Ligand Isomerism in Endohedrally Low-Symmetry Discrete Coordination Cages and Their Differential Switching Modes

Complexation of Pd(II) with a symmetrical bis-monodentate ligand produces a cage or dynamic equilibrium of more than one cage of the Pd_mL_2m formulation. However, orientational isomers are expected when ligand is unsymmetrical. Dynamic equilibrium of Pd₂L₄- and Pd₃L₆-type endohedrally low-symmetry cages containing orientational isomers (13 possibilities) were obtained from Pd(II) and pyridine-3-yl appended unsymmetrical ligand L1 having amide-phenylene-ester spacer. Crafting geometric constraint (unequal-sized coordination-arms) in ligand should bring isomeric selectivity but with altered cage-shape/size. To retain cage-shape/size, a steric constraint was implanted by peripheral substitution on L1. Quinolin-3-yl was placed instead of pyridine-3-yl at the amide-end and ester-end of L1 to obtain regioisomeric ligands L2 and L3, respectively. Solvent-dependent nuclearity control (Pd₂L₄ in CH₃CN; Pd₃L₆ in dimethyl sulfoxide (DMSO)) and substituent-driven isomeric control (all-cis(2,2) isomer) have been obtained by mixing Pd(II) with L2 or L3. The obtained all-cis(2,2)-Pd₃L₆ of L2 and L3 are designated as isomer-1 and isomer-2, under ligand isomerism. The isomer-1 in DMSO could be switched to Pd₂L₄ form by adding CH₃CN and reverted to Pd₃L₆ form by evaporation of CH₃CN. However, the isomer-2 in DMSO showed insignificant changes upon addition of CH₃CN. Thus, we demonstrated differential switching modes of a pair of isomeric cages that are endohedrally low-symmetry.