Structure and Reactivity of Chelating Imido−Amido Complexes of Tantalum. Mechanistic Studies on the Addition of Silanes to Ta−N Multiple Bonds

Abstract

The synthesis, structural characterization, and reactivity of tantalum complexes with chelating imido−amido ligands are reported. The highly bent imido Cp* = η5-C5Me5), with a TaN−C bond angle of 116.3(4)°, was synthesized from Cp*TaCl4 and the lithiated bis(silylamino)biphenyl (C6H3Me)2(NLiSiMe3)2 (3). Compound 4 undergoes reactions with electrophiles at the nucleophilic imido nitrogen atom. The methyl reacts with xylyl isonitrile to give an insertion product, 6, which was structurally characterized. Addition of MeI to 5 gives a cationic diamide tantalum the ionic structure of which was confirmed by X-ray crystallography. Reactions of 4 and 5 with unhindered silanes result in addition of the silane Si−H bond across the TaN double bond. Addition of PhSiH3 to 4 and 5 gave the respectively. The crystal structure of 9 was determined. Compounds 8 and 9 are unstable and decompose via elimination of HSiMe3. In the presence of CH2Cl2 and PhSiH3, 4 was slowly converted to another hydrido A mechanism for this transformation, involving a sequence of silane addition/elimination reactions, is proposed. X-ray structural characterization of 12 revealed the presence of a nonclassical bonding interaction between the hydride ligand and a neighboring silyl group, leading to a short H−Si contact of 1.86(4) Å and a distorted pentagonal bipyramidal geometry at silicon. Reactions of PhSiH3 and (CH2)3SiH2 (silacyclobutane) with 5 follow second-order kinetics, and an inverse deuterium isotope effect of k H/k D = 0.78(1) for the addition of PhSiH3 to 5 was observed. The elimination of HSiMe3 from 9 was found to follow a first-order rate law with approach to equilibrium (K H = 0.025(2) mol/L) and exhibit an inverse isotope effect of k H/k D = 0.85(2). A study of the temperature dependence of the first-order rate constant for HSiMe3 elimination from 9 provided the activation parameters ΔH ⧧= 25.5(3) kcal/mol and ΔS ⧧ = −0.3(1.0) cal/(mol·K). These findings are interpreted in terms of a mechanism involving slow, rate-determining formation of pentacoordinate silicon intermediates, coupled with a fast hydride shift between Ta and Si.

Isocyanide insertion reactivity of dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [{Nb(η5-C5H4SiMe3)(CH2Ph)2}2(μ-1,4-NC6H4N)]

“…This selective migration of the amide rather than the methyl group is in agreement with that previously reported for similar niobium and tantalum complexes [14]. However this behaviour is unusual, as shown in the reactivity of alkyl amide complexes of titanium [15], zirconium [16], tungsten [17], uranium [18], tantalum [19] and niobium [20], where the isocyanide insertion takes place at the M-C(alkyl) bond. The presence of the g 2 -iminocarbamoyl in 13 was confirmed by 13 C NMR spectroscopy.…”

Oxo and Imido Complexes of Rhenium and Molybdenum in Catalytic Reductions

“…Addition of silane and dihydrogen to metal-ligand multiple bonds has been reported for few early transition metal systems, such as Ti IV [50], and Ta V =NR with silane [51]. Although all of these reactions afford 2+2 addition products containing a metalhydride bond, their reaction mechanisms can be quite different.…”

“…Although all of these reactions afford 2+2 addition products containing a metalhydride bond, their reaction mechanisms can be quite different. For example, reaction of Cp* 2 (py)Ti IV =S with silanes is believed to go through a four-centered transition state (as in E) [47], whereas the Ta imido complex (Ta V =NR) reacts with silane through a pentacoordinate silicon intermediate (type D) resulting from the nucleophilic attack of an exposed imido nitrogen at silane [51]. In an imido molybdenum (IV) system, a stretched-dihydrogen complex (type A) was observed upon exposure of the complex to H 2 at low temperature [52].…”

Nonclassical Titanocene Silyl Hydrides

“…[1f] These include interligand hypervalent interactions (IHI) [6][7][8] and a variety of multicenter SiÀH interactions found in some complexes of Ru and Os, the bonding in which is the subject of continuing debate. [9][10][11][12] However, for a given ligand set and metal, usually only one type of Si À H interaction (if any) occurs.…”