Monotropic Polymorphism in Copper(II) Decanoate

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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 single-crystal 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; α = 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; α = 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, P1̅. 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.

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