δ-Bonding and Electron Localisation in Crystalline Divalent Rare Earth Arene Complexes

Landmark advances in rare earth (RE) chemistry have shown that formally divalent complexes can be isolated with non-Aufbau 4fn5d(dz2)1 or 4fn{5d/6s}1 electron configurations, which facilitate novel 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}), where 2Sc, 2Y, and 2La show metal-arene δ-bonding via nd(x2-y2)- and π* orbital mixing, while 2Sm, 2Eu, 2Tm, and 2Yb contain 4fn+1 ions without δ-bonds. Fluid solution EPR spectroscopy gives giso = 1.9995, 1.998, 1.989 for 2Sc, 2Y, and 2La, respectively, consistent with formal nd1 configurations, while metal hyperfine interactions suggest extensive delocalisation. Solution and crystalline phase EPR and UV-Vis-NIR spectroscopy for 2Y show incongruities due to minor structural changes between these phases that markedly alter the metal 4d(x2-y2) character to the SOMO (22% vs 12%, respectively). In contrast to existing 4fn5d(dz2)1 or 4fn{5d/6s}1 complexes where the valence d-based electron resides in a non-bonding orbital, the sensitivity of 2Y to its local environment suggests RE(II) arene δ-bonding represents a hitherto unexplored opportunity to tune the electronic structure properties of nd1 rare earth ions.