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

Abstract

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 (H 4 L), has been rationally designed to encapsulate Eu 2+ and to enable direct formation of the first mixed Eu(II)/Ln(III) complexes, namely, Eu II 2 Ln III 4 (OH) 4 (NIC) 4 L 2 (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 Eu 2+ . Cyclic voltammetry experiments showed that the oxidation potential of Eu 2+ 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 Eu 2+ -containing complexes. Further development of Eu(II) complexes has been limited because Eu 2+ could be easily oxidized to Eu 3+ . 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.

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

“…The comparable emission intensity of the Laporte‐allowed 4f 6 5d 1 → 4f 7 transition and 5 D 0 → 7 F J transitions may be due to energy transfer from Eu 2+ to Eu 3+ . To the best of our knowledge, this is the first reported heterovalent europium complex that clearly exhibits emissions from both Eu 2+ and Eu 3+ …”

“…ities of the designed ligands L (Scheme 1) and are further isolated from the environment by peripheral phenyl rings and the tetrahedral 4Ln III cluster, which results in the high oxidative stability of Eu 2+ . [6] The oxidation potential of Eu 2+ in the heterovalent 2Eu II /4Ln III cluster is larger than that of the ferrocene/ ferrocenium redox couple (E ox = 0.34 V vs. Fc/Fc + ), which has never been reported previously for Eu 2+ -containing complexes. As a continuation of this work, we report herein the synthesis of two europium clusters, namely 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), by replacing the auxiliary ligand nicotinic acid by benzoic acid (Hbza).…”

“…General: Ligand H 4 L was synthesized according to the previously reported procedure. [6] All other reagents were commercially available and were used without further purification. Elemental analyses (C, H, N) were determined with a Vario MICRO CHNOS elemental analyzer.…”

“…To the best of our knowledge, this is the first reported heterovalent europium complex that clearly exhibits emissions from both Eu 2+ and Eu 3+ . [3][4][5][6]10]…”

“…Owing to the higher oxidation potential of Eu 2+ (the standard reduction potential for trivalent europium to its divalent species is –0.35 V) compared with other divalent lanthanide ions and its special magnetic, optical, and catalytic properties, a large number of stable Eu 2+ ‐containing macrocyclic complexes have been reported over the past few decades, for example, 4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane europium(II) and the more stable {5,6‐(benzo)‐4,7‐dioxa‐13,16,21,24‐tetrathia‐1,10‐diazabicyclo[8.8.8]hexacos‐5‐ene}europium(II) . We have recently reported a series of novel heterovalent complexes with the composition Eu II 2 Ln III 4 (OH) 4 (nic) 4 L 2 [Ln = Eu, Sm, Tb; Hnic = nicotinic acid, H 4 L = N , N′ , N′′ , N′′′ ‐tetrakis(2‐hydroxy‐3‐methoxy‐5‐methylbenzyl)‐1,4,7,10‐tetraazacyclododecane] in which the two divalent europium ions in the cluster are trapped within the macrocyclic cavities of the designed ligands L (Scheme ) and are further isolated from the environment by peripheral phenyl rings and the tetrahedral 4Ln III cluster, which results in the high oxidative stability of Eu 2+ . The oxidation potential of Eu 2+ in the heterovalent 2Eu II /4Ln III cluster is larger than that of the ferrocene/ferrocenium redox couple ( E ox = 0.34 V vs. Fc/Fc + ), which has never been reported previously for Eu 2+ ‐containing complexes.…”