δ-Bonding versus Electron Localization in Formally Divalent Rare Earth Complexes

Herein we present a series of room-temperature stable, structurally analogous rare-earth complexes of the form M (NHAriPr6) 2 (M = Sc, Y, La, Sm, Eu, Tm, Yb; NHAriPr6 = N (H) C6H3-2, 6- (C6H2-2, 4, 6-iPr3) 2). All seven complexes contain formal M (II) ions with close M···arene contacts. The Sc (II) and La (II) complexes display an open-book deformation (ca. 11–13°) of one metal-bound arene, while for Y (II) there are two symmetry-equivalent Y···arene interactions with smaller deformations (ca. 7°). In the case of Sc (II), Y (II), and La (II), quantum chemical calculations reveal the delocalization of a metal d-electron from M (II) into ligand π-orbitals forming δ-bonding interactions. In the case of Y (II), EPR and UV-Vis-NIR spectroscopies combined with quantum chemical calculations show a structural change in solution to the single-open-book deformation structure in common with Sc (II) and La (II). While the Sm (II), Eu (II), Tm (II), and Yb (II) complexes retain similar close M···arene contacts, their 4fn+1 valence electron configurations do not lead to δ-bonding interactions; however, NMR studies of diamagnetic 4f14 Yb (II) reveal 171Yb···1H coupling to arene ring-protons at room temperature and show that the M···arene interaction is preserved in solution across a wide temperature range. These complexes represent a rare structurally analogous M (II) series which extends from the group 3 elements into the lanthanide series and includes an extremely rare neutral formal Sc (II) complex.