Looking for the origin of the switch between coordination-captured helicates and catenates

Abstract

Self-assembly of the linear segmental ligand L5, consisting of a tridentate binding unit flanked with two bidentate binding units, with a mixture of Fe^II/Ag^I yields the trinuclear coordination-captured [2]catenate [AgFeAg(L5)₂]⁴⁺ instead of the planned isomeric double-stranded helicate. Replacing the octahedral (Fe^II) and tetrahedral (Ag^I) cations with Zn^II, which is compatible with both geometries, gives intricate mixtures of homometallic complexes upon reaction with the twin ligand L6, from which the macrocyclic dinuclear complex [Zn₂(L6)]⁴⁺ can be isolated. Application of the thermodynamic site binding model attributes the origin of the ligand preference for producing single-stranded macrocycles, the precursors of the trinuclear catenate, to the abnormally low value of the effective molarity controlling the intramolecular connection leading to the usual double-stranded helical isomer.