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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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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