Mechanochemical Access to a Short‐Lived Cyclic Dimer Pd<sub>2</sub>L<sub>2</sub>: An Elusive Kinetic Species En Route to Molecular Triangle Pd<sub>3</sub>L<sub>3</sub>and Molecular Square Pd<sub>4</sub>L<sub>4</sub>
Yan Liu, Fang‐Zi Liu, KaKing Yan
Abstract
Abstract Pd‐based molecular square Pd 4 L 4 and triangle Pd 3 L 3 represent the molecular ancestors of metal‐coordination polyhedra that have been an integral part of the field for the last 30 years. Conventional solution‐based reactions between cis ‐protected Pd ions and 2,2′‐bipyridine exclusively give Pd 4 L 4 and/or Pd 3 L 3 as the sole products. We herein show that, under solvent‐free mechanochemical conditions, the self‐assembly energy landscape can be thermodynamically manipulated to form an elusive cyclic dimer Pd 2 L 2 for the first time. In the absence of solvent, Pd 2 L 2 is indefinitely stable in the solid‐state, but converts rapidly to its thermodynamic products Pd 3 L 3 and Pd 4 L 4 in solution, confirming Pd 2 L 2 as a short‐lived kinetic species in the solution‐based self‐assembly process. Our results highlight how mechanochemistry grants access to a vastly different chemical space than available under conventional solution conditions. This provides a unique opportunity to isolate elusive species in self‐assembly processes that are too reactive to both “see” and “capture”.