H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

Abstract

The silica-grafted Ta(V) imido amido complex [(≡SiO)2Ta(NH)(NH2)], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silica-grafted hydrides [(≡SiO)2Ta(III)H], 1a, and [(≡SiO)2Ta(V)H3], 1b, undergoes H/D exchange with D2. In situ IR spectroscopy shows that the fully labelled compound [(≡SiO)2Ta(ND)(ND2)], 2-d, can be obtained by moderate heating (60 °C, 3 h) under D2 atmosphere (550 torr, 300 eq. with respect to Ta), and that the exchange is reversible. The observed stretching and bending frequencies of 2-d are in agreement with the expected isotopic shift upon H/D replacement with respect to literature values reported for 2 and have been corroborated by the independent synthesis of 2-d by reaction of deuterated 1a and 1b with N2 and D2. Density functional theory (DFT) calculations, performed using a periodic or a cluster model, explored the structures and energetics of all minima involved in the reaction with H2 and showed that among the explored pathways the energetically preferred mechanisms for H2 reaction with [{(μ-O)[(HO)2SiO]2}Ta(V)(NH)(NH2)], 2q, is the heterolytic cleavage of either the imido Ta=N or the amido Ta-N bonds, to yield respectively [{(μ-O)[(HO)2SiO]2}TaH(NH2)2], 3q (ΔE = −9.5 kcal mol−1 and ΔG298K = +2.6 kcal mol−1 with respect to 2q) and [{(μ-O)[(HO)2SiO]2}Ta(NH)(NH3)], 4q (ΔE = −6.0 kcal mol−1 and ΔG298K = +7.9 kcal mol−1 with respect to 2q). All activation barriers are moderate (between 17.7 and 30.2 kcal mol−1) in agreement with the observed mild heating conditions necessary for the reaction to occur.