Design, Syntheses, Physicochemical and Biological Properties of Rhenium(I) tricarbonyl complexes of isatin derivative

AA Ikotun1, MP Coogan2,3, AA Owoseni4, N Bhuvanesh5, GO Egharevba6. 1Bowen University, Iwo, Osun State, Nigeria, 2Cardiff University, Cardiff, Wales, UK, 3Lancaster University, Bailrigg, Lancaster, UK, 4Bowen University, Iwo, Osun State, Nigeria, 5Texas A & M University, College Station, Texas, USA, 6Obafemi Awolowo University, Ile-ife, Osun State, Nigeria

One emerging area which has only recently begun to receive increasing attention due to cell imaging is the medicinal application of luminescent d6 transition metal complexes (1). And only recently, has Coogan and co-workers (2, 3), turned the direction of their research group to developing d6 transition organometallic based agents which could target specific biological entities. Using the method of Coogan et al (2), with slight modifications, the reactions of the Schiff base of isatin and aniline (L1; C14H10N2O; 41 %) with Re(CO)5X (X = Cl; a and Br; b) in hot toluene yielded the complexes [Re(C14H10N2O)(CO)3X] (2a, 2b; 96 % and 90 % respectively). Refluxing in dry toluene under nitrogen and recrystallization afforded the neutral complex crystallizing as a solvate compound [Re(C14H10N2O)(CO)3Br].C2H5OH (3; 72 %) in ethanol. Attempts to prepare the cationic pyridine derivative of 2b, gave the intermediate compound 4 and further attempts at crystallization in pyridine rather removed the ligand giving a new compound 5; [Re(C6H5N)3(CO)3]+BF4 -. Crystallographic analyses of compound 5 showed pyridine assuming the coordination positions of nitrogen and oxygen donor atoms previously from L1 to Re(I). Characterization of the prepared compounds was done by Fourier Transform Infra-red Spectroscopy (FTTIR), 1H and 13C NMR, Mass Spectra, Electronic spectra, Magnetic susceptibility measurements, melting point determinations and X-ray analyses. All synthesized compounds were screened for in vitro antibacterial activities against three Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis and Haemolytic Staphylococcus aureus) and three Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli and Klebsiella sp.) (4), (5). The antifungal activities of the compounds were evaluated against three fungi (Aspergillus niger, Trichoderma viride and Penicillium citrinum) (4), (5). The rhenium(I) tricarbonyl chloride complex (2a) showed a broad-spectrum activity, much better than tetracycline (standard clinical antibiotic) against the tested bacteria. All other complexes showed selective activities in a distinct manner from the ligand and mostly with higher zones of inhibition against tested organisms than tetracycline. Complex 2b had the minimum inhibitory concentration (MIC) value of 1.25 μg/ml against Pseudomonas aeruginosa. The antifungal studies revealed that L1 and its complexes were inactive against the tested fungi. This report serves as the first to provide profound information on the coordination of isatin derived Schiff bases to such d6 transition metals like Re(I), their physicochemical properties, as well as the medicinal implication of such metal complexes.

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(3) Fernandez-Moreira V. (2010). Chem. Commun., 46 : 186-202.

(4) Cheesbrough M. (2002). In: Cheesbrough M (ed.). Laboratory practice in tropical countries. Cambridge Edition, pp 63-87.

(5) Emeruwa K.C. (1982). J. Nat. Prod. 45: 123-127.