Biasing the Formation of Solution-Unstable Intermediates in Coordination Self-Assembly by Mechanochemistry

Due to the reversible nature of coordination bonds and solvation effect, coordination self-assembly pathways are often difficult to elucidate experimentally in solution, as intermediates and products are in constant equilibration. The present study shows that some of these transient and high-energy self-assembly intermediates can be accessed by means of ball-milling approaches. Among them, highly aqueous-unstable Pd3L11 and Pd6L14 open-cage intermediates of the framed Fujita Pd6L14 cage and Pd2L22, Pd3L21 and Pd4L22 intermediates of Mukherjee Pd6L24 capsule are successfully trapped in solid-state, where Pd=tmedaPd2+, L1=2,4,6-tris(4-pyridyl)-1,3,5-triazine and L2=1,3,5-tris(1-imidazolyl)benzene). Their structures are assigned by a combination of solution-based characterization tools such as standard NMR spectroscopy, DOSY NMR, ESI-MS and X-ray diffraction. Collectively, these results highlight the opportunity of using mechanochemistry to access unique chemical space with vastly different reactivity compared to conventional solution-based supramolecular self-assembly reactions. A solvent-free ball-mill approach is developed to modulate coordination self-assembly energy landscapes, allowing access to self-assembly intermediates in the solid state that are too short-lived to be observed in traditional solution-based approaches.image