Chemistry of ruthenium with some dioxime ligands. Syntheses, structures and reactivities

Article abstract of DOI:10.1016/S0277-5387(00)00613-6

Reaction of two dioxime ligands, viz. dimethylglyoxime (H₂dmg) and diphenylglyoxime (H₂dpg). (abbreviated in general as H₂L. where H stands for the oxime protons) with [Ru(PPh₃)₃Cl₂] in 1:1 mole ratio affords complexes of type [Ru{PPh₃)₂(H₂L)Cl₂]. Structure of the [Ru(PPh₃)₂(H₂dpg)Cl₂] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium, is N₂P₂Cl₂ with the two PPh₃ ligands in frans and the two chlorides in cis positions. Reaction of the dioxime ligands with [Ru(PPh₃)₃Cl₂] in 2:1 mole ratio in the presence of a base affords complexes of type [Ru(PPh₃)₂(HL)₂]. Structure of the [Ru(PPh₂(Hdmg)₂] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is N₄P₂ with the two PPh₃ ligands in trans positions. Reaction of the [Ru(PPh₃)₂H₂dpg)Cl₂ complex with a group of bidentate acidic ligands, viz. picolinic acid (Hpic), quinolin-8-ol (Hq) and 1-nitroso-2-naphthol (Hnn), (abbreviated in general as HL', where H stands for the acidic proton) in the presence of a base affords complexes of type [Ru(PPh₃)₂(H₂dpg)(L')]⁺ isolated as perchlorate salts. All the complexes are diamagnetic (low-spin d⁶, S = 0) and in dichloromethane solution show several intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a reversible ruthenium(II)-ruthenium(III) oxidation within 0.36-0.98 V versus SCE followed by a quasi-reversible ruthenium(III)-ruthenium(IV) oxidation within 0.94- 1.60 V versus SCE.

Full text of DOI:10.1016/S0277-5387(00)00613-6

www.elsevier.nl/locate/poly
Polyhedron 20 (2001) 327–335
Chemistry of ruthenium with some dioxime ligands.
Syntheses, structures and reactivities
Anjan Kumar Das a, Shie-Ming Peng b, Samaresh Bhattacharya a,*
a Department of Chemistry, Inorganic Chemistry Section, Jada6pur Uni6ersity, Calcutta 700 032, India
b Department of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC
Received 24 August 2000; accepted 8 November 2000
Abstract
Reaction of two dioxime ligands, viz. dimethylglyoxime (H2dmg) and diphenylglyoxime (H2dpg), (abbreviated in general as
H2L, where H stands for the oxime protons) with [Ru(PPh3)3Cl2] in 1:1 mole ratio affords complexes of type [Ru(PPh3)2(H2L)Cl2].
Structure of the [Ru(PPh3)2(H2dpg)Cl2] complex has been solved by X-ray crystallography. The coordination sphere around
ruthenium is N2P2Cl2 with the two PPh3 ligands in trans and the two chlorides in cis positions. Reaction of the dioxime ligands
with [Ru(PPh3)3Cl2] in 2:1 mole ratio in the presence of a base affords complexes of type [Ru(PPh3)2(HL)2]. Structure of the
[Ru(PPh3)2(Hdmg)2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is N4P2 with
the two PPh3 ligands in trans positions. Reaction of the [Ru(PPh3)2(H2dpg)Cl2] complex with a group of bidentate acidic ligands,
viz. picolinic acid (Hpic), quinolin-8-ol (Hq) and 1-nitroso-2-naphthol (Hnn), (abbreviated in general as HL%, where H stands for
the acidic proton) in the presence of a base affords complexes of type [Ru(PPh3)2(H2dpg)(L%)]+ isolated as perchlorate salts. All
the complexes are diamagnetic (low-spin d6, S=0) and in dichloromethane solution show several intense MLCT transitions in the
visible region. Cyclic voltammetry on all the complexes shows a reversible ruthenium(II)–ruthenium(III) oxidation within
0.36–0.98 V versus SCE followed by a quasi-reversible ruthenium(III)–ruthenium(IV) oxidation within 0.94–1.60 V versus SCE.
© 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ruthenium; Dioxime ligands; Syntheses; Structures; Reactivities
the groups linked to the two carbons and the two
imine-nitrogens. The presence of other p-acceptor lig-
ands within the coordination sphere may also have
significant influence on the p-interaction between the
diimine ligands and ruthenium(II).
1. Introduction
The ruthenium chemistry of diimine ligands (1) is an
area of significant current interest [1–7], particularly
with regard to the photophysical and photo-chemical
properties exhibited by such complexes. Diimine lig-
ands are strong p-acceptors and are recognized stabiliz-
ers of the +2 state of ruthenium (low-spin d6, S=0).
As a consequence, an interesting aspect of the ruthe-
In the present study, we have chosen dioximes (2;
nium–diimine chemistry has been to study the remark-
able p-interaction between the filled t2 orbitals of
ruthenium(II) and the low-lying vacant p*-orbital of
the diimine chromophore. The extent of p-interaction in
these complexes depends primarily on the nature of the
diimine ligands, which again depends on the nature of
abbreviated in general as H2L, where H stands for the
oxime protons) as the principal ligand, not only be-
cause they carry the diimine chromophore but also for
their different coordination modes. In our recent stud-
ies on the chemistry of ruthenium with some
monooxime ligands we have witnessed interesting oxo-
transfer and polynucleation reactions [8,9], and these
have also encouraged us to study the ruthenium chem-
istry of the dioxime ligands. The dioxime ligands are
known to coordinate metal ions as neutral dioximes
* Corresponding author. Tel.: +91-33-483-6223; fax: +91-33-473-
4266.
E-mail address: samaresh b@hotmail.com (S. Bhattacharya).
–
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00613-6

328
A.K. Das et al. / Polyhedron 20 (2001) 327–335
and also as monoanionic dioximates via dissociation of
one oxime proton [10,11]. They are also known to act
as bridging ligands via coordination through the oxy-
gens [12–15]. While coordination chemistry of the
dioxime ligands has been extensively studied with the
3d metal ions [16–20], the dioxime chemistry of ruthe-
nium has not been much explored [21–26]. Herein we
report the chemistry of some mono and bis-dioxime
complexes of ruthenium(II), where triphenylphosphine
(PPh3) has been used as the coligand. Triphenylphos-
phine is also a familiar p-acceptor ligand and hence its
coordination is expected to result in some interesting
effect on the p interaction with the dioxime ligand as
well as on the stereochemistry of the complexes. In the
present work, the chemistry of dioxime complexes of
type [Ru(PPh3)2(H2L)Cl2] and [Ru(PPh3)2(HL)2] has
been studied. The chemistry of the bis-dioximate com-
plexes of transition metal ions has been attracting con-
tinuous attention because of their importance with
reference to models for vitamin B12 [27–29], dioxygen
carriers [30], catalysis in chemical transformations [31–
33], intramolecular hydrogen bonding and metal–metal
interaction [34–36]. The synthesis, structure, spectro-
scopic and electron-transfer properties of these com-
plexes have been described in this paper with special
emphasis on the p-interaction between dioxime ligands
and bivalent ruthenium, and reactivities of the mono-
dioxime complexes involving Ru–Cl bond cleavage.
was obtained which was washed thoroughly with etha-
nol and dried in air. Recrystallization from dichloro
methane–hexane solution gave [Ru(PPh3)2(H2dmg)Cl2]
as a crystalline brown solid. The yield was 83 mg (70%).
Anal. Calc. for C40H38Cl2N2O2P2Ru: C, 59.11; H, 4.68;
N, 3.45. Found: C, 59.14; H, 4.71; N, 3.43%.
2.2.2. [Ru(PPh3)2(H2dpg)Cl2]
This was synthesized by following the same above
procedure and in the same above scale using diphenyl-
glyoxime instead of dimethylglyoxime. [Ru(PPh3)2-
(H2dpg)Cl2] was obtained as a microcrystalline brown
solid. The yield was 71 mg (73%). Anal. Calc. for
C50H42Cl2N2O2P2Ru: C, 64.17; H, 4.49; N, 2.99.
Found: C, 64.14; H, 4.46; N, 2.97%.
2.2.3. [Ru(PPh3)2(Hdmg)2]
Method A: [Ru(PPh3)3Cl2] (100 mg 0.10 mmol) was
taken in ethanol (50 cm3) and dimethylglyoxime (30
mg, 0.26 mmol) was added to it followed by triethy-
lamine (16 mg, 0.16 mmol). The mixture was refluxed
for 1 h. Upon partial evaporation of the solution,
[Ru(PPh3)2(Hdmg)2] started to precipitate as a bright
orange crystalline solid. It was collected by filtration,
washed thoroughly with ethanol and dried in air. Re-
crystallization from dichloromethane–hexane solution
gave [Ru(PPh3)2(Hdmg)2] as a bright orange crystalline
solid. The yield was 56 mg (63%). Anal. Calc. for
C44H44N4O4P2Ru: C, 61.75; H, 5.15; N, 6.55. Found: C,
61.78; H, 5.13; N, 6.57%.
2. Experimental
Method B: to a solution of [Ru(PPh3)2(H2dmg)Cl2]
(100 mg, 0.10 mmol) in dichloromethane (30 cm3) was
added dimethylglyoxime (18 mg, 0.15 mmol) followed
by triethylamine (16 mg, 0.16 mmol). The resulting
solution was stirred for 1 h. The color of the solution
gradually changed from brown to orange. Upon evap-
orating the solvent a bright orange crystalline solid
was obtained, which was thoroughly washed with wa-
ter and dried in vacuo over P4O10. Recrystallization
2.1. Materials
Commercial ruthenium trichloride (Arora Matthey,
Calcutta, India) was converted to RuCl3·3H2O by re-
peated evaporation with concentrated hydrochloric
acid. Triphenylphosphine (PPh3) was purchased from
Loba Chemie, Mumbai, India. Dimethylglyoxime
(H2dmg) and diphenylglyoxime (H2dpg) were obtained
from s.d. fine-chem, Mumbai, India. [Ru(PPh3)3Cl2]
was prepared by following a published procedure [37].
Purification of dichloromethane and preparation of te-
trabutylammonium perchlorate (TBAP) for electro-
chemical work were performed as reported in the
literature [38,39]. All other chemicals and solvents were
reagent grade commercial materials and were used as
received.
from
dichloromethane–hexane
solution
gave
[Ru(PPh3)2(Hdmg)2] as a bright orange crystalline red
solid. The yield was 53 mg (60%).
2.2.4. [Ru(PPh3)2(Hdpg)2]
This was synthesized by following the same above
procedures and in the same above scales. In method A
diphenylglyoxime was used instead of dimethylgly-
oxime and in method B [Ru(PPh3)2(H2dpg)Cl2] and
H2dpg were used instead of [Ru(PPh3)2(H2dmg)Cl2]
and H2dmg, respectively. [Ru(PPh3)2(Hdpg)2] was ob-
tained as a bright-orange crystalline solid. The yield
was 69 mg (60%). Anal. Calc. for C64H52N4O4P2Ru: C,
69.62; H, 4.89; N, 5.08. Found: C, 69.62; H, 4.91; N,
5.06%.
2.2. Preparations
2.2.1. [Ru(PPh3)2(H2dmg)Cl2]
Dichloromethane (50 cm3) was added to a mixture of
[Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) and H2dmg (18
mg, 0.15 mmol). The resulting red solution was stirred
for 1 h. Upon evaporating the solvent, a solid residue

A.K. Das et al. / Polyhedron 20 (2001) 327–335
329
2.2.5. [Ru(PPh3)2(Hdpg)(Hdmg)]
residue was obtained, which was washed thoroughly
with cold water and dried in vacuo over P4O10. Recrys-
Method A: to a solution of [Ru(PPh3)2(H2dpg)Cl2]
(100 mg, 0.10 mmol) in dichloromethane (30 cm3) was
added dimethylglyoxime (18 mg, 0.15 mmol) followed
by triethylamine (16 mg, 0.16 mmol). The resulting
solution was stirred for 1 h. The color of the solution
gradually changed from yellowish–orange to red. Upon
evaporating the solvent a solid residue was obtained,
which was thoroughly washed with water and dried in
vacuo over P4O10. Recrystallization from dichloro-
methane–hexane solution gave [Ru(PPh3)2(Hdpg)-
(Hdmg)] as a crystalline red solid. The yield was 80 mg
(82%). Anal. Calc. for C54H50N4O4P2Ru: C, 66.05; H,
5.09; N, 5.70. Found: C, 66.07; H, 5.05; N, 5.68%.
Method B: the same above procedure and scale were
followed using [Ru(PPh3)2(H2dmg)Cl2] and diphenyl-
glyoxime instead of [Ru(PPh3)2(H2dpg)Cl2] and
tallization
from
dichloromethane–hexane
gave
[Ru(PPh3)2(H2dpg)(pic)]ClO4 as a crystalline red solid.
The yield was 87 mg (75%). Anal. Calc. for
C56H46ClN3O8P2Ru: C, 61.90; H, 4.23; N, 3.87. Found:
C, 61.93; H, 4.22; N, 3.90%.
2.2.7. [Ru(PPh3)2(H2dpg)(q)]ClO4
This was synthesized by following the above proce-
dure and scale using 8-hydroxyquinoline (Hq) instead
of picolinic acid. [Ru(PPh3)2(H2dpg)(q)]ClO4 was ob-
tained as a microcrystalline red solid. The yield was 86
mg (73%). Anal. Calc. for C59H48ClN3O7P2Ru: C,
63.92; H, 4.33; N, 3.79. Found: C, 63.90; H, 4.35; N,
3.78%.
dimethylglyoxime
respectively.
[Ru(PPh3)2(Hdpg)-
2.2.8. [Ru(PPh3)2(H2dpg)(nn)]ClO4
(Hdmg)] was obtained as a crystalline red solid. The
yield was 79 mg (78%).
This was synthesized by following the same proce-
dure and scale used for the synthesis of [Ru(PPh3)2-
(H2dpg)(pic)]ClO4 using 1-nitroso-2-naphthol (Hnn) in-
stead of picolinic acid. [Ru(PPh3)2(H2dpg)(nn)]ClO4
was obtained as a crystalline red solid. The yield was 92
mg (76%). Anal. Calc. for C60H48ClN3O8P2Ru: C,
63.40; H, 4.22; N, 3.70. Found: C, 63.41; H, 4.25; N,
3.68%.
2.2.6. [Ru(PPh3)2(H2dpg)(pic)]ClO4
An ethanolic solution (30 cm3) of picolinic acid (13
mg, 0.10 mmol) was added to
a solution of
[Ru(PPh3)2(H2dpg)Cl2] (100 mg, 0.10 mmol) in
dichloromethane (50 cm3). To it triethylamine (16 mg,
0.16 mmol) was added and the resulting solution was
initially heated gently to expel dichloromethane as
much as possible. The solution was then refluxed for 6
h. The color of the solution changed from yellowish–
orange to red. The solution was allowed to cool to
room temperature (25°C) and a saturated aqueous solu-
tion of sodium perchlorate (0.5 cm3) was added to it.
Upon partial evaporation of the solution, a solid
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a
Perkin–Elmer 240C elemental analyzer. IR spectra
were obtained on a Perkin–Elmer 783 spectrometer
with samples prepared as KBr pellets. Electronic spec-
tra were recorded on a Shimadzu UV 240 spectropho-
tometer. Magnetic susceptibilities were measured using
1
Table 1
Crystallographic data
a PAR 155 Vibrating sample magnetometer. H NMR
spectra were recorded on a Bruker AC-200 NMR spec-
trometer using TMS as the internal standard. Solution
electrical conductivities were measured using a Phillips
PR 9500 bridge with a solute concentration of 10−3 M.
Electrochemical measurements were made using a PAR
model 273 potentiostat. A platinum disc working elec-
trode, a platinum wire auxiliary electrode and an
aqueous saturated calomel reference electrode (SCE)
were used in a three electrode configuration. All electro-
chemical data were collected at 298 K and are uncor-
rected for junction potentials. A RE 0089 X–Y
recorder was used to trace the voltammograms.
[Ru(PPh3)2(H2dpg)Cl2] [Ru(PPh3)2(Hdmg)2]
Empirical formula C50H44N2O3P2Cl2Ru
C
44H44N4O4P2Ru
855.87
orthorhombic, P212121 monoclinic, P21/c
Formula weight
Space group
954.82
,
a (A)
12.026(3)
17.5853(22)
21.194(3)
4482.0(13)
4
8.6568(21)
16.024(3)
13.8305(22)
1884.2(6)
2
,
b (A)
,
c (A)
V (A )
3
,
Z
,
u (A)
0.71073
0.7107
0.10×0.15×0.38
25
Crystal size (mm) 0.50×0.13×0.05
T (°C)
v (cm−1
Rf
25
)
5.727
0.039 a
0.036 b
1.21 c
5.394
0.036 a
2.4. Crystallography
Rw
0.037 b
Goodness-of-fit
1.36 c
[Ru(PPh3)2(H2dpg)Cl2]. Single crystals of [Ru(PPh3)2-
(H2dpg)Cl2] were grown by slow diffusion of hexane
into a dichloromethane solution of the complex. Se-
lected crystal data and data collection parameters are
given in Table 1. Data were collected on an Enraf–
a Rf=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
b Rw=[S w(ꢀFoꢀ−ꢀFcꢀ)2/S w(Fo)2]1/2
.
c Goodness-of-fit=[S w(ꢀFoꢀ−ꢀFcꢀ)2/(M−N)]1/2
, where M is the
number of reflections and N is the number of parameters refined.

330
A.K. Das et al. / Polyhedron 20 (2001) 327–335
Nonius CAD-4 diffractometer using graphite-
,
monochromated Mo Ka radiation (u=0.71073 A).
Three standard reflections measured every 3600 s of
X-ray exposure showed no significant intensity varia-
tion over the course of data collection. X-ray data
reduction, structure solution and refinement were done
using the NRCVAX package [40].
[Ru(PPh3)2(Hdmg)2]. Single crystals of [Ru(PPh3)2-
(Hdmg)2] were grown by slow diffusion of hexane into
a dichloromethane solution of the complex. Selected
crystal data and data collection parameters are given in
Table 1. Data collection and structure solution and
refinement were done as described above.
3. Results and discussion
3.1. Synthesis and characterization
Fig. 1. View of the [Ru(PPh3)2(H2dpg)Cl2] molecule.
3.1.1. [Ru(PPh3)2(H2L)Cl2] complexes
Reaction of the dioxime ligands (H2L) with
[Ru(PPh3)3Cl2] affords, under different experimental
conditions, different products. Simple reaction of the
dioxime ligands with [Ru(PPh3)3Cl2] in 1:1 mole ratio in
dichloromethane solution at ambient temperature yields
complexes of type [Ru(PPh3)2(H2L)Cl2]. It is interesting
to note that the dioxime ligands do not undergo any
proton loss during complexation. Elemental (C, H, N)
analytical data agree well with the proposed composi-
tion of the complexes. These complexes are diamagnetic
which corresponds to the bivalent state of ruthenium
(low-spin d6, S=0) in these complexes. As the dioxime
ligands are symmetric in nature, the [Ru(PPh3)2-
(H2L)Cl2] complexes may exist in three geometrical
isomeric forms (3–5). To find out the stereochemistry,
structure of [Ru(PPh3)2(H2dpg)Cl2] has been deter-
mined by X-ray crystallography.
ably shorter than the known Ru(II)–N lengths where
the N-donor ligand is not involved in p-interaction with
the metal [45–49]. The C–N lengths within the coordi-
nated dioxime ligands are also significantly longer than
localized CꢀN bond [50]. The decrease in Ru–N dis-
tance and increase in C–N distance within the ruthe-
nium–dioxime chelate clearly indicate strong
p-interaction between ruthenium and the diimine frag-
ment of the dioxime ligands. In complexes of rutheniu-
m(II) containing the Ru(PPh3)2 moiety, the PPh3
ligands usually take up mutually cis positions because
of favorable p-interaction [51–53]. In [Ru(PPh3)2-
(H2dpg)Cl2], the H2dpg ligand appears to function as a
better p-acid by virtue of having the diimine fragment
(as also observed in the structural characterization of
[Ru(PPh3)2(H2dpg)Cl2]) and hence forces the bulky
PPh3 ligands to mutually trans positions for less steric
hindrance. As properties of the two [Ru(PPh3)2-
(H2L)Cl2] complexes are similar (vide infra), the
[Ru(PPh3)2(H2dmg)Cl2] complex is assumed to have a
similar structure (3) like [Ru(PPh3)2(H2dpg)Cl2].
Infrared spectra of the [Ru(PPh3)2(H2L)Cl2] com-
plexes show strong vibrations due to the Ru(PPh3)2
fragment near 520, 695 and 740 cm−1 [54,55]. A sharp
band observed near 1030 cm−1 in both the complexes
is assigned to the w(N–O) vibration. The w(O–H)
stretches appear near 3400 cm−1 and the w(Ru–Cl)
The structure is shown in Fig. 1 and selected bond
parameters are listed in Table 2. The diphenylglyoxime
ligand is coordinated to ruthenium as a neutral biden-
tate N,N-donor ligand forming five-membered chelate
ring with a bite angle of 76.7(2)°. The two PPh3 ligands
are mutually trans and the two chlorides are mutually
cis and therefore Ru(PPh3)2(H2dpg)Cl2] has structure 3.
Ruthenium is sitting in a N2P2Cl2 coordination sphere
which is distorted significantly from ideal octahedral
geometry as reflected in the bond parameters around
ruthenium. While the Ru–P and Ru–Cl bond distances
are quite normal [41–44], the Ru–N lengths are notice-
vibrations are observed at 310–335 cm−1
. The
[Ru(PPh3)2(H2L)Cl2] complexes are moderately soluble
in common organic solvents like dichloromethane,
1
chloroform, acetone, etc. producing red solutions. H
NMR spectra of these complexes have been recorded in
CDCl3 solution. An isolated signal, observed near 10.5
ppm in both the complexes, is assigned to the oxime

A.K. Das et al. / Polyhedron 20 (2001) 327–335
331
Table 2
Selected bond lengths (A) and bond angles (°) for [Ru(PPh3)2(H2dpg)Cl2] and [Ru(PPh3)2(Hdmg)2]
,
[Ru(PPh3)2(H2dpg)Cl2]
[Ru(PPh3)2(Hdmg)2]
Ru–Cl(1)
Ru–Cl(2)
Ru–P(1)
Ru–P(2)
Ru–N(1)
Ru–N(2)
C(2)–C(9)
2.442(2)
O(1)–N(1)
O(2)–N(2)
N(1)–C(1)
N(2)–C(2)
C(1)–C(2)
C(1)–C(3)
1.376(8)
1.379(8)
1.321(10)
1.322(10)
1.45(1)
Ru–P
2.417(1)
2.030(3)
2.002(3)
1.329(5)
1.387(5)
1.15(5)
O(1)–Ha
1.68(5)
2.461(2)
2.387(2)
2.410(2)
1.973(5)
1.962(6)
1.48(1)
Ru–N(1)
Ru–N(2)
O(1)–N(1)
O(2)–N(2)
O(2)–H
N(1)–C(2)
N(2)–C(3)
C(2)–C(3)
C(1)–C(2)
C(3)–C(4)
1.317(6)
1.290(6)
1.466(7)
1.484(7)
1.488(7)
1.47(1)
P(1)–Ru–P(2)
Cl(1)–Ru–N(1)
Cl(2)–Ru–N(2)
173.1(8)
167.3(2)
166.6(2)
N(1)–Ru–N(2)
Cl(1)–Ru–Cl(2)
76.7(2)
102.79(8)
P–Ru–Pa
N(1)–Ru–N(1a)
179.9
180.0
N(2)–Ru–N(2a)
N(1)–Ru–N(2)
180.0
77.1(1)
O–H proton. The aromatic protons are observed
within 6.1–7.6 ppm. The methyl signal of the coordi-
nated dimethylglyoxime ligand in [Ru(PPh3)2-
(H2dmg)Cl2] is observed as a sharp resonance at 1.92
ppm. Electronic spectra of the [Ru(PPh3)2(H2L)Cl2]
complexes have been recorded in dichloromethane solu-
tion. Spectral data are presented in Table 3. Each
complex shows several intense absorptions in the visible
and ultraviolet region. The absorptions in the ultravio-
let region are assignable to transitions involving ligand
orbitals. Three absorptions are displayed by both the
[Ru(PPh3)2(H2L)Cl2] complexes in the visible region, of
which the lowest energy one is much weaker in
intensity1 and is assigned to the d–d (1A1“1T1) transi-
tion. The other probable d–d (1A1“1T2) transition
could not be identified due to intense absorptions at
higher energies. The other two intense absorptions in
the visible region are probably due to allowed metal-to-
ligand charge-transfer transitions. Multiple charge-
transfer transitions in such mixed-ligand complexes
may result from lower symmetry splitting of the metal
level, the presence of different acceptor orbitals and
from the mixing of singlet and triplet configurations in
the excited state through spin–orbit coupling [56–59].
To have an insight into the nature of the observed
charge-transfer transitions, qualitative EHMO calcula-
tions have been performed [60,61] on a model of the
[Ru(PPh3)2(H2L)Cl2] complexes replacing the methyl/
phenyl groups of the dioxime ligands and the phenyl
groups of triphenylphosphines by hydrogen. Partial
MO diagram is shown in Fig. 2. The highest occupied
molecular orbital (HOMO) and the next two occupied
orbitals (HOMO-1 and HOMO-2) have major contri-
butions from the metal t2 orbitals. Hence these three
filled orbitals may be attributed to the metal t2 orbitals.
The lowest unoccupied molecular orbital (LUMO) and
the next unoccupied orbital (LUMO+1) are relatively
closely spaced and these are localized almost completely
on the diimine part of the dioxime ligand. Hence these
two vacant orbitals may be assumed to be imine p*-or-
bital of the dioxime ligands. The two charge-transfer
transitions observed in the [Ru(PPh3)2(H2L)Cl2] com-
plexes may therefore be assigned to transitions occur-
ring from the filled metal t2 levels to the
p*-(imine)orbitals of the dioxime ligands.
Table 3
Electronic spectral and cyclic voltammetric data
Compounds
Electronic spectral data a
Cyclic
umax (nm) (m, M−1 cm−1
)
valtammetric
data a,b
E1/2 (V)
(DEp, mV)
[Ru(PPh3)2(H2dmg)Cl2] 222 (44 900), 247 (44 200), 0.84(80),
300 c(9100), 360(9200),
440 c(1500)
1.31(140)
[Ru(PPh3)2(H2dpg)Cl2] 232 (40 100), 278 (39 600), 0.98(80),
312 c(6900), 406(10 200),
467 c(1100)
1.38(110)
[Ru(PPh3)2(Hdmg)2]
[Ru(PPh3)2(Hdpg)2]
235(34 800), 260(37 400),
300 c(5700), 375(7400),
440 c(1000)
0.36(80),
0.94(120)
240(44 400), 270 c(35 400), 0.52(70),
315 c(11 800), 406(7300),
1.03(120)
438 c(5000)
[Ru(PPh3)2(Hdmg)-
(Hdpg)]
240(40 200), 254(37 600),
324 c(9400), 390(9400),
440 c(2600)
0.46(70),
1.00(100)
[Ru(PPh3)2(H2dpg)-
(pic)]ClO4
[Ru(PPh3)2(H2dpg)-
(q)]ClO4
[Ru(PPh3)2(H2dpg)-
(nn)]ClO4
224(82 800), 256(81 400),
0.66(70),
328 c(18 100), 392(18 100) 1.60(250)
224(56 600), 252 c(44 900), 0.60(70),
292(34 200), 364(11 500)
224(84 100), 252(71 000),
308 c(22 700), 400(5500),
490(22 200)
1.50(240)
0.68(80),
1.42(320)
a In dichloromethane solution.
b Supporting electrolyte, TBAP; reference electrode, SCE; E1/2
=
0.5(Epa+Epc), where Epa and Epc are anodic and cathodic peak
potentials respectively; DEp=Epa−Epc; scan rate, 50 mV s−1
.
1 After deducting the contribution of the next intense absorption.
c Shoulder.

332
A.K. Das et al. / Polyhedron 20 (2001) 327–335
Fig. 2. Partial MO diagram of [Ru(PPh3)2(H2L)Cl2].
fragment are quite interesting. The two sets of Ru–N,
3.1.2. [Ru(PPh3)2(HL)2] complexes
C–N and N–O distances within the Ru(Hdmg) chelate
are all very different. All these features indicate that
deprotonation of one oxime function has made the
Hdmg ligand unsymmetrical and the diimine character
of it is also reduced to a great extent. The other two
bis-dioximato complexes are assumed to have similar
structure as [Ru(PPh3)2(Hdmg)2] as all three bis-dioxi-
mato complexes display similar spectroscopic and elec-
tron-transfer properties (vide infra).
Reaction of two moles of the dioxime ligands with
one mole of [Ru(PPh3)3Cl2] proceeds smoothly in
refluxing ethanol in the presence of a base to afford
bis-dioximato complexes of type [Ru(PPh3)2(HL)2] in
decent yields. The bis-dioximato complexes can also be
synthesized from [Ru(PPh3)2(H2L)Cl2] by reacting them
with the respective dioxime ligand in presence of a base.
A
mixed-bis-dioximato complex, viz. [Ru(PPh3)2-
(Hdpg)(Hdmg)], has been prepared by reacting either
[Ru(PPh3)2(H2dpg)Cl2] or [Ru(PPh3)2(H2dmg)Cl2] with
H2dmg or H2dpg respectively in the presence of a base.
Composition of the bis-dioximato complexes has been
verified by their microanalytical data. All the three bis
dioximato complexes are diamagnetic, which corre-
sponds to the +2 state of ruthenium in these com-
plexes. The structure of [Ru(PPh3)2(Hdmg)2] has been
determined by X-ray crystallography. The structure is
shown in Fig. 3 and relevant bond distances and angles
are presented in Table 2. The dioxime ligands are
coordinated to ruthenium, via loss of one oxime pro-
ton, as monoanionic bidentate N,N-donor ligands
forming five-membered chelate ring with a bite angle of
77.1(1)°. The two Hdmg ligands which share the equa-
torial plane, are hydrogen bonded in the usual in-
tramolecular fashion. The two PPh3 ligands occupy
mutually trans positions, the two oxime-nitrogens and
the two oximato-nitrogens are also mutually trans.
From the bond parameters around ruthenium, the
RuN4P2 core appears to be much less distorted from
ideal
octahedral
geometry
relative
to
the
[Ru(PPh3)2(H2L)Cl2] complexes. While the Ru–P dis-
tance is normal, bond distances within the Ru(Hdmg)
Fig. 3. View of the [Ru(PPh3)2(Hdmg)2] molecule.

A.K. Das et al. / Polyhedron 20 (2001) 327–335
333
Infrared spectra of the bis-dioximato complexes show
strong absorptions due to the Ru(PPh3)2 fragment near
520, 700 and 750 cm−1 as before. A strong band
displayed near 1225 cm−1 by all these complexes is
assigned to the w(N–O) stretch. The bis dioximato
complexes are soluble in common organic solvents like
dichloromethane, chloroform, etc., producing orange
of the [Ru(PPh3)2(H2dpg)Cl2] complex with the two
PPh3 ligands in mutually trans positions and the two
chlorides replaced by L%.
Infrared spectra of the [Ru(PPh3)2(H2dpg)(L%)]ClO4
complexes are mostly similar to the spectrum of
[Ru(PPh3)2(H2dpg)Cl2], which is obviously due to pres-
ence of the common Ru(PPh3)2(H2dpg) fragment. Some
new vibrations are of course observed in the
[Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes, of which the
two vibrations uniformly displayed by all the
[Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes near 1100 and
620 cm−1, are due to the presence of perchlorate ion.
The [Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes are soluble
1
solutions. H NMR spectra, recorded in CDCl3 solu-
tion, show the aromatic proton signals within 7.0–8.0
ppm. An isolated resonance observed near 10.0 ppm is
assigned to the oxime O–H signal. The methyl signal of
the dimethylglyoximate ligand in [Ru(PPh3)2(Hdmg)2] is
observed at 2.19 ppm. Electronic spectra of these com-
plexes have been recorded in dichloromethane solution.
Spectral data are listed in Table 3. Intense absorptions
are observed in both visible and ultraviolet regions. The
absorptions in the ultraviolet region are believed to be
occurring within the ligand orbitals. Qualitative EHMO
calculations on the bis-dioximato complexes show simi-
lar results as obtained in the [Ru(PPh3)2(H2L)Cl2] com-
plexes. The top three filled orbitals having predomi-
nantly ruthenium t2 character and the first two vacant
orbitals having ligand (HL) p*-character. Hence the
absorptions in the visible region are assigned to metal
(t2)-to-ligand (p* of HL) charge-transfer transitions.
in
polar
organic
solvents
like
acetonitrile,
dichloromethane, etc. Conductivity studies in acetoni-
trile solution show that these complexes behave as 1:1
1
electrolytes (\M=1140–155 V−1 cm2 M−1). H NMR
spectra of the [Ru(PPh3)2(H2dpg)(L%)]+ complexes
recorded in CDCl3 solution, show the oxime OH reso-
nance near 10.4 ppm. The aromatic protons are ob-
served within 6.0–8.0 ppm as overlapping signals.
However, intensity measurements correspond nicely to
the total number of aromatic protons in the respective
complexes. Electronic spectra of the [Ru(PPh3)2-
(H2dpg)(L%)]+ complexes has been recorded in
dichloromethane solution (Table 3). Each complex
shows intense absorptions in the visible region probably
due to the allowed metal-to-ligand charge-transfer tran-
sitions. Intense absorptions are also observed in the
ultraviolet region as before.
3.1.3. The [Ru(PPh3)2(H2L)(L%)]ClO4 complexes
Complexes of ruthenium(II) having a cis-RuCl2 frag-
ment have always been of particular interest with refer-
ence to their possible reactivities arising from the
dissociation of the Ru–Cl bonds. The coordinated
chlorides are found to be displaceable by chelating
bidentate ligands under relatively mild condition. Such
reactivities have already been utilized in the preparation
of the bis-dioximate complexes (vide supra). Though
both of the [Ru(PPh3)2(H2L)Cl2] complexes display sim-
ilar reactivities with regard to displacement of the chlo-
rides by bidentate chelating ligands, only the reactions
of [Ru(PPh3)2(H2dpg)Cl2] are reported here. A group of
three acidic ligands, viz., picolinic acid(Hpic), quinolin-
8-ol(Hq) and 1-nitroso-2-napthol(Hnn), (abbreviated in
general as HL%, where H stands for the acidic hydrogen)
have been used for these reactions. All these ligands are
known to coordinate ruthenium, via loss of the acidic
proton, as bidentate N,O-donors forming five-mem-
bered chelate rings [9,62,63]. Reaction of these ligands
with [Ru(PPh3)3Cl2] proceeds smoothly in stirring
dichloromethane in the presence of a base to afford
complexes of type [Ru(PPh3)2(H2dpg)(L%)]+ which have
been isolated as perchlorate salts in the solid state.
Elemental (C, H, N) analytical data of these complexes
agree well with their compositions. The [Ru(PPh3)2-
(H2dpg)(L%)]ClO4 complexes are diamagnetic, which in-
dicates that ruthenium is in its +2 oxidation state in
these complexes. The [Ru(PPh3)2(H2dpg)(L%)]+ com-
plexes are assumed to have a geometry similar to that
3.2. Cyclic 6oltammetric studies
Electrochemical properties of all the complexes have
been studied by cyclic voltammetry in dichloromethane
solution (0.1 M TBAP). Voltammetric data are pre-
sented in Table 3 and selected voltammograms are
shown in Fig. 4. Each complex shows two one-electron
oxidative oxidative responses on the positive side of
SCE. The first response is assigned to ruthenium(II)–
ruthenium(III) oxidation and the second to rutheniu-
m(III)–ruthenium(IV) oxidation. One-electron nature
of both the responses has been established by compar-
ing current height of each response with that of fer-
rocene–ferrocenium couple under identical ex-
perimental conditions. The ruthenium(II)–rutheniu-
m(III) oxidation is reversible, characterized by a peak-
to-peak separation of 70–80 mV and the anodic-
peak-current (ipa) is almost equal to the cathodic-peak-
current (ipc). The ruthenium(III)–ruthenium(IV) oxida-
tion is quasi-reversible. It is interesting to note here that
the ruthenium(III)–ruthenium(IV) oxidation potential
is rather close to the ruthenium(II)–ruthenium(III) oxi-
dation potential than is usually observed. This closeness
indicates that this second electron-transfer reaction is
probably associated with proton-transfer from the ox-

334
A.K. Das et al. / Polyhedron 20 (2001) 327–335
4. Conclusions
The present study shows that reaction of dioxime
ligands with [Ru(PPh3)3Cl2] affords stable complexes of
ruthenium(II) where the dioxime ligands bind to ruthe-
nium(II) either in the neutral dioxime form or in the
monoanionic dioximate form. While the [Ru(PPh3)2-
(H2L)Cl2] complexes have been found to be useful start-
ing material for the synthesis of complexes of type
[Ru(PPh3)2(H2L)(L%)]+, the bis-dioximato complexes
appear to be suitable as ‘bridging ligand’ (via disso-
ciation of the remaining two oxime protons), in the
synthesis of trinuclear complexes of type 6.
Such possibilities are currently under exploration.
5. Supplementary material
Fig. 4. Cyclic voltammograms of (a) [Ru(PPh3)2(H2dmg)Cl2]; (b)
[Ru(PPh3)2(Hdmg)2]; and (c) [Ru(PPh3)2(H2dpg)(nn)]ClO4 in
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 148069 and CCDC no.
148070. Copies of this information may be obtained
from The Director, CCDC, 12 Union Road, Cam-
bridge, CB2 1EZ, UK (fax: +44-1233-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.
cam.ac.uk).
dichloromethane solution (0.1 M TBAP) at a scan rate of 50 mV s−1
.
ime function. Potential of both the oxidation is found
to be quite sensitive to the composition of the com-
plexes. In the [Ru(PPh3)2(H2L)Cl2] complexes, the
dioxime ligands behave as efficient diimine ligands with
strong p-acid character and therefore stabilizes the
bivalent state of ruthenium to a great extent. This is
reflected in the relatively high magnitude of rutheniu-
m(II)–ruthenium(III) oxidation potential in these com-
plexes. In the bis dioximato complexes the diimine
character of the dioxime ligand is reduced because of
deprotonation of one oxime function which results in
the destabilization of the bivalent state of ruthenium,
and this is reflected in the negative shift of the rutheni-
um(II)–ruthenium(III) oxidation potential in these
complexes by more than 400 mV relative to that in the
[Ru(PPh3)2(H2L)Cl2] complexes. The ruthenium(II)–
ruthenium(III) oxidation potential in [Ru(PPh3)2-
(H2dpg)(L%)]+ complexes is observed to be lower than
that in [Ru(PPh3)2(H2L)Cl2]. This shows that replace-
ment of the chloride ligands by these N,O-donor lig-
ands has stabilized the trivalent state of ruthenium and
this may be primarily attributed to coordination of
ruthenium by the hard oxygen (phenolate or carboxy-
late) center of the N–O ligands. The ruthenium(III)–
ruthenium(IV) oxidation potential follows a similar
trend as the ruthenium(II)–ruthenium(III) oxidation
potential.
Acknowledgements
Financial assistance received from the Department of
Science and Technology, New Delhi (grant no. SP/S1/
F33/98) is gratefully acknowledged. Thanks are also
due to the Third World Academy of Sciences for finan-
cial support enabling the purchase of an electrochemi-
cal cell system. The authors thank Professor Ramgopal
Bhattacharyya of Jadavpur University and Dr Sree-
brata Goswami of the Indian Association for the Culti-
vation of Science for their help.
References
[1] S.I. Gorelsky, E.S. Dodsworth, A.B.P. Lever, A.A. Vlcek, Co-
ord. Chem. Rev. 174 (1998) 469.
[2] S.-M. Lee, W.-T. Wong, Coord. Chem. Rev. 164 (1997) 415.
[3] V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni,
Chem. Rev. 96 (1996) 759.
[4] J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C.
Coudret, V. Balzani, F. Barigelletti, L. De Cola, L. Flamigni,
Chem. Rev. 94 (1994) 993.

A.K. Das et al. / Polyhedron 20 (2001) 327–335
335
[33] M.M. Taqui Khan, G. Ramachandraiah, S.H. Mehta, J. Mol.
Catal. 50 (1989) 123.
[5] A.B.P. Lever, H. Masui, R.A. Metcalfe, D.J. Stufkens, E.S.
Dodsworth, P.R. Auburn, Coord. Chem. Rev. 125 (1993) 317.
[6] A. Anillo, C. Bario, S.G. Granda and R.O. Rosete, J. Chem.
Soc., Dalton Trans. (1993) 1125 and refs. cited therein.
[7] H. Masui, A.B.P. Lever, E.S. Dodsworth, Inorg. Chem. 32
(1993) 258 and refs. cited therein.
[8] A.K. Das, S.M. Peng, S. Bhattacharya, J. Chem. Soc., Dalton
Trans. (2000) 181.
[9] A.K. Das, A. Rueda, L.R. Falvalo, S.M. Peng, S. Bhattacharya,
Inorg. Chem. 38 (1999) 4365.
[10] V.I. Ovcharenko, S.V. Fokin, V.A. Reznikov, V.V. Ikorskii,
G.V. Romanenko, R.Z. Sagdeev, Inorg. Chem. 37 (1998) 2104.
[11] A. Chakravorty, Coord. Chem. Rev. 13 (1974) 1.
[12] D. Burdinski, F. Birkelbach, T. Weyhermuller, U. Florke, H.-J.
Hampt, M. Lengen, A.X. Trautwein, E. Bill, K. Wieghardt, P.
Chaudhuri, Inorg. Chem. 37 (1998) 1009.
[13] D. Xu, J. Gu, L. Xu, K. Liang, Y. Xu, Polyhedron 17 (1998)
231.
[14] F. Birkelbach, T. Weyhermuller, M. Lengen, M. Gerdan, A.X.
Trautwein, K. Wieghardt, P. Chaudhuri, J. Chem. Soc., Dalton
Trans. (1997) 4529.
[34] E.O. Schlemper, Acta Crystallogr., Sect. B 33 (1977) 2482.
[35] T.W. Thomas, A.E. Underhill, Chem. Soc. Rev. 1 (1972) 99.
[36] K.G. Caulton, F.A. Cotton, J. Am. Chem. Soc. 93 (1971) 1941.
[37] T.A. Stephenson, G. Wilkinson, J. Inorg. Nucl. Chem. 28 (1966)
945.
[38] D.T. Sawyer, J.L. Roberts, Jr., Experimental Electrochemistry
for Chemists, Wiley, New York, 1974, pp. 167–215.
[39] M. Walter, L. Ramaley, Anal. Chem. 45 (1973) 165.
[40] E.J. Gabe, Y. Le. Page, J.P. Charland, F.L. Lee, P.S. White, J.
Appl. Crystallogr. 22 (1989) 384.
[41] M. Menon, A. Pramanik, S. Chattopadhyay, N. Bag, A.
Chakravorty, Inorg. Chem. 34 (1995) 1361.
[42] G.A. Fox, S. Bhattacharya, C.G. Pierpont, Inorg. Chem. 30
(1991) 2895.
[43] M.C. Barral, R.J. Aparicio, E.C. Royer, M.J. Saucedo, F.A.
Urbanos, E.G. Puebla, C.R. Valero, J. Chem. Soc., Dalton
Trans. (1991) 1609.
[44] S. Bhattacharya, C.G. Pierpont, Inorg. Chem. 30 (1991) 1511.
[45] K.N. Mitra, P. Majumder, S.M. Peng, A. Castineiras, S.
Goswami, Chem. Commun. (1997) 1267.
[15] R. Ruiz, M. Julve, J. Faus, F. Lioret, M.C. Munoz, Y. Jour-
naux, C. Bois, Inorg. Chem. 36 (1997) 3434.
[16] A. Bakac, M.E. Brynildson, J.H. Espenson, Inorg. Chem. 25
(1986) 4108.
[46] R.C. Elder, M. Trkula, Inorg. Chem. 16 (1977) 1048.
[47] F.C. March, G. Ferguson, Can. J. Chem. 49 (1971) 3590.
[48] L.H. Vogt, Jr., J.L. Katz, S.E. Wiberley, Inorg. Chem. 4 (1965)
1157.
[17] F. Pomposo, D.V. Stynes, Inorg. Chem. 22 (1983) 569.
[18] D.V. Stynes, I. Pang, Inorg. Chem. 16 (1977) 590.
[19] J.E. Caton, C.V. Banks, Talanta 13 (1966) 967.
[20] G.N. Schrauzer, Chem. Ber. 95 (1962) 1438.
[21] L.F. Szczepura, J.G. Muller, C.A. Bessel, R.F. Sec, T.S. Janik,
M.R. Churchill, K.J. Takeuchi, Inorg. Chem. 31 (1992) 859.
[22] J.G. Muller, K.J. Takeuchi, Inorg. Chem. 29 (1990) 2185.
[23] J.G. Muller, K.J. Takeuchi, J.J. Grzybowski, Polyhedron 8
(1989) 1391.
[24] T. Fukuchi, E. Miki, K. Mizumachi, T. Isimori, Chem. Lett.
(1987) 1133.
[25] J.G. Muller, J.J. Grzybowski, K.J. Takeuchi, Inorg. Chem. 25
(1986) 2665.
[49] R.J. Sundberg, R.F. Bryan, I.F. Taylor, Jr., H. Taube, J. Am.
Chem. Soc. 96 (1974) 381.
[50] D. Chattopadhya, S.K. Majumdar, T. Banerjee, S. Ghosh, Acta.
Crystallogr., Sect. C 44 (1988) 1025.
[51] F. Basuli, M. Ruf, C.G. Pierpont, S. Bhattacharya, Inorg. Chem.
37 (1998) 6113.
[52] F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 36 (1997)
5645.
[53] M. Menon, A. Pramanik, N. Bag, A. Chakravorty, J. Chem.
Soc., Dalton Trans. (1995) 1543.
[54] N.C. Pramanik, S. Bhattacharya, Transition Met. Chem. 24
(1999) 95.
[55] J. Chakravarty, S. Bhattacharya, Polyhedron 13 (1994) 2671.
[56] B.J. Pankuch, D.E. Lacky, G.A. Crosby, Phys. Chem. 84 (1980)
2061.
[26] A.R. Middleton, J.R. Thornback, G. Wilkinson, J. Chem. Soc.,
Dalton Trans. (1980) 174.
[27] N.B. Pahor, R. Dreos-Garlatti, S. Geremia, L. Randaccio, G.
Tauzher, E. Zangrando, Inorg. Chem. 29 (1990) 3437.
[28] S.M. Polson, L. Hansen, L.G. Marzilli, Inorg. Chem. 36 (1997)
307.
[57] S. Decurtis, F. Felix, J. Ferguson, H.U. Gudel, A. Ludi, J. Am.
Chem. Soc. 102 (1980) 4102.
[58] A. Ceulemans, L.G. Vanquickenborne, J. Am. Chem. Soc. 103
(1981) 2238.
[29] N.B. Pahor, M. Forcolin, L.G. Marzilli, L. Randaccio, M.F.
Summers, P.J. Toscano, Coord. Chem. Rev. 63 (1985) 1.
[30] K.A. Lance, K.A. Goldsby, D.H. Busch, Inorg. Chem. 29 (1990)
4537.
[31] M. Kijma, K. Miyamori, T. Nakamura, T. Sato, Bull. Chem.
Soc. Jpn. 63 (1990) 2549.
[59] E.M. Kober, T.J. Meyer, Inorg. Chem. 21 (1982) 3967.
[60] C. Mealli, D.M. Proserpio, CACAO version 4.0, Firenze, Italy,
July 1994.
[61] C. Mealli, D.M. Proserpio, J. Chem. Educ. 67 (1990) 399.
[62] A. Das, F. Basuli, S.M. Peng, S. Bhattacharya, Polyhedron 18
(1999) 2729.
[32] M.M. Taqui Khan, G. Ramachandraiah, S.H. Mehta, S.H.R.
Abdi, S. Kumar, J. Mol. Catal. 58 (1990) 199.
[63] M. Menon, A. Pramanik, N. Bag, A. Chakravorty, J. Chem.
Soc., Dalton Trans. (1995) 1417.
.