Solid-State End-On to Side-On Isomerization of (NN)^2– in {[(R₂N)₃Nd]₂N₂}^2– (R = SiMe₃) Connects In Situ Ln^III(NR₂)₃/K and Isolated [Ln^II(NR₂)₃]^1– Dinitrogen Reduction

Abstract

Examination of the reduction chemistry of Nd(NR2)3 (R = SiMe3) under N2 has provided connections between the in situ Ln(III)-based LnIII(NR2)3/K reductions of N2 that form side-on bound neutral (N=N)2– complexes, [(R2N)2(THF)Ln]2[μ-η2:η2-N2], and the Ln(II)-based [LnII(NR2)3]1– reductions by Sc, Gd, and Tb that form end-on bound (N=N)2– complexes, {[(R2N)3Ln]2[μ-η1:η1-N2]}2–, which are dianions. The reduction of Nd(NR2)3 by KC8 under dinitrogen in Et2O in the presence of 18-crown-6 (18-c-6) forms dark yellow solutions of [K2(18-c-6)3]{[(R2N)3Nd]2N2} at low temperatures that become green as they warm up to −35 °C in a glovebox freezer. Green crystals obtained from the solution turn yellow-brown when cooled below −100 °C, and the yellow-brown compound has an end-on Nd2(μ-η1:η1-N2) structure. The yellow-brown crystals isomerize in the solid state on the diffractometer upon warming, and at −25 °C, the crystals are green and have a side-on Nd2(μ-η2:η2-N2) structure. Collection of X-ray diffraction data at 10 °C intervals from −50 to −90 °C revealed that the isomerization occurs at temperatures below −100 °C. In the presence of tetrahydrofuran (THF), the dianionic {[(R2N)3Nd]2N2}2– system can lose an amide ligand to provide the monoanionic [(R2N)3NdIII(μ-η2:η2-N2)NdIII(NR2)2(THF)]1–, characterized by X-ray crystallography. These data suggest a connection between the in situ Ln(III)/K reductions and Ln(II) reductions that depends on solvent, temperature, the presence of a chelate, and the specific rare-earth metal