No abstract

Angew. Chem. lnt. Ed. Engl. 26 (1987) No. 9 Angen Chem. I n f . Ed. Engl. 26 (1987) No. 9 0 VCH Verlagsgesellschafi mbH. 0-6940 Weinheim. 1987 0570-0833/87/0909-0931 $ 02.50/0

No abstract

Solid state, variable temperature 15N‐NMR studies make it possible to predict the behavior of the four N atoms and the two internal H atoms in the isomeric porphine and porphycene. The spectra of porphine indicate a statistical disorder of the internal H atoms. In the case of porphycene it can be concluded that there are two non‐equivalent unsymmetric proton transfer systems. Since the NH…N distances are very short, the energy barrier for the rearrangement is very small. Therefore, other than in the case of porphine, in porphycene the interconversion of the tautomers is so rapid that the rate constants for the proton transfer could not be determined by the 15N‐CPMAS‐NMR method.

Porphycene (2), a planar structural isomer of porphyrin (1) recognized and synthesized only recently, exhibits short N … N distances allowing exceptionally strong N‐H … N hydrogen bonds to be formed. Although the N4‐coordination hole of 2 is smaller than that of 1, the new tetrapyrrolic macrocycle is capable of complexation with a broad spectrum of metal ions to give the respective metalloporphycenes. In accord with model considerations, both N‐H … N hydrogen bonding and complexation of 2 may be modulated by peripheral alkyl groups through changes in the geometry of the porphycene skeleton arising from non‐bonded repulsions between the substituents. In this publication are delineated some of the consequences of structural variation of 2 by introduction of alkyl groups at the 9,10,19,20‐positions.