NMR spectra of the porphyrins. 30—Calibration and application of a ring current model for cobalt(III) meso‐tetraphenylporphyrin (CoTPP) complexes

The geometry of metalloporphyrin‐ligand complexes [mono‐(1:1) and bis‐(1:2)] produced by cobalt(III) meso ‐tetraphenylporphyrin (CoTPP) with amine ligands has been investigated using the diamagnetic ring current shifts of the porphyrin on the ligand nuclei. The observed ring current shifts are corrected for complexation effects by the use of cobalt(III) dimethylglyoxime (DMG) complexes. Significant changes in ligand chemical shifts are observed on formation of a DMG complex, although saturated ligands only show changes at points close to the site of complexation. Crystallographic data and corrected ring current shifts for pyridine and N 1 ‐methylimidazole are used to parameterize a double‐dipole model of the CoTPP ring current. For 4‐methylpiperidine a geometry calculated from corrected ring current shifts is in reasonable agreement with that observed in the solid state. The geometry calculated for a cyclohexylamine CoTPP complex can be rationalized in terms of a minimization of steric interactions with the porphyrin. Corrected 13 C ring current shifts are still in poor agreement with calculated values, although there is a considerable improvement compared with uncorrected values.