Long-range metal–metal coupling in transition-metal 5,10,15,20-tetraferrocenylporphyrins

Abstract

Redox properties of several transition-metal MTFcP complexes (TFcP2− = 5,10,15,20-tetraferrocenylporphyrin, M = Co2+, Ni2+, Cu2+, and Zn2+) were investigated using electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation approaches. Electrochemical experiments conducted in a low-polarity solvent using a non-coordinating electrolyte are crucial for the sequential oxidation of ferrocene substituents along with porphyrin-based single-electron oxidation and reduction processes. The first ferrocene oxidation process in all MTFcP complexes is separated by at least 140 mV from the next three ferrocene based oxidations. The second, third, and fourth redox processes in the ferrocene region are more closely spaced, with the largest separation observed in CuTFcP and CoTFcP complexes. Mixed-valence compounds were observed and characterized by spectroelectrochemical and chemical oxidation approaches. In all cases, intervalence charge transfer (IVCT) bands were detected confirming the existence of the iron-based mixed-valence [MTFcP]n+ (n = 1–3) species and suggesting long-range metal–metal coupling in the target systems. The resulting data from the mixed-valence [MTFcP]n+ (n = 1–3) complexes matched very closely to the metal-free poly(ferrocenyl)porphyrins previously reported and were assigned as Robin and Day Class II mixed-valence compounds. In the case of CoTFcP, the selective oxidation of a central metal was also demonstrated in the presence of the strong ligand-field anions using oxygen as an oxidant.