Neighboring Cage Participation for Assisted Construction of Self-Assembled Multicavity Conjoined Cages and Augmented...

A set of Pd₂L₄, Pd₃L₄, and Pd₄L₄-type single-, double-, and triple-cavity cages are prepared by complexation of Pd(NO₃)₂ with designer bis-monodentate (L1), tris-monodentate (L2), and tetrakis-monodentate (L3) ligands. The Pd₂L₄ cage exists in equilibrium with a Pd₃L₆ cage; the equilibrium shifted to Pd₂L₄ at 70 °C or upon addition of pyrazine-N,N'-dioxide (PZDO). The Pd₂L₄ cage binds a PZDO molecule using electrostatic, bifurcated H-bonding and overcoordinated H-bonding interactions. The discrete Pd₃L₄ and Pd₄L₄ compounds are conjoined cages comprising of unequal sized Pd₂L₄ cages (bigger and smaller). The bigger unit of Pd₃L₄ cage selectively binds a PZDO, and the smaller one binds a nitrate, fluoride, chloride, or bromide. The Pd₄L₄ cage, having a central bigger Pd₂L₄ cavity and two smaller peripheral Pd₂L₄ cavities, binds one PZDO and two nitrate, fluoride, chloride, or bromide. The smaller cavity can be prepared individually from Pd(II) and bis-monodentate ligand (L4), however, in the presence of template like a nitrate, fluoride, chloride, or bromide; otherwise, it forms an oligomeric mixture. Notably, the conjoined Pd₃L₄ and Pd₄L₄ cages could be prepared with (preferably) or without using a template for smaller cavity, and the bigger Pd₂L₄ is formed by sacrificing the possibility of the Pd₃L₆ moiety. Thus, the conjoined cages are formed in a symbiotic manner where the neighboring cages participate in the formation of each other. The binding of PZDO shows that the presence of one neighboring cage (as in Pd₃L₄) augments the binding affinity and that is further augmented in the presence of two neighboring cages (as in Pd₄L₄).