Irreversible transformation of chiral to achiral polymorph of K[Co(HCOO)3]: synthesis, structures, and magnetic properties

Abstract

We report the rare observation of an irreversible transformation from a chiral to an achiral crystal through the syntheses of two polymorphs of K[Co(HCOO)₃], and their characterization using single and powder crystal X-ray diffractions, infrared spectroscopy, thermal analyses, and the study of their magnetic properties. The kinetic phase (1), obtained under a high concentration of reactants, has a chiral (4⁹·6⁶) framework and adopts the hexagonal space-groupP 6₃22. The thermodynamic phase (2), obtained from dilute solution, has an achiral (4¹²·6³) framework and adopts a monoclinic space groupC 2/c. Once 1 has crystallized out from the concentrated solution, it dissolves slowly at the expense of the formation of 2. In the solid state 1 is irreversibly transformed to 2 very slowly at ambient conditions. The transformation is accompanied by an increase of the crystal density, a change of coordination mode of some formates from anti-anti to syn-anti and a change in the coordination number of K from 6 to 8. 1 is a spin-canted antiferromagnet (T_N = 8.3 K) displaying two steps in the isothermal magnetization at 2 K, a spin-flop at 1.2 kOe and a flip to paramagnetic phase at 42.0 kOe. 2 is an antiferromagnet (T_N = 2.0 K). Thermal properties and IR spectra of the two polymorphs have been studied, showing some differences.