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 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 2 O at −35 °C to form the end-on bridging dinitrogen complex [K(crypt)] 2 {[(R 2 N) 3 Tb] 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 2 N) 3 Tb] 2 [μ-η 1:η 1 -N 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 2 N) 3 Gd] 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 2 N) 3 Gd] 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 2 N) 2 Ln] 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 2 N) 2 Gd][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 2 O under N 2, the irreversible formation of an (N 2 ) 3– complex [K(crypt)][(THF)(R 2 N) 2 Gd] 2 [μ-η 2:η 2 -N 2 ], 9-Gd, is observed.