Polymorphism of a Planar Neutral Radical: Magnetic and Optical Implications.

The synthesis and characterization of a new paramagnetic ligand, 4-( N′-methyl-2′-benzimidazolyl)-1,2,3,5-dithiadiazolyl (MebimDTDA), is presented. MebimDTDA exhibits polymorphism in the solid state. The red microcrystalline form (Form I-red) is diamagnetic below T = 370 K, consistent with typical multi-center two-electron bonding interactions (i.e., pancake bonds) between planar π-radicals. By contrast, the green–black crystalline form (Form II-green) exhibits paramagnetic properties consistent with the observed unusual crystal packing in which some, but not all, of the radicals are arranged in a typical pancake-bonded geometry. Crystallographic analysis, magnetic data, and computational methods are employed to elucidate the structure-property relationship, with an emphasis on the role of crystal packing.