Self-assembled molecular squares as supramolecular tectons

Abstract

Concentration-dependent equilibria of molecular squares [Pd4(L′)4(L)4](NO3)8 and triangles [Pd3(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 [Pd4(L′)4(L)4](OTs)8 revealed a pair of tosylate anions encapsulated in the hydrophobic cavity of the square. Further, [Pd4(2,2′-bpy)4(L)4](OTs)8 and [Pd4(phen)4(L)4](OTs)8 exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular π···π stacking interactions wherein the cis-protecting units (i.e., 2...