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 phenomenal magnetic properties. We report an unprecedented series of divalent bis-tethered arene complexes, [RE(NHAriPr6)2] (2RE; RE = Sc, Y, La, Sm, Eu, Tm, Yb; NHAriPr6 = {N(H)C6H3-2,6-(C6H2-2,4,6-iPr3)2}). Fluid solution EPR spectroscopy gives giso < 2.022 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 2Ybcontain highly-localised 4fn+1 ions with no appreciable metal–arene bonding by density functional calculations. However, a variable-temperature 1H NMR study of 2Yb reveals the Yb–arene is not fluxional at room temperature, and this is reflected in the 1H-171Yb HMBC spectrum which shows coupling between arene protons and the 171Yb nucleus. 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.