Mechanochemical Access to a Short-Lived Cyclic Dimer Pd 2 L 2 : An Elusive Kinetic Species En Route to Molecular Triangle Pd 3 L 3 and Molecular Square Pd 4 L 4 .

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".