Self-Assembled Molecular Squares as Supramolecular Tectons

Concentration-dependent equilibria of molecular squares [Pd-4(L')(4)(L)(4)](NO3)(8) and triangles [Pd-3(L')(3)(L)(3)](NO3)(6) were obtained when cis-protected Pd(II) units [PdL'(NO3)(2)] (L' = tmeda, 2,2'-bpy, and phen) were combined independently with 4,4'-bipyridine (L) in water. However, complexation of [PdL'(OTs)(2)] with L resulted in exclusive formation of the corresponding molecular squares. The addition of AgOTs to each mixture of square and triangle led to a shift in the equilibrium, resulting in the disappearance of the triangles and exclusive formation of the corresponding squares. The crystal structures of the molecular squares [Pd-4(L')(4)(L)(4)](OTs)(8) revealed a pair of tosylate anions encapsulated in the hydrophobic cavity of the square. Further, [Pd-4(2,2'-bpy)(4)(L)(4)](OTs)(8) and [Pd-4(phen)(4)(L)(4)](OTs)(8) exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular pi center dot center dot center dot pi stacking interactions wherein the cis-protecting units (i.e., 2,2'-bpy and phen) of adjacent molecules overlap via side-on or end-on modes. Thus, the cations may be considered as "tectons", each of which contains four peripheral 2,2'-bpy/phen units, which behave as "supramolecular synthons" in the self-assembly of the squares. The tosylates interact with the cations through C-H center dot center dot center dot O and C-H center dot center dot center dot pi interactions and play a role in the packing of the molecular squares.