Stepwise construction of self-assembled heterometallic cages showing high proton conductivity

Abstract

Two neutral tripodal metalloligands (CoL and FeL where L=C₁₈H₂₁N₁₀) 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.