End-On Bridging Dinitrogen Complex of Scandium

The first (N═N)^2- complex of a rare-earth metal with an end-on dinitrogen bridge, {K(crypt)}₂{[(R₂N)₃Sc]₂[μ-η¹:η¹-N₂]} (crypt = 2.2.2-cryptand, R = SiMe₃), has been isolated from the reduction of Sc(NR₂)₃ under dinitrogen at -35 °C and characterized by X-ray crystallography. The structure differs from the characteristic side-on structures previously observed for over 40 crystallographically characterized rare-earth metal (N═N)^2- complexes of formula [A₂Ln(THF)_x]₂[μ-η²:η²-N₂] (Ln = Sc, Y, and lanthanides; x = 0, 1; A = anionic ligand such as amide, cyclopentadienide, and aryloxide). The 1.221(3) Å N-N distance and the 1644 cm^-1 Raman stretch are consistent with the presence of an (N═N)^2- bridge. The observed paramagnetism of the complex by Evans method measurements is consistent with DFT calculations that suggest a triplet (³A₂) ground state in D₃ symmetry involving two degenerate Sc-N₂-Sc bonding orbitals. Upon brief exposure of the orange Sc³⁺ bridging dinitrogen complex to UV-light, photolysis to form the monomeric Sc²⁺ complex, [K(crypt)][Sc(NR₂)₃], was observed. Conversion of the Sc²⁺ complex to the Sc³⁺ dinitrogen complex was not observed with this crypt system, but it did occur with the 18-crown-6 (crown) analog which formed {K(crown)}₂{[(R₂N)₃Sc]₂[μ-η¹:η¹-N₂]}. This suggests the importance of the alkali metal chelating agent in the reversibility of dinitrogen binding in this scandium system.