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

Abstract

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).

A pair of conjoined trinuclear sub-frameworks in a pentanuclear double-cavity discrete coordination cage

“…In the framework of a Pd 3 L 4 type cage there are two units of Pd 2 L 4 type sub-framework that are apparently conjoined with each other using a common square planar coordination environment of a Pd( ii ) centre. Low-symmetry Pd 3 L 4 cages possessing two unequal sized Pd 2 L 4 sub-frameworks have been reported 40–42 as shown in Fig. 1D(i).…”

Multicomponent, Multicavity Metallacages That Contain Different Binding Sites for Allosteric Recognition

“…Metal-coordination-driven self-assembly has proved to be a reliable strategy for the construction of supramolecular coordination complexes (SCCs) with multifunctional geometric structures owing to the directionality and moderate bond strength of metal-coordination bonds. − Specially, metallacages, as three-dimensional SCCs, can be finely designed to encapsulate a series of guest molecules, exhibiting various applications in guest encapsulation, sensing, catalysis, stabilizing reactive intermediates, etc. − Recently, multicavity metallacages have been developed to encapsulate multiple guests cooperatively to achieve multifunctionalities. − In these systems, the multidentate ligands are designed to be sufficiently rigid to avoid the formation of thermodynamic byproducts, so most of the multicavity metallacages only show binding affinity similar to the sum of their individual metallacage precursors. The construction of multicavity metallacages with certain flexibility to allow intramolecular communication between different guest molecules for allosteric recognition has been rarely reported.…”