Multielectron chemistry of zinc porphyrinogen: A ligand-based platform for two-electron mixed valency

The synthesis, electronic structure, and oxidation-reduction chemistry of a homologous series of Zn(II) porphyrinogens are presented. The fully reduced member of the series, [LZn](2-), was prepared in two steps from pyrrole and acetone. The compound undergoes consecutive two-electron, ligand-based, oxidations at +0.21 and +0.63 V vs NHE to yield [(LZn)-Zn-Delta] and [(LZn)-Zn-DeltaDelta](2+), which also have been independently prepared by chemical means. X-ray diffraction analysis of the redox intermediary, [(LZn)-Zn-Delta], shows that the partly oxidized macrocycle is composed of a methylene-bridged dipyrrole that is doubly strapped to a two-electron oxidized dipyrrole bridged by a cyclopropane ring (L-Delta). The localization of two hole equivalents on the oxidized side of the porphyrinogen framework is consistent with a two-electron mixed valency formulation for the [(LZn)-Zn-Delta] species. Electronic structure calculations and electronic spectroscopy support this formalism. Density functional theory computations identify the HOMO to be localized on the reduced half of the macrocycle and the LUMO to be localized on its oxidized half. As implicated by the energy level diagram, the lowest energy transition in the absorption spectrum of [(LZn)-Zn-Delta] exhibits charge-transfer character. Taken together, these results establish the viability of using a ligand framework as a two- and four-electron/hole reservoir in the design of multielectron redox schemes.