Mechanochemical Access to a Short‐Lived Cyclic Dimer Pd2L2: An Elusive Kinetic Species En Route to Molecular Triangle Pd3L3and Molecular Square Pd4L4

Abstract

AbstractPd‐based molecular squarePd4L4and trianglePd3L3represent 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 betweencis‐protected Pd ions and 2,2′‐bipyridine exclusively givePd4L4and/orPd3L3as 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 dimerPd2L2for the first time. In the absence of solvent,Pd2L2is indefinitely stable in the solid‐state, but converts rapidly to its thermodynamic productsPd3L3andPd4L4in solution, confirmingPd2L2as 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”.