Solid-State End-On to Side-On Isomerization of (N═N)<sup>2–</sup> in {[(R<sub>2</sub>N)<sub>3</sub>Nd]<sub>2</sub>N<sub>2</sub>}<sup>2–</sup> (R = SiMe<sub>3</sub>) Connects In Situ Ln<sup>III</sup>(NR<sub>2</sub>)<sub>3</sub>/K and Isolated [Ln<sup>II</sup>(NR<sub>2</sub>)<sub>3</sub>]<sup>1–</sup> Dinitrogen Reduction
Amanda B. Chung, Dmitrij Rappoport, Joseph W. Ziller, Roger E. Cramer, Filipp Furche, William J. Evans
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.