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.