Synthesis, Structure and Thermal Decomposition of a New Iodine Inclusion Compound in the 2,2-Dimethylpropane-1,3-diamine/HI/I<sub>2</sub> System

The reaction of 2,2-dimethylpropane-1,3-diamine with hydroiodic acid in the presence of iodine gave a new polyiodide best described as bis(2,2-dimethylpropane-1,3-diazanium) tetraiodide-diiodine (1:1), (C₅H₁₆N₂)₂I₄ ·I₂ (1). The title compound can be transformed into the known 2,2-dimethylpropane-1,3-diazanium diiodide, (C₅H₁₆N₂)I₂ (2), upon iodine release at 433 K and 2 × 10⁻³ mbar. Both compounds have been characterised by spectroscopic methods (Raman and IR) and powder diffraction. For 1 the single-crystal structure determination has been successful. The asymmetric unit of 1 consists of one half 2,2-dimethylpropane-1,3-diazanium cation and two half iodide anions, all lying in a mirror plane. In addition, there is one quarter of an iodine molecule located near to a centre of inversion (2/m site) which is disordered over two positions with occupancy factors of 0.22 and 0.78. The structure of the title compound contains cube-shaped structural units in which the -NH₃⁺ groups of four 2,2-dimethylpropane-1,3-diazanium cations occupy the corners. The iodide anions lie near the midpoints of eight of the twelve edges, and the disordered iodine molecule fills the void in the centre. Weak I···I interactions between the disordered iodine molecules and adjacent iodide anions may also allow a description as conceivable I₄²⁻ dianions. The cube-type building units are further connected to adjacent ones by weak to medium strong N–H···I hydrogen bonds resulting in a two-dimensional layered structure parallel to the ab plane.