A soluble cyanide-bridged {Fe₄Ni₄} box encapsulating a Cs⁺ ion: synthesis, structure and electronic properties

Cyanide coordination clusters have received attention for their electronic and magnetic properties. Here, we synthesized a new octametallic cyanide box encapsulating a Cs⁺. The cationic complex Cs⊂[Fe^III₄(Tp)₄Ni^II₄(^NMe2Tp)₄(μ-CN)₁₂]⁺ was crystallized as a BF₄⁻ salt and its structure reveals that the cesium ion interacts with the twelve cyanide edges. These interactions are likely responsible for the remarkable stability of the supramolecular assembly, which maintains its integrity in solution, as shown by NMR spectroscopy, mass spectrometry, and cyclic voltammetry experiments. The ¹H NMR spectrum of 1 shows well-resolved signals, which are strongly shifted due to the paramagnetic nature of the cage. The encapsulated caesium ion is also affected by the paramagnetic cage and shows a strongly shifted ¹³³Cs paramagnetic NMR signal. The {Fe₄Ni₄} box shows remarkable redox flexibility with five accessible oxidation states, which correspond to the reversible oxidation/reduction of the four Fe^III/Fe^II ions. The separation of the four consecutive redox waves accounts for the occurrence of an efficient electronic communication between the metal ions through the cyanide bridges. The cyanide bridges are also efficient in transmitting a ferromagnetic interaction between the iron and nickel ions as shown by the magnetic properties.