Ru(III) complexes containing 3,5-pyrazole dicarboxylic acid and triphenylphosphine/triphenylarsine: Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1016/j.poly.2011.01.020

New hexa-coordinated Ru(III) complexes of the type [Ru(H ₂Pzdc)(EPh₃)₃X₂] have been synthesized by reacting 3,5-pyrazole dicarboxylic acid (H₃Pzdc) with the appropriate starting complexes [RuX₃

Full text of DOI:10.1016/j.poly.2011.01.020

Polyhedron 30 (2011) 1108–1113
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ru(III) complexes containing 3,5-pyrazole dicarboxylic acid and
triphenylphosphine/triphenylarsine: Synthesis, characterization and catalytic
activity
Duraiswamy Sukanya a, Duraisamy Senthil Raja a, Nattamai S.P. Bhuvanesh b, Karuppannan Natarajan a,
⇑
a Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
b Department of Chemistry, Texas A&M University, College Station, TX 77842, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
New hexa-coordinated Ru(III) complexes of the type [Ru(H2Pzdc)(EPh3)3X2] have been synthesized by
reacting 3,5-pyrazole dicarboxylic acid (H3Pzdc) with the appropriate starting complexes [RuX3(EPh3)3]
(where X = Cl or Br; E = P or As). The ligand behaves as a bidentate monobasic chelate. All the complexes
have been characterized by analytical and spectroscopic (IR, electronic and EPR) data. Single-crystal X-ray
analysis of the complex [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH revealed that the coordination environ-
ment around the ruthenium center consists of an NOP2Cl2 octahedron. The planar ligand occupies the
equatorial position along with two chlorine atoms, while the triphenylphosphine groups occupy the axial
positions. The electrochemical behavior of the new complexes was studied using cyclic voltammetry. The
new mononuclear ruthenium complexes are capable of acting as catalysts for the oxidation of alcohols.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 21 October 2010
Accepted 12 January 2011
Available online 31 January 2011
Keywords:
Ruthenium(III) complexes
3,5-Pyrazole dicarboxylic acid
Crystal structure
Electrochemistry
Catalysis
1. Introduction
complexes containing 3,5-pyrazole dicarboxylic acid and PPh3/
AsPh3.
The use of transition metal complexes as catalysts has become
indispensable and reactions being environmentally benign are
mandatory. In recognition of their widespread importance over
the years, many synthetic metal complexes have been employed
as catalysts for simple single step reactions to complicated multi-
step processes [1–8]. The cross coupling reaction is found to be
one of the efficient methods for the easy synthesis of certain com-
pounds which would otherwise be tedious [9–11]. The new syn-
thetic methods that have emerged for the formation of CAC
bonds play an important role in pharmacological industries. A
number of ruthenium metal complexes have been found to cata-
lyze such aryl–aryl coupling reactions [12–15]. Another industri-
ally important reaction is the oxidation of organic compounds,
ranging from simple alcohols to complicated compounds. Ruthe-
nium complexes can also act as versatile catalysts in the oxidation
of alcohols to aldehydes or ketones [1,16–18].
2. Experimental work
2.1. Materials
RuCl3Á3H2O purchased from Hi-media and 3,5-pyrazole dicar-
boxylic acid purchased from Acros chemicals were used as such,
without further purification. Solvents were dried before use.
The starting complexes [RuCl3(PPh3)3], [RuCl3(AsPh3)3], [RuBr3-
(AsPh3)3] and [RuBr3(PPh3)3] were prepared as reported earlier
[19–22].
2.2. Physical measurements
The infrared spectra of the ligand and the complexes have been
recorded on a Nicolet Avatar model FT-IR spectrophotometer using
KBr discs in the range 4000–400 cmÀ1. Electronic spectra were re-
corded on a Systronics 119 UV–Vis spectrophotometer using meth-
anol as the solvent. Elemental analyses were performed with a
Vario EL III Elementar analyzer. Room temperature electron para-
magnetic resonance (EPR) spectra were recorded with an ER 200
D-SRC Bruker spectrometer. Cyclic voltammetric experiments were
carried out using a BAS CV-27 electrochemical analyzer with a
glassy carbon working electrode. A platinum wire and a silver–
With this background in mind, we aimed at developing metal
complexes that can act as a dual catalyst for both aryl–aryl cou-
pling and oxidation. Herein we report the synthesis, characteriza-
tion and catalytic activity of hexacoordinated ruthenium(III)
⇑
Corresponding author. Tel.: +91 422 2428319; fax: +91 422 2422387.
E-mail address: k_natraj6@yahoo.com (K. Natarajan).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.020

D. Sukanya et al. / Polyhedron 30 (2011) 1108–1113
1109
silver chloride electrode were used as counter and reference elec-
trodes respectively. Melting points were recorded with a Lab India
apparatus.
tion of [RuX3(EPh3)3] (where E = P or As; X = Cl or Br) (0.1 mmol)
was added to a refluxing solution of 3,5-pyrazole dicarboxylic acid
monohydrate (0.1 mmol) in ethanol (20 ml). The solution was fil-
tered while hot, reduced to half of its volume and left for slow
evaporation. The crystalline product that separated out was fil-
tered off, washed with ethanol and dried under vacuum. The prod-
2.3. Crystallography
uct was recrystallized from
individual syntheses are given below.
a benzene/ethanol mixture. The
Single crystals of [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH suitable
for X-ray diffraction studies were obtained from slow evaporation
of a solution of the complex in a benzene/ethanol mixture.
2.4.1. [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH (1)
[Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH (1) was prepared from
[RuCl3(PPh3)3] (0.0994 g, 0.1 mmol) and 3,5-pyrazole dicarboxylic
acid monohydrate (0.017 g, 0.1 mmol) as dark green crystals. Yield:
70%. Anal. Calc. for RuC41H33N2O4Cl2P2ÁC6H6ÁC2H5OH: C, 60.31; H,
4.65; N, 2.87. Found: C, 60.28; H, 4.71; N, 2.82%. IR (cmÀ1): 3332,
1715, 1652, 1482, 1434, 1091, 694. UV kmax (nm (L molÀ1 cmÀ1)):
248(25,200), 270(17,260), 289(18,450), 301(19,220). M.p.:
>200 °Cd (d = decomposition).
2.3.1. Data collection
A Bruker SMART 1000 three-circle X-ray diffractometer (Mo K
a
radiation, 50 kV, 40 mA) fitted with a graphite monochromator and
a CCD detector (CCD-PXL-KAF2, SMART 1000, 512 Â 512 pixel) was
employed [23]. The crystal, coated with rotectant (mineral oil), was
mounted on a nylon loop and was maintained at 110 K using a cold
nitrogen stream. The crystal to detector distance was set at 5.0 cm.
2.3.2. Data reduction, structure solution and refinement
2.4.2. [Ru(H2Pzdc)(AsPh3)2Cl2] (2)
SAINTPLUS [24] was used for data integration and data were cor-
rected for Lorentz and polarization factors. SADABS [25] was em-
ployed to correct the absorption effects. Systematic reflection
conditions for the data set suggest the space group Pca21 for
[Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH. The structure was solved
readily by Patterson map interpretation. All non-hydrogen atoms
were located with the Fourier difference map analysis and refined
with anisotropic thermal parameters. The hydrogen atoms were
placed in idealized positions. The structure was refined (weighted
least squares refinement on F2) to convergence [26]. Table 1 gives
further details of the data collection, refinement and the structural
details of [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH.
[Ru(H2Pzdc)(AsPh3)2Cl2] (2) was prepared from [RuCl3(AsPh3)3]
(0.112 g, 0.1 mmol) and 3,5-pyrazole dicarboxylic acid monohy-
drate (0.017 g, 0.1 mmol) as dark green crystals. Yield: 75%. Anal.
Calc. for RuC41H33N2O4Cl2As2: C, 52.40; H, 3.53; N, 2.98. Found:
C, 52.16; H, 3.35; N, 2.63%. IR (cmÀ1): 3334, 1724, 1652, 1481,
1434, 1076, 692. UV kmax (nm (L molÀ1 cmÀ1)): 284(24,610),
306(24,950). M.p.: >190 °Cd.
2.4.3. [Ru(H2Pzdc)(AsPh3)2Br2] (3)
[Ru(H2Pzdc)(AsPh3)2Br2] (3) was prepared from [RuBr3(AsPh3)3]
(0.125 g, 0.1 mmol) and 3,5-pyrazole dicarboxylic acid monohy-
drate (0.017 g, 0.1 mmol) as reddish brown crystals. Yield: 75%.
Anal. Calc. for RuC41H33N2O4Br2As2: C, 47.87; H, 3.23; N, 2.72.
Found: C, 47.62; H, 3.11; N, 2.83%. IR (cmÀ1): 3326, 1720, 1652,
1482, 1434, 1076, 692. UV kmax (nm (L molÀ1 cmÀ1)): 268(4236),
306(16,910). M.p.: >200 °Cd.
2.4. Synthesis of [Ru(H2Pzdc)(EPh3)2X2] (X = Cl or Br; E = P or As)
All the new mononuclear ruthenium(III) complexes were pre-
pared by the following general procedure: a benzene (20 ml) solu-
2.4.4. [Ru(H2Pzdc)(PPh3)2Br2] (4)
Table 1
[Ru(H2Pzdc)(PPh3)2Br2] (4) was prepared from [RuBr3(PPh3)3]
(0.1128 g, 0.1 mmol) and 3,5-pyrazole dicarboxylic acid monohy-
drate (0.017 g, 0.1 mmol) as dark green crystals. Yield: 80%. Anal.
Calc. for RuC41H33N2O4Br2P2: C, 52.35; H, 3.53; N, 2.97. Found: C,
52.06; H, 3.93; N, 2.72%. IR (cmÀ1): 3324, 1713, 1651, 1481,
1434, 1090, 693. UV kmax (nm (L molÀ1 cmÀ1)): 248(25,000),
268(16,450), 306(14,590). M.p.: >230 °Cd.
Crystal data and structure refinement parameters for [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6Á-
C2H5OH (1).
Crystal data
1
Empirical formula
Formula weight
T (K)
C
49H45Cl2N2O5P2Ru
975.78
110(2)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
0.71073
orthorhombic, Pca2(1)
2.5. Application studies
20.972(2)
b (Å)
11.9684(11)
18.0511(17)
4530.8(7)
4, 1.431
0.584
2.5.1. Catalytic oxidation
c (Å)
V (Å3)
To a solution of the alcohol (1 mmol) in CH2Cl2 (20 ml), N-
methyl morpholine-oxide (1 mmol) and the ruthenium complex
(0.01 mmol) were added. The solution was heated under reflux
for 3 h. The mixture was evaporated to dryness and extracted with
petroleum ether (60–80 °C). The combined petroleum ether mix-
ture was filtered and evaporated to give the corresponding alde-
hyde, which was then quantified as the 2,4-dinitrophenyl
hydrazone derivative [27].
Z, Dcalc (Mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
2004
Crystal size (mm)
h Range for data collection (°)
Limiting indices
0.29 Â 0.19 Â 0.16
1.70–27.52
À27 6 h 6 27, À10 6 k 6 15,
À20 6 l 6 23
33 976/10 055 [Rint = 0.0433]
99.4
Reflections collected/unique
Completeness (to h = 24.98) (%)
Absorption correction
SADABS
Maximum and minimum
transmission
Refinement method
0.9129 and 0.8495
2.5.2. Aryl–aryl coupling
Magnesium turnings (0.320 g; 0.013 g atom) were placed in a
two necked round bottomed flask with a calcium chloride guard
tube. A crystal of iodine was added to activate the magnesium. Bro-
mobenzene [0.746 g of a total 1.884 g (0.012 mol)] in anhydrous
diethylether (5 ml) was added with stirring and the mixture was
heated under reflux. The appearance of turbidity after 5 min indi-
cated initiation of the reaction. The remaining bromobenzene in
full-matrix least-squares on F2
10 055/4/558
1.036
R1 = 0.0345, wR2 = 0.0715
R1 = 0.0391, wR2 = 0.0729
0.847 and À0.363
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r(I)]
Largest difference in peak and hole
(e AÀ3
)

1110
D. Sukanya et al. / Polyhedron 30 (2011) 1108–1113
diethylether (5 ml) was added dropwise and the mixture was re-
fluxed for 40 min. To this mixture, 1.03 ml (0.01 mol) of bromoben-
zene in anhydrous diethylether (5 ml) and the ruthenium complex
(0.05 mmol) chosen for investigation were added and heated under
reflux for 6 h. The reaction mixture was cooled and hydrolyzed
with a saturated solution of aqueous ammonium chloride. The
diethylether extract, on evaporation of the solvent, gave the crude
product which was chromatographed on a silica gel column. Light
petroleum (60–80 °C) eluted the pure biphenyl, which compared
well with an authentic sample (TLC plates, IR and 1H NMR M.p.
69–72 °C) [28].
600000
400000
200000
0
-200000
-400000
-600000
3. Results and discussion
2.0
2.2
2.4
Field (g-factor)
New hexa-coordinated Ru(III) complexes of the type [Ru(H2Pzd-
c)(EPh3)2X2] have been prepared by reacting 3,5-pyrazole
dicarboxylic acid (H3Pzdc) with [RuCl3(PPh3)3], [RuCl3(AsPh3)3],
[RuBr3(AsPh3)3] and [RuBr3(PPh3)3] in a 1:1 ratio (Scheme 1). The
ligand replaces one EPh3 and one X atom (E = As or P; X = Cl or
Br) from the starting complex to yield [Ru(H2Pzdc)(EPh3)2X2],
and the analytical data conform to this stoichiometry.
Fig. 1. EPR spectrum of [Ru(H2Pzdc)(PPh3)2Br2] (4) in the solid state (solid line) and
in DMF solution (dotted line) at room temperature.
tal [33]. The nature of spectra obtained is in good agreement with
that of previously reported ruthenium(III) complexes [33,34]. A
representative spectrum is depicted in Fig. 1.
3.1. Infrared spectroscopy
3.3. Electrochemistry
The IR spectrum of the free ligand (H3Pzdc) shows a strong band
around 1700 cmÀ1 assignable to m(C@O) of the ACOOH moiety [29].
The redox behavior of the complexes was studied using cyclic
voltammetry at a glassy carbon working electrode at a scan rate
of 100 mV sÀ1. [NBu4]ClO4 (0.1 M) was used as supporting electro-
lyte in an acetonitrile solution of 0.001 M of the complexes. The
cyclic voltammetric data are given in Table 2. The E1/2 of the oxida-
tion process was in the range from 0.82 to 0.99 V and reduction
process was in the range from À0.79 to À0.82 V. The oxidation
and reduction waves are due to the metal centered Ru(III) ? Ru(IV)
and Ru(III) ? Ru(II) processes respectively. Moreover, the potential
difference between the two successive oxidation processes is
ꢀ1.5 V, which agrees well with the average potential difference be-
In all the complexes, new bands due to
the region 1652–1651 cmÀ1 and that due to
1480 cmÀ1. A large difference of 171–170 cmÀ1 between the mas
and s vibrations indicates a monodentate coordination of the car-
boxylic group in all the complexes [30]. A broad band observed in
m
as(COOꢀ) were observed in
m
s(COOꢀ) at about
m
the region 3400–3000 cmÀ1 due to
m(OH) of the carboxyl group and
that due to m(C@O) around 1700 cmÀ1 in the free ligand remains as
such in the new complexes, indicating that one ACOOH group re-
mains uncoordinated [31]. All the characteristic peaks due to PPh3/
AsPh3 were observed in the usual regions [32].
tween the redox processes of the ruthenium center (RuII/III–RuIII/IV
)
(ꢀ1.0–2.0 V) observed for other mononuclear complexes [35]. All
3.2. EPR spectroscopy
the complexes exhibit quasi-reversible oxidative couples with
peak to peak separations (DEp) of 180–280 mV [36].
The EPR spectra of all the complexes have been recorded in the
solid state and in DMF solution at room temperature. The nature of
the spectra revealed the absence of any hyperfine splitting due to
interactions with any other nuclei present in the complexes. All
the complexes exhibit a single isotropic resonance with g values
in the range 2.21–2.35. EPR spectra of all the complexes in DMF
solution indicate that the complexes retain their structures in the
solution state. Although the complexes have some distortion in
their octahedral geometries, the observation of isotropic lines in
the EPR spectra may be due to intermolecular spin exchange and
due to the occupancy of the unpaired electron in a degenerate orbi-
3.4. Crystal structure of [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH
The ORTEP diagram of [Ru(H2Pzdc)(PPh3)2Cl2] is shown in Fig. 2.
Selected bond lengths and bond angles are given in Table 3. The
bite angles around Ru(III) are N(1)–Ru(1)–O(1) = 76.07(9); N(1)–
Ru(1)–Cl(2) = 89.52(7); O(1)–Ru(1)–Cl(1) = 96.36(6) and Cl(2)–
Ru(1)–Cl(1) = 98.04(3)°, summing up the in-plane angles to be
360.09°. This shows the high planarity of the two Cl, N and O
atoms. The ligand occupies the equatorial position, along with
HO
O
C
EPh3
Ru
N
NH
HN
N
X
X
EPh3
X
HO
OH
Ethanol/Benzene
Reflux, 6h
EPh3
+
C
C
X
X
EPh3
Ru
O
O
C
O
EPh3
O
E = As, P
X = Cl, Br
Scheme 1. Synthesis of [Ru(H2Pzdc)(EPh3)2X2].

D. Sukanya et al. / Polyhedron 30 (2011) 1108–1113
1111
Table 2
Cyclic voltammetric dataa of new Ru(III) complexes.
Complex
Ru(III)–Ru(IV)
Ru(III)–Ru(II)
Epc (V)
Epc (V)
Epa (V)
Ef (V)
D
Ep (mV)
Epa (V)
Ef (V)
DEp (mV)
1
2
3
4
0.78
0.72
0.89
0.79
1.05
0.93
1.10
1.07
0.91
0.82
0.99
0.93
270
210
210
280
À0.88
À0.91
À0.91
À0.92
À0.70
À0.71
À0.70
À0.72
À0.79
À0.81
À0.80
À0.82
180
200
210
200
a
Supporting electrolyte: [NBu4]ClO4 (0.1 M); complex concentration: 0.001 M; scan rate: 100 mV sÀ1; all the potentials are referenced to Ag/AgCl.
Fig. 2. ORTEP diagram of [Ru(H2Pzdc)(PPh3)2Cl2]ÁC6H6ÁC2H5OH (1). Hydrogen atoms and solvent molecules have been omitted for clarity.
the two chlorine atoms. The P(2)–Ru(1)–P(1) bond angle is
175.41(3)°, showing that the two PPh3 groups are trans to each
other, occupying the axial positions. The Ru–O [33], Ru–N [6],
Ru–P [37] and Ru–Cl [38] bond lengths found in the complex agree
well with those reported for similar ruthenium complexes. Since
the 3,5-pyrazole dicarboxylic acid moiety has both hydrogen bond
donors and hydrogen bond acceptors, the species provide the pos-
sibility of forming hydrogen bonds in the crystal. The complexes
are joined through hydrogen bonds involving the O2 and O3 oxy-
gen atoms of 3,5-pyrazole dicarboxylic acid, giving a complex three
dimensional lattice for [Ru(H2Pzdc)(PPh3)2Cl2].
3.5. Catalytic activity
The oxidation of alcohols was carried out with the new ruthe-
nium(III) complexes as catalysts in the presence of N-methyl mor-
pholine-N-oxide (NMO) as a co-oxidant in CH2Cl2, and the results
are summarized in Table 4. In no case was there any detectable
oxidation of alcohols in the presence of NMO alone, without the
Ru(III) complexes. All the complexes oxidize primary alcohols to
the corresponding aldehyde and secondary alcohols to the corre-
sponding ketone in high yield. The results of the present investiga-
tion suggest that the complexes efficiently react with NMO to yield

1112
D. Sukanya et al. / Polyhedron 30 (2011) 1108–1113
Table 3
it is evident that triphenylphosphine complexes possess more cat-
alytic activity than triphenylarsine complexes because the RuAP
bond is more labile than the RuAAs bond.
Selected geometrical parameters for 1.
Bond distances (Å)
Ru(1)–N(1)
Ru(1)–O(1)
Ru(1)–Cl(2)
Ru(1)–Cl(1)
Ru(1)–P(2)
2.073(2)
2.084(2)
2.3145(7)
2.3226(7)
2.4027(8)
2.4137(7)
The new complexes have been used as catalysts for the coupling
of phenyl magnesium bromide with bromobenzene to give biphe-
nyl. An insignificant amount of biphenyl was formed when the
reaction was carried out without the catalyst. The percentage yield
of biphenyl obtained from the reactions catalyzed by the new
Ru(III) complexes are low compared to NiCl2(PPh3)2 [42]. This
may be due to the fact that the lifetime of the active species (d7)
formed from ruthenium(III) complexes is less than that of the d10
species formed from the nickel(II) complexes, as the effectiveness
of the catalysts is directly related to their ability to generate the
corresponding active species [43,44]. The nature of the ligand
was found to have no positive effect on the reaction rate when
compared to our previously reported binuclear Ru(III) complexes
containing an O,N donor ligand and PPh3/AsPh3 [28].
Ru(1)–P(1)
Bond angles (°)
N(1)–Ru(1)–O(1)
N(1)–Ru(1)–Cl(2)
O(1)–Ru(1)–Cl(2)
N(1)–Ru(1)–Cl(1)
O(1)–Ru(1)–Cl(1)
Cl(2)–Ru(1)–Cl(1)
N(1)–Ru(1)–P(2)
O(1)–Ru(1)–P(2)
Cl(2)–Ru(1)–P(2)
Cl(1)–Ru(1)–P(2)
N(1)–Ru(1)–P(1)
O(1)–Ru(1)–P(1)
Cl(2)–Ru(1)–P(1)
Cl(1)–Ru(1)–P(1)
P(2)–Ru(1)–P(1)
76.07(9)
89.52(7)
165.55(6)
172.41(7)
96.36(6)
98.04(3)
93.21(7)
89.86(6)
89.96(3)
86.19(3)
90.31(7)
88.13(6)
93.01(3)
89.93(3)
175.41(3)
4. Conclusion
Some new hexa-coordinated ruthenium(III) complexes contain-
ing 3,5-pyrazole dicarboxylic acid and PPh3/AsPh3 have been syn-
thesized. The new complexes have been characterized by
analytical and spectral (IR, electronic and EPR) studies. The redox
behavior of the complexes has been studied by cyclic voltammetry.
The new complexes were found to catalyze the oxidation of alco-
hols to give reasonably good yields, but were not effective catalysts
for aryl–aryl coupling.
Table 4
Catalytic activity of new Ru(III) complexes.
Complex
Substrate
Yielda (%)
Turnover numberb
1
benzyl alcohol
cinnamyl alcohol
cyclohexanol
74
82
26
75
80
25
Acknowledgment
2
3
4
benzyl alcohol
cinnamyl alcohol
cyclohexanol
54
75
16
53
74
16
The Department of Science and Technology (DST-SERC), New
Delhi, India is gratefully acknowledged for the financial assistance.
benzyl alcohol
cinnamyl alcohol
cyclohexanol
65
80
18
67
79
17
Appendix A. Supplementary data
CCDC 621596 contains the supplementary crystallographic data
for compound 1. 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, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
benzyl alcohol
cinnamyl alcohol
cyclohexanol
73
83
25
74
82
26
a
Yield based on substrate.
Moles of product per mole of catalyst.
b
References
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to alcohols. This was supported by spectral changes that occur on
the addition of NMO to a CH2Cl2 solution of the Ru(III) complexes.
The appearance of a peak at 390 nm is attributed to the formation
of Ru(V)@O, which is in conformity with other Ru(V) complexes
[39,40]. Further support in favor of the formation of such a species
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showed a band at 860 cmÀ1, characteristic of Ru(V)@O species
[18,39,41]. It has already been established that the catalytic cycle
passes through a Ru(V)@O species [41]. The relatively higher yield
for oxidation of cinnamyl alcohol than benzyl alcohol or cyclohex-
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