Synthesis and crystal structure of rare-earth biuret complexes with linear pentaiodide ions: Infinite polyiodide chains in a cationic framework

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Isostructural compounds [Ln(BU)4][I5][I]2 (Ln = La, Nd, Gd) were prepared by reactions of rare-earth iodides with biuret (BU) and iodine. Their crystals contain [Ln(BU)4]3+ complex cations and non-coordinated iodide and pentaiodide ions. Complex cations and isolated iodide ions form a 3-D framework with infinite square channels via weak bonds of the N–H...O and N–H...I types. In the channels, infinite chains of almost linear pentaiodide ions are arranged. Calculated distribution of the charge in the pentaiodide ion explains the absence of weak interactions of its iodine atoms with other atoms of a coordinate compound. The energies of the inner I–I bonds correspond to formation of the anion from two iodine molecules and central iodide ion. Distances between iodine atoms and charge distribution in the polyiodide anion support Grotthuss-like charge transfer mechanism. Band gaps for polyiodide chains correspond to semi-conducting character of conductivity.