First Eu(II)/Ln(III) Mixed Complex with High Oxidative Stability

Due to the instability of divalent europium ions, the heterometallic Eu­(II)/Ln­(III) complex has not yet been reported. By utilizing coordination chemistry principles, a macrocyclic ligand, N,N′,N″,N‴-tetra­(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetra­azacyclo­dodecae (H4L), has been rationally designed to encapsulate Eu2+ and to enable direct formation of the first mixed Eu­(II)/Ln­(III) complexes, namely, EuII2LnIII4(OH)4(NIC)4L2 (Ln = Sm, Eu, Tb; HNIC = nicotinic acid). Two divalent europium ions are trapped within the macrocyclic cavities of designed ligands L and are further isolated from the environment by outside phenyl rings and the tetrahedral 4Ln­(III) cluster, resulting in the enhanced stability of Eu2+. Cyclic voltammetry experiments showed that the oxidation potential of Eu2+ in the heterovalent 2Eu­(II)/4Ln­(III) cluster is larger than that for the ferrocene/ferrocenium redox couple, which has never been reported previously for Eu2+-containing complexes. Further development of Eu­(II) complexes has been limited because Eu2+ could be easily oxidized to Eu3+. The dramatic oxidative stability of as-synthesized complexes not only verifies the synthetic feasibility but also highlights the prospective applications of mixed Eu­(II)/Ln­(III) coordination complexes.