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 4fn5d/6s1 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 4fn5d/6s1 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.