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

Abstract

Combination of Pd(II) with selected bis-monodentate ligands produces the familiar multinuclear Pd_mL_2m type self-assembled “single-cavity discrete coordination cages” (SCDCC). If the ligand provides parallel coordination vectors, then it forms a binuclear Pd₂L₄ type cage, whereas utilization of ligands having appropriately divergent coordination vectors results in specific higher nuclear complexes. In contrast, preparation of emergent “multi-cavity discrete coordination cages” (MCDCC) using Pd(II) and designer ligands is quite captivating where the neighboring cavities of the framework are conjoined with each other through a common metal center. A pair of conjoined binuclear Pd₂L₄ type sub-frameworks are present in a trinuclear Pd₃L₄ type double-cavity cage prepared from Pd(II) and a tris-monodentate ligand having parallel coordination vectors. The present work envisioned a design to make double-cavity coordination cages having a pair of conjoined trinuclear Pd₃L₆ type sub-frameworks. To fulfill the objective we combined Pd(II) with a mixture of designer bis-monodentate ligand (L) and tris-monodentate ligand (L′) in a 5 : 4 : 4 ratio in one pot to afford the targeted pentanuclear type cage. The choice of bis-monodentate ligand L is based on the divergent nature of the coordination vectors suitable to produce a Pd₃L₆ type SCDCC. The tris-monodentate ligand L′ having two arms is designed in such a manner that each of the arms reasonably resembles L. Study of the complexation behavior of Pd(II) with L′ provided additional guiding factors essential for the successful making of type MCDCC by integrative self-sorting. A few other MCDCC including lower symmetry versions were also prepared in the course of the work.