Phthalocyanine analogues. Part 1. Synthesis, spectroscopy, and theoretical study of 8,18-dihydrodibenzo[b,l]-5,7,8,10,15,17,18,20-octa-azaporphyrin and MNDO calculations on its related Hückel heteroannulene

Abstract

Three hemiporphyrazines containing two 1,2,4-triazole subunits, namely the N,N′-unsubstituted, the N,N′-didodecyl-, and N,N′-diphenyl-substituted derivatives, have been synthetised and fully characterized (m.s., u.v., and n.m.r.). In particular, high-resolution solid-state ¹³C n.m.r. spectroscopy has proved to be a suitable method for the study of this highly insoluble class of compounds, giving valuable information on their tautomerism. An MNDO theoretical study (fully optimized geometries) has been carried out in order to understand why the didehydro derivative (7)(an 18π-electron aromatic porphyrin) is much less stable than the corresponding N,N′-unsubstituted hemiporphyrazine (6). The explanation lies in the peculiar structure of 1,2,4-triazoline-3,5-di-imine that one triazoline ring of (7) must adopt in order to allow a fully conjugated system. This structure has also been calculated, and shows a strong tendency to lose bound fragments (molecular dinitrogen and the azomethyne ylide HNC–NH–CNH).