Role of Secondary Interactions Involving Iodine Atoms in the Crystals of 1,2-Diamino-4,5-Diiodobenzene Polymorphs

Two crystal modifications of 1,2-diamino-4,5-diiodobenzene (P212121 space group, Z′ = 1), exemplifying packing polymorphism, are prepared and characterized by X-ray diffraction. The polymorphs contain an identical double chain (1D-motif) formed via N–H⋯N H-bonds and I⋯Cg contacts. The main difference arises from the relative arrangement of the 1D motifs (“distinctly herringbone” and “smoothed herringbone” types) and is governed by secondary interactions involving iodine atoms (N–H⋯I, N⋯I, I⋯I). The estimated crystal field energies and DSC data show that the polymorph with the “distinctly herringbone” arrangement of 1D motifs (1a) is thermodynamically stable and can be reproduced from a wider range of solvents. The polymorph with the “smoothed herringbone” arrangement (1b) exhibits higher calculated crystal density and molecular packing coefficient values than polymorph 1a and can be prepared only from acetonitrile at 7 °C.