Redox induced colour changes between red-violet and blue in hetero-metal complexes of the type [Co^II(4′-ferrocenyl-2,2′;6′2′′-terpyridine)₂]X₂ (X = counter anion)

The hetero-metal complexes [Co^II(Fctpy)₂]X₂ (Fctpy = 4′-ferrocenyl-2,2′;6′2′′-terpyridine, X = PF₆ (1), BF₄ (2), ClO₄ (3), BPh₄ (4)) were prepared and their structures and properties were investigated by single crystal X-ray structural analysis, cyclic voltammetry, Mössbauer spectroscopy, UV-vis spectroscopy measurements and DFT calculations. These compounds show reversible redox behaviour in the solid state and switching of the colour between red-violet and blue in response to the change of the oxidation state of the cobalt ion centres. [Co^II(Fctpy)₂](PF₆)₂ (1) and [Co^III(Fctpy)₂](PF₆)₃ (5) were isolated as the red and blue crystalline products, and the crystal structures were determined. From the results of UV-vis spectroscopy and the DFT calculations, we assigned the respective colours as resulting from MLCT involving the ferrocenyl substituents, with the shift in the wavelength of the MLCT reflecting the respective HOMO/LUMO stabilizations induced by the change in the oxidation state of the cobalt centres. Furthermore, these compounds 1-4 also showed counter anion-dependent spin crossover behaviours.