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.