Structural and spectroscopic investigations of nine-coordinate redox active lanthanide complexes with a pincer O,N,O ligand

Abstract

The lanthanide complexes EuL₃, GdL₃, YbL₃ and LuL₃ of the N,N′-bis(2-hydroxy-di-3,5-tert-butylphenyl)amine were prepared. The X-Ray crystal structures of GdL₃ and LuL₃ demonstrated a nine-coordinate sphere with three ligand molecules under their anionic diamagnetic form (Cat-N-BQ)⁻. The complexes showed three oxidation events (E^ox1_1/2 = 0.15–0.16 V, E_1/2² = 0.51–55 V, and E_1/2³ = 0.75–0.78 V vs. Fc⁺/Fc) via cyclic voltammetry, corresponding to the successive oxidation of the aminophenolate moeities to iminosemiquinone species. The complexes GdL₃ and YbL₃ were characterized by EPR spectroscopy, allowing for the determination of the zero field splitting (ZFS) parameters in the first case. The monocations (LnL₃)⁺ and monoanions (LnL₃)⁻ were electrochemically generated (Ln = Eu, Gd, Yb, Lu), as well as the dications YbL₃²⁺ and LuL₃²⁺. The spins are antiferromagnetically exchange coupled in the diradical species LuL₃²⁺ (|D| = 260 MHz, E = 0). All the complexes (incl. neutral) possess a strong absorption band in the NIR region (730–840 nm, ε > 19 mM⁻¹ cm⁻¹) corresponding to ligand-based transitions.