Electronic Properties of [Ru(NH₃)₆][M(CN)₆] Double-Complex Crystals and Their Thermal Decomposition to Prussian Blue Analogs

The complex ions, [Ru(NH 3 ) 6 ] 2+/3+ and [Fe(CN) 6 ] 3-/4- , are well known redox couples exhibiting fast interfacial and homogeneous electron transfer rates in aqueous solution. Here we show that when mixed together, they form insoluble double-complex A x [Ru(NH 3 ) 6 ][Fe(CN) 6 ] crystals with rhombohedral symmetry, where A is a cationic species and 0≤x≤2. The analogous interactions of [Ru(NH 3 ) 6 ] 2+/3+ with [Ru(CN) 6 ] 3-/4- produces A x [Ru(NH 3 ) 6 ][Fe(CN) 6 ] crystals. Electrical measurements of these single crystals exhibit ohmic current-voltage responses, with corresponding conductivities on the order of 0.1 mS/m. We propose a solid-state charge transport mechanism based on Marcus theory for homogeneous outer-sphere electron transfer. Thermal decomposition at mild temperatures removes ammine ligands and transforms these crystals into nanopolycrystalline Prussian blue analogs (PBAs) that retain the macroscopic geometry of the double-complex crystals but adopt the familiar highly defected porous crystal structure and intervalence charge transfer characteristics of PBAs.