Reversible Transformation between a [PdL ₂ ] ²⁺ “Figure-of-Eight” Complex and a [Pd ₂ L ₂ ] ⁴⁺ Dimer: Switching On and Off Self-Recognition

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with Pd ^II metal ions, a spiro-type [PdL ₂ ] ²⁺ “Figure-of-eight” complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of Pd ^II metal ion results in the conversion to a dimeric [Pd ₂ L ₂ ] ⁴⁺ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of Pd ^II metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL ₂ ] ²⁺ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:Pd ^II stoichiometry.