New structure, reconstruction, and behaviour of the coordination polymer framework [CuZn(CN)₄]^- towards hydrated alkali metal ions

A new framework structure of the coordination polymer [CuZn(CN)₄]^- has been discovered in the newly synthesized [Na(H₂O)_n][CuZn(CN)₄] (n = 12.7; 1). X-ray crystal structure analysis revealed a 3D framework comprised of tetrahedral Cu(I) and Zn(II) centres with bridging cyanide ligands containing both octagonal and square channel cavities; these channels contain hydrated Na⁺ ions as guests. In open air, complex 1 is extremely unstable and loses water molecules to form [Na(H₂O)_n][CuZn(CN)₄] (n = 5.2-6.8; 2) within 30 min. Powder X-ray diffraction patterns showed that this change results in the conversion of the framework of complex 1 to the same framework observed in [K(H₂O)_n][CuZn(CN)₄]. The [CuZn(CN)₄]^- framework of [K(H₂O)_n][CuZn(CN)₄] has a tridymite structure with hexagonal channel cavities occupied by hydrated K⁺ ions. Like 1, 2 is also unstable under atmospheric conditions, yielding [Na(H₂O)_n][CuZn(CN)₄] (n = 3), which has an unknown structure. While the framework of [K(H₂O)_n][CuZn(CN)₄] is structurally flexible and robust, the framework of 2 is unstable and fragile. This contrast comes from the difference between the nature of the hydrated K⁺ and Na⁺ ions present in the frameworks. (author)