Mild Hydrothermal Synthesis of the Complex Hafnium-Containing Fluorides Cs₂[M(H₂O)₆][Hf₂F₁₂] (M = Ni, Co, Zn), CuHfF₆(H₂O)₄, and Cs₂Hf₃Mn₃F₂₀ Based on HfF₇ and HfF₆ Coordination Polyhedra

A series of new Hf­(IV)-containing fluorides with three different compositions, Cs₂[M­(H₂O)₆]­[Hf₂F₁₂] (M = Ni, Co, Zn), CuHfF₆(H₂O)₄, and Cs₂Hf₃Mn₃F₂₀, were synthesized as high-quality single crystals via a mild hydrothermal route. The compounds with compositions of Cs₂[M­(H₂O)₆]­[Hf₂F₁₂] (M = Ni, Co, Zn) and CuHfF₆(H₂O)₄ crystallize in the monoclinic space groups P2₁/n and P2₁/c, respectively, while the Cs₂Hf₃Mn₃F₂₀ phase crystallizes in the orthorhombic space group Pmmn. Cs₂[M­(H₂O)₆]­[Hf₂F₁₂] (M = Ni, Co, Zn) exhibits a complex three-dimensional (3D) crystal structure consisting of edge-sharing dimers of HfF₇ polyhedra, which are linked to the divalent metal octahedra via hydrogen bonding. Cs₂Hf₃Mn₃F₂₀ features corner-sharing HfF₇ and MnF₇ dimers as well as isolated MnF₆ octahedra, while the CuHfF₆(H₂O)₄ phase exhibits a 3D structure that consists of HfF₆ octahedra linked with neighboring copper octahedral units by hydrogen-bonding interactions. UV–vis spectra of the title compounds were collected and exhibit absorption bands due to electronic transitions in the divalent metal cations (Ni²⁺, Co²⁺, Cu²⁺, and Mn²⁺). Magnetic susceptibility measurements revealed paramagnetic behavior in the compounds containing the magnetic cations Ni, Co, Cu, and Mn.