Water‐Soluble Cobalt(III)–Polypyridyl Complexes: Synthesis, Structures and Properties

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(NO 3 ) 6 ) complexes. To further showcase the synthetic utility of the method, two [Co(N–N–N) 2 ] (Ce(NO 3 ) 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 1 H and 13 C{ 1 H} 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(NO 3 ) 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.