Mechanochemical Access to a Short-Lived Cyclic Dimer Pd2L2: An Elusive Kinetic Species En Route to Molecular Triangle Pd3L3 and Molecular Square Pd4L4

Pd-based molecular square Pd4L4 and triangle Pd3L3 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 Pd4L4 and/or Pd3L3 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 Pd2L2 for the first time. In the absence of solvent, Pd2L2 is indefinitely stable in the solid-state, but converts rapidly to its thermodynamic products Pd3L3 and Pd4L4 in solution, confirming Pd2L2 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.

Automated summary

The study successfully synthesized the elusive cyclic dimer Pd2L2 for the first time using mechanochemical methods, and revealed the rapid conversion of Pd2L2 to Pd3L3 and Pd4L4 in solution as a kinetic process.