A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet.

Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet-triplet energy gaps (ΔE ST) over 0.59 kcal mol-1 (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu2-PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet-triplet energy gap ΔE ST = 0.94 kcal mol-1 (473 K). This is a rare example of stable diradicals with singlet-triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu2-PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K2, single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.