Synthesis, characterisation and antibacterial activity of novel Ga(III) polypyridyl catecholate complexes.

Ga(III) polypyridyl catecholate complexes of type [Ga(bipy)2(O,O)](NO3) or [Ga(phen)2(O,O)](NO3) respectively were readily synthesised on reaction of Ga(NO3)3 in methanol with 1 equivalent of catecholate ligand (2,3-DHBA, 3,4-DHBA, 2,3,4-THBA or CafA) and then 2 equivalents of either bipy or phen. The complexes were characterised in full including by X-ray crystallography, which established that the catecholate ligands coordinate the Ga(III) centres in a bidentate manner via the two deprotonated hydroxy groups. All Ga(III) complexes exhibited good in vitro antibacterial activity against the Gram-negative pathogenic bacteria Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. The complexes were inactive against the Gram-positive pathogenic bacteria Staphylococcus aureus including against a methicillin-resistant Staphylococcus aureus strain (MRSA). [Ga(bipy)2(2,3-DHBA-2H)](NO3)·1.5H2O (1) was shown to be non toxic in vivo in larvae of Galleria mellonella at doses up to 2000 µg mL-1 and to offer protection at doses of 100 and 250 µg mL-1 at 48 and 96 h to larvae infected with P. aeruginosa.