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

Dalton Trans. 2012 Jun 28;41(24):7218-26. doi: 10.1039/c2dt30414g. Epub 2012 May 11.

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 [Zn(2)(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.