On the microporous nature of transition metal nitroprussides

Nitroprussides of divalent transition metals form a family of microporous molecular materials. Their properties in this sense depend on the transition metal cation involved and also on the preparative method, which determine their crystal structures. The stable phases of this family of materials belong to one of the following crystal structures: orthorhombic (Pnma) (Mn²⁺, Fe²⁺, Cu²⁺, Zn²⁺, and Cd²⁺), cubic (Fm3m) (Co²⁺ and Ni²⁺), and orthorhombic (Amm2) (Cu²⁺). These materials are stable up to above 160°C, while their dehydration takes place around 100°C. On dehydration, Amm2 copper complex changes into a tetragonal (I4mm) phase. The microporous nature of these materials is discussed according to their crystal structure and correlating structural and adsorption data. The accessibility to the pore system was evaluated through adsorption of H₂O, CO₂, and N₂. Pores of both orthorhombic and cubic structures are accessible to H₂O and CO₂ in experiments carried out at 23 and 0°C, respectively; however, they are inaccessible to N₂ at -196°C. This behavior is discussed as related to the large polarizing power of the nitrosyl (NO) ligand which distorts the local environment of the iron atom and reduces the effective window cross section. The small pores of tetragonal copper nitroprusside were inaccessible to the adsorbates used.