Timothy Lash and colleagues at Illinois State University, in the US, demonstrate that N-methyl and N-phenylpyrazole dialdehydes readily form porphyrin analogues using the '3 + 1' methodology - a variant on the MacDonald condensation, where an aromatic aldehyde is condensed with a tripyrrane. 

                    

These new porphyrinoid structures are less aromatic than N-confused porphyrins (NCPs), but easily form organometallic derivatives under mild conditions. 

The term 'N-confused porphyrin' means that one of the pyrrole nitrogens in the porphyrin is confused about which way the unit should be pointing. NCPs have many remarkable properties. They form organometallic derivatives and can generate highly ordered coordination complexes. They also have long wavelength absorptions resulting in potential applications as photosenstizers in photodynamic therapy. 

Lash and his team suggest that these porphyrin analogues are of great interest in the area of carboporphyrin and NCP chemistry. 'In the future we hope to prepare these azaNCPs without the external N-methyl or phenyl group. This should allow formation of fully aromatic analogues,' says Lash. 

This will open the way to observations on the tautomeric properties of the compounds as well as introducing a symmetrical framework that will facilitate investigations into the formation of supramolecular complexes,' he adds. 

Michael Brown