Seven-coordinate lanthanide complexes with a tripodal redox active ligand: structural, electrochemical and spectroscopic investigations

Abstract

The tripodal ligand TREN-(3,5-di-tert-butylsalicylidene)₃ (H₃L) was synthesized and its tris(phenolato) lanthanide complexes L-Ln (Ln = Nd^III, Eu^III, Tb^III, Gd^III, Er^III, Yb^III and Lu^III) were prepared. The X-Ray crystal structures confirm that each metal ion resides in a similar monocapped octahedral geometry, excluding water molecules from the coordination sphere. The coordination bond distances are in agreement with the lanthanide contraction, with Ln–O bond lengths in the range 2.139–2.216 Å. The complexes show three reversible monoelectronic oxidation waves, which are assigned to the successive oxidation of the phenolate moieties to phenoxyl radicals. The L-Nd complex is the easiest to oxidize, with E¹_1/2 = 0.11, E²_1/2 = 0.21 and E³_1/2 = 0.34 V vs. Fc⁺/Fc, due to the larger size of the lanthanide ion. The ΔE_1/2 value (ΔE_1/2 = E²_1/2 − E¹_1/2) is correlated to the lanthanide radius, with values of 0.10 V for L-Nd and 0.22 V for L-Lu. The monoradical species were persistent in solution, allowing for their characterisation. All exhibit a distinct absorption band at around 445 nm due to the phenoxyl π–π* transitions. The EPR spectrum of L-Lu⁺ consists of a single resonance at g_iso = 1.999, confirming the radical nature of the oxidized product. Most of the other complexes (L-Gd, L-Er, L-Yb) show a quenching of the Ln^III-based resonances upon oxidation, indicative of magnetic interactions between the metal and the radical spins. The L-Ln (L = Nd, Er, Yb) complexes exhibit a metal-based luminescence upon excitation of the ligand. A significant quenching of the luminescence was observed upon radical formation: 92%, 83% and 79% respectively for L-Nd⁺, L-Er⁺ and L-Yb⁺.