Novel cyclen-polyiodide complexes: a reappraisal of I–I covalent and secondary bond limits

Abstract

Supramolecular stabilization of polyiodides and iodine-dense phases is of high interest: this study explores the possibilities offered in this sense by diprotonated cyclen, affording two novel crystal structures. One of them contains at least one peculiar I⋯I interatomic distance (3.305(1) Å), falling well below the region commonly described by secondary bonding (3.4–3.7 Å) and essentially equal to the accepted limit for covalent bonding (3.30 Å): in other words, according to threshold distance values, we are relatively free to regard this interaction either as a bond or as contact. Lest the flip of a coin decides if we should or should not draw a bond in a polyiodide, statistical insights based on CSD surveys were used to put in perspective literature material and work out a meaningful assignment (as I₈²⁻). In doing so, we address how currently accepted threshold distance values came to be in the first place, their significance, soundness, and shortcomings in describing I₈²⁻ and its formal fragments (I₂, I₃⁻, I₅⁻). Discussion of the chemical meaning of the line representing bonding in I–I fragments in similar fringe cases, relating CSD data herein presented with the previous literature, is provided. Available information coincides quite well in supporting the necessity of a revision of broadly accepted threshold distance values.