Synthesis, spectroscopic characterization, redox properties and catalytic activity of some ruthenium(II) complexes containing aromatic aldehyde and triphenylphosphine or triphenylarsine.

Article abstract of DOI:10.1016/S1386-1425(03)00191-4

A series of new mixed ligand penta-coordinated square pyramidal ruthenium(II) complexes containing benzaldehyde or its substituents and triphenylphosphine or triphenylarsine have been synthesized and characterized. In the electronic spectra, three well-defined peaks in the visible region were observed and assigned to d-d transitions in D(4h) and low spin axially distortion from O(h) symmetry. The spectrochemical parameters of the complexes were calculated and placed the ligands in the middle of the spectrochemical series. The redox properties and stability of the complexes toward oxidation were related to the electron-withdrawing or releasing ability of the substituent in the phenyl ring of the benzaldehyde. The electron-withdrawing substituents stabilized Ru(2+) complexes, while electron-donating groups favored oxidation to Ru(3+). The mechanism and kinetics of the catalytic oxidation of benzyl alcohol by the complex [RuCl(2)(Pph(3))(C(6)H(5)CHO)(2)] in the presence of N-methylmorpholine-N-oxide have also been studied.

Full text of DOI:10.1016/S1386-1425(03)00191-4

Spectrochimica Acta Part A 60 (2004) 121–127
Synthesis, spectroscopic characterization, redox properties and catalytic
activity of some ruthenium(II) complexes containing aromatic aldehyde
and triphenylphosphine or triphenylarsine
M.S. El-Shahawi∗, A.F. Shoair
Chemistry Department, Faculty of Science at Damietta, Mansoura University, Mansoura, Egypt
Received 6 January 2003; received in revised form 11 April 2003; accepted 14 April 2003
Abstract
A series of new mixed ligand penta-coordinated square pyramidal ruthenium(II) complexes containing benzaldehyde or its substituents
and triphenylphosphine or triphenylarsine have been synthesized and characterized. In the electronic spectra, three well-defined peaks in the
visible region were observed and assigned to d–d transitions in D4h and low spin axially distortion from Oh symmetry. The spectrochemical
parameters of the complexes were calculated and placed the ligands in the middle of the spectrochemical series. The redox properties and
stability of the complexes toward oxidation were related to the electron-withdrawing or releasing ability of the substituent in the phenyl ring
of the benzaldehyde. The electron-withdrawing substituents stabilized Ru2+ complexes, while electron-donating groups favored oxidation to
Ru3+. The mechanism and kinetics of the catalytic oxidation of benzyl alcohol by the complex [RuCl2(Pph3)(C6H5CHO)2] in the presence of
N-methylmorpholine-N-oxide have also been studied.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Ruthenium(II); Spectroscopic characterization; Redox properties; Reactivity
1. Introduction
triphenylarsine) and ␲ acceptor aromatic aldehyde (Fig. 1).
The nature of the electrode reactions of the prepared com-
plexes and the oxidative behavior of one of the complexes
were discussed.
Transition metal complexes with some ligand systems
containing N- and N,O-donors have been recognized as inor-
ganic catalysts for olefin epoxidation, alcohols and aliphatic
and aromatic hydroxylations [1–8]. The ruthenium(II or III)
complexes containing weak donor ligands are lacking in
comparison to other donor ligands [9–12]. Ruthenium(II)
complexes have been characterized by their high stability
mainly when ligands with donor atoms such as N, P, S, As
or O are present in the coordination sphere [13,14].
Recent years have seen an upsurge of interest in find-
ing versatile ruthenium(II) complexes that allow both cat-
alytic and electrochemical behavior. In view of that and as
a part of our continuing work on O,O-donor ligands we re-
port herein the synthesis and characterization of some novel
ruthenium(II) complexes containing triphenylphosphine (or
2. Experimental
2.1. Reagents and materials
All the chemicals used are of analytical reagent
grade. The reagent N-methylmorpholine-N-oxide (NMO),
RuCl3·3H2O, benzaldehyde, 4-Br-benzaldehyde, anisalde-
hyde, o-phthaldehyde, triphenylphosphine (Pph3) and
triphenylarsine (Asph3) were obtained from Aldrich. The
complex [RuCl2(Pph3)2] was prepared as reported [15].
The supporting electrolyte tetrabutylammonium-hexafluoro-
phosphate (TBA)+PF6 was dried in vacuum before use.
−
The solvents used were degassed and the ruthenium per-
centage in the complexes was determined by the reported
methods [16,17]. The electrochemical measurements were
carried out at 30 ◦C.
∗
Corresponding author. Tel.: +20-5-740-3867; fax: +20-5-740-3868.
E-mail address: mohammad el shahawi@hotmail.com
(M.S. El-Shahawi).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1386-1425(03)00191-4

122
M.S. El-Shahawi, A.F. Shoair / Spectrochimica Acta Part A 60 (2004) 121–127
stirring. The reaction mixture was refluxed for 2 h. Green
precipitate was separated out, filtered off, washed with
ethanol and finally dried in vacuum.
2.3.1.3. [RuCl2(MPh3)L 2], M=P or As, L =2-CHO
C6H4CHO. To a solution of RuCl3·3H2O (0.21 g, 1 mmol)
in degassed methanol (20 cm3), the reagent o-phthaldehyde
(0.27 g, 2 mmol) in 10 cm3 methanol was added and the
reaction mixture was refluxed for 1 h. A solution of triph-
enylphosphine (0.25 g, 1 mmol) or triphenylarsine (0.29 g,
1 mmol) in methanol (10 cm3) was then added slowly to the
hot solution with constant stirring. The resulting reaction
mixture was refluxed for 2 h. On concentrating and cooling
the solution, a brown or purple precipitate was separated
out, filtered off, washed with diethylether and finally dried
in vacuum.
Fig. 1. Atom numbering scheme of the aldehyde molecules. X=H,
Y=CH3O; X=H, Y=Br and X=CHO, Y=H.
2.2. Physical measurements
2.3.2. Catalytic oxidation
Infrared (IR) spectra (200–4000 cm−1) as KBr discs
were measured on a Matson 500 FT-IR spectrometer at
room temperature. 1H NMR in DMSO-d6 and electronic
spectra in CH2Cl2 (∼5×10−4 M) were recorded on a Var-
ian Gemini VM-200 and a Unicam UV2–100 UV–visible
spectrometers, respectively. Cyclic voltammograms (CVs)
were recorded on a potentiostate/wave generator (Oxford
Electrodes) equipped with a 7000 AM X-Y recorder. A
conventional three-electrode electrochemical cell was used
comprising a platinum micro-cylinder working electrode, a
spiral platinum wire (0.5 mm diam) as a counter electrode
and Ag/AgCl reference electrode to which all potentials are
referred. The electrochemical experiments were performed
using a concentration of 1×10−3 mol dm−3 of the complexe
and 1×10−2 mol dm−3 of the supporting electrolyte in
CH2Cl2. The solutions of the complexes were bubbled with
dry N2 for 20 min before CV measurements. All microanal-
yses (C, H, Cl) analyses were performed on a Perkin–Elmer,
240 C elemental analyzer at Plymouth University, UK.
The catalytic activity of the prepared complexes for
the oxidation of benzyl alcohol (phCH2OH) to ben-
zaldehyde (phCHO) was tested in the presence of NMO
as a co-oxidant. A typical reaction using the complex
[RuCl2(Pph3)(C6H5CHO)2] as a catalyst and BA as a sub-
strate at 1:200 molar ratio is described as follows: A solu-
tion of [RuCl2(Pph3)(C6H5CHO)2] (0.001 g, 0.01 mmol)
in 10 cm3 CH2Cl2 was added to a solution of benzyl alco-
hol (2 mmol) and NMO (20 cm3, 3 mmol) with constant
stirring. The solution mixture was refluxed for 2 h and the
solvent was then evaporated from the mother liquor under
reduced pressure. The solid residue was then extracted with
diethylether (3×10 cm3) and the ethereal extract was then
filtered off and quantified as 2,4-dinitrophenylhydrazone
derivative. The catalytic activity of the complex was deter-
mined from the percent yield (Y%) and the turnover (T.O)
conversion of BA to benzaldehyde as follows:
Weight of BA oxidized to
benzaldehyde
Y (%) =
× 100
(1)
(2)
Total weight of BA
mmoles of product
2.3. Recommended procedures
T.O =
2.3.1. Synthesis of ruthenium(II) complexes
mmoles of catalyst
2.3.1.1. [RuCl2(Pph3)L2], L=HC6H4CHO, 4-CH3OC6H4-
CHO or 4Br-C6H5CHO. A suspension of L (2 mmol) and
RuCl2(Pph3)3 (0.95 g, 1 mmol) in degassed ethanol (40 cm3)
was refluxed for 3 h during which the complex RuCl2(Pph3)3
was dissolved and the reaction mixture was turned to brown-
ich (or green) color. A brown or green precipitate appeared
after slow evaporation of the solvent, filtered off, washed
with ethanol, diethylether and finally dried in vacuum.
3. Results and discussion
The prepared complexes are listed in Table 1 to-
gether with their elemental analyses and colors. The
complexes are green/brown, dark green and purple in
color. Two types of complexes having the general for-
mulae [RuCl2(MPh3)L2] and [Ru2Cl4(H2O)2(MPh3)2L 2]
where M=P or As, L=H–C6H4CHO, p-Br–C6H4CHO
or p-CH3O·C6CH4CHO and L =o-CHO–C6H4CHO are
formed. The proposed chemical structures of the complexes
are in good agreement with the stoichiometries concluded
from their analytical data (Table 1). During the course
of these synthetic reactions, ruthenium(III) undergoes a
2.3.1.2. [RuCl2(Asph3)L2], L=HC6H4CHO or 4-CH3-
OC6H4CHO. An accurate weight (0.104 g; 0.5 mmol)
of hydrated RuCl3 in ethanol (10 cm3) was mixed with
benzaldehyde or anisaldehyde (1 mmol) and triphenylar-
sine (0.15 g, 0.5 mmol) in 10 cm3 ethanol under constant

M.S. El-Shahawi, A.F. Shoair / Spectrochimica Acta Part A 60 (2004) 121–127
123
Table 1
Analytical data and color of the prepared ruthenium(II) complexes
Complex
Color
Calculated (found) %
C
H
Ru
Cl
[RuCl2(Pph3)(C6H5CHO)2]
Brown
Brown
Green
Brown
Green
Brown
Brown
Green
59.4 (60.2)
57.7 (58.4)
47.7 (48.8)
54.4 (55.8)
55.6 (55.1)
54.9 (54.2)
66.6 (66.1)
49.7 (48.9)
4.1 (4.2)
4.3 (4.1)
3.1 (3.4)
4.1 (4.3)
3.9 (3.6)
3.6 (3.9)
3.4 (3.2)
3.4 (3.2)
15.6 (15.34)
14.3 (14.0)
12.5 (12.1)
13.4 (12.9)
14.6 (14.1)
17.1 (17.4)
16.5 (15.8)
23.2 (24.1)
10.9 (11.2)
10.0 (10.3)
8.8 (8.6)
[RuCl2(Pph3)(p-CH3OC6H4CHO)2]
[RuCl2(Pph3)(p-BrC6H4CHO)2]
[RuCl2(Asph3)(p-CH3OC6H4CHO)2]
[RuCl2(Asph3)(C6H5CHO)2]
[Ru2Cl4(Pph3)2(H2O)2(o-CHOC6H4CHO)2]
[Ru2Cl4(Asph3)2(H2O)2(o-CHOC6H4CHO)2]
[RuCl2(Pph3)3]
9.4 (9.8)
10.2 (9.7)
12.5 (12.1)
11.6 (11.2)
16.3 (15.7)
The 1H NMR spectra of the free aldehydes and their
ruthenium(II) complexes (Table 2) showed a similar pattern
and are in accordance with the reported data [12,13]. The
–CHO proton signal in the free aldehydes was appeared as
a sharp singlet in the region δ 9.8–10.2 ppm. In the ruthe-
nium(II) complexes, the aldehyde proton signal was shifted
down field as a singlet at δ 10.0–10.25 and 10.10–10.3 ppm
for mono- and o-dialdehyde, respectively. These data con-
firm participation of the –CHO group upon complex for-
mation [17,18] and the structure (Fig. 2) proposed for the
o-phthaldehyde complexes. Similar features have been ob-
served for the 2-hydroxy-1-naphthaldehyde ruthenium com-
plexes [12,23]. The de-shielding caused by the donation of
the lone pair of electrons of the aldehyde oxygen atom to
the central metal ion could account for the observed chemi-
cal shift in the1H NMR spectra [22,24]. The observed signal
one-electron reduction and the solvent may serves as the
reductant. The isolated solid complexes are stable in air at
room temperature, non-hygroscopic in nature and almost
insoluble in common organic solvents. The complexes are
easily soluble in dichloromethane, DMF and DMSO. The
conductivity measurements of the complexes fall in the
range for non-electrolytes [18]. Magnetic susceptibility
measurements showed that, all the complexes are diamag-
netic at room temperature as expected for ruthenium(II)
(low spin d6, S=0) complexes.
3.1. Spectroscopic studies
The IR spectra of the complexes (Table 1) displayed the
characteristic bands due to triphenylphosphine at 440–455
cm−1 or triphenylarsine at 272–295 cm−1 [19]. The free
aldehydes showed their characteristic strong carbonyl fre-
quency ν(C–O) in the region 1675–1710 cm−1 [13,19]. In
the IR spectra of the complexes, the νCO vibrational mode
was shifted (∼30–40 cm−1) towards the lower frequency
region from that observed for the free ligands confirming
participation of the oxygen atom of the aldehyde (–CHO)
group in the complex formation [13,19]. The medium IR
bands between 450 and 520 cm−1 in the spectra of the com-
plexes are assigned to ν(Ru–O) vibration [13,1,9]. The ob-
served bands in the regions 430–470 and 320–330 cm−1 in
the mononuclear complexes (1–5) are tentatively assigned to
the νRu–Cl vibrations [19]. The two bands located in the re-
gion 320–330 cm−1 are consistent with νRu–Cl vibrations of
the di-chloro ligands in cis- geometry as previously reported
for cis-[RuCl2(Pph3)2(RCN)2] [20]. In the O-phthaldehyde
complexes the presence of coordinated water molecules in
the complexes (6 and 7) was confirmed by the appearance
of a sharp band around 3440–3445 cm−1 and two weaker
bands around 700 and 850 cm−1. The latter two bands are
safely assigned to wagging and/or rock vibrations of aqua
ligands [21,22]. Therefore, in the o-phthaldehyde complexes
we postulated that, the two O-phthaldehyde molecules (or
the two chlorine atoms) bridge the two ruthenium(II) atoms
as shown Fig. 2. This structure was also confirmed on the
basis of the spectroscopic data and by the analogy with re-
lated complexes [12–14].
Fig. 2. Proposed chemical structure of the binuclear O-phthaldehyde
ruthenium(II) complex [RuCl2(MPph3)(o-CHO-C6H4CHO)]2, M=P or
As.

124
M.S. El-Shahawi, A.F. Shoair / Spectrochimica Acta Part A 60 (2004) 121–127
Table 2
Significant IR-vibrational frequencies (cm−1) and 1H NMR (δ ppm) of the ruthenium(II) complexes with relevant bands of the free aldehyde in parentheses
Complex
νCO
νRu–O
νRu–Cl
νRu–P
νRu–As
Complexed H2O
δ (ppm)
1
2
1685(s), (1715)
1675(s),(1710)
480(m)
450(m)
460(s)
430(s)
445
451
10.12, 7.2, 7.58, 7.5
10.20, 6.90, 7.36, 7.3, 3.9, (10.1), (7.12), (7.4),
(3.92)
3
4
5
6
7
8
1700(s), (1730)
1795, (1735)
1690, (1730)
1710(s), (1745)
1710(s), (1745)
505(m)
520(m)
490(m)
510(m)
510(m)
450(s)
445(s)
465(s)
470
470(s)
475
455
440
10.20, 6.98, 7.5, 7.46
10.25, 7.38, 7.4, 7.3, 3.8
10.2, 7.25, 7.52, 7.5
10.16, 7.28, 7.6, 7.6, (9.85), (7.10), 7.56, 7.50
10.3, 7.01, 7.6, 7.5
272
285
295
295
3440(s), 850(w), 700(w)
3445(m, br), 860(w), 705(w)
455
* s, strong; m, medium; br, broad and w, weak.
(19.4–23.6)×103 and (24.9–27.6)×103 cm−1. The molar
extinction coefficients of these bands are low relative to
those of metal to ligand charge transfer (MLCT) transi-
tion [27]. Assuming that, the complexes belong to D4 or
low spin axially distortion from octahedral symmetry, the
first two bands (Table 3) are safely assigned to the ν1
at δ 3.8–3.94 ppm in the anisaldehyde and its complexes is
attributed to the methoxy proton [22,25]. This proton under-
goes deshielding to a magnitude of 0.45 ppm in the com-
plexes supporting the involvement of the –CHO group in
the complex formation. The resonance arising from H3 and
H5 (see Fig. 1 for atom numbering scheme) in the mono
aldehydes were shifted as double–doublet (7.1–7.6 ppm) to
high field to a greater extent than H2 and H6 (7.4–7.9 ppm).
A sharp multiplet signal in the range δ 6.90–7.29 ppm was
observed and was attributed to the pph3 or Asph3 protons.
In the free benzaldehyde and O-phthaldehyde ligands, the
H3, H4 and H5 protons appeared as doublets at 7.3, 8.1 and
7.9 ppm. In the complexes, the signals arising from H3, H5
and H6 protons were observed at high field similar to that
reported for Sb3+ and Zn2+ complexes with similar ligands
[23]. This behavior is most likely due to the decreasing in
the electronic density of the aromatic ring of the coordina-
tion. The multiple peaks of the substituted aromatic protons
resonate in the region δ 7.15–7.20 ppm due to one bound
triphenylphosphine (or triphenylarsine) and two inequiva-
lent aldehyde ligands. These data confirmed that the Pph3
or AsPh3 and chloride ligands are mutually cis- as reported
for similar geometry of ruthenium(II) complexes [26,27].
The UV–visible spectra of the complexes are simi-
lar to those observed for other analogous of low spin
penta-coordinated square pyramidal ruthenium(II) com-
plexes [26,27]. The spectral of the complexes with their lig-
and field parameters (10Dq, B) in CH2Cl2 are summarized
in Table 3. The spectra of these complexes are similar and
displayed three well bands in the range (16.9–17.8)×103,
(1A1g→ E2g) and ν2 (1A1g→ A2g) d–d transitions in the
range corresponding to the spin allowed transitions from
the lower frequency side [26,28]. The third peak is possibly
assigned to a contribution form the spin forbidden d–d ν3
1
1
1T2(1A1g→ E) besides the CT transition (Ru2+→ligand),
1
respectively [28].
The bands (or shoulder) observed in the range (34.01–
40.5)×103 and (32.1–32.8)×103 cm−1 in the UV-region are
safely assigned to intraligand (IL) ␲→␲* type of CT tran-
sitions of the carbonyl group of the ligands or an allowed
Ru2+→Cl transitions [29,30]. The in-equivalence of the lig-
ands decreased the ligand field strength of the complexes as
compared with [RuCl2(MPh3)3]. This effect reduces the en-
ergy gap between Ru (d␲) and ligand (␲*) orbitals [27,30].
Thus, substitution of one or two ␲ acidic MPh3 ligand in the
complex [RuCl2(MPh3)] by one di or two mono-aldehyde
ligand decreased the MLCT energy.
The spectrochemical parameters 10Dq and B (Table 3)
were found in the range 1461–1614 and 616.2–465.6 cm−1
,
respectively. The Dq values are far closer to the range
observed for RuCl2P3 (or As3) and slightly closer to the
RuO2Cl2P2 or RuO2Cl2As2 [27]. These data confirm the
coordination of chlorine, oxygen and P (or As) atoms. The
10Dq values also placed the ligands in the middle range of
Table 3
Electronic spectral data (cm−1) and molar absorptivity in parentheses of the prepared complexes with ligand field parameters in CH2Cl2 solutiona
1
1
1
Complex
␲→␲*, n→␲* ×103
1A1→ E2, ν1 ×103 (log ε)
1A1→ A2, ν2 ×103 (log ε)
1A1→ E, ν3 ×103 (log ε)
B
10Dq (cm−1
)
1
2
3
4
5
6
7
40.48, 35.84
40.84, 35.84
40.48, 35.84
34.01, 32.15
35.33, 32.78
40.0, 35.71
34.48, 32.36
17.85 (2.6)
17.24 (2.57)
17.70 (2.72)
16.92 (2.74)
17.24 (2.6)
17.54 (2.91)
17.24 (2.98)
19.80 (3.32)
19.42 (3.47)
19.80 (3.21)
23.20 (4.3)
23.58 (4.12)
22.22 (sh) (3.96)
21.27 (4.21)
24.69 (3.51)
24.69 (3.6)
24.69 (4.1)
27.10 (4.8)
27.77
427.5
465.6
436.8
636.2
658.1
446.8
616.2
1614
1538
1595
1445
1461
1575
1475
24.69 (4.92)
27.10 (4.96)
a
Logarith molar absorptivity of each absorption band is given in parentheses.

M.S. El-Shahawi, A.F. Shoair / Spectrochimica Acta Part A 60 (2004) 121–127
125
Table 4
Cyclic voltammetric data of the prepared ruthenium(II) complexes in
dichloromethane–(TBA)PF6 solutions
Complex
Mph3 reduction
RuII/RuIII
RuIII/RuIV
E◦
Ep
E◦
Ep
E◦
Ep
1
2
3
4
5
6
7
8
−0.460
−0.630
−0.80
−0.70
−0.92
−0.28
−0.570
−0.65
0.68
0.130
0.80
0.9
0.84
0.140
0.180
0.210
0.15
−0.54
−0.45
0.25
−0.170
0.075
−0.30
0.26
0.50
0.40
0.38
0.86
0.60
0.73
0.550
0.98
0.66
0.360
0.660
0.52
0.62
0.47
0.80
0.216
0.960
0.20
0.30
0.26
0.06
0.170
0.180
0.210
the spectrochemical series and provided reassurance that
the aldehyde oxygen of the ligands was coordinated to
ruthenium(II) ions. The B values of the complexes (Table 3)
were found in the range 56–74% of that of the free ion
[27] indicating considerable overlap with strongly cova-
lent metal–ligand bond [27,28]. The decrease in B values
is most likely associated with the reduction in the nuclear
charge on the cation.
3.2. Redox properties
The CVs of the complexes were studied versus Ag/AgCl
in CH2Cl2–(TBA)PF6 solutions. The electrochemical re-
sponse of the complexes RuCl2(Mph3)L2 were found simi-
lar and displayed three well defined electrode couples in the
range E◦=−0.92 to −0.46 V. E◦=−0.54 to +0.26 V and
E◦=0.52 to 0.98 V versus Ag/AgCl electrode (Table 4), re-
spectively. Representative voltammograms are also shown in
Fig. 3. The ratio of ic/ia due to cathodic and anodic sweeps
was found not close to unity at the scan rates (50–200 mV
s
−1). The potential difference ( Ep=Ea−Ec) of the first
electrode couple increased with increasing the scan rate.
This behavior was reported for related complexes confirm-
ing the occurrence of a slow chemical reaction following the
electrode process [26]. Thus, the electron transfer process
is irreversible, the mass transfer is limited and the species
that initially formed in the electrode process may also react
further to give products that are not reoxidized at the same
potential as the first formed species [31]. A possible mech-
anism account for one electron reduction of the coordinated
Mph3 could be proceeded as follows:
Fig.
3.
CVs
of
[RuCl2(Pph3)(p-BrC6H4CHO)2],
(a)
and
RuCl2(Asph3)(p-CH3OC6H4CHO)2, (b) in dichloromethane–(TBA)PF6
solution vs. Ag/AgCl electrode.
complexes. Thus, this electrode couple is safely assigned to
RuII/RuIII oxidation as follows:
RuIICl2(Mph3)L2 + e− → [RuIICl2(Mph3)L2]−
(3)
RuIICl2(Mph3)L2 → [RuIIICl2(Mph3)L2]+ + e−
(4)
The reduction wave, which is expected to occur at much
more negative potential for the aldehyde ligand (L) is not
observed owing to solvent cut-off.
The one electron nature of the second irreversible elec-
trode couple (0.54–0.26 V) has been established by com-
paring its current height with those of the electrode couple
RuII/RuIII displayed by other analogous ruthenium(II) [30]
The potential of this couple was found not sensitive to the
nature of the substituent in the para-position of the aldehyde
ligand. The electrode potential (E◦) slightly increased with
increasing electron-withdrawing character. The plot of E◦
versus Hammett constant (2∂) [32] of the para-substituent
in the aldehyde was found linear.

126
M.S. El-Shahawi, A.F. Shoair / Spectrochimica Acta Part A 60 (2004) 121–127
The irreversible one electron metal-centered oxidation in
4. Conclusions
the potential range 0.52–0.98 V is safely assigned to the
RuIII/RuIV oxidation [30] as follows:
The weak O-donor aromatic aldehydes coordinate to
ruthenium(II) ions. The mono- and di-aldehydes tend to
impose penta-coordinated square-based pyramidal and octa-
hedral coordination sphere, respectively, at the metal centers
as a consequence of the reduced conformational flexibility
caused by the presence of Pph3 (or AsPh3). Substitution
of the hard O- or Br-donor atom in the para-position of
the benezaldehyde facilitates the redox properties. The cat-
alytic reactivity of the complex [RuCl2(Pph3)(C6H5CHO)2]
towards oxidation of benzyl alcohol was found higher
than that reported for N,N- and some N,O-neutral ligands
under similar conditions [33]. Further experiments are cur-
rently in progress to explore the mechanism and kinetics
of the catalytic activity of some other complexes of this
series.
[RuIIICl2(Mph3)L2]+ → [RuIVCl2(Mph3)L2]2+ + e− (5)
The oxidation potential of this couple correlates linearly
with the Hammett constant (∂) of p-substituent and the E◦
of this couple is sensitive to the nature of p-substituent
compared with RuII/RuIII. The data also confirm that the
electron-withdrawing groups attached to the aldehyde moi-
ety stabilize the RuII complexes while electron-donating
groups favor oxidation of RuIII.
3.3. Catalytic oxidation
The applications of ruthenium(II) complexes as an in-
expensive and easy to handle co-oxidant for selective
oxidation of alcohols are well known in the literature
[12,33]. No oxidation of benzyl alcohol to benzaldehyde
was achieved employing NMO only. Thus, the catalytic
oxidation of benzyl alcohol to phCHO by the precur-
sor catalyst [RuCl2(Pph3)(C6H5CHO)2] in the presence
of NMO (1:200 molar ratio of catalyst to substrate) at
room temperature in dry CH2Cl2 was carried out. Ex-
cellent experimental yield (70 5%) with good turnover
(140 2) of BA to BCHO was successfully achieved.
These results are better as compared with the data re-
ported for [RuII(bpy)2acac]PF6, [RuII(bpy)2Cl2]IO4,
[RuIICl2(Pph3)3] and RuII complexes of 1,1 -bis iso-
quinoline (BIQN) [32,33]. The formation of benzaldehyde
was confirmed by the formation of 2,4-dinitrophenyl-
hydrazone.
The mechanism of oxidation of benzyl alcohol to ben-
zaldehyde by the catalyst [RuCl2(Pph3)(C6H5CHO)2] in the
presence of the co-oxidant could be proceeded via the forma-
tion of ␮-peroxoruthenium(IV) intermediate species which
are capable to abstract hydrogen atom from the OH group in
benzyl alcohol (Scheme 1). This mechanism is similar to that
reported for [RuBr2(Pph3)(C6H5CHO)2], [RuIICl2(Pph3)3],
[RuII(terpy)(BIQN)Cl]ClO4 and [R(trPy)(R2dppi)O]2+
complexes [34,35], where terpy=2,2,6,2-terpyridine;
H2dppi=3,6-bis(pyrid-2-yl) pyridazine and trpy=2,22-
terpyridine The reaction velocity of this type of reactions
always shows first-order kinetics in terms of the amount of
unconsumed benzyl alcohol [36].
Acknowledgements
The author M.S. El-Shahawi wishes to thank Dr E.P.
Achterberg, Plymouth University, UK and the British
Council, for time and support in the form of chemicals
1
(RuCl3·3H2O and o-phthaldehyde) and H NMR and ele-
mental analysis.
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