Exploration and characterisation of novel bronze patinas derived from simple coordination complexes

Abstract

The chemical reactions of aqueous solutions containing simple transition-metal salts and bidentate nitrogen ligands on bronze surfaces results in a series of brightly coloured patinas. Chemically and physically robust patinas involve a chemical oxidation of the copper atoms within the bronze surface, either by an applied ferric salt or atmospheric dioxygen. The combination of Fe(NO₃)₃·9H₂O and 2,2′-bipyridine produces a coloured patina that is either red or blue, depending on whether oxygen was present during application, due to the presence of [Fe(bpy)₃]²⁺ or [Fe(bpy)₃)]³⁺, respectively. Application of pyrazine produces a bright orange patina, due to the formation of Cu(I) coordination polymers on the bronze surface. Application of Fe(NO₃)₃·9H₂O and tetramethylethylenediamine (TMEDA) yields a forest green coloration, believed to be due to a bimetallic copper–iron complex, while TMEDA alone affords the sapphire blue of [Cu(TMEDA)₂]²⁺. These patinas were characterized with ESI-TOF MS, UV-Vis spectrophotometry, and IR spectrophotometry, to determine the molecular basis for the patina chromophores.