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 {[(R 2 N) 3 Sc] 2 [μ-η 1:η 1 -N 2 ]} (crypt = 2.2.2-cryptand, R = SiMe 3 ), has been isolated from the reduction of Sc(NR 2 ) 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 [A 2 Ln(THF) x ] 2 [μ-η 2:η 2 -N 2 ] (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 A 2 ) ground state in D 3 symmetry involving two degenerate Sc—N 2 —Sc bonding orbitals. Upon brief exposure of the orange Sc 3+ bridging dinitrogen complex to UV-light, photolysis to form the monomeric Sc 2+ complex, [K(crypt)][Sc(NR 2 ) 3 ], was observed. Conversion of the Sc 2+ complex to the Sc 3+ 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 {[(R 2 N) 3 Sc] 2 [μ-η 1:η 1 -N 2 ]}. This suggests the importance of the alkali metal chelating agent in the reversibility of dinitrogen binding in this scandium system.