Synthesis, Structure And Antimicrobial Properties Of New Silver(I)- And Palladium(II)-N-Heterocyclic Carbene Complexes Derived From [BENZ]Imidazol-2-Ylidenes
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Date
2015-05
Authors
Asekunowo, Patrick O.
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Abstract
This thesis reports the synthesis, structure and antimicrobial properties of various
silver(I)- and palladium(II)-NHC complexes bearing [benz]imidazolin-2-ylidene
ligands. The findings of the research are presented in four chapters. Two types of
[benz]imidazolium salts with different substituents were prepared. The first type is
mono-[benz]imidazolium salts (1-15) with ether, nitrile and allyl functionalities.
They are conveniently tuned, using different alkyl/alkyl aryl substituents at the 3-
position of the [benz]imidazolium ring. While the second type is bis-
[benz]imidazolium salts (16-25) derived from propyl, pyridine and para-xylene
linked systems. The complexes were prepared by in situ reaction of Ag2O with the
corresponding [benz]imidazolium salts by varying the metal to salt molar ratio,
resulting in the isolation of twenty five new Ag(I)-NHC complexes (26-50). X-ray
crystal structures for fifteen of these complexes were determined showing a variation
in the structural motifs across the series with mononuclear and binuclear species
being generated. The salts and their complexes were characterized by spectroscopic
methods (FTIR, 1H and 13C NMR), elemental analysis (CHN) and single crystal Xray
diffraction techniques. The effect of substitutions on antibacterial activity of
these compounds has been investigated. Compounds were screened for their
antibacterial efficacy against Staphylococcus aureus (ATCC 12600) as a Gram
positive bacterium and Escherichia coli (ATCC 25922) as a Gram negative
bacterium. All the Ag(I)-NHC complexes, in general, showed good antibacterial
activities in the range 10.0±0.5-32.9±2 mm for the zone of bacterial growth
inhibition and the MIC values of the complexes are in the range of 6.25-100 μg/ mL. However, their corresponding salts were essentially inactive against both strains of
bacteria. It was observed that the derivatives of the Ag(I)-NHC complexes with
longer alkyl chain were more bioactive. Furthermore the results also suggest that
binuclear Ag(I)-NHC complexes have relatively better antibacterial potential
compared with their mononuclear counterparts. In order to gain preliminary insights
into their actual mode of action(s), circular plasmid pTS414 DNA/RNA was exposed
to gel electrophoresis and it was found that the mononuclear Ag(I)-NHC (31, 34, 37
and 38) and binuclear Ag(I)-NHC (41, 44, 45, 46 and 48) are extremely efficient in
promoting the cleavage/degradation of RNA and DNA in the absence of coreactants
i.e., in the absence of H2O2 and added reductant.
The synthesis of CNC pincer Pd(II)-NHC complexes has been achieved by
the transmetallation (NHC transfer) reaction between Pd(cod)Cl2 and the
corresponding Ag(I)-NHC complexes derived from bis-[benz]imidazolium salts. Xray
studies revealed mononuclear structures for three of the complexes (52, 54 and
55) in solid state. The prepared Pd(II)-NHC complexes were tested against the E. coli
and S. aureus. The examined complexes showed an activity against the mentioned
bacteria, but much lower than that of the Ag(I)-NHC complexes. The results of the
preliminary nuclease activities demonstrate that Pd(II)-NHC (51, 52, 54 and 55)
complexes are efficient in the cleavage of nucleic acids via non-oxidative
mechanism.
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Chemistry