15N NMR chemical shifts for the identification of dipyrrolic structures

A variety of dipyrromethanes and dipyrromethenes have been prepared, and their N-15 NMR chemical shifts have been measured by two-dimensional correlation to H-1 NMR signals. The nitrogen atoms in five examples of dipyrromethanes consistently exhibit chemical shifts a-round -231 ppm, relative to nitromethane. Seven examples of hydrobromide salts of meso-unsubstituted dipyrromethenes consistently display N-15 chemical shifts around -210 ppm, while their corresponding zinc(II) complexes exhibit chemical shifts around - 170 ppm. The presence of electron-withdrawing substituents on one of the pyrrolic rings of dipyrromethenes affects the chemical shifts of both of the nitrogen nuclei in the molecule. Boron difluoride complexes of meso-unsubstituted dipyrromethenes display N-15 chemical shifts around - 190 ppm. Two examples of free-base dipyrromethenes bearing substituents at the meso-position exhibit N-15 chemical shifts at approximately - 156 ppm, and for the zinc complexes of these compounds at - 162 ppm. One-bond nitrogen-hydrogen coupling constants, when measurable, were consistently in the range of -96 Hz. Since the measured N-15 chemical shifts have such a high regularity correlated to structure, they can be used as diagnostic indications for identifying the structure of dipyrrolic compounds.