New Cubic Phases for T₂M[CN]₆·xH₂O with T = Ni, Cu and M = Ru, Os: Improving the Robustness and Modulating the Electron Density at the Cavity Surfaces

The four Prussian blue analogs (PBAs) under study in this contribution, T₂[M(CN)₆]·xH₂O, have an open framework structure with 50 % of vacancies for the hexacyanometallate building block. In this series, the metal T is always found at the surface of the formed system of cavities and has a partially naked coordination environment. In the as-synthesised materials, such available coordination sites are occupied by water molecules, which, in turn, stabilize additional water molecules inside the cavity through hydrogen bonding interactions. Herein, we report the synthesis, crystal structure, and related properties for the new title members of this family of coordination polymers, particularly for T = Ni, Cu and M = Ru, Os. To improve the material's crystallinity, the powders formed from the precipitation reaction were submitted to solvothermal recrystallization. Their crystal structure was solved and refined from the corresponding X-ray diffraction (XRD) patterns. Unlike their Fe^II analogs, which crystallize in a Pm3m cubic structure, two types of cubic structures (depending on the external metal) were obtained: Fm3m for Ni₂[M(CN)₆]·xH₂O and Pm3m for Cu₂[M(CN)₆]·xH₂O. Such structural differences are properly supported by a detailed analysis of their IR and UV/Vis spectra. The stability of their porous network after water removal was evaluated by combining XRD, H₂ adsorption isotherms, and thermogravimetric curves. Hydrogen adsorption was also used to explore the adsorption potential at the cavity surface. Except Cu₂[Ru(CN)₆]·xH₂O, the other three materials appear to be stable after water removal under soft heating, which was ascribed to the role of the inner metal (Ru^II and Os^II) on the framework robustness. These two metals have extended t_2g orbitals, which favor the occurrence of a strong coupling between the inner and outer metals in the –T–N≡C-M–C≡N-T– chain. Apparently, such overlapping for charge density contributes to the material's stability.