Metallation effects on the thermal interconversion of atropisomers of di(orthomethylarene)-substituted porphyrins

Abstract

A new series of meso-substituted diaryl free-base and metalloporphyrins have been prepared. Each arene has been substituted with both a methyl group in the ortho position and a formyl group in the meta position. Rotation of the arene units is prevented at room temperature due to the steric restrictions imposed by the flanking methyl groups at the porphyrin β-pyrrolic positions on the methyl groups at the ortho position on the meso-substituted arene unit. This allowed the αα and αβ atropisomers of this porphyrin to be separated and characterised. X-Ray crystallographic determination of the structure of the free-base porphyrin revealed a very flat porphyrin core. Metallation resulted in the isolation and characterisation of the nickel, zinc and copper derivatives. The assignments of the αα and αβ isomers are confirmed by X-ray crystallographic determination of the structures of the Cu(II) analogues. The copper αα structure exhibits a very twisted porphyrin core, the copper αβ structure is also distorted, but to a lesser degree. The activation energy for rotation has been calculated for each of the 2H, Ni and Zn derivatives. The energy required to rotate the arene ring increases in the order Ni < Zn ∼ 2H. No significant difference in the free energy of rotation was observed between experiments carried out with the αα and αβ isomers.