Characteristics of mononuclear ruthenium-oxo complexes adjusted by axial ligand for the catalysis of oxygen-transfer reactions

Article abstract of DOI:10.1246/bcsj.80.507

Ruthenium(II) complexes, [Ru(babp)(dmso)L], consisting of tetradentate square-planar ligand (babp = 6,6′-bis-(benzoylamino)-2,2′- bipyridinato) and two monodentate axial ones, DMSO and L (L = dmso or heterocycles), were synthesized and structurally charac

Full text of DOI:10.1246/bcsj.80.507

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 3, 507–517 (2007)
507
Characteristics of Mononuclear Ruthenium–Oxo Complexes Adjusted
by Axial Ligand for the Catalysis of Oxygen-Transfer Reactions
Takeshi Okumura, Yuji Morishima, Hiroyoshi Shiozaki, Takeyoshi Yagyu,
ꢀ
Yasuhiro Funahashi, Tomohiro Ozawa, Koichiro Jitsukawa, and Hideki Masuda
Department of Materials Science and Engineering, Graduate School of Engineering,
Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555
Received July 14, 2006; E-mail: jitsukawa.koichiro@nitech.ac.jp
0
Ruthenium(II) complexes, [Ru(babp)(dmso)L], consisting of tetradentate square-planar ligand (babp = 6,6 -bis-
0
(
benzoylamino)-2,2 -bipyridinato) and two monodentate axial ones, DMSO and L (L = dmso or heterocycles), were
synthesized and structurally characterized using UV–vis, NMR, ESI-MS, and IR spectroscopies and X-ray crystallogra-
phy. These complexes showed catalytic activity toward the oxygen-atom-transfer reactions, such as epoxidation, allylic
oxidation, cleavage reaction of olefins, and sulfoxidation of thioethers, in the presence of oxidant, PhIO. The relationship
between the coordination structure and the catalytic activities of [Ru(babp)(dmso)L] was investigated. The active species
V
VI
for the reaction was assigned to be a high valent Ru =O or Ru =O species. The oxo ligand was generated from the
ligand-exchange reaction involving the dmso attached to [Ru(babp)(dmso)L]. Some complexes having a labile ligand at
VI
an axial position gave a Ru =O species as an active intermediate, and others having moderate coordinating one gave
V
a Ru =O one. The oxidation activities of these complexes were affected by the axial ligand, L, through the trans-
influence.
Ruthenium(IV, V, or VI)–oxo (Ru=O) species, generated
from the reaction of low-valent ruthenium(II or III) complexes
and various oxidants, are considered as active intermediates in
oxygen-transfer reactions.1 In order to characterize the oxo
species generated upon ruthenium complex, mechanistic in-
vestigations have been carried out using tripodal polypyridyl-
For investigating the characteristics of the Ru=O species,
porphyrin has been used as a square-planar ligand.8
,13,17–19
The oxo species on a ruthenium porphyrin complex is located
at the axial position of the octahedron. Dissociation of the ax-
ial ligand modulated by changes in the field strength of the li-
gand at the trans-position has been discussed using ruthenium
–8
9
–11
12,13
20
amine derivatives
[
or porphyrin ligands.
In the case of
Ru (bpy)2(py)(O)] complex, the Ru =O species had only
slight epoxidation activity, while it had good hydrogen atom
nitrosyl complexes with tetradentate planar ligand. The ac-
IV
IV
tivity of the metal–oxo species at the axial position is regulated
by the ligand coordinated at the trans-position. Moreover, the
catalytic activity in cytochrome P-450 is known to be affected
14
abstraction activity. Meyer et al. have also reported that
IV
the Ru =O species attacked the C–H bond rather than olefin
double bond and that the low-valent metal–oxo species had
radical character.1 On the other hand, the higher oxidation
by the axially coordinated proximal ligand to the iron–oxo
species.1
9–22
Taking into account of the trans-influence, the
5
catalytic activity of the metal–oxo species with square-planar
ligand has been investigated using non-porphyrin ligand
(Scheme 1). In this article, we report the relationship between
the coordination structures of the ruthenium complexes with a
V
VI
state of ruthenium–oxo species, such as Ru =O or Ru =O,
has epoxidation activity.2 Recently, we have reported that
the ruthenium complexes with tripodal tris(2-pyridylmethyl)-
amine derivatives catalyzed the hydroxylation of adamantane
and epoxidation of cyclohexene in the presence of PhIO, in
which the oxidation activity was affected by the electronic
and steric character of the ruthenium–oxo species.16 This com-
plementary reactivity for hydroxylation or epoxidation cata-
lyzed by the Ru=O species is controlled by the substituent
group on the pyridylmethylamine ligands. However, this is
not sufficient for understanding the characteristics of the
Ru=O species, because the bulky pivalamido or neopentyl-
–5
Metal-oxo species
O
L
L
M
Substrate
oxygen
L
L
trans-influence
transfer
reaction
amino groups attached at the 6-position of the pyridine steri-
cally suppress the reactions16 and the ꢀ-position of the pyri-
L
L
Oxidant
Oxidation product
dine group is often hydroxylated during the reaction.9 In
order to avoid self-decomposition of the complex and to have
the intermolecular oxygen-transfer reaction occur smoothly,
rational design of the coordination structure of the complex
is required.
,10
M
L
L
Axial Ligand
Scheme 1. Oxygen-transfer reaction adjusted by the trans-
influence of axially coordinating ligand.
Published on the web March 13, 2007; doi:10.1246/bcsj.80.507

5
08 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Characterization of Ruthenium–Oxo Complex
NH
dmso
O
N
H
H
2
RuCl (dmso)4
recrystallization
DMSO / acetone
N
O
N
O
N
O
N
Ru
N
HN
NaH / DMF
hydrogen binding interaction
H babp
dmso
2
[
Ru(babp)(dmso) ] (1)
2
dmso
dmso
N
N
Heterocycle (L)
DMSO / acetone
N
O
N
O
N
Ru
N
Ru
L
N
N
O
O
dmso
(
1)
[
[
[
[
[
[
[
Ru(babp)(dmso)(im)] (2)
Ru(babp)(dmso)(py)] (3)
Ru(babp)(dmso)(phpy)] (4)
Ru(babp)(dmso)(mepy)] (5)
Ru(babp)(dmso)(bupy)] (6)
Ru(babp)(dmso)(dmapy)] (7)
Ru(babp)(dmso)(cnpy)] (8)
Scheme 2. Intramolecular interaction of BABP and preparation of the ruthenium(II) complexes with babp, [Ru(babp)(dmso)L].
square-planar ligand and their catalytic activities toward oxy-
gen-transfer reactions, which are adjusted by the axial ligand.
Ruthenium complexes used here consist of a tetradentate
gand at the axial positions is as shown in Scheme 2. Com-
plexes with heterocycles coordinated at both axial positions
were not obtained even with prolonged reaction times or in
the presence of excess amount of the ligand. Complexes 1–8
were fully characterized by spectroscopic and elemental analy-
ses. Complexes 1 and 5–7 were characterized by X-ray diffrac-
tion. In less-polar solvents, such as dichloromethane, the color
of solution of the ruthenium complexes gradually changed
from red to green, but in polar solvents, such as DMSO, this
color change did not occur under atmospheric conditions.
The solution color did not return to the original even upon ad-
dition of dmso. In other words, the coordinated dmso ligand
dissociates irreversibly in the less-polar solvent. Accordingly,
spectroscopic measurements were carried out in polar solvents
in order to determine the structure of the complexes, and inves-
tigations on the oxidation activity of the complexes were car-
ried out in less-polar solvents so that catalysis proceeded
smoothly. These complexes 1–8 were used as catalysts for var-
ious oxygen-transfer reactions.
0
0
square-planar ligand, 6,6 -bis(benzoylamino)-2,2 -bipyridinato
2
3
(babp), and two monodentate axial ligands. The babp ligand
should not decompose during oxidation,
24–26
which is a key
factor for the preparation of a high-performance catalyst.
The effect of the axial ligand on the reactivity for the ruthe-
nium-catalyzed oxidation is investigated.
Results and Discussion
Synthesis and Characterization of Ruthenium Com-
plexes. The reaction of cis-[RuCl2(dmso)4] and BABP in
the presence of NaH in DMF solution gave [Ru(babp)(dmso)2]
complex (1). When RuCl3 was used instead of cis-[RuCl2-
(dmso)4], complex 1 was not obtained. In the positive ion elec-
trospray ionization mass spectrum (ESI-MS) of 1 in methanol
solution, peak clusters were observed at m=z ¼ 495:0 (100%),
II
þ
þ
5
27.0 (17%), 573.2 (7%), corresponding to [Ru (babp) + H] ,
II
þ
II
[Ru (babp)(CH3OH) + H] , and [Ru (babp)(dmso) + H] ,
respectively.
In the IR spectra of 1–8, characteristic C=O stretching
ꢁ
1 24
1
bands were observed at about 1550 cm , while that of free
ꢁ
Complexes 2–8 were synthesized from complex 1 in DMSO
solution by the addition of heterocycles, such as imidazole
BABP was observed at 1660 cm
that two carbonyl groups of BABP coordinate to the metal cen-
.
This finding indicates
(
(
im), pyridine (py), 4-phenylpyridine (phpy), 4-methylpyridine
mepy), 4-tert-butylpyridine (bupy), 4-N,N-dimethylamino-
ter. The ruthenium ion in 1–8 was a +II oxidation state be-
1
cause the complexes were ESR silent, so that the H NMR
pyridine (dmapy), or 4-cyanopyridine (cnpy), respectively.
The synthesis of the ruthenium(II) complexes [Ru(babp)-
spectra of 1–8 in DMSO solution could be acquired. The oxi-
dation state of ruthenium is also verified by X-ray analysis,
i.e., no counter ions were found in the crystals of the com-
plexes 1 and 5–7. In the H NMR spectra, summarized in
Table 1, the large up-field shifts (about 0.9 ppm) of babp-H5
in 1–8 were interpreted as the loss of the hydrogen bonding be-
(dmso)(im)] (2), [Ru(babp)(dmso)(py)] (3), [Ru(babp)(dmso)-
(phpy)] (4), [Ru(babp)(dmso)(mepy)] (5), [Ru(babp)(dmso)-
(bupy)] (6), [Ru(babp)(dmso)(dmapy)] (7), and [Ru(babp)-
(dmso)(cnpy)] (8), having one heterocycle and one dmso li-
1

T. Okumura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
509
Table 1. 1H NMR Data of Complexes 1–8 and Free BABPa)
Complex
babp-H3
8.19
babp-H4
7.98
babp-H5
7.35
babp-phenyl
8.40
[
[
[
[
[
[
[
[
Ru(babp)(dmso)2]
1
2
3
4
5
6
7
8
7
7
8.38
7
7
8.39
7
7
8.41
7
7
8.38
7
7
8.39
7
7
8.38
7
7
8.38
7
7
8.08
.50
.48
Ru(babp)(dmso)(im)]
Ru(babp)(dmso)(py)]
Ru(babp)(dmso)(phpy)]
Ru(babp)(dmso)(mepy)]
Ru(babp)(dmso)(bupy)]
Ru(babp)(dmso)(dmapy)]
Ru(babp)(dmso)(cnpy)]
8.10
8.26
8.28
8.24
8.24
8.18
8.31
8.24
7.88
7.95
7.97
7.94
7.93
7.90
7.97
8.03
7.27
7.28
7.30
7.26
7.28
7.25
7.35
8.21
.46
.45
.46
.45
.47
.45
.46
.45
.47
.45
.46
.44
.47
.45
BABP
7
7
.56
.64
a) ꢃ ppm from TMS in dmso-d6 solution.
tween benzoyl carbonyl oxygen and babp-H5 (8.21 ppm in free
ligand and 7.35 ppm in 1).27 In 2, the chemical shift values of
the bipyridine moiety of BABP were observed in the highest
region of all complexes, because im, which has both ꢁ and
ꢂ electron-donating abilities, has more electron-donating char-
acter than the other nitrogen heterocycles.28 Complex 7 with
dmapy, which is an electron-donating ligand, has the highest
chemical shift values among complexes 3–8, which have a
pyridine ligand, and complex 8 with cnpy as an electron-with-
drawing ligand, shows the lowest values. Based on the NMR
results, the nature of the axial ligand affects the electrochem-
ical properties of the complexes.
650
600
8
(cnpy)
4
(phpy)
6
(bupy)
5
5
4
4
50
00
50
00
3
(py)
5
(mepy)
7
(dmapy)
-0.5
-1
0
0.5
1
The electrochemical properties of 1–8 were investigated by
cyclic voltammetry in acetonitrile solution. Quasi-reversible
Hammett σ constant
Fig. 1. Relationship between the redox potentials of the
complexes (3–8) and Hammett ꢁ constants of the 4-sub-
stituent group of pyridine.
II
III
redox couples (Ru /Ru ) were observed between 460 and
10 mV (vs SCE). The E1=2 value of 2 (460 mV) was lower
6
than that of 1 (521 mV). This negative shift in 2 indicates
the stabilization of the higher oxidation state through the coor-
dination of the electron-donating im. The redox potential of 7
is lower than that of 8, indicating that the electron-donating
substituent at 4-posion of pyridine more easily adopts the high-
er oxidation state similar to im. In Fig. 1, 3 (E1=2 ¼ 531 mV vs
SCE), 4 (568 mV), 5 (528 mV), 6 (540 mV), 7 (464 mV), and 8
Structural Elucidation of Ruthenium Complexes with
Square-Planar Ligand. Complexes 1 and 5–7 were obtained
as single crystals suitable for X-ray analysis. The crystal data
for these complexes except 5, which has been reported previ-
ously,26 are summarized in Table 2, and selected bond lengths
and angles including 5 are listed in Table 3. Complexes 1 and
5–7 had an octahedral structure, in which babp coordinated to
ruthenium in a square plane and two monodentate ligands,
such as dmso or heterocycles, occupied the axial positions
(Fig. 2). As expected from their IR spectra, the carbonyl oxy-
gen atoms of the dianionic form of babp23 were coordinated to
(613 mV), which have pyridine-based ligands, show a good re-
lationship between their redox potentials and the Hammett ꢁ
constants for the 4-position of pyridine. In other words, the
electronic properties of these ruthenium complexes can be ad-
justed by the axial ligand.

5
10 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Table 2. Crystallographic Data for Complexes 1, 6, and 7
Characterization of Ruthenium–Oxo Complex
[
Ru(babp)(dmso)2]
[Ru(babp)(dmso)(bupy)]
6
[Ru(babp)(dmso)(dmapy)]
7
1a)
Formula
Fw
Color
(C14H14N2O2Ru0:5S)2
ð324:87Þ ꢃ 2
red
C36H39N5O4RuS
738.87
red
C33H34N6O4RuS
711.80
red
Crystal system
Space group
monoclinic
C2=c (#15)
17.597(1)
tetragonal
I41=a (#88)
20.3664(7)
tetragonal
I41=a (#88)
20.158(1)
˚
a/A
˚
b/A
17.1222(9)
8.8793(5)
˚
c/A
32.792(2)
32.238(2)
ꢀ/deg
ꢄ/deg
ꢅ/deg
90.590(3)
˚
V/A
3
2675.3(3)
8
1.613
7.85
1328.00
0.032
0.069
0.92
13602.0(9)
16
13099(1)
16
Z
ꢁ3
ꢆcalcd/g cm
1.443
5.69
1.444
5.89
ꢁ
1
ꢇ/cm
F000
Rb)
6112.00
0.071
0.243
1.48
5856.00
0.083
0.248
1.87
Rwc)
GOF
a) Complex 1 has a crystallographic two-fold axis passing through the Ru atom and the center of C(1)–
2 2 1=2
0
2
0
2 2
c
2
0
2
2
C(1 ) bond. b) R ¼ ꢀjjF0j ꢁ jFcjj=ꢀjF0j. c) Rw ¼ ½ꢀwðF ꢁ F Þ =ꢀwðF Þ ꢄ ; w ¼ 4F =ꢁ ðF0Þ .
0
˚
Table 3. Selected Bond Lengths (A) and Angles ( ) of 1, 5, 6, and 7
ꢂ
a)
1
dmso
5
mepy
6
bupy
7
dmapy
axial ligand
Ru–S
2.3055(9)
2.3055(9)
2.072(3)
2.000(3)
2.000(3)
2.072(3)
2.239(3)
2.132(8)
2.082(7)
1.987(9)
1.992(9)
2.071(7)
2.240(1)
2.135(1)
2.053(4)
1.984(2)
1.985(1)
2.0694(2)
2.240(2)
2.158(6)
2.076(5)
1.986(6)
1.992(6)
2.063(5)
Ru–N(1c) or Ru–S
Ru–O(1a)
Ru–N(1a)
Ru–N(1b)
Ru–O(1b)
S–Ru–N(1c) or S–Ru–S
O(1a)–Ru–N(1a)
O(1a)–Ru–N(1b)
N(1a)–Ru–N(1b)
O(1a)–Ru–O(1b)
177.64(7)
93.1(1)
174.4(1)
81.5(2)
92.3(2)
174.9(2)
92.8(3)
173.2(3)
81.3(4)
93.9(3)
177.42(6)
92.67(9)
174.48(7)
82.48(6)
92.25(8)
177.5(2)
91.9(2)
174.3(2)
82.7(3)
92.8(2)
a) Space group was C2=c. The molecule 1 has a crystallographic two-fold axis.
II
˚
the Ru–N bond lengths of 1 and 5–7 (1.984(2)–2.000(3) A)
were slightly shorter than those of Ru–salen (1.997(4)–
˚
2.039(5) A), and the Ru–O bond lengths (2.053(4)–2.082(7)
the Ru ion in each complex.
Complex 1 has a crystallographic two-fold axis passing
0
through the Ru atom and the center of C(1)–C(1 ) bond. The
˚
Ru–S(dmso) bond length was 2.3055(9) A, which was slightly
longer than those observed in [Ru(NH3)5(dmso)] (2.188(3)
˚
A) were similar or slightly longer than the salens (2.020(4)–
33
2
þ
˚
2.063(3) A).
˚
29
˚
30
A) and cis-[RuCl2(dmso)4] (2.252(1)–2.277(1) A). The
longer Ru–S bond lengths, which also occur in the case of
Oxidation of Olefins and Thioether. The oxidation of cis-
cyclooctene, cis- and trans-stilbenes, and thioanisole catalyzed
by the typical ruthenium complexes (1–4) was carried out in
the presence of iodosylbenzene (PhIO) as a two-electron oxi-
dant (Ru:substrate:PhIO = 1:100:100) in 1,2-dichloroethane
cis-[Ru(bpy)2(dmso)2]2 (2.292(1)–2.293(2) A), is due to the
þ
˚
electron density from back-bonding with the pyridine li-
3
1
þ
gand, and that in the amine complex [Ru(tpy)(tren)(dmso)]
˚
2.251(1) A) is slightly shorter than that of the pyridine com-
ꢂ
(
solution at 40 C for 4 h under argon atmosphere (Scheme 3).
þ
˚
32
plex [Ru(tpy)(bpy)(dmso)] (2.282(1) A). The bond lengths
of Ru–S(dmso) in 5–7 were slightly shorter than that of 1, be-
cause of the trans-influence of the heterocycles. The relatively
longer Ru–S(dmso) bond length in the complex 1 indicates a
ligand-exchange reaction in the presence of PhIO. In addition,
These results are summarized in Table 4. Epoxidation of cis-
cyclooctene catalyzed by 2 gave a 30% yield of 1,2-epoxy-
cyclooctane. Generally, epoxidation of cis-cyclooctene cata-
lyzed by ruthenium complexes does not proceed quantitative-
ly. For example, 1,2-epoxycyclooctane is obtained in a 19%

T. Okumura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
511
6
yield when a mononuclear ruthenium(III) complex with Schiff
base ligand is used as a catalyst,34 and Ru –salen complexes
give the epoxide in poor yield in comparison with the corre-
sponding MnIII complexes. In this case, PhIO is known to
be an efficient 2-electron oxidant for the generation of
III
Ru=O species,1
,2,5,6,35,36
but has relatively lower epoxidation
ꢁ
37
activity compared with periodate oxidant (IO4 ). When 4
was used as a catalyst in the place of 2, only a 4.2% yield
of 1,2-epoxycyclooctane was obtained under the same reaction
conditions. The order of the yield of 1,2-epoxycyclooctane was
2 > 1 > 3 > 4, which was identical with the case of oxida-
S
O(1)
O
Ru
N(2)
N(1)
C(1)
t
N
N
tion of stilbenes and cyclohexene. When BuOOH was used
C(1')
as an oxidant in the presence of 1, the yield of the correspond-
ing epoxide was 26%. The reactivity of alkylhydroperoxide is
similar to that of PhIO. m-Chloroperbenzoic acid (MCPBA),
which is often used to generate metal–oxo species, was not uti-
S
(a)
lized in our oxidation experiments to avoid further complicat-
ed reactions.3
,5,12,13,17,38,39
For our investigations to determine
the relationship between the structure and the catalytic activi-
ties of the ruthenium complexes, PhIO was used because of
ease of handling.41 In the oxidation of cyclooctene, moreover,
no cis-cyclooctane-1,2-diol was detected, which means that
cis-dioxoruthenium(VI) intermediate was not involved in the
reaction.40
S
O(1a)
O(1b)
Ru
N(1b)
N(1a)
N(1c)
Oxidation of cis-stilbene catalyzed by 1–4 gave cis-stilbene
oxide, a small amount of trans-epoxide, and large amount of
benzaldehyde, which was a C=C bond cleavage product. For-
mation of a thermally stable trans-stilbene oxide in the oxida-
,13,42
tion of cis-stilbene suggests a radical intermediate.2
In
acetonitrile, the epoxidation activity of 1 decreased compared
to that in 1,2-dichloroethane, and the ratio for cis-stilbene ox-
(
b)
16
ide to trans-one decreased. Radical pathway is accelerated in
halogeno-hydrocarbon.4
2,43
In the case of 3 or 4 with pyridine
derivative, a significant decrease in the yield of stilbene oxide
and increase in that of benzaldehyde were observed. This find-
S
O(1b)
O(1a)
Ru
N(1b)
N(1a)
O
N(1c)
Ph
Ph
Ru / PhIO
O
+
+
PhCHO
N
Ph
Ph
Ph
Ph
O
S
O
O
(c)
S
S
Ph
Fig. 2. (a) X-ray structure of 1. (b) X-ray structure of 6.
(
c) X-ray structure of 7.
Scheme 3. Oxidation of substrates.
Table 4. Oxidation of Olefins and Thioether Catalyzed by 1–4a)
Cyclooctene
Epoxide
cis-Stilbeneb)
trans- Benzaldehyde
/%
trans-Stilbeneb)
trans- Benzaldehyde Sulfoxidec) Sulfoned)
Thioanisole
[
Ru(babp)(dmso)L]
cis-
Epoxide Epoxide
cis-
Epoxide Epoxide
/%
/%
/%
/%
axial ligand (L)
dmso
/%
/%
/%
ndg)
/%
1
24
2
20
2.2
48
19
34
41
5.1
e)
6
8
28
.2f)
1.8f)
2.3
1.3
1.4
40f)
32
50
53
2
3
4
im
py
phpy
30
13
4.2
nd
nd
nd
28
14
12
28
44
45
23
37
22
9.6
6.8
10
7.8
6.6
ꢂ
a) Reaction conditions: Ru–babp complex:substrate:PhIO = 1:100:100 in 1,2-dichloroethane, 40 C, 4 h, under Ar. b) Ref. 26. c)
t
Methylphenylsulfoxide. d) Methylphenylsulfone. e) BuOOH was used as an oxidant instead of PhIO. f) Acetonitrile was used as
a solvent instead of 1,2-dichroloethane. g) nd = not detected.

5
12 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Characterization of Ruthenium–Oxo Complex
O
(
Epoxidation)
Ru-babp / PhIO
OH
O
+
(Allylic Oxidation)
OH Ru-babp / PhIO
O
(
Dehydrogenation)
Scheme 4.
ing is similar to a previous report that the epoxidation of trans-
stilbene catalyzed by the combined system of RuCl3, NaIO4,
and chiral bis(dihydrooxazolylphenyl)oxalamide ligand gave
a moderate yield (4–43%) of the corresponding epoxide ac-
companied by significant amount (21–65% yield) of benzalde-
Table 5. Oxidation of Cyclohexene Catalyzed by 1–8a)
Complex
[Ru(babp)(dmso)L]
Epoxidation Allylic oxidation E/A
Yield (A)c)
/%
Yield (E)
/%
L
4
4
b)
hyde. In the oxidation of stilbenes catalyzed by the high-val-
ent ruthenium–oxo species with a non-porphyrin ligand, C=C
bond cleavage reactions predominantly have been reported to
dmso
b)
1
2
19
22
54
10
7.5
11
10
5.6
6.1
12d)
7.3
8.6
5.4
5.3
3.6
3.3
3.7
4.7
1.4
0.9
2.0
1.9
0.5
2.6
2.6
im
d)
d)
4
pyb)
phpy
occur. Using ruthenium–porphyrin complex, however, selec-
3
4
5
6
7
tivity for the cleavage reaction is quite low.45 This finding
indicates that babp acts as a non-porphyrin ligand, even though
it adopts square-planar 4-coordination geometry with the ex-
panded resonant system. In the oxidation of trans-stilbene,
b)
mepy
bupy
dmapy
1.8
d)
d)
d)
5
19
1
20
7.1
1
–4 had similar reactivity to that with cis-stilbene, although
cnpy
8
cis-stilbene oxide was not detected. In these reactions, forma-
tion of benzaldehyde relatively decreased in comparison with
the cases of cis-one.
a) Reaction conditions were the same as those in Table 4.
b) Ref. 26. c) Total yields of 2-cyclohexen-1-ol and 2-cyclo-
ꢂ
Oxidation of thioanisole catalyzed by 1–4 gave methyl-
phenylsulfoxide and methylphenylsulfone. The sulfone is suc-
cessive oxidation product of the sulfoxide. In the oxidation of
thioether, an electrophilic oxygen species gave a sulfoxide, but
a nucleophilic one did not show any reactivity.46 On the other
hand, sulfone is generated from the corresponding sulfoxide
using either an electrophilic or nucleophilic active oxygen spe-
hexen-1-one. d) 80 C, 2 h.
complexes except 7 showed moderate reactivity for epoxida-
ꢂ
ꢂ
tion at 40 C. The epoxidation activity of 2 increased at 80 C.
In the reaction catalyzed by 7, the yield of the oxidation prod-
ꢂ
ucts was very low at 40 C. The yield of 1,2-epoxycyclooctane
ꢂ
was 0.8%. At 80 C, however, 7 showed higher catalytic activ-
ity, and the yield of 1,2-epoxycyclohexane rose to 51% accom-
4
7
cies. Formation of methylphenylsulfoxide and methylphenyl-
sulfone in this system indicates that the active species derived
from 1–4 has electrophilic and nucleophilic characters.
Effect of Axial Ligands. Oxidation of cyclohexene cata-
ꢂ
panied by 20% of the allylic oxidation products. At 80 C, the
reactivity of 7 is comparable to that of 2. The dmapy ligand
coordinated at the trans-position might suppress the ligand-
exchange reaction from dmso to oxo species. At higher tem-
perature, however, dissociation of dmso from 7 gave an active
species leading to the same reactivity as demonstrated in 2. In
complexes 3–6, their oxidation activities are similar to each
other, of which redox potentials are within a range of 40 mV
as shown in Fig. 1. From these findings, we believe that the
active species generated from the complexes 2–7 is common
in the catalysis. In the case of 8, we may consider that the re-
activity was the lowest of all complexes here employed be-
cause of the coordination of the electron-withdrawing cnpy
ligand at the trans-position. However, the oxidation activity
of 8 was comparable to that of 1 as listed in Table 5. This find-
ing is interpreted in terms of the easy dissociation of cnpy from
ruthenium, which is discussed later.
ꢂ
lyzed by 1–8 was carried out at 40 C for 4 h under argon in
the presence of PhIO (Ru:olefin:PhIO = 1:100:100) in 1,2-
dichloroethane. This reaction gave 1,2-epoxycyclohexane as
an epoxidation product and both 2-cyclohexen-1-ol and 2-
cyclohexen-1-one as allylic oxidation products as shown in
Scheme 4. In this ruthenium–PhIO system, alcohol was easily
4
8
oxidized to ketone. It is well known that cyclohexene is sen-
49
sitive toward allylic oxidation, but cis-cyclooctene hardly
takes place allylic oxidation.50 Using iron(III)– or manga-
nese(III)–porphyrin and PhIO system, influence of the ring
size of the cycloalkenes on the selectivity for epoxidation
and allylic oxidation has been studied.51 Epoxidation activity
of 2 was superior to those of the other complexes as summa-
rized in Table 5. The yields of 1,2-epoxycyclohexane decreas-
ed in the order of 2 > 1 > 3 > 4, which was similar to the
case of cis-cyclooctene as mentioned above. All ruthenium
The ratio (E/A) of epoxidation to allylic oxidation describes
an interesting catalytic activity of the ruthenium complexes.

T. Okumura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
513
Here, the allylic oxidation products include both 2-cyclo-
hexen-1-ol and 2-cyclohexen-1-one, because the allylic alco-
hol once formed is further oxidized to give an ꢀ,ꢄ-unsaturated
ketone through the ruthenium-catalyzed oxidative dehydrogen-
(
a)
394
1
6,36,48
ation pathway.
In the reactions catalyzed by 1–8, E/A in
3
77
(
b)
Table 5 was in range of about 0.5–3.7, which was almost con-
stant during the initial stage of the reaction. Using tripodal
II
pyridylmethylamine ligands, the ratios 21 using [Ru (bppa)-
III
Cl]PF6 and 6.3 using [Ru (bnpa)Cl2]PF6 have been report-
(c)
1
6
ed. Such a difference in the reactivity has been interpreted
in terms of the trans-influence of tertiary amine or pyridine
of the tripodal ligands. We think that the oxo-species generated
V
with bppa complex is an electrophilic Ru =O species and that
IV
with bnpa complex is a radical Ru –Oꢅ species. The catalytic
intermediate in these reactions is thought to be combination of
IV
V
both Ru =O and Ru –Oꢅ, which are electronically equiva-
5
2
Fig. 3. Spectral change of 1 in 1,2-dichloroethane solution
with successive addition of PhIO and cyclohexene. (a)
Starting solution of 1 (0.1 mM). (b) Addition of PhIO
lent to each other. The amine- or pyridine-nitrogen atom that
occupies trans-position of the oxo species affects the inter-
5
2,53
mediate.
In the reactions catalyzed by 1–8, the axial ligand
(
(
100 equiv) to (a). (c) Successive addition of cyclohexene
100 equiv) to (b).
coordinated at trans-position to the ruthenium–oxo species ob-
viously causes trans-influence for the reactivity. The ratio in 2
was higher than those in 3–6. When the electron-donating im
was used as an axial ligand, the epoxidation activity for cyclo-
hexene was relatively high, but the allylic oxidation activity
was low. On the other hand, such selectivity was not observed
when pyridine derivatives were used. The effect of donor
ligand has also been reported in the case of Cr(salen)- or
Mn(salen)-catalyzed epoxidation of olefins.53
III
þ
metastable transient species, in a mixture of [Mn (salen)]
and PhIO has been reported.56 In positive mode ESI-MS in a
1/10 (v/v) 1,2-dichloroethane/acetonitrile solution, several
spectral patterns were observed in the reaction mixture of
the [Ru(babp)(dmso)L] and PhIO. In the case of 1, peak clus-
ters were observed at m=z ¼ 527:1 (100%), 510.1 (20%), and
VI
þ
494.1 (35%), corresponding to the [Ru (babp)(O)2 + H] ,
IV
þ
III
þ
Spectroscopic Characterization of Ruthenium–Oxo Spe-
cies. Characterization of the transient species generated from
the starting ruthenium complex and PhIO was carried out using
the UV–vis and ESI-MS spectroscopic methods. The color
change of the reaction solution from reddish-yellow of the
starting ruthenium(II) species to reddish-purple after addition
of PhIO indicates a transient ruthenium species. In the reaction
of 1 and PhIO, typical absorption bands in UV–vis spectrum
were observed at 377 and 394 nm (" ¼ 5700 and 6400
[Ru (babp)(O) + H] , [Ru (babp)] ions, respectively, al-
though unidentified higher molecular peak clusters were de-
tected at m=z ¼ 563:2 and 552.1. The isotopic patterns of
VI
Ru species at m=z ¼ 527:1 agreed with those calculated for
VI
þ
VI
þ
[Ru (babp)(O)2 + H] or [Ru (babp)(O)(OH)] , as shown
in Fig. 4. Although we have no information whether formula
VI
þ
VI
þ
of [Ru (babp)(O)2 + H] or [Ru (babp)(O)(OH)] is cor-
VI
rect, formation of Ru =O species as expected from the
UV–vis and ESR spectra is confirmed by ESI-MS. In the pres-
ence of PhIO, the starting complex 1 easily released two dmso
ligands from ruthenium to give oxo species accompanied with
ꢁ
1
ꢁ1
cm , respectively). They are assigned to spin-allowed
M
2
ꢁ
VI
pꢂ(O ) ! Ru charge-transfer (CT) band, because the
characteristic absorption bands of Ru=O species with nitro-
gen-containing ligands are generally observed at about 380–
II
VI
oxidation of Ru to Ru . In the case of 2, a peak cluster was
V
observed at m=z ¼ 578:3 (100%) corresponding to [Ru -
3
9,54
þ
4
00 nm.
This CT band disappeared after addition of cyclo-
(babp)(im)(O)] ions, as shown in Fig. 5. Substitutionally
hexene. Spectral changes in the reaction solution of 1 with suc-
cessive addition of PhIO and cyclohexene in 1,2-dichloro-
ethane are shown in Fig. 3. Moreover, the ESR signal of the
solution containing 1 and PhIO was silent. These findings in-
dicate that the oxidation state of the starting ruthenium 1 is
labile dmso in 2 was exchanged with an oxo ligand, and sub-
stitutionally inert im remained in the transient species. Some
of the differences observed in the reactivity of 1 and 2 are in-
terpreted in terms of the oxidation state of the transient ruthe-
nium species.
+
II and that of active species generated by the reaction with
The ESI-MS of the reaction of 7 and PhIO had a peak clus-
III
PhIO is +IV or +VI. On the other hand, such color change
as observed with 1 did not occur with 2–4. The Ru=O species
generated from 2–4 and PhIO is different from that from 1.
Detailed characterization of the active species from 2–4 was
impossible using NMR spectroscopy because of their para-
magnetic characters. Accordingly, we think that the oxidation
state of the active species generated from 2–4 is +V.
ter at m=z ¼ 693:9 (100%), corresponding to [Ru (babp)-
þ
(dmapy)(dmso)] ions. No Ru=O species thought to be the ac-
tive oxygen species were detected in the same analysis condi-
tion in 1 and 2. It is concluded that the poor catalytic activity
ꢂ
of 7 at 40 C is because no active species forms. Complex 7
having electron-donating dimethylamino group was expected
to have similar activity to that of 2 because of their E1=2 val-
ꢂ
ESI-MS is known to be a useful tool for characterization of
large size biomolecules and multiply charged metal complexes
ues, but it had almost no reactivity at 40 C. At higher temper-
ature, 7 became reactive, because dmso dissociated to afford
the active species in the presence of PhIO. Complex 8, which
has an electron-withdrawing cyano group, was expected to
55
in solution because of its mild ionization method. Detection
þ
V
of prominent peak clusters of [O=Mn (salen)] complex, as a

5
14 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Characterization of Ruthenium–Oxo Complex
5
27.0
isotopic distribution patterns
578.1
(b)
(b)
100
V
+
1
1
00
for [Ru (babp)(im)(O)]
isotopic distribution patterns for
[
Ru (babp)(O)2 + H] or [RuVI(babp)(O)(OH)]+
VI
+
m/z
0
00
0
00
m/z
1
5
27.1
578.3
measured
measured
0
m/z
520
525
530
535
540
0
m/z
(a)
100
527.1
(a)
100
5
78.3
5
76.4
50
5
0
494.1
5
63.2
5
65.3
615.4
5
10.1
552.1
5
51.3
0
m/z
460
480
500
520
540
560
580
0
m/z
5
20
540
560
580
600
620
640
Fig. 4. Positive mode ESI-MS of the reaction mixture of
and PhIO. (a) Mass spectral pattern within the range
1
Fig. 5. Positive mode ESI-MS of the reaction mixture of
and PhIO. (a) Mass spectral pattern within the range
VI
m=z ¼ 460{580. (b) Peak clusters assigned to Ru =O
2
species.
V
m=z ¼ 520{640. (b) Peak clusters assigned to Ru =O
species.
have the lowest reactivity, because it has the most positive
E1=2 value of all complexes. In the reaction catalyzed by 8,
however, the oxidation activity as listed in Table 5 was similar
to that of 1. The changes in the spectra of the reaction solu-
tion containing 8 upon successive additions of PhIO and cyclo-
hexene were similar to those of 1. ESI-MS spectrum of the
generated from the ligand-exchange reaction of dmso and ox-
idant, was affected by the trans-influence of the axial ligand.
The order of epoxidation activity of [Ru(babp)(dmso)L] com-
plexes (1–8) was as follows: 2 (L = im) > 1 (L = dmso) >
3
such as dmso or cnpy, are easily exchanged to afford the
–8 (L = pyridine derivatives). Weakly coordinated ligands,
mixture of 8 and PhIO also had peak clusters at m=z ¼ 638:9
VI
(
(
8%) and 676.9 (100%). The former was assigned to [Ru -
þ
VI
oxo species Ru =O in the presence of PhIO. Strongly coordi-
nated dmapy prevents ligand-exchange reaction at low temper-
ature. Moderately coordinated ligand, such as im or pyridine
babp)(O)2(C2H6SO2)(H2O)] ion, with which isotopic pattern
fully agreed, but the formation pathway is not clear yet. The
þ
III
latter was assigned to [Ru (babp)(cnpy)(dmso)] . Formation
VI
V
derivatives, gives Ru =O intermediate. The catalytic activities
of Ru =O from 8 was confirmed by ESI-MS, even though
the starting complex remained. The weakly coordinated axial
of mononuclear ruthenium–oxo complexes with the square-
planar ligand are controlled by axial ligand coordinating at
the trans-position. Rational design of the coordination struc-
ture of the catalyst precursor, leading to the active species, is
important for selective oxygen-transfer reactions.
VI
ligand, cnpy, dissociates from 8 to generate the Ru =O
species.
Conclusion
Eight ruthenium(II) complexes, [Ru(babp)(dmso)L], con-
sisting of the tetradentate babp and two monodentate ligands,
DMSO and L (L = DMSO or heterocycles), were prepared.
All complexes had an octahedral geometry with square-planar
coordination of babp. Complexes 1–8 showed moderate cata-
lytic activities toward the oxygen-transfer reactions, such as
epoxidation, allylic oxidation, and bond cleavage of olefins
and sulfoxidation of thioethers, in the presence of PhIO. The
catalytic activity of the high-valent ruthenium–oxo species,
Experimental
Materials and Measurement. Reagents used for synthesis
were of the highest grade available and were used without further
purification. All solvents for spectroscopic measurements, except
NMR spectroscopy, were purified by distillation before use. BABP
was prepared according to the method previously reported.24
UV–vis spectra were recorded on a JASCO Ubest-570. H NMR
spectra were measured on a Varian VXR-300S spectrometer with
TMS as an internal standard. The redox potentials (RuII/III) of
1

T. Okumura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
515
the complexes were measured in CH3CN solution using a BAS
CV-1 cyclic voltammetry unit. All measurements were carried
selected bond lengths and angles are listed in Table 3.
[Ru(babp)(dmso)(im)](2):
To complex 1 (37 mg, 0.057
ꢁ1
out at room temperature with a sweep rate of 50 or 100 V s un-
n
mmol) under nitrogen atmosphere was added a DMSO solution
(3 mL) of im (39 mg, 0.57 mmol). The mixture was stirred for 3
days at room temperature. The cloudy reaction solution became
clear dark red, to which acetone was added. On standing, a small
amount of precipitate was obtained. In the preparation of 2–8, iso-
lated yields of the ruthenium(II) complexes precipitated from the
reaction solution were about 50% based on the starting 1.
der Ar using Bu4NPF6 as a supporting electrolyte and referenced
to SCE. ESR spectra were measured with a JEOL JES-REIX spec-
trometer at 77 K. IR spectra measured for a KBr pellet were per-
formed on a JASCO FT/IR-410. GC analysis using an internal
standard method was performed on a Shimadzu GC8F apparatus
equipped with PEG-20M or silicon OV-17 column. Positive mode
of ESI-MS spectra for characterization of Ru=O species generat-
ed from the mixture of ruthenium complex and 10 equiv of PhIO
in 1,2-dichloromethane solution were aquired on a Micromass
LCT, to which 10-fold amount of acetonitrile was added for
easy ionization. The isotopic patterns of the complexes agree with
those calculated for the given formulations.
IR (KBr, cm 1): 1550 (C=O). Anal. Calcd for 2 (C29H26N6-
ꢁ
O3SRu 0.5H2O): C, 53.69; H, 4.20; N, 12.96%. Found: C, 53.66;
ꢆ
H, 4.09; N, 12.74%. 1H NMR data for 2 are summarized in
Table 1.
[Ru(babp)(dmso)L] (3–8): Complexes 3–8 with pyridine de-
1
rivatives were synthesized similar to 2. H NMR data are summa-
rized in Table 1.
Crystal Structure Analysis. Single crystals of 1 and 5–7
suitable for X-ray diffraction measurements were mounted on a
glass capillary, and the diffraction data were collected for 1, 6,
and 7 on a Rigaku MSC Mercury CCD using graphite monochro-
ꢁ1
[Ru(babp)(dmso)(py)] (3): Red crystal. IR (KBr, cm ): 1550
(C=O). Anal. Calcd for 3 (C31H27N5O3SRu 0.2DMSO): C,
ꢆ
56.60; H, 4.27; N, 10.51%. Found: C, 56.84; H, 4.23; N, 10.24%.
ꢁ1
˚
mated Mo Kꢀ radiation (ꢈ ¼ 0:71070 A) and for 5 on a Rigaku
[Ru(babp)(dmso)(phpy)] (4): Dark red crystal. IR (KBr, cm ):
1548 (C=O). Anal. Calcd for 4 (C37H31N5O3SRu 0.4H2O): C,
RAXIS-II imaging plate area detector using graphite monochro-
mated Mo Kꢀ radiation. All of the structures were solved by a
combination of direct method and Fourier techniques. Non-
hydrogen atoms were anisotropically refined by full-matrix least-
squares calculations. Hydrogen atoms were included but not
refined. Refinements were continued until all shifts were smaller
than one-third of the standard deviations of the parameters in-
ꢆ
60.54; H, 4.37; N, 9.54%. Found: C, 60.28; H, 4.23; N, 9.25%.
ꢁ
1
[Ru(babp)(dmso)(mepy)] (5): Orange crystal. IR (KBr, cm ):
1549 (C=O). Anal. Calcd for 5 (C32H29N5O3SRu 0.5acetone): C,
ꢆ
58.00; H, 4.65; N, 10.09%. Found: C, 57.84; H, 4.45; N, 9.99%.
The crystal structure of 5 obtained from recrystallization in ace-
tone solution was reported previously.26 Selected bond lengths
and angles are listed in Table 3.
5
7
volved. R and Rw values were defined as follows: R ¼ ꢀjjF0j ꢁ
2
2 2
2 2 1=2
2
ꢁ1
jFcjj=ꢀjF0j and Rw ¼ ½ꢀwðF0 ꢁ Fc Þ =ꢀwðF0 Þ ꢄ , w ¼ 4F0 =
[Ru(babp)(dmso)(bupy)] (6): Orange crystal. IR (KBr, cm ):
1551 (C=O). Anal. Calcd for 6 (C35H35N5O3SRu DMSO): C,
2
2
ꢁ
ðF0Þ . Atomic scattering factors and anomalous dispersion
ꢆ
terms were taken from International Tables for X-ray Crystallog-
raphy.58 All calculations were carried out on a Japan SGI worksta-
tion computer using the teXsan crystallographic software pack-
age.59 Crystallographic data has been deposited with Cambridge
Crystallographic Data Centre: Deposition number CCDC-622758
for compound 1, -622759 for 6, and -622760 for 7. Copies of the
data can be obtained free charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax:
56.61; H, 5.62; N, 8.92%. Found: C, 56.90; H, 5.43; N, 8.87%.
Crystallographic data are listed in Table 2, and selected bond
lengths and angles are listed in Table 3.
ꢁ1
[Ru(babp)(dmso)(dmapy)] (7): Orange crystal. IR (KBr, cm ):
1548 (C=O). Anal. Calcd for 7 (C33H32N6O3SRu DMSO): C,
ꢆ
54.46; H, 4.96; N, 10.89%. Found: C, 54.67; H, 5.07; N, 11.04%.
Crystallographic data are listed in Table 2, and selected bond
lengths and angles are listed in Table 3.
ꢁ1
[Ru(babp)(dmso)(cnpy)] (8): Orange Crystal. IR (KBr, cm ):
1551 (C=O). Anal. Calcd for 8 (C32H26N6O3SRu H2O): C,
+
44 1223 336033; e-mail: deposite@ccdc.cam.ac.uk).
Synthesis and Characterization of the Ruthenium Com-
ꢆ
55.40; H, 4.07; N, 12.11%. Found: C, 55.33; H, 4.00; N, 12.00%.
Oxidation of Substrates. Oxidation of substrate (cyclooctene
55 mg, cis- and trans-stilbenes 90 mg, cyclohexene 41 mg, or thio-
anisole 62 mg (0.5 mmol)) was carried out in the presence of PhIO
(110 mg, 0.5 mmol) and the ruthenium complex (0.005 mmol) in
II
plexes. [Ru (babp)(dmso)2] (1): To a solution of BABP (0.26
g, 0.65 mmol) in DMF (80 mL) was added excess amount of NaH
(
0.07 g) and equimolar amount of cis-[RuCl2(dmso)4] (0.32 g).
The mixture was refluxed for 3 h under nitrogen atmosphere. After
evaporation of the solvent, small amounts of DMSO and acetone
were added to the residue. An orange crystal (0.23 g, 55% yield)
ꢂ
1,2-dichloroethane (5 mL) under Ar atmosphere at 40 C. The re-
action products identified by comparison with the authentic sam-
ples were monitored by GC analysis at appropriate time.
Measurement of Spectral Change of the Complex 1. UV–
vis spectra of the complex 1 was measured in the 1,2-dichloro-
ethane solution (0.1 mM) at room temperature, to which 100-fold
amount of PhIO was added and stirred at room temperature under
Ar atmosphere. Absorption maxima as CT bands were observed at
377 and 394 nm. ESI-MS was measured under these conditions
with addition of acetonitrile. To examine the reactivity of this re-
action solution, 100-fold amount of cyclohexene was added. The
CT bands were disappeared after addition of cyclohexene. These
results are shown in Fig. 3.
II
of [Ru (babp)(dmso)2] complex (1) suitable for X-ray analysis
precipitated from the solution.
IR (KBr, cm 1): 1552 (C=O). Anal. Calcd for 1 (C28H28N4-
ꢁ
O4S2Ru 0.1H2O): C, 51.62; H, 4.36; N, 8.60%. Found: C, 51.41;
ꢆ
H, 4.47; N, 8.84%. Positive mode ESI-MS (methanol): m=z ¼
II
4
95:0 (100%), 527.0 (17%), 573.2 (7%), corresponding to [Ru -
II
þ
II
þ
(
(
babp) + H] , [Ru (babp)(CH3OH) + H] , and [Ru (babp)-
dmso) + H] ions, respectively. H NMR data are summarized
þ
1
in Table 1, in which the chemical shift values of the coordinating
CH3)2SO moiety of all complexes are not characterized because
(
of scrambling of the ligand (dmso-h6) and the solvent (dmso-d6).
Complex 1 has a crystallographic two-fold axis passing through
0
the Ru atom and the center of C(1)–C(1 ) bond, so the environ-
ments of the above and below structures of the complex are the
same each other. Crystallographic data are listed in Table 2, and
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology Japan, to which our thanks are due.

5
16 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Characterization of Ruthenium–Oxo Complex
2
3
9
0
F. C. March, G. Ferguson, Can. J. Chem. 1971, 49, 3590.
A. Mercer, J. Trotter, J. Chem. Soc., Dalton Trans. 1975,
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