Atropisomerism of Zinc Tetrakis(o-cyanophenyl)porphyrins. The Crystal Structure of the αβαβ-Isomer and the Atropisomerization Rates

The structure of one of the four atropisomers of tetrakis(o-cyanophenyl)porphyrin has been determined as the zinc complex by X-ray crystallographic methods. The five-coordinate zinc porphyrin with pyridine as the axial ligand also has a pyridine solvate in the crystal lattice. The conformation of the o-cyanophenyl rings is αβαβ as predicted previously. The two nitrogen atoms of the CN groups on the same side of the coordinated pyridine have close intramolecular contacts to pyridine at the van der Waals distance. However, the structural parameters, bond distances and angles of this molecule do not indicate distinct steric effects of the CN groups. Thermal atropisomerization among the four zinc porphyrin atropisomers occurs in solution. The quantitative composition of the atropisomers at equilibrium and the rates of the isomerization reaction have been determined with aliquots separated by TLC. A nonstatistical equilibrium distribution of the atropisomers suggests some mutual interactions among the CN group...