Heterovalent 2Eu II /4Eu III and Trivalent 5Eu III Clusters Assembled with a Macrocyclic Cyclen Derivative Ligand: Molecular Structures, Oxidation Stabilities, and Luminescence Properties

Two coordination complexes Eu II 2 Eu III 4 (OH) 4 (bza) 4 L 2 ( 1 ) and Eu III 5 (H 2 O)(OH) 4 (NO 3 ) 3 (bza) 4 L ( 2 ) have been assembled with the tetrasubstituted cyclen ligand N , N′ , N′′ , N′′′ ‐tetrakis(2‐hydroxy‐3‐methoxy‐5‐methylbenzyl)‐1,4,7,10‐tetraazacyclododecane (H 4 L) and the auxiliary ligand benzoic acid (Hbza). Complex 2 is a pentanuclear Eu III cluster whereas complex 1 is a huge heterovalent 2Eu II /4Eu III cluster in which two divalent europium ions are trapped within the macrocyclic cavities of the designed ligands L and are further isolated from the environment by peripheral phenyl rings and the tetrahedral 4Eu III cluster, which results in the enhanced oxidative stability of Eu 2+ . The luminescence of complex 1 was investigated and is discussed in comparison with the luminescence of ligand H 4 L and complex 2 . Owing to the protection of Eu 2+ by the macrocyclic ligands L, the heterovalent complex exhibits a strong broad emission band with a maximum at 533 nm as well as the characteristic sharp emissions of Eu 3+ . The broad emission is redshifted compared with that of ligand H 4 L and has been attributed to the Laporte‐allowed 4f 6 5d 1 → 4f 7 transition of Eu 2+ .