R. Ramesh / Inorganic Chemistry Communications 7 (2004) 274–276
275
features characteristic of magnetic anisotropy and the
presence of three ÔgÕ values ðgx ¼ gy ¼ gzÞ reveals
rhombic distortion in these complexes (average hgi2.02–
2.16). Cyclic voltammogram of most of the complexes
(2 ꢂ 10ꢁ3) exhibits a reversible oxidation (DEp ¼ 60–80
mV) and irreversible reduction peaks at the scan rate of
100 mV Sꢁ1. The oxidation and reduction of each
complex are characterized by well defined waves with Ef
values in the range +0.70 to +0.75 and )0.34 to )0.74 V
vs. Ag/AgCl, respectively. An octahedral geometry has
been tentatively proposed for all the complexes as
shown in Fig. 1.
Catalytic oxidation of primary alcohols by some of
the ruthenium(III) Schiff base complexes was carried out
in CH2Cl2 in the presence of NMO. All the complexes
oxidize primary alcohols to their corresponding alde-
hydes with high yield and the results are summarized in
Table 1. The aldehydes formed after 3 h of reflux were
quantified as their 2,4-dinitrophenylhydrazone deriva-
tives and the oxidation did not take place in the absence
of catalyst. Results of the present investigation suggest
that the source of oxygen transferred in the formation of
aldehydes from primary alcohols via Ru(v)-oxo com-
plex. This was further supported by spectral changes
that occur by addition of NMO to a dichloromethane
solution of the Ru(III) complexes. The appearance of
the peak at 388 nm is attributed to the formation of
high-valent Ru(v)-oxo species which is in conformity
with other oxo-ruthenium(v) complexes [23,24]. Fur-
thermore, support in favor of the formation of such
species came from the IR spectrum of solid mass (ob-
tained by evaporation of resultant solution to dryness),
which showed a band at 862 cmꢁ1, characteristic of
ruthenium(v)-oxo species [23,24].
Unsaturated alcohol like cinnamyl alcohol is selec-
tively oxidized at the alcoholic group with high yield
without competing double bond. This is an important
characteristic of ruthenium/NMO system. The relatively
higher product yield obtained for oxidation of cinnamyl
alcohol than benzyl alcohol is due to the fact that a-CH
unit of cinnamyl alcohol is more acidic than benzyl al-
cohol. The catalytic efficiency of these ruthenium(III)
Schiff base complexes in the case of oxidation of benzyl
and cinnamyl alcohol is more when compared to the
earlier report [11,25] on similar ruthenium complexes as
catalysts in the presence of NMO/t-BuOOH. Hence, it is
relevant from the cyclic voltammetric data that oxida-
tion effected by catalysts is likely to occur via ruthenium
high oxidation states, which should be easily accessible
through chemical oxidation with common co-oxidants
[11,18].
Acknowledgements
The author thanks Council of Scientific and Indus-
trial Research (CSIR) New Delhi for the financial sup-
port in the form of research grant (Ref. No. 1(1725/02/
EMR-II).
W
X
N
N
Ru
O
O
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Fig. 1. Ruthenium(III) Schiff base complexes.
Table 1
Catalytic oxidation data of alcohols by ruthenium(III) complexes/
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Complex
Substrate
Product Yieldb (%)
[RuCl(PPh3)
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Benzyl alcohol
A
52.7
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Cinnamyl alcohol
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72.3
61.5
[RuCl(AsPh3)
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Cinnamyl alcohol
Benzyl alcohol
A
A
71.9
58.3
[RuBr(AsPh3)
(Naphpn)]
Cinnamyl alcohol
A
81.1
A ¼ corresponding aldehyde.
a Alcohol (1 mmol); NMO (3 mmol); catalyst (0.01 mmol).
b Yield based on substrate.
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