Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates

Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solid-state electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of α- and β-CaB₁₀H₁₀, which are structurally and energetically similar, both showing a tetrahedral coordination of Ca²⁺ to four closo-borate cages. The main distinctions between the α- and β-polymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca²⁺ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation-anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, and discovered an order-disorder polymorphic transition, α- to β-CaB₁₀H₁₀·6H₂O. The hydrates reveal a rich structural diversity with ordered structures, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7-8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B-H^δ-···^+δH-O; hydrogen-hydrogen interactions, B-H···H-B; and hydrogen bonds, O-H^δ+···^-δO-H. A rather short B-H···H-B (2.14 Å) interaction is observed for CaB₁₀H₁₀·5H₂O, which is locally stabilized by four hydrogen bonds.