+ B-H₂amido] and

C-H₃. In the case of the methoxy-substituted PNP complexes,

the TON was further improved when another chloride was

introduced in place of hydride (C-HCl₂, TON = 160000). A

comparison of A-HCl₂, B-HCl₂, and C-HCl₂shows that the

electron-donating ability of the methoxy group enhanced the

i

base. However, Ru-^tBuPNP and the Pr congener Ru-ⁱPrPNP

showed signiﬁcantly lower activities under the aqueous con-

ditions we employed here (TON 4000 for Ru-^tBuPNP and 9000

for Ru-ⁱPrPNP). The introduction of one chloride into B-H₃,

C-H₃, and C-H₂Cl increased the TONs, but further substitution

of hydride with chloride anions resulted in considerable

decreases in the activities (B-HCl₂and C-Cl₃). The TON of

C-HCl₂(38000) was higher than those of A-HCl₂and B-HCl₂

Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

H

Cl

ⁱPr₂P

Ir

PⁱPr₂

H

N

recrystallization with

THF/pentane

recrystallization with

THF/pentane

mother liquid

(iii)

OMe

C-H₂Cl

crude product

(i)

crystal

(ii)

1

Figure 3. Structural changes in C-H₂Cl during recrystallization, monitored by H NMR (hydride region, left) and ³¹P NMR (right)

spectroscopies.

(12000 and 8100, respectively) even in the ﬁrst hour, suggest-

ing that the activity (TOF) itself, not only the robustness, was

improved by the introduction of a methoxy group. The catalyst

activity shows good correlation with the observed and calcu-

lated physical properties listed in the Table 1, namely, the

chemical shift of hydride in ¹H NMR, Ir-H vibration in IR, and

natural atomic charges of Ir and hydride, exhibiting the possi-

bility to design more active catalysts. However, no signiﬁcant

increase was observed for a prolonged reaction time, i.e., 40 h

(Figure 4, red and blue bars) while it did not reach the

equilibrium (see the Supporting Information), indicating that

deactivation is a potential problem for C-HCl₂.

study, C-HCl₂gave the highest TON with an aqueous amine as

a base.

Experimental

General Procedures. All manipulations involving the air-

and moisture-sensitive compounds were carried out in an

argon-ﬁlled glove box or using standard Schlenk technique

under argon puriﬁed by passing through a hot column packed

with BASF catalyst R3-11. All the solvents used for reactions

were distilled under argon after drying over an appropriate

drying agent or passed through solvent puriﬁcation columns.

Most reagents were used without further puriﬁcation unless

otherwise speciﬁed. NMR spectra were recorded on 500 MHz

or 400 MHz spectrometers (JEOL JNM-ECP500, JEOL JNM-

ECS400 and Bruker Ascend^TM500). Chemical shifts are

reported in ppm relative to the residual protiated solvent for

¹H, deuterated solvent for ¹³C, and external 85% H₃PO₄for

³¹P nuclei. Data are presented in the following space: chemical

shift, multiplicity (s = singlet, d = doublet, t = triplet, vt =

virtual triplet, q = quartet, m = multiplet, br = broad), cou-

pling constant in hertz (Hz), and signal area integration in

natural numbers. IR spectra were recorded on Shimadzu FTIR-

8400. [IrCl(coe)₂]₂,⁵⁸py-PNP (1),⁵⁹(py-PNP)IrH₃(A-H₃),¹²

(py-PNP)IrH₂Cl (A-H₂Cl),¹²2,6-bis(chloromethyl)pyrazine

Conclusion

In summary, we developed pyrazine-based PNP com-

plexes B and p-methoxy-substituted PNP complexes C, which

catalyse the hydrogenation of carbon dioxide in the presence of

triethanolamine. These complexes showed distinct behaviours,

namely facile hydrogen release (B-H₃) and disproportionation

of anionic ligands (C-HCl₂). These complexes were used in

catalytic CO₂hydrogenation. The TON recorded by A-H₃was

higher than those by A-H₂Cl and A-HCl₂. The activities of B-

H₃and C-H₃were promoted by replacement of hydride ligands

with chloride ligands. Among the complexes examined in this

118 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

OH

1.0 M

N

aq.(5.0 mL)

3

CO₂+ H₂

HCOO

/HBase

cat. (0.010 mmol), THF (0.1 mL),

150 °C, time

(2.5 MPa) (2.5 MPa)

1

a) Yield of ammonium formate was calculated based on H NMR spectrum in D₂O with sodium

3-(trimethylsilyl)-1-propanesulfonate as an internal standard. Error bar shows standard errors.

H

Cl

H

Cl

H

ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂

H

N

B-H₂amido

A-H₃

A-H₂Cl

A-HCl₂

B-H₃

H

Cl

H

Cl

H

Cl

ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ir

PⁱPr₂

H

N

OMe

C-H₃

OMe

C-H₂Cl

OMe

C-HCl₂

B-H₂Cl

B-HCl₂

CO

Cl

CO

Cl

ⁱPr₂P

Ir

PⁱPr₂ⁱPr₂P

Ru

PⁱPr₂

^tBu₂P

Ru

P^tBu₂

Cl

H

N

Ru-tBuPNP

Ru-iPrPNP

OMe

C-Cl₃

Figure 4. Hydrogenation of carbon dioxide catalyzed by Ir(III) pincer complexes using triethanolamine as base.

(3),^{60 i}Pr₂PH-BH₃,⁶¹(^tBu)PNP-RuHCl(CO),⁶²and (ⁱPr)PNP-

RuHCl(CO)^63,64were synthesized according to the literature.

Standard Procedure for Hydrogenation of Carbon

Dioxide. Catalyst (20 ¯mol) was dissolved in THF (4.0 mL)

and diluted to 0.10 ¯mol mL^¹1. To a 50 mL stainless autoclave,

the solution of catalyst (100 ¯L, 10 nmol) and degassed aque-

ous triethanolamine (1.00 M, 5.00 mL) were charged and

pressurized by CO₂(2.5 MPa) and H₂(2.5 MPa). The reaction

mixture was stirred at 150 °C for 13 h. The yield of ammonium

in vacuo. The residue was washed with THF (5 mL) and

recrystallized from CHCl₃/benzene to give colourless crystal

of A-HCl₂(24 mg, 42 ¯mol, 32%). Single crystals suitable for

X-ray crystallography were obtained by recrystallization from

CHCl₃/hexane at ¹35 °C. ¹H NMR (CDCl₃, 400 MHz): ¤ 7.45

3

(t, J_HH= 8 Hz, 1H, pyr-H4), 7.16 (d, J_HH= 8 Hz, 2H, pyr-

2

H3,H5), 3.78 (d vt, J_HH= 17 Hz, apparent J = 4 Hz, 2H, P-

CH₂-pyr), 3.32 (d vt, ²J_HH= 17 Hz, apparent J = 4 Hz, 2H, P-

CH₂-pyr), 3.16 (m, 2H, P-CH(CH₃)₂), 2.40 (m, 2H, P-

1

3

formate was calculated by H NMR in D₂O with sodium 3-

CH(CH₃)₂), 1.50 (d vt, J_HH= 8 Hz, apparent J = 8 Hz, 6H,

3

(trimethylsilyl)-1-propanesulfonate as an internal standard.

Preparation of (PNP)IrH(Cl)₂(A-HCl₂). A solution of A-

H₂Cl (74 mg, 130 ¯mol) in CHCl₃(5 mL) was stirred at 70 °C

for 20 h. After the reaction time, the solvent was removed

CH(CH₃)₂), 1.42 (d vt, J_HH= 7 Hz, apparent J = 7 Hz, 6H,

3

CH(CH₃)₂), 1.37 (d vt, J_HH= 8 Hz, apparent J = 8 Hz, 6H,

3

CH(CH₃)₂), 1.03 (d vt, J_HH= 7 Hz, apparent J = 7 Hz, 6H,

CH(CH₃)₂), ¹21.81 (t, ²J_PH= 12 Hz, 1H, Ir-H); ¹³C NMR

Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

Cl

Ir

Cl

ⁱPr₂P

PⁱPr₂

H

CO₂

N

Cl

Ir

Cl

Ir

Cl

ⁱPr₂P

PⁱPr₂

O

ⁱPr₂P

HCOO

PⁱPr₂

H

N

C-HCl₂

N

O

C-f

C-a

HCOO

Cl

H

ⁱPr₂P

O

Cl

Ir

PⁱPr₂

O

N

H

Ir

Cl

O

ⁱPr₂P

HCOO

PⁱPr₂

ⁱPr₂P

HCOO

Ir

PⁱPr₂

C-c

N

O

C-e

C-b

H H

Cl

ⁱPr₂P

HCOO

Ir

PⁱPr₂

H₂

HBase

N

Base

O

C-d

Scheme 5. Possible catalytic cycle for CO₂hydrogenation catalyzed by C-HCl₂.

i

(CDCl₃, 126 MHz): ¤ 165.6 (vt, apparent J = 4 Hz, pyr-C2,C6),

135.9 (s, pyr-C4), 120.3 (vt, apparent J = 5 Hz, pyr-C3,C5),

39.9 (vt, apparent J = 12 Hz, P-CH₂-pyr), 25.2 (vt, apparent

J = 13 Hz, P-CH(CH₃)₂), 22.8 (vt, apparent J = 15 Hz, P-

CH(CH₃)₂), 19.9 (m, CH(CH₃)), 18.7 (s, CH(CH₃)), 18.3 (s,

CH(CH₃)), 18.2 (s, CH(CH₃)); ³¹P{¹H} NMR (CDCl₃, 162

MHz): ¤ 29.42; IR (powder, cm^¹1): ¯_Ir-H2204. HRMS(ESI)

[M ¹ Cl]⁺Calcd for C₁₉H₃₆ClIrNP₂: 568.1641. Found:

568.1629.

To a solution of Pr₂PH-BH₃(3.46 g, 26.2 mmol) in THF

n

(40 mL), BuLi (1.64 M in hexane, 16.8 mL, 27.6 mmol) was

added dropwise at ¹78 °C and the mixture was stirred at room

temperature for 13 h. A solution of the chlorinated pyrazine

product 3 (2.07 g, total 11.6 mmol) in THF (18 mL) was added

to the reaction mixture at ¹78 °C and stirred at room tem-

perature for 12 h. After the reaction time, the reaction was

quenched by adding water (0.5 mL) and the solvent was

evaporated in vacuo. Water (30 mL) was added to the mixture

and the product was extracted with CH₂Cl₂(40 mL, three

times). After removal of the solvent in vacuo, reprecipitation of

the residue from hot methanol gave a pale yellow solid 4

(2.20 g, 5.98 mmol, 20% yield from 2,6-dimethylpyrazine).

¹H NMR (500 MHz, CDCl₃): ¤ 8.45 (s, 2H, pyz-H3,H5),

Preparation of pyz-PNP-BH₃(4). pyz-PNP-BH₃(4) was

i

synthesized from 2,6-bis(chloromethyl)pyrazine (3) and Pr₂-

PLi-BH₃according to a similar procedure in literature.^59,65

2,6-bis(chloromethyl)pyrazine (3) was synthesized from

2,6-dimethylpyrazine (2) (3.24 g, 30 mmol) according to the

literature.⁶¹Puriﬁcation with silica-gel column chromatography

(dichloromethane, hexane = 3:1, R_f= 0.21) gave brown oil

(2.07 g, gradually decomposed under air), which contaminated

2-(dichloromethyl)-6-methylpyrazine and the crude product

was used in the next step without further puriﬁcation.

2

3.19 (d, J_PH= 11 Hz, 4H, P-CH₂-pyz), 2.17-2.09 (m, 4H, P-

CH(CH₃)₂), 1.20 (ddd, J = 16, 9, 6 Hz, 24H, CH(CH₃)₂),

¹0.08 to 0.73 (br m, 6H, BH₃); ¹³C NMR (101 MHz, CDCl₃):

¤ 150.2 (²J_PC= 6 Hz, pyz-C2,C6), 143.7 (d, J_PC= 2 Hz, pyz-

3

1

C3,C5), 27.8 (d, J_PC= 25 Hz, P-CH₂-pyz), 22.1 (d, J_PC

=

120 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

31 Hz, P-CH(CH₃)₂), 17.2 (d, ²J_PC= 11 Hz, CH(CH₃)₂);

³¹P{¹H} NMR (202 MHz, CDCl₃):

38.95-37.63 (m);

¹¹B{¹H} NMR (160 MHz, CDCl₃): ¤ ¹43.7 to ¹45.3 (m);

Anal. Calcd for C₁₈H₄₀B₂N₂P₂: C, 58.73; H, 10.95; N,

7.61%. Found: C, 58.55; H, 10.98; N, 7.36%; HRMS(ESI)

[M ¹ 2BH₃+ Na]⁺Calcd for C₁₈H₃₄N₂NaP₂: 363.2095.

Found: 363.2097; mp 143.0-146.5 °C (decomp.).

To a solution of B-H₂Cl (13 mg, 198 mmol) in THF (4 mL),

ﬁnely ground CsOH-H₂O (100 mg, 596 mmol) was added and

the resulting mixture was stirred at room temperature for one

day. During that time, the suspension turned to dark red

solution. After completion of the reaction was conﬁrmed by

³¹P NMR, the solvent was removed under reduced pressure.

The product was extracted with toluene and recrystallized from

benzene/heptane to give red solid B-H₂amido (65 mg, 121

¯mol, 62%). The signals of NMR spectra were broadened

and it implied an equilibrium between multiple species. The

full assignment of the each signal was unsuccessful because

of the broadness, and ³¹P NMR is shown in the Supporting

Information.

¤

Preparation of pyz-PNP Ligand (5). Ligand 5 was synthe-

sized from pyz-PNP-BH₃(4) according to a similar procedure

in literature.⁶⁶A suspension of pyz-PNP-BH₃(4) (478 mg, 1.30

mmol) in degassed morpholine (10 mL) was heated at 90 °C

for 15 h. The reaction progress was monitored by ³¹P and

¹¹B NMR. The reaction mixture was cooled to room temperature

and concentrated under reduced pressure. The product was

extracted with toluene (5 mL, 2 mL © 3). The combined organic

phase was washed with degassed water (2 mL). Concentration

of the organic phase gave brown oil (360 mg, 81%), which was

used in the next step without further puriﬁcation.

Preparation of Mixture of (pyz-PNP*)Ir(H)₂(B-H₂amido)

and (pyz-PNP)IrH₃(B-H₃).

This reaction is performed

according to a similar procedure in literature.¹²

To a solution of B-H₂Cl (113 mg, 0.20 mol) in THF (4 mL),

NaH (100 mg, 4.2 mmol) was added and the resulting mixture

was stirred at room temperature for one day. During that time,

the suspension turned to dark red solution. After conﬁrma-

tion of the reaction completion by ³¹P NMR, the solvent was

removed in vacuo. The residue was extracted with toluene and

recrystallized from cold hexane to give red crystal (65 mg,

121 ¯mol, 62%), mixture of B-H₃and B-H₂amido. The ratio

of the B-H₃and B-H₂amido was diﬀerent in each experiment.

The ratio of B-H₃increased by treating the reaction mixture (23

mg) in THF-d₈(1.5 mL) with H₂(5.0 MPa) at room temperature

for 17 h. Data of B-H₃is shown below. ¹H NMR (THF-d₈,

500 MHz): ¤ 8.16 (s, 2H, pyz-H3,H5), 3.30 (vt, apparent J =

4 Hz, P-CH₂-pyz, 4H), 1.91 (m, P-CH(CH₃)₂, 4H), 1.14 (d vt,

¹H NMR (400 MHz C₆D₆): ¤ 8.43 (s, 2H, pyz-H5), 2.75 (d,

²J_PH= 1 Hz, 4H, P-CH₂-pyz), 1.62 (m, 4H, P-CH(CH₃)₂),

1.01-0.95 (m, 24H, CH(CH₃)₂); ¹³C NMR (101 MHz, CDCl₃):

2

3

¤ 155.9 (d, J_PC= 9 Hz, pyz-C2,C6), 142.2 (d, J_PC= 7 Hz,

pyz-C3,C5), 29.9 (d, ¹J_PC= 26 Hz, P-CH₂-pyz), 23.9 (d,

¹J_PC= 16 Hz, P-CH(CH₃)₂), 19.9 (d, ²J_PC= 15 Hz, CH(CH₃)₂),

19.2 (d, J_PC= 12 Hz, CH(CH₃)₂); ³¹P{¹H} NMR (162 MHz,

C₆D₆): ¤ 12.80 (s); HRMS(ESI) [M + H]⁺Calcd for

C₁₈H₃₅N₂P₂: 341.2275. Found: 341.2266.

2

Preparation of (pyz-PNP)IrH₂Cl (B-H₂Cl). B-H₂Cl was

synthesizd from pyz-PNP (5) according to a similar procedure

in literature.¹²To a 50 mL stainless autoclave, [IrCl(coe)₂]₂

(342 mg, 0.38 mmol), pyz-PNP ligand (5, 368 mg, 1.08 mmol),

and THF (15 mL) were charged under argon atmosphere. The

mixture was pressurized by hydrogen (2.5 MPa) and stirred at

80 °C for seven hours. The solvent was removed in vacuo and

the residue was reprecipitated from THF/hexane. After wash-

ing the residue with hexane, colourless solid was obtained. B-

H₂Cl (354 mg, 750 ¯mol, 80%). Single crystals suitable for

X-ray crystallography were obtained by recrystallization from

THF/pentane at ¹35 °C. ¹H NMR (C₆D₆, 400 MHz): ¤ 8.04 (s,

³J_HH= 8 Hz, apparent J = 8 Hz, 12H), 1.07 (d vt, ³J_HH= 7 Hz,

2

apparent J = 7 Hz, 12H), ¹10.70 (td, ²J_HH= 5 Hz, J_PH

=

17 Hz, 2H), ¹19.83 (tt, ²J_HH= 5 Hz, ²J_PH= 14 Hz, 1H);

¹³C NMR (THF-d₈, 101 MHz): ¤ 158.3 (vt, apparent J = 5 Hz,

pyz-C2,C6), 139.5 (vt, apparent J = 5 Hz, pyz-C3,C5), 40.9 (vt,

apparent J = 12 Hz, P-CH₂-pyz), 27.6 (vt, apparent J = 15

Hz, P-CH(CH₃)₂), 19.7 (m, CH(CH₃)₂), 18.8 (s, CH(CH₃));

³¹P{¹H} NMR (THF-d₈, 202 MHz): ¤ 57.9; IR (KBr, cm^¹1):

¯

_Ir-H2094, 1717. HRMS(ESI) [B-H₃+ MeCN ¹ H]⁺Calcd for

2

2H, pyz-H3,H5), 3.18 (d vt, J_HH= 17 Hz, apparent J = 4 Hz,

C₂₀H₃₉IrN₃P₂: 576.2248. Found: 576.2273.

2H, P-CH₂-pyz), 2.88 (m, 2H, P-CH(CH₃)₂), 2.59 (d vt,

²J_HH= 17 Hz, apparent J = 4 Hz, 2H, P-CH₂-pyz), 1.59 (m,

Preparation of (yz-PNP)IrH(Cl)₂(B-HCl₂). A solution

of B-H₂Cl (79 mg, 140 ¯mol) in CHCl₃(5 mL) was stirred at

70 °C for one day. After the reaction, the solvent was removed

in vacuo. The residue was reprecipitated from THF/pentane

and recrystallized from THF/pentane to give light brown crys-

tals of B-HCl₂(18 mg, 32 ¯mol, 23%). Single crystals suitable

for X-ray crystallography were obtained by recrystallization

from toluene/hexane at ¹35 °C.

3

2H, P-CH(CH₃)₂), 1.49 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

3

6H, CH(CH₃)₂), 1.12 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

3

6H, CH(CH₃)₂), 1.03 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

3

6H, CH(CH₃)₂), 0.68 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

2

6H, CH(CH₃)₂), ¹17.77 (td, J_HH= 8 Hz, J_PH= 13 Hz, 1H,

Ir-H), ¹21.37 (td, ²J_HH= 8 Hz, ²J_PH= 15 Hz, 1H, Ir-H);

¹³C NMR (C₆D₆, 101 MHz): ¤ 157.8 (vt, apparent J = 5 Hz,

pyz-C2,C6), 140.5 (vt, apparent J = 5 Hz, pyz-C3,C5), 40.0

(vt, apparent J = 12 Hz, P-CH₂-pyz), 24.9 (m, P-CH(CH₃)₂),

21.1 (br s, CH(CH₃)), 20.6 (vt, apparent J = 3 Hz, CH(CH₃)),

19.0 (s, CH(CH₃)), 17.2 (s, CH(CH₃)); ³¹P NMR (C₆D₆, 162

MHz): ¤ 48.9; IR (KBr, cm^¹1): ¯_Ir-H2176, 2098. HRMS(EI)

[M + MeCN ¹ Cl]⁺Calcd for C₂₀H₃₉IrN₃P₂: 576.2248.

Found: 576.224.

¹H NMR (CDCl₃, 500 MHz): ¤ 8.35 (s, 2H, pyz-H3,H5),

2

3.67 (d vt, J_HH= 17 Hz, apparent J = 4 Hz, 2H, P-CH₂-pyz),

2

3.24 (d vt, J_HH= 17 Hz, apparent J = 4 Hz, 2H, P-CH₂-pyz),

3.19 (m, 2H, P-CH(CH₃)₂), 2.42 (m, 2H, P-CH(CH₃)₂), 1.51 (d

vt, ³J_HH= 8 Hz, apparent J = 8 Hz, 6H, CH(CH₃)₂), 1.43 (d vt,

³J_HH= 7 Hz, apparent J = 7 Hz, 6H, CH(CH₃)₂), 1.37 (d vt,

³J_HH= 8 Hz, apparent J = 8 Hz, 6H, CH(CH₃)₂), 1.05 (d vt,

³J_HH= 8 Hz, apparent J = 8 Hz, 6H), ¹20.44 (t, ²J_PH= 12 Hz,

1H, Ir-H); ¹³C NMR (CDCl₃, 126 MHz): ¤ 160.5 (vt, apparent

J = 4 Hz, pyz-C2,C6), 141.0 (vt, apparent J = 5 Hz, pyz-

C3,C5), 36.6 (vt, apparent J = 11 Hz, P-CH₂-pyz), 25.4 (vt,

Preparation of (pyz-PNP*)Ir(H)₂(B-H₂amido).

B-

H₂amido was synthesize from B-H₂Cl according to a similar

procedure in literature.¹²

Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

apparent J = 14 Hz, P-CH(CH₃)₂), 23.1 (vt, apparent J = 15

Hz, P-CH(CH₃)₂), 19.8 (s, CH(CH₃)), 18.6 (s, CH(CH₃)), 18.3

(s, CH(CH₃)), 18.1 (s, CH(CH₃)); ³¹P{¹H} NMR (CDCl₃, 162

MHz): ¤ 30.82; IR (powder, cm^¹1): ¯_Ir-H2214. HRMS(ESI)

[M ¹ HCl₂]⁺Calcd for C₁₈H₃₄IN₂P₂: 533.1826. Found:

533.1805.

Preparation of 4-Methoxy-2,6-dimethylpyridine (7). 4-

Methoxy-2,6-dimethylpyridine (7) was synthesizd from 4-

hydroxy-2,6-dimethylpyridine (6) (3.24 g, 30 mmol) according

to a similar procedure in literature.⁶⁷

A suspension of 6 (1.85 g, 15.0 mmol), dimethyl sulfate

(1.47 mL, 15.5 mmol) and potassium carbonate (6.09 g, 44.1

mmol) in anhydrous acetone (56 mL) was heated under reﬂux

for 2 h. After cooling to 0 °C the mixture was ﬁltered through a

short plug of silica on a glass ﬁlter. The ﬁltrate was concen-

trated to provide 7 (1.53 g, 11.1 mmol, yield) as yellow spec-

trooscopic data were consistent with the literature.⁶⁸

Preparation of pOMe-PNP Ligand 8. 4-Methoxy-PNP

was synthesized from 4-methoxy-2,6-dimethylpyridine (7)

according to a similar procedure in literature.^69,70

¹H NMR (THF-d₈, 500 MHz): ¤ 6.87 (s, 2H, pyr-H3,H5),

2

3.80 (s, 3H, OCH₃), 3.75 (d vt, J_HH= 16 Hz, apparent J =

2

4 Hz, 2H, P-CH₂-pyr), 3.48 (d vt, J_HH= 17 Hz, apparent J =

4 Hz, 2H, P-CH₂-pyr), 2.77 (m, 2H, P-CH(CH₃)₂), 1.98 (m,

3

2H, P-CH(CH₃)₂), 1.34 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

3

6H, CH(CH₃)₂), 1.30 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

3

6H, CH(CH₃)₂), 1.09 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

3

6H, CH(CH₃)₂), 0.88 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

2

6H, CH(CH₃)₂), ¹19.83 (td, J_HH= 8 Hz, J_PH= 13 Hz, 1H,

Ir-H), ¹23.01 (td, ²J_HH= 8 Hz, ²J_PH= 15 Hz, 1H, Ir-H);

¹³C NMR (C₆D₆, 126 MHz): ¤ 164.9 (s, pyr-C4), 164.4 (vt,

apparent J = 4 Hz, pyr-C2,C6), 105.4 (vt, apparent J = 5 Hz,

pyr-C3,C5), 54.9 (s, OCH₃), 43.9 (vt, apparent J = 12 Hz,

P-CH₂-pyr), 24.8 (m, P-CH(CH₃)₂), 21.2 (vt, apparent J =

3 Hz, CH(CH₃)), 20.8 (vt, apparent J = 3 Hz, CH(CH₃)), 19.3

(s, CH(CH₃)), 17.4 (s, CH(CH₃)); ³¹P{¹H} NMR (C₆D₆, 202

MHz): ¤ 48.7; IR (KBr, cm^¹1): ¯_Ir-H2176, 2098. HRMS(ESI)

[M ¹ Cl]⁺Calcd for C₂₀H₃₉IrNOP₂: 564.2136. Found:

564.2134.

Preparation of (pOMe-PNP)Ir(H)₃(C-H₃). C-H₃was

synthesized from C-H₂Cl according to a similar procedure in

literature.¹²

To a solution of 2,6-dimethyl-4-methoxypyridine 7 (686

mg, 5.00 mmol) and N,N,N¤,N¤-tetramethylethylenediamine

(1.6 mL, 11 mmol) in Et₂O (13 mL), nBuLi (2.66 M in hexane,

4.0 mL, 10.6 mmol) was added dropwise at 0 °C and the mix-

ture was allowed to warm to room temperature with stirring for

To a solution of C-H₂Cl (+C-HCl₂) (100 mg, 0.17 mmol)

in THF (10 mL), NaH (50 mg, 2 mmol) was added and the

resulting mixture was stirred at room temperature for 1 day.

During that time, the suspension turned to a dark red solution.

After conﬁrmation of the completion by ³¹P NMR, the solvent

was removed in vacuo. The residue was extracted with hexane

and recrystallized from cold hexane to aﬀord oﬀ-white solid

(C-H₃, 21 mg, 37 ¯mol, 22%).

i

5 h. A solution of Pr₂PCl (1.70 g, 11.2 mmol) in Et₂O (10 mL)

was added to the resulting mixture dropwise at ¹78 °C and

stirred at room temperature for 21 h. After the reaction, the

resulted solid was removed by ﬁltration through a pad of Celite,

and the ﬁltrate was concentrated by evaporation. The obtained

crude product was puriﬁed by passing through a short plug of

silica gel on a glass ﬁlter with Et₂O as an eluent. The solvent

was removed to give light brown oil of 8 (1.64 g), which was

used in the next step without further puriﬁcation. ¹H NMR

(400 MHz, C₆D₆): ¤ 6.78 (s, 2H, pyr-H3,H5), 3.20 (s, 3H,

O-CH₃), 2.98 (d, ²J_PH= 1 Hz, 4H, P-CH₂-pyr), 1.79-1.67 (m,

¹H NMR (C₆D₆, 500 MHz): ¤ 6.25 (s, 2H, pyr-H3,H5), 3.12

(s, 3H, OCH₃), 3.01 (vt, apparent J = 3 Hz, 2H, P-CH₂-pyr),

3

1.85 (m, 2H, P-CH(CH₃)₂), 1.28 (d vt, J_HH= 8 Hz, apparent

3

J = 8 Hz, 12H, CH(CH₃)₂), 1.18 (d vt, J_HH= 7 Hz, apparent

2

J = 7 Hz, 12H, CH(CH₃)₂), ¹10.47 (td, J_PH= 17 Hz, J_HH

=

2

5 Hz, 2H, Ir-H), ¹20.53 (tt, J_PH= 14 Hz, J_HH= 5 Hz, 1H,

Ir-H); ¹³C NMR (C₆D₆, 126 MHz): ¤ 163.9 (vt, apparent J = 4

Hz, pyr-C2,C6), 163.5 (s, pyr-C4), 104.6 (vt, apparent J = 5

Hz, pyr-C3,C5), 54.7 (s, OCH₃), 45.1 (vt, apparent J = 12

Hz, P-CH₂-pyr), 27.1 (vt, apparent J = 15 Hz, P-CH(CH₃)₂),

20.0 (vt, apparent J = 3 Hz, CH(CH₃)₂), 19.1 (s, CH(CH₃)₂);

³¹P{¹H} NMR (C₆D₆, 162 MHz): ¤ 57.8; IR (KBr, cm^¹1):

4H, P-CH(CH₃)₂), 1.11-1.03 (m, 24H, CH(CH₃)₂); ¹³C NMR

2

(101 MHz, CDCl₃): ¤ 166.5 (s, C-O-CH₃), 162.0 (d, J_PC

=

3

10 Hz, pyr-C2,C6), 106.8 (d, J_PC= 7 Hz, pyr-C3,C5), 54.3 (s,

1

O-CH₃), 33.2 (d, J_PC= 23 Hz, P-CH₂-pyr), 23.9 (d, J_PC

=

2

16 Hz, P-CH(CH₃)₂), 19.9 (d, J_PC= 14 Hz, CH(CH₃)₂), 19.3

(d, ²J_PC= 11 Hz, CH(CH₃)₂); ³¹P{¹H} NMR (162 MHz, C₆D₆):

¤ 11.64; HRMS(ESI) [M + H]⁺Calcd for C₂₀H₃₈NOP₂:

370.2429. Found: 370.2417.

¯

2083, 1686. HRMS(ESI) [M ¹ H]⁺Calcd for C₂₀H₃₉-

Ir-H

IrNOP₂: 564.2136. Found: 564.2111.

Preparation of (pOMe-PNP)IrH₂Cl (C-H₂Cl). C-H₂Cl

was synthesized from p-OMe-PNP (5) according to a similar

procedure in literature.¹²

Preparation of (pOMe-PNP)IrH(Cl)₂(C-HCl₂).

A

solution of C-H₂Cl (+C-HCl₂) (50 mg, 85 ¯mol) in CHCl₃

(5 mL) was stirred at room temperature for 20 h. After the

completion of the reaction conﬁrmed by ³¹P NMR, the solvent

was removed in vacuo. The product was recrystallized from

CHCl₃/pentane to give colourless crystal of C-HCl₂(17 mg,

27 ¯mol, 32%). Single crystals suitable for X-ray crystallog-

raphy were obtained by recrystallization from CHCl₃/pentane.

¹H NMR (CDCl₃, 500 MHz): ¤ 6.72 (s, 2H, pyr-H3,H5),

To a 50 mL stainless autoclave, [IrCl(coe)₂]₂(448 mg, 0.500

mmol), pOMe-PNP (8, 406 mg, 1.10 mmol), and THF (20 mL)

were charged under argon atmosphere. The mixture was pres-

surized by hydrogen (3.0 MPa) and stirred at 80 °C for 12 h.

The solvent was removed in vacuo and the residue was

reprecipitated from THF/hexane. The residue was washed with

hexane to give a pale-yellow solid. Because of the facile

disproportionation in the reaction condition and high crys-

tallinity of C-HCl₂, a mixture of C-H₂Cl and C-HCl₂(414 mg)

was obtained (C-H₂Cl/C-HCl₂= 1/0.16). Subsequent recrys-

tallizations from THF/pentane twice and extraction with THF

gave a mixture of C-H₂Cl and C-HCl₂at the ratio of 1/0.07.

2

3.81 (s, 3H, OCH₃), 3.72 (d vt, J_HH= 16 Hz, apparent J =

2

2 Hz, 2H, P-CH₂-pyr), 3.27 (d vt, J_HH= 17 Hz, apparent J =

2 Hz, 2H, P-CH₂-pyr), 3.14 (m, 2H, P-CH(CH₃)₂), 2.38 (m,

3

2H, P-CH(CH₃)₂), 1.49 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

3

6H, CH(CH₃)₂), 1.40 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

3

6H, CH(CH₃)₂), 1.35 (d vt, J_HH= 8 Hz, apparent J = 8 Hz,

122 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

3

6H, CH(CH₃)₂), 1.03 (d vt, J_HH= 7 Hz, apparent J = 7 Hz,

and 1402673 for complex C-HCl₂. Copies of the data can be

obtained free of charge via http://www.ccdc.cam.ac.uk/conts/

retr i ev i ng.htm l (or from the Cambridge Crystallographic Data

Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44

1223 336033; e-mail: deposit@ccdc.cam.ac.uk).

6H, CH(CH₃)₂), ¹22.50 (t, ²J_PH= 12 Hz, 1H, Ir-H); ¹³C NMR

(CDCl₃, 126 MHz): ¤ 166.1 (vt, apparent J = 4 Hz, pyr-C2,C6),

165.4 (s, pyr-C4), 106.5 (vt, apparent J = 5 Hz, pyr-C3,C5),

56.0 (s, OCH₃), 40.2 (vt, apparent J = 12 Hz, P-CH₂-

pyr), 25.2 (vt, apparent J = 14 Hz, P-CH(CH₃)₂), 22.8 (vt,

apparent J = 15 Hz, P-CH(CH₃)₂), 19.9 (m, CH(CH₃)₂), 18.7

(s, CH(CH₃)₂), 18.2 (s, CH(CH₃)₂), 18.1 (s, CH(CH₃)₂);

³¹P{¹H} NMR (CDCl₃, 202 MHz): ¤ 28.81; IR (powder,

cm^¹1): ¯_Ir-H2214. HRMS(ESI) [M ¹ Cl]⁺Calcd for C₂₀H₃₈-

ClIrNOP₂: 598.1746. Found: 598.1750.

A part of this work was conducted in Research Hub for

Advanced Nano Characterization, The University of Tokyo,

supported by MEXT, Japan. The computations were performed

using Research Centre for Computational Science, Okazaki,

Japan.

Preparation of (pOMe-PNP)Ir (Cl)₃(C-Cl₃). To a solu-

tion of hexachloroethane (52 mg, 0.22 mmol) in THF (1 mL) in

a 20-mL Schlenk tube was added a solution of [IrCl(coe)₂]₂

(89 mg, 0.1 mmol) and pOMe-PNP ligand (8, 74 mg, 0.2 mmol)

in THF (2 mL). After stirring for 1 day at 65 °C, the mix-

ture was dried in vacuo. After the repetitive and exhaustive

puriﬁcation, (reprecipitation from CH₂Cl₂/hexane, extraction

with benzene, reprecipitation from CHCl₃/benzene and then

from CH₂ClCH₂Cl/benzene) a pale yellow powder of C-Cl₃

was obtained (10 mg, 0.015 mmol, 7.5%).

Supporting Information

Experimental procedure, additional experimental results,

detail for the X-ray structural analysis, and the detail for the

theoretical calculations. This material is available on http://

dx.do i .org/10.1246/bcsj.20150311.

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124 | Bull. Chem. Soc. Jpn. 2016, 89, 113–124 | doi:10.1246/bcsj.20150311

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