Electrochemistry of Tri‐substituted Porphyrins with β‐Appended Ethyl Acetoacetate and Acetylacetone in Neutral and Basic Nonaqueous Solvents

Abstract

AbstractThe electrochemistry of β‐substituted tetraphenylporphyrins with a single ethyl acetoacetate (EAA), acetylacetone (acac), or ethyl acetate (EA) group and two antipodal H, Br or Ph groups was investigated in CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate (TBAP) before and after addition of base to solution in the form of tetrabutylammonium hydroxide (TBAOH). The tri‐substituted porphyrins in their neutral form are represented as MTPP(EAA)X2, MTPP(acac)X2 and MTPP(EA)X2, where TPP is the dianion of tetraphenylporphyrin, M=H2, NiII, CuII or ZnII and X=H, Br or Ph. The singly reduced porphyrins are relatively stable on the cyclic voltammetry timescale, but this is not the case for the doubly reduced EAA and acac derivatives, which are highly reactive at room temperature, leading to a porphyrin product assigned as containing a deprotonated EAA− or acac− (enolate) substituent. Enolate‐appended porphyrins were also generated in‐situ by the addition of TBAOH, resulting in mono‐anionic porphyrins that are harder to reduce by 110–160 mV as compared to the neutral compounds with the same β‐substituents, suggesting a high degree of interaction between the deprotonated EAA− or acac− substituent and the porphyrin π‐ring system. The chemically or electrochemically generated enolate‐appended porphyrin could also be irreversibly oxidized by one electron at peak potentials between 0.10 and 0.30 V; the exact value depending upon solvent and type of β‐diketo substituent.