Structural and spectroscopic investigations of redox active seven coordinate luminescent lanthanide complexes

Abstract

The ligand H3L and its corresponding tris(phenolato) lanthanide complexes L-Tb, L-Gd, L-Yb and L-Lu were synthesized. The X-Ray crystal structures demonstrated a monocapped octahedron environment of the lanthanide ion, with exclusion of all solvent molecules from the coordination sphere due to the protection by the tert-butyl groups. The coordination bond distances followed the lanthanide contraction, with Ln-O bond lengths in the range 2.148–2.217 Å and Ln-Nimine bond lengths in the range 2.405–2.535 Å. Each of the complexes showed three one-electron oxidation processes. These potentials changed as a function of the lanthanide due to the difference in size of the metal ions. Both chemical and electrochemical oxidations of these complexes permitted the one-electron oxidised species to be generated. They all show a sharp absorption band at around 420 nm in their UV-Vis spectra, which demonstrated phenoxyl radical formation. Consistently, the radical species L-Lu+ displays a resonance at g = 2.001 in its EPR spectrum. The other cations are EPR-silent or difficult to detect due to magnetic interactions, confirming the proximity of the phenoxyl radical to the metal centre. For both L-Lu+ and L-Yb+ DFT calculations predict the radical to be delocalized over the three arms. The L-Tb and L-Yb complexes experienced a quenching of the luminescence upon oxidation to the radical species whereby the quenching of up to 83% in the visible region and 93% in the NIR was observed.