Chiral anionic layers in tartramide spiroborate salts and variable solvation for [NR₄][B(TarNH₂)₂] (R = Et, Pr or Bu)

The spiroborate anion, namely, 2,3,7,8-tetracarboxamido-1,4,6,9-tetraoxa-5λ⁴-boraspiro[4.4]nonane, [B(TarNH₂)₂]^-, derived from the diol L-tartramide TarNH₂, [CH(O)(CONH₂)]₂, shows a novel self-assembly into two-dimensional (2D) layer structures in its salts with alkylammonium cations, [NR₄]⁺ (R = Et, Pr and Bu), and sparteinium, [HSpa]⁺, in which the cations and anions are segregated. The structures of four such salts are reported, namely, the tetrapropylazanium salt, C₁₂H₂₈N⁺·C₈H₁₂BN₄O₈^-, the tetraethylazanium salt hydrate, C₈H₂₀N⁺·C₈H₁₂BN₄O₈^-·6.375H₂O, the tetrabutylazanium salt as the ethanol monosolvate hemihydrate, C₁₆H₃₆N⁺·C₈H₁₂BN₄O₈^-·C₂H₅OH·0.5H₂O, and the sparteinium (7-aza-15-azoniatetracyclo[7.7.1.0^2,7.0^10,15]heptadecane) salt as the ethanol monosolvate, C₁₅H₂₇N₂⁺·C₈H₁₂BN₄O₈^-·C₂H₅OH. The 2D anion layers have preserved intermolecular hydrogen bonding between the amide groups and a typical metric repeat of around 10 × 15 Å. The constraint of matching the interfacial area organizes the cations into quite different solvated arrangements, i.e. the [NEt₄] salt is highly hydrated with around 6.5H₂O per cation, the [NPr₄] salt apparently has a good metric match to the anion layer and is unsolvated, whilst the [NBu₄] salt is intermediate and has EtOH and H₂O in its cation layer, which is similar to the arrangement for the chiral [HSpa]⁺ cation. This family of salts shows highly organized chiral space and offers potential for the resolution of both chiral cations and neutral chiral solvent molecules.