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

Abstract

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. Herein, we showed 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 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, our result highlights the opportunity of using mechanochemistry to access unique chemical space with vastly different reactivity compared to conventional solution‐based supramolecular self‐assembly reactions.

Solid-State Self-Assembly: Exclusive Formation and Dynamic Interconversion of Discrete Cyclic Assemblies Based on Molecular Tweezers

“…Recently, a few energetically unfavored coordination-driven self-assemblies have been constructed in the solid state via crystallization 31 − 34 or under ball-milling conditions. 35 − 38 As shown in these representative studies, the resulting self-assembled structures are essentially inaccessible in solution. Thus, solid-state self-assembly beyond typical crystal engineering 14 , 15 represents an unconventional strategy that can potentially lead to the development of sophisticated well-defined supramolecules with unusual functions.…”

“…The term “solid-state self-assembly” encompasses two meanings here: (i) the construction of assemblies through crystallization (from solution to the solid state) and (ii) self-assembly in the solid state (from the solid state to the solid state), similar to solid-state synthesis. − Indeed, some macrocyclic organic molecules coincidentally form unique cyclic or capsule-shaped assemblies such as dimers, ,, trimers, and hexamers ,− through crystallization. Recently, a few energetically unfavored coordination-driven self-assemblies have been constructed in the solid state via crystallization − or under ball-milling conditions. − As shown in these representative studies, the resulting self-assembled structures are essentially inaccessible in solution. Thus, solid-state self-assembly beyond typical crystal engineering , represents an unconventional strategy that can potentially lead to the development of sophisticated well-defined supramolecules with unusual functions.…”

Mechanochemistry for Organic and Inorganic Synthesis

“…Using a similar approach, Yan and co-workers used the same self-assembly mechanochemical approach to prepare and isolate several unstable Pd cages and their intermediates. 119 Regarding N,N-diaryl NHC metal complexes, mechanochemistry has proven to be a handy tool for preparing these types of compounds, which usually require prolonged reactions in reflux conditions to achieve metalation. 120 To this end, Lamaty's group described a very convenient preparation of Cu, Au, and Pd NHC complexes under mechanochemical conditions.…”