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

A set of Pd 2 L 4 , Pd 3 L 4 , and Pd 4 L 4 -type single-, double-, and triple-cavity cages are prepared by complexation of Pd(NO 3 ) 2 with designer bis-monodentate ( L1 ), tris-monodentate ( L2 ), and tetrakis-monodentate ( L3 ) ligands. The Pd 2 L 4 cage exists in equilibrium with a Pd 3 L 6 cage; the equilibrium shifted to Pd 2 L 4 at 70 °C or upon addition of pyrazine- N,N' -dioxide (PZDO). The Pd 2 L 4 cage binds a PZDO molecule using electrostatic, bifurcated H-bonding and overcoordinated H-bonding interactions. The discrete Pd 3 L 4 and Pd 4 L 4 compounds are conjoined cages comprising of unequal sized Pd 2 L 4 cages (bigger and smaller). The bigger unit of Pd 3 L 4 cage selectively binds a PZDO, and the smaller one binds a nitrate, fluoride, chloride, or bromide. The Pd 4 L 4 cage, having a central bigger Pd 2 L 4 cavity and two smaller peripheral Pd 2 L 4 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 Pd 3 L 4 and Pd 4 L 4 cages could be prepared with (preferably) or without using a template for smaller cavity, and the bigger Pd 2 L 4 is formed by sacrificing the possibility of the Pd 3 L 6 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 Pd 3 L 4 ) augments the binding affinity and that is further augmented in the presence of two neighboring cages (as in Pd 4 L 4 ).