Formazanate Complexes of Hypervalent Group 14 Elements as Precursors to Electronically Stabilized Radicals

The stability of molecular radicals containing main-group elements usually hinges on the presence of bulky substituents that shield the reactive radical center. We describe a family of Group 14 formazanate complexes whose chemical reduction allows access to radicals that are stabilized instead by geometric and electron-delocalization effects, specifically by the square-pyramidal coordination geometry adopted by the Group 14 atom (Si, Ge, Sn) within the framework of the heteroatom-rich formazanate ligands. The reduction potentials of the Si, Ge, and Sn complexes as determined by cyclic voltammetry become more negative in that order. Examination of the solid-state structures of these complexes suggested that their electron-accepting ability decreases with increasing size of the Group 14 atom because a larger central atom increases the nonplanarity of the ligand-based conjugated π-electron system of the complex. The experimental findings were supported by density-functional calculations on the parent complexes and the corresponding radical anions.