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 D ₂

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 silicagrafted hydrides [(≡SiO)^(III)H], 1a, and [(≡SiO)_2- Ta ^(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 H ₂ 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(NH ₂)₂], 3q (Δ= -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 (Δ = -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.