Exploiting a template effect of 1,4-diazacycloheptane (also known as homopiperazine, Hpipe), four new hybrid iodides, (HpipeH₂)₂Bi₂I₁₀·2H₂O, (HpipeH₂)I(I₃), (HpipeH₂)₃I₆·H₂O, and (HpipeH₂)₃(H₃O)I₇, were prepared and their crystal structures were solved using single crystal X-ray diffraction data. All four solid-state crystal structures feature the HpipeH₂²⁺ cation alternating with Bi₂I₁₀^4–, I₃^–, or I^– anions and solvent water or H₃O⁺ cation. HpipeH₂²⁺ 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 (HpipeH₂)₂Bi₂I₁₀·2H₂O, 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.

In searching for a tool for optimizing the band gap of a hybrid compound capable of serving as a light-harvesting material in lead-free photovoltaics, we synthesized a new polyiodoantimonate (HpipeH₂)₂[Sb₂I₁₀](I₂) and analyzed its crystal and electronic structure by application of X-ray crystal structure analysis, Raman and diffuse reflectance spectroscopies, and quantum chemical calculations. It was demonstrated that I₂ molecules link Sb₂I₁₀ edge-sharing octahedra into zig-zag chains, whereas the organic cations link inorganic anionic chains into a 3D structure featuring a complex pattern of covalent bonds and non-covalent interactions. Overall, these features provide the background for forming the electronic structure with a narrow band gap of 1.41 eV, therefore being a versatile tool for optimizing the band gap of a potential light-harvesting hybrid compound.

Metal-inorganic frameworks are all-inorganic counterparts of well-known and extensively investigated metal-organic frameworks. Metal atoms connected by linkers form structures of different dimensionality with channels filled with atoms, ions or molecules. The review is devoted to metal-inorganic frameworks assembled from transition metal atoms, mainly mercury and silver, linked to phosphorus, arsenic or antimony atoms or groups of atoms of different shapes (dumbbells, rings, clusters, chains or helices). This assembly gives rise to frameworks of different geometry and with different topology. In all cases, the positively charged framework is composed of transition metal cations and pnictogen atoms as linkers. The positive charge of the framework is compensated by guest anions. Depending on the charge of the framework and the nature of the anion, compounds with different structures and properties can be produced. Specific features of metal-inorganic frameworks, the influence of weak interactions on the framework architecture and possibilities of designing new frameworks are discussed. The bibliography includes 82 references.