Experimental Observation of Thermally Excited Triplet States of Heavier Group 15 Element Centered Diradical Dianions

One-electron reductions of Mes*As=Fl* (1; Fl*=2,7-di-tert-butylfluorenylidene, Mes*=2,4,6-tBu₃ C₆ H₂ ) and diarsaalkenes [1,2-b]-IF(=AsMes*)₂ (2; IF=indenofluorene) with potassium led to the isolation of the arsenic-centered radical anion salts 1K and 2K, respectively. The diradical dianion salts 2K₂ and 3K₂ were afforded by the reduction of 2 and 2,8-tBu₂ -[2,1-b]-IF(=AsMes*)₂ (3) with an excess amount of KC₈ . The radicals have been investigated by single-crystal X-ray crystallography, EPR, and UV/Vis absorption spectroscopy, along with theoretical calculations. The calculations revealed that 2K₂ and 3K₂ feature open-shell singlet ground states with singlet-triplet energy gaps of 2.1 and 1.0 kcal mol^-1 , respectively. They are readily thermally excited to triplet states as demonstrated by EPR spectroscopy. The obtained diradicals represent the first examples of heavier Group 15 element centered diradicals with experimentally observable triplet states.