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

IFP-Lyon, Rond-point de l’échangeur de Solaize BP 3 69360 Solaize France
Topics in Catalysis (Impact Factor: 2.37). 10/2009; 52(11):1482-1491. DOI: 10.1007/s11244-009-9295-0


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. Insitu IR spectroscopy shows that the fully labelled compound [(≡SiO)2Ta(ND)(ND2)], 2-d, can be obtained by moderate heating (60°C, 3h) under D2 atmosphere (550torr, 300eq. 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=−9.5kcalmol−1 and ΔG298K=+2.6kcalmol−1 with respect to 2q) and [{(μ-O)[(HO)2SiO]2}Ta(NH)(NH3)], 4q (ΔE=−6.0kcalmol−1 and ΔG298K=+7.9kcalmol−1 with respect to 2q). All activation barriers are moderate (between 17.7 and 30.2kcalmol−1) in agreement with the observed mild heating conditions necessary for the reaction to occur.

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    ABSTRACT: Ammonia N–H bond is cleaved at room temperature by the silica–supported tantalum imido amido complex [(≡SiO)2Ta(NH)(–NH2)], 2, if excess ammonia is present, but requires 150 °C to achieve the same reaction if only one equivalent NH3 is added to 2. MAS solid-state 15N NMR and in situ IR spectroscopic studies of the reaction of either 15N or 2H labeled ammonia with 2 show that initial coordination of the ammonia is followed by scrambling of either 15N or 2H among ammonia, amido and imido groups. Density functional theory (DFT) calculations with a cluster model [{(μ-O)[(H3SiO)2SiO]2}Ta(NH)(–NH2)(NH3)], 2q·NH3, show that the intramolecular H transfer from Ta–NH2 to TaNH is ruled out, but the H transfers from the coordinated ammonia to the amido and imido groups have accessible energy barriers. The energy barrier for the ammonia N–H activation by the Ta-amidogroup is energetically preferred relative to the Ta-imidogroup. The importance of excess NH3 for getting full isotope scrambling is rationalized by an outer sphere assistance of ammonia acting as proton transfer agent, which equalizes the energy barriers for H transfer from coordinated ammonia to the amido and imido groups. In contrast, additional coordinated ammonia does not favor significantly the H transfer. These results rationalize the experimental conditions used.
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    ABSTRACT: Ammonia N–H bond activation and dinitrogen N≡N cleavage with dihydrogen on an isolated metal atom have been achieved with the silica-grafted TaIII and TaV hydrides [(≡SiO)2TaH] (2a) and [(≡SiO)2TaH3] (2b), accessed through surface organometallic chemistry (SOMC). The synthesis of the starting tantalum hydrides 2a and 2b by grafting tris(neopentyl)neopentylidenetantalum(V), Ta(=CH–tBu)Np3, on silica yields well-defined, isolated tantalum atoms. Silsesquioxane molecular modelling shows that the mechanism of the grafting reaction implies a tetraalkyl intermediate [(≡SiO)TaNp4]. The starting hydrides 2a and 2b react stoichiometrically and catalytically with alkanes in reactions such as alkane metathesis, cross-metathesis between ethane and toluene, and methane coupling to form ethane. Mechanistic studies show the relevance of tantalum carbenes and Chauvin-like metallacyclobutane intermediates in most of these reactions. Finally, the stoichiometric N2 cleavage and NH3 activation to the final imido amido tantalum(V) complex [(≡SiO)2Ta(NH)(NH2)] (3) are reviewed and discussed mechanistically. In the N≡N cleavage reaction, dihydrogen adducts on silica-grafted isolated tantalum atoms appear to play a central role. The ammonia reaction occurs by bifunctional activation through the Lewis acid/Lewis base couple formed by a metal centre and a coordinated nitrogen atom, the so-called “NH effect”. Such bifunctional activation is also observed for the heterolytic cleavage of H2 by [(≡SiO)2Ta(NH)(NH2)] (3).
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