Formation of the End-on Bound Lanthanide Dinitrogen Complexes [(RN)Ln-N═N-Ln(NR)] from Divalent [(RN)Ln] Salts (R = SiMe).

Lanthanide-based dinitrogen reduction chemistry has been expanded by the discovery of the first end-on Ln(μ-η:η-N) complexes, whose synthesis and reactivity help explain the reduction of N by the combination of trivalent Ln(NR) complexes (R = SiMe) and potassium. The formation of end-on versus the more common side-on Ln(μ-η:η-N) complexes is possible by using recently discovered Ln(II) complexes ligated by three NR amide ligands (R = SiMe). The isolated Ln(II) tris(amide) complex [K(crypt)][Tb(NR)] (crypt = 2.2.2-cryptand), , reacts with dinitrogen in EtO at -35 °C to form the end-on bridging dinitrogen complex [K(crypt)]{[(RN)Tb][μ-η:η-N]}, . The 18-crown-6 (18-c-6) Tb(II) analogue, [K(18-c-6)][Tb(NR)], , also reacts with N to form an end-on product, [K(18-c-6)]{[(RN)Tb][μ-η:η-N]}, . The reaction of with dinitrogen forms a complex with the same composition as but with both side-on and end-on bonding of the N unit in the same crystal, [K(crypt)]{[(RN)Gd][μ-η:η-N]} ( = 1 and 2), . Similarly, the 18-c-6 Gd(II) complex, , generates a product with both binding modes: [K(18-c-6)]{[(RN)Gd][μ-η:η-N]} ( = 1, 2), . All of these new reduced dinitrogen complexes, , , , and , have three ancillary amide ligands per metal. In contrast, the side-on bound complexes, [(THF)(RN)Ln][μ-η:η-N], , observed previously in Ln(NR)/K/N 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)]{[(THF)(RN)Gd][μ-η:η-N][Gd(NR)]}, , synthesized at -196 °C. Reaction conditions are crucial in this dinitrogen reaction system. When and are warmed above -15 °C, they reform Gd(II) complexes. If is dissolved in THF instead of EtO under N, the irreversible formation of an (N) complex [K(crypt)][(THF)(RN)Gd][μ-η:η-N], , is observed.