Synthesis, structures, and transfer hydrogenation catalysis of bifunctional iridium complexes bearing a C-N chelate oxime ligand

Article abstract of DOI:10.1002/ejic.201100800

We have synthesized a series of organometallic oxime complexes as novel metal-ligand cooperating bifunctional catalysts. The reaction of [{Cp*IrCl(μ₂-Cl)}₂] with ketoximes in the presence of sodium acetate afforded the half-sandwich chlorido iridium complexes 6 bearing a C-N chelate oxime ligand with a protic OH group in the β-position to the metal. Complex 6a, derived from acetophenone oxime, reacted with silver triflate to give the triflate complex 7 and cationic nitrile complex 8 depending upon the reaction solvent. Complexes 6 also reacted with a base in dichloromethane to afford the oximato-bridged dinuclear complexes 9, which were converted back to the chlorido complexes 6 upon treatment with an amine hydrochloride. In contrast, dehydrochlorination of 6 in 2-propanol as well as the reaction of 9 with 2-propanol yielded the hydrido-bridged dinuclear oxime-oximato complexes 10. Crossover experiments revealed that 10 dissociates into the mononuclear hydrido-oxime complex 11 and unsaturated oximato complex 12, which are interconvertible by reactions with hydrogen donors and acceptors. Owing to the metal-ligand cooperation, 10 effectively catalyzed transfer hydrogenation of ketones with 2-propanol. Copyright

Full text of DOI:10.1002/ejic.201100800

FULL PAPER
DOI: 10.1002/ejic.201100800
Synthesis, Structures, and Transfer Hydrogenation Catalysis of Bifunctional
Iridium Complexes Bearing a C–N Chelate Oxime Ligand
Megumi Watanabe,[a] Yohei Kashiwame,[a] Shigeki Kuwata,*[a] and Takao Ikariya*[a]
Dedicated to Prof. Dr. Christian Bruneau on the occasion of his 60th birthday
Keywords: Homogeneous catalysis / Iridium / Hydrogen bonds / Hydride ligands / Cooperative effects
We have synthesized a series of organometallic oxime com- verted back to the chlorido complexes 6 upon treatment with
plexes as novel metal–ligand cooperating bifunctional cata-
lysts. The reaction of [{Cp*IrCl(μ2-Cl)}2] with ketoximes in
the presence of sodium acetate afforded the half-sandwich
chlorido iridium complexes 6 bearing a C–N chelate oxime
ligand with a protic OH group in the β-position to the metal.
Complex 6a, derived from acetophenone oxime, reacted with
silver triflate to give the triflate complex 7 and cationic nitrile
complex 8 depending upon the reaction solvent. Complexes
6 also reacted with a base in dichloromethane to afford the
oximato-bridged dinuclear complexes 9, which were con-
an amine hydrochloride. In contrast, dehydrochlorination of
6 in 2-propanol as well as the reaction of 9 with 2-propanol
yielded the hydrido-bridged dinuclear oxime–oximato com-
plexes 10. Crossover experiments revealed that 10 dissoci-
ates into the mononuclear hydrido–oxime complex 11 and
unsaturated oximato complex 12, which are interconvertible
by reactions with hydrogen donors and acceptors. Owing to
the metal–ligand cooperation, 10 effectively catalyzed trans-
fer hydrogenation of ketones with 2-propanol.
Introduction
Hydrogen transfer reactions catalyzed by metal com-
plexes offer a practical approach for the reduction of
ketones and oxidation of alcohols with the advantages of
operational simplicity and mild reaction conditions.[1] Bi-
functional metal hydride catalysts bearing a protic cooper-
ating ligand often dramatically accelerate such reactions
through the coupled transfer of the proton and hydride with
the substrates. For example, the protic amine complex with
a chiral chelate auxiliary, 1, efficiently catalyzes the asym-
metric transfer hydrogenation of ketones as a result of the
facile and reversible conversion to the coordinatively unsat-
urated amido complex 2 (Scheme 1, top).[2] The proton-de-
livering functional group and nucleophilic metal hydride are
more separated in Shvo’s catalyst 3, a heterodimer of the
reduced (4) and oxidized (5) forms, which are interconvert-
ible through proton-coupled hydride transfer with the sub-
strate (Scheme 1, bottom).[3] In these metal–ligand bifunc-
tional catalysts, the Brønsted acidity of the protic group
and the relative positions of the two functionalities are judi-
ciously designed to realize high catalytic performance.
Scheme 1. Representative examples of bifunctional transfer hydro-
genation catalysts.
We have continued our interest in the metal–ligand coop-
erating bifunctional catalysis of protic amine complexes
typified by 1.[4] Our recent efforts to modulate the catalytic
activity and selectivity led to an investigation of C–N che-
late amine complexes, in which the electron-donating σ-car-
[a] Department of Applied Chemistry, Graduate School of Science
and Engineering, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Fax: +81-3-5734-2637
E-mail: skuwata@apc.titech.ac.jp
tikariya@apc.titech.ac.jp
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Transfer Hydrogenation Catalysis of Bifunctional Ir Complexes
bon donor in the chelate increased the nucleophilic charac- C(cyclopentadienyl) bonds [2.155(5)–2.160(5) Å] in line
ter of the substrate ligand;[5] these complexes are easily ob- with the strong trans influence. It should be noted that half-
tained by acetate-promoted cyclometalation of benzylic sandwich-type organometallic oxime complexes are rare[15]
amines.[5,6] The cyclometalation methodology is also appli- in contrast to the many Werner-type analogues reported to
cable to N-heteroaromatics, which leads to the formation of date.[14,16]
the structurally diverse bifunctional N-heterocyclic car-
bene[7] and pyrazole[8] complexes bearing an NH group in
the β-position to the metal.[9] We have further explored such
β-protic bifunctional catalysts[10] derived from the cyclo-
metalation of appropriately designed ligand precursors. In
this paper, we report cyclometalated ketoxime complexes as
new β-OH bifunctional catalysts for the transfer hydrogena-
tion of ketones.
Figure 1. Dimeric structure of 6a connected with hydrogen bonds.
Hydrogen atoms except for the OH protons are omitted for clarity.
Results and Discussion
Asterisks denote atoms generated by a symmetry operation (1 – x,
1 – y, 2 – z).
Synthesis and Structure of C–N Chelate Oxime Complexes
6
Table 1. Selected bond lengths [Å] and angles [°] for 6a, 7, and 8.
We have recently demonstrated that the complex cationic
N–N chelate oxime [Cp*IrCl(Py–NOH)]Cl (Cp* = η5-
6a
7
8
C5Me5; Py–NOH = 2-acetylpyridine oxime) undergoes re-
versible deprotonation of the β-OH group.[11] The isoelec-
tronic C–N chelate oxime complexes 6 were successfully ob-
tained from the reaction of [{Cp*IrCl(μ2-Cl)}2] with aceto-
phenone oximes in the presence of sodium acetate
(Scheme 2). Quite recently, Davies and coworkers have syn-
thesized a cyclometalated aldoxime complex in a similar
manner.[9g] We also note that the cyclometalation of aro-
matic oximes has been long known[12] and is a key step in
their C–H functionalization catalyzed by Cp*Rh com-
[a]
Ir1–Xa
2.4103(13)
2.059(4)
2.041(6)
1.410(7)
1.292(7)
0.87(8)
2.224(5)
2.068(6)
2.052(6)
1.394(7)
1.299(8)
0.82
2.044(2)
2.067(3)
2.040(3)
1.395(4)
1.295(4)
1.00(6)
Ir1–N1
Ir1–C18
N1–O1
N1–C11
O1–H1
Xb[a]···H1
Xb[a]···O1
Ir1–N1–O1
Ir1–N1–C11
O1–N1–C11
2.31(8)
2.08
1.77(6)
3.126(4)
122.5(3)
121.5(4)
116.0(4)
2.807(6)
122.3(4)
120.8(4)
116.9(5)
2.748(4)
122.8(2)
121.2(2)
115.9(3)
1
plexes.[13] The H NMR spectra of 6 exhibit a sharp singlet
[a] Xa = Cl1 (6a), O2 (7), N2 (8); Xb = Cl1* (6a), O3 (7), O2 (8).
at around 9 ppm, which disappeared upon treatment with
D2O. The azomethine carbon atom resonance of 6a is ob-
served at 169.0 ppm. The detailed structure of 6a was deter-
mined by X-ray analysis and is shown in Figure 1; selected
interatomic distances and angles are listed in Table 1. Com-
plex 6a forms a centrosymmetric dimer connected by inter-
molecular hydrogen bonds between the crystallographically
refined oxime hydrogen atom and the chlorido ligand. The
hydrogen bond and the facile H–D exchange of the oxime
hydrogen atom indicate the Brønsted acidity of the β-OH
group, which should increase upon coordination.[14] The
bond lengths in the chelate are very close to those in the
related aldoxime complex mentioned above.[9g] The Ir–C1/
C2 bonds trans to the σ-aryl ligand [2.250(5) and
2.252(6) Å] are noticeably longer than the rest of the Ir–
Reactions of 6a with Silver Triflate
When the chlorido–oxime complex 6a was treated with
silver triflate in dichloromethane, the triflato complex 7 was
obtained in 45% yield, whereas the reaction in acetonitrile
afforded the cationic nitrile complex 8 in 79% yield
(Scheme 3). The OH resonance at 8.70 (7) and 10.20 (8)
1
ppm in the H NMR spectra was identified by D2O ex-
change. Figure 2 depicts the crystal structures of 7 and 8.
Both complexes have a three-legged piano-stool structure
with a C–N chelate oxime ligand. The bond lengths and
angles, shown in Table 1, are unexceptional. The OH proton
is engaged in a hydrogen bonding interaction with a triflate
oxygen atom inside (7) or outside (8) the coordination
sphere.
Scheme 2. Synthesis of C–N chelate oxime complexes 6.
Scheme 3. Reactions of 6a with silver triflate.
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505

M. Watanabe, Y. Kashiwame, S. Kuwata, T. Ikariya
FULL PAPER
Figure 2. Structures of a) 7 and b) 8. Hydrogen atoms except for
the OH protons are omitted for clarity.
Dehydrochlorination of 6
Similar to the related β-protic pyrazole complex,[8] the
chlorido–oxime complex 6 underwent facile dehydrochlor-
ination with an equimolar amount of a base to afford the
oximato-bridged dinuclear complexes 9, as shown in
Scheme 4. Complex 9a reacted with an amine hydrochloride
to regenerate 6a as expected. An X-ray analysis of 9a sub-
stantiated the dimeric structure with approximate C2 sym-
metry (Figure 3 and Table 2). The oximato nitrogen and
azomethine carbon atoms are perfectly planar as seen in 6a.
The N–O distances of 1.375 Å (mean) in the μ-oximato-
κN,κO 9a are close to those in the oxime complexes 6a, 7,
and 8 [1.394(4)–1.410(7) Å] rather than that in the oximato-
κN complex [Cp*IrCl(Py–NO)] [1.291(5) Å], in which the
N–O bond has some double bond character.[11] On the
other hand, the Ir–O bond lengths of 2.131(2) and
2.141(2) Å are longer than those in [Cp*Ir-
(Ph)(OEt)(PMe3)] [2.082(4) Å][17] and [Cp*Ir(H2O)-
(Me2CO)2](BF4)2 [Ir–OCMe2: 2.105(7) and 2.130(7) Å].[18]
The large Ir–Ir separation of 3.9892(6) Å precludes any di-
rect metal–metal bonding. The μ-κN,κO ligation of the oxi-
mate anion is found in a number of coordination com-
pounds, some of which exhibit intriguing magnetic proper-
ties,[19] yet remains rare in organometallic compounds.[20] In
the 1H NMR spectra of 9, the methyl group of the oximato
ligand is shielded (1.13 ppm) due to the arene ring in the
other half of the molecule, which suggests that the dimeric
structure of 9 is preserved in solution. In fact, the reaction
of 9a and 9b in [D6]benzene at 55 °C did not yield the cross-
over product [(Cp*Ir)2(Ph–NO)(Tol–NO)] (9c) at all after
23 h. The 13C{1H} NMR shift of the azomethine carbon
atom in 9a (169.9 ppm) is almost the same as that in the
corresponding oxime complex 6a, which contrasts with the
upfield shift observed with the deprotonation of the cat-
ionic oxime complex [Cp*IrCl(Py–NOH)]Cl to the mono-
nuclear oximato-κN complex [Cp*IrCl(Py–NO)].[11]
Figure 3. Structure of 9a. Hydrogen atoms are omitted for clarity.
Table 2. Selected bond lengths [Å] for 9a and 10a.
9a
10a
Ir1–N1/Ir2–N2
Ir1–O2/Ir2–O1
Ir1–C18/Ir2–C36
N1–O1/N2–O2
N1–C11/N2–C29
2.069(3)/2.066(3)
2.131(2)/2.141(2)
2.033(4)/2.026(4)
1.368(4)/1.382(4)
1.316(5)/1.297(5)
2.057(5)/2.036(7)
–
2.013(8)/2.024(7)
1.368(9)/1.355(9)
1.296(10)/1.315(11)
Synthesis and Reactions of Hydrido-Bridged Oxime–
Oximato Complexes 10
In contrast, dehydrochlorination of 6 in a hydrogen do-
nor solvent, 2-propanol, resulted in the formation of the
hydrido-bridged oxime–oximato complexes 10 as shown in
Scheme 5. The 1H NMR spectra of 10 exhibit characteristic
high- and low-field resonances with a 1 H intensity, which
are ascribed to the hydrido and oxime ligands, respectively.
The 30 H singlet of the Cp* ligand is somewhat shielded at
1.1 ppm, possibly due to the neighboring arene rings in the
oxime ligands. The 13C{1H} signal of the azomethine car-
bon atoms in 10a (159.2 ppm) is slightly upfield-shifted
compared with those in the oxime complex 6a and the oxi-
mato complex 9a. The approximately C2-symmetric dinu-
clear structure of 10a was determined by single-crystal X-
ray analysis as depicted in Figure 4. Although the hydrido
and oxime hydrogen atoms could not be found in the differ-
ence Fourier map, these hydrogen atoms most likely bridge
the two iridium atoms and two oxygen atoms through a
three-center–two-electron bond and an OH···O hydrogen
Scheme 4. Reversible dehydrochlorination of 6.
Scheme 5. Formation of 10.
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Transfer Hydrogenation Catalysis of Bifunctional Ir Complexes
bond, respectively, judging from the short Ir–Ir and O–O
contacts [3.0026(4) and 2.455(9) Å, respectively]. The oxime
and oximato ligands are almost indistinguishable from their
metric parameters; the N–O distances of 1.368(9) and
1.355(9) Å in 10a lie in the middle of those in the oxime
and oximato-κN complexes (vide supra).
Scheme 6. Proposed mechanism for transfer hydrogenation of 9a.
The crossover reaction of 10a and 10b demonstrated that
the reaction of 11 and 12 to give 10 is reversible as in Shvo’s
catalyst (Scheme 1). We thus examined the dehydrogenation
of the hydrido complex 10a with a ketone as the oxidant.
Treatment of 10a with an equimolar amount of acetophen-
one in methanol at 50 °C afforded 1-phenylethanol in 63%
Figure 4. Structure of 10a. Hydrogen atoms are omitted for clarity.
1
yield (NMR). The H NMR spectrum of the reaction mix-
Complex 10a could also be obtained cleanly by transfer
hydrogenation of the oximato-bridged dinuclear complex 9a
with 2-propanol at 50 °C (Scheme 5). The fully-hydrogen-
ated mononuclear hydrido–oxime complex [Cp*IrH(Ph–
NOH)] (11a) was not produced even with extended reaction
times. To gain more insight into the mechanism of this reac-
tion, we carried out the following crossover experiments un-
der milder conditions. When a mixture of the oximato-
bridged complexes 9a and 9b in 2-propanol was stirred at
room temperature for 17 h, ca. 15% of these complexes
were converted into the mixed-ligand complex [(Cp*Ir)2-
(Ph–NOH)(Tol–NO)] (10c; the OH position is arbitrary)
along with 10a and 10b in the expected statistical ratio, 10a/
10c/10b = 1:2:1. It is to be emphasized that the remaining
9 did not contain the crossover product 9c, which suggests
that cleavage of the dimeric structure of 9 is irreversible at
least under these conditions. Furthermore, a separate cross-
over reaction of the hydrido-bridged complexes 10a and 10b
in 2-propanol under the same conditions did not reach the
statistical mixture; the observed ratio of 10a/10c/10b was
ture also revealed quantitative recovery of 10a instead of
dehydrogenation product 9a, which implies that oxidation
of methanol with 12a is involved in the reaction. Notably,
the reaction performed in toluene, a solvent without hydro-
gen donating ability, did not produce 1-phenylethanol.
These observations indicate that hydrido–oxime 11a, which
would be generated by the splitting of 10a, favors recombi-
nation with 12a rather than the reaction with acetophenone.
In methanol, however, 12a is converted into 11a through
transfer hydrogenation with the solvent. The increased ratio
of 11a to 12a leads to the hydrogen transfer from 11a to
acetophenone, which gives 1-phenylethanol and 12a. Fi-
nally, 10a would be regenerated from 11a and 12a.
Catalytic Transfer Hydrogenation of Ketones with 10a
The stoichiometric reactions mentioned above encour-
approximately 2:1:2.[21] Thus, the ligand scramble in the aged us to examine the catalytic transfer hydrogenation of
transfer hydrogenation of 9 is much faster than the cross- ketones with the bifunctional oxime complexes. When a
over reactions of 9 and 10 themselves, indicating the pres- mixture of acetophenone and 10a with a substrate/catalyst
ence of mononuclear intermediate(s) in the transfer hydro- (S/C) ratio of 20:1 in 2-propanol was stirred at 50 °C, 1-
genation. Scheme 6 illustrates the possible mechanism. phenylethanol was obtained quantitatively (Table 3, Entry
Fragmentation of 9a, which hardly occurs in nonpolar sol- 1). The reaction with an increased S/C ratio resulted in a
vents as evidenced by the crossover experiment of 9a and lower yield (Entry 2). Various functionalized ketones with
9b in [D6]benzene (vide supra), may be facilitated by pro- nitro, bromo, cyano, ester, and ether groups were reduced
tonation with 2-propanol. Subsequent hydrogen transfer in to the corresponding alcohols with these functional groups
a concerted or stepwise manner would generate the mono- preserved (Entries 3–7). On the other hand, transfer hydro-
nuclear hydrido–oxime complex 11a, which immediately re- genation of 4-aminoacetophenone did not proceed at all,
acts with the concomitantly produced oximato complex 12a possibly due to the coordination of the amino group (Entry
to afford stable 10a as the final product.
8).
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507

M. Watanabe, Y. Kashiwame, S. Kuwata, T. Ikariya
FULL PAPER
Table 3. Transfer hydrogenation of ketones with 10a.
give the parent chlorido complex 6a and hydrido-bridged
dinuclear oxime–oximato complex 10a, respectively. We
have further demonstrated that 10a catalyzes the transfer
hydrogenation of ketones with 2-propanol. A series of
crossover experiments and stoichiometric reactions sub-
stantiated that 10a splits into two spectroscopically elusive
mononuclear species, i.e. 11a and 12a; these two species are
interconvertible through reactions with hydrogen donors
and acceptors and are responsible for the bifunctional catal-
ysis. Further studies on the detailed mechanism of the hy-
drogen transfer from/to the mononuclear active species 11a
and 12a as well as the development of new β-protic bifunc-
tional catalysts are underway.
Entry
X
S/C
Yield [%]
1
2
3
4
5
6
7
8
H
H
20:1
100:1
20:1
20:1
20:1
20:1
20:1
20:1
100[a]
57[a]
100[b]
99[b]
98[b]
87[b]
60[b]
0[b]
NO2
Br
CN
OAc
OMe
NH2
1
[a] Determined by GC. [b] Determined by H NMR spectroscopy.
Experimental Section
Scheme 7 outlines the proposed mechanism for the cata-
lytic transfer hydrogenation with 10a. The hydrido-bridged
dinuclear precatalyst 10a dissociates into two catalytically
active species 11a and 12a. The unsaturated oximato com-
plex 12a gains protons from 2-propanol to give the hyd-
rido–oxime complex 11a. Complex 11a then reduces the
ketone to the corresponding alcohol. The overall mecha-
nism is quite similar to that of Shvo’s catalyst, although
formal redox of the metal and ligand is not involved in the
β-protic oxime catalyst. It should be noted that cooperation
General: All manipulations were performed under an atmosphere
of argon using standard Schlenk technique unless otherwise speci-
fied. Solvents were dried by heating to reflux with sodium benzo-
phenone ketyl (THF, toluene, diethyl ether, and hexane) or CaH2
(acetonitrile, dichloromethane, and 2-propanol) and distilled before
use. Ketoximes[24] and [{Cp*IrCl(μ2-Cl)}2][25] were prepared ac-
cording to the literature. 1H and 13C{1H} NMR spectra (399.78
and 100.53 MHz) were obtained with a JEOL JNM-ECX400 spec-
trometer. The azomethine carbon signals were assigned by H-13C
heteronuclear multiple quantum coherence (HMQC) and hetero-
1
of the oxime ligand is evident and essential in this catalysis nuclear multiple-bond connectivity (HMBC) spectroscopy. Infrared
spectra were recorded with a JASCO FT/IR-610 spectrometer. GC
analyses were carried out using a Shimadzu GC-17A with an IN-
NOWAX column. Elemental analyses were performed with a Per-
kin–Elmer 2400II CHN analyzer.
in contrast to previously reported catalytic reactions with
oxime complexes.[22]
[Cp*IrCl(Ph–NOH)] (6a): To a solution of [{Cp*IrCl(μ2-Cl)}2]
(955.3 mg, 1.1991 mmol) and acetophenone oxime (325.0 mg,
2.4045 mmol) in dichloromethane (40 mL) was added sodium acet-
ate (217.0 mg, 2.6452 mmol), and the mixture was stirred for 19 h
at room temperature. After removal of the solvent in vacuo, the
orange residue was extracted with dichloromethane (3ϫ 10 mL)
and filtered through a Celite pad. The resultant orange solution
was evaporated to dryness. Recrystallization from dichlorometh-
ane/hexane (30 mL/50 mL) afforded 6a as orange crystals
(670.0 mg, 1.3480 mmol, 56%). 1H NMR (CD2Cl2, –80 °C): δ =
1.74 (s, 15 H, Cp*), 2.54 (s, 3 H, N=CMe), 7.03, 7.13 (m, 1 H each,
aryl), 7.33 (d, J = 8.6 Hz, 1 H, aryl), 7.67 (d, J = 7.3 Hz, 1 H, aryl),
9.42 (s, 1 H, OH) ppm. 13C{1H} NMR (CD2Cl2): δ = 8.9 [C5-
(CH3)5], 11.5 [N=C(CH3)], 89.6 [C5(CH3)5], 121.6, 126.0, 130.3,
Scheme 7. Proposed mechanism for catalytic transfer hydrogena-
tion with 10a.
134.9, 142.8, 162.7, 169.0 (C=NOH) ppm. IR (KBr): ν = 1607,
˜
1576, 1547, 1513 cm–1. C18H23ClIrNO (497.06): calcd. C 43.50, H
4.66, N 2.82; found C 43.49, H 4.65, N 2.90.
[Cp*IrCl(Tol–NOH)] (6b): This complex was obtained from 1-(p-
tolyl)ethanone oxime by a method similar to that for 6a. Yield:
40%. H NMR (CDCl3): δ = 1.74 (s, 15 H, Cp*), 2.40, 2.54 (s, 3
Conclusions
1
We have synthesized C–N chelate oxime complexes fea-
turing a Brønsted acidic β-OH group as new metal–ligand
bifunctional catalysts. Dehydrochlorination of the
chlorido–oxime complex 6a afforded the oximato complex
12a, which is also bifunctional, with a Lewis acidic metal
center and basic oximato ligand in a rigid framework. The
organometallic frustrated Lewis pair[23] in 12a is quenched
in the dimer 9a yet exerts a bifunctional effect in the reac-
H each, N=CMe and C6H3Me), 6.83, 7.20 (d, J = 7.7 Hz, 1 H
each, aryl), 7.49 (s,
1 H, aryl), 8.14 (s, 1 H, OH) ppm.
C19H25ClIrNO (511.08): calcd. C 44.65, H 4.93, N 2.74; found C
44.44, H 4.96, N 2.95.
[Cp*Ir(OTf)(Ph–NOH)] (7):
A
mixture of 6a (74.4 mg,
0.150 mmol) and silver triflate (42.2 mg, 0.164 mmol) in dichloro-
methane (10 mL) was stirred for 14 h at room temperature. After
removal of the solvent in vacuo, the orange residue was extracted
tion of 9a with an amine hydrochloride and 2-propanol to with dichloromethane (3ϫ 2 mL). Evaporation of the extract and
508 Eur. J. Inorg. Chem. 2012, 504–511
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Transfer Hydrogenation Catalysis of Bifunctional Ir Complexes
subsequent recrystallization from THF/hexane (1 mL/20 mL) af-
forded 7 as orange crystals (41.6 mg, 0.0681 mmol, 45%). 1H NMR
(CDCl3): δ = 1.72 (s, 15 H, Cp*), 2.51 (s, 3 H, N=CMe), 7.12, 7.25
(m, 1 H each, aryl), 7.35 (d, J = 7.4 Hz, 1 H, aryl), 7.89 (d, J =
7.6 Hz, 1 H, aryl), 8.70 (s, 1 H, OH) ppm. C19H23F3IrNO4S
(610.67): calcd. C 37.37, H 3.80, N 2.29; found C 37.25, H 3.86, N
2.31.
120.7, 123.0, 126.1, 137.1, 146.6, 158.4, 159.2 (C=NOH) ppm. IR
(KBr): ν = 1574, 1557 cm–1. C H Ir N O (923.21): calcd. C
˜
36 46
2
2
2
46.84, H 5.02, N 3.03; found C 46.68, H 5.03, N 3.16.
[(Cp*Ir)2(μ2-H)(Tol–NO)(Tol–NOH)] (10b): This complex was ob-
tained from 6b in a manner similar to that for 10a. 1H NMR
(CD2Cl2): δ = –19.33 (s, 1 H, IrH), 1.07 (s, 30 H, Cp*), 2.37 (s, 6
H, C6H3Me), 2.59 (s, 6 H, N=CMe), 6.72, 7.00 (d, J = 7.7 Hz, 2
H each, aryl), 7.50 (s, 2 H, aryl), 18.41 (s, 1 H, OH) ppm.
[Cp*Ir(MeCN)(Ph–NOH)](OTf) (8): A mixture of 6a (225.3 mg,
0.4533 mmol) and silver triflate (140.0 mg, 0.5449 mmol) in aceto-
nitrile (20 mL) was stirred for 3 h at room temperature. After re-
moval of the solvent in vacuo, the yellow residue was extracted
with dichloromethane (4ϫ 4 mL). Evaporation of the extract and
subsequent recrystallization from dichloromethane/hexane (2 mL/
20 mL) afforded 8 as yellow crystals (234.3 mg, 0.3595 mmol,
Reaction of 9a with 2-Propanol: A solution of 9a (29.2 mg,
0.0317 mmol) in 2-propanol (5 mL) was stirred for 36 h at 50 °C.
Evaporation of the solvent and subsequent recrystallization from
THF/hexane (0.8 mL/20 mL) afforded 10a as red crystals (4.4 mg,
0.0048 mmol, 15%).
1
Transfer Hydrogenation of 9a and 9b with 2-Propanol: A mixture
of 9a (6.5 mg, 0.0071 mmol) and 9b (6.7 mg, 0.0071 mmol) in 2-
propanol (10 mL) was stirred for 17 h at room temperature. The
solvent was removed in vacuo, and the residue dissolved in CD2Cl2.
The 1H NMR spectrum using 1,3,5-trimethoxybenzene as an in-
ternal standard revealed that ca. 15% of complexes 9 were con-
verted into 10a, 10b, and the mixed-ligand product 10c with the
10a/10c/10b ratio of 1:2:1.
10c: 1H NMR (CD2Cl2): δ = –19.32 (s, 1 H, IrH), 1.07 (s, 30 H,
Cp*), 2.37 (s, 3 H, C6H3Me), 2.61 (s, 6 H, N=CMe), 6.73 (d, J =
7.8 Hz, 1 H, aryl), 6.84, 6.90 (m, 1 H each, aryl), 7.02 (d, J =
7.8 Hz, 1 H, aryl), 7.13 (m, 1 H, aryl), 7.50 (s, 1 H, aryl), 7.67 (d,
J = 7.4 Hz, 1 H, aryl), 18.41 (s, 1 H, OH) ppm.
79%). H NMR (CDCl3): δ = 1.78 (s, 15 H, Cp*), 2.44, 2.58 (s, 3
H each, N=CMe and MeCN), 7.13, 7.18 (m, 1 H each, aryl), 7.36
(d, J = 7.2 Hz, 1 H, aryl), 7.59 (d, J = 6.9 Hz, 1 H, aryl), 10.20 (s,
1 H, OH) ppm. IR (KBr): ν = 2313 (CϵN), 1615, 1580, 1553, 1508
˜
cm–1. C21H26F3IrN2O4S (651.73): calcd. C 38.70, H 4.02, N 4.30;
found C 38.69, H 4.00, N 4.26.
[{Cp*Ir(Ph–NO)}2] (9a): A mixture of 6a (109.0 mg, 0.2193 mmol)
and KOtBu (29.7 mg, 0.265 mmol) in dichloromethane (18 mL)
was stirred for 17 h at room temperature. After removal of the sol-
vent in vacuo, the red residue was extracted with diethyl ether (4ϫ
2 mL). The extract was concentrated to ca. 2 mL and stored at
–30 °C. The red crystals formed were collected by filtration and
1
dried in vacuo (69.0 mg, 0.0749 mmol, 68%). H NMR (CD2Cl2):
δ = 1.13 (s, 6 H, N=CMe), 1.62 (s, 30 H, Cp*), 6.53 (d, J = 7.3 Hz,
2 H, aryl), 6.77, 6.82 (m, 2 H each, aryl), 7.84 (d, J = 7.3 Hz, 2
H, aryl) ppm. 13C{1H} NMR (CD2Cl2): δ = 9.3 [C5(CH3)5], 11.4
[N=C(CH3)], 87.0 [C5(CH3)5], 120.8, 123.3, 126.6, 136.0, 145.8,
Catalytic Transfer Hydrogenation of Ketones with 10a: A mixture
of 10a (2.9 mg, 0.0031 mmol) and ketone (0.0628 mmol) in 2-pro-
panol (2 mL) was stirred at 50 °C for 15 h. The reaction mixture
was filtered through a silica plug and subjected to GC and 1H
NMR analysis.
163.9, 169.9 (C=NOH) ppm. IR (KBr): ν = 1573, 1565 cm–1.
˜
C36H44Ir2N2O2 (921.19): calcd. C 46.94, H 4.81, N 3.04; found C
47.33, H 5.14, N 3.13.
X-ray Diffraction Studies: Diffraction experiments were performed
with a Rigaku Saturn CCD area detector with graphite-monochro-
mated Mo-Kα radiation (λ = 0.71070 Å). Single crystals suitable for
X-ray analysis were mounted on glass fibers. Intensity data were
[{Cp*Ir(Tol–NO)}2] (9b): This complex was obtained from 6b in a
1
manner similar to that for 9a. H NMR (CD2Cl2): δ = 1.13 (s, 6
H, N=CMe), 1.61 (s, 30 H, Cp*), 2.36 (s, 6 H, C6H3Me), 6.43, 6.63 corrected for Lorentz–polarization effects and for absorption. De-
(d, J = 7.6 Hz, 2 H each, aryl), 7.66 (s, 2 H, aryl) ppm.
tails of crystal and data collection parameters are summarized in
Table 4. Structure solution and refinements were carried out using
the CrystalStructure program package.[26] The heavy atom posi-
tions were determined by a direct method program (SIR92;[27] for
6a, 7, 8, and 10a) or a Patterson method program (DIRDIF99
PATTY;[28] for 9a) and remaining non-hydrogen atoms were found
by subsequent Fourier syntheses. The non-hydrogen atoms were
refined anisotropically by full-matrix least-square techniques based
on F2. The severely disordered Cp* ligands in 10a were refined as
follows: the methyl carbon atoms in one Cp* group [C(6)–C(10)]
were refined with isotropic thermal parameters, and the other Cp*
group was located at two disordered positions with 60% and 40%
occupancies and included in the refinements as rigid groups. The
oxime hydrogen atoms in 6a and 8 were refined isotropically,
whereas that in 7 was included in the refinement with fixed iso-
tropic thermal parameters. The rest of the hydrogen atoms, except
for the hydrido and oxime hydrogen atoms in 10a, were placed at
calculated positions and included in the refinements using the ri-
ding model. The absolute structures of 9a and 10a were determined
on the basis of the Flack absolute structure parameter.[29]
Crossover Experiments with 9a and 9b: In an NMR tube equipped
with a J-Young valve, a mixture of 9a (4.7 mg, 0.0051 mmol) and
9b (4.8 mg, 0.0051 mmol) in C6D6 (0.5 mL) was kept at 55 °C. The
1H NMR spectrum of the mixture exhibited no significant change
during a period of 23 h.
Reaction of 9a with Ph2NH·HCl: To a solution of 9a (31.2 mg,
0.0339 mmol) in dichloromethane (4 mL) was added Ph2NH·HCl
(14.1 mg, 0.0686 mmol) at –78 °C, and the mixture was slowly
warmed to room temperature with stirring. After 17 h, the mixture
was evaporated to dryness. Recrystallization from dichlorometh-
ane/hexane (1.5 mL/20 mL) afforded 6a (24.7 mg, 0.0497 mmol,
73%).
[(Cp*Ir)2(μ2-H)(Ph–NO)(Ph–NOH)] (10a):
A mixture of 6a
(53.1 mg, 0.107 mmol) and KOtBu (14.4 mg, 0.128 mmol) in 2-pro-
panol (10 mL) was stirred for 18 h at room temperature. After re-
moval of the solvent in vacuo, the red residue was extracted with
toluene (4ϫ 2 mL). Evaporation of the extract and subsequent
recrystallization from THF/hexane (1 mL/20 mL) afforded 10a as
red crystals (11.3 mg, 0.0122 mmol, 23%). 1H NMR (CD2Cl2): δ =
–19.31 (s, 1 H, IrH), 1.06 (s, 30 H, Cp*), 2.62 (s, 6 H, N=CMe),
6.85, 6.90 (m, 2 H each, aryl), 7.13 (d, J = 7.4 Hz, 2 H, aryl), 7.66
(d, J = 7.0 Hz, 2 H, aryl), 18.50 (s, 1 H, OH) ppm. 13C{1H} NMR
CCDC-837050 (for 6a), -837051 (for 7), -837052 (for 8), -837053
(for 9a), and -837054 (for 10a) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
(CD2Cl2): δ = 8.3 [C5(CH3)5], 12.0 [N=C(CH3)], 92.6 [C5(CH3)5], www.ccdc.cam.ac.uk/data_request/cif.
Eur. J. Inorg. Chem. 2012, 504–511
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
509

M. Watanabe, Y. Kashiwame, S. Kuwata, T. Ikariya
FULL PAPER
Table 4. X-ray crystallographic data for 6a, 7, 8, 9a, and 10a.
6a
7
8
9a
10a
Emprical formula
MW
C18H23ClIrNO
497.06
C19H23F3IrNO4S C21H26F3IrN2O4S C36H44Ir2N2O2
C36H46Ir2N2O2
923.21
610.67
651.72
921.20
Crystal system
monoclinic
P21/c
0.3ϫ0.1ϫ0.1
14.926(6)
14.875(6)
8.334(4)
90
monoclinic
P21/n
0.5ϫ0.5ϫ0.3
10.407(5)
20.157(9)
10.471(5)
90
monoclinic
P21/c
0.3ϫ0.1ϫ0.1
12.463(2)
13.157(3)
15.710(3)
90
orthorhombic
P212121
0.4ϫ0.1ϫ0.1
10.9515(17)
14.237(2)
20.811(3)
90
monoclinic
P21
0.4ϫ0.2ϫ0.2
11.130(3)
11.290(3)
12.929(4)
90
Space group
Crystal size [mm3]
a [Å]
b [Å]
c [Å]
α [°]
β [°]
112.447(6)
90
1710.1(13)
4
110.669(6)
90
2055.0(16)
4
113.863(2)
90
2356.0(8)
4
90
90
3244.7(9)
4
95.964(4)
90
1615.9(7)
2
γ [°]
V [Å3]
Z
T [K]
93
1.930
7.986
123
1.974
6.669
123
1.837
5.824
123
1.886
8.251
123
1.897
8.284
Density (calcd.) [gcm–3]
μ [mm–1]
F(000)
960
1184
1272
1776
892
Transmission factors
Independent reflections (Rint
Parameters
0.266–0.450
3844 (0.040)
225
0.065–0.135
4711 (0.060)
287
0.365–0.559
5389 (0.029)
319
0.196–0.438
7383 (0.037)
425
0.105–0.191
7064 (0.029)
327
)
R1 [I Ͼ2σ (I)]
wR2 (all data)
Goodness-of-fit
0.0250
0.0917
1.000
0.0415
0.1069
1.002
2.86/–2.06
0.0214
0.0749
1.001
1.49/–1.12
0.0184
0.0452
1.005
1.48/–1.94
0.0334
0.0913
1.002
9.49/–4.59
Largest difference peak/hole [eÅ–3] 1.96/–1.50
Eur. J. Inorg. Chem. 1998, 485–493; b) D. L. Davies, O. Al-
Duaij, J. Fawcett, M. Giardiello, S. T. Hilton, D. R. Russell,
Dalton Trans. 2003, 4132–4138; c) C. Scheeren, F. Maasarani,
A. Hijazi, J.-P. Djukic, M. Pfeffer, S. D. ZariЈc, X.-F. Le Goff,
L. Ricard, Organometallics 2007, 26, 3336–3345; d) L. Li,
W. W. Brennessel, W. D. Jones, J. Am. Chem. Soc. 2008, 130,
12414–12419; e) Y. Boutadla, O. Al-Duaij, D. L. Davies, G. A.
Griffith, K. Singh, Organometallics 2009, 28, 433–440; f) J.-P.
Djukic, J.-B. Sortais, L. Barloy, M. Pfeffer, Eur. J. Inorg. Chem.
2009, 817–853; g) J. F. Hull, D. Balcells, J. D. Blakemore, C. D.
Incarvito, O. Eisenstein, G. W. Brudvig, R. H. Crabtree, J. Am.
Chem. Soc. 2009, 131, 8730–8731; h) Y. Boutadla, D. L. Davies,
R. C. Jones, K. Singh, Chem. Eur. J. 2011, 17, 3438–3448; i)
E. F. Flegeau, C. Bruneau, P. H. Dixneuf, A. Jutand, J. Am.
Chem. Soc. 2011, 133, 10161–10170; j) L. Bernet, R. Lalrem-
puia, W. Ghattas, H. Mueller-Bunz, L. Vigara, A. Llobet, M.
Albrecht, Chem. Commun. 2011, 47, 8058–8060.
Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research
(S) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (22225004).
[1] a) T. Ikariya, A. J. Blacker, Acc. Chem. Res. 2007, 40, 1300–
1308; b) J. S. M. Samec, J.-E. Bäckvall, P. G. Andersson, P.
Brandt, Chem. Soc. Rev. 2006, 35, 237–248.
[2] T. Ikariya, K. Murata, R. Noyori, Org. Biomol. Chem. 2006, 4,
393–406.
[3] a) M. C. Warner, C. P. Casey, J.-E. Bäckvall, Top. Organomet.
Chem. 2011, 37, 85–125; b) B. L. Conley, M. K. Pennington-
Boggio, E. Boz, T. J. Williams, Chem. Rev. 2010, 110, 2294–
2312.
[4] a) T. Ikariya, Bull. Chem. Soc. Jpn. 2011, 84, 1–16; b) M. Ito,
T. Ootsuka, R. Watari, A. Shiibashi, A. Himizu, T. Ikariya, J.
Am. Chem. Soc. 2011, 133, 4240–4242; c) M. Ito, A. Shiibashi,
T. Ikariya, Chem. Commun. 2011, 47, 2134–2136.
[5] a) S. Arita, T. Koike, Y. Kayaki, T. Ikariya, Angew. Chem. 2008,
120, 2481–2483; Angew. Chem. Int. Ed. 2008, 47, 2447–2449;
b) S. Arita, T. Koike, Y. Kayaki, T. Ikariya, Organometallics
2008, 27, 2795–2802; c) S. Arita, T. Koike, Y. Kayaki, T. Ika-
riya, Chem. Asian J. 2008, 3, 1479–1485.
[6] a) J.-B. Sortais, V. Ritleng, A. Voelklin, A. Holuigue, H. Small,
L. Barloy, C. Sirlin, G. K. M. Verzijl, J. A. F. Boogers,
A. H. M. de Vries, J. G. de Vries, M. Pfeffer, Org. Lett. 2005, 7,
1247–1250; b) J.-B. Sortais, N. Pannetier, A. Holuigue, L. Bar-
loy, C. Sirlin, M. Pfeffer, N. Kyritsakas, Organometallics 2007,
26, 1856–1867; c) J.-B. Sortais, N. Pannetier, N. Clément, L.
Barloy, C. Sirlin, M. Pfeffer, N. Kyritsakas, Organometallics
2007, 26, 1868–1874.
[10]
a) S. Kuwata, T. Ikariya, Chem. Eur. J. 2011, 17, 3542–3556;
b) Y. Kashiwame, M. Watanabe, K. Araki, S. Kuwata, T. Ika-
riya, Bull. Chem. Soc. Jpn. 2011, 84, 251–258.
M. Watanabe, Y. Kashiwame, S. Kuwata, T. Ikariya, Chem.
Lett. 2010, 39, 758–759.
[11]
[12]
a) H. Onoue, K. Minami, K. Nakagawa, Bull. Chem. Soc. Jpn.
1970, 43, 3480–3485; b) H. Onoue, I. Moritani, J. Organomet.
Chem. 1972, 44, 189–195; c) M. Nonoyama, J. Organomet.
Chem. 1975, 92, 89–95; d) A. J. Nielson, J. Chem. Soc., Dalton
Trans. 1981, 205–211; e) A. D. Ryabov, G. M. Kazankov, A. K.
Yatsimirsky, L. G. Kuz’mina, O. Yu. Burtseva, N. V. Dvort-
sova, V. A. Polyakov, Inorg. Chem. 1992, 31, 3083–3090; f) H.
Werner, A. Höhn, M. Dziallas, T. Dirnberger, Dalton Trans.
2006, 2597–2606.
a) A. S. Tsai, M. Brasse, R. G. Bergman, J. A. Ellman, Org.
Lett. 2011, 13, 540–542; b) X. Zhang, D. Chen, M. Zhao, J.
Zhao, A. Jia, X. Li, Adv. Synth. Catal. 2011, 353, 719–723.
V. Yu. Kukushkin, D. Tudela, A. J. L. Pombeiro, Coord. Chem.
Rev. 1996, 156, 333–362.
[13]
[7] K. Araki, S. Kuwata, T. Ikariya, Organometallics 2008, 27,
2176–2178.
[14]
[15]
[8] Y. Kashiwame, S. Kuwata, T. Ikariya, Chem. Eur. J. 2010, 16,
766–770.
a) U. Kölle, E. Raabe, C. Krüger, F. P. Rotzinger, Chem. Ber.
1987, 120, 979–985; b) E. Steckhan, S. Herrmann, R. Ruppert,
E. Dietz, M. Frede, E. Spika, Organometallics 1991, 10, 1568–
1577; c) H. Werner, T. Daniel, W. Knaup, O. Nürnberg, J. Or-
[9] β-NH-free azametallacycles derived from base-promoted cyclo-
metalation of N-heterocycles are much more prevalent: a) W.
Bauer, M. Prem, K. Polborn, K. Sünkel, W. Steglich, W. Beck,
510
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 504–511

Transfer Hydrogenation Catalysis of Bifunctional Ir Complexes
ganomet. Chem. 1993, 462, 309–318; d) T. Daniel, W. Knaup,
M. Dziallas, H. Werner, Chem. Ber. 1993, 126, 1981–1993; e)
R. Lampeka, R. Bergs, R. Krämer, K. Polborn, W. Beck, Z.
Naturforsch. B 1994, 49, 225–232; f) R. Bergs, R. Lampeka,
C. Robl, W. Beck, Z. Naturforsch. B 1994, 49, 483–488; g) R.
Lampeka, R. Bergs, R. Fernández de Bobadilla, K. Polborn, S.
Mihan, W. Beck, J. Organomet. Chem. 1995, 491, 203–214; h)
P. Kumar, A. K. Singh, S. Sharma, D. S. Pandey, J. Organomet.
Chem. 2009, 694, 3643–3652.
Wong, New J. Chem. 2002, 26, 94–104; c) J. A. Cabeza, I.
del Río, V. Riera, M. Suárez, C. Álvarez-Rúa, S. García-
Granda, S. H. Chuang, J. R. Hwu, Eur. J. Inorg. Chem. 2003,
4159–4165; d) J. S.-Y. Wong, Y.-J. Gu, L. Szeto, W.-T. Wong,
CrystEngComm 2008, 10, 29–33.
The ligand scrambling completed to give the statistical mixture
after 17 h in both 2-propanol and toluene at 50 °C.
a) D. A. Alonso, C. Nájera, Chem. Soc. Rev. 2010, 39, 2891–
2902; b) D. Yang, H. Fu, Chem. Eur. J. 2010, 16, 2366–2370;
c) S. Mukherjee, B. A. Patel, S. Bhaduri, Organometallics 2009,
28, 3074–3078.
G. Erker, Dalton Trans. 2011, 40, 7475–7483.
N. Jain, A. Kumar, S. M. S. Chauhan, Tetrahedron Lett. 2005,
46, 2599–2602.
C. White, A. Yates, P. M. Maitlis, Inorg. Synth. 1992, 29, 228–
234.
[21]
[22]
[16] a) M. E. Keeney, K. Osseo-Asare, K. A. Woode, Coord. Chem.
Rev. 1984, 59, 141–201; b) A. G. Smith, P. A. Tasker, D. J.
White, Coord. Chem. Rev. 2003, 241, 61–85; c) C. J. Milios,
T. C. Stamatatos, S. P. Perlepes, Polyhedron 2006, 25, 134–194.
[17] J. C. M. Ritter, R. G. Bergman, J. Am. Chem. Soc. 1998, 120,
6826–6827.
[18] H. Amouri, C. Guyard-Duhayon, J. Vaissermann, M. N.
Rager, Inorg. Chem. 2002, 41, 1397–1403.
[19] a) P. Chaudhuri, Coord. Chem. Rev. 2003, 243, 143–190; b) [26] CrystalStructure 3.8, Rigaku/MSC, The Woodlands TX, USA,
[23]
[24]
[25]
T. C. Stamatatos, G. Christou, Inorg. Chem. 2009, 48, 3308–
3322; c) M. Prinz, K. Kuepper, C. Taubitz, M. Raekers, S.
Khanra, B. Biswas, T. Weyhermüller, M. Uhlarz, J. Wosnitza,
J. Schnack, A. V. Postnikov, C. Schröder, S. J. George, M. Neu-
mann, P. Chaudhuri, Inorg. Chem. 2010, 49, 2093–2102; d) B.
Cordero, O. Roubeau, S. J. Teat, A. Escuer, Dalton Trans. 2011,
40, 7127–7129.
2000–2006.
[27] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.
Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994, 27,
435.
[28] P. T. Beurskens, G. Admiraal, H. Behm, G. Beurskens, J. M. M.
Smits, C. Smykalla, Z. Kristallogr. (Suppl. 4) 1991, 99.
[29] H. D. Flack, Acta Crystallogr., Sect. A 1983, 39, 876.
Received: July 30, 2011
[20] a) M. Langenbahn, H. Stoeckli-Evans, G. Süss-Fink, J. Or-
ganomet. Chem. 1990, 397, 347–358; b) J. S.-Y. Wong, W.-T.
Published Online: October 10, 2011
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