Hydrazine N–N Bond Cleavage over Silica-Supported Tantalum-Hydrides

Hydrazine reacts with silica-supported tantalum-hydrides [(≡SiO)₂TaH_<i>x</i>] (<i>x</i> = 1, 3), <b>1</b>, under mild conditions (100 °C). The IR <i>in situ</i> monitoring of the reaction with N₂H₄ or ¹⁵N₂H₄, and the solid-state MAS NMR spectra of the fully ¹⁵N labeled compounds (CP ¹⁵N, ¹H–¹⁵N HETCOR, ¹H–¹H double-quantum, and ¹H–¹H triple-quantum spectra) were used to identify stable intermediates and products. DFT calculations were used for determining the reaction pathway and calculating the ¹⁵N and ¹H NMR chemical shifts. Combining the experimental and computational studies led to the following results. At room temperature, only hydrazine adducts, <b>1-N</b>_<b>2</b><b>H</b>_<b>4</b>, are formed. Upon heating at 100 °C, the hydrazine adducts are converted to several species among which [(≡SiO)₂Ta­(NH)­(NH₂)], <b>2</b>, [(≡SiO)₂TaH­(NH₂)₂], <b>3</b>, and [(≡SiO)₂TaH₂(NHNH₂)], <b>4</b>, were identified. The final product <b>2</b> is also formed in the reaction of N₂ with the same silica-supported tantalum-hydride complexes, and the species identified as <b>3</b> and <b>4</b> had been previously suggested by DFT studies as intermediates on the reaction pathway for N–N cleavage in N₂. The present computational studies (cluster models with M06 functional complemented by selected calculations with periodic calculations) show that <b>2</b> is formed via <b>3</b> and <b>4</b>, with either N₂ or N₂H₄. This strengthens the previous proposal of the existence of <b>3</b> and <b>4</b> as intermediates in the reaction of N₂ with the tantalum-hydrides. However, the reaction of N₂ does <i>not</i> imply the formation of N₂H₄ or its hydrazido monoanionic or dianionic ligand as an intermediate. For this reason, this study informs both on the similarities and differences of the reaction pathways involving N₂ and N₂H₄ with tantalum-hydrides