Fig. 1: The crystal structure of four Eu2+ complexes.

ORTEP drawings of the crystal structures of (a) EuBr2–N4, (b) EuI2–N4, (c) EuBr2–N8, and (d) EuI2–N8, respectively. e The coordination polyhedrons (from top to bottom): square antiprism in EuX2–N4, distorted cube in EuX2–N4, distorted “hula-hoop” in EuBr2–N8 and EuI2–N8. The nitrogen atoms are labeled from N1–N8 for the data in Table 1. The solvent methanol (in EuBr2–N4 and EuI2–N8) and all the hydrogens are omitted for clarification. Atom notation: Eu (cyan), C (gray), N (blue), Br (brown), I (purple).