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)(3)Sc](2)[mu-eta(1):eta(1)-N-2]} (crypt = 2.2.2-crypt- and, R = SiMe3), has been isolated from the reduction of Sc(NR2)(3) under dinitrogen at -35 degrees 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 [A(2)Ln(THF)(x)](2)[mu-eta(2):eta(2)-N-2] (Ln = Sc, Y, and lanthanides; x = 0, 1; A = anionic ligand such as amide, cyclopentadienide, and aryloxide). The 1.221(3) angstrom 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)A(2)) ground state in D-3 symmetry involving two degenerate Sc-N-2-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)(2)][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)(3)Sc](2)[mu-eta(1):eta(1)-N-2]. This suggests the importance of the alkali metal chelating agent in the reversibility of dinitrogen binding in this scandium system.