Stable 1,3,2-Benzodithiazolyl Radicals: Modification ofReactivity, Crystal Packing, and Solid State MagneticProperties by Fluorination

Abstract

Impact of fluorination on crystal and molecular structure, heteroatom reactivity, and solid-state magnetic properties of thermally-stable π-radicals is studied experimentally and computationally with 1,3,2-benzodithiazolyl 1· and its 4,7-difluoro, 4,5,6,7-tetra-fluoro, and 4,7-difluoro-5,6-(hexafluoropropane-1,3-diyl) derivatives 2-4, respectively. Radicals 2 -4 are isolated by vacuum thermolysis of their unusual covalent 2:1 adducts with 7,7,8,8-tetracyanoquinodimethane. The impact of fluorination on reactivity is evidenced by transformation of 2-4 and 2 +-4 + into corresponding 2H-1-oxo-1,3,2-benzodithiazoles under the influence of air’s or solvents’ moisture; back transformation into the cations under the action of protic acids; and formation of a paramagnetic molecular complex between 3 and naphthalene, whereas 1 and octafluoronaphthalene do not exhibit complexation. The crystal structures of 3 and 4 reveal a novel packing motif featuring radical pairs linked by four-center interactions that stack into offset π-columns, forming a unique zip-π-stack synthon that incorporates head-over-tail π-pairs of radicals. Despite the formation of π-pairs, polycrystalline 3 and 4 display a nonzero effective magnetic moment that rises with temperature above 200 K, although the values remain significantly lower than those of the high-temperature polymorphs of magnetically-bistable 1 and 2·. This behavior can be rationalized by different magnetic topologies and values of spin exchange between the radicals.