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) 2 XL] (Hdmg dimethylglyoximate, X CN, NO 2 ) 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 P2 1 with two molecules per asymmetric unit, the racemates in P2 1 /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 P2 1 . 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.

Chirality and Order in Binary Molecular Crystals of Inert Cobalt(III) ComplexesDedicated to Professor Joachim Strähle on the Occasion of his 65thBirthday

“…Only a few examples [4,20] for this type of solid have been studied by X-ray diffraction. All structures of the phenylethylamine complexes, 1؊6 are closely related crystallizing in space group P2 1 or the centric supergroup P2 1 /c with similar lattice parameters; they contain four molecules per unit cell and show comparable packing [16]. The centric structures 5 and 6 show no dipole moment for reasons of symmetry.…”

“…Complementary chirality represents the driving force for the formation of well-ordered binary crystals. The syntheses and characterization of 1؊3, 5, and 6 have been published [16]. Self recognition between molecules is an ubiquitous phenomenon occuring in all solids with translational symmetry.…”

“…In addition to quasiracemates more complicated solids with a 3:1 ratio between the components may result. Bis(dimethylglyoximato)nitro(S-1-phenylethylamine)cobalt(III) was synthesized by the analogous procedure described for its R enantiomer [16]. Interaction between opposite enantiomers has been claimed to be favourable for more than a century [23].…”

“…1b) are expected to occur more frequently [11Ϫ13] than conglomerates (Fig. A preliminary account on our first results in the context of this project has been published [16]. For more than a century opposite chirality has been known as a driving force for the formation of compounds [14].…”

Crystal engineering based on mirror image recognition

“…Opposite chirality offers the possibility to design binary crystals based on the favourable interaction between opposite enantiomers. We have exploited the statistical preference for heterochiral crystals [1][2][3] as a design tool to generate predictable molecular assemblies [4][5][6][7]. Preferred mirror image recognition in the solid extends to closely related compounds of opposite chirality, so-called quasiracemates [8,9].…”