Porphyrazinehexamines and dinitroporphyrazines: Synthesis, characterization, and complementary electrochemistry

Unsymmetrical porphyrazines, M[pz(H)(2)(R)(6)], with three substituted pyrr oles (substituents, R = -NMe2, propyl, or 4-tert-butylphenyl) at the periph ery and one unsubstituted pyrrole have been synthesized from inexpensive an d readily available materials. Linstead crossover macrocyclization of 8 wit h 9, 11, and 12 gave pigments 1, 3, and 5, respectively. Subsequent demetal ation by treatment with TFA gave free-base porphyrazines 2, 4, and 6. Gener ally, pyrrolines 11 and 12 are better co-cyclization partners and give high er yields for the desired unsymmetrical porphyrazines compared to maleonitr iles 9 and 10 when co-cyclized with 8. Nickel porphyrazine 7 was obtained b y reaction of 6 with Ni(OAc)(2) in chlorobenzene and dimethylformamide. Bot h free-base and nickel porphyrazines (6 and 7) were readily nitrated by nit rogen dioxide in CH2Cl2, yielding dinitroporphyrazines 18 and 19, respectiv ely. Electrochemical studies have shown that porphyrazinehexamine 2 is easy to oxidize, having the first oxidation at E-1/2 = - 0.18 V, which is 0.91 and 1.04 V lower than that of 4 and. 6, respectively. Dinitroporphyrazine 1 8, however, displays a 0.6 V of cathodic shift for the two reversible reduc tion waves compared to those observed for the parent porphyrazine 6.