Synthesis of and catalytic nitrile hydration by a cationic tris(μ-hydroxo)diruthenium(II) complex having PMe3ligands

Article abstract of DOI:10.1016/j.poly.2016.04.006

While phenyl vinyl ether does not react with [Ru(η⁴-1,5-COD)(η⁶-1,3,5-COT)] (1)/PMe₃, the C–O bond cleavage of phenyl vinyl ether occurs by 1/PMe₃in the presence of water to give a tris(μ-hydroxo)diruthenium(II) complex [(Me₃P)₃Ru(μ-OH)₃Ru(PMe₃)₃]⁺[OPh]^?·HOPh (3·HOPh) with evolution of ethylene. The molecular structure of 3·HOPh is unequivocally determined by X-ray analysis. The most likely mechanism for the formation of 3·HOPh is protonation of [Ru(η⁴-1,5-COD)(PMe₃)₃] (2c) by water and subsequent insertion of phenyl vinyl ether into the resulting Ru–H bond followed by the β-phenoxide elimination and hydrolysis and dimerization of the phenoxoruthenium(II) species. Complex 3 acts as a catalyst for nitrile hydration. As a typical example, the hydration of benzonitrile was achieved by 3 (1.0 mol%) in 1,4-dioxane at 120 °C for 6 h to give benzamide quantitatively.

Full text of DOI:10.1016/j.poly.2016.04.006

Accepted Manuscript
Synthesis of and catalytic nitrile hydration by a cationic tris(μ-hydroxo)diru-
thenium(II) complex having PMe ligands
3
Sayori Kiyota, Takako Kobori, Hirofumi Soeta, You-ichi Ichikawa, Nobuyuki
Komine, Sanshiro Komiya, Masafumi Hirano
PII:
S0277-5387(16)30078-X
http://dx.doi.org/10.1016/j.poly.2016.04.006
POLY 11932
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
25 January 2016
5 April 2016
Please cite this article as: S. Kiyota, T. Kobori, H. Soeta, Y-i. Ichikawa, N. Komine, S. Komiya, M. Hirano, Synthesis
of and catalytic nitrile hydration by a cationic tris(μ-hydroxo)diruthenium(II) complex having PMe ligands,
3
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.04.006
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Synthesis of and catalytic nitrile hydration by a cationic tris(µ-
hydroxo)diruthenium(II) complex having PMe ligands
3
Sayori Kiyota, Takako Kobori, Hirofumi Soeta, You-ichi Ichikawa, Nobuyuki Komine, Sanshiro
§
Komiya and Masafumi Hirano*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of
Agriculture and Technology, 2-24-26 Nakacho, Koganei, Tokyo 184-8588, Japan
RECEIVED DATE
XX, XXXX, 2016
CORRESPONDING AUTHOR FOOTNOTE: Phone and facsimile: +81 423 887 044. email:
§
hrc@cc.tuat.ac.jp (MH). Current address: Gakushuin University. Abbreviation used in this text:
COD = cyclooctadiene (C
8
H
12), COT = cyclooctatriene (C
8
H12), Cp* = pentamethylcyclopentadienyl
-
(
C
5
Me
5
). DMPE = 1,2-bis(dimethylphosphino)ethane (Me
PC PEt ).
2
PC PMe ), DEPE = 1,2-
2
H
4
2
bis(diethylphosphino)ethane (Et
2
2
H
4
2
KEYWORDS. C−O Bond activation, Tris(hydroxo)diruthenium(II), Hydration of nitriles
th
DEDICATION. This article celebrates Martin A. Bennett’s 80 birthday and numerous path
breaking contributions to organometallic chemistry.
1

ABSTRACT
4
6
While phenyl vinyl ether does not react with [Ru(η -1,5-COD)(η -1,3,5-COT)] (1)/PMe
C−O bond cleavage of phenyl vinyl ether occurs by 1/PMe in the presence of water to give a tris(µ-
hydroxo)diruthenium(II) complex [(Me P) Ru(µ-OH) Ru(PMe
3
, the
3
+
-
3
3
3
3 3
) ] [OPh] ·HOPh (3·HOPh) with
evolution of ethylene. The molecular structure of 3·HOPh is unequivocally determined by X-ray
4
analysis. The most likely mechanism for the formation of 3·HOPh is protonation of [Ru(η -1,5-
3 3
COD)(PMe ) ] (2c) by water and subsequent insertion of phenyl vinyl ether into the resulting Ru−H
bond followed by the β-phenoxide elimination and hydrolysis and dimerization of the
phenoxoruthenium(II) species. Complex 3 acts as a catalyst for nitrile hydration. As a typical
example, the hydration of benzonitrile was achieved by 3 (1.0 mol%) in 1,4-dioxane at 120 ˚C for 6
h to give benzamide quantitatively.
2

1
. Introduction
C−O bond cleavage reactions by transition-metal complexes are attractive in relation to
development of environmentally benign halogen-free process [1]. In fact, these reactions are widely
used as an inlet for the molecular transformation in modern organic synthesis, but the C−O bond
cleavage of ethers still remains difficult. The limited examples involve the Ar−OMe bond cleavage
using a directing group [2], synthesis of biphenyl from anisol [3], the Ar−OMe cleavage in a pincer
ligand [4], and the C−O bond cleavage in dialkyl ethers by Lewis acidic compounds [5]. The C−O
bond cleavage in alkenyl ethers is also very difficult but the stoichiometric bimolecular C−O bond
oxidative addition by a Lewis acidic Sm(II) complex [6] (Scheme 1, (A)), and β-siloxide elimination
[
(
7] from a putative β-siloxyethyl intermediate giving a siloxocobalt(I) are documented (Scheme 1,
B)) [8]. The definitive C−O bond oxidative addition of alkenyl ethers to a mono-nuclear complex
has not yet been reported to our best knowledge.
Scheme 1. C-O Bond cleavage reactions of alkenyl ethers.
We have reported allylic C−O bond oxidative additions of carboxylates [9] and ethers [10] to
4
6
[
Ru(η -1,5-COD)(η -1,3,5-COT)] (1) in the presence of tertiary phosphine [11] (Scheme 2). While
similar vinylic C−O oxidative addition of carboxylates took place to give a
3

(
carboxylato)(vinyl)ruthenium(II) [12], no C−O bond oxidative addition occurred when vinyl ether
was used for the reaction [13].
4
6
Scheme 2. Reactions of [Ru(η -1,5-COD)(η -1,3,5-COT)] (1) with allyl and vinyl carboxylates and
ethers.
However, we accidentally found that water promoted the C−O bond cleavage reaction of phenyl
vinyl ether by 1/PMe
3
3 3
to give a cationic tris(µ-hydroxo)diruthenium(II) [(Me P) Ru(µ-
+
-
OH)
3
Ru(PMe
3
)
3
] [OPh] (3) with evolution of ethylene. In this paper, we disclose the reaction of
3
phenyl vinyl ether with 1/PMe in the presence of water, and hydration of nitriles catalyzed by 3.
2
. Experimental Section
4

2
.1. General
All procedures described in this paper were carried out under a nitrogen or argon atmosphere by
use of Schlenk and screw-cap tubes and vacuum line techniques. Acetone was dried over Drierite
and distilled under nitrogen. Benzene, toluene, hexane, THF, 1,4-dioxane and Et O were distilled
over sodium benzophenone ketyl. These solvents were stored under a nitrogen atmosphere.
Acetone-d was dried over Drierite and was distilled under reduced pressure, and stored under
vacuum. Benzene-d was dried over sodium wire and stored under vacuum, and it was used by the
vacuum distillation prior to use. Chloroform-d was dried over calcium chloride and stored under
2
6
6
1
4
6
vacuum. [Ru(η -1,5-COD)( η -1,3,5-COT)] (1) was prepared according to the literature method
except for using magnetic stirring instead of ultrasonic irradiation [14]. Preparations of cis-
1
1
[
[
Ru(CH=CH
2
)(OAc-κ O)(dmpe)
2
] (5a), cis-[Ru(CH=CH
2
)(OAc-κ O)(PMe
3
) ] (5b) and trans-
4
1
Ru(CH=CH
2
)(OAc-κ O)(depe)
2
] (5c) were reported previously [9b]. Nitriles were used after
degassed by freeze-pump-thaw cycles or purified by distillation under reduced pressure. NMR
spectra were recorded on a JEOL LA-300 or a JEOL ECX-400P spectrometer (300 MHz and 400
1
MHz for H, respectively) with chemical shifts reported in ppm downfield from TMS (the solvent
1
peak as an internal standard) for H. IR spectra were recorded on a JASCO FT/IR-410 or JASCO
FT/IR4100 spectrometer using KBr disks. GLC analysis was performed on a Shimadzu GC-14B
with FID detector equipped with a capillary column (TC-1, 0.25 mmφ x 30 m) for catalytic reactions.
GC-8A with TCD detector equipped with a Porapak-Q column (5 mmφ x 2 m) for the qualitative
and quantitative analyses of ethylene. GC-MS spectra were performed on a Shimadzu QP2000
equipped with a capillary column (TC-wax and TC-1, 0.25 mmφ x 30 m). Elemental analysis was
performed with a Perkin-Elmer 2400 series II CHN analyzer.
+
-
2
.2. [(Me
3
P)
3
3 3 3 n n
Ru(µ-OH) Ru(PMe ) ] [OPh] ·(HOPh) (3·(HOPh) ).
5

4
6
(
A): Into a toluene solution (8 ml) of [Ru(η -1,5-COD)(η -1,3,5-COT)] (1) (1.0382 g, 3.29
mmol), PMe (2.4 ml, 23 mmol), phenyl vinyl ether (1.7 ml, 14 mmol) and H O (600 µl, 33.3 mmol)
3
2
were added in a Schlenk tube. The mixture was stirred at 50 ˚C for 1 day. The solution was added
hexane to give complex 3·1/3 HOPh as white powder in 62% yield (851.8 mg, 1.021 mmol).
(
B): Into a toluene (2 ml) solution of 1 (231.1 mg, 0.734mmol), PMe
phenyl vinyl ether (360.0 µl, 2.93 mmol) and H O (132.0 µl, 7.33 mmol) were added in a Schlenk
tube. The mixture was stirred at 50 ˚C for 2 days to give 3·HOPh as white powder in 43% yield
3
(381 µl, 3.67 mmol),
2
3
1
1
1
(
143 mg, 0.160 mmol). P{ H} NMR (121.5 MHz, acetone-d
MHz, acetone-d , 298 K): δ 1.41 (m, 54H, PMe ), 6.24 (t, JH-H = 6.9 Hz, 2H, OPh(-p)), 6.69 (d, JH-H
8.1 Hz, 4H, OPh(-o)), 6.88 (JH-H = 8.1, 6.9 Hz, 4H, OPh(-m)). IR (KBr, cm ): 3652(m), 3436(br),
6
, 298 K): δ 24.4 (s). H NMR (300
6
3
-1
=
3
1
054(m), 2965(m), 2904(m), 1637(br), 1590(m), 1475(m), 1433(m), 1300(m), 1280(s), 1146(w),
067(w), 1021(w), 936(vs), 822(w), 753(s), 716(s), 695(s), 667(s), 409(vs). Anal. Found: C. 40.18;
H, 7.36. Calcd for C30
C): A mixture of 1 (20.2 mg, 0.0641 mmol), PMe
31.0 µl, 0.252 mmol) and H O (11.5 µl, 0.638 mmol) in benzene-d
H
68
O
5
P
6
Ru
2
: C, 40.18; H, 7.64.
(
3
(33.0 µl, 0.321 mmol), phenyl vinyl ether
(
2
6
(500 µl) was heated for 2 days
)] (2b) was
] (2a) and
4
4
at 50 ˚C in an NMR tube. During the reaction, [Ru(η -1,5-COD)(η -1,3,5-COT)(PMe
3
1
3
initially formed but it gradually decreased with increase of [Ru(η :η -C
8
3 3
H10)(PMe )
4
[
(
Ru(η -1,5-COD)(PMe
3
)
3
] (2c). The final distributions of the ruthenium species were as follows: 2a
46%), 2b (~0%), and 2c (45%). Right after addition of water into the mixture of 2a-c with phenyl
, white precipitate deposited (Note that tris(hydroxo)diruthenium complex 3
is almost insoluble to benzene). After removal of volatile matters, acetone-d was introduced to
vinyl ether in benzene-d
6
6
make a homogeneous solution. The NMR experiment suggested formation of 3 in 36%/Ru (0.0233
mmol) with complete conversion of 2c and recovery of 2a in 40%/1 (0.0254 mmol).
2
.3. Analysis of evolved ethylene
6

(
A): 1 (28.3 mg, 0.0898 mmol) was placed in a Schlenk tube to which dry toluene (2.00 ml) was
added by a volumetric pipette and the solution was frozen using liquid nitrogen. On to the frozen
solution, PMe (46.5 µl, 0.449 mmol), H O (16.2 µl, 0.898 mmol) and phenyl vinyl ether (44.1 µl,
.359 mmol) were added and these reagents were also frozen by liquid nitrogen. After evacuation of
3
2
0
gases, the closed system was heated at 50 ˚C for 2 days. After addition of methane as an internal
standard, the generated gas was analyzed by GLC. Ethylene was generated in 20%/Ru.
(
B): 1 (210.9 mg, 0.6695 mmol) was placed in a Schlenk tube to which dry toluene (2.00 ml)
was added by a volumetric pipette and the solution was frozen using liquid nitrogen. On to the
frozen solution, PMe (348 µl, 3.35 mmol), D O (184 µl, 10.2 mmol) and phenyl vinyl ether (329 µl,
.68 mmol) were added and these reagents were also frozen by liquid nitrogen. After evacuation of
3
2
2
gases, the closed system was heated at 50 ˚C for 2 days. The evolved gases were collected by
Toepler pump and the gas was transferred to a gas cell for IR. The IR bands clearly suggested
-1
formation of ethylene-d
911(w), 1887(w), 1867(w), 1463(w), 1444 (m), 1419(br), 1005(m), 948(vs), 808(m). The solution
involving 3 was evaporated to dryness and the IR spectrum of the residue was measured. IR (KBr,
1
. IR (gas cell, cm ): 3053(m), 3420(m), 3053(m), 2965(m), 2904(m),
1
-1
cm ): 3653(w), 3420(br), 3053(w), 2965(s), 2904(s), 2810(w), 2694(m), 2539(br), 1595(s), 1468(s),
1
5
429(s), 1299(s), 1280(s), 936(vs), 852(m), 821(m), 754(s), 718(s), 694(s), 666(s), 610(s), 570(w),
14(s).
2
3
.4. Reaction of 1 with 3 equiv of PMe without addition of phenyl vinyl ether
Into an NMR tube containing 1 (23.3 mg, 0.0739 mmol), CHPh
H NMR, and a sealed capillary containing P(OPh) in benzene-d as an internal standard for the
(600 µl) was introduced into the NMR tube. After the
(23.0 µl, 0.222 mmol) was added. The NMR tube was
3
as an internal standard for the
1
3
6
3
1
1
P{ H} NMR were added. Then benzene-d
6
first measurement of the NMR, PMe
3
warmed at 50 ˚C and the NMR was measured periodically. After 32 h, the yield of 2a, 2b, and 2c
were estimated as 66%, 8%, and 20%, respectively.
7

+
-
2
3 3 3 3 3 2 2 2
.5. [(Me P) Ru(µ-OH) Ru(PMe ) ] [O CMe] ·(HO CMe) (4·HO CMe).
A toluene solution (2.00 ml) of 1 (29.2 mg, 0.0927 mmol) with PMe
O (16.7 µl, 0.927 mmol), and allyl acetate (37.0 µl, 0.371 mmol) was heated at 50 ˚C for 2 days.
Propylene was generated in 38%/Ru. 4·HO CMe was produced in 37%/Ru yield with concomitant
3
(53.5 µl, 0.556 mmol),
H
2
2
3
1
1
formation of 2a in 58%/Ru yield. 4·HO
2
CMe was characterized by spectroscopies: P{ H} NMR
1
(
121.5 MHz, acetone-d
6
): δ 24.5 (s). H NMR (300 MHz, acetone-d
6
): δ 1.41 (br. s, 54H, PMe ),
3
-
1
1
1
2
.77 (s, 6H, O CMe). IR (KBr, cm ): 3651(sh), 3412(br), 2970(m), 2908(m), 1573(vs), 1408(vs),
302(m), 1282(s), 1018(w), 943(vs), 853(m), 721(s), 667(s), 462(w).
2
.6. Reaction of vinylruthenium(II) complexes with water
1
To a benzene-d
6
solution (600 µl) of cis-[Ru(CH=CH
2
)(OAc-κ O)(dmpe)
2
] (5a: 13.55 mg,
0
.02780 mmol), water (5.0 µl, 0.27 mmol) was added by a hypodermic syringe. After heating the
reaction mixture at 50 ˚C for 1 day, unidentified deposition was observed with trans-
1
[
(
Ru(CH=CH
2
)(OAc-κ O)(dmpe)
2
] in 37%. After injection of pure methane as an internal standard
98.5 µl, 0.00439 mmol), the GLC analysis showed evolution of ethylene (17%, 0.00484 mmol) and
1
ethane (0.5%, 0.000152 mmol). A similar reaction using cis-[Ru(CH=CH
2
)(OAc-κ O)(PMe
3
)
4
] (5b)
was also performed but 5b did not react with water at all under these conditions.
2
.7.Reaction of vinylruthenium(II) complexes with hydrochloric acid
1
trans-[Ru(CH=CH
2
)(OAc-κ O)(depe)
2
] (5c: 9.0 mg, 0.015 mmol) was placed in an NMR tube
. 5c was
and excess amount of hydrochloric acid (12 µl, 0.16 mmol) was added into the benzene-d
6
1
+
-
quantitatively converted to trans-[Ru(=CHMe)(OAc-κ O)(depe)
2
] Cl or trans-
+
-
1
[
Ru(=CHMe)Cl(depe)
2
] [OAc] . H NMR (300 MHz, benzene-d
6
): δ 0.8 (m, 12H, PCH
2
Me), 1.0 (m,
8

1
4
2H, PCH
H, PCH CH
H-P = 7 Hz, 1H, Ru=CHMe). P{ H} NMR (122 MHz, benzene-d
2
Me), 1.24 (m, 4H, PCH
2
Me), 1.56 (s, 3H, OAc), 1.5-1.8 (m, 12H, PCH
2
Me), 1.9-2.0 (br,
2
2
P), 2.2-2.3 (m, 4H, PCH
2
CH
2
P), 2.59 (d, JH-H = 7 Hz, 3H, Ru=CHMe), 16.7 (oct, JH-H
3
1
1
=
J
6
): δ 46.7 (s).
2
.8. General procedure for the catalytic hydration of nitriles
(
6
A): Complex 3 (21.2 mg, 0.0237 mmol) was placed in a Schlenk tube and acetone-d (2.00 ml)
was added by a volumetric pipette. A portion of the solution (500 µl) was moved to an NMR tube
by a hypodermic syringe, and benzonitrile (30.3 µl, 0.296 mmol), water (18.0 µl, 1.0 mmol) and 1,4-
dioxane (5.9 µl, 1.0 mmol) as an internal standard were added. The reaction mixture was warmed at
5
0 ˚C for 88 h.
B): Complex 3 (18.3 mg, 0.0204 mmol), 1,4-dioxane (1.0 ml), benzonitrile (103.7 mg, 1.18
(
mmol), water (100 µl, 5.53 mmol) were added in a screw-cap tube. The reaction mixture was heated
at 100 ˚C for 12 h. After cooling the reaction mixture to room temperature, dibenzyl and methanol
(
1 ml) were added to give a homogeneous solution and the products were analyzed by GLC.
+
-
2
3 3 3 3 3
.9. X-ray analysis of [(Me P) Ru(µ-OH) Ru(PMe ) ] [OPh] ·(HOPh) (3·HOPh).
Rigaku AFC-7R-Mercury II diffractometer with graphite monochromated Mo-Kα radiation (λ =
.71069 Å) were used for data collection at 200.0 K. A selected single crystal of 3·HOPh was
0
mounted on the top of glass capillary by use of Paraton N oil. The collected data were solved by
direct methods (SIR92), and refined by a full-matrix least square procedure using SHELXL in the
CrystalStructure (ver.4.2) package program [15,16]. All non-hydrogen atoms were refined with
anisotropic displacement parameters. A hydrogen atom attached to O(1) was found from the
differential Fourier map and it was solved isotropically. The other O-H protons were not found and
were excluded from the calculations. The other hydrogen atoms were placed in calculated positions
9

and were not refined. The molecular structure was depicted with POV-Ray [17]. The
crystallographic data of 3·HOPh were deposited and available as CCDC1449576.
3
. Results and Discussion
3
.1. Synthesis of Cationic Tris(µ-hydroxo)diruthenium(II) Complex.
4
6
3
When phenyl vinyl ether was treated with [Ru(η -1,5-COD)(η -1,3,5-COT)] (1)/PMe in
1
3
4
4
benzene at 50 ˚C, a mixture of fac-[Ru(η :η -C
8
H10)(PMe
3
)
3
] (2a) [18], [Ru(η -1,5-COD)(η -1,3,5-
4
COT)(PMe
3
)] (2b) [18, 19] and [Ru(η -1,5-COD)(PMe
)
3 3
] (2c) [20] was obtained in 46%, ~0% and
4
5% yields, respectively, and the added phenyl vinyl ether remained unreacted [21]. Because such a
bunch of these phosphine complexes 2a-c was formed from 1/PMe without addition of phenyl vinyl
3
ether under the similar conditions (2a: 66%, 2b: 8%, 2c: 20%) [18, 22], phenyl vinyl ether did not
engage with the formation of 2a-c at all. However, we accidentally found that addition of water into
the mixture involving 2a-c and phenyl vinyl ether caused complete conversion of 2c into white
+
-
precipitates of [(Me
81% yield based on 2c), while 2a remained intact. Because neither phenyl vinyl ether nor water
apparently reacts with 2c individually, both of them must engage with this reaction. Thus, we
intentionally added water into the starting solution involving 1/PMe phenyl vinyl ether, and the
3
P)
3
Ru(µ-OH)
3
Ru(PMe
3
)
3
] [OPh] ·HOPh (3·HOPh) in 36% yield based on 1
(
3
reaction at 50 ˚C for 2 days afforded 3⋅HOPh in 43-62% yields, which was isolated as analytically
pure white powder (Scheme 3).
10

4
6
Scheme 3. Reactions of [Ru(η -1,5-COD)(η -1,3,5-COT)] (1)/PMe
3
with phenyl vinyl ether in the
presence of water.
Complex 3 was characterized by NMR, IR and elemental analysis but most unambiguously by
3
1
1
6
X-ray analysis (vide infra). P{ H} NMR spectrum of 3 in acetone-d showed a singlet at δ 24.4,
1
suggesting all PMe
3
to be equivalent. The H NMR suggested that 3 had 1 equiv of associated
phenol, which was indistinguishable from the phenoxide anion. By the subsequent experiments, 3
1
was found to have arbitrary amount of phenol by hydrogen bonding. The H NMR showed a
3 3 2
multiplet resonance of PMe at δ 1.41 (54H) probably due to the A XX’ spin system, and
6
phenoxide/phenol resonances at δ 6.24 (t, 2H), 6.68 (d, 4H), and 6.88 (t, 4H) in acebtone-d . All
1
hydroxo resonances have not been clearly observed in our complex in the H NMR, but the IR
spectrum suggests the presence of the hydroxo groups. Nakamoto reported compounds containing
the hydroxo groups to exhibit the ν(OH), δ(MOH) and ν(MO) at 3760-3000, 1200-700, and 900-300
-1
-1
cm , respectively [23]. In fact, 3⋅HOPh showed characteristic bands at 3652 and 3436 cm , which
were assignable to the ν(OH) bands.
3
.2. Molecular structure of 3
11

Single crystals of 3 were obtained by the fractional crystallization from cold THF as analytically
pure colorless needles. The molecular structure by the X-ray crystallographic analysis clearly
indicated formation of 3⋅HOPh, though the two of three hydroxo protons and a phenol proton could
not be observed from the differential Fourier map (Fig. 1).
+
-
Fig. 1. Molecular structure of [(Me
3
P)
3
Ru(µ-OH)
3
Ru(PMe
3
) ] [OPh] ·HOPh (3·HOPh) with
3
selected numbering schemes. All hydrogen atoms are omitted for clarity. Ellipsoids represent 50%
probability (R = 0.0475, wR = 0.1077).
Fig. 1 shows 3 being a cationic tris(µ-hydroxo)diruthenium(II) complex having six PMe
The Ru−O bond lengths are in the range 2.138-2.162 Å and the Ru(1)−O−Ru(2) angles are in 89.18-
9.98˚. The short bond length between O(4) and O(5) in phenol-phenoxide (2.39 Å) suggests an
interaction through the hydrogen bond. Reetz and coworkers also reported the short O⋅⋅⋅O length in
3
ligands.
8
+
-
[
NBu
4
] [PhO⋅HOPh] to be 2.471(5) Å [24]. The Ru(1)−Ru(2) is 3.0306(16) Å, and this issue is
discussed later.
12

A handful of cationic tris(µ-hydroxo)diruthenium(II) complexes have been known. The first
6
6
+
-
example was prepared as the arene complex [(η -C
6
H
6
)Ru(µ-OH)
3
Ru(η -C
6
H )] Cl [25,26] by the
6
6
reaction of [RuCl
2
(η -C
6
H
6
)]
2
with aqueous NaOH, and the X-ray structure of the mesitylene
+
-
analogue was also reported [27]. The Cp* analogue [Cp*Ru(µ-OH)
from [RuCp*Cl in refluxing water and AgNO [28], and the triazacyclononane analogue [LRu(µ-
(L = 1,4,7-trimethyl-1,4,7-triazacyclononane) was prepared by treatment of
3 3
RuCp*] [NO ] was prepared
2
]
2
3
2
+
-
OH)
LRu(µ-O)(µ-OAc)
of the mixed valence complex with Na
corresponding Ru(III) analogues [LRu(µ-OH)
corresponding tertiary phosphine complexes have also been documented. Singleton and coworkers
3 6 2
RuL] [PF ]
+
-
[
2
RuL] [PF
6
]
2
with H
2
SO
4
in water and zinc amalgam, and subsequent oxidation
2 2 8
S O followed by the anion exchange produced
3
+
-
3
6 3
RuL] [PF ] and the perchlorate analogue [29]. The
+
-
prepared the PMe
2
Ph complex, [(Me
2
PhP)
3
Ru(µ-OH)
3
Ru(PMe
2
Ph)
3
] [BPh ] by hydrolysis of
4
4
+
-
[
RuH(η -1,5-COD)(PMe
2
Ph)
3
] [BPh
Ru(PMe
Ru(PMe
4
] in refluxing acetone-methanol [30]. The striking resemble
+
-
example [(Me
3
P)
3
Ru(µ-OH)
Ru(µ-CH
3
3
)
3
] [BF
4
] was reported by treatment of a trivalent diruthenium
complex [(Me
3
P)
3
2
)
3
3
)
3
] with Ph
3
CBF in THF [31], although the formation
4
mechanism was not discussed in this paper. Bergman and coworkers briefly commented that
treatment [Ru(C
2
H
4
)(PMe
3
)
4
] with excess water may give a compound formulated as [(Me
3
3
P) Ru(µ-
+
-
OH)
3
Ru(PMe
3
)
3
] [OH] as a side product [32].
Table 1 shows selected distances and angles for 3·HOPh and reported tris(µ-
hydroxo)diruthenium(II) complexes. Wilkinson and coworkers pointed relation between the Ru−Ru
lengths in tris(µ-bridged)diruthenium complexes and the formal oxidation state [33]. In fact, the
IV
IV
2+
-
III
III
Ru −Ru in [(Me
(Me P) Ru(µ-CH
bond length in [(Me
3
P)
Ru(PMe
P) Ru(µ-OH)
Ru −Ru [31]. Wieghhardt and coworkers also documented the shorter Ru −Ru lengths in
3
Ru(µ-CH
2
)
3
Ru(PMe
3
)
3
] [BF
4
]
2
and the Ru −Ru bond length in
[
3
3
2
)
3
3 3
)
] fall in 2.641(1) Å and 2.650(1) Å, respectively, while the Ru−Ru
+
-
3
3
3 3 3 4
Ru(PMe ) ] [BF ] [3.004(1) Å] was a good evidence for the
II
II
II
III
2
+
-
III
III
3+
-
[
[
LRu(µ-OH)
3 6 2 3 4 3
RuL] [PF ] [2.401(2) Å] and the Ru −Ru length in [LRu(µ-OH) RuL] [ClO ]
2.505(3) Å] (L = 1,4,7-trimethyl-1,4,7-triazacyclononane) [29]. The present Ru(1)−Ru(2) length is
13

II
II
3
.0306(16) Å and is consistent with a Ru −Ru complex. These features are explained that the
II
II
II
III
III
III
IV
IV
formal Ru−Ru bond order increases on going from Ru −Ru , Ru −Ru , Ru −Ru and Ru −Ru ,
because electrons from the anti-bonding orbitals are stepwise removed (Fig. 2) [29]. These
complexes shown in Table 1 roughly reflect this feature.
Table 1. Selected bond and angles (deg) and distances (Å) for some tris(µ-hydroxo)diruthenium(II)
complexes [25-31, 33].
Complex
Ru(1)-O-Ru(2)
89.18-89.98
89.0-89.5
90
Ru-O
Ru(1)-Ru(2)
3.0306(16)
3.004(1)
3.08
+
-
[
[
[
[
[
[
[
(Me
3
3
P)
3
3
Ru(µ-OH)
3
Ru(PMe
3
)
3
] [OPh] ⋅(HOPh) (3⋅HOPh)
2.138-2.162
2.134-2.144
2.15-2.21
2.071-2.092
2.087-2.109
1.955(10)
2.06
+
-
(Me
P)
Ru(µ-OH)
3
Ru(PMe
3
)
3
] [BF
4
]
+
-
(PhMe
2
P)
)Ru(µ-OH)
Me )Ru(µ-OH)
3
Ru(µ-OH)
3
2 3 4
Ru(PMe Ph) ] [BF ]
6
6
+
-
(η -C
6
H
6
3
Ru(η -C
6
6
H )] Cl ⋅3H
2
O
90.6-92.1
90.9
2.9812(7)
2.989(3)
2.401(2)
2.505(3)
6
6
+
-
(η -C
6
H
3
3
3
Ru(η -C
6
H
3
Me
3
)] Cl ⋅3H
2
O
2
+
-
a)
LRu(µ-OH)
LRu(µ-OH)
3
RuL] [PF
]
6 2
2
⋅H O
90.9(4)
3
+
- a)
75.0
3
4 3
RuL] [ClO ]
a)
L = 1,4,7-trimethyl-1,4,7-triazacyclononane
Fig. 2. Orbital diagrams for diruthenium complexes having D3h symmetry.
3
.3. Formation mechanism of 3
14

In order to understand the mechanism, we performed this reaction in a closed system. The
reaction produced 3⋅HOPh in 25% yield with evolution of ethylene (20%) and concomitant
formation of 2a (74%) [Eq. (1)] [34].
We also used D
2
O for this reaction and the evolved gas was collected in a gas cell for the IR
spectrum. The IR spectrum of the collected gas involved characteristic bands at 1419(br), 1005(m),
-
1
9
48(vs), and 808(m) cm in the fingerprint region. The vibration modes for ethylene and a series of
deuterated ethylenes are shown in Table 2 [35]. The most consistent understanding of this data is
that the collected gas involves ethylene-d . With agreement, a proton is coming from water to give
ethylene-d . Because phenyl vinyl ether is not hydrolyzed by water under these conditions, a
1
1
ruthenium species must concern with the formation of ethylene.
Table 2. Vibration modes for ethylene and deuterated ethylenes.
mode
this work
ν
4
1027
1000
809
988
727
764
727
726
720
1005
808
ν
7
752
944
843
ν
8
943
943
780
948
ν
12
1443
1404
1384
1344
1300
1300
1078
1419
+
-
The IR spectrum of deuterated analogue [(Me
3
P)
3
Ru(µ-OD)
3
Ru(PMe
3
)
3
] [OPh] ·(DOPh) (3-
-1
d
3
⋅DOPh) suggests that the absorption bands at 3652 and 3463 cm in 3⋅HOPh shift to 2694 and
-1
2
539 cm , respectively. This is consistent with replacement of O−H with O−D.
15

Similar treatment of 1/PMe
3
with allyl acetate was performed. As we have reported previously,
3
the C−O bond oxidative addition of allyl acetate to 1/PMe
3
gives a η -allylruthenium(II) complex,
3
[
Ru(OAc)(η -C
3
H
5
)(PMe
3
)
3
] under the water-free conditions [9]. However, treatment of 1/PMe
3
with allyl acetate in the presence of water produced the corresponding cationic tris(µ-
+
-
hydroxo)diruthenium complex [(Me
3
P)
3
Ru(µ-OH)
3
Ru(PMe
3
)
3
] [O
2
CMe] ⋅HO
2
CMe (4⋅HO
2
CMe) in
3
7% yield along with evolution of propylene (38%).
Scheme 4. Possible mechanism for the formation of 3⋅HOPh.
Possible formation mechanism of 3 is shown in Scheme 4. The initial reaction of 1 with PMe
3
4
gives an active zero-valent Ru species [Ru(η -1,5-COD)(PMe
3
3
) ] (2c) with concomitant formation
of the other Ru species 2a and 2b. A simple pathway is the C−O bond oxidative addition of phenyl
vinyl ether to 2c giving (phenoxo)(vinyl)ruthenium(II) A, and subsequent hydrolysis of A to give B
with evolution of ethylene followed by hydrolysis and dimerization of B to give 3⋅HOPh. However,
because 2c does react with phenyl vinyl ether, it is not clear about why water promotes the C−O
oxidative addition to 2c in this mechanism. We performed the model reactions of cis-
16

1
[
Ru(CH=CH
2 2
)(OAc-κ O)(dmpe) ] (5a) with water at 50 ˚C for 1 day but evolution of ethylene and
ethane was observed in only 17% and 0.5% yields, respectively. Moreover, cis-[Ru(CH=CH
2
)(OAc-
1
κ O)(PMe
3
)
4
] (5b) did not react with water at all under these conditions. When a much stronger acid,
1
hydrochloric acid was added to trans-[Ru(CH=CH
2
)(OAc-κ O)(depe)
2
] (5c) in benzene-d
6
, 5c was
1
quantitatively converted into a new species having characteristic H NMR resonances at δ 2.59 (d, J
=
7 Hz, 3H) and 16.7 (oct, J = 7 Hz, 1H) assignable to the methyl and methine protons in Ru=CHMe,
respectively. This fact suggests the proton has attacked the β-carbon in the vinyl group to give a
1
+
-
carbene complex trans-[Ru(=CHMe)(OAc-κ O)(depe)
2
] Cl or trans-
+
-
[
Ru(=CHMe)Cl(depe) ] [OAc] [36]. These results suggest acidity of water being not enough for
2
protonation the vinylruthenium(II) species under these conditions, and a proton favors to attack the
β-carbon in the vinylruthenium(II) species rather than evolution of ethylene [37].
The second scenario is the protonation of 2c by water to give C (Scheme 4), insertion of phenyl
vinyl ether into the resulting Ru−H bond to give D, and subsequent β-phenoxide elimination with
evolution of ethylene followed by the hydrolysis and dimerization [38]. Komiya and Shindo
reported the β-phenoxide elimination from 2-phenoxyethyl group in trans-[PtBr(C
6) to give ethylene [39]. Komiya and Yamamoto also documented the related β-acetate elimination
to give ethylene by the reaction of cis-[RuH (PPh ] with vinyl acetate [40]. One of the concerns
2 4 3 2
H OPh)(PPh ) ]
(
2
)
3 4
about this mechanism is that the β-phenoxide elimination from D may compete with the β-hydride
elimination to reproduce C. The β-hydride elimination from C establishes reversibility between C
and D, and this reversibility enables introduction of more than one deuterium atom in ethylene,
which is inconsistent with the observed result. In fact, thermolysis of 6 at 100 ˚C is reported to
liberate ethylene and phenyl vinyl ether in 1:2 ratio [39], suggesting the β-hydride elimination to
proceed at the double rate for the β-phenoxide elimination. Caulton and coworkers also documented
rapid and exclusive β-hydride elimination from β-ethoxyethylruthenium(II) [41]. However, the
cationic ruthenium(II) center in C would have high electrophilicity and it therefore favors the β-
phenoxide elimination rather than the β-hydride elimination. We favor this scenario, although we
17

could not get any mechanistic evidence by stoichiometric reaction of 2c with water/phenyl vinyl
ether by NMR.
3
.4. Catalytic hydration of nitriles catalyzed by cationic tris(µ-hydroxo)diruthenium(II) Complex 3
Hydroxo complexes of late transition-metals have been postulated as critical intermediates for
the Wacker type oxidation reaction [42] and hydration reaction [43]. Ruthenium complexes are
particularly attractive for potential catalysts for hydration [44]. Although tris(µ-
hydroxo)diruthenium(II) is a common structural motif of hydroxoruthenium species as described
above, their catalytic activities are unprecedented so far. We commenced our catalytic study by
subjecting benzonitrile and water to catalytic amounts of 3. By the preliminary studies, we found
the optimized conditions to be the reaction in 1,4-dioxane at 120 ˚C for 6 h with 1.0 mol% of
catalyst. With the optimized conditions, we have screened several Ru complexes as shown in Table
3
.
18

Table 3. Hydration of benzonitrile catalyzed by some Ru complexes and related compounds
entry
Cat
3
time/h
6
yield/%
1
2
3
4
5
100
7
1
18
6
4
[Ru(acac) (η -1,5-COD)]
2
0
4
[RuCl (η -1,5-COD)]
2
n
6
0
RuCl
3
·3 H
2
O
6
1
a
6
3
88
88
88
100
0
a
7
PMe
KOPh
as solvent at 50 ˚C.
3
a
8
0
a
acetone-d
6
Tris(µ-hydroxo)diruthenium(II) complex 3 catalyzed hydration of benzonitrile in 1,4-dioxane at 120
C to give benzamide quantitatively. Among these Ru catalysts screened, 3 was the most active
˚
catalyst for the hydration. PMe
3
and KOPh do not catalyze the hydration, that excludes the catalysis
-
performed by a liberated base such as PMe
3
or OPh .
With the optimized conditions in hand, we examined the tolerance and electronic effect of
functional groups (Table 4). Functional groups including p-nitro (σ = 0.78), p-chloro (σ = 0.23), p-
methyl (σ = −0.17), p-methoxy (σ = −0.27) and p-amino (σ = −0.66) groups were tolerated, and the
reaction seems to be accelerated by an electron-withdrawing group. This result supports that an
electrophilic attack of nitrile concerns with the rate-determining step. The p-hydroxo and p-formyl
groups discouraged the hydration. The substituents at the ortho position also discouraged the
reaction.
19

Table 4. Hydration of substituted benzonitrile catalyzed by 3.
Then we turned our attention to hydrolysis of alkyl and alkenyl nitriles (Table 5). Benzyl nitrile
and butyronitile also produced corresponding amides in high yield, and acetonitrile was also
converted into acetamide in a moderate yield. When acrylonitrile was used for this hydration, a
mixture of 3-(2-cyanoethoxy)propanenitrile and acrylamide was produced. 3-(2-
Cyanoethoxy)propanenitrile is probably formed via β–cyanohydrine. One of the features of this
catalysis is that the low catalyst loading provides high turnover numbers (TONs). For examples, the
acetonitrile hydration proceeds with 2 mol% of 3 in 56% yield, whose TON is 28. With 0.04 mol%
of 3, the TONs reached to 200. In hydration reaction of acrylonitrile, although the TONs for 3-(2-
cyanoethoxy)propanenitrile and acrylamide fall in only 0 and 3.5, respectively, with 2.00 mol% of 3,
their TONs (and also their yields) dramatically increased to 1020 and 600, respectively, with 0.03
mol% catalyst loading.
20

Table 5. Hydration of alkyl and alkenyl nitriles catalyzed by 3.
a
b
c
d
3
(2.00 mol%) without solvent. 3 (0.04 mol%) without solvent. 3 (2.00 mol%) at 80 ˚C without solvent. 3 (0.03
mol%) at 80 ˚C without solvent.
3
.5. Mechanism for catalytic hydration
A simple mechanism for the present hydration catalyzed by 3 is that a nitrile coordinates to one
of the Ru centers in diruthenium species and a hydroxo group comes to the coordinated nitrile from
another Ru center [45]. However, because TON of the present hydration reaction dramatically
increases in low catalyst loading conditions, a rational hypothesis is that a mono-nuclear
hydroxoruthenium species is an active catalyst, which constitutes an equilibrium with 3 under the
catalytic conditions [46]. In the catalytic benzonitrile hydration, electron-deficient nitriles
encouraged the catalytic hydration and the ortho substituents discouraged the reaction. These
features are consistent with the mechanism involving the prior coordination of nitrile to the resulting
coordinatively unsaturated Ru species and nucleophilic attack of the hydroxo group to the
coordinated nitrile.
In relation to the Ru-catalyzed nitrile hydrations, Murahashi and coworkers reported a wide
range of nitrile hydrations catalyzed by cis-[RuH
RuH(N=C(OH)R)L ] [47]. Sola, Joó and coworkers documented an interesting stoichiometric
3 4
reaction of trans-[Ru(OH)(NCMe) (P Pr )] [BF ] to give the amidate complex fac-[Ru(NHCOMe-
2 3 4
(PPh ) ] and they proposed an imino intermediate
[
n
i
+
-
4
21

2
+
-
3 4
κ N,O)(NCMe) ] [BF ] [48]. In the case of hydroxoplatinum(II), Bennett and coworkers
3 2
documented nucleophilic syn addition of the hydroxo group in [PtMe(OH)(PR ) ] to an electron-
deficient alkene [49].
The consistent catalytic cycle starting from 3 is depicted in Scheme 5. Complex 3 is converted
into a coordinatively unsaturated hydroxo species E to which a nitrile coordinates to give F. The
hydroxo group is nucleophilic and the internal reaction gives G, which isomerizes to the more stable
H. Finally, the hydrolysis of H produces amide with reproduction of the hydroxo complex E.
Scheme 5. A possible catalytic cycle for the hydration of nitrile.
4
. Conclusions
3
In summary, the C−O bond cleavage of phenyl vinyl ether occurred by 1/PMe in the presence of
+
-
water to give a tris(hydroxo)diruthenium(II) complex [(Me
3
P)
3
Ru(µ-OH)
3
Ru(PMe
3
)
3
] [OPh] ·HOPh
4
(
3·HOPh) in moderate yield. The actual precursor in the formation of 3·HOPh is [Ru(η -1,5-
3 3
COD)(PMe ) ] (2c) but water is indispensable for the C−O bond cleavage of phenyl vinyl ether. A
possible mechanism for the formation of 3·HOPh is protonation of 2c with water, insertion of phenyl
vinyl ether into the resulting Ru−H bond, β-phenoxide elimination with evolution of ethylene, and
22

subsequent hydrolysis and dimerization. Although such tris(hydroxo)diruthenium frameworks have
been prepared by different routes, we establish a new direct access to 3 from a Ru(0) complex. We
also found 3 to show high catalytic performance for the nitrile hydration reaction for the first time.
This finding opens a new horizone for the catalytic applications of hydroxoruthenium complexes.
Acknowledgments
We thank Y. Ogawa for a part of supporting reactions. A part of this work was finnancially
supported by Japan Science and Technology Agency (JST), ACT-C, and the JSPS Grant-in-Aid for
Scientific Research on Innovative Areas “3D Active-Site Science” 26105003.
23

References and notes
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[
[
[
[
[
[
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6
20] 2c was originally reported by Bennett and coworkers by the treatment of [Ru(η -
4
naphthalene)(η -1,5-COD)] with PMe
3
: M.A. Bennett, Z. Lu, X.-Q. Wang, M. Bown, D.C.R.
Hockless, J. Am. Chem. Soc. 120 (1998) 10409. We confirmed the formation of 2c by the
reaction of 1 with PMe under these conditions.
3
24

[
[
[
21] At the initial stage, 1 was converted into 2b almost quantitatively.
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[
[
[
[
[
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28] R.O. Gould, C.L. Jones, D.R. Robertson, D.A. Tocher, T.A. Stephenson, J. Organomet. Chem.
2
26 (1982) 199.
[
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(
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[
[
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[
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34] Although 3⋅HOPh is considered to be produced from 2c, it is not clear about why formation of
2
a was promoted in this reaction. A significant difference in these conditions from the typical
conditions is that this reaction was operated under reduced pressure for qualitative and
quantitative analysis of ethylene.
[
[
35] B.L. Crawford, J.E. Lancaster, R.G. Inskeep, J. Chem. Phys. 21 (1953) 678.
36] Similar protonation of [FeCp(CH=CH
2
)(CO)
] has been reported: K.A.M. Kremer, G.-H. Kuo, E.J. O’Connor,
P. Helquist, R.C. Kerber, J. Am. Chem. Soc. 194 (1982) 6119.
2 4
] with HBF giving
+
-
[
2 4
FeCp(=CHMe)(CO) ] [BF
3
[
[
37] Contrary to vinyl ether, allyl acetate readily reacts with 1/PMe
3
and protonation of [Ru(η -
C
3
H
5
)(OAc)(PMe ] is easily protonated by HCl to evolve propylene (91%). We cannot rule
out the formation mechanism of 4 through the protonation of (acetate)(η -allyl)ruthenium(II)
)
3 3
3
intermediate in the case of allyl acetate.
1
31
1
38] However, the H and P{ H} NMR resonances of 2c remained unchanged in the presence of
water without addition of phenyl vinyl ether. Probably, the equilibrium between 2c and C
greatly lies to 2c side.
[
[
39] S. Komiya, T. Shindo, J. Chem. Soc., Chem. Commun. (1984) 1672.
40] S. Komiya, A. Yamamoto, J. Organomet. Chem. 87 (1975) 333.
25

[
41] J.N. Coalter III, G.J. Spivak, H. Gérard, E. Clot, E.R. Davidson, O. Eisenstein, K.G. Caulton, J.
Am. Chem. Soc. 120 (1998) 9388.
[
[
[
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[
[
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2
] and cis-
[
3
)
4
2
2
[
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[
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26

Synopsis
4
6
Treatment of [Ru(η -1,5-COD)(η -1,3,5-COT)] (1)/PMe
water gives a tris(µ-hydroxo)diruthenium(II) complex [(Me
3·HOPh) with evolution of ethylene. The molecular structure of 3·HOPh is unequivocally determined
3
with phenyl vinyl ether in the presence of
+
-
3
P)
3
Ru(µ-OH)
3
Ru(PMe ) ] [OPh] ·HOPh
3
3
(
by X-ray analysis. Complex 3 acts as a catalyst for nitrile hydration. The hydration of benzonitrile
was achieved by 3 (1.0 mol%) in 1,4-dioxane at 120 ˚C for 6 h to give benzamide quantitatively.