Redox, spectroscopic and magnetic properties of C₃-symmetric rare earth complexes featuring atypical ortho-dioxolene...

The molecular symmetry in rare earth (RE) coordination chemistry is critically important for controlling the electronic structure of the RE ion and the resulting magnetic and photophysical properties. Here, we report a family of complexes with unusual C₃-point symmetry: [RE^III(Br₄catH)₃(tpa)] (Br₄catH^- = tetrabromocatecholate, tpa = tris(2-pyridylmethyl)amine). The synthesis and solid-state characterisation of eleven analogues (RE = Y, Sm to Lu) were performed, enabling a systematic investigation of the effect of symmetry on various physical properties across the RE series. The crystal structures reveal a unique cooperative coordination motif, featuring a cyclic hydrogen-bonding network between the atypical monodentate monoprotonated Br₄catH^- ligands. Electrochemical analysis reveals a single oxidation process that suggests a concerted three-electron oxidation of all tetrabromocatecholate ligands to semiquinonate. Furthermore, single-molecule magnet (SMM) behaviour was investigated, revealing unexpected in-field slow magnetic relaxation for both Dy and Yb analogues, which can be rationalised by the effect of C₃-symmetry. Finally, luminescence measurements were performed to probe the CF splitting of the Yb analogue and quantify the error in the overall CF splitting predicted by ab initio calculations. The governing effects of C₃-symmetry are consistent observations in all RE³⁺ metals studied in this work, manifesting in the concerted three-electron oxidation, SMM behaviour, ground state composition, and luminescence properties.