Selecting Topology and Connectivity through Metal-Directed Macrocyclization Reactions:  A Square Planar Palladium [2]Catenate and Two Noninterlocked Isomers

We report the synthesis of a [2]catenate using a square planar palladium(II) template, together with two isomers of the interlocked structure:  a single tetradentate macrocycle that adopts a “figure of eight” conformation to encapsulate the metal and a complex in which the two macrocycles of the catenane are not interlocked. The three isomers can each be selectively formed depending on how the building blocks are assembled and cyclized. Olefin metathesis of both building blocks while they are attached to the metal gives the single large macrocycle in 77% yield. Cyclizing the monodentate unit prior to attaching both ligands to the metal gives the [2]catenate in 78% yield. Preforming the tridentate macrocycle produces a complex in two atropisomeric formsthreaded and nonthreadedin a 2:3 ratio, which do not interconvert in dichloromethane at room temperature over 7 days. RCM of the nonthreaded atropisomer affords the complex with two noninterlocked macrocyclic ligands; RCM of the threaded atropisomer generates the topologically isomeric [2]catenate. Heating the acyclic atropisomers in acetonitrile provides a mechanism for their interconversion via ligand exchange, allowing the threaded:nonthreaded ratio to be varied from 2:3 to 8:1. All three fully ring-closed complexes were characterized unambiguously by ¹H NMR spectroscopy and X-ray crystallography. As far as we are aware, this is the first time such a set of three formal topological and constitutional isomers has been described.