Paramagnetic ruthenium(III) complexes bearing O,O chelating ligands: Synthesis, spectra, molecular structure and electron transfer properties

Article abstract of DOI:10.1016/j.poly.2011.09.019

Paramagnetic Ru(III) complexes of the type [RuX₂(EPh ₃)₂(L)] (where X = Cl or Br; E = P or As; L = monobasic bidentate benzophenone ligand) have been synthesized from the reaction of ruthenium(III) precursors, viz. [RuX₃(EPh₃)₃] (where X = Cl, E = P; X = Cl or Br, E = As) or [RuBr₃(PPh ₃)₂(CH₃OH)] and substituted hydroxy benzophenones in a 1:1 molar ratio in benzene under reflux for 6 h. The hydroxy benzophenone ligands behave as monoanionic bidentate O,O donors and coordinate to ruthenium through the phenolate oxygen and ketonic oxygen atoms, generating a six-membered chelate ring. The compositions of the complexes have been established by analytical and spectral (FT-IR, UV-Vis, EPR) and X-ray crystallography methods. The single crystal structure of the complex [RuCl ₂(PPh₃)₂(L₁)] (1) has been determined by X-ray crystallography and indicates the presence of a distorted octahedral geometry in these complexes. The magnetic moment values of the complexes are in the range 1.75-1.89 μ_B, which reveals the presence of one unpaired electron in the metal ion. EPR spectra of liquid samples at liquid nitrogen temperature (LNT) show a rhombic distortion (g _x ≠ g_y ≠ g_z) around the ruthenium ion. The complexes are redox active and display quasi-reversible oxidation and quasi-reversible reduction waves versus Ag/AgCl.

Full text of DOI:10.1016/j.poly.2011.09.019

Polyhedron 31 (2012) 196–201
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Polyhedron
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Paramagnetic ruthenium(III) complexes bearing O,O chelating ligands:
Synthesis, spectra, molecular structure and electron transfer properties
b
Nandhagopal Raja a, Rengan Ramesh a, , Yu Liu
⇑
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India
b Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2011
Accepted 11 September 2011
Available online 29 September 2011
Paramagnetic Ru(III) complexes of the type [RuX2(EPh3)2(L)] (where X = Cl or Br; E = P or As; L = monoba-
sic bidentate benzophenone ligand) have been synthesized from the reaction of ruthenium(III) precur-
sors, viz. [RuX3(EPh3)3] (where X = Cl, E = P; X = Cl or Br, E = As) or [RuBr3(PPh3)2(CH3OH)] and
substituted hydroxy benzophenones in a 1:1 molar ratio in benzene under reflux for 6 h. The hydroxy
benzophenone ligands behave as monoanionic bidentate O,O donors and coordinate to ruthenium
through the phenolate oxygen and ketonic oxygen atoms, generating a six-membered chelate ring. The
compositions of the complexes have been established by analytical and spectral (FT-IR, UV–Vis, EPR)
and X-ray crystallography methods. The single crystal structure of the complex [RuCl2(PPh3)2(L1)] (1)
has been determined by X-ray crystallography and indicates the presence of a distorted octahedral geom-
Keywords:
Hydroxy benzophenone
Ru(III) complex
Crystal structure
EPR
etry in these complexes. The magnetic moment values of the complexes are in the range 1.75–1.89 lB
,
Redox property
which reveals the presence of one unpaired electron in the metal ion. EPR spectra of liquid samples at
liquid nitrogen temperature (LNT) show a rhombic distortion (gx – gy – gz) around the ruthenium ion.
The complexes are redox active and display quasi-reversible oxidation and quasi-reversible reduction
waves versus Ag/AgCl.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
nium(III) tetradentate Schiff base complexes have been reported as
catalyst for transfer hydrogenation of ketones in the presence of iso-
Ruthenium is well known for its ability to cover a wide range of
oxidation states (ꢀ2 to +8) and to adopt various coordination
geometries. Coordination complexes of ruthenium are well known
for their versatile electron-transfer and energy-transfer properties
[1–3]. The coordination chemistry of trivalent ruthenium(III) has
been developed tremendously in recent years, yet the correspond-
ing paramagnetic ruthenium(III) organometallic species are rare
[4]. Ruthenium complexes have been recently gaining importance
in a broad range of applications, and consequently there is a con-
tinuous endeavor to synthesize new complexes of ruthenium with
different types of ligands, such as Schiff base [5], azo [6], pincer [7],
tripodal [8] and carbenes [9,10]. In particular, emphasis has been
placed on ligands containing a phenolate oxygen, which is a recog-
nized hard donor, and hence coordination by a phenolate oxygen is
of importance with regard to stabilization of the higher oxidation
states of ruthenium [11–14].
propanol/KOH [18]. A series of air stable low spin Ru(III) monobasic
bidentate Schiff base complexes has been reported as a catalyst for
the oxidation alcohols in the presence of N-methylmorpholine-N-
oxide as a co-oxidant [19]. Polymer Ru(III) complexes with chemi-
cally modified Schiff base ligands have been reported as catalysts
for selective epoxidation of olefins in presence of tert-butyl hydro-
peroxide as an oxidant[20]. Further, the presence of ruthenium–hal-
ogen bonds in several ruthenium complexes exhibiting anticancer
activitysuggeststhatthese bondsmayalsoplaysomeimportantrole
[21]. Recently, in vitro and in vivo biological activity screening of
ruthenium(III) complexes involving benzylaminopurine derivatives
havebeen studied[22]. In addition, a polymer-anchored Ru(III) com-
plex has been reported as a catalyst for the oxidation of benzyl alco-
hol [23]. Paramagnetic cyclometallated ruthenium(III) complexes
containing phenolic Schiff bases and acid hydrazone ligands
(H2apahR) have been reported [24,25]. A mixed-ligand ruthe-
nium(III) complex containing a sugar based ligand has been used
asa catalystfortheepoxidationof alkenesusing tert-butylhydroper-
oxide as a terminal oxidant [26]. Similarly, mononuclear ruthe-
nium(III) Schiff base complexes have been used as catalysts for the
transfer hydrogenation of ketones in the presence of isopropanol
and KOH at 82 °C [27]. Recently, mononuclear ruthenium(III) com-
plexes bearing benzoquinoline have been reported along with their
Metal complexes of ruthenium containingtriphenylphosphine or
arsine, oxygen and nitrogen donor ligands are found to be effective
catalysts for organic transformation reactions [15–17]. Ruthe-
⇑
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.019

N. Raja et al. / Polyhedron 31 (2012) 196–201
197
catalytic activity for the aerobic oxidative dehydrogenation of ben-
zyl amines [28].
[32], [RuBr3(AsPh3)3] [33] and [RuBr3(PPh3)2(CH3OH)] [34] were
prepared according to the literature procedures.
In view of the catalytic and biological applications, we have de-
scribed here the synthesis of eight mononuclear ruthenium(III)
complexes containing hydroxy benzophenones obtained by the
Fries rearrangement of benzophenone esters. The molecular and
electronic structure of the trivalent ruthenium complexes are
probed with the help of X-ray crystallography, in combination with
elemental analysis, susceptibility measurement, FT-IR, UV–Vis and
EPR spectroscopic methods. Further, the electron transfer proper-
ties of complexes were examined by cyclic voltammetry.
2.3. Preparation of the hydroxy benzophenone ligands
Hydroxy benzophenone was prepared from the benzoylation of
4-substituted phenol followed by the Fries rearrangement reaction
(Scheme 1). Benzoyl chloride (0.075 mol) was added to a solution
of 4-substituted phenol (0.050 mol) in aqueous sodium hydroxide
and was shaken vigorously for 20 min until a white precipitate was
obtained. The solid benzoyl ester product was filtered, washed
with water and dried in a vacuum. The crude product was recrys-
tallized from the rectified spirit to get white crystals. Then anhy-
drous aluminum chloride (56.0 mol) was heated in an oil bath at
140 °C for 5 min and the benzoate ester (8.1 mol) was slowly
added to it with constant stirring. The temperature was allowed
to rise up to 170–180 °C and the temperature was maintained for
20 min. After cooling, the reaction mixture was added to 200 cm3
of 2 N HCl. The insoluble material that formed was filtered, washed
with water and dried. The crude product was recrystallized from
hot ethanol to give pale green or light yellow crystals.
2. Experimental
2.1. Reagents and materials
All the reagents used were chemically pure and were of analyt-
ical reagent grade. The solvents were dried and distilled before use
following the standard procedures [29]. RuCl3ꢁ3H2O was purchased
from Loba-Chemie Pvt. Ltd., and was used without further purifica-
tion. 4-Chloro-3-methyl phenol, 4-methoxy phenol and benzoyl
chloride were obtained from Aldrich. The supporting electrolyte,
tetrabutyl ammonium perchlorate, (TBAP) was dried in a vacuum
prior to use.
2.3.1. 5-Chloro-4-methyl-2-hydroxy benzophenone (HL1)
HL1 (R1 = CH3, R2 = Cl): yellow; yield: 60%; mp: 140–142 °C; IR
(KBr) m
/cmꢀ1; 3064 (OH), 1625 (C@O); 1H NMR (400 MHz, CDCl3);
d/ppm: 11.93 (s, 1H, OH), 7.48–7.59 (m, 6H, Ar–H), 6.97 (s, 1H, Ar–
H), 2.40 (s, 3H, CH3).
2.2. Physical measurements
2.3.2. 4-Methoxy-2-hydroxy benzophenone (HL2)
The carbon and hydrogen microanalysis content of each sample
was determined at STIC, Cochin University of Science and Technol-
ogy, Cochin, India by using an analytic function testing Vario EL III
CHNS elemental analyzer. Infrared spectra were collected using
KBr pellets on a Jasco 400 Plus, FT-IR spectrophotometer in the
range 4000–400 cmꢀ1. Electronic spectra of the complexes in ace-
tonitrile were recorded on a Cary 300 Bio UV–Vis Varian spectro-
photometer. X-ray data were collected by a Bruker smart APEX II
CCD X-ray diffractometer. The room temperature magnetic suscep-
tibilities were measured on an EG and G model 155 vibrating sam-
ple magnetometer at SAIF, IIT, Chennai, and diamagnetic
corrections were calculated from Pascal’s constants [30]. EPR spec-
tra were recorded in the X-band frequency on a JEOL JES-FA200
EPR spectrometer with a microwave power of 1 mW and a modu-
lation amplitude of 160.00. The EPR spectra at RT and LNT were
simulated using Simfonia programs. NMR spectra were recorded
on a Bruker Avance-III 400 MHz NMR spectrometer. Electrochemi-
cal studies were performed using a Princeton EG and G-PARC
model potentiostat in 0.001 M acetonitrile solutions of [(n-
C4H9)4N]ClO4 (TBAP) as the supporting electrolyte under a nitrogen
atmosphere. A three electrode cell was employed with a glassy car-
bon working electrode, a platinum wire counter electrode and an
Ag/AgCl reference electrode. Melting points were recorded with a
Boetius micro-heating table and are uncorrected. The precursor
ruthenium(III) complexes [RuCl3(PPh3)3] [31], [RuCl3(AsPh3)3]
HL2 (R1 = OCH3, R2 = H): yellow; yield: 60%; mp: 150–160 °C, IR
(KBr) m
/cmꢀ1; 3064 (OH), 1634 (C@O); 1H NMR (400 MHz, CDCl3);
d/ppm: 12.7 (s, 1H, OH), 7.49–7.66 (m, 6H, Ar–H), 6.54 (s, 1H, Ar–
H), 3.88 (s, 3H, OCH3).
2.4. Synthesis of the ruthenium(III) hydroxy benzophenone complexes
All the reactions were performed under strictly anhydrous condi-
tions and the complexes were prepared by the following general
procedure. To
a
benzene (20 cm3) solution of [RuX3(EPh3)3]
(0.124–0.157 g, 0.125 mmol) (E = P, X = Cl; E = As; X = Cl or Br) or
[RuBr3(PPh3)2(CH3OH)] (0.112 g, 0.125 mmol) was added the appro-
priate benzophenone (0.022–0.031 g, 0.125 mmol) (HL1–HL2). The
solution was allowed to heat under reflux for 8 h. The color of the
reaction mixture gradually deepened and the resulting solution
was concentrated to 3 ml and cooled. Light petroleum ether (60–
80 °C) (5 cm3) was then added, whereupon the complex separated
out. The thus obtained solid product was recrystallized from a
CH2Cl2/light petroleum ether mixture and dried in a vacuum. The
purity of the complexes was checked by TLC (yield: 65–78%).
2.5. X-ray crystallography
Single crystals of [RuCl2(PPh3)2(L1)] (1) were grown by slow
evaporation from a methanol solution at room temperature. A
OH
O
O
Shaken 20 min
NaOH
1800C
O
C O
Cl
AlCl3
Fries
R1
OH
R2
R2
R2
rearrangement
R1
R1
HL1 (R1= CH3, R2 =Cl)
HL2 (R1=OCH3, R2 =H )
Scheme 1. Preparation of o-hydroxy benzophenone ligands.

198
N. Raja et al. / Polyhedron 31 (2012) 196–201
[RuX3(EPh3)3]
EPh3
Ru
X
X
Benzene
Reflux / 8h
O
O
O
or
[RuBr3(PPh3)2(CH3OH)]
R2
OH
R2
EPh3
R1
R1
(E = P, X = Cl; E = As; X = Cl or Br)
HL1 (R1= CH3, R2 =Cl)
HL2 (R1= H, R2 = OCH3)
Scheme 2. Synthesis of Ru(III) benzophenone complexes.
Table 2
IR and electronic spectral data of the ruthenium(III) benzophenone complexes.
single crystal of suitable size was covered with Paratone oil,
mounted on the top of a glass fiber, and transferred to a Stoe IPDS
Complex
m(C@O) (cmꢀ1
)
m(C–O) (cmꢀ1
)
kmax
616a(979), 406b(2117),
(e )
) (dm3 molꢀ1 cmꢀ1
diffractometer using monochromated Mo
K
a
radiation
1
1604
1394
(k = 0.71073 Å). Corrections were made for Lorentz and polariza-
tion effects, as well as for absorption (numerical). The structure
was solved with the direct method using SIR-97 [35] and was re-
fined by the full matrix least-squares method [36] on F2 with SHEL-
XL-97. All hydrogen atoms were geometrically fixed and allowed to
refine using a riding model.
360c(21590), 281d(45068)
607a(1978), 410b(3928),
370c(33890), 284d(45440)
592a(758), 405b(1990),
373c(24565), 285d(46594)
587a(658), 408b(1490),
378c(26868), 286d(48868)
623a(842), 413b(1358),
369c(20640), 286d(48180)
606a(2849), 411b(3078),
371c(14556), 285d(43422)
583a(860), 403b(4098),
370c(42490), 285d(49177)
616a(2967), 403b(4098),
366c(14500), 280d(45522)
2
3
4
5
6
7
8
1595
1600
1602
1606
1605
1600
1597
1400
1400
1401
1390
1398
1400
1399
3. Results and discussion
The reactions of [RuX3(EPh3)3] (E = P, X = Cl; E = As; X = Cl or Br)
or [RuBr3(PPh3)2(CH3OH)] with the hydroxy benzophenone ligands
(HL1–HL2) in a 1:1 molar ratio in dry benzene afforded new hexa-
coordinated low-spin ruthenium(III) benzophenone complexes
(Scheme 2). The proposed molecular formulae for all the com-
plexes are in good agreement with the stoichiometry concluded
from their analytical data (Table 1). In all these reactions, the ben-
zophenone ligands behave as monobasic bidentate ligands, replac-
ing one triphenylphosphine or triphenylarsine molecule, one
chloride or bromide and one methanol ligand from the precursors.
All the complexes are air stable, non-hygroscopic in nature, insol-
uble in water and highly soluble in common solvents such as
dichloromethane, chloroform, acetonitrile and dimethylsulfoxide,
producing intense dark colored solutions.
a
b
c
d–d transition.
Charge transfer (LMCT) transition.
n–p⁄
.
p– .
p⁄
d
benzophenone to the metal through the ketonic oxygen atom is ex-
pected to reduce the electron density in the ketonic group
frequency. The band due to the ketonic oxygen at around 1606–
1595 cmꢀ1 shows a modest decrease in the stretching frequency
for the complexes, and being shifted to lower frequencies indicates
the coordination of the ketonic oxygen. Also, there is an upward
shift in the stretching frequency of phenolic oxygen of the com-
plexes, to the range 1390–1400 cmꢀ1, and this shows that the
other coordination is through the phenolate oxygen atom. This fact
3.1. IR spectra
The IR bands for the metal complexes derived from the hydroxy
benzophenone ligands are most useful in attempting to determine
the mode of coordination, and are listed in Table 2. The IR spectra
of the free benzophenone ligands show a strong band in the re-
gions 1640–1659 and 1335–1347 cmꢀ1, corresponding to the ke-
tonic group and phenolic C–O, respectively. Coordination of the
is further supported by the disappearance of the (cOH) band in the
range 3415–3434 cmꢀ1 for all the complexes, indicating the depro-
tonation of the phenolic proton prior to coordination. Bands in the
510–550 cmꢀ1 region are ascribed to the formation of M–O bonds.
Further, the ruthenium(III) benzophenone complexes show strong
vibrations near 520, 695, 740, 1445 and 1532 cmꢀ1, which are
attributable to the triphenylphosphine or triphenylarsine groups
[37,38].
Table 1
Analytical data of the ruthenium(III) benzophenone complexes.
Complex
Colour
Mp (°C)a
Calculated (found) %
3.2. Electronic spectra
C
H
[RuCl2(PPh3)2(L1)] (1)
[RuCl2(AsPh3)2(L1)] (2)
[RuBr2(AsPh3)2(L1)] (3)
[RuBr2(PPh3)2(L1)] (4)
[RuCl2(PPh3)2(L2)] (5)
[RuCl2(AsPh3)2(L2)] (6)
[RuBr2(AsPh3)2(L2)] (7)
[RuBr2(PPh3)2(L2)] (8)
Violet
Brown
Green
Green
Brown
Green
Green
Green
169
245
160
228
220
250
199
196
63.74(63.69)
58.30(58.36)
53.67(58.59)
58.24(58.20)
65.01(65.06)
59.36(59.35)
54.57(54.57)
59.30(59.34)
4.28(4.19)
3.91(3.98)
3.60(3.58)
3.91(3.86)
4.47(4.49)
4.08(4.09)
3.75(3.78)
4.08(4.11)
The electronic spectra of all the complexes have been recorded
in acetonitrile solution in the range 800–200 nm, and the spectral
data are displayed in Table 2. The ground state of ruthenium(III) in
2
5
an octahedral environment is T2g, arising from the t2g configura-
tion, and the first excited doublet levels in the order of increasing
2
2
4
1
energy are A2g and T1g, arising from the t2g eg configuration.
Hence, two bands corresponding to 2T2g ? 2A2g and 2T2g ? 2T1g
are possible. All the complexes show four intense absorptions in
a
Decomposition temperature.

N. Raja et al. / Polyhedron 31 (2012) 196–201
199
the region 633–242 nm. The intense absorptions in the region of
the metal center at O(1) and O(2), forming a six-membered chelate
ring with a bite angle of 86.29(1)° (O(1)–Ru–O(2)) and bond
lengths of 2.106(3) Å for Ru(1)–O(1) and 2.064(3) Å for Ru(1)–
O(2). The Ru(1)–Cl(2) and Ru(1)–Cl(3) bond distances are
616–583 nm are attributed to d–d transitions
(e = 658–
2987 dm3 molꢀ1 cmꢀ1) and the absorptions observed in the region
403–413 nm are probably due to charge transfer transitions
(LMCT) taking place from the filled ligand (HOMO) orbital to the
singly-occupied ruthenium t2 orbital (LUMO). The absorptions in
the ultraviolet region below 380 nm are very similar and are attrib-
utable to the transitions within the ligand orbitals (n–p⁄ p⁄),
, p–
taking place in the hydroxyl benzophenone ligands. The pattern
of the electronic spectra of all the complexes indicates the presence
of an octahedral environment around the ruthenium(III) ion simi-
lar to that of other ruthenium(III) octahedral complexes [39–41].
3.3. X-ray structure
To understand the coordination mode of the hydroxy benzophe-
none ligands and the stereochemistry of ruthenium(III) complexes,
the molecular structure of one of the complexes, [RuCl2(PPh3)2(L1)]
(1), has been determined using the single crystal X-ray diffraction
method, and the ORTEP view of this complex is shown in Fig. 1. The
summary of the single crystal data and refinement are shown in S1
and selected bond angles and bond lengths are given in S2. The hy-
droxy benzophenone ligand acts in a bidendate fashion, binding
Fig. 2a. EPR spectrum of complex 1 at RT (A, experimental and B, simulated).
Fig. 1. The ORTEP diagram of the complex [Ru(Cl)(PPh3)2(L1)] (1). For reasons of
clarity, hydrogen atoms have been omitted.
Table 3
EPR spectral and magnetic susceptibility data of the ruthenium(III) benzophenone
complexes.
Complex
gx
gy
gz
hgi⁄
leff (BM)
1
2
3
4
5
6
7
8
2.37
2.48
2.55
2.49
2.36
2.36
2.45
2.54
2.37
2.48
2.55
2.49
2.36
2.36
2.45
2.54
1.98
1.99
1.96
1.88
1.91
1.84
2.00
1.99
2.24
2.32
2.37
2.30
2.22
2.19
2.31
2.37
1.89
1.86
1.77
1.78
1.79
1.75
1.83
1.79
1=2
hgiꢂ ¼ ½1=3gx2 þ 1=3gy2 þ 1=3g2z ꢃ
.
Fig. 2b. EPR spectrum of complex 1 at LNT (A, experimental and B, simulated).

200
N. Raja et al. / Polyhedron 31 (2012) 196–201
Table 4
The solid state EPR spectra of all the complexes were recorded
at X-band frequencies at room temperature. The ‘g’ values are gi-
ven in Table 3 and a representative spectrum is shown in Fig. 2a.
The low spin d5 configuration is a good probe of molecular struc-
ture and bonding since the observed ‘g’ values are very sensitive
to small changes in the structure and metal-ligand covalency.
The EPR spectra of all the complexes exhibit an anisotropic spec-
trum with g\ around 2.36–2.55 and g|| around 1.84–2.00. For an
octahedral field with tetragonal distortion gx = gy – gz, and hence
two ‘g’ values indicate an axial symmetry for these complexes
and hence trans positions are assigned to the triphenylphosphine
groups. However, the EPR spectrum of complex 1 (Fig. 2b) recorded
in dichloromethane solution at 77 K shows a rhombic spectrum
with three distinct ‘g’ values (gx – gy – gz; gx = 2.44, gy = 2.25,
gz = 1.98) in decreasing order of magnitude. Of these three major
resonances, two are lower and one is higher than the field
(ꢄ3200 G). The observed rhombicity of the EPR spectrum is under-
standable in terms of the gross molecular symmetry of these com-
plexes containing non-equivalent axes. The presence of rhombic
distortion is apparent in the splitting of the perpendicular reso-
nance into two spaced components (gx and gy). Further, the rhomb-
icity of the spectrum reflects the asymmetry of electronic
environment around ruthenium in these complexes [44]. Further,
we have simulated the EPR spectra of both powder (Fig. 2a) and li-
quid (Fig. 2b) samples. It has been observed that both the experi-
mental and the simulated ‘g’ values are very close to each other.
Hence, the results from the EPR spectral analysis indicate that
these ruthenium(III) hydroxy benzophenone complexes are signif-
icantly distorted from an ideal octahedral geometry, as observed in
the crystal structure of complex 1.
Electrochemical behavior of the ruthenium(III) benzophenone complexes.a
Complex RuIII/RuIV
RuIII/RuII
b
c
b
c
Epa
Epc
D
EP
E1/2
Epc
Epa
D
EP
E1/2
(V)
(mV)
(V)
(mV)
1
2
3
4
5
6
7
8
0.42 0.33
0.37 0.28
0.39 0.29 100
0.49 0.36 130
0.46 0.35 110
0.45 0.36
0.41 0.31 100
0.44 0.31 130
90
90
0.59
0.51
0.54
0.67
0.64
0.63
0.57
0.60
ꢀ0.29 ꢀ0.19 100
ꢀ0.38 0.28 120
90
ꢀ0.32
ꢀ0.45
ꢀ0.35
ꢀ0.39
ꢀ0.34
ꢀ0.38
ꢀ0.33
ꢀ0.36
ꢀ0.29 ꢀ0.20
ꢀ0.34 ꢀ0.22 120
ꢀ0.31 ꢀ0.18 130
ꢀ0.33 ꢀ0.21 120
90
ꢀ0.28 ꢀ0.19
90
ꢀ0.32 ꢀ0.20 120
a
Supporting electrolyte: NBu4ClO4 (0.005 M); complex: 0.001 M; solvent:
acetonitrile.
b
D
Ep = Epa ꢀ Epc where Epa and Epc are anodic and cathodic potentials,
respectively.
c
E1/2 = 0.5(Epa + Epc); scan rate: 100 mV sꢀ1
.
2.429(4) and 2.423(2) Å, respectively. The coordinated benzophe-
none ligand and two chlorine ions constitute one equatorial plane
with the metal at the center, where each chlorine ion is trans to the
coordinated ketonic and phenolate oxygen atoms. The two PPh3 li-
gands have taken up the remaining two axial positions and hence
they are mutually trans; the Ru(1)–P(1) (2.3972(10) Å) and Ru(1)–
P(2) (2.3969(10) Å) bond distances are virtually equal. The Ru–O,
Ru–Cl and Ru–P bond distances are comparable with other re-
ported ruthenium(III) complexes [42,43]. Ruthenium is therefore
sitting in a 2O2Cl2P coordination environment which is distorted
octahedral in nature as reflected in all the bond parameters around
the ruthenium center. We believe that there is no change in the
coordination sphere around ruthenium when we change the ligand
from HL1 to HL2 by simple substitutions. As all the complexes dis-
play similar spectral properties, the other seven complexes are as-
sumed to have similar structures to that of complex 1.
3.5. Electron transfer property
The electron transfer properties of all the complexes were stud-
ied in acetonitrile solution by cyclic voltammetry under a N2 atmo-
sphere using a glassy-carbon working electrode, and the redox
potentials are expressed with reference to Ag/AgCl. All the com-
plexes (1 ꢅ 10ꢀ3 M) are electro active with respect to the metal
centers and exhibit two redox processes in the potential range
+1.5 to ꢀ1.5 V (0.05 M tetrabutyl ammonium perchlorate as a sup-
porting electrolyte at 298 K). The cyclic voltammograms of all the
complexes display quasi-reversible oxidation (RuIV/RuIII) and qua-
si-reversible reduction (RuIII/RuII) peaks at the scan rate of
100 mV sꢀ1. The potentials are summarized in Table 4 and a repre-
sentative voltammogram is shown in Fig. 3. All the complexes
showed well-defined waves with E1/2 lying at 0.51–0.67 V for oxi-
dation (RuIV/RuIII) and ꢀ0.32 to ꢀ0.45 V for reduction (RuIII/RuII).
3.4. Magnetic moment and EPR spectra
The room temperature (300 K) magnetic moment of the mono-
nuclear ruthenium(III) complexes (1–8) in the powder phase are in
the range 1.75–1.89 lB, and are listed in Table 3. These values are
consistent with the low spin character and S = 1/2 ground state of
the metal ion, which correspond to one unpaired electron (low spin
RuIII, t2g5) in an octahedral environment. In a low-spin d5 system,
2
the T2g (octahedral) ground state is orbitally degenerate and will
be affected by spin–orbit coupling and low-symmetry ligand-field
effects, both of which will remove this degeneracy.
Fig. 3. Cyclic voltammogram of complex 1; supporting electrolyte: [Bu4N](ClO4) (0.05 M); solvent = acetonitrile; scan rate: 100 mV sꢀ1
.

N. Raja et al. / Polyhedron 31 (2012) 196–201
201
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½RuIIIX2ðEPh3Þ2ðLÞꢃ ꢀ ½RuIVX2ðEPh3Þ2ðLÞꢃ þ eꢀ
½RuIIIX2ðEPh3Þ2ðLÞꢃ þ eꢀ ꢀ ½RuIIX2ðEPh3Þ2ðLÞꢃ
ð1Þ
ð2Þ
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4. Conclusions
ˇ
ˇ
ˇ
ˇ
´
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Acknowledgements
One of the authors (N.R.) thanks the University Grants Commis-
sion (UGC), New Delhi for the award of the UGC-SAP RFSMS Schol-
arship. We thank DST-FIST for X-ray and NMR data and UGC-SAP
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CCDC 807530 contains the supplementary crystallographic data
for this paper. These data can be obtained free of 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: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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