δ-Bonding modulates the electronic structure of formally divalent nd1 rare earth arene complexes

Abstract

Landmark advances in rare earth (RE) chemistry have shown that divalent complexes can be isolated with non-Aufbau 4fⁿ{5d/6s}¹ electron configurations, facilitating remarkable bonding motifs and magnetic properties. We report a series of divalent bis-tethered arene complexes, [RE(NHAr^iPr₆)₂] (2RE; RE = Sc, Y, La, Sm, Eu, Tm, Yb; NHAr^iPr₆ = {N(H)C₆H₃-2,6-(C₆H₂-2,4,6-iPr₃)₂}). Fluid solution EPR spectroscopy gives g_iso < 2.002 for 2Sc, 2Y, and 2La, consistent with formal nd¹ configurations, calculations reveal metal–arene δ-bonding via mixing of nd_(x²−y²) valence electrons into arene π* orbitals. Experimental and calculated EPR and UV-Vis-NIR spectroscopic properties for 2Y show that minor structural changes markedly alter the metal d_(x²−y²) contribution to the SOMO. This contrasts 4fⁿ{5d/6s}¹ complexes where the valence d-based electron resides in a non-bonding orbital. Complexes 2Sm, 2Eu, 2Tm, and 2Yb contain highly-localised 4fⁿ⁺¹ ions with no appreciable metal–arene bonding by density functional calculations. These results show that the physicochemical properties of divalent rare earth arene complexes with both formal nd¹ and 4fⁿ⁺¹ configurations are nuanced, may be controlled through ligand modification, and require a multi-pronged experimental and theoretical approach to fully rationalise.