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

Abstract

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 (meta-xylyl or para-tolyl) and a bidentate trans-2,3-butanediolate. The meta-xylyl and para-tolyl complexes of either enantiomerically pure 2R,3R-butanediolate or 2S,3S-butanediolate (compounds 1-4) 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 meta-xylyl complexes each crystallized in space group P1 (approximating P1̅), and the racemic compound did indeed crystallize in P1̅. The enantiomerically pure para-tolyl complexes each crystallized in space group P2₁ (approximating P2₁/c), but the racemic compound crystallized in P1̅. Although the enantiomerically pure and racemic compounds are not isostructural, there are similarities in their 3D structures that are analyzed.