Synthesis, structure and catalytic activity of cycloruthenated carbonyl complexes containing arylazo phenolate ligands

Article abstract of DOI:10.1016/j.molcata.2006.10.015

A simple and convenient synthesis of a new class of air-stable mononuclear cyclometallated ruthenium(II) carbonyl 2-(arylazo)phenolate complexes bearing triphenylarsine [Ru(ap-R)(AsPh₃)₂(CO)] has been described. The 2-(arylazo)phenolate ligands behave as dianionic tridentate ligands and are coordinated to ruthenium through C, N and O by dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring forming two five-membered chelate rings. These complexes have been characterized by elemental analysis, and FT-IR, ¹H NMR and UV-vis spectroscopies. In dichloromethane solution all the metal complexes exhibit characteristic metal-to-ligand charge transfer (MLCT) absorption and emission bands in the visible region. One of the complexes [Ru(ap-Cl)(AsPh₃)₂(CO)] was successfully characterized by X-ray crystallography. Cyclic voltammetric data of all the complexes show Ru^III/Ru^II oxidation and Ru^II/Ru^I reduction within the range of +0.75 to +0.86 V and -0.50 to -0.57 V respect to Ag/AgCl, respectively. The potentials are observed with respect to the electronic nature of substituents (R) in the 2-(arylazo)phenolate ligands. Further, a complex (2) was tested as a new catalyst in the oxidation of primary and secondary alcohols in the presence of NMO as a more viable oxidant with moderate to high conversion. The formation of high valent Ru^IV = O species as a catalytic intermediate is proposed for the catalytic process.

Full text of DOI:10.1016/j.molcata.2006.10.015

Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
Synthesis, structure and catalytic activity of cycloruthenated carbonyl
complexes containing arylazo phenolate ligands
K. Naresh Kumara, R. Ramesha,∗, Yu Liub
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India
b Institute of Inorganic Chemistry, University of Zu¨rich, Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland
Received 12 May 2006; received in revised form 29 September 2006; accepted 10 October 2006
Available online 14 October 2006
Abstract
A simple and convenient synthesis of a new class of air-stable mononuclear cyclometallated ruthenium(II) carbonyl 2-(arylazo)phenolate
complexes bearing triphenylarsine [Ru(ap-R)(AsPh3)2(CO)] has been described. The 2-(arylazo)phenolate ligands behave as dianionic tridentate
ligands and are coordinated to ruthenium through C, N and O by dissociation of the phenolic proton and the phenyl proton at the ortho position of
the phenyl ring forming two five-membered chelate rings. These complexes have been characterized by elemental analysis, and FT-IR, 1H NMR
and UV–vis spectroscopies. In dichloromethane solution all the metal complexes exhibit characteristic metal-to-ligand charge transfer (MLCT)
absorption and emission bands in the visible region. One of the complexes [Ru(ap-Cl)(AsPh3)2(CO)] was successfully characterized by X-ray
crystallography. Cyclic voltammetric data of all the complexes show RuIII/RuII oxidation and RuII/RuI reduction within the range of +0.75 to
+0.86 V and −0.50 to −0.57 V respect to Ag/AgCl, respectively. The potentials are observed with respect to the electronic nature of substituents
(R) in the 2-(arylazo)phenolate ligands. Further, a complex (2) was tested as a new catalyst in the oxidation of primary and secondary alcohols
in the presence of NMO as a more viable oxidant with moderate to high conversion. The formation of high valent RuIV = O species as a catalytic
intermediate is proposed for the catalytic process.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Cyclometalation; Ruthenium(II) C, N, O donor; Characterization; Catalytic oxidation
1. Introduction
methodologies have been developed with them [10–13]. How-
ever, stoichiometric chemical oxidants were needed in most
cases. From an environmental viewpoint, it is quite important
to develop a mild and less toxic oxidation reaction systems.
Numerous ruthenium complexes can selectively catalyze a large
variety of oxidative transformations including the generation of
dioxygen from water, the dihydroxylation of olefins, the oxida-
tive dehydrogenation of alcohols to carbonyl compounds and
the asymmetric epoxidation of alkenes, thus opening the way
to the conversions of petroleum products to valuable oxygenate
chemicals as well as, potentially, to the extraction of energy
from renewable sources [14]. In early studies Oshima and co-
workers reported that primary alcohols are selectively oxidized
to aldehydes in the presence of secondary ones by using stoi-
chiometric Ru(PPh3)3Cl2 or a Ru(PPh3)3Cl2 and TMSOOTMS
system [15]. Ishii and co-workers successfully achieved aerobic
oxidation using Ru(PPh3)3Cl2 as the catalyst in the presence
of hydroquinone without diminishing chemoselectivity [16].
Several ruthenium catalytic systems have been reported with
a wide range of oxidants viz., tert-butyl hydroperoxide [17],
The cyclometallation reaction, i.e., the intramolecular activa-
tion of aromatic C–H bonds of coordinated ligands by transition
metals has been widely investigated in view of their growing
chemistry [1–6]. Transition metal complexes that contain an
azo ligand or its derivatives are well known and there are sev-
eral coordination modes documented [7]. Formation of a metal
carbon ␴ bond is important research targets in organometallic
chemistry and the application of organometallic compounds to
organic transformation has been of considerable interest [8]. The
oxidation of primary and secondary alcohols to corresponding
aldehydes and ketones is of tremendous importance in organic
synthesis and many transition metal compounds have been used
for this purpose [9]. It is to be noted that ruthenium compounds
are intensively studied transition metal compounds and many
∗
Corresponding author. Fax: +91 431 2407045/2407020.
E-mail address: ramesh bdu@yahoo.com (R. Ramesh).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.10.015

K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
219
ton EG and G-PARC model potentiostat using a glassy carbon
working electrode, Pt wire as counter electrode and all the poten-
tials were referenced to the Ag/AgCl electrode. Melting points
were recorded with a Boetius micro-heating table and are uncor-
rected. The catalytic yields were determined using a HP 6890
series GC-FID with a DP-5 column of 30 m length, 0.32 mm
diameter and 0.25 ␮m film thickness.
2.2. Materials
Fig. 1. Structure of 2-(arylazo)phenol ligands, where R = H: H2ap-H; Cl: H2ap-
Cl; Me: H2ap-Me; OMe: H2ap-OMe; OEt: H2ap-OEt.
All the reagents used were chemically pure or analytical
reagent grade. Solvents were purified and dried according to
standard procedures. RuCl3·3H2O was purchased from Loba
Chemie Pvt. Ltd., Bombay, India and was used without further
purification. The alcohols and their corresponding ketones or
aldehydes, which were used in the catalytic studies, were pur-
chased from Merck and Aldrich.
chloramine-T [18], benzoquinone [19], hydrogen peroxide [20],
molecular oxygen [21], iodosylbenzene [22], NaIO4 [23] and N-
methylmorpholine-N-oxide (NMO) [18,24]. Further, the –N–N–
and –O–N donors of the ligands play an important role in the
catalytic reactions. In addition, large number of transition metal
phosphine/arsine complexes have been used in catalysis due to
their characteristic steric and electronic properties [25]. In con-
trast to the considerable growth of literature on the chemistry of
orthometallated complexes, to the best of our knowledge there
are no reports available for catalytic oxidation of alcohols by
cyclometallated ruthenium(II) 2-(arylazo)phenolate complexes
incorporating C–, N– and O– donors and triphenylarsine in the
presence of NMO.
2.3. Synthetic methods
The ruthenium(II) precursor complex [RuHCl(CO)
(AsPh3)3], was prepared according to literature method [26].
The 2-(arylazo)phenol ligands were prepared by coupling
diazotized aniline and p-substituted anilines with p-cresol [27].
2.3.1. Synthesis of ruthenium(II) carbonyl
2-(arylazo)phenolate complexes
In view of the rich interest to understand these simple and
inexpensive C, N, O donor ligands towards ruthenium, we have
synthesized and characterized series of orthometallated ruthe-
nium(II) carbonyl complexes containing 2-(arylazo)phenol lig-
ands and triphenylarsine. The molecular and electronic structure
of the bivalent complexes are probed with the help of X-ray crys-
Synthesis of ruthenium(II) carbonyl 2-(arylazo)phenolate
complexes was carried out under strictly anhydrous con-
ditions. The ruthenium(II) complex [RuH(Cl)(CO)(AsPh3)3]
[0.16 mmol; 0.150 g] and 2-(arylazo)phenol ligands (H2ap-
R) (where R = –H, –Cl, –Me, –OMe, –OEt) (0.16 mmol;
0.033–0.041 g) were taken in 20 ml benzene. The mixture was
heated under reflux for 5 h until a deep green solution was
obtained and the reactions were monitored by thin layer chro-
matography. The solvent was then evaporated under reduced
pressure, which was subjected to purification by silica gel col-
umn chromatography. On elution with chloroform the sepa-
rated green band was collected. The ruthenium complexes were
obtained as crystalline solid of yield ∼70% upon complete evap-
oration of the solvent.
1
tal structure in combination with FT-IR, H NMR, electronic
spectroscopy and emission properties. The relative stabilities of
oxidation and reduction states are monitored electrochemically.
Further, the synthesized complex effectively catalyze towards
oxidation of primary and secondary alcohols in the presence of
NMO as co-oxidant. The following 2-(arylazo)phenol ligands
(Fig. 1) were used to synthesis the mononuclear ruthenium(II)
carbonyl complexes.
2. Experimental
2.3.1.1. [Ru(ap-H)(AsPh3)2(CO)] (1). Anal. Calcd. for
C50H40N2O2As2Ru: C, 63.09; H, 4.23; N, 2.94; found: C,
62.87; H. 4.06; N, 2.83. FT-IR (KBr disc, cm−1): 1934, 1382,
1299. 1H NMR (400 MHz, CDCl3, δ, ppm): 6.75–8.15 (m,
Ar-H), 1.24 (s, 3H, CH3). UV λmax (nm) (ε (M−1 cm−1)): 545
(1328), 402 (5572), 323 (11,632), 269 (14,772). Emission λmax
(nm): 464.
2.1. Physical methods
The analysis of carbon, hydrogen and nitrogen were per-
formed in a Carlo-Erba 1106-model 240 Perkin-Elmer analyzer
atSophisticatedTestandInstrumentationCentre(STIC), Cochin
University, Kochi, India. FT-IR spectra were recorded in KBr
pellets with a JASCO 400 plus spectrophotometer. A CARY 300
Bio UV-visible Varian spectrophotometer was used to record the
electronic spectra. 1H NMR spectra were recorded on a Bruker
400 MHz instrument using tetramethylsilane (TMS) as an inter-
nal reference. Emission intensity measurements were carried
out by using a Jasco FP-6500 spectrofluorimeter with 5 nm exit
slit. Electrochemical measurements were made using a Prince-
2.3.1.2. [Ru(ap-Cl)(AsPh3)2(CO)] (2). Anal. Calcd. for
C50H39N2O2ClAs2Ru: C, 60.89; H, 3.98; N, 2.83; found: C,
60.73; H, 3.78; N, 2.62. FT-IR (KBr disc, cm−1): 1940, 1380,
1310. 1H NMR (400 MHz, CDCl3, δ, ppm): 6.90–8.61 (m,
Ar-H), 1.18 (s, 3H, CH3). UV λmax (nm) (ε (M−1 cm−1)): 541
(1464), 404 (9180), 328 (14,836), 274 (17,682). Emission λmax
(nm): 502.

220
K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
2.3.1.3. [Ru(ap-Me)(AsPh3)2(CO)] (3). Anal. Calcd. for
C51H42N2O2As2Ru: C, 63.42; H, 4.38; N, 2.89; found: C,
63.13; H, 4.17; N, 2.65. FT-IR (KBr disc, cm−1): 1937, 1375,
1285. 1H NMR (400 MHz, CDCl3, δ, ppm): 6.85–8.15 (m,
Ar-H), 2.24 (s, 3H, CH3), 1.30 (s, 3H, CH3). UV λmax (nm)
(ε (M−1 cm−1)): 549 (1960), 400 (4920), 335 (11,965), 272
(14,944). Emission λmax (nm): 440.
2.3.1.4. [Ru(ap-OMe)(AsPh3)2(CO)] (4). Anal. Calcd. for
C51H42N2O3As2Ru: C, 62.38; H, 4.31; N, 2.85; found: C, 62.17;
H, 4.16; N, 2.73. FT-IR (KBr disc, cm−1): 1937, 1389, 1315. 1H
NMR (400 MHz, CDCl3, δ, ppm): 7.2–8.1 (m, Ar-H), 1.34 (s,
3H, CH3), 3.95 (s, 3H, OCH3). UV λmax (nm) (ε (M−1 cm−1)):
544 (1924), 399 (6040), 336 (12,980), 275 (15,392). Emission
λmax (nm): 482.
Scheme 1. Structure of cyclometallated ruthenium(II) complexes, where R = H;
ap-H (1), R = Cl; ap-Cl (2), R = Me; ap-Me (3), R = OMe; ap-OMe (4), R = OEt;
ap-OEt (5).
3. Results and discussion
3.1. Synthesis
2.3.1.5. [Ru(ap-OEt)(AsPh3)2(CO)] (5). Anal. Calcd. for
C52H44N2O3As2Ru: C, 62.71; H, 4.45; N, 2.81; found: C, 62.59;
H, 4.29; N, 2.73. FT-IR (KBr disc, cm−1): 1945, 1373, 1311. 1H
NMR (400 MHz, CDCl3, δ, ppm): 6.75–7.85 (m, Ar-H), 1.20
(s, 3H, CH3), 4.01 (q, 2H, OCH2), 1.45 (t, 3H, CH3). UV λmax
(nm) (ε (M−1 cm−1)): 537 (1328), 395 (10,668), 331 (11,704),
263 (14,832). Emission λmax (nm): 496.
Organoruthenium complexes of the general formula
[Ru(ap-R)(AsPh3)2(CO)] (where ap-R = CNO donors of 2-
(arylazo)phenolate ligands) were synthesized by the reaction of
[RuHCl(CO)(AsPh3)3] precursor with equal molar of respec-
tive 2-(arylazo)phenolate ligands under reflux in dry benzene.
The 2-(arylazo)phenolate ligands were observed to undergoing
C–H activation at one ortho position of the phenyl ring in the
arylazo fragment leading to the formation of five-membered
cyclometallated complexes (Scheme 1). It has been observed
that arylazo phenolate ligands replace one hydride, one chloride
and one AsPh3 from ruthenium(II) precursor and the oxidation
state of ruthenium remains unchanged during the formation of
orthometallated species. All the complexes are green in colour
and found to be air stable in both the solid and liquid states
at room temperature, and non-hygroscopic in nature. The syn-
thesized ruthenium(II) carbonyl complexes are highly soluble in
common solvents such as chloroform, dichloromethane, toluene
and benzene producing intense green solutions. Elemental anal-
yses(C, HandN)wereconsistentwiththecompositionproposed
for all complexes.
2.4. X-ray crystallography
Single crystal suitable for X-ray diffraction was grown from
mixture of chloroform/acetonitrile solution at room tempera-
ture. The single crystals of suitable size were covered with
Paratone oil, mounted on the top of a glass fiber, and trans-
ferred to a Stoe IPDS diffractometer using mono-chromated
˚
Mo K␣ radiation (λ = 0.71073 A). Data were collected at 183 K.
Corrections were made for Lorentz and polarization effects as
well as for absorption (numerical). Structures were solved by
a direct method using SHELXS-97 for Sir 97 and were refined
by the full-matrix least-squares method on F2 with SHELXL-97
[28]. Non-hydrogen atoms were refined with anisotropy thermal
parameters. All hydrogen atoms were geometrically fixed and
allowed to refine using a riding model.
3.2. FT-IR spectroscopy
2.5. Catalytic oxidation procedure
The IR spectra of all the ligands exhibit significant bands
around 1420–1435 and 1268–1278 cm−1 which are tentatively
Catalytic oxidation of primary alcohols to correspond-
ing aldehydes and secondary alcohols to ketones by ruthe-
nium(II) carbonyl complex was studied in the presence of
NMO as co-oxidant. A typical reaction using the complex
[Ru(ap-Cl)(AsPh3)2(CO)] (2) as a catalyst and primary or
secondary alcohol as substrates at a 1:100 molar ratio was
conducted as follows. A solution of ruthenium complex (2)
(0.01 mmol) in 20 cm3 CH2Cl2 was added to the solution of
substrate (1 mmol) and NMO (3 mmol). The solution mixture
was refluxed for 3 h and the solvent was then evaporated from
the mother liquor under reduced pressure. The residue was
then extracted with pet. ether (60–80 ◦C) (20 cm3) and was
analyzed by GC. The oxidation products are commercially avail-
able, and were identified by GC co-injection with authentic
samples.
assigned to ν
On complexation ν
and phenolic νC–O stretching, respectively.
–N N–
–N N–
appears at 1373–1389 cm−1 and the
red shift corroborates the N (azo) coordination. In the spectra
of all the complexes the coordination through phenolic oxy-
gen is confirmed by the increase in C–O at higher frequencies
in the region 1285–1315 cm−1. The positive shift is further
supported by the disappearance of the νOH band in the range
3430–3454 cm−1 in all the complexes. The spectra of each com-
plex shows a strong band in the region 1934–1945 cm−1 due to
the coordinated νC O groups. Further, these complexes show a
strong band near 530, 695, 740 and 1557 cm−1 indicating the
presence of coordinated AsPh3 ligands. Besides, all the com-
plexes show strong bands corresponding to νM–N and νM–O in
the region 440–464 and 510–545 cm−1, respectively [29–31].

K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
221
visible region at 549–537 nm and the absorption at 395–404 nm
are probably due to metal-to-ligand charge transfer transitions.
The charge transfer transitions taking place from the highest
filled ruthenium t2 orbital (highest occupied molecular orbital)
to the vacant ␲*(–N N–) orbital of the 2-(arylazo)phenolate
ligand (lowest unoccupied molecular orbital) or to the higher
energy vacant orbitals of other fragment of ligands. It has been
observed that multiple charge transfer transition in such mixed
ligand complexes may result from the splitting of d orbitals
of the metal and thus lower symmetry. Therefore, these may
be a possibility of mixing of singlet and triplet configurations
in the excited state through spin-orbit coupling [33]. In addi-
tion, the other high intensity bands in the 334–263 nm regions
were characterized by ligand-centered (LC) transitions taking
place in the three coordinated 2-(arylazo)phenolate ligand. The
absorption in the ultraviolet region may be attributed to usual
Fig. 2. 1H NMR spectrum of complex (4).
3.3. NMR spectroscopy
The 1H NMR spectra of the [Ru(ap-R)(AsPh3)2(CO)] com-
plexes have been recorded in CDCl3 solution (Fig. 2). All
the complexes show multiplets at δ 6.75–8.60 ppm due to the
coordinated AsPh3 ligands. Among the expected aromatic pro-
ton signals for the coordinated ap-R ligand, most have been
clearly observed while a few could not be detected due to their
overlap with their AsPh3 signals. The distinct methyl reso-
nance is displayed near δ 1.18–1.34 ppm for p-cresol fragment
of 2-(arylazo)phenolate ligands. Additional methyl signals are
observed as singlet for complex 3 at δ 2.24 ppm and the methoxy
signals are observed as singlet for complex 4 at δ 3.95 ppm. A
triplet is observed for methyl protons for complex 5 at δ 1.45 ppm
and OCH2 protons appears as quartet in the region δ 4.01 ppm.
A sharp singlet appeared for OH proton of all 2-(arylazo)phenol
ligands in the region δ 13.0 ppm is absent in all the complexes
[32].
*
n–␲*, ␲–␲ transitions occurring within ligand orbitals. The
pattern of the electronic spectra of all the complexes indicated
the presence of an octahedral environment around ruthenium(II)
ion, similar to that of other ruthenium(II) octahedral complexes
[34].
3.5. Emission study
Emission study of ruthenium(II) carbonyl complexes of lig-
ands containing strong chromophoric groups often show in
addition to intense MLCT transitions interesting luminescence
properties [35,36], such properties have also been explored in the
[Ru(ap-R)(AsPh3)2(CO)] complexes. The luminescence studies
were carried out in dichloromethane solutions at ambient tem-
perature (298 K). When an excitation wavelength corresponding
to the lowest energy absorption (∼500 nm) was used, no emis-
sion was observed. However, when an excitation wavelength of
∼400 nm was used, all the [Ru(ap-R)(AsPh3)2(CO)] complexes
showed strong emission (Fig. 3) in the range of 440–502 nm.
This indicates that transition of a metal electron to the met-
allated phenyl ring (or) p-cresol ring leads to photochemical
excited state, which is much more stable compared to excitation
of metal electron to ␲*(azo –N = N–) [37].
3.4. Electronic absorption spectroscopy
Electronic spectra of the complexes have been recorded
in dichloromethane solution and a representative spectrum is
showninFig. 3. Alltheruthenium(II)2-(arylazo)phenolatecom-
plexes show three to four intense absorptions in the visible and
ultraviolet region 549–263 nm. The intense absorption in the
3.6. X-ray crystallography
To find out the coordination mode of the 2-(arylazo)phenolate
ligands in these complexes as well as the stereochemistry of
the complexes, the structure of one of the complexes of this
family [Ru(ap-Cl)(AsPh3)2(CO)] (2) has been determined by
X-ray crystallography and ORTEP view of the complex is
shown in Fig. 4. The summary of single crystal X-ray struc-
ture refinement is shown in Table 1 and selected bond param-
eters are listed in Table 2. The structure shows that the 2-
(arylazo)phenolate ligand is coordinated to ruthenium in the
expected tridentate fashion through C, N and O in addition
two AsPh3 and one carbonyl groups. Ruthenium is therefore
sitting in a C2N1O1As2 coordination environment, which is
distorted octahedral in nature as reflected in all the bond param-
eters around ruthenium. The 2-(arylazo)phenolate ligand binds
the metal center at O, N and C forming two five-membered
Fig. 3. (a) Electronic and (b) emission spectrum of complex 2.

222
K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
Table 2
◦
˚
Selected bond lengths (A) and bond angles ( ) of complex 2
˚
Bond lengths (A)
Ru(1)–C(14)
Ru(1)–N(1)
Ru(1)–C(13)
Ru(1)–O(1)
Ru(1)–As(20)
Ru(1)–As(30)
C(1)–O(1)
C(4)–C(5)
C(7)–N(1)
C(8)–N(2)
C(14)–O(2)
1.910(4)
2.048(3)
2.055(3)
2.218(2)
2.4527(4)
2.4616(4)
1.299(4)
1.518(5)
1.409(4)
1.416(4)
1.118(4)
Bond angles (◦)
C(14)–Ru(1)–N(1)
C(14)–Ru(1)–C(13)
N(1)–Ru(1)–C(13)
C(14)–Ru(1)–O(1)
N(1)–Ru(1)–O(1)
C(13)–Ru(1)–O(1)
C(14)–Ru(1)–As(20)
N(1)–Ru(1)–As(20)
C(13)–Ru(1)–As(20)
O(1)–Ru(1)–As(20)
C(14)–Ru(1)–As(30)
N(1)–Ru(1)–As(30)
C(13)–Ru(1)–As(30)
O(1)–Ru(1)–As(30)
As(20)–Ru(1)–As(30)
175.82(13)
98.70(14)
77.26(12)
105.92(12)
78.17(10)
155.25(10)
88.11(9)
92.71(7)
87.16(8)
91.04(6)
90.64(9)
88.60(7)
94.00(8)
88.36(6)
178.419(15)
Fig. 4. ORTEP view of [Ru(ap-Cl)(AsPh3)2(CO)] (2), the hydrogen atoms were
omitted for clarity.
Table 1
Crystal data and structure refinement for complex [Ru(ap-Cl)(AsPh3)2(CO)] (2)
chelate rings with a bite angle of N(1)–Ru–O(1) = 78.17(17)◦
and N(1)–Ru–C(13) = 77.26(12)◦ and a bond length of
Empirical formula
Formula weight
Temperature
C50H39As2ClN2O2Ru
986.19
˚
˚
Ru–C(13) = 2.055(3) (A), Ru–O(1) = 2.218(2) (A) and Ru–
183(2) K
˚
˚
N(1) = 2.048(3) (A). The two AsPh3 groups are mutually
Wavelength
0.71073 A
Crystal system
Space group
Monoclinic
P21/n
trans to each other and are almost equal from the metal
˚
center, the bond distance being Ru–As(20) = 2.452(4) (A)and
Ru–As(30) = 2.461(4) (A). The RuII–CO(Ru–C(14)) bond
˚
◦
˚
Unit cell dimensions
a = 31.8258(13) A, α = 90
◦
˚
˚
˚
b = 12.4205(4) A, β = 91◦.869(5)
length is found to be 1.910(4) (A) which is much shorter than
c = 10.9318(4) A, γ = 90
the Ru–C(13) bond length.
3
˚
Volume
4319.0(3) A
Z
4
3.7. Cyclic voltammetry
Density (calculated)
Absorption coefficient
F(0 0 0)
1.517 Mg/m3
1.988 mm−1
1984
The electron transfer properties of the [Ru(ap-R)(AsPh3)2
(CO)] complexes have been studied in dichloromethane solu-
tion by cyclic voltammetrically at a glassy carbon working
electrode and all the potentials are expressed with reference to
Ag/AgCl. Voltammetric data are presented in Table 3 and a rep-
resentative cyclic voltammogram is shown in Fig. 5. All these
complexes (1 × 10−3) show two voltammetric response, one
quasi-reversible oxidation(RuIII/RuII) with E1/2 values +0.75
to +0.86 V and one quasi-reversible reduction with E1/2 val-
ues −0.50 to −0.57 V with respect to Ag/AgCl at a scan rate
100 mV s−1. In general, the changes in oxidation and reduc-
tion potentials are probably due to the relative stabilization of
ruthenium(II) over ruthenium(III) by a combination of ␴ and
␲ effects exhibited by the ligand. The oxidation discerned is
quasi-reversible in nature, characterized by a rather large peak-
to-peak separation ( Ep) of 140–200 mV and the cathodic peak
current (ipc) is less than that of the anodic peak current (ipa).
Crystal size
0.25 mm × 0.22 mm × 0.11 mm
Crystal description
Theta range for data collection
Index ranges
Green block
2.77–30.45◦
−45 ≤ h ≤ 45, −17 ≤ k ≤ 17, −13 ≤ l ≤ 15
Reflections collected
Independent reflections
Reflections observed
Criterion for observation
Completeness to theta = 30.45◦
Absorption correction
Maximum and minimum
transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
41,435
13,084 [R(int) = 0.0561]
9433
>2sigma(I)
99.6%
Numerical
0.8292 and 0.6619
Full-matrix least-squares on F2
13084/1/523
1.035
R1 = 0.0474, wR2 = 0.1289
R1 = 0.0629, wR2 = 0.1346
−3
˚
1.075 and −0.534 Einstein A

K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
223
Table 3
Electrochemical data of ruthenium(II) carbonyl 2-(arylazo)phenolate complexes
Complexes
RuIII/RuII
RuII/RuI
Epa (V)
Epc (V)
E1/2 (V)
Ep (mV)
140
180
180
190
140
Epa (V)
Epc (V)
E1/2 (V)
Ep (mV)
1
2
3
4
5
0.75
0.95
0.90
0.84
0.84
0.89
0.77
0.72
0.65
0.70
0.82
0.86
0.81
0.75
0.77
−0.44
−0.40
−0.48
−0.45
−0.43
−0.60
−0.60
−0.66
−0.59
−0.59
−0.52
−0.50
−0.57
−0.52
−0.51
160
200
180
140
160
Supporting electrolyte: NBu4ClO4 (0.005 M); complex: 0.001 M; solvent: CH2Cl2; Ep = Epa − Epc where Epa and Epc are anodic and cathodic potentials, respec-
tively; E1/2 = 0.5(Epa + Epc); scan rate: 100 mV s−1
.
The one electron nature of this oxidation has been verified
by comparing its current height (ipa) with that of the standard
ferrocene/ferrocenium couple under identical experimental con-
ditions [38]. The redox potentials (E1/2) are independent of the
various scan rates, supporting quasi-reversibility. The couples
at more positive and less negative potentials are believed to be
corresponding to metal oxidation and metal reduction, respec-
tively:
state of ruthenium ion and that the electron transfer reactions
take place without gross changes in the stereochemistry of the
complexes [40].
3.8. Catalytic oxidation
Ruthenium mediated oxidations are finding increasing appli-
cation due to the unique properties of this extremely versa-
tile transition metal, whose oxidation states can vary from
−II to +VIII and this prompted us to carry out this type
of reaction. The present work describes the catalytic oxi-
dation of primary and secondary alcohols by the synthe-
sized ruthenium(II) 2-(arylazo)phenolate complex [Ru(ap-
Cl)(AsPh3)2(CO)] in CH2Cl2 in the presence of NMO and the by
product water was removed by using about 0.25 g of molecular
sieves. Complex [Ru(ap-Cl)(AsPh3)2(CO)] (2) oxidizes primary
alcohols to corresponding aldehydes and secondary alcohols
to ketones with moderate to high conversion and the results
are listed in Table 4 (entries 1–15). The aldehydes or ketones
formed after 3 h of refluxing were determined by GC, and 2,4-
[RuII(ap-R)(AsPh3)2(CO)]
oxiꢀdation[RuIII(ap-R)(AsPh3) (CO)] + e−
2
[RuII(ap-R)(AsPh3)2(CO)] + e−
redꢀuction[RuI(ap-R)(AsPh3) (CO)]−
2
The oxidation and reduction potential is found to be sen-
sitive to the nature of the substituents (R) present in the
2-(arylazo)phenolate ligand increasing linearly (Fig. 6) with
increasing electron withdrawing character of the substituents
(expressed in terms of the Hammett substituent constant σ). The
plot of E1/2 versus σ (σ-Hammett constant of R: OCH3 = −0.27,
OC2H5 = −0.24, CH3 = −0.17, H = 0.00, Cl = 0.23) [39] is lin-
ear for both couples with slopes (ρ) of 0.57 V (RuIII/RuII) and
0.21 V (RuII/RuI) of all complexes. The linear correlation of
E1/2 versus σ with reasonable slopes (ρ) clearly shows that a
single substituent on the 2-(arylazo)phenolate ligands, which is
four bonds away from the metal center, can influence the metal
centered redox potential in a predictable manner. Hence, it is
inferred from the electrochemical data that the present ligand
system is ideally suitable for stabilizing the higher oxidation
Fig. 6. Least-squares plot of E1/2 values oxidation (RuIII/RuII) and reduction
(RuII/RuI) potentials of complexes 1–5 (where a = oxidation, b = reduction).
Fig. 5. Cyclic voltammogram of complex (4).

224
K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
Table 4
Catalytic oxidation of alcohols to corresponding aldehydes and ketones with complex [Ru(ap-Cl)(AsPh3)2(CO)] (2) in the presence of NMO
Entry
Substrate
Product
Yielda (%)
Turnoverb
60
1
60
2
3
>93
93
19
19c (8)d
4
45c
45
5
6
>99
>98
99
98
7
8
9
65e
65
94
98
>94
>98
10
11
12
98
98
99
93
>99
>93
13
19c (8)d, 38c (14)d
19, 38
14
15
11c (8)d, 38c (14)d
11, 38
19c (8)d
19
Substrate (1 mmol); NMO (3 mmol); complex (0.01 mmol); solvent dichloromethane.
a
Yield of product was determined using a HP 6890 series GC-FID with a DP-5 column of 30 m length, 0.32 mm diameter and 0.25 ␮m film thickness and by
comparison with authentic samples.
b
Moles of product per mole of catalyst.
Isolated yield and confirmed by derivative.
c
d
Reaction hours.
Isolated yield, characterized by melting point, 1H NMR and IR.
e

K.N. Kumar et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 218–226
225
dinitrophenylhydrozone derivatives and there was no detectable
oxidation in the absence of ruthenium complex.
Cl)(AsPh3)2(CO)]. Further, the complexes effectively catalyze
the aliphatic alcohols like butan-2-ol and 4-methylpentan-2-ol
to the corresponding ketones with high conversions higher than
93% and the conversion of borneol is found to be lower. More-
over, the five, six and substituted six-membered cyclic alcohols
are efficiently converted into corresponding ketones with high
conversions higher than 92%. The N-methylmorpholine and
water are the only by products during the course of the reaction.
It has been observed that the complex 2 has high catalytic effi-
ciencyinthecaseofoxidationofprimaryandsecondaryalcohols
when compared to similar ruthenium complexes as catalysts in
the presence of NMO/t-BuOOH reported earlier [24(b),44–47].
Results of the present investigation suggest that the com-
plexes are able to react efficiently with NMO to yield high
valent ruthenium-oxo species responsible for dehydrogenation
of alcohols. This was further supported by spectral changes
that occur by addition of NMO to a dichloromethane solu-
tion of the ruthenium(II) complexes. The appearance of peak
at 390 nm is attributed to the formation of RuIV = O species
(Fig. 7), which is in conformity with other oxo ruthenium(II)
complexes [41–43]. Further support in favor of the formation of
such species is identified by the IR spectrum of the solid mass
(obtained by evaporation of the resultant solution to dryness),
which shows a band at 860 cm−1, characteristic of RuIV = O
species [41,42].
The oxidation of benzylalcohol to benzaldehyde results in
60% conversion and 4-benzyloxy-3-methoxy benzyl alcohol is
converted to 4-benzyloxy-3-methoxy benzaldehyde yielded in
45%. Unsaturated cinnamyl alcohol is oxidized to cinnamalde-
hyde with >93% conversion without cleavage of carbon–carbon
double bonds. This is an important characteristic of the ruthe-
nium/NMO system. In addition to phenyl ethylalcohol, citronel-
lol and geraniol takes longer reaction time for the conversion
to corresponding aldehydes, which are confirmed by NMR
and their yields are determined by 2,4-dinitrophenylhydrazone
derivatives. The complex catalyzes the oxidation of aromatic
secondary alcohols such as benzhydrol and 1-phenyl ethanol
effectively into corresponding ketones in >97% yield. In the case
of benzoin the oxidation product benzil is isolated and character-
ized by melting points, IR and 1H NMR. In view of comparative
evaluation, oxidation of benzoin is carried out in the presence of
PPh3 analogue [Ru(ap-Cl)(PPh3)2(CO)] and the yield of 63% is
attained, which is almost equal to catalytic efficiency of [Ru(ap-
4. Conclusion
A new class of cyclometallated ruthenium(II) carbonyl 2-
(arylazo)phenolate complexes has been synthesized. The X-ray
crystal structure of one of the complexes reveals a distorted octa-
hedralenvironmentaroundruthenium. Allthecomplexesexhibit
characteristic metal-to-ligand charge transfer (MLCT) absorp-
tion and emission bands in the visible region. The complex (2)
has proved to be a new and efficient catalyst in the oxidation of
primary and secondary alcohols in the presence of NMO.
5. Supplementary material
Crystallographic data for the structural analysis have been
deposited with Cambridge crystallographic center, CCDC no.
261331. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk).
Acknowledgements
We express sincere thanks to Dr. B. Spingler, Institute of
¨
Inorganic Chemistry, University of Zurich, Switzerland for crys-
tallographic discussion. The authors also thanks to Prof. P.R.
Athappan, School of Chemistry, Madurai Kamaraj University,
for cyclic voltammetric facilities.
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