Crystal Growth from Anhydrous HF Solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]^-, Not Only Simple M(AuF₆)₂ Salts

Crystal growth from anhydrous HF solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]^- (molar ratio 1:2) gave [Ca(HF)₂](AuF₆)₂, [Sr(HF)](AuF₆)₂, and Ba[Ba(HF)]₆(AuF₆)₁₄. [Ca(HF)₂](AuF₆)₂ exhibits a layered structure in which [Ca(HF)₂]²⁺ cations are connected by AuF₆ units, while the crystal structure of Ba[Ba(HF)]₆(AuF₆)₁₄ exhibits a complex three-dimensional (3-D) network consisting of Ba²⁺ and [Ba(HF)₂]²⁺ cations bridged by AuF₆ groups. These results indicate that the previously reported M(AuF₆)₂ (M = Ca, Sr, Ba) compounds, prepared in the anhydrous HF, do not in fact correspond to this chemical formula. When the initial M²⁺/[AuF₆]^- ratio was 1:1, single crystals of [M(HF)](H₃F₄)(AuF₆) were grown for M = Sr. The crystal structure consists of a 3-D framework formed by [Sr(HF)]²⁺ cations associated with [AuF₆]^- and [H₃F₄]^- anions. The latter exhibits a Z-shaped conformation, which has not been observed before. Single crystals of M(BF₄)(AuF₆) (M = Sr, Ba) were grown when a small amount of BF₃ was present during crystallization. Sr(BF₄)(AuF₆) crystallizes in two modifications. A high-temperature α-phase (293 K) crystallized in an orthorhombic unit cell, and a low-temperature β-phase (150 K) crystallized in a monoclinic unit cell. For Ba(BF₄)(AuF₆), only an orthorhombic modification was observed in the range 80-230 K. An attempt to grow crystals of Ca(BF₄)(AuF₆) failed. Instead, crystals of [Ca(HF)](BF₄)₂ were grown and the crystal structure was determined. During prolonged crystallization of [AuF]₆^- salts, moisture can penetrate through the walls of the crystallization vessel. This can lead to partial reduction of Au(V) to A(III) and the formation of [AuF₄]^- byproducts, as shown by the single-crystal growth of [Ba(HF)]₄(AuF₄)(AuF₆)₇. Its crystal structure consists of [Ba(HF)]²⁺ cations connected by AuF₆ octahedra and square-planar AuF₄ units. The crystal structure of the minor product [O₂]₂[Sr(HF)]₅[AuF₆]₁₂·HF was also determined.