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

Dalton Transactions Pub Date: 2011-03-17 DOI: 10.1039/C0DT01701A

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 (TN = 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 (TN = 2.0 K). Thermal properties and IR spectra of the two polymorphs have been studied, showing some differences.

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