Synthesis, characterisation and biological studies of copper(I) and copper(II) pyridinyl Schiff base complexes.

Abstract

Coordination complexes have numerous applications ranging from industrial processes to pharmaceuticals. Schiff base ligands, along with copper(I) and copper(II) metal complexes have been investigated and reported in the literature for their potential as antibacterial agents. This thesis reports the synthesis, characterisation of Schiff base ligands and their corresponding copper(I) and copper(II) complexes for antibacterial application investigations. A total of fourteen Schiff base ligand derivatives (L1 – L14) were synthesised, purified and characterised. They were analysed by nuclear magnetic resonance (1H-NMR & 13C-NMR) spectroscopy, UV-Vis spectroscopy, infrared (IR) spectroscopy and also melting point determinations. L1 – L14 were synthesised via the solvent-free grinding technique, whilst L3 and L11 – L14 were also synthesised by the microwave-assisted heating for comparative purposes. It was found that both the solvent-free grinding and microwave-assisted techniques were efficient, timeous, clean and high yielding, (83 – 98%) and (85 – 91%), respectively. L1 – L10 were used in the synthesis (two-stage) of fifteen copper complexes in reactions done under an inert atmosphere. Of the fifteen complexes, 1 – 7 were copper(I) pyridinyl Schiff base complexes, while 8 – 15 were copper(II) Schiff base complexes. Additionally, 8 – 15 were also prepared using a one-pot approach in which the respective ligand precursors and the copper(I) or copper(II) metal salts were mixed together. Both techniques resulted in high yields, (84 – 94%) and (77 – 89%) for two-stage and one-pot reactions, respectively. The two-stage reactions showed to be efficient with higher yields and is suitable for both copper(I) and copper(II) Schiff base synthesis by allowing controlled reaction conditions. The Schiff base complexes were analysed by nuclear magnetic resonance (1H-NMR & 13C-NMR), UV-Vis spectroscopy, infrared (IR) spectroscopy, elemental analysis (EA), electrospray ionisation mass spectroscopy (ESI-MS) and their melting points were determined. Finally, the single-crystal structures of derivatives of 10 and 12, (10py and 12py) were solved using the single-crystal X-ray diffraction (SC-XRD). The structures of 10py and 12py adopt the distorted trigonal bipyramidal geometry around Cu(II) centre. All complexes (1 – 15) were tested for their antimicrobial potency against four gram-negative, i.e., Escherichia coli ATCC 25922 (E. coli), Pseudomonas aeruginosa ATCC 27853 (P. aeruginosa), Klebsiella pneumoniae, ATCC 31488 (K. pneumonia) Salmonella typhimurium ATCC 14028 (S. typhimurium); two gram-positive, i.e., Staphylococcus aureus ATCC 25923 (S. aureus) and Methicillin-resistant Staphylococcus aureus ATCC 700699 (MRSA) bacteria. The copper(I) complexes (1 – 7) were inactive against all tested bacteria, while copper(II) complexes (8 – 15) showed bactericidal activities but did not demonstrate a wide spectrum of activities. Notably, 10, 11 and 12 were appreciably active against five of six bacterial strains, except for the Staphylococcus aureus bacterium. Some of the Cu(II) Schiff base complexes displayed lower minimum inhibitory concentration (MIC) values in comparison with the standard reference, ciprofloxacin.