A Macrocyclic Polyoxomolybdate with Phosphate and Phosphonate Linkers: Synthesis, Structure, and Proton Conductivity.

A coordination macrocycle composed of eight identical [PMo 8 O 27 ] - ({PMo 8 }) clusters connected by both organic tetraphosphonates and inorganic phosphates, (C 3 N 2 H 5 ) 29 (NH 4 ) 6 H 12 [(PMo 8 O 27 ) 8 (C 10 P 4 O 12 N 2 H 20 ) 4 (PO 4 ) 4 Cs(Mo 4 O 10 (H 2 O) 4 )] (C 3 N 2 H 5 + = imidazolium), is presented here. The primary building block, {PMo 8 }, is a tetravacant Keggin-type phosphomolybdate that has never been observed before. The compound shows a high proton conductivity of 9.70 × 10 -3 S cm -1 at 373 K and 98% relative humidity. Control experiments on an imidazolium-free sample demonstrate the critical role of the imidazolium counterions as mobile proton carriers. The contribution of imidazolium necessitates a high activation energy ( E a = 0.502 eV) for proton conduction via the vehicle mechanism.