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 m L 2m formulation. However, orientational isomers are expected when ligand is unsymmetrical. Dynamic equilibrium of Pd 2 L 4 - and Pd 3 L 6 -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 (Pd 2 L 4 in CH 3 CN; Pd 3 L 6 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)-Pd 3 L 6 of L2 and L3 are designated as isomer-1 and isomer-2, under ligand isomerism. The isomer-1 in DMSO could be switched to Pd 2 L 4 form by adding CH 3 CN and reverted to Pd 3 L 6 form by evaporation of CH 3 CN. However, the isomer-2 in DMSO showed insignificant changes upon addition of CH 3 CN. Thus, we demonstrated differential switching modes of a pair of isomeric cages that are endohedrally low-symmetry.