Stabilization of interpenetrating cluster-based frameworks promoted by N–H⋯X hydrogen bonds: synthesis, structures and properties of {[Cd(NH₃)₄]₃[Re₃Mo₃Se₈(CN)₆]}X (X = Cl, Br and I)

Control of covalent coordination polymer structures using non-covalent interactions is a promising way for obtaining functional materials by self-assembly in solution. Here we report the crucial role of halide anions in the formation of interpenetrating frameworks based on cyanometalate clusters. It was found that the interaction of [Re₃Mo₃Se₈(CN)₆]⁵⁻ cluster anions and Cd²⁺ cations in aqueous ammonia led to the formation of the 1D polymeric compound {[Cd(NH₃)₅]₂[Cd(NH₃)₄]₃[Re₃Mo₃Se₈(CN)₆]₂}·5H₂O (1). Compound 1 is unstable outside the mother liquor due to the rapid loss of NH₃ and H₂O molecules. Addition of KX (X = Cl, Br, I) to the reaction mixture led to selective formation of 3D framework compounds {[Cd(NH₃)₄]₃[Re₃Mo₃Se₈(CN)₆]}X (2–4 for X = Cl, Br, I, respectively) stabilized by N–H⋯X hydrogen bonding. Compounds 2–4 demonstrate high thermal stability as well as reversible loss of ammonia and reversible oxidation in the solid state.