The discovery that combinations of Ln(III) salts with alkali metals would reduce dinitrogen to (N=N)2-and (N2)3-complexes via a so-called LnA3/M reaction (Ln = La, Ce, Pr, Gd, Tb, Ho, Er, Y, and Lu; A = anionic ligand; M = K, Na) initiated a search for isolable Ln(II) complexes of nine metals listed above. Subsequently, it was discovered that crystallographically-characterizable molecular complexes of these nine metals in the +2 oxidation state can be isolated in tris(cyclopentadienyl) coordination environments like (Cp'3Ln)1−and (Cp"3Ln)1−, where Cp' = C5H4SiMe3and Cp" = C5H3(SiMe3)2. These complexes were unusual in that reduction of the 4fnCp'3Ln and Cp"3Ln Ln(III) precursors of La, Ce, Pr, Gd, Tb, Ho, Er, and Lu formed 4fn5d1products rather than the traditional 4fn+1Ln(II) ions previously found for Eu, Yb, Sm, Tm, Dy, and Nd. The fifth period rare-earth Y(II) ion was a 4d1ion. The discovery of the new 4fn5d1Ln(II) ions in these tris(cyclopentadienyl) complexes has stimulated a worldwide search for more examples and for information on the factors that allow for their isolation. Recent results relating to the steric and electronic factors that allow isolation of molecular complexes of these 4fn5d1Ln(II) ions will be presented. In addition, the connection of these isolated Ln(II) complexes with the LnA3/M dinitrogen reduction reported earlier will be described based on the recent isolation of end-on and side-on bound (N=N)2-complexes.

Recent developments in reductive rare-earth metal chemistry

ACS Spring Meeting 2021

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