Assembling Polyiodides and Iodobismuthates Using a Template Effect of a Cyclic Diammonium Cation and Formation of a Low-Gap Hybrid Iodobismuthate with High Thermal Stability.

Exploiting a template effect of 1,4-diazacycloheptane (also known as homopiperazine, Hpipe), four new hybrid iodides, (HpipeH ₂ ) ₂ Bi ₂ I ₁₀ ·2H ₂ O, (HpipeH ₂ )I(I ₃ ), (HpipeH ₂ ) ₃ I ₆ ·H ₂ O, and (HpipeH ₂ ) ₃ (H ₃ O)I ₇ , were prepared and their crystal structures were solved using single crystal X-ray diffraction data. All four solid-state crystal structures feature the HpipeH ₂ ²⁺ cation alternating with Bi ₂ I ₁₀ ^4- , I ₃ ^- , or I ^- anions and solvent water or H ₃ O ⁺ cation. HpipeH ₂ ²⁺ assembles anionic and neutral building blocks into polymer structures by forming four strong (N)H···I and (N)H···O hydrogen bonds per cation, with the H···I distances ranging from 2.44 to 2.93 Å and H···O distances of 1.88-1.89 Å. These hydrogen bonds strongly affect the properties of compounds; in particular, in the case of (HpipeH ₂ ) ₂ Bi ₂ I ₁₀ ·2H ₂ O, they ensure narrowing of the band gap down to 1.8 eV and provide high thermal stability up to 240 °C, remarkable for a hydrated molecular solid.