Study of the phase transition between orthorhombic modifications Ln tris-2,2,6,6-tetramethylheptane-2,4-dionates

The crystal structure of new isostructural complex Tb(thd)3 (1), Ho(thd)3 (2) (sp. gr. Pmn21) and Tb(thd)3 (3), Dy(thd)3 (4), Er(thd)3 (5), Yb(thd)3 (6) (sp. gr. Pna21) are determined. The crystal structures of 1–6 are molecular and consist of discrete monomeric Ln(thd)3 molecules (Ln = Tb, Ho, Dy, Er, Yb). The samples of Ln(thd)3 in the solid state have been investigated over the temperature range from 120 to 300 K using differential scanning calorimetry. DSC curves Ln(thd)3 exhibit a single endothermic peak of the crystal-to-crystal reversible phase transition. First-order phase transition is accompanied by an abrupt change in the volume of the unit cell. The quantum chemical calculations allow one to conclude that the electronic structures of the three modifications for Tb tris-2,2,6,6-tetramethylheptane-2,4-dionates are some different. It has been shown that for Tb(III) complexes with thd-ligands the modification I is energetically more stable.