Cocrystallization of chiral cobalt complexes via formation of quasiracemates

Abstract

Starting from the enantiomerically pure or racemic nucleophile 1-phenylethylamine (L) chiral cobalt complexes with the general formula [Co(Hdmg)2XL] (Hdmg = dimethylglyoximate, X = CN, NO2) were obtained as homochiral or heterochiral crystalline solids, respectively. The crystal structures of the resulting cyano and nitro complexes were determined by single crystal X-ray diffraction. The homochiral solids crystallize in space group P21 with two molecules per asymmetric unit, the racemates in P21/c with only one independent molecule. From a solution containing equimolar amounts of enantiomerically pure S-L cyano and R-L nitro complexes a stable and well-ordered binary solid could be crystallized and structurally characterized. The molecular constituents of opposite chirality are arranged in pairs in the vicinity of pseudo inversion centers forming a quasi-racemate in space group P21. The driving force for the formation of this cocrystal is discussed in terms of intermolecular geometry and energy: The crystal structures of all five complexes are similar with respect to lattice parameters and intermolecular arrangement, but comparisons based on space filling and lattice energy confirm higher packing efficiency for the racemates and the quasi-racemate than for the enantiopure compounds.