New expanded porphyrinoids: Synthesis, structure and properties of hemihexaphyrazines and their reduced metal containing derivatives

Hemihexaphyrazines can be considered as hexamembered porphyrinoids of the ABABAB type, where thiadiazole rings (A) and pyrrole-containing subunits (B) joined via aza bridges to form a macrocyclic system with an expanded coordination cavity capable of holding at once three atoms of transition metals that are formed a triangular cluster. It was established for the first time that unsubstituted hemihexaphyrazine and its metal complexes can be reduced to form crystalline salts. These salts were characterized by UV-Vis-NIR and IR spectroscopy, MS, and X-Ray diffraction data. The magnetic properties of these compounds were studied for the first time using EPR and SQUID techniques within the 1.9-300 K temperature range. It was shown that the three-electron reduction of starting {(M3O)-O-II(Hhp)}(+) yields {cryptand(Cs+)}(2){(Ni2NiO)-Ni-II-O-I(Hhp(5-))}(2-)center dot 2C(7)H(8) and {cryptand(Cs+)}(2){Cu-3(II)-O(Hhp(center dot 6-))}(2-)center dot C7H8 crystalline salts. An unusual doublet-quartet transition was found for the {(Ni2NiO)-Ni-II-O-I-(Hhp(5-))}(2-) dianions. Salt {cryptand(Cs+)}(2){(Ni2NiO)-Ni-II-O-I(Hhp(5-))}(2-)center dot 2C(7)H(8) also shows an unusually strong low energy NIR absorption, which was observed at 1000-2200 nm with maxima located at 1283 and 1980 nm. A phenomenon consisting of equalization of bond lengths among exocyclic nitrogen atoms (N-ex) and neighboring atoms C(pyrrole) and C(thiadiazole) was observed under reduction conditions.

Automated summary

This study demonstrates for the first time that unsubstituted hemihexaphyrazine and its metal complexes can be reduced to form crystalline salts. The properties of these salts were characterized using various spectroscopic and analytical techniques. The magnetic and absorption properties of these compounds were also investigated, revealing their unique features. Additionally, an equalization phenomenon in bond lengths was observed under reduction conditions.