Synthesis of Thiadiazole Hemihexaporphyrazine Bearing iso-Propyloxy Groups and its Trinuclear NiII Complex

Abstract

Expanded conjugated macrocycles are of a deep interest for the last decades due to their specific spectral features.[1,2] These macromolecules evolve perfect models for the amplification of the Huckel rule of aromaticity[3] and computational studies[4] due of their structural variety, versatility and complicity, resulting in rich photophysics and electrochemistry.[5,6] From the functional viewpoint, expanded porphyrins possess exceptional coordination properties consisting in selective binding to certain metal ions with further homoand heteropolynuclear complexes formation. [7] Thus, they are good candidates for application in catalysis, anion recognition, ion extraction and transport.[8-10] Multiple trials to the modification of the tetraazaporphyrin structure have been undertaken in order to obtain other macrocycles structurally related to phthalocyanines and porphyrazines, but exhibiting nonconventional physicochemical features. Superphthalocyanine pentapyrrolic macrocycles have been remained the only reported example of expanded azaporphyrins for many years,[11] until different heterocyclic building blocks were incorporated into structures of azaporphyrins. That has fairly enriched synthetic sources toward the desirable structures and provided chemists with a big variety of objects for a further depth study. In the last decades, chemistry of azaporphyrins has been widely advanced due to the persistent work on this field. [12-14] Herein, the one problem for the rational design of expanded azaporphyrins, i.e. the spontaneous formation of macrocycles of the favorable structures and open-chain intermediates, has been overcome by the working out of some synthetic approaches. Following the stepwise procedure, the first example of an expanded 28 π-electron heteroazaporphyrinoid 1 of ABBABBtype, made up of four diiminoisoindoline (B) subunits and two face-to-face triazole (A) rings, was prepared.[15] (Figure 1). Its expanded macrocyclic cavity with two metal ions inside was constructed by the dimerization of metal complex of opened trimeric intermediate of two diiminoisoindoline and one triazole subunits (BAB). Later, another expanded metal-free 30 π-electron hemiporphyrazine 2 of ABABABtype has been communicated[16] as a product of the crossover condensation of three 2,5-diamino-1,3,4-thiadiazole and three substituted diiminoisoindoline. This macrocycle is considered to be a hexaphyrin analogue where its subunits are bonded via six aza-bridges and three pyrrolic rings are replaced by three thiadiazoles. So, a term that reproduces these features seems to be acceptable to define Mc of ABABAB-type. Recently, hemihexaporphyrazine has been unequivocally structurally characterized, and complete spectroscopic, redox, photophysical and thermal stability characterization newly was given.[17] Furthermore, the copper(II) template crossover condensation was applied successfully to the preparation of the novel binuclear thiadiazole-derived azaporphyrin-like hexamer 3 of AABAAB-type, made up of four thiadiazole subunits, two face-to-face pyrrole rings and one metal ion. [18] However, authors noted that under those conditions corresponding ABABAB complexes have been also obtained. Our previous attempts to prepare a metal free ABABAB heteroazaporphyrinoid bearing six strong electron-donating isopropyloxy substituents in n-pentanol were failed. Recently, we succeed in synthesis of compound 5 in an extremely low 2 % yield when isopropanol was used for the cyclomerization of precursors (Scheme 1). Complex 6 was prepared by crossover condensation in the presence of NiII salt as a template in 23% yield (Scheme 2). Thus, we report here the template condensation as a fitting method for the preparation of complexes of ABABAB hemiporphyrazines.