Clustering of molecular spins in the crystals of nitronylnitroxide and iminonitroxide triradicals based on benzene-1,3,5-triyl frameworks

Abstract

We propose a strategy of crystal-engineering for heteromolecular crystals composed of organic open-shell molecules, in which two kinds of open-shell molecular entity are co-crystallized and, at the same time, distinct molecular entities with differing spin multiplicities are adjacent to each other in a crystalline solid state. Stable organic triradicals of nitronylnitroxide and iminonitroxide based on the benzene-1,3,5-triyl framework, trisNN (3) and trisIN (4), have been designed and synthesized. These molecules consist of a m-phenylene-substituted biradical and a monoradical moiety united by an ester bridge. From X-ray crystallography, these molecules have been found to possess an approximately isosceles triangular geometry and fully head-to-tail molecular packing in three dimensions has been achieved in the crystalline solid state. Thus, the co-crystallization is attained by binding the two π-conjugated radicals by σ-bonds, retaining the magnetic degree of freedom for the two distinct open-shell moieties. The alternating aggregation of the heteromolecular assembly is achieved by tuning the molecular geometry of the building-block molecules, leading to the close packing of isosceles triangular molecules in the head-to-tail fashion. Local contacts of the nitroxide groups with large spin densities between the nearest-neighbor molecules give a six-spin molecular cluster on a tetrad of the bi-, mono-, mono-, and biradical moieties in the head-to-tail stacking. Temperature dependence of magnetic susceptibility of the crystalline solids of 3 and 4 has been analyzed on the basis of their crystal structures. The six-spin cluster model has reasonably explained the susceptibility.