Exploring the Use of Pseudosymmetry in the Design of Higher-Symmetry Crystals of Racemic Compounds

Organometallic antimony(V) complexes were prepared as model compounds to better understand the interactions of chiral chelating diols with this metalloid. These complexes feature three aryl groups (<i>meta</i>-xylyl or <i>para</i>-tolyl) and a bidentate <i>trans</i>-2,3-butanediolate. The <i>meta</i>-xylyl and <i>para</i>-tolyl complexes of either enantiomerically pure 2<i>R</i>,3<i>R</i>-butanediolate or 2<i>S</i>,3<i>S</i>-butanediolate (compounds <b>1</b>-<b>4</b>) crystallized in Sohncke space groups, as expected. In each case, though, pseudoinversion centers were present that mimic higher-symmetry space groups through global pseudosymmetry. We hypothesized that the crystallization of 1:1 mixtures of the enantiomeric complexes would produce crystals in the centrosymmetric space group approximated by the pseudosymmetry. The enantiomerically pure <i>meta</i>-xylyl complexes each crystallized in space group <i>P</i>1 (approximating <i>P</i>1̅), and the racemic compound did indeed crystallize in <i>P</i>1̅. The enantiomerically pure <i>para</i>-tolyl complexes each crystallized in space group <i>P</i>2₁ (approximating <i>P</i>2₁/<i>c</i>), but the racemic compound crystallized in <i>P</i>1̅. Although the enantiomerically pure and racemic compounds are not isostructural, there are similarities in their 3D structures that are analyzed.