Unsymmetrical porphyrazines

Figure 3. Synthesis of unsymmetrical porphyrazines via cocyclization of two dinitriles.

An important development in pz chemistry was the successful preparation of unsymmetrical porphyrazines achieved by the cocyclization of two different dinitriles, Fig. 4. This synthetic advance was part of an effort to prepare sulfur appended porphyrazines designed to bind metal ions at the pz periphery as well as in the central cavity (4-6, 21, 22). To avoid the statistical mix of six different isomers from such cyclizations, Hoffman and co-workers (13) developed techniques to selectively prepare specific isomers. Mixed cyclizations are described in all sections of this review. 

Optical Spectra. The optical spectra of the unsymmetrical porphyrazines 65 and 66a-66d exhibit Soret bands between 330 and 375 nm and a split Q band, due to reduced symmetry between 650 and 700 nm. The peripherally metalated species 67d-67h do not have a split Q band, reflecting the extension of the pz n system into the five-membered chelate ring mediated by the bridging dithiolene. Table XVII compares the Q-band region of the optical spectra for 65, 66a-66c, 67b-67d to the symmetrical octa. V-bcnzyl porphyrazines 58, 59, 60a, 60b, 61a, and 61b to the analogous phthalocyanines (4). 

Synthesis. To prepare unsymmetrical porphyrazines with n = 1-3 appended thia-oxo crowns or thia crowns, the strategy of stacking the stoichiometry... 

A series of unsymmetrical porphyrazines with thio-ethoxy(ethoxy) ethanol groups (S-polyetherol) appended to the periphery, which both coordinate metals and impart solubility to the macrocycle have been reported (50). These macrocycles show selective optical changes in response to specific metal ions, thus are of potential use in metal ion sensing. 

The coordination of PdCl2 and PtCl2 to the single peripheral bis(dimethylamino) chelating site of the unsymmetrical porphyrazines 128 (Ni) and 129 (Cu) results in the formation of the bimetalic solitaire porphyrazines 143 (95%), 144 (72%), 145 (no yield reported), 146 (no yield reported), indicated by a color change from blue to purple and a sharpening of the g-band region (Scheme 27) (29). 

Nitrogen substituted porphyrazines were the second type of heteroatom-deriva-tized pz macrocycle reported and were prepared from the readily derivatized diami-nomalconitrilc (DAMN) (7). Octakis(dimethylamino)porphyrazines are extremely electron-rich systems and have been used to prepare charge-transfer complexes with Cgo, as well as to peripherally chelate metals or convert to crown appended systems (38, 39). The unsymmetrical dimethylaminoporphyrazine analogues have also been reported (29), as well as the first example of the desymmetrized seco-pz from the dimethylaminoporphyrazine (8, 40). The nitrogen substituted porphyrazines are discussed in Section V. 

Porphyrazines with alkyl or aryl substituents are considerably more soluble than their unsubstituted counterparts (Section III. A). Consequently, various pz isomers with alkyl and aryl substituents, for example, symmetrical M[pz(A4)] and unsym-metrical M[pz(A3B)], have been reported. In particular, the symmetrical species M[pz( A4)] have been used both as vehicles to study the fundamental physical properties of metalated porphyrazines (52) as well as to make double decker or sandwich porphyrazines, cofacial dimers linked with lanthanide metal ions (34), while the unsymmetrical species M[pz(A3B)] have utilized the alkyl-aryl substituents as solubilizing groups and have been applied to all areas of pz chemistry. 

M[pz(A3B)]. The preparation of unsymmetrical (A3B) porphyrazines can be achieved by a mixed cyclization of two different substituted dinitriles however, as outlined in Section II, the result of this method is a statistical mix of the six possible isomers formed by all possible combinations of dinitriles. To avoid such a result, it has been shown that stacking the stoichiometry to favor one of the two dinitrile precursors will result in the formation of mostly A4 and A3B (4, 21). 

Scheme 12. Unsymmetrical dithiol appended porphyrazines and their peripherally metalated analogues. [Adapted from (4), (21).]...

Scheme 18. Unsymmetrical thia-oxa and thia crown porphyrazines. [Adapted from Lange, unpublished results.]...

The unsymmetrical thiacrown porphyrazines Mg[pz(A3B)], where A = Bn, B = S3 (92a) and Mg[pz(A3B)], where A = Bn, B = S4 crown (93a) were formed by a cross-condensation of excess 3,6-dibutyl-1,2-dicyanobenzene and the corresponding thiaether crown dithiomaleonitrile derivative (91). The porphyrazines were demetalated with TFA and remetalated with either copper or nickel acetate to form Compounds 92c, 93c and 93d (16-18%). 

Optical Spectra. The optical spectra of the unsymmetrical crown porphyrazines are given in Table XXV. The spectra are typical pz spectra with two intense optical absorbance bands, a Soret band at "k < 400 nm and a Q band at X > 600 nm. The Q band is split for both the metalated and free base species, with the latter exhibiting the greater splitting due to decreased symmetry. 

Electronic Absorption Maxima for Unsymmetrical Crown Porphyrazines... 

The absorption spectra of the unsymmetrical magnesium porphyrazines 133-136 are very similar to the symmetrically substituted Mg pz(/V-Me2)x ... 

The same chemical oxidation strategy can be applied to unsymmetrical por-phyrazines, but a stronger oxidant than manganese dioxide is required. Reaction of the porphyrazines (127 and 130) with potassium permanganate gives the expected seco-porphyrazines 164 and 160, respectively, in 93 and 96% yield (Scheme 31). 

Scheme 31. Formation of unsymmetrical ieco-porphyrazines. [Adapted from (8).]...

Over the past few years, in terms of atom economy and extension the conjugated system of dithienylethene, we fused photochromic dithienylethene directly to the position of the pyrrole units of porphyrazines (tetra-azaporphyrines) or phthalocyanines, and developed a class of unsymmetrical and symmetrical phthalocyanine and porphyrazine hybrids containing 2,4, 6, and 8 thiophenyl groups (BTE-TAPs), as illustrated in Scheme 9 [37-39],... 

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