Anion packing, hole filling, and HF solvation in A2(HF)nB12F12 and K2(HF)TiF6 (A = K, Cs)

Abstract

The crystal structures of three new HF solvates of fluoroanion salts of alkali metal ions are reported, K2(HF)TiF6, K2(HF)3B12F12, and Cs2(HF)B12F12. The anion packing in K2(HF)TiF6 (P21/m) is distorted cubic close-packed with Ti⋯Ti distances that range from 5.717(1) to 7.394(1)Å (average 6.18Å). Half of the K+ ions are in Td holes and half are in Oh holes (i.e., this is a distorted version of the Cs2S structure). Each HF molecule is bonded to a K+ ion in the Oh holes (KF(H)=2.679(5)Å) and also weakly interacts with two other K+ ions in adjacent Oh holes (K⋯F(H)=3.238(2)Å). The anion packing in K2(HF)3B12F12 (Fm3¯m) is simple cubic. The (B12 centroid)⋯(B12 centroid) distance (⊙⋯⊙ distance) is 7.242Å, and disordered K2(μ-HF)32+ cations occupy each cube. The anion packing in Cs2(HF)B12F12 (P21/c) is distorted hexagonal close-packed with ⊙⋯⊙ distances that range from 7.217 to 9.408Å (average 8.304Å). The HF molecule bridges Cs+ ions in adjacent Oh holes, forming infinite Cs+(μ-HF)Cs+(μ-HF) chains. The other half of the Cs+ ions are in Td holes, displaced nearly 1Å from the center of those holes. This structure is similar to the distorted Ni2In structure exhibited by Cs2(H2O)B12F12. The new results are used to compare and contrast the strength of M–F(H) interactions with M–F interactions involving F atoms from fluoroanions as well as the solid-state packing of icosahedral B12F122− anions and octahedral MF62− anions in alkali-metal salts, both with and without the inclusion of weakly-basic HF solvent molecules.