Tetraazaporphyrins porphyrazines

Meso substitution of porphyrines to give tetraazaporphyrines, so-called porphyrazines, modulates the electronic character of the macrocycle. While porphyrazines have received considerably less attention than porphyrines over many years, this has changed due to the development of efficient syntheses of soluble derivatives.1 6-1809 Also, various porphyrazines (and phtalocyanines) with peripheral groups for metal ion coordination have been prepared and used for the construction of multimetallic complexes.1806 Ni porphyrazines (695) typically show absorptions spectra with a strong Q band at around 615nm. 

The effect of peripheral substituents on absorption spectrum and in vitro PDT activity has been studied for the zinc(II) porphyrazine (5,10,15,20-tetraazaporphyrin) system.276 277 Water-soluble zinc(II) polyazaphthalocyanines have been synthesized for PDT applications.278,279 A zinc(II) bis(dimethylamino)porphyrazine is photooxidised to give the zinc(II)-,seco-porphyrazine, the reaction being autocatalytic.280 The zinc(II)-complex of a hexaalkyltexaphyrin chloride has Amax (MeOH) 732 nm and A 0.61,281 but the system does not appear to have been developed for PDT. 

Porphyrazines (pz), or tetraazaporphyrins, are compounds that can be viewed as porphyrin variants in which the meso carbon atoms are replaced with nitrogen atoms, as Fig. 1 shows (1). This difference intrinsically gives porphyrazines discrete physiochemical properties from the porphyrins. In addition, despite their similar molecular architecture, porphyrazines are prepared by an entirely different synthetic route than porphyrins—by template cyclization of maleonitrile derivatives, as in Fig. 2, where the open circle with the A in it represents the peripheral substituent of the pz—rather than by the condensation of pyrrole and aldehyde derivatives (1). The pz synthetic route allows for the preparation of macrocycles with chemical and physical properties not readily accessible to porphyrins. In particular, procedures have been developed for the synthesis of porphyrazines with S, N, or O heteroatom peripheral functionalization of the macrocycle core (2-11). It is difficult to impossible to attach the equivalent heteroatoms to the periphery of porphyrins (12). In addition, the preparation and purification of porphyrazines that bear two different kinds of substituents is readily achievable through the directed cocyclization of two different dinitriles, Fig. 3 (4, 5, 13). 

Cationic porphyrinic macrocycles, in particular the archetypical tetracationic, meso-telra-W-melhylpyridyl (porphyrin, have applications in biology, medicine, catalysis, and materials (95-103). Cationic tetraazaporphyrins, or porphyrazines, which represent a novel alternative and class of cationic porphyrinic compounds, were recently reported (37). 

Let us conclude shortly with tetraazaporphyrins, the most popular members of which are constituted by phthalocyanines and porphyrazines, Scheme 12. 

They have also prepared mono- 147-149 and tris-crown ethers 150-152 containing the tetraazaporphyrin unit. Singly crowned porphyrazine 147 was prepared by the reaction of >15 equiv of 1,2-dicyanobenzene and 1 equiv of 33 under classic Linstead macrocyclization conditions. On the other hand, tris-crowned porphyrazine 150 was prepared by the reaction of 3 equiv of 32 and 1 equiv of l,2-dicyano-l,2-bis(l 1-hydroxyundecylthio)ethylene. Rapid complexa-tion of Ag+ was observed for 147-149, with weaker complexation of Hg2+ and Na+ observed only by MS. There was... 

Tetraazaporphyrin (3) (or porphyrazine) differs from Pc only in the absence of the four benzo units. The Q-band absorption of tetraporphyrins is found at shorter wavelengths ( 600 nm)69 and they are less prone to oxidation, which is advantageous for their use in catalysis. The cyclo-tetramerization of maleonitrile by the Linstead method, usually initiated by magnesium alkoxide, is by far the most important route to tetraazaporphyrins (Table l).70 The preparation and properties of substituted maleonitriles have been reviewed.71... 

Because of space limitations, we will only briefly discuss the most basic application in the area of porphyrin optical spectra. Much work has already been done in this area, both with TDDFT and related methods. The TDDFT applications include free base porphin and its ss-octahalogenated derivatives, the porphyrinato-porphyrazinato-zirconiumhV) complex, NiP, NiPz, NiTBP, and NiPc, zinc phthalocyanine, chlorophyll a, zinc complexes of porphyrin, tetraazaporphyrin, tetrabenzoporphyrin, phthalocyanine, phenylene-linked free-base and zinc porphyrin dimers, metal bis(porphyrin) complexes a series of porphyrin-type molecules, and many more. We refer to ref. 75 for an extensive discussion of TDDFT calculations on the spectra of porphyrins and porphyrazines, as well as their interpretation. For further theoretical work on porphyrines, we mention ref. 76 and other papers in that special issue. 

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