All Four Atropisomers of Iron Tetra(-,,-trimethylanilinium)porphyrin in Both the Ferric and Ferrous States.

Electrostatic effects are key to many biological and (electro)chemical transformations, especially those that involve charged species. The position and orientation of the electric field with respect to the molecules undergoing charge rearrangement are often crucial to the progress of the reaction. Recently, several molecular (electro)catalysts have been designed to contain spatially positioned charged groups that can engage in specific intramolecular electrostatic interactions. For instance, iron complexes of the tetra(-,,-trimethylanilinium)porphyrin ligand, which has four cationic groups, have been used to great effect for both CO and O reduction. Because of the substitution pattern on the porphyrin ligand, there are four possible atropisomers-such as the αβαβ isomer with trimethylanilinium groups on alternating faces of the porphyrin-and thus four unique electrostatic environments. This study details the synthesis and characterization (H NMR spectroscopy, single crystal X-ray diffraction, and cyclic voltammetry) of these four metalloporphyrin isomers in both the ferric (Fe) and ferrous (Fe) forms by using a synthetic route that preserves atropisomeric purity. The atropisomers are different in some respects but show remarkable similarities in others, such as their reduction potentials. This study also shows that the widely-cited literature method used previously to prepare the molecular electrocatalyst for CO and O reduction yields a mixture of atropisomers rather than a single one, as was previously assumed. These results identify the ways in which intra- and intermolecular electrostatic effects affect both solution and solid-state properties as well underscoring the challenges associated with preparing metalloporphyrins with high atropisomeric purity.