Chiral Cr(iii) and Co(iii) complex cations as building blocks for ordered and disordered salts

Abstract

Cobalt and chromium compounds of the composition [M(chxn)₃]Cl₃ (chxn = trans-1,2-diaminocyclohexane) have been used as building blocks to synthesize ternary salts. Chirality of the cations allows control of the nature of the products. Joint crystallisation of Co(III) and Cr(III) complexes of the same configuration, i.e. both derived from either the same enantiomer or the racemate of the chiral chelating ligand, results in either homochiral or heterochiral solid solutions. Complex salts [M(rac-chxn)₃]Cl₃·H₂O (M = Co, Cr) crystallise in the tetragonal racemic space group I2d. The preferred formation of this heterochiral structure offers the oppurtunity to synthesize an ordered solid of predictable geometry via the quasi-racemate approach: when Co(III) and Cr(III) cationic complexes of opposite chirality are cocrystallised, no solid solutions but a well-ordered ternary salt is obtained. X-Ray diffraction reveals a t₂ group–subgroup relationship between the racemic parent structure and the mixed crystals. The latter crystallise in the orthorhombic space group I2₁2₁2₁ with lattice parameters reflecting the different size requirements of trivalent cobalt and chromium cations.