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.