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Methods for the formation of maleic dinitrile have been studied and a routine method of preparation has been devised. Reaction of this nitrile with magnesium n-propoxide in n-propyl alcohol (and with similar reagents) leads to magnesium tetrazaporphin (I1 ; M = Mg) in yields of up to 15%. Removal of the metal gives metal-free tetrazaporphin (I). Copper and nickel derivatives have also been prepared. The absorption spectra of these substances are given. The structure of tetrazaporphin and its relation to porphin and phthalocyanine are discussed.THE four structural cornerstones of the group of macrocyclic tetrapyrrolic pigments are porphin, tetrazaporphin, tetrabenzoporphin and phthalocyanine. Of these the last two (and compounds intermediate between them) have been described in earlier ,papers in this series." Porphin has been investigated in a preliminary way by Hans Fischer and Gleim (Annulen, 1936, 521, 157; cf. Rothemund, J . Amer. Chem. SOC., 1936, 58, 625). We now report the discovery of the fourth parent compound, tetrazaporphin (I) (the prefixes p p' p" p"' systematically required to indicate the positions of the four entering aza-groups are omitted in this series of papers).Many attempts to make this substance and its simple derivatives were made before the war but our only success was the preparation of its octaphenyl substitution product (A. H. Cook and Linstead, J., 1937,929). At about the same time Fischer and Endermann (Annaleut, 1937, 531, 245) prepared a reIated compound containing four methyl and four ethyl substituents in the p-positions of the pyrrole rings, which they called tetraimidoztioporphyrin .The earlier general title of the series (Phthalocyanines and Related Compounds) has now become too restrictive.

1 : 3-Di-iminoisoindoline readily reacts with 2 : 6-diaminopyridine, to give a red compound, C,,H,,NS. The structure of this has been established as ( 11) by analytical and synthetical evidence. It contains a cross-conjugated macrocyclic system of a novel type, related to some extent to the azaporphins. The macrocyclic compound forms metallic derivatives of high thermal stability, in which the metal occupies the centre of the ring. Lightabsorption data are discussed.

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