Crystal structure of molecular complexes of zinc(II) tetraphenylporphyrin with pyridine and quinoline N-oxides

Abstract

The structure of molecular complexes of zinc(II) tetraphenylporphyrin (Zn-TPP) with 4-methoxy- and trans-2-(4-dimethylaminostyryl)pyridine and 4-methoxy-, 4-nitro, and 4-chloroquinoline N-oxides was studied by X-ray diffraction analysis. Heteroaromatic N-oxides, like aniline derivatives, are coordinated at an angle of 26°–32° to the porphyrin ring plane, whereas pyridine ligands are coordinated almost perpendicularly to this plane. It is suggested that just such coordination peculiarities are responsible for the difference in the thermodynamic parameters of Zn-TPP with pyridine-based ligands (linear relationship between ΔH0 and ΔS0, high heat effect of the process), and N-oxides, aniline derivatives (low ΔH0 values, quasi-isenthalpic processes).