Formation and distortion of iodidoantimonates(III): the first isolated [SbI₆]^3- octahedron

The ability to intentionally construct, through different types of interactions, inorganic-organic hybrid materials with desired properties is the main goal of inorganic crystal engineering. The primary deformation, related to intrinsic interactions within inorganic substructure, and the secondary deformation, mainly caused by the hydrogen bond interactions, are both responsible for polyhedral distortions of halogenidoantimonates(III) with organic cations. The evolution of structural parameters, in particular the Sb-I secondary- and O/N/C-H...I hydrogen bonds, as a function of temperature assists in understanding the contribution of those two distortion factors to the irregularity of [SbI₆]^3- polyhedra. In tris(piperazine-1,4-diium) bis[hexaiodidoantimonate(III)] pentahydrate, (C₄H₁₂N₂)₃[SbI₆]₂·5H₂O (TPBHP), where the isolated [SbI₆]^3- units were found, distortion is governed only by O/N/C-H...I hydrogen bonds, whereas in piperazine-1,4-diium bis[tetraiodidoantimonate(III)] tetrahydrate, (C₄H₁₂N₂)[SbI₄]₂·4H₂O (PBTT), both primary and O-H...I secondary factors cause the deformation of one-dimensional [{SbI₄}_n]^n- chains. The larger in spatial dimensions piperazine-1,4-diium cations, in contrast to the smaller water of crystallization molecules, do not significantly contribute to the octahedral distortion, especially in PBTT. The formation of isolated [SbI₆]^3- ions in TPBHP is the result of specific second coordination sphere hydrogen bond interactions that stabilize the hybrid structure and simultaneously effectively separate and prevent [SbI₆]^3- units from mutual interactions. The temperature-induced changes, further supported by the analysis of data retrieved from the Cambridge Structural Database, illustrate the significance of both primary and secondary distortion factors on the deformation of octahedra. Also, a comparison of packing features in the studied hybrids with those in the non-metal containing piperazine-1,4-diium diiodide diiodine (C₄H₁₂N₂)I₂·I₂ (PDD) confirms the importance and hierarchy of different types of interactions.