Formation of the End-on Bound Lanthanide Dinitrogen Complexes [(R2N)3Ln–NN–Ln(NR2)3]2– from Divalent [(R2N)3Ln]1– Salts (R = SiMe3)

Lanthanide-based dinitrogen reduction chemistry has been expanded by the discovery of the first end-on Ln2(μ-η1:η1-N2) complexes, whose synthesis and reactivity help explain the reduction of N2 by the combination of trivalent Ln­(NR2)3 complexes (R = SiMe3) and potassium. The formation of end-on versus the more common side-on Ln2(μ-η2:η2-N2) complexes is possible by using recently discovered Ln­(II) complexes ligated by three NR2 amide ligands (R = SiMe3). The isolated Ln­(II) tris­(amide) complex [K­(crypt)]­[Tb­(NR2)3] (crypt = 2.2.2-cryptand), 1-Tb, reacts with dinitrogen in Et2O at −35 °C to form the end-on bridging dinitrogen complex [K­(crypt)]2{[(R2N)3Tb]2[μ-η1:η1-N2]}, 2-Tb. The 18-crown-6 (18-c-6) Tb­(II) analogue, [K­(18-c-6)2]­[Tb­(NR2)3], 3-Tb, also reacts with N2 to form an end-on product, [K2(18-c-6)3]­{[(R2N)3Tb]2[μ-η1:η1-N2]}, 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 N2 unit in the same crystal, [K­(crypt)]2{[(R2N)3Gd]2[μ-ηx:ηx-N2]} (x = 1 and 2), 5-Gd. Similarly, the 18-c-6 Gd­(II) complex, 3-Gd, generates a product with both binding modes: [K2(18-c-6)3]­{[(R2N)3Gd]2[μ-ηx:ηx-N2]} (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)­(R2N)2Ln]2[μ-η2:η2-N2], 7-Ln, observed previously in Ln­(NR2)3/K/N2 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)­(R2N)2Gd]­[μ-η2:η2-N2]­[Gd­(NR2)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 Et2O under N2, the irreversible formation of an (N2)3– complex [K­(crypt)]­[(THF)­(R2N)2Gd]2[μ-η2:η2-N2], 9-Gd, is observed.