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

Two neutral tripodal metalloligands ( CoL and FeL where L=C 18 H 21 N 10 ) containing a clathrochelate core were synthesized and characterized in one‐step. Reactions of these ligands with three different metal acceptors cis ‐(tmen)Pd(NO 3 ) 2 (tmen = tetramethylethylenediamine), Zn(NO 3 ) 2 and Mn(ClO 4 ) 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.