Formation of the End-on Bound Lanthanide Dinitrogen Complexes [(R_2N)_3Ln-N=N-Ln(NR_2)_3]~(2-) from Divalent [(R_2N)_3Ln]~(1-) Salts (R =SiMe_3)

摘要

Lanthanide-based dinitrogen reduction chemistry has been expanded by the discovery of the first end-on Ln_2(μ-η~1:η~1-N_2) complexes, whose synthesis and reactivity help explain the reduction of N_2 by the combination of trivalent Ln(NR_2)_3 complexes (R = SiMe_3) and potassium. The formation of end-on versus the more common side-on Ln_2(μ-η~2:η~2-N_2) complexes is possible by using recently discovered Ln(Ⅱ) complexes ligated by three NR_2 amide ligands (R = SiMe_3). The isolated Ln(Ⅱ) tris(amide) complex [K(crypt)][Tb(NR_2)_3] (crypt = 2.2.2-cryptand), 1-Tb, reacts with dinitrogen in Et_2O at -35 °C to form the end-on bridging dinitrogen complex [K(crypt)]_2{[(R_2N)_3Tb]_2[μ-η~1:η~1-N_2]}, 2-Tb. The 18-crown-6 (18-c-6) Tb(Ⅱ) analogue, [K(18-c-6)_2][Tb(NR_2)_3], 3-Tb, also reacts with N_2 to form an end-on product, [K_2(18-c-6)_3]{[(R_2N)_3Tb]_2[μ-η~1:η~1N_2}, 4-Tb. The reaction of 1-Gd with dinitrogen forms a complex with the same composition as 2-Tb but with both side-on and end-on bonding of the N_2 unit in the same crystal, [K(crypt)]_2{[(R_2N)_3Gd]_2[μ-η~x:η~x-N_2]} (x = 1 and 2), 5-Gd. Similarly, the 18-C-6 Gd(Ⅱ) complex, 3-Gd, generates a product with both binding modes: [K_2(18-c-6)_3]{[(R_2N)_3Gd]_2[μ-η~x:η~x-N_2]} (x = 1, 2), 6-Gd. All of these new reduced dinitrogen complexes, 2-Tb, 4-Tb, 5-Gd, and 6-Gd, have three ancillary amide ligands per metal. In contrast, the side-on bound complexes, [(THF)(R_2N)_2Ln]_2[μ-η~2:η~2-N_2], 7-Ln, observed previously in Ln(NR_2)_3/K/N_2 reactions, have only two amides per metal. A connection between these systems related to their formation was observed in the structure of the bimetallic penta-amide complex, [K(THF)_6]{[(THF)(R_2N)_2Gd][μ-η~2:η~2-N_2][Gd(NR_2)_3]}, 8-Gd, synthesized at -196 °C. Reaction conditions are crucial in this dinitrogen reaction system. When 5-Gd and 6-Gd are warmed above -15 °C, they reform Gd(Ⅱ) complexes. If 1-Gd is dissolved in THF instead of Et_2O under N_2, the irreversible formation of an (N_2)~(3-) complex [K(crypt)][(THF)(R_2N)_2Gd]_2[μ-η~2:η~2-N_2], 9-Gd, is observed.