Gas and vapor adsorption in octacyanometallate-based frameworks Mn2[M(CN)8] (M = W, Mo) with exposed Mn2+ sites

Dehydration of the isostructural three-dimensional (3D) octacyanometallate-based materials Mn2M(CN)8·7H2O (M = Mo, 1·7H2O; W, 2·7H2O) generates robust porous frameworks (1 and 2). In the structure, the [M(CN)8]4− units are linked via octahedral Mn2+ centers to form an open 3D framework with 1D channels, in which the non-coordinated and coordinated water molecules are involved. The permanent porosities have been confirmed by thermogravimetric analysis, variable-temperature X-ray diffraction and Raman spectra, and adsorption (H2O, N2 and H2) measurements. H2 adsorption at 1.1 bar and 77 K was 0.60 wt% for 1 and 0.49 wt% for 2. At initial loading ΔHads has the value of ca. 10.0 kJ mol−1 for both materials, which represents the highest value reported for any cyanide-based assemblies. The high enthalpy can be attributed to the presence of coordinatively-unsaturated Mn2+ sites left exposed by the removal of coordinated water molecules in the structure.