Electrochemistry of Platinum(II) Porphyrins: Effect of Substituents and π-Extension on Redox Potentials and Site of Electron Transfer

Fourteen platinum(II) porphyrins with different pi-conjugated macrocycles and different electron-donating or electron-withdrawing substituents were investigated as to their electrochemical and spectroscopic properties in nonaqueous media. Eight compounds have the formula (Ar4P)Pt-II, where Ar4P = the dianion of a tetraarylporphyrin, while six have pi-extented macrocycles with four beta,beta'-fused benzo or naphtho groups and are represented as (TBP)Pt-II and (TNP)Pt-II where TBP and TNP are the dianions of tetrabenzoporphyrin and tetranaphthoporphyrin, respectively. Each Pt(II) porphyrin undergoes two reversible one-electron reductions and one to three reversible one-electron oxidations in nonaqueous media. These reactions were characterized by cyclic voltammetry, UV-visible thin-layer spectroelectrochemistry and in some cases by ESR spectroscopy. The two reductions invariably occur at the conjugated pi-ring system to yield relatively stable Pt(II) pi-anion radicals and dianions. The first oxidation leads to a stable pi-cation radical for each investigated porphyrin; but in the case of tetraarylporphyrins containing electron-withdrawing substituents, the product of the second oxidation may undergo an internal electron transfer to give a Pt(IV) porphyrin with an unoxidized macrocycle. The effects of macrocycle structure on UV-visible spectra, oxidation/reduction potentials, and site of electron transfer are discussed.