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.基于镧系元素（lanthanide）的氮气还原（dinitrogen reduction）化学，随着首例端基配位（end-on）Ln₂(μ-η¹:η¹-N₂)配合物的发现得到了拓展。该类配合物的合成与反应活性，有助于解释三价Ln(NR₂)₃配合物（R = SiMe₃）与钾组合实现氮气还原的过程。通过使用近期发现的、由三个NR₂酰胺配体配位的Ln(II)配合物（R = SiMe₃），可制备得到端基配位型而非更常见的侧基配位（side-on）Ln₂(μ-η²:η²-N₂)型配合物。分离得到的Ln(II)三(酰胺)配合物[K(crypt)]⁺[Tb(NR₂)₃]⁻（crypt = "2.2.2-cryptand"，记为1-Tb），在乙醚（Et₂O）中于-35 ℃下与氮气反应，生成端基桥连氮气配合物[K(crypt)]₂{[(R₂N)₃Tb]₂[μ-η¹:η¹-N₂]}，记为2-Tb。18-冠-6（18-crown-6，简称18-c-6）的Tb(II)类似物[K(18-c-6)₂]⁺[Tb(NR₂)₃]⁻，记为3-Tb，同样可与氮气反应生成端基配位产物[K₂(18-c-6)₃]⁺{[(R₂N)₃Tb]₂[μ-η¹:η¹-N₂]}⁻，记为4-Tb。1-Gd与氮气的反应则生成组成与2-Tb一致的配合物，但在同一晶体中同时存在N₂单元的端基与侧基两种成键模式，即[K(crypt)]₂{[(R₂N)₃Gd]₂[μ-ηˣ:ηˣ-N₂]}（x = 1和2），记为5-Gd。类似地，18-冠-6的Gd(II)配合物3-Gd也会生成同时存在两种配位模式的产物[K₂(18-c-6)₃]⁺{[(R₂N)₃Gd]₂[μ-ηˣ:ηˣ-N₂]}⁻（x = 1、2），记为6-Gd。上述所有新型还原型氮气配合物2-Tb、4-Tb、5-Gd及6-Gd，每个金属中心均带有三个辅助酰胺配体。与之相反，此前在Ln(NR₂)₃/K/N₂反应中观测到的侧基配位型配合物[(THF)(R₂N)₂Ln]₂[μ-η²:η²-N₂]（记为7-Ln），每个金属中心仅带有两个酰胺配体。在-196 ℃下合成的双金属五酰胺配合物[K(THF)₆]⁺{[(THF)(R₂N)₂Gd][μ-η²:η²-N₂][Gd(NR₂)₃]}⁻（记为8-Gd）的结构中，观测到了与这些体系形成相关的关联。该氮气反应体系中反应条件至关重要：当5-Gd与6-Gd被升温至-15 ℃以上时，它们会重新转化为Gd(II)配合物。若将1-Gd在氮气氛围下溶于四氢呋喃（THF）而非乙醚中，则会不可逆地生成(N₂)³⁻配合物[K(crypt)]⁺[(THF)(R₂N)₂Gd]₂[μ-η²:η²-N₂]⁻，记为9-Gd。