A1: Synthesis of divalent lanthanide precursors

The synthetic and stabilization of divalent lanthanide complexes represent a burgeoning field in 4f chemistry. One of the major reasons is their remarkable reactivities towards both organic and inorganic substrates.¹ Pioneering work by Evans and Andersen demonstrated the versatile reactivity of Ln(C₅Me₅)₂ (Ln = Sm, Eu, Yb).^2-4
Although the chemistry of the divalent lanthanide species comprising cyclopentadienyl-based ligand systems has been well documented, there is a growing interest of alternative supporting ligands. In recent years, the focus has expanded to homoleptic and heteroleptic Ln(II) complexes with various ligands such as alkoxides,⁵ phosphides,⁶ amidinates,⁷ macrocycles,⁸ each displaying interesting reactivities. The key to obtain a novel homoleptic or heteroleptic Ln(II) complex lies in selecting a suitable Ln(II) precursor.
This article provides an overview of some traditional Ln(II) starting materials. A more comprehensive summary can be found in the article “Rare Earth Starting Materials and Methodologies for Synthetic Chemistry” by F. Ortu.⁹ It is important to note that all the reaction discussed below are conducted under inert atmosphere, using argon is recommended. All solvents were dried prior to use.