Monotropic polymorphism in Copper(II) Decanoate

Abstract

Copper(II) decanoate was synthesized, purified and studied by DSC, optical microscopy, XRD, and FTIR and C-13 NMR spectroscopies. Depending on the solvent used for crystallization two polymorphs were obtained. A combination of singlecrystal and powder XRD was used to study their structures. One of the polymorphs (recrystallized in n-heptane) has similar structure to that previously reported at room temperature: triclinic (a ) 7.94(1) Å, b ) 5.28(3) Å, c ) 28.13(5) Å; R ) 94.6(10)°, β = 97.1(5)°, γ ) 98.6(10)°). A single crystal of the second polymorph was obtained from an ethanol solution and has a triclinic structure (a ) 5.2035(1) Å, b ) 14.331(3) Å, c ) 16.440(4) Å; R ) 65.390(4)°, β = 86.889(4)°, γ ) 82.886(4)°). This polymorph is monotropic and was identified by calorimetry as the metastable form. Both room temperature crystalline polymorphs belong to the same space group, P1j . However, they differ in (1) unit cell parameters; (2) one is bilayered whereas the other presents a columnar distribution of the binuclear complexes inside a net of two-by-two interdigitated chains. Thermal coefficients for both polymorphs have been estimated by indexation of their powder X-ray patterns versus temperature. The FTIR and C-13 NMR techniques confirm the structural differences. Both polymorphs melt to the same columnar discotic liquid crystal (identified by optical microscopy), decomposing before reaching the clearing point.