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 {[(R2 N)3 Sc]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 [A2 Ln(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 (3 A2 ) 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 {[(R2 N)3 Sc]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.