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) 2 Ta(NH)(NH 2 )], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silicagrafted hydrides [(:SiO) 2 Ta (III) H], 1a, and [(:SiO) 2-Ta (V) H 3 ], 1b, undergoes H/D exchange with D 2 . In situ IR spectroscopy shows that the fully labelled compound [(:SiO) 2 Ta(ND)(ND 2 )], 2-d, can be obtained by moderate heating (60°C, 3 h) under D 2 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 N 2 and D 2 . 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 H 2 reaction with [{(l-O)[(HO) 2 SiO] 2 }Ta (V) (NH) (NH 2 )], 2q, is the heterolytic cleavage of either the imido Ta=N or the amido Ta-N bonds, to yield respectively [{(l-O)[(HO) 2 SiO] 2 }TaH(NH 2 ) 2 ], 3q (DE = -9.5 kcal mol -1 and DG 298K = ?2.6 kcal mol -1 with respect to 2q) and [{(l-O)[(HO) 2 SiO] 2 }Ta(NH) (NH 3 )], 4q (DE = -6.0 kcal mol -1 and DG 298K = ?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.

Ammonia and Dinitrogen Activation by Surface Organometallic Chemistry on Silica‐Grafted Tantalum Hydrides

“…This behaviour has been observed through the appropriate isotopic shifts in the IR spectra of the complexes and is reversible, since exposing the deuterated species to a hydrogen atmosphere regenerates the original imido amido species. [63] The mechanism of this exchange reaction has been explored by DFT calculations on the model cluster compound [{(μ-O)(HO) 2 SiO} 2 Ta V (NH)(NH 2 )] (Figure 5b), with the Gaussian 03 package. [63] Heterolytic splitting of H 2 is the most favoured pathway.…”

“…[63] The mechanism of this exchange reaction has been explored by DFT calculations on the model cluster compound [{(μ-O)(HO) 2 SiO} 2 Ta V (NH)(NH 2 )] (Figure 5b), with the Gaussian 03 package. [63] Heterolytic splitting of H 2 is the most favoured pathway. This involves protonation of either the imido or amido ligand through a transient dihydrogen adduct and simultaneous hydride migration onto the tantalum centre ( Figure 6).…”

“…Together with the reverse reaction, α-hydrogen abstraction, these processes lead to complete exchange in the presence of the appropriate gas. [63] The moderate activation barriers for the formation of these intermediates {for [(ϵSiO) 2 NH 3 . [64] Investigation by DFT shows an accessible route through heterolytic splitting of N-H bonds of ammonia across Ta=NH and Ta-NH 2 .…”

“…Alternative pathways for the exchange reaction were also considered computationally; intramolecular TaNH 2 -to-TaNH H-transfer in [(ϵSiO) 2 Ta-(=NH)(NH 2 )] has a prohibitively high activation barrier, and oxidative addition of H 2 onto the formally d 0 Ta V centre is unlikely. [63] In the heterolytic activation of ammonia, the tantalum centre acts as a Lewis acid and the imido or amido moiety as the Lewis basic cooperating ligand (Scheme 4). Such cooperative tandem or bifunctional activation [65,66] through the Lewis acid/Lewis base couple formed by a metal centre and a ligated nitrogen atom, termed "1,2-NH bond addition" or the "NH effect", [67] has already been highlighted in the heterolytic splitting of H 2 [68] and is crucial to catalytic systems such as asymmetric hydrogenation of ketones.…”

C–H Bond Activation of Benzene, Toluene, 3,3‐Dimethyl‐1‐butene and Methane by Silica‐Supported TaV Imido Amido Surface Complex

“…11,12 Further studies on the reactivity of complex 2 showed its capacity to react with the H–H bond of H 2 and the N–H bond of ammonia. Both reactions occur across the Ta–N and Ta=NH bonds of 2 via heterolytic bond cleavage 13,16 …”