Reaction of Pd(BF4)2 with L (L = bis(pyridin-3-yl-propyl)pyridine-3,5-dicarboxylate) in the 1 : 2 mole ratio gives rise to a spiro-type [PdL2]·(BF4)2·2C6H6·2CH3CN, and further self-assembly of [PdL2]·(BF4)2·2C6H6·2CH3CN with Pd(NO3)2 in the 2 : 1 mole ratio in Me2SO at 90 °C produces a uniquely pliable double cage of [(NO3)2(H2O)2@Pd3L4](BF4)4·6C3H7NO. Both the encapsulated NO3− and the outside BF4− anions are exchanged by X− to form [(X)2@Pd3L4](X′)4 (X− = PF6−, ClO4−, and/or NO3−; X’− = BF4−, PF6−, ClO4−, and NO3−) with all-inclusive pure or mixed anions. The pliable and characteristic properties of the double cages were confirmed by anion exchange of the nestled or outside anions in the present study. This system can be used as a ruler for recognition of ubiquitous polyatomic anions.
![Graphical abstract: Diverse anion exchange of pliable [X2@Pd3L4]4+ double cages: a molecular ruler for recognition of polyatomic anions](http://hg.y866.cn/compound/lib/scimg/usr/1/D0DT01027H.jpg)
