Highly Nonplanar, Electron Deficient, N-Substituted tetra-Oxocyclohexadienylidene Porphyrinogens:  Structural, Computational, and Electrochemical Investigations

The structures and electrochemistry of N-benzylated meso-tetrakis (3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogens have been investigated. Structural determinations reveal the isomeric identity of the products obtained from the N-alkylation of the parent meso-tetra (oxo-cyclohexadienylidene) porphyrinogen. The compounds are subject to increased macrocyclic deformations upon increasing N-substitution culminating in the tetra-N-benzyl derivative, which has a buckling superimposed on the already highly puckered macrocycle. The electrochemical analyses emphasize the electron deficiency of the N-benzylated meso-tetra(oxo-cyclohexadienylidene) porphyrinogens and indicate that they can be considered as quinones conjugated via the unsaturated tetrapyrrolic macrocycle. The N-benzylated compounds studied form stable and well-defined π-cation radical and π-anion radical species because of their highly conjugated nature. Ab initio molecular orbital calculations at the B3LYP/3-21G(*) level confirmed the high degree of conjugation between tetrapyrrole and meso substituents and also gave good agreement between calculated and experimentally determined HOMO−LUMO band gap energies.