FIGURE 1 Optimized structures of the mesogenic Eu(III) complexes in the ground state: (а) Eu(CPDK3­5)3Bpy17-17, (b) Eu(CPDK3-5)3Phen, (c) Eu(CPDK5-Th)3Bpy17-17, (d) Eu(CPDK5­Th)3Phen, (e) Eu(DK12­14)3Bpy17­17, (f) Eu(CPDK3-Ph)3Phen
From Figure 1 one can see that all of the studied complexes have the coordination number 8. Their coordination polyhedra includes the Eu(III) ion and 8 atoms from its first coordination sphere - six O centers of the β-diketones and two N centers of the Lewis bases. Initial geometries of the polyhedra were taken from the X-ray diffraction data for similar Eu(III) complexes without long terminal substituents[49-51] from the Cambridge Structural Database.[52]
For the start of the optimization process the geometries of model Eu(III) complexes[49-51] from the Cambridge Structural Database were modified by replacement of initial ligands by various substituted β­diketones and Lewis bases (Figure 1).
The presence of a heavy Eu(III) ion and a large number of atoms results in a high computational costs especially when optimizing the geometry of excited states. The geometry optimization of eight isomers for each of the studied Eu(III) complexes with different arrangement of β-diketones relative to the plane formed by the Eu(III) ion and the Lewis base[8,9,11,27] was made. According to the calculations, the isomers with the crosswise arrangement of β­diketones have the lowest energy. In this energetically most advantageous arrangement of β­diketones the long alkyl substitutes do not sterically hinder each other. Therefore, only one stereoisomer for each of the coordination polyhedra of the studied Eu(III) complexes was considered. Exemplarily, eight optimized geometries of Eu(CPDk3­5)3Phen isomers are presented in the Supporting Information (Table S1).
The resulting optimized structures and geometry parameters of the studied mesogenic Eu(III) complexes are shown in Figure 2. Steric hindrances caused by long alkyl substituents in the ligands led to insignificant distortions of the Eu(III) coordination polyhedra during optimization. Structural parameters of the optimized Eu(III) complexes, the average Eu–O, C–O, and Eu–N bond lengths, as well as some characteristic angles are presented in Table 1. Coordination polyhedra of almost all studied Eu(III) complexes, as well as model compounds, were defined by the SHAPE program as a slightly distorted square antiprism. Eu(CPDK3­5)3Phen, in contrast to Eu(CPDK3­5)3Bpy17­17, has a triangular dodecahedron, as well as Eu(DK12­14)3Bpy17-17with long substituents in three β-diketones DK12­14. They are all chiral and correspond to the C1 point group that often occurs in Ln(III) complexes with a coordination number of 8.[53-55] Therefore, it can be assumed that Lewis base determines the type of coordination polyhedron in the some of the mesogenic Eu(III) complexes.