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.