A. Manimaran, V. Chinnusamy and C. Jayabalakrishnan
yield a high valency ruthenium-oxo species, capable of oxygen Conclusion
atom transfer to alcohols. This was further supported by
spectral changes that occur by addition of NMO, H2O2 and
in presence of dioxygen to a dichloromethane solution of the
ruthenium(III) complexes and it was monitored using a UV–vis
spectrophotometer and the appearance of the peak at 390 nm.
The relatively higher product yield obtained for the oxidation of
cinnamyl alcohol is due to the presence of a more acidic α-CH unit
in cinnamyl alcohol. Further, the oxidation of cinnamyl alcohol to
cinnamaldehydetakesplacewithretentionofaC Cdoublebond,
which is an important characteristic of a ruthenium–NMO or H2O2
or dioxygen system. The primary and secondary aliphatic alcohols
were effectively oxidized into their corresponding aldehydes
and ketones with high yield. These reactions were also studied
without using catalysts to determine the effect of catalyst in
oxidation of primary and secondary alcohols to its carbonyl
compounds. The amount of reaction products was compared with
percentage conversion of these alcohols. The oxidation products
of hydrozones showed separation peaks corresponding to the
alcohols in the FT-IR spectrometer; hence, the carboxylic acid
formed in the oxidation of alcohols was discarded.
In conclusion, we have synthesized and characterized a series of
ruthenium(III) Schiff base complexes of N2O2 donor ligands and
these complexes were developed as a new and efficient catalyst
for the C–C coupling and oxidation reactions of primary and
secondary alcohols into their corresponding carbonyl compounds
in the presence of NMO/H2O2 co-oxidants and in O2 atmosphere
at ambient temperature. It was shown that complexes of the
type [RuX(EPh3)(L)] (where, X D Cl/Br; E D P/As; L D N2O2
donors of Schiff bas ligands) can be turned from moderate to
good catalysts with proper modification of their ligands under
the usually applied concentrations (1 : 100 catatlyst : substrate).
Furthermore, the organometallic ruthenium systems allow easy
ligand modifications, which may lead to further improvements of
their catalytic performance. Superior selectivity combined with a
high activity and easy synthetic modifications are the advantages
of the ruthenium catalysts described here in comparison to
previously described coupling and oxidation catalysts. From
antibacterial activity study, it was found that the activity of
the ruthenium(III) Schiff base complexes almost reaches the
effectiveness of the conventional bacteriocide standards such as
Amoxycilin, ampicillin, erythromycin and streptomycin, for which
the concentrations used in this study were 0.5, 1.0, 1.5, 2.0 and
2.5%.
Antibacterial Activities
The Schiff base ligands and their ruthenium(III) complexes were
tested in vitro to access their growth inhibitory activity against
K.pneumoniae,Shigella sp., M.luteus, E. coliand S.typhi by the Kirby
Bauer method.[46] The test organisms were grown on nutrient
agar medium in Petri plates. The compounds to be tested were
dissolved in DMSO and soaked on a filter paper disk of 5 mm
diameter and 1 mm thickness. The concentrations used in this
study were 0.5, 1.0, 1.5, 2.0 and 2.5% (Table 6). The disks were
placed on the previously seeded plates and incubated at 37 ŽC for
24 h. Amoxycilin, ampicillin, erythromycin and streptomycin were
used as standards with different concentrations. The variation in
the effectiveness of the different compounds against different
organisms depends on their impermeability of the microbial cells
or on the difference in the ribosome of the microbial cells.[47] In
general, the complexes are more active than those of the parent
ligands and ruthenium(III) starting complexes. The increase in
the antibacterial activity of the metal chelates with increase in
concentration is due to the effect of metal ion on normal cell
process. Such an increase in activity of the metal chelates can be
explained on the basis of Overtone’s concept[48] and chelation
theory.[49,50] According to Overtone’s concept of cell permeability,
the lipid membrane that surrounds the cell favors the passage
of only rapid soluble materials, due to which liposolubility is
an important factor that controls the antimicrobial activity. On
chelation, the polarity of the metal ion will be reduced to a greater
extent due to the overlap of the ligand orbital and partial sharing
of the positive charge of the metal ion with donor groups. Further,
it increases the delocalization of π-electrons over the whole
chelate ring and enhances the liphophilicity, which enhances
the penetration of the complexes. This increased lipophilicity
enhances the penetration of the complexes into lipid membrane
and blocks the metal binding sites on enzymes of microorganism.
These complexes also disturb the respiration process of the cell
and thus block the synthesis of proteins, which restricts the further
growth of the organism. Furthermore, the mode of action of the
complexes may involve formation of a hydrogen bond through
an azomethine group with the active centers of cell constituents,
resulting in interference with the normal cell process.[51,52]
Acknowledgment
WearethankfultoNMRResearchCentre,IndianInstituteofScience
(IISc), Bangalore, India for providing 13Cf Hg-NMR spectra.
1
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Appl. Organometal. Chem. 2011, 25, 87–97
Manimaran, Arumugam
