Stepwise Construction of Self-Assembled Heterometallic Cages Showing High Proton Conductivity

Two neutral tripodal metalloligands (CoL and FeL where L=C₁₈ H₂₁ N₁₀ ) containing a clathrochelate core were synthesized and characterized in one-step. Reactions of these ligands with three different metal acceptors cis-(tmen)Pd(NO₃ )₂ (tmen = tetramethylethylenediamine), Zn(NO₃ )₂ and Mn(ClO₄ )₂ separately yielded a series of heterometallic coordination cages (1 a-3 a and 1 b-3 b) in high yields. Depending on the nature of coordination geometry of the acceptors, the resulting assemblies have trigonal- bipyramidal (1 a/1 b), open-cubic (2 a/2 b), and closed-cubic structures (3 a/3 b). The structures of the complexes 1 a, 2 a, 2 b, 3 a, and 3 b were confirmed by single-crystal X-ray diffraction studies. Analysis of crystal packing of the complexes 3 a and 3 b revealed the presence of several coordinated and lattice water molecules in the intermolecular channels. Both these complexes (3 a and 3 b) showed very high water adsorption under humid conditions. In addition, 3 a and 3 b exhibited promising proton conductivity of 3.31×10^-3 and 1.05×10^-4  S cm^-1 at 70 °C under 98 % relative humidity (RH) respectively, with activation energy of 1.00-0.78 eV.