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.