Design of a double-decker coordination cage revisited to make new cages and exemplify ligand isomerism

The complexation study of cis-protected and bare palladium(II) components with a new tridentate ligand, i.e., pyridine-3,5-diylbis(methylene) dinicotinate (L1) is the focus of this work. Complexation of cis-Pd(tmeda)(NO(3))(2) with L1 at a 1:1 or 3:2 ratio produced [Pd(tmeda)(L1)](NO(3))(2) (1a). The reaction mixture obtained at 3:2 ratio upon prolonged heating, produced a small amount of [Pd(3)(tmeda)(3)(L1)(2)](NO(3))(6) (2a). Complexation of Pd(NO(3))(2) with L1 at a 1:2 or 3:4 ratios afforded [Pd(L1)(2)](NO(3))(2) (3a) and [(NO(3))(2)@Pd(3)(L1)(4)](NO(3))(4) (4a), respectively. The encapsulated NO(3)(–) ions of 4a undergo anion exchange with halides (F(–), Cl(–) and Br(–) but not with I(–)) to form [(X)(2)@Pd(3)(L1)(4)](NO(3))(4) 5a–7a. The coordination behaviour of ligand L1 and some dynamic properties of these complexes are compared with a set of known complexes prepared using the regioisomeric ligand bis(pyridin-3-ylmethyl)pyridine-3,5-dicarboxylate (L2). Importantly, a ligand isomerism phenomenon is claimed by considering complexes prepared from L1 and L2.