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

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 angstrom. 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 E11/2 = 0.11, E21/2 = 0.21 and E31/2 = 0.34 V vs. Fc(+)/Fc, due to the larger size of the lanthanide ion. The E-1/2 value (E-1/2 = E21/2 - E11/2) 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 -* 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+.