Abstract
A family of [Co(N-N)3]3+ complexes (where N-N = 2,2 '-bipyridine or 1,10-phenanthroline derivatives) was developed as earth-abundant multifunctional antimicrobial photoinitiators for the generation of biocompatible hydrogels. A facile, robust assembly-followed-by-oxidation approach was used to generate eight [Co(N-N)3](Ce(NO3)6) complexes. To further showcase the synthetic utility of the method, two [Co(N-N-N)2] (Ce(NO3)6) complexes (where N-N-N = tridentate polypyridyl ligand) were also synthesised. All the complexes studied proved to be water-soluble. The Co(III) complexes were characterised using 1H and 13C{1H} nuclear magnetic resonance spectroscopy, elemental analysis and electrospray ionisation mass spectrometry. Additionally, all ten complexes were structurally characterised using X-ray crystallography. The electronic properties of the family of complexes were examined using UV-Vis and Raman spectroscopy, density functional theory calculations and cyclic voltammetry and could be tuned based on the ligand substituents. Efforts to use the [Co(N-N)3](Ce(NO3)6) complexes as photoinitiators for the generation of biocompatible hydrogels from either gelatin methacrylate (Gel-MA) or polyvinyl alcohol methacrylate (PVA-MA) were unsuccessful. However, [Co(dmobipy)3]3+ (where dmobipy = 4,4 '-dimethoxy-2,2 '-bipyridine) was found to promote gel formation from a gelatin-norbornene (GelNB) macromer under step-growth thiol-ene crosslinking conditions. The antibacterial properties of the Co(III) complexes were also examined. Several of the more hydrophobic complexes were active against a range of Gram-positive and Gram-negative bacteria.