Isolation and crystal structure of a water-soluble iridium hydride: A robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds in acidic media

Article abstract of DOI:10.1021/ja0288237

This paper reports the isolation and structural determination of a water-soluble hydride complex [Cp*Ir^III(bpy)H]⁺ (1, Cp* = η⁵-C₅Me₅, bpy = 2,2′-bipyridine) that serves as a robust and highly active

Full text of DOI:10.1021/ja0288237

Published on Web 03/12/2003
Isolation and Crystal Structure of a Water-Soluble Iridium
Hydride: A Robust and Highly Active Catalyst for
Acid-Catalyzed Transfer Hydrogenations of Carbonyl
Compounds in Acidic Media
Tsutomu Abura,†,‡ Seiji Ogo,*,† Yoshihito Watanabe, and Shunichi Fukuzumi*,†
§
Contribution from the Department of Material and Life Science,
Graduate School of Engineering, Osaka UniVersity, PRESTO & CREST,
Science and Technology Corporation (JST), Suita 565-0871, Japan,
Department of Structural Molecular Science, The Graduate UniVersity for AdVanced Studies,
Myodaiji, Okazaki 444-8585, Japan, and Department of Chemistry, Graduate School of Science,
Nagoya UniVersity, Chikusa-ku, Nagoya 464-8602, Japan
Received October 5, 2002; E-mail: ogo@ap.chem.eng.osaka-u.ac.jp
Abstract: This paper reports the isolation and structural determination of a water-soluble hydride complex
III
+
5
[
Cp*Ir (bpy)H] (1, Cp* ) η -C
5 5
Me , bpy ) 2,2′-bipyridine) that serves as a robust and highly active catalyst
for acid-catalyzed transfer hydrogenations of carbonyl compounds at pH 2.0-3.0 at 70 °C. The catalyst 1
III
2+
was synthesized from the reaction of a precatalyst [Cp*Ir (bpy)(OH
X ) H or Na) in H O under controlled conditions (2.0 < pH < 6.0, 25 °C) which avoid protonation of the
hydrido ligand of 1 below pH ca. 1.0 and deprotonation of the aqua ligand of 2 above pH ca. 6.0 (pK value
2
)] (2) with hydrogen donors HCOOX
(
2
a
of 2 ) 6.6). X-ray analysis shows that complex 1 adopts a distorted octahedral geometry with the Ir atom
5
coordinated by one η -Cp*, one bidentate bpy, and one terminal hydrido ligand that occupies a bond position.
The isolation of 1 allowed us to investigate the robust ability of 1 in acidic media and reducing ability of 1
in the reaction with carbonyl compounds under both stoichiometric and catalytic conditions. The rate of the
acid-catalyzed transfer hydrogenation is drastically dependent on pH of the solution, reaction temperature,
and concentration of HCOOH. The effect of pH on the rate of the transfer hydrogenation is rationalized by
the pH-dependent formation of 1 and activation process of the carbonyl compounds by protons. High turnover
frequencies of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0 are ascribed not only to
nucleophilicity of 1 toward the carbonyl groups activated by protons but also to a protonic character of the
hydrido ligand of 1 that inhibits the protonation of the hydrido ligand.
Introduction
conditions is a worthy endeavor owing to applications in
synthesis in the presence of functional groups that are sensitive
to basic (or acidic) conditions.
The chemistry in aqueous media is presently undergoing very
rapid growth because of many potential advantages such as
For the reduction of carbonyl compounds under acidic
conditions in aqueous media, sodium cyanoborohydride (NaBH3-
CN) is frequently utilized as a noncatalytic reducing agent.3
However, serious precautions in handling are required since
NaBH3CN is flammable and a very hygroscopic solid and highly
toxic HCN is formed as a byproduct of the reduction. Hydro-
industrial applications (e.g., introduction of new biphasic
processes) and reaction-specific pH selectivity.1,2 In this area,
development of reagents to be used under acidic (or basic)
†
Osaka University, PRESTO & CREST, JST.
The Graduate University for Advanced Studies.
Nagoya University.
‡
§
genation and transfer hydrogenation are useful methods for
(
1) Reviews: (a) Jo o´ , F. In Catalysis by Metal Complexes; James, B. R.,
Leeuwen, P. W. N. M., Series Eds.; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 2001; Vol. 23, Aqueous Organometallic
Catalysis, p 300. (b) Cornils, B.; Herrmann, W. A. In Aqueous-Phase
Organometallic Catalysis; Wiley-VCH: Weinheim, Germany, 1998; p 615.
catalytic reductions of carbonyl compounds in organic solvents,4
though instability of metal-hydride species in acidic media has
precluded definitive characterization of active catalysts in
aqueous media.
(c) Li, C.-J.; Chan, T.-H. In Organic Reactions in Aqueous Media; John
Wiley & Sons: New York, 1997; p 199. (d) Koelle, U. Coord. Chem.
ReV. 1994, 135/136, 623-650. (e) Herrmann, W. A.; Kohlpaintner, C. W.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1524-1544.
We preliminarily reported pH-dependent transfer hydrogena-
tions of a variety of carbonyl compounds catalyzed by an in-
(
2) (a) Ogo, S.; Nakai, H.; Watanabe, Y. J. Am. Chem. Soc. 2002, 124, 597-
6
01. (b) Ogo, S.; Abura, T.; Watanabe, Y. Organometallics 2002, 21, 2964-
III
+
5
situ-formed hydride species [Cp*Ir (bpy)H] (1, Cp* ) η -
2
969 and references therein; (c) Nakai, H.; Ogo, S.; Watanabe, Y.
Organometallics 2002, 21, 1674-1678. (d) Makihara, N.; Ogo, S.;
Watanabe, Y. Organometallics 2001, 20, 497-500. (e) Ogo, S.; Makihara,
N.; Watanabe, Y. Organometallics 1999, 18, 5470-5474. (f) Ogo, S.; Chen,
H.; Olmstead, M. M.; Fish, R. H. Organometallics 1996, 15, 2009-2013.
(3) (a) Borch, R. F.; Bernstein, M. D.; Durst, H. D. J. Am. Chem. Soc. 1971,
93, 2897-2904. (b) Lane, C. F. In Selections from the Aldrichimica Acta;
Aldrich Chemical Co., Inc.: Milwaukee, WI, 1984; pp 67-74. (c) Lane,
C. F. Synthesis 1975, 135-146. (d) Carruthers, W. In Some Modern
Methods of Organic Synthesis; Cambridge University Press: New York,
1987; pp 475-476.
(
g) Lo, H. C.; Leiva, C.; Buriez, O.; Kerr, J. B.; Olmstead, M. M.; Fish, R.
H. Inorg. Chem. 2001, 40, 6705-6716. (h) Jo o´ , F.; Kov a´ cs, J.; B e´ nyei, A.
C.; Kath o´ , AÄ . Angew. Chem., Int. Ed. Engl. 1998, 37, 969-970.
10.1021/ja0288237 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 4149-4154
9
4149

A R T I C L E S
Abura et al.
5
C5Me5, bpy ) 2,2′-bipyridine). However, the definitive structure
of 1 has yet to be disclosed. Besides, low turnover frequencies
(TOFs ) (mol of product formed/mol of catalyst)/1 h) of the
transfer hydrogenations in acidic media should have been
improved.
Herein, we report isolation of 1, whose structure was
unequivocally determined by X-ray analysis. Complex 1 acts
as a robust and highly active catalyst for the transfer hydrogena-
tions of carbonyl compounds under optimized conditions. The
robust ability of 1 in acidic media is discussed on the basis of
pH-dependent properties of the hydrido ligand and stability of
1
under acidic conditions. The reducing ability of 1 is
investigated under stoichiometric and catalytic conditions that
are optimized for the pH of the solution, reaction temperature,
and concentration of HCOOH as a hydrogen donor. The
isolation and structural determination of 1 provide excellent
opportunity to elucidate the mechanism of high TOFs of the
pH-dependent transfer hydrogenations catalyzed by 1.
Figure 1. pH-dependent formation ratio of [1]+ prepared from the reaction
of 2(SO4) (5 µmol) with 100 equiv (500 µmol) of HCOOH in D2O (1 mL)
at 25 °C for 5 min.
in H
2
O (20 mL) at pH 5.0 at 70 °C for 15 min provides a yellow solution
+
of water-soluble mononuclear hydride complex [1] . To the solution
Experimental Section
was added NaPF
was stirred for 1 min giving an orange powder of 1(PF
collected by filtration, washed with water, and dried in vacuo. The
powder was recrystallized from methanol/diethyl ether to provide air-
6
(17 mg, 0.1 mmol) in H
2
O (20 mL), and the mixture
), which was
6
Materials and Methods. All experiments were carried out under
an Ar atmosphere by using standard Schlenk techniques and a globebox.
The manipulations in the acidic media were carried out with plastic-
and glasswares (without metals). D
Cambridge Isotope Laboratories, and 65% DNO
1
sensitive orange crystals of 1(PF
NMR (300 MHz, in D
.78 (s, 15H), 7.62 (t, 2H), 8.11 (t, 2H), 8.29 (d, 2H), 8.90 (d, 2H).
Transfer Hydrogenations. Water-soluble and water-insoluble car-
bonyl compounds (0.32 mmol), 2(SO ) (1.0 mg, 1.6 µmol), and
HCOOH (6.7 M HCOOH 0.25 mL, 1.6 mmol) were dissolved in H
6
). Yield: 35% based on 2(SO
4
). H
2
O (99.9% D) was purchased from
/D O (99% D) and
2
O, reference to TPS, 25 °C): δ -11.80 (s, 1H),
3
2
1
DCOONa (99% D) were purchased from Isotec Inc.; these reagents
were used as received. All carbonyl compounds used in this study
4
(highest purity available) were purchased from Aldrich Chemicals Co.
2
O
Purification of water (18.2 MΩ cm) was performed with a Milli-Q
system (Millipore; Milli-RO 5 plus and -Q plus). The H NMR spectra
were recorded on Varian VRX-300S and JEOL JNM-AL300 spec-
2 2
trometers at 20 °C. H and CO gases were determined by a Shimadzu
GC-8A (He carrier, Unibeads column, 60/80 2 m, GL Sciences Inc.)
equipped with a thermal conductivity detector. A Nissin magnetic stirrer
1
(3 mL) at pH 1.0-10.0 under Ar atmosphere. The mixture was
vigorously stirred (1000 rpm, Nissin magnetic stirrer model SW-R700)
at 25-90 °C. After 1-4 h, it was cooled to 0 °C, and the resulting
mixture was analyzed by H NMR. Water-soluble products (entries a,
d, and e) were isolated by salting-out with NaCl and extraction with
1
Et
extraction with Et
c and g) were also isolated by using Whatman phase separators 1PS
silicone-treated filter paper) without organic solvents. It was confirmed
2
O. Water-insoluble products (entries b, c, and g) were isolated by
(model SW-R700) was used.
2
O. Furthermore, the water-insoluble products (entries
pH Adjustment. In a pH range of 1.0-10.0, the pH value of the
solutions was determined by a pH meter (TOA, HM-18E) equipped
with a pH combination electrode (TOA, GS-5015C). The pH of the
solution was adjusted by using 0.01-3.00 M HNO
(
that the transfer hydrogenations do not occur in the absence of 2 or
HCOOH (as blank experiments).
3 2
/H O and 0.01-
3
.00 M NaOH/H O without buffer. During the reaction, a stainless steel-
2
micro pH probe (IQ Scientific Instruments, Inc., PH15-SS) was dipped
in the reaction mixture at 70 °C under Ar atmosphere, and the pH of
the solution was monitored by a pH meter (IQ Scientific Instruments,
Inc., IQ200). It was confirmed that the pH of the solution does not
change during the course of the reactions under the conditions of this
6
X-ray Crystallographic Analysis. Crystallographic data for 1(PF )
have been deposited with the Cambridge Crystallographic Data Center
as supplementary publication No. CCDC-193541. Copies of the data
can be obtained free of charge on application to the CCDC, 12 Union
Road, Cambridge CB21EZ, U.K. [fax, (+44)1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk]. Measurements were made on a Rigaku/MSC
Mercury CCD diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.7107). All calculations were performed using the
teXsan crystallographic software package of the Molecular Structure
Corp. Crystal data, data collection parameters, structure solution and
refinement, atomic coordinates, anisotropic displacement parameters,
bond lengths, and bond angles are given in the Supporting Information.
1
study. To determine the exact pH values, the H NMR experiments
were performed by dissolving the samples in HNO
tube (diameter ) 5.0 mm) with a sealed capillary tube (diameter )
.5 mm) containing 3-(trimethylsilyl)propionic-2,2,3,3-d acid sodium
salt (TSP, 500 mM, as the reference with the methyl proton resonance
set at 0.00 ppm) dissolved in D O (for deuterium lock). Values of pD
3 2
/H O in an NMR
1
4
2
6
were corrected by adding 0.4 to the observed values. In the case of
biphasic media, the pH value of the aqueous phase is adopted.
III
III
[
Cp*Ir (bpy)H](PF
6
) [1(PF
6
)]. A reaction of [Cp*Ir (bpy)(OH
2
)]-
Results and Discussion
(
SO ) [2(SO ), 60 mg, 0.1 mmol] and HCOONa (2720 mg, 40 mmol)
4
4
Isolation and Characterization of 1. In a pH 2.0-6.0 region
(
4) (a) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40-73. (b)
+
at 25°C, a mononuclear hydride complex [1] was synthesized
Ohkuma, T.; Ishii, D.; Takeno, H.; Noyori, R. J. Am. Chem. Soc. 2000,
III
from the reaction of [Cp*Ir (bpy)(OH2)](SO4) [2(SO4)] with
1
22, 6510-6511. (c) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997,
0, 97-102. (d) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R.
3
HCOOX (X ) H or Na) through â-hydrogen elimination with
K.; Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090-1100. (e) Casey, C.
P.; Singer, S. W.; Powell, D. R. Can. J. Chem. 2001, 79, 1002-1011. (f)
S u¨ ss-Fink, G.; Faure, M.; Ward, T. R. Angew. Chem., Int. Ed. 2002, 41,
the evolution of CO2 that was confirmed by GC analysis (see
+
Experimental Section). Complex [1] has high solubility in
9
9-101.
(
(
5) Ogo, S.; Makihara, N.; Kaneko Y.; Watanabe, Y. Organometallics 2001,
water (20 mg/mL at pH 5.0 at 25 °C). In this study, HCOOH
was used not only as a hydrogen donor but also as an acid to
prepare the acidic pH of the solution. Figure 1 shows the pH-
2
0, 4903-4910.
6) (a) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188-190. (b)
Mikkelsen, K.; Nielsen, S. O. J. Phys. Chem. 1960, 64, 632-637.
4150 J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003

A Water-Soluble Iridium Hydride
A R T I C L E S
Table 1. Summary of Crystal Data, Data Collection Parameters,
and Structure Refinement for 1(PF6)
empirical formula
fw
C20H24F6IrN2P
629.61
cryst color
cryst dimens (mm)
cryst system
a (Å)
b (Å)
c (Å)
orange
0.20 × 0.25 × 0.24
monoclinic
8.924(1)
15.025(2)
15.823(2)
96.569(6)
2107.7(4)
P21/c (14)
4
1.984
1216
64.89 K
Mo KR (0.7107)
-150
54.9
numerical
4674
274
0.049
0.062
0.030
0.98
0.002
â (deg)
3
V (Å )
space group (No.)
Z
Dcalc (g cm-3)
F000
µ(Mo KR) (cm-1)
radiation (λ, Å)
temp (°C)
2
θmax (deg)
abs corr method
Figure 2. ORTEP drawing (50% probability) of 1(PF6). The anion (PF6)
is omitted for clarity.
no. of reflcns obsd (all, 2θ < 54.9°)
no. of params
a
R
dependent formation of 1 from the reaction of 2 with 100 equiv
of HCOOH (in the absence of reducible ketones) at 25°C for 5
min.
Rw
R1
b
c
goodness of fit indicator, Sd
max shift/error in final cycle
max peaks in final diff map (e Å
The influence of pH on the formation of 1 is explained by
pH-dependent structural changes of 1, 2, and HCOOH as
follows: (i) Below pH ca. 1.0, the protonation of the hydrido
ligand of 1 leads to the formation of 2 with the evolution of H2
-3
)
1.45
-0.72
min peaks in final diff map (e Å-3)
a
R ) ∑(Fo - Fc )/∑Fo . Rw ) [{∑ω(Fo - Fc ) /∑ω(Fo ) }]1/2. c R1
2
2
2
b
2
2 2
2 2
d
)
∑||Fo| - |Fc||/∑|Fo| [for >2.0σ(I) data]. Goodness of fit indicator, S
III
2
1/2
that was confirmed by GC analysis (eq 1, M ) Cp*Ir bpy).
) [∑w(|Fo| - |Fc|) /(No - Nv)]
(No ) number of observations; Nv )
-
number of variables).
(
ii) Above pH 3.6, HCOOH acts as HCOO to bind the iridium
center since the pKa value of HCOOH at the studied concentra-
tion is 3.6 (eq 2). (iii) Above pH 6.6 () pKa value of 2), complex
Table 2. Selected Bond Lengths (l/Å) and Angles (φ/deg) for
1(PF6)
2
is predominantly deprotonated to form a hydroxo complex
Ir1-H1
Ir1-C1
Ir1-C4
C1-C5
C4-C5
1.73(5) Ir1-N1
2.235(5) Ir1-C2
2.171(5) Ir1-C5
1.453(7) C2-C3
2.069(4) Ir1-N2
2.254(5) Ir1-C3
2.160(5) C1-C2
1.445(7) C3-C4
2.069(4)
2.179(5)
1.495(7)
1.430(7)
III
+
[Cp*Ir (bpy)(OH)] that hardly reacts with HCOOH (eq 3).
pH < ca. 1.0
+
H+
8 [M-OH2]2+ + H2
+
(1)
1.437(7) N1-Ir1-N2 77.0(2) H1-Ir1-N1 84(1)
[M-H]
H2O
H1-Ir1-N2 84(1)
C1-Ir1-C2 36.9(2) C1-Ir1-C5 38.6(2)
1
2
C2-Ir1-C3 38.0(2) C3-Ir1-C4 38.4(2) C4-Ir1-C5 38.7(2)
pK ) 3.6
a
-
+
7
HCOOH {
} HCOO + H
(2)
(3)
media. Crystal data, data collection parameters, and structure
refinement for 1 are listed in Table 1. Selected bond lengths
and angles for 1 are listed in Table 2.
Complex 1 adopts a distorted octahedral coordination which
pK ) 6.6
a
2+
+
+
[
M-OH2]
{
} [M-OH] + H
2
5
is surrounded by one η -Cp* ligand, one bidentate bpy ligand,
and one terminal hydrido ligand (H1) that occupies a bond
position. The atomic coordinates (x, y, z) of H1 were refined in
the X-ray analysis. The Ir1-H1 bond length of 1 is 1.73(5) Å,
which is close to Ir-H bond lengths (average 1.77 Å) of
1
It was confirmed by H NMR that complex 1 is quite stable
below 70 °C under an Ar atmosphere at pH 2.0-6.0 in the
absence of reducible carbonyl compounds. Above 90 °C,
however, complex 1 is slowly decomposed to unidentified
compounds accompanied by Ir-C(Cp*) bond cleavage. It is
noteworthy that the thermostability of M-C(Cp*) bonds of
III
+
dinuclear µ-hydride complex [(Cp*Ir )2(µ-H)3] previously
2a
determined by X-ray analysis. The distances between Ir atom
and carbons of the Cp* ring of complex 1 in the solid state are
not equivalent; the distances of Ir1-C1 and Ir1-C2 [2.235(5)
and 2.254(5) Å, respectively] trans to the hydrido ligand are
longer than those of Ir1-C3, Ir1-C4, and Ir1-C5 [2.179(5),
III
III
III
[
(
Cp*M (bpy)(H2O)]2+ (M ) Co , Rh , and Ir ) in acidic media
pH 2.0-6.0) is in order Ir-C(Cp*) > Rh-C(Cp*) . Co-
C(Cp*). It is expected that the Ir-N bonds are stabilized by a
chelate effect of the bpy ligand compared with those of a
monodentate pyridine ligand.
Crystal Structure of 1(PF6). Orange crystals of 1(PF6) used
in X-ray analysis (Figure 2) were obtained by diffusion of
diethyl ether into a methanol solution of 1(PF6) at ambient
temperature (see Experimental Section).
To our knowledge, this is the first crystal structure of a
mononuclear highly active catalyst for the transfer hydrogena-
tions of carbonyl compounds under acidic conditions in aqueous
2
.171(5), and 2.160(5) Å, respectively] trans to the bpy ligand.
This result indicates that the hydrido ligand has a greater trans
influence than the bpy ligand. The torsion angle between the
least-squares plane of Cp* and that of bpy is 66.3(3)°.
(
7) (a) Slugovc, C.; Mereiter, K.; Trofimenko, S.; Carmona, E. Angew. Chem.,
Int. Ed. 2000, 39, 2158-2160. (b) Klei, S. R.; Tilley, T. D.; Bergman, R.
G. J. Am. Chem. Soc. 2000, 122, 1816-1817. (c) Gruet, K.; Crabtree, R.
H.; Lee, D.-H.; Liable-Sands, L.; Rheingold, A. L. Organometallics 2000,
1
9, 2228-2232. (d) Dorta, R.; Togni, A. Organometallics 1998, 17, 3423-
3428.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 14, 2003 4151

A R T I C L E S
Abura et al.
Figure 4. (b) pH-dependent profile of TOFs for the transfer hydrogenation
of a (0.32 mmol) with 2(SO4) (1.6 µmol) and 1000 equiv (1.6 mmol) of
HCOOH in water (3 mL) at 70 °C for 15 min. (O) pH-dependent profile of
TOFs for the transfer hydrogenation of b (0.32 mmol) with 2(SO4) (1.6
µmol) and 1000 equiv (1.6 mmol) of HCOOH in water (3 mL) at 70 °C for
1
5 min. (9) pH-dependent profile of TOFs for the transfer hydrogenation
6
II
of b (0.32 mmol) with [(η -C6Me6)Ru (bpy)(H2O)] (SO4) (1.6 µmol) and
1000 equiv (1.6 mmol) of HCOOH in water (3 mL) at 70 °C for 15 min.
example of water-insoluble ketones in water and in biphasic
media (b + water), respectively. Under stoichiometric conditions
(1/ketones ) 1/1) in the absence of HCOOH, a and b are
reduced to cyclohexanonol (yield: 43%) and 1-phenylethanol
Figure 3. (a) H NMR spectrum of [1]+ in D2O at 25 °C at pD 5.0. TSP:
1
the reference with the methyl proton resonance set at 0.00 ppm. The inset
shows the H NMR spectrum of the D-labeled [1] in D2O (reference to
D2O at 4.78 ppm). (b) H/D exchange of 1 (5 µmol) with D (0.1 M DNO3/
D2O, 1 mL, at pD 5.0) at 25 °C monitored by H NMR for 60 min. The
2
+
(
yield: 40%) in 20 min at pH 2.0 at 70 °C as shown in eq 6
+
III
1
(where M ) Cp*Ir bpy).
signal at -11.80 ppm corresponds to the hydrido ligand of 1.
pH-Dependent Properties of the Hydrido Ligand of 1.
1
+
Figure 3a shows H NMR spectrum of [1] in D2O at pD 5.0
(
6
pD ) pH meter reading + 0.4) at 25 °C.
On the other hand, under catalytic conditions (eq 7, 1/ketones/
HCOOH ) 1/200/1000), complex 1 catalyzes the transfer
hydrogenations of a and b to yield the corresponding alcohols
The signal at -11.80 ppm corresponds to the hydrido ligand
of 1. To establish the origin of the hydrido ligands of 1, the
III
+
synthesis of D-labeled 1 {[Cp*Ir (bpy)D] } by a reaction of 2
with DCOONa in H2O at pH 5.0 for 15 min at 25 °C has been
(yield: 99%) in 2 h at pH 2.0 at 70 °C.
III
carried out (M ) Cp*Ir bpy):
pH 5.0
2
+
-
8 [M-D]+
D-labeled 1
[
M-OH2] + DCOO
+
H2O
2
CO + H O (4)
2
2
1H NMR experiments revealed that the labeled hydrogen atoms
D) are incorporated into the ketones when DCOONa is used
as the hydrogen donor in the transfer hydrogenations.
Optimized Conditions of Transfer Hydrogenations. The
TOFs of the acid-catalyzed transfer hydrogenation are drastically
dependent on the pH of the solution, reaction temperature, and
concentration of HCOOH. Because of operational simplicity,
optimization of the conditions of the transfer hydrogenation is
carried out with the air-stable precatalyst 2 and HCOOH. Figure
2
The H NMR spectrum shows that the deuterium atom is
(
incorporated in 1 (Figure 3a). Complex 1 undergoes an H/D
+
8
exchange with D in a pD range of 2.4-6.4 (eq 5). Figure 3b
shows the result of H/D exchange in 60 min at pD 5.0 at 70
°
C.
In the pD range of 2.4-6.4, the lower the pD of the solution,
the faster is the rate of the H/D exchange; i.e., the rate of the
H/D exchange is dependent on D concentration. These results
suggest that the hydrido ligand of 1 exhibits a protonic character.
+
4
shows pH-dependent profiles of TOFs of the transfer
hydrogenations of cyclohexanone (a, closed circle) and ac-
etophenone (b, open circle).
pD 2.4-6.4
+
} [M-D]+
D-labeled 1
9
[
M-H] {
(5)
1
The rate of transfer hydrogenations of carbonyl compounds
examined in this study shows a sharp maximum around pH 2.0.
The pH dependence should be explained not only by the stability
of 1 in the acidic media but also by the activation process of
the carbonyl compounds by protons as follows: (i) Compound
b is not reduced by 1 under stoichiometric conditions (1/b/
Reducing Ability of 1 in Acidic Media. We have investi-
gated the reducing ability of 1 toward cyclohexanone (a) as an
example of water-soluble ketones and acetophenone (b) as an
(
8) (a) Balzarek, C.; Weakley, T. J. R.; Kuo, L. Y., Tyler D. R. Organometallics
000, 19, 2927-2931. (b) Balzarek, C.; Tyler, D. R. Angew. Chem., Int.
Ed. 1999, 38, 2406-2408. (c) Trost, B. M. Chem.sEur. J. 1998, 4, 2405-
412.
2
(9) Conditions: 2/carbonyl compounds/HCOOH ) 1/200/1000, pH 2.0, 70
°C.
2
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A Water-Soluble Iridium Hydride
A R T I C L E S
Table 3. Transfer Hydrogenation of Water-Soluble Carbonyl
Compounds (a, d, e, and f) and Water-Insoluble Carbonyl
III
2+
Compounds (b, c, and g) with [Cp*Ir (bpy)(H2O)] as a Catalyst
Precursor and HCOOH as a Hydrogen Donor in Water and in
a
Biphasic Media at pH 2.0
Figure 5. (b) Temperature-dependent TOFs of the transfer hydrogenation
of a. (O) Temperature-dependent TOFs of the transfer hydrogenation of b.
a
The reaction was carried out at 70 °C using a ketone (0.32 mmol) in
Figure 6. (b) TOFs depending upon the number of moles of HCOO- of
the transfer hydrogenation of a. (O) TOFs depending upon the number of
moles of HCOO of the transfer hydrogenation of b.
b
H2O (3 mL) with 2/ketones/HCOOH ) 1/200/1000. Turnover frequency:
mol of product/mol of 2)/h. Determined by H NMR. Isolated yield after
salting-out with NaCl and extraction with Et2O. Isolated yield after
extraction with Et2O. Isolated yield by using Whatman phase separators
c
1
d
(
-
e
f
g
1
PS (silicone-treated filter paper) without organic solvents. Not isolated
by extraction with Et2O.
HCOOH ) 1/1/0) in methanol at 60 °C, though an addition of
1
% (v/v) of trifluoroacetic acid (TFA) as a proton source to the
methanol solution initiates a reduction of b to 1-phenylethanol
1
0
in 10% yield. (ii) The TOF (525) of transfer hydrogenation
of an acetophenone derivative containing an electron-withdraw-
ing group (2,2,2-trifluoroacetophenone, c) is higher than the TOF
(343) of transfer hydrogenation of acetophenone (b) at pH 2.0
at 70 °C (Table 3).
In addition, as shown in Figure 4, the TOF (343) of the
transfer hydrogenations of b catalyzed by 2 and HCOOH at
9
pH 2.0 (open circle) is significantly higher than the TOF (75)
Figure 7. (b) Time course of the TONs of the transfer hydrogenation of
a. (O) Time course of the TONs of the transfer hydrogenation of b.
of previously reported transfer hydrogenation of b catalyzed
6
II
by a ruthenium complex [(η -C6Me6)Ru (bpy)(H2O)](SO4) and
(closed circle) and b (open circle) at pH 2.0 at 70 °C. The TOFs
HCOONa at pH 4.0 (open triangle).2
b,11
of the transfer hydrogenations of carbonyl compounds examined
in this study saturate at a 1/200/1000 ratio of 2/ketones/HCOOH.
Figure 7 shows the time course of the turnover numbers
Figure 5 reveals temperature-dependent TOFs in the transfer
hydrogenations of a (closed circle) and b (open circle). The
TOFs of the transfer hydrogenations are drastically increased
above 70 °C. The catalytic reactions were carried out at 70 °C
because complex 1 is slowly decomposed above 90 °C under
even Ar atmosphere (vide supra).
() TONs, mol of product formed/mol of catalyst) in the transfer
hydrogenations of a (closed circle) and b (open circle) under
the optimized conditions (pH 2.0, 70 °C, 1/200/1000 ratio of
2
/carbonyl compounds/HCOOH).
Furthermore, Figure 6 shows TOFs depending upon concen-
trations of HCOOH used in the transfer hydrogenations of a
Table 3 demonstrates the transfer hydrogenations of a variety
of water-soluble and water-insoluble carbonyl compounds under
the optimized conditions. The water-soluble cyclic ketone
(
10) The addition of TFA mainly induces the protonation of the hydrido ligand
of 1 to generate H
11) Conditions: Ru catalyst/b/HCOONa ) 1/200/6000, pH 4.0, 70 °C.
(cyclohexanone, a) is converted to the corresponding alcohol
2
.
(
much more efficiently than a water-soluble straight chain ketone
J. AM. CHEM. SOC.
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A R T I C L E S
Abura et al.
Scheme 1
water-soluble and water-insoluble aldehydes can be reduced
similarly, but the water-soluble and water-insoluble esters cannot
be reduced.
Mechanism of High TOFs. As shown in Figure 4, the TOFs
of the acid-catalyzed transfer hydrogenation are drastically
dependent on the pH of the solution. Scheme 1 summarizes the
effect of pH on the formation of 1 (at pH 2.0-6.0) and also on
the acid-catalyzed hydride transfer (at pH 2.0-3.0). The
remarkable stability of 1 is attributed to the protonic character
of the hydrido ligand that prevents the protonation of the hydrido
ligand. In conclusion, the high TOFs of the acid-catalyzed
transfer hydrogenations should result not only from the nucleo-
philicity of 1 toward the carbonyl groups activated by protons
but rather from the robust ability of 1 under the present acidic
conditions.
Acknowledgment. Financial support of this research by the
Ministry of Education, Science, Sports, and Culture, Japan
Society for the Promotion of Science, Grant-in-Aid for Scientific
Research to S.O. (13640568) and S.F. (11228205), is gratefully
acknowledged. We thank Professor K. Isobe (Osaka City
University) and emeritus Professor A. Nakamura (Osaka
University) for valuable discussions.
(
2-butanone, d). The transfer hydrogenation of a water-soluble
keto acid (pyruvic acid, e) also occurs easily. In our previous
paper, we reported that compounds d and e cannot be reduced
by in-situ-generated 1 under unoptimized conditions (pH 5.0,
2
9
5 °C, 1/d or e/HCOOH ) 1/10/50). The rate of transfer
hydrogenation of an acetophenone derivative with a water-
soluble ligand (4-acetylbenzenesulfonic acid sodium salt, f) in
water is faster than that of acetophenone (b) in biphasic media.
A water-insoluble bulky cyclic ketone (R-tetralone, g) is also
reduced. In addition, under the conditions of this study, the
Supporting Information Available: Crystallographic infor-
mation. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0288237
4154 J. AM. CHEM. SOC.
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