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

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 E = 0.11, E = 0.21 and E = 0.34 V vs. Fc⁺/Fc, due to the larger size of the lanthanide ion. The ΔE_1/2 value (ΔE_1/2 = E-E) 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⁺.