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

Hydrazine reacts with silica-supported
tantalum-hydrides [(≡SiO)2TaHx] (x = 1, 3), 1, under mild conditions
(100 °C). The IR in situ 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, 1-N2H4, are formed. Upon heating at 100
°C, the hydrazine adducts are converted to several species among
which [(≡SiO)2Ta­(NH)­(NH2)], 2, [(≡SiO)2TaH­(NH2)2], 3, and [(≡SiO)2TaH2(NHNH2)], 4, were identified. The final product 2 is also formed in the reaction of N2 with the
same silica-supported tantalum-hydride complexes, and the species
identified as 3 and 4 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 2 is
formed via 3 and 4, with either N2 or N2H4. This strengthens the previous proposal
of the existence of 3 and 4 as intermediates
in the reaction of N2 with the tantalum-hydrides. However,
the reaction of N2 does not 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.