Chiral anionic layers in tartramide spiroborate salts and variable solvation for [NR ₄][B(TarNH₂)₂] (R = Et, Pr or Bu)

Abstract

<jats:p>The spiroborate anion, namely, 2,3,7,8-tetracarboxamido-1,4,6,9-tetraoxa-5λ<jats:sup>4</jats:sup>-boraspiro[4.4]nonane, [B(TarNH<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>]<jats:sup>−</jats:sup>, derived from the diol L-tartramide TarNH<jats:sub>2</jats:sub>, [CH(O)(CONH<jats:sub>2</jats:sub>)]<jats:sub>2</jats:sub>, shows a novel self-assembly into two-dimensional (2D) layer structures in its salts with alkylammonium cations, [N<jats:italic>R</jats:italic> <jats:sub>4</jats:sub>]<jats:sup>+</jats:sup> (<jats:italic>R</jats:italic> = Et, Pr and Bu), and sparteinium, [HSpa]<jats:sup>+</jats:sup>, in which the cations and anions are segregated. The structures of four such salts are reported, namely, the tetrapropylazanium salt, C<jats:sub>12</jats:sub>H<jats:sub>28</jats:sub>N<jats:sup>+</jats:sup>·C<jats:sub>8</jats:sub>H<jats:sub>12</jats:sub>BN<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> <jats:sup>−</jats:sup>, the tetraethylazanium salt hydrate, C<jats:sub>8</jats:sub>H<jats:sub>20</jats:sub>N<jats:sup>+</jats:sup>·C<jats:sub>8</jats:sub>H<jats:sub>12</jats:sub>BN<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> <jats:sup>−</jats:sup>·6.375H<jats:sub>2</jats:sub>O, the tetrabutylazanium salt as the ethanol monosolvate hemihydrate, C<jats:sub>16</jats:sub>H<jats:sub>36</jats:sub>N<jats:sup>+</jats:sup>·C<jats:sub>8</jats:sub>H<jats:sub>12</jats:sub>BN<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> <jats:sup>−</jats:sup>·C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub>OH·0.5H<jats:sub>2</jats:sub>O, and the sparteinium (7-aza-15-azoniatetracyclo[7.7.1.0<jats:sup>2,7</jats:sup>.0<jats:sup>10,15</jats:sup>]heptadecane) salt as the ethanol monosolvate, C<jats:sub>15</jats:sub>H<jats:sub>27</jats:sub>N<jats:sub>2</jats:sub> <jats:sup>+</jats:sup>·C<jats:sub>8</jats:sub>H<jats:sub>12</jats:sub>BN<jats:sub>4</jats:sub>O<jats:sub>8</jats:sub> <jats:sup>−</jats:sup>·C<jats:sub>2</jats:sub>H<jats:sub>5</jats:sub>OH. 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, <jats:italic>i.e.</jats:italic> the [NEt<jats:sub>4</jats:sub>] salt is highly hydrated with around 6.5H<jats:sub>2</jats:sub>O per cation, the [NPr<jats:sub>4</jats:sub>] salt apparently has a good metric match to the anion layer and is unsolvated, whilst the [NBu<jats:sub>4</jats:sub>] salt is intermediate and has EtOH and H<jats:sub>2</jats:sub>O in its cation layer, which is similar to the arrangement for the chiral [HSpa]<jats:sup>+</jats:sup> 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.</jats:p>