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

The silica-grafted Ta-(V) imido amido complex [(a parts per thousand SiO)(2)Ta(NH)(NH2)], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silica-grafted hydrides [(a parts per thousand SiO)(2)(TaH)-H-(III)], 1a, and [(a parts per thousand SiO)(2)(TaH3)-H-(V)], 1b, undergoes H/D exchange with D-2. In situ IR spectroscopy shows that the fully labelled compound [(a parts per thousand SiO)(2)Ta(ND)(ND2)], 2-d, can be obtained by moderate heating (60 A degrees 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 [{(mu-O)[(HO)(2)SiO](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 [{(mu-O)[(HO)(2)SiO](2)}TaH(NH2)(2)], 3q (Delta E = -9.5 kcal mol(-1) and Delta G(298K) = +2.6 kcal mol(-1) with respect to 2q) and [{(mu-O)[(HO)(2)SiO](2)}Ta(NH)(NH3)], 4q (Delta E = -6.0 kcal mol(-1) and Delta G(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.