Introduction

Early transition-metal chemistry has received increased attention in recent years, particularly as new applications in catalysis and polymer chemistry have been developed.^1-9 Much of this new chemistry involves metal complexes which possess cyclopentadienyl ligand sets. Such complexes are readily modified to adjust electronic and steric properties for the metal center, and therefore activities and selectivities for the catalyst.^2,3 Many of the systems of interest involve electrophilic and coordinatively unsaturated d⁰ metal centers that behave as Lewis acids in their chemistry. For this reason, effort has been devoted to development of alternative ligand sets that may increase the electrophilicity of the metal center.^10-14 One approach involves use of ancillary multidentate amido ligands to support reactive and highly electrophilic metal centers.^13-26

While investigating alternative ancillary ligands for d⁰ metal centers, a number of complexes with the bis(triisopropylsilyl)-o-phenylenediamido (o-C₆H₄(NSiⁱPr₃)₂) ligand^27,28 have been synthesized and studied. A common structural feature for these complexes is the presence of a secondary bonding interaction between the aromatic ring and the metal center. While confirming the electron-poor character of the metal center in complexes of this ligand, this donation of electron density from the ligand has the potentially undesirable effect of reducing electrophilicity at the metal. To reduce the possibility of such an interaction, and also to create a chiral environment at the reactive site, an exploration of alternative bis(silylamido) ligands based on C₂-symmetric biphenyl backbones was undertaken. The use of similar ligands has recently been reported by Cloke²⁹ and Lappert.³⁰ One aspect of this work, reported in this chapter, concerns a set of tantalum complexes containing both Cp* (h⁵-C₅Me₅) and bis(silylamido) ligands.³¹ This system is characterized by facile cleavage of N-Si bonds in the ligand, and formation of Ta=N double bonds with unusually acute Ta=N-C bond angles. While this reactivity implies that these silylamides cannot always be regarded as innocent spectator ligands, it presents us with the opportunity to explore the chemistry of a reactive d⁰ Ta=N bond. The elimination of silyl groups from silylamido complexes has previously been reported as a limitation in the use of these types of ligands,^14,23 and the cleavage of N-Si bonds has been employed as a route to transition metal imido species,^32-34 but the mechanism of this process has not been investigated. Understanding the reactivity of metal-heteroatom multiple bonds is a subject of both theoretical and practical importance. Transition metal imido species^32,33,35 are involved or suspected as intermediates in hydrocarbon activation,^36-40 catalytic hydrodenitrogenation,⁴¹ and hydroamination.⁴² In this chapter the synthesis and reactivity of multiply bonded tantalum - nitrogen species are described, supplemented by detailed kinetic and mechanistic studies on the reversible additions of silanes to Ta=N bonds.

Results and Discussion

Ligand synthesis. The starting material for the synthesis of the target silylamine ligand, 2,2'-diamino-6,6'-dimethylbiphenyl (1), was prepared according to literature procedures.⁴³ N-Silylation of this diamine was achieved via deprotonation of 1 in THF followed by reaction of the resulting dianion with Me₃SiCl (eq 1). N,N'-Bis(trimethyl-silyl)-2,2'-diamino-6,6'-dimethylbiphenyl (2) was obtained in 84% yield as colorless crystals from pentane. The lithium salt (3) was then prepared in 63% yield by treating 2 with two equiv of ⁿBuLi in pentane.

Syntheses and characterization of bent tantalum imido complexes. Reaction of 3 with Cp*TaCl₄ in refluxing benzene for 4 h yielded a dark red solution. After evaporation of the solvent and extraction with pentane, compound 4 was isolated in 68% yield as a red crystalline powder. ¹H NMR spectroscopy indicated that loss of one trimethylsilyl group had occurred to afford a tantalum imido complex, as indicated in eq 2. After 3 h (80 ^oC, benzene-d₆), the reaction had proceeded to >90% conversion with formation of an equimolar mixture of Me₃SiCl and 4. This reaction therefore differs from that between Cp*TaCl₄ and o-C₆H₄(NLiSiⁱPr₃)₂, which produces the stable bis(amido) complex Cp*[o-C₆H₄(NSiⁱPr₃)₂]TaCl₂.²⁸

The molecular structure of 4 is shown in Figure 1. The N(1)-Ta-Cl and N(2)-Ta-Cl bond angles are 102.6(2)^o and 108.5(2)^o, and the bite angle (N(1)-Ta-N(2)) for the imido-amido ligand is 98.8(2)^o. The most interesting feature of the molecule is the unusually small Ta-N(1)-C(1) bond angle of 116.3(4)^o, which is surprisingly close to the corresponding angle of 114.6(4)^o associated with the Ta-N(2)-C(12) amide linkage. The amido N is planar (sum of angles around N(2) = 359.9^o), as expected for a metal complex of this type. The Ta=N(1) bond length of 1.830(5) Å is considerably shorter than the Ta-N(2) bond distance of 1.988(6) Å.

The majority of structurally characterized imido complexes exhibit linear, or near linear, geometries with M-N-C bond angles in the range 160-180^o.^32,33 The nature of metal-nitrogen bonding in transition metal imido complexes has been the subject of a number of theoretical studies,^44-48 which suggest that the metal-ligand bond order is usually intermediate between two and three, and that there is a rather soft potential for bending giving rise to the relatively wide range of observed bond angles. In valence-bond

Figure 1. ORTEP diagram of Cp*Ta[=N(C₆H₃Me)₂NSiMe₃]Cl (4).

terms, the nitrogen in linear imidos is sp-hybridized, resulting in a formally triple M-N bond. There are only a few reported examples of strongly bent imido complexes, and the bending in these is attributed either to the steric constraints of a chelate ring,^41,49,50 or to electronic effects (in particular the absence of available empty metal d-orbitals of appropriate symmetry to interact with the nitrogen lone pair).^51,52 In the limiting case of a strongly bent imido ligand, the N atom would be sp²-hybridized with a formally double M-N bond, and a non-bonding electron pair localized on the nitrogen. There are also a number of linear imidos for which a formally triple M-N bond would result in a violation of the 18-electron rule, and such compounds are best described as possessing a double M-N bond.^45,47,53

To the best of our knowledge, compound 4 is the most strongly bent transition metal imido complex reported so far. The observed bending is attributed to the steric restrictions imposed on the imido linkage by the chelating biphenyl ligand. The Ta=N bond is rather long compared to the typical range of 1.6 to 1.8 Å for organoimido complexes of tantalum,^32,33 but is comparable to the long Ta=N bond of 1.831(10) Å found in Cp*₂Ta(=NC₆H₅)H,^46,54 which is considered to possess a Ta-N bond order between two and three.

Reaction of 4 with MeMgBr in Et₂O gave the methyl derivative 5, isolated in 85% yield as orange-red crystals from pentane (eq 2). The TaMe group gives rise to a new singlet at 0.72 ppm in the ¹H NMR spectrum and a signal at 39.5 ppm in the ¹³C NMR spectrum. Unfortunately, Ta=N stretches in infrared spectra of 4 and 5 could not be positively identified, presumably because they are coupled with other vibrations in the molecules.^33,54

Reactivity of 4 and 5 towards small molecules. While many transition metal imido complexes are stable and unreactive, some exhibit high reactivities towards electrophiles,^55,56 nucleophiles,⁵⁷ or both.^33,46,58 Nucleophilic imido ligands tend to be associated with the early transition metals (and more strongly polarized M-N bonds), while later transition metals exhibit more covalent metal-nitrogen bonding and less nucleophilic imido nitrogen centers.^33,46 In addition, some imido complexes have been observed to react with unsaturated substrates, to give [2+2] or [2+4] cycloaddition products,^38,42,56 and certain d⁰ imido complexes have been found to activate the C-H bonds in hydrocarbons.^36,38,40,59 It is expected, according to the bonding model described above, that bending of the imido ligand should lead to a reduced metal-nitrogen bond order and localization of a lone pair on nitrogen. This should lead to increased nucleophilicity of the imido nitrogen atom and increased reactivity of the complex due to a weaker, and more exposed, Ta=N bond.

Contrary to these expectations, compound 4 was found to be rather stable. This compound does not decompose in refluxing toluene-d₈ for 24 h, and does not react with H₂, CO, C₂H₄, PhCCPh, or Me₃SiCCH within 24 h in refluxing benzene-d₆. The methyl derivative 5 also did not react with H₂ or Me₃SiCCH under similar conditions. Both 4 and 5 are inert towards the nucleophiles PPh₃, PhPH₂, and p-(N,N)-dimethyl-aminopyridine (24 h, 80 ^oC, benzene-d₆). Although transition-metal imido complexes typically react readily with organic carbonyl compounds,³³ no reaction was observed between 4 or 5 and benzophenone (4 h, 80 ^oC, benzene-d₆). Benzaldehyde reacted slowly with 4 (ca. 40% conversion after 20 h) and with 5 (70% conversion after 24 h) at room temperature in benzene-d₆ to give a mixture of products. Complexes 4 and 5 also reacted with weak acids such as PhOH and p-toluidine (but not with the more sterically hindered Ph₂NH), giving mixtures of products.

Complex 5 reacts very slowly with carbon monoxide under 40-80 psi, at 80 ^oC in benzene. However, the accumulation of decomposition products precluded the isolation and identification of the product of this reaction. While 4 did not react with xylyl isonitrile, compound 5 was found to undergo a clean reaction at room temperature to give the insertion product 6 (eq 3), isolated as bright yellow crystals in 68% yield. The methyl group of the iminoacyl ligand in 6 appears at 1.26 ppm in the ¹H NMR spectrum, and the ¹³C resonance for the iminoacyl carbon (N=CMe) was observed at 255.0 ppm, in the low field region of the 195-268 ppm range reported for h²-iminoacyl complexes.^60-62 In the IR spectrum, the C=N stretch appears at 1579 cm^-1.

The structure of 6 was determined by X-ray crystallography (Figure 2). The iminoacyl nitrogen N(3) is coordinated to the metal with a Ta-N(3) bond length of 2.259(4) Å, which is longer than the typical values of 2.12-2.17 Å reported for tantalum h²-iminoacyl complexes,^60-62 but the Ta-C(26) and N(3)-C(26) bond lengths of 2.134(5) Å and 1.273(6) Å are within the typical range for such complexes.⁶² The imido Ta-N(1)-C(1) bond angle of 133.2(3)^o is considerably expanded relative to the corresponding value for 4, while the bite angle of the chelating ligand (N(1)-Ta-N(2) = 94.2(2)^o) is slightly reduced from that in 4. These metrical changes may be attributed to the higher coordination number for the metal center, which leads to increased steric crowding in 6 relative to 4. The Ta-N(1) bond length, 1.819(4) Å, is slightly shorter than the Ta=N bond in 4, which is consistent with the greater bond angle at N(1).

The nucleophilic properties of the imido nitrogens in 4 and 5 were demonstrated by their reactions with MeI. Compound 4 did not react with MeI at room temperature in benzene-d₆, and upon heating the reaction mixture gave a number of decomposition products. The methyl derivative 5, on the other hand, reacts readily with MeI at room temperature. The product of this reaction (7), isolated as yellow crystals in 74% yield, is highly insoluble in nonpolar organic solvents (Et₂O, benzene) but is soluble in CH₂Cl₂. Reactions of imido complexes with MeI or MeBr have often been observed to result in

Figure 2. ORTEP diagram of Cp*Ta[=N(C₆H₃Me)₂NSiMe₃][h²-(2,6-Me₂C₆H₃)N=CMe] (6).

complete cleavage of the metal-nitrogen bond to produce ammonium ions,^55,56 but spectroscopic characterization of 7 revealed that only one equiv of MeI had added to the Ta=N bond (eq 4). Furthermore, the insolubility of 7 in nonpolar solvents and the downfield shifts for the MeN-Ta group in the ¹H (3.39 ppm) and ¹³C (53.4 ppm) NMR spectra suggested that 7 might be cationic, and this formulation was confirmed by X-ray crystallography.

An ORTEP drawing of 7 is shown on Figure 3. The Ta-I distance of 5.311(1) Å, and the undistorted three-legged piano stool geometry about the metal center confirmed that the iodide is not coordinated to Ta. The ligand bite angle in this case is expanded to 104.5(4)^o. The Ta-N(Me) bond of 1.92(1) Å is intermediate in length between the multiple Ta=N bonds found in 4 and 6 (1.830(5) and 1.819(4) Å, respectively), and those found for the amido nitrogens in 4, 6, 9 and 12, ranging from 1.988(6) in 4 to 2.105(4) in 6, while the Ta-N(Si) bond length of 1.99(1) Å is within the latter range. The unusually large Ta-N(1)-C(15) bond angle of 147.2(9)^o and the small Ta-N(1)-C(1) angle of 101.8(8)^o can be attributed to steric crowding caused by the MeN group interacting with the Cp* ring, and probably the presence of a bonding interaction involving the ipso carbon, leading to a close Ta...C(1) contact of 2.64(1) Å.

The ionic structure of 7 is attributed to steric crowding about Ta, which prevents coordination of the iodide anion. The stability of the cation can also be rationalized by the possibility for stabilization by donation of pi-electron density from the two amido nitrogens.

Figure 3. ORTEP diagram of {Cp*Ta[MeN(C₆H₃Me)₂NSiMe₃]Me}⁺I^- (7).

While coordinatively unsaturated, cationic early transition metal complexes are potentially active as olefin polymerization catalysts, compound 7 did not react with ethene over 24 h (room temperature, dichloromethane-d₂).

Reactions of 4 and 5 with silanes. Compounds 4 and 5 were found to react with hydrosilanes, to form tantalum hydride complexes via addition of the Si-H bond across the Ta=N bond. While the primary silanes PhSiH₃ and PhCH₂SiH₃ reacted at measurable rates at room temperature, reactions with most secondary silanes (such as PhMeSiH₂) could be observed only after prolonged heating, which led to decomposition and the formation of HSiMe₃ (by ¹H NMR spectroscopy). The tertiary silane MeEt₂SiH and the sterically hindered primary silane MesSiH₃ did not react with 4 even at 80 ^oC over 24 h in benzene-d₆. Addition of PhSiH₃ to 4 and 5 yielded the hydrido chloride and hydrido methyl complexes 8 and 9, respectively, as shown in eq 5. The moderate rates of these reactions and the thermal instabilities of the products required use of neat PhSiH₃ and mild reaction conditions (room temperature) for isolation of the products.

The TaH groups in 8 and 9 appear as singlets in the ¹H NMR spectra, at 20.44 and 17.15 ppm, respectively. The methyl ligand in 9 gives rise to a doublet in the ¹H NMR spectrum at 0.88 ppm, with coupling to the hydride ligand (³J_HH = 1.9 Hz; coupling was not resolved for the TaH resonance). No significant changes were observed in the ¹H NMR spectrum of 9 on cooling to -80 ^oC. The Ta-H stretches in the IR spectra of 8 and 9 are observed at 1790 and 1778 cm^-1, respectively, and the deuteride 9-d₃, prepared from 5 and PhSiD₃, exhibits a Ta-D stretch at 1278 cm^-1. No exchange of the hydride ligand in 9 was observed upon exposure to D₂ (1 atm, 25 ^oC, 24 h in benzene-d₆). Compound 9 also did not react with ethene under similar conditions.

The structure of 9 was determined by single-crystal X-ray crystallography (Figure 4). The tantalum adopts a four-legged piano stool coordination geometry, with approximately equal Ta-N(amide) distances of 2.060(3) and 2.028(3) Å. The hydride ligand H(1) was located in the Fourier difference map, and its position was refined. Interestingly, it adopts a position that is trans to the phenylsilyl group from which it is derived. The Ta-H(1) bond length in 9 is 1.67(3) Å, and the H(1)-Ta-N(2), H(1)-Ta-C(24), and N(1)-Ta-C(24) angles are 74(1)^o, 70(1)^o, and 87.8(1)^o, respectively. The bite angle associated with the chelating diamide ligand is only 88.6(1)^o. Similarly to compound 7, the angles about N(1) show some deviation from the expected 120^o for sp²-hybridized N, with a Ta-N(1)-Si(1) angle of 136.0(1)^o and a Ta-N(1)-C(1) angle of 113.8(2)^o. This distortion is attributed to steric crowding associated with the Cp* ring. In contrast, the angles about N(2) do not deviate significantly from 120^o.

The Ta hydrides 8 and 9 are stable in the solid state under nitrogen and in the dark, but slowly decompose in solution with clean elimination of HSiMe₃ (by ¹H NMR spectroscopy), to give the imido species 10 and 11, respectively (eq 5). Other potential elimination products (PhSiH₃, PhSiH₂Cl, PhMeSiH₂, Me₃SiCl or Me₄Si) were not observed in these decompositions. The hydrido chloride 8 begins to decompose after several hours in benzene-d₆ at room temperature, while the methyl hydride 9 is more stable and exhibits a half-life of a few days in benzene-d₆. Identification of 10 and 11 was based on their ¹H NMR spectra. Isolation of 11 was not possible as the elimination of HSiMe₃ from 9 did not go to completion but reached equilibrium, and attempts to remove the HSiMe₃ by prolonged reflux in benzene only resulted in production of a mixture of decomposition products.

Figure 4. ORTEP diagram of Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiMe₃](H)Me (9).

In dichloromethane solution, the reaction of 4 with PhSiH₃ occurred over four days at room temperature, as the color of the reaction mixture slowly changed from dark red to orange to light yellow. The isolated pale yellow product (12) was found to possess a hydride ligand, observed by ¹H NMR spectroscopy as a doublet (J = 6.0 Hz) at 14.85 ppm in dichloromethane-d₂. Structural characterization of 12 by X-ray crystallography revealed the presence of a diamide ligand containing both -SiH₂Ph and -SiHClPh groups bound to the nitrogens (Figure 5). The structure of 12 is similar to that of the methyl hydride 9, with angles about Ta of 92.3(1)^o (Cl(1)-Ta-N(1)), 89.4(1)^o (N(1)-Ta-N(2)), 76(1)^o (Cl(1)-Ta-H(1)), and 67(1)^o (N(2)-Ta-H(1)). The Ta-N bond lengths of 2.015(3) and 2.016(3) Å are essentially the same. Again, due to the steric hindrance of the Cp* ring, the angles at N(1) show significant deviation from 120^o, the Ta-N(1)-Si(1) angle is 140.1(2)^o and the Ta-N(1)-C(1) angle is 111.7(2)^o. The hydride ligand, located in the Fourier difference map and refined isotropically, is 1.83(4) Å from tantalum (compared to the Ta-H bond length of 1.67(3) Å in 9). In addition, the distance between H(1) and the neighboring silicon Si(2) in 12, 1.86(4) Å, is rather small and significantly shorter than the sum of the van der Waals radii (ca. 3.1 Å), suggesting the presence of a nonclassical bonding interaction between these atoms. For comparison, the distance between the hydride H(1) and the silicon Si(2) of the Me₃Si group in 9 is 2.56(3) Å. The Ta-N(2)-Si(2) angle is reduced to 108.8(2)^o, while in complexes 4, 6, 7, and 9 the corresponding Ta-N-Si angles are greater than 120^o. Further support for a significant H(1)^...Si(2) interaction is found in the bond distances and angles about Si(2), which suggest a distortion from tetrahedral geometry. The N(2)-Si(2)-C(21) angle of 120.2(2)^o is consistent with the nitrogen and the phenyl group occupying equatorial positions in a distorted trigonal bipyramid. The chlorine atom appears to occupy an axial site, with Cl(2)-Si(2)-N(2) and Cl(2)-Si(2)-C(21) angles of 103.1(1)^o and 100.5(1)^o. The Si(2)-Cl(2) bond length of 2.149(2) Å is also rather long, approaching values observed for axial chlorine in pentacoordinate silicon compounds.⁶³ Similar nonclassical bonding interactions

Figure 5. ORTEP diagram of Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiPhHCl](H)Cl (12).

have been characterized for a number of transition metal hydrido silyl complexes,^64-71 based on short Si-H distances and distorted geometries at silicon. Such complexes can be viewed as exhibiting three-center M-H-Si bonding with limiting metal silyl hydride and sigma-H-Si resonance structures.⁷² The bonding situation in compound 12 is different, since the silyl group is not directly bonded to the metal. Thus, the bonding in 12 is probably best described by the resonance structures shown in Scheme 1, involving a pentacoordinate Si center in one of the canonical forms. This interaction may also be differentiated from those found in a number of beta-Si-H agostic complexes^73-75 in which an Si-H sigma-bond is coordinated to the metal, without an increase of the coordination number at the silicon.

The existence of a bonding interaction between the hydride ligand and the neighboring silicon atom in 12 may also account for the low Ta-H stretching frequency (1678 cm^-1 vs. 1790-1778 cm^-1 for 8, 9, and 16). Finally, evidence for such an interaction is seen in the unusually large ¹H NMR coupling constant (6.0 Hz) between H(1) and H(41) (the hydrogen bonded to Si(2)), which are otherwise separated by four bonds. The magnitude of this coupling and the appearance of the spectrum do not observably change on cooling to -80 ^oC (in dichloromethane-d₂). In the ²⁹Si (¹H coupled) NMR of 12, the PhSiH(Cl)N silicon appears as a doublet at -66.8 ppm (¹J_SiH = 272 Hz), and the PhSiH₂N silicon gives rise to a triplet at -26.9 ppm (¹J_SiH = 208 Hz). Unresolved coupling (J ~ 6-7 Hz) to hydrogens of the phenyl groups was observed for both resonances, and this effect on the line width appears to obscure coupling to the hydride ligand.

The formation of 12 apparently involves a series of addition and elimination steps, combined with partial chlorination of a silicon atom. A possible mechanism is given in Scheme 2.

Monitoring the reaction by ¹H NMR spectroscopy provided evidence for some of the intermediates in this scheme. When 4 was reacted with excess PhSiH₃ (50 equiv) in dichloromethane-d₂, compound 8 was observed to form as an intermediate (the main TaH-containing species after 12 h), and then gradually disappear (complete conversion to 12 after 2 days). This reaction also produced Me₃SiCl and HSiMe₃ in a 1.2:1 ratio (by ¹H NMR integration). Another tantalum hydride signal, observed to grow and then decay during the reaction, is tentatively assigned to intermediate 13 (vide infra). Reaction of isolated 8 with PhSiH₃ in dichloromethane-d₂ (24 h, room temperature) also produced 12, along with HSiMe₃, Me₃SiCl, and CHD₂Cl, identified by ¹H NMR spectroscopy. The formation of CHD₂Cl indicates that solvent is the source of the silicon-bound chlorine in 12. Compound 10 was observed independently as resulting from the elimination of HSiMe₃ from 8 (benzene-d₆, room temperature), but the formation of both HSiMe₃ and Me₃SiCl suggests the intermediacy of 14 in an alternative pathway when the reaction occurs in dichloromethane (see Scheme 2). While a number of PhSiH₂ signals are observed during the transformation (some associated with 10 and 13), it is not possible to unambiguously assign resonances to species such as 14 and 15.

In an attempt to isolate some of the intermediates in Scheme 2, and also to gain information concerning the silicon-chlorination step, the reaction of 8 with PhSiH₃ in benzene (3 days at room temperature) was examined. Removal of the volatiles from the resulting dark yellow solution and recrystallization of the residue from pentane afforded a yellow solid. The ¹H NMR spectrum (500 MHz, benzene-d₆) of this product mixture revealed a set of signals which represent a major component of the mixture and appear to be associated with 13, the expected final product in the absence of CH₂Cl₂. The TaH in 13 appears as a singlet at 19.76 ppm. One of the PhSiH₂ groups gives rise to two doublets (²J_HH = 10.1 Hz), while a set of signals assigned to the other PhSiH₂ group (two doublets of doublets) exhibits further small splittings of 3.1 and 1.5 Hz, presumably due to coupling to the TaH hydride ligand. Other resonances for 13 were obscured by signals from other species. The elimination product 10 was also identified in this spectrum. Addition of dichloromethane-d₂ to this intermediate mixture resulted in its complete conversion to 12 after 16 h at room temperature, implying that 13 lies on the reaction pathway. Finally, a facile ligand exchange between Ta and Si via a pentacoordinate Si intermediate can be invoked to explain the migration of chlorine from tantalum to silicon. Evidence for the involvement of such intermediates in this system also comes from kinetic studies on the silane addition/elimination process (vide infra).

Kinetic studies of the silane addition/elimination reactions. The kinetics of the PhSiH₃ addition to the tantalum imido complex 5 was studied by ¹H NMR spectroscopy at 35.0 ^oC (benzene-d₆ solvent). The disappearance of 5 in the presence of a large excess of the silane (25-70 equivalents) was found to be first order in 5, as shown by the linear decay of ln([5]/[5]₀) vs. time (Figure 6). A plot of the observed pseudo-first order rate constants kobs vs. PhSiH₃ concentrations (Figure 7) established a second-order rate law for the reaction, and a rate constant of k_H = 5.57(6) x 10^-5 L/(mol.s). The measured rate constant (kobs) was not affected by addition of one equiv of the product (9).

To gain more insight into the mechanism of the reaction of 5 with PhSiH₃, the H/D kinetic isotope effect was measured. The second-order rate constant for the reaction of 5 with PhSiD₃, measured under the same conditions, is kD = 7.15(10) x 10^-5 L/(mol.s), which corresponds to a kinetic isotope effect (KIE) of k_H/k_D = 0.78(1). This inverse isotope effect suggests that the rate-determining transition state does not involve significant breaking or making of bonds to hydrogen. Although it has been shown⁷⁶ that primary isotope effects can be very small or even inverse in cases of extremely exo- or endothermic reactions, this does not seem likely for the reaction under consideration. Inverse KIEs are usually secondary in nature and associated with changes in the bond strengths and the vibrational frequencies of bonds to H/D in the transition state.^77,78 The formation of an intermediate adduct, involving a pentacoordinate Si center, is proposed to be the rate-determining step in the addition of the silane to the Ta imido complex 5, as indicated in

Figure 6. Pseudo-first order plots for disappearance of 5 at PhSiH₃ concentrations of 0.505, 0.668, 0.795^a, 1.483, and 1.588 mol/L; kobs x 10⁵ = 2.75, 3.53, 4.67^a, 8.06, and 8.90 s^-1, respectively. ^a conducted in presence of 1 equiv of 9

Figure 7. Observed pseudo-first order rate constant for disappearance of 5 (kobs) as a function of the phenylsilane concentration.

Scheme 3. Analogous pentacoordinate silicon species have often been invoked as intermediates in nucleophilic substitution at silicon,^63,79,80 and the formation of such intermediates has been shown in some cases to be the rate-determining step.⁸¹ While no bonds to H/D are formed or broken in this step, an increase in the Si-H out-of-plane bending frequencies in the pentacoordinate Si intermediate relative to the free silane can lead to a larger zero-point energy difference between the proteated and the deuterated species in the transition state, relative to the zero-point energy difference in the reactants. Such an increase in the Si-H bending force constants can be explained by the increased "tightness" of the transition state relative to the reactants. The resulting lower activation barrier for the deuterated species is consistent with the observed inverse KIE. Related arguments have been used to explain the secondary isotope effects in the very extensively studied SN2-type reactions at electrophilic sp³ C centers, which proceed through a five-coordinate transition state.^77,78,82,83 We are not aware, however, of a similar observation of inverse secondary isotope effects for nucleophilic substitution reactions of hydrosilanes, although very small (1-1.3) primary kinetic isotope effects have been reported for nucleophilic substitution of the hydrogen in tertiary silanes with organolithium reagents.⁸¹ Formation of the pentacoordinate Si intermediate is probably followed by a fast intramolecular hydride shift from Si to Ta to give product 9. The observation of an H^...Si bonding interaction in the related complex 12 further supports the idea that such a hydride shift can occur without a significant energy barrier, and thus cleavage of the Si-H bond is not a rate-determining step. Species analogous to 12 probably lie on the reaction coordinate of this migration. An alternative, one-step mechanism can also be envisaged, involving a concerted [2+2] addition of the Si-H bond to the Ta=N bond, with a transition state resembling the structure of 12. Such a mechanism, however, would be difficult to reconcile with the observed inverse isotope effect, since it would involve weakening of the bonds to H in the rate-determining transition state.

Additional evidence for rate-determining formation of a pentacoordinate silicon intermediate comes from the observation that silacyclobutane, (CH₂)₃SiH₂, reacts with 5 much faster than PhSiH₃, in contrast to other secondary silanes which were unreactive even at elevated temperatures. The disappearance of 5 in the presence of excess silane was followed by ¹H NMR spectroscopy, and the reaction was again found to follow a second-order rate law (Figure 8) with a rate constant (k' = 1.23(2) x 10^-2 L/(mol.s) at 35.0 ^oC) which is 220 times greater than that for PhSiH₃. The product of this reaction, Cp*Ta[(CH₂)₃SiHN(C₆H₃Me)₂NSiMe₃](H)Me (16), was isolated and characterized. Its ¹H and ¹³C NMR spectra suggest a structure analogous to that for 8, with the strained silacyclobutane ring remaining intact. The higher reactivity of this silane is therefore not due to release of ring strain in the reaction, but rather to the more favored formation of the intermediate with pentacoordinate silicon, which can readily accommodate an imposed 90^o bond angle between axial and equatorial substituents in a trigonal bipyramid.

Figure 8. Pseudo-first order rate constant for disappearance of 5 as a function of silacyclobutane concentration. kobs x 10⁴ = 1.22 s^-1 at [(CH₂)₃SiH₂] = 0.0106 mol/L; 2.61 s^-1 at 0.0214 mol/L; 3.90 s^-1 at 0.0322 mol/L; 5.69 s^-1 at 0.0441 mol/L; 6.46 s^-1 at 0.0540 mol/L.

The lower kinetic barrier for addition of (CH₂)₃SiH₂ (compared to PhSiH₃) also results in relatively facile elimination of silacyclobutane from 16. For comparison, compound 9 thermally decomposes exclusively via HSiMe₃ elimination and no PhSiH₃ was detected. For 16, two competitive decomposition pathways were observed. Heating 16 for 4 h at 80 ^oC in benzene-d₆ resulted in about 50% decomposition of 16 to 17 and 5 in a ratio of 1.1:1 (eq 6; identification of 17 is based on its ¹H NMR spectrum). HSiMe₃ was also observed, but no free silacyclobutane could be detected in the reaction mixture, presumably due to its high reactivity leading to further reactions.

The kinetics of the HSiMe₃ elimination from 9, presumed to occur via the mechanistic reverse of silane addition to 5, were also studied by ¹H NMR spectroscopy. Figure 9, which is a plot of reactant and product concentrations and the ratio [11][HSiMe₃]/[9] as a function of time, shows that an equilibrium is established after about 7 h. From data taken during the time frame 8-14 h, the equilibrium constant was estimated to be K_H = 0.025(2) mol/L. Note that a more accurate determination of K_H is not possible due to the slow decomposition of 11, which is reflected in an estimated systematic error in K_H of ca. 10%. For an initial period (2000-3000 s at 60.6 ^oC), the plot of ln[9]/[9]₀ vs. time gave a straight line with a slope independent of the initial concentration of 9 over a broad concentration range (0.0052-0.041 mol/L). This implies a first order rate law for the forward reaction. The rate constant, calculated from the average of five measurements, was found to be k_H = 1.09(2) x 10^-4 s^-1.

Figure 9. Plot of concentrations of 9, 11, and HSiMe₃ vs. time for the elimination of HSiMe₃ from 9. (Q = [11][HSiMe₃]/[9]).

The first-order rate constant for elimination of DSiMe₃ from 9-d3 at 60.6 ^oC was measured to be kD = 1.28(2) x 10^-4 s^-1 (average of five runs), therefore the deuterium KIE for this reaction is k_H/k_D = 0.85(2). This inverse isotope effect again suggests that no bonds to H/D are being broken in the rate-determining step, which is consistent with our hypothesis of an intermediate pentacoordinate Si species. Such an intermediate is likely formed in a pre-equilibrium with the starting material (9), by a rapid hydride shift from Ta to Si. A rate-determining cleavage of the N-Si bond to liberate HSiMe₃ would then lead to product (see Scheme 3). Deuterium substitution appears to shift the pre-equilibrium towards the intermediate, as a result of the stronger Si-H (vs. Ta-H) bond. Thus, the difference in relative zero-point energies for the Si-D and Si-H (vs. the Ta-D and Ta-H) bonds results in an equilibrium isotope effect. The secondary isotope effect for the HSiMe₃-dissociation step, which is expected to be normal and small, apparently does little to offset the pre-equilibrium isotope effect. The net result is again a lower activation energy for reaction of the deuterated species, in agreement with the observed inverse KIE. The equilibrium constant for the reaction was estimated to be K_D = 0.026(3) mol/L. While it can be expected that deuterium substitution will shift the equilibrium towards 11 + DSiMe₃, the difference between K_H and K_D is clearly within experimental error, and we are therefore prevented from drawing further quantitative conclusions about the equilibrium isotope effect, or the isotope effect for the reverse reaction.

The temperature dependence on the first-order rate constant for elimination of HSiMe₃ from 9 was investigated by conducting the reaction at five different temperatures between 25.0 and 75.0 ^oC. A plot of ln(k/T) vs. 1/T (Figure 10) produced a straight line from which the activation parameters delta H_act = 25.5(3) kcal/mol and delta S_act = -0.3(1.0) cal/(mol.K) were extracted. The very small entropy of activation can be rationalized as resulting from the combined influences of a small positive value for the dissociative, rate-determining step (with an early transition state), and a small negative entropy change for the

Figure 10. Eyring plot for the rate of decomposition of 9 at different temperatures. k = 9.8(6) x 10^-7 s^-1 at 25.0 ^oC; 4.5(3) x 10^-6 s^-1 at 34.8 ^oC; 1.66(5) x 10^-5 s^-1 at 44.8 ^oC; 1.09(2) x 10^-4 s^-1 at 60.6 ^oC; 6.7(3) x 10^-4 s^-1 at 75.9 ^oC.

pre-equilibrium (more restricted rotation about the N-Si bond in the intermediate compared to that in the starting material).

Conclusions

In the exploration of the chemistry of some Ta complexes with C₂-symmetric silylamido ligands, we have observed cleavage of the N-Si bond and loss of a silyl group, to form Ta=N multiply bonded species. These highly bent tantalum imidos are rather stable, but react with silanes via an interesting two-step process involving an intermediate which features a pentacoordinate silicon center. Thus, the bending of an imido ligand in this system appears to enhance the nucleophilicity of the nitrogen center. Formation of pentacoordinate intermediates has often been invoked for reactions of silanes with nucleophiles,^63,79-81 but this is the first documented example of the involvement of such species in the addition/elimination reactions of silanes with transition metal imidos. The mechanism of these transformations appears to reflect the tendency of silicon to easily expand its coordination sphere, and is thus rather different from that of C-H bond activations by zirconium or tantalum imidos,^36,40 as might be expected in view of the fundamental differences between carbon and silicon. The tendency of silicon to expand its coordination sphere is also exhibited in the nonclassical bonding interaction found in compound 12. The loss of silyl groups in such tantalum complexes seems to be facilitated by the sterically crowded coordination environment created by the chelating ligand, and this steric crowding is also reflected in the ease of formation of the cationic complex 7.

Experimental Section

General. All reactions with air-sensitive compounds were performed under dry nitrogen, using standard Schlenk and glove box techniques. Reagents were obtained from commercial suppliers and used without further purification, unless otherwise noted. Olefin-free pentane, benzene, and toluene were prepared by pretreating with conc. H₂SO₄, 0.5 N KMnO₄ in 3 M H₂SO₄, NaHCO₃ and finally anhydrous MgSO₄. Solvents (pentane, diethyl ether, benzene, toluene, tetrahydrofuran) were distilled under nitrogen from sodium benzophenone ketyl. Benzene-d₆ was distilled from Na/K alloy. Dichloromethane-d₂ was distilled under nitrogen from calcium hydride. Commercial silanes were distilled and dried over molecular sieves before use. PhSiD₃ was prepared by reduction of PhSiCl₃ with LiAlD₄ (Aldrich, 98% D). ⁿBuLi was used as a 1.6 M solution in hexanes and MeMgBr as a 1.4 M solution in Et₂O, as supplied by Aldrich. 2,2'-Diamino-6,6'-dimethylbiphenyl (1)⁴³ and Cp*TaCl₄⁸⁴ were prepared according to published literature procedures. NMR spectra were recorded at 300 or 500 MHz (¹H) with Bruker AMX-300 and DRX-500 spectrometers, at 100 MHz (¹³C{¹H}) with an AMX-400 spectrometer, or at 99.4 MHz (²⁹Si) with a DRX-500 spectrometer, at ambient temperature and in benzene-d₆, unless otherwise noted. Signal multiplicities are reported as follows: s - singlet, d - doublet, t - triplet, q - quartet, qn - quintet, m - multiplet. Elemental analyses were performed by the Microanalytical Laboratory at UC Berkeley. Infrared spectra were recorded with a Mattson Instruments Galaxy Series FTIR spectrometer, as Nujol mulls with CsI plates or as KBr pellets.

N,N'-Bis(trimethylsilyl)-2,2'-diamino-6,6'-dimethylbiphenyl (2). To a cold (0 deg.C) solution of 1 (7.75 g; 36.5 mmol) in THF (100 mL) was added dropwise 48 mL (76.8 mmol) of 1.6 M ⁿBuLi. A white precipitate formed initially, but dissolved completely after all of the ⁿBuLi had been added. The solution was stirred at 0 ^oC for 2 hours, and was then allowed to warm to room temperature, resulting in a color change from pale yellow to green. Me₃SiCl (10.2 mL, 80.4 mmol) was then added dropwise. A white precipitate formed immediately, and the evolution of heat was observed. After 2 h of heating at reflux, the solution was stirred overnight at room temperature. The THF was removed under vacuum to give an oily white solid. Extraction with pentane (2 x 100 mL) gave a bright yellow solution, which was concentrated in vacuo until crystals appeared, and then cooled to -78 ^oC. The resulting crystalline product was recrystallized from pentane and dried to afford 10.93 g (30.6 mmol, 84% yield) of the product as colorless crystals, mp 66.5-67.5 ^oC. ¹H NMR: d 7.15 (t, overlaps with solvent peak, J = 7.8 Hz), 6.87 (d, 2 H, J = 7.8 Hz), 6.77 (d, 2 H, J = 7.5 Hz biphenyl H's), 3.53 (br s, 2 H, NH), 2.02 (s, 6 H, Me), 0.06 (s, 18 H, SiMe₃). ¹³C{¹H} NMR: d 146.3, 138.7, 129.2, 125.2, 120.5, 113.5 (aromatic C's), 20.5 (Me), 0.2 (SiMe₃). IR (Nujol, cm^-1): 3371 (m), 1579 (m), 1301 (s), 1252 (s), 962 (m), 868 (s), 840 (s), 773 (m), 750 (m). Anal. Calcd for C₂₀H₃₂N₂Si₂: C, 67.35; H, 9.04; N, 7.85. Found: C, 67.15; H, 9.21; N, 7.78.

Li(Me₃Si)N(C₆H₃Me)₂N(SiMe₃)Li (3). To a solution of 10.87 g (30.48 mmol) of 2 in 150 mL of pentane at 0 ^oC was added dropwise 40.0 mL (64.0 mmol) of 1.6 M ⁿBuLi. Formation of a white precipitate was observed within 1 h. The mixture was allowed to warm to room temperature and was stirred overnight, during which time the precipitate gradually dissolved. The slightly cloudy solution was filtered, concentrated in vacuo, and cooled to -78 ^oC to afford 5.02 g of the product as white crystals (mp 120-125 ^oC, dec). Concentration and cooling of the filtrate afforded another 2.02 g of product, for a total yield of 63%. ¹H NMR: d 6.98 (t, 2 H, J = 7.7 Hz), 6.71 (d, 2 H, J = 7.8 Hz), 6.61 (d, 2 H, J = 7.3 Hz, biphenyl H's), 1.75 (s, 6 H, Me), 0.06 (s, 18 H, SiMe₃). ¹³C{¹H} NMR: d 159.4, 139.2, 133.6, 130.1, 127.1, 120.6 (aromatic C's), 21.0 (Me), 3.7 (SiMe₃). IR (Nujol, cm^-1): 3053 (m), 1571 (s), 1562 (s), 1269 (s), 1255 (s), 1245 (s), 1038 (s), 958 (s), 856 (s), 825 (s), 793 (s), 746 (s), 582 (m), 569 (m), 472 (m), 455 (m). Anal. Calcd for C₂₀H₃₀N₂Si₂Li₂: C, 65.18; H, 8.21; N, 7.60. Found: C, 63.84; H, 8.94; N, 5.76. Satisfactory elemental analysis data could not be obtained, even after repeated recrystallization of the spectroscopically pure compound.

Cp*Ta[=N(C₆H₃Me)₂NSiMe₃]Cl (4). Cp*TaCl₄ (3.43 g, 7.48 mmol) and 3 (2.76 g, 7.48 mmol) were dissolved in 150 mL of benzene and the mixture was heated at reflux for 4 h, resulting in a dark red solution. After cooling to room temperature, the solvent was removed under vacuum and the residual solid was extracted with about 200 mL pentane until the extracts were colorless. The dark red pentane solution was concentrated to about 70 mL, which caused precipitation of red, prism-shaped crystals. After cooling to -78 ^oC, the solution was filtered, and the isolated crystals were washed with pentane and dried under vacuum to afford 3.21 g of 4 (68%, mp 176-179 ^oC). ¹H NMR: d 7.26 (m, 1 H), 7.02 (m, 1 H), 6.91 (m, 1 H), 6.65 (m, 2 H), 6.58 (m, 1 H, aromatic H's), 2.04 (s, 3 H, Me), 2.04 (s, 3 H, Me), 1.89 (s, 15 H, Me₅C₅), 0.04 (s, 9 H, Me₃Si). ¹³C{¹H} NMR (dichloromethane-d₂): d 156.7, 140.1, 138.7, 135.9, 135.4, 128.1, 127.5, 127.3, 127.1, 123.5, 121.5, 121.2, 113.3 (aromatic C's), 21.0 (Me, Me'), 11.6 (Me₅C₅), 1.8 (Me₃Si). IR (KBr, cm^-1): 3049 (w), 2953 (m), 2916 (m), 1574 (m), 1446 (m), 1250 (s), 1211 (m), 1012 (m), 945 (m), 900 (m), 854 (s), 769 (m), 727 (m). Anal. Calcd for C₂₇H₃₆N₂ClSiTa: C, 51.23; H, 5.73; N, 4.43. Found: C, 51.21; H, 5.90; N, 4.34.

Cp*Ta[=N(C₆H₃Me)₂NSiMe₃]Me (5). To a solution of 4 (1.01 g, 1.59 mmol) in diethyl ether (80 mL) was added 1.20 mL of 1.4 M MeMgBr (1.68 mmol) at room temperature. The color of the solution gradually changed from dark red to light red-orange, with formation of a white precipitate. After stirring for 5 h, the solvent was removed in vacuo and the residue was extracted with about 100 mL pentane, until extracts were colorless. The pentane solution was concentrated to less than 10 mL and left at -78 ^oC for 12 h, to obtain 0.83 g of 5 as a bright red-orange crystalline powder (85% yield, mp 114-118 ^oC). ¹H NMR: d 7.28 (m, 1 H), 7.05 (m, 1 H), 6.98 (m, 1 H), 6.69 (m, 2 H), 6.59 (m, 1 H, aromatic H's), 2.09 (s, 3 H, Me), 2.05 (s, 3 H, Me), 1.78 (s, 15 H, Me₅C₅), 0.72 (s, 3 H, MeTa), -0.08 (s, 9 H, Me₃Si). ¹³C{¹H} NMR: d 159.2, 140.1, 139.5, 135.9, 133.5, 130.6, 128.8, 127.6, 127.4, 124.9, 120.9, 117.9, 113.6 (aromatic C's), 39.5 (MeTa), 21.5 (Me), 21.3 (Me), 11.4 (Me₅C₅), 1.9 (Me₃Si). IR (KBr, cm^-1): 3045 (w), 2954 (m), 2912 (m), 1571 (m), 1446 (s), 1377 (w), 1281 (s), 1250 (s), 1207 (m), 1041 (w), 945 (w), 903 (m), 845 (s), 771 (m), 756 (m), 728 (m). Anal. Calcd for C₂₈H₃₉N₂SiTa: C, 54.89; H, 6.42; N, 4.57. Found: C, 54.24; H, 6.47; N, 4.21.

Cp*Ta[=N(C₆H₃Me)₂NSiMe₃][h²-(2,6-Me₂C₆H₃)N=CMe] (6). A solution of xylyl isonitrile (0.12 g, 0.92 mmol) in 20 mL of pentane was added to a solution of 5 (0.57 g, 0.92 mmol) in 40 mL of pentane. The red, transparent solution was stirred for 12 h at room temperature resulting in the formation of a bright yellow, crystalline precipitate which was filtered and dried in vacuo. Cooling the filtrate to -78 ^oC afforded a second crop of crystals, giving a total yield of 0.47 g of 6 (68% yield). The product was purified by recrystallization from Et₂O, which results in incorporation of 0.5 equiv of solvent in the crystals (mp > 180 ^oC, dec). ¹H NMR (400 MHz): d 7.35 (m, 1 H), 7.21 (m, 1 H), 7.03 (m, 2 H), 6.85 (m, 2 H), 6.80 (m, 1 H), 6.52 (d, 1 H), 6.18 (d, 1 H, aromatic H's), 3.25 (q, 2 H, Et₂O), 2.18 (s, 3 H, Me), 2.16 (s, 3 H, Me), 2.05 (s, 3 H, Me), 1.94 (s, 15 H, Me₅C₅), 1.86 (t, 3 H, Me), 1.26 (s, 3 H, N=C-Me), 1.11 (t, 3 H, Et₂O), 0.03 (s, 9 H, Me₃Si). ¹³C{¹H} NMR: d 255.0 (N=C-Me), 158.2, 154.7, 145.8, 138.5, 136.1, 135.8, 131.7, 129.9, 129.3, 129.0, 128.5, 127.1, 126.9, 126.6, 125.3, 125.2, 120.5, 116.8, 112.1 (aromatic C's), 66.2 (Et₂O), 23.1, 22.0, 21.3, 19.9, 19.2, 15.9, 12.3 (Me's), 5.2 (Me₃Si). IR (KBr, cm^-1): 3043 (w), 2953 (m), 2916 (m), 1579 (N=C, m), 1446 (s), 1313 (s), 864 (s), 837 (s), 773 (m). Anal. Calcd for C₃₉H₅₃N₃O_0.5SiTa: C, 59.99; H, 6.84; N, 5.38. Found: C, 59.88; H, 7.01; N, 5.18.

{Cp*Ta[MeN(C₆H₃Me)₂NSiMe₃]Me}⁺I^- (7). To a solution of 5 (0.46 g, 0.75 mmol) in 20 mL of Et₂O was added 3 mL of MeI at room temperature. The mixture instantly became cloudy. After stirring for 24 h, the yellow precipitate was filtered, washed with 20 mL of Et₂O and then 20 mL of pentane, and finally dried under vacuum to afford 0.42 g (74% yield; mp 240-243 ^oC, dec, subl) of 7 as a yellow, microcrystalline powder. ¹H NMR (dichloromethane-d₂): d 7.52 (m, 2 H), 7.39 (m, 1 H), 7.29 (m, 1 H), 7.21 (m, 1 H), 6.99 (m, 1 H, aromatic H's), 3.39 (s, 3 H, MeN), 2.19 (s, 15 H, Me₅C₅), 2.16 (s, 3 H, Me), 2.10 (s, 3 H, Me), 0.78 (s, 3 H, TaMe), -0.08 (s, 9 H, Me₃Si). ¹³C{¹H} NMR (dichloromethane-d₂): d 141.1, 138.5, 138.4, 135.0, 134.2, 132.3, 131.7, 131.3, 131.0, 130.0, 128.8, 127.4, 124.0 (aromatic C's), 55.1 (MeTa), 53.4 (MeN), 21.4 (Me), 20.4 (Me), 12.4 (Me₅C₅), 1.5 (Me₃Si). IR (KBr, cm^-1): 2953 (m), 2916 (m), 1581 (w), 1443 (m), 1381 (w), 1250 (m), 839 (s). Anal. Calcd for C₂₉H₄₂N₂ISi₂Ta: C, 46.16; H, 5.61; N, 3.71. Found: C, 45.81; H, 5.59 N, 3.75.

Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiMe₃](H)Cl (8). A solution of 4 (0.56 g, 0.88 mmol) in 1.5 mL of neat PhSiH₃ was stirred at room temperature. The inhomogeneous red mixture turned light yellow after one day. The volatile material was removed under vacuum, the resulting residue was washed with pentane (2 x 20 mL) and extracted with toluene (about 70 mL). The toluene solution was concentrated to 10 mL and cooled to -78 ^oC, and the resulting precipitate was washed with pentane, to obtain 0.39 g of 8 (in two crops, 60% yield) as a pale yellow powder (mp 138-141 ^oC, dec). The product is weakly light-sensitive and is best kept in the dark. ¹H NMR: d 20.44 (s, 1 H, TaH), 7.76 (m, 2 H), 7.60 (m, 1 H), 7.08 (m, 5 H), 6.95 (m, 1 H), 6.80 (m, 2 H, aromatic H's), 5.46 (d, 1 H, ²J_HH = 9.8 Hz, ¹J_HSi = 195 Hz, PhSiH₂N), 4.80 (d, 1 H, ²J_HH = 9.8 Hz, ¹J_HSi = 201 Hz, PhSiH₂N), 2.10 (s, 3 H, Me), 2.04 (s, 3 H, Me), 2.02 (s, 15 H, Me₅C₅), 0.03 (s, 9 H, Me₃Si). ¹³C{¹H} NMR: d 156.9, 151.5, 138.6, 136.8, 136.0, 135.7, 134.2, 130.4, 129.7, 128.9, 127.3, 127.1, 126.6, 126.3, 126.1, 123.7, 121.0 (aromatic C's), 21.4 (Me), 20.7 (Me), 13.3 (Me₅C₅), 4.7 (Me₃Si). IR (KBr, cm^-1): 3055 (w), 2987 (m), 2916 (m), 2216 and 2150 (Si-H, s), 1790 (Ta-H, m), 1564 (m), 1433 (s), 1238 (s), 1213 (s), 1111 (m), 955 (m), 910 (m), 850 (s). Anal. Calcd for C₃₃H₄₄N₂ClSi₂Ta: C, 53.47; H, 5.98; N, 3.78. Found: C, 53.84; H, 6.10; N, 3.73.

Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiMe₃](H)Me (9). A solution of 5 (0.37 g, 0.60 mmol) in 0.5 mL of neat PhSiH₃ was stirred at room temperature for one day. The clear, bright red-orange mixture gradually turned pale yellow, with the formation of a precipitate. The volatile material was removed under vacuum and the residue was extracted with Et₂O (80 mL). The extract was filtered, concentrated to about 10 mL and cooled to -78 ^oC. The resulting white crystalline precipitate was isolated, recrystallized again from Et₂O, washed with cold pentane and dried under vacuum to obtain 0.25 g of 9 (58%; mp 148-149 ^oC, dec). The product is weakly light-sensitive and is best kept in the dark. ¹H NMR: d 17.15 (s, 1 H, TaH), 7.49 (m, 2 H), 7.10 (m, 5 H), 6.98 (m, 1 H), 6.94 (m, 1 H), 6.83 (m, 2 H, aromatic H's), 5.29 (d, 1 H, ²J_HH = 9.6 Hz, ¹J_HSi = 179 Hz, PhSiH₂N), 4.68 (d, 1 H, ²J_HH = 9.6 Hz, ¹J_HSi = 184 Hz, PhSiH₂N), 2.11 (s, 3 H, Me), 2.08 (s, 3 H, Me), 1.91 (s, 15 H, Me₅C₅), 0.88 (d, 3 H, ³J_HH = 1.9 Hz, MeTa), 0.00 (s, 9 H, Me₃Si). ¹³C{¹H} NMR: d 156.3, 153.4, 138.3, 137.3, 136.6, 135.5, 134.7, 134.3, 130.2, 128.5, 127.1, 126.6, 126.6, 126.2, 125.2, 124.6, 116.2 (aromatic C's), 48.4 (MeTa), 21.5 (Me), 21.0 (Me), 12.7 (Me₅C₅), 4.6 (Me₃Si). IR (KBr, cm^-1): 3057 (w), 2958 (m), 2922 (m), 2214 and 2150 (Si-H, m), 1778 (Ta-H, m), 1564 (m), 1431 (m), 1236 (s), 948 (m), 910 (m), 870 (m), 845 (s). Characteristic IR bands for 9-d₃: 1605 and 1565 (Si-D, m), 1278 (Ta-D, m). Anal. Calcd for C₃₄H₄₇N₂Si₂Ta: C, 56.65; H, 6.57; N, 3.89. Found: C, 57.17; H, 6.68; N, 3.85.

Cp*Ta[=N(C₆H₃Me)₂NSiH₂Ph]Cl (10). A sample of 8 (ca. 10 mg) was dissolved in 0.5 mL benzene-d₆ and left at room temperature. After 18 h, about 60% conversion of 8 to 10 and HSiMe₃ was found by ¹H NMR spectroscopy, and after 48 h the conversion was complete. ¹H NMR data for 10: d 7.29-7.26 (m, 3 H), 7.21 (m, 2 H), 7.13-6.93 (m, 3 H), 6.68-6.61 (m, 3 H), 5.51 (d, 1 H, ²J_HH = 10.5 Hz, PhSiH₂), 5.31 (d, 1 H, ²J_HH = 10.5 Hz, PhSiH₂), 2.04 (s 15 H, C₅Me₅), 1.83 (s, 3 H, Me), 1.70 (s, 3 H, Me).

Cp*Ta[=N(C₆H₃Me)₂NSiH₂Ph]Me (11). A sample of 9 (ca. 10 mg) was dissolved in 0.5 mL benzene-d₆ and heated to 80 ^oC for 2 h, to give a mixture of 11 and HSiMe₃. ¹H NMR data for 11: d 7.26-7.21 (m, 3 H), 7.12-6.91 (m, 5 H), 6.75-6.65 (m, 3 H, aromatic H's), 5.44 (d, 1 H, ²J_HH = 10 Hz, PhSiH₂), 5.33 (d, 1 H, ²J_HH = 10 Hz, PhSiH₂), 1.95 (s, 3 H, Me), 1.92 (s, 15 H, C₅Me₅), 1.71 (s, 3 H, Me), 0.34 (s, 3 H, TaMe).

Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiPhHCl](H)Cl (12). To a solution of 4 (1.00 g, 1.58 mmol) in 25 mL of CH₂Cl₂ was added 1.6 mL of PhSiH₃, and the mixture was stirred at room temperature. The solution slowly changed color from dark red through orange to yellow over a period of four days. The volatile material was removed under vacuum, and the residual solid was washed with pentane (2 x 25 mL) and dried to afford 1.08 g of 12 as a pale yellow powder (84%; mp 156-159 ^oC, dec). The crude product was purified by recrystallization from toluene. ¹H NMR (dichloromethane-d₂): d 14.85 (d, 1 H, ⁴J_HH = 6.0 Hz, TaH), 7.1 - 7.5 (m, 8 H), 7.03 (m, 5 H), 6.88 (m, 1 H), 6.60 (m, 2 H, aromatic H's), 6.28 (d, 1 H, ⁴J_HH = 6.0 Hz, PhSiH(Cl)N), 5.37 (d, 1 H, ²J_HH = 10.4 Hz, PhSiH₂N), 4.54 (d, 1 H, ²J_HH = 10.4 Hz, PhSiH₂N), 2.32 (s, 15 H, Me₅C₅), 2.01 (s, 3 H, Me), 1.98 (s, 3 H, Me). ¹³C{¹H} NMR (benzene-d₆): d 153.8, 149.8, 140.2, 138.6, 137.9, 135.7, 134.9, 134.2, 133.8, 133.7, 130.7, 129.4, 128.3, 127.9, 127.4, 127.1, 126.2, 126.2, 125.0, 122.3 (aromatic C's), 21.1 (Me), 20.7 (Me), 12.7 (Me₅C₅). ²⁹Si NMR (direct detection, 99.4 MHz, benzene-d₆): d -26.9 (t, ¹J_SiH = 208 Hz, PhSiH₂N), -66.8 (d, ¹J_SiH = 272 Hz, PhSiHClN). IR (KBr, cm^-1): 3053 (w), 2999 (w), 2916 (w), 2189 and 2152 (Si-H, m), 1678 (Ta-H, w br), 1585 (w), 1566 (w), 1429 (m), 1113 (m), 835 (s). Anal. Calcd for C₃₆H₄₁N₂Cl₂Si₂Ta: C, 53.40; H, 5.10; N, 3.46. Found: C, 53.44; H, 5.12; N, 3.46.

Cp*Ta[PhSiH₂N(C₆H₃Me)₂NSiH₂Ph](H)Cl (13). A solution of 4 (0.17 g, 0.27 mmol) in 1.5 mL PhSiH₃ was stirred at room temperature for 4 days and the volatiles were removed in vacuo. The crude product (8, as identified by ¹H NMR) was redissolved in 10 mL of benzene and 1.0 mL of PhSiH₃ was added. The yellow solution was stirred at room temperature for 3 days. Removal of the volatiles in vacuo followed by extraction of the resulting brown oil with 30 mL of pentane and crystallization at -78 ^oC gave 0.12 g of yellow powder which was found to contain 13 as a major component, along with unidentified decomposition products. ¹H NMR data for 13: d 19.76 (br s, TaH), 6.05 (dd, J₁ = 3.1 Hz, J₂ = 11.4 Hz, PhSiH₂ ) and 5.72 (dd, J₁ = 1.5 Hz, J₂ = 11.4 Hz, PhSiH₂), 5.42 (d, J = 10.1 Hz, PhSiH₂), 4.84 (d, J = 10.1 Hz, PhSiH₂), 2.01 (s, C₅Me₅).

Cp*Ta[(CH₂)₃SiHN(C₆H₃Me)₂NSiMe₃](H)Me (16). To a solution of 5 (0.37 g, 0.60 mmol) in 25 mL of pentane was added 0.1 mL of (CH₂)₃SiH₂, and the mixture was stirred overnight at room temperature. The solution color changed from orange-red to light yellow. The volatile material was removed in vacuo and the resulting residue was redissolved in pentane (30 mL), concentrated, and crystallized at -78 ^oC to afford 0.25 g of 16 as a crystalline, off-white powder (60% yield). ¹H NMR: d 17.02 (s, 1 H, TaH), 7.12-7.04 (m, 3 H), 6.93 (m, 2 H), 6.79 (m, 1 H, aromatic H's), 5.14 (m, 1 H, SiH), 2.13 (s, 3 H, Me), 2.06 (s, 3 H, Me), 1.91 (s, 15 H, Me₅C₅), 1.60-1.40, 1.27-1.22 and 0.86-0.84 (m, 6 H, (CH₂)₃SiH), 0.75 (d, 3 H, ³J_HH = 2.1 Hz, MeTa), 0.01 (s, 9 H, Me₃Si). ¹³C{¹H} NMR: d 156.4, 153.0, 137.7, 137.2, 134.7, 134.4, 127.1, 126.4, 126.2, 125.9, 124.5, 124.3, 116.2 (aromatic C's), 48.4 (MeTa), 21.1 (Me), 20.9 (Me), 20.5, 18.7, 15.1 ((CH₂)₃SiH), 12.6 (Me₅C₅), 4.5 (Me₃Si). IR (KBr, cm^-1): 3051 (w), 2960 (m), 2914 (m), 2154 (Si-H, w), 1780 (Ta-H, m), 1566 (w), 1437 (m), 1236 (s), 1118 (m), 916 (s), 841 (s). Anal. Calcd for C₃₁H₄₇N₂Si₂Ta: C, 54.37; H, 6.92; N, 4.09. Found: C, 54.59; H, 7.23; N, 4.12.

Cp*Ta[=N(C₆H₃Me)₂N(SiH(CH₂)₃)]Me (17). A sample of 16 (ca. 10 mg) was dissolved in 0.5 mL of benzene-d₆ and heated at 80 ^oC. The reaction was monitored by ¹H NMR. After 4 h, 50% of 16 had decomposed to give 17 and 5 in a 1.1:1 ratio, along with some HSiMe₃. After 24 h, 16 had completely decomposed and the ratio of 17 to 5 had decreased to 0.5:1, and after heating for two more days, the reaction mixture was found to contain only 5 and HSiMe₃. ¹H NMR data for 17: d 5.33 (m, 1 H, SiH), 2.00 and 2.08 (s, 3 H each, biphenyl Me's), 1.95 (s, 15 H, C₅Me₅), 0.32 (s, 3 H, TaMe), aromatic H's and CH₂'s not assignable.

X-ray structure determinations. X-ray diffraction measurements were made on a Siemens SMART diffractometer with a CCD area detector, using graphite monochromated Mo-Kalpha radiation. The crystal was mounted on a glass fiber using Paratone N hydrocarbon oil. A hemisphere of data was collected using omega scans of 0.3^o. Cell constants and an orientation matrix for data collection were obtained from a least-squares refinement using the measured positions of reflections in the range 4 < 2theta < 45^o. The frame data were integrated using the program SAINT (SAX Area-Detector Integration Program; V4.024; Siemens Industrial Automation, Inc.: Madison, WI, 1995). An empirical absorption correction based on measurements of multiply redundant data was performed using the programs XPREP (Part of the SHELXTL Crystal Structure Determination Package; Siemens Industrial Automation, Inc.: Madison, WI, 1995) or SADABS. Equivalent reflections were merged. The data were corrected for Lorentz and polarization effects. A secondary extinction correction was applied if appropriate. The structures were solved using the teXsan crystallographic software package of the Molecular Structure Corporation, using direct methods,^85,86 and expanded with Fourier techniques.⁸⁷ All non-hydrogen atoms were refined anisotropically and the hydrogen atoms were included in calculated positions but not refined unless otherwise noted. The function minimized in the full-matrix least-squares refinement was Sigma w(|Fo|-|Fc|)². The weighting scheme was based on counting statistics and included a p-factor to downweight the intense reflections.

For 4: Crystals were grown from a dilute pentane solution for 5 days at -40 ^oC. The systematic absences of: h0l: l = 2n+1 and 0k0: k = 2n+1 uniquely determine the space group to be P21/c (#14).

For 6^.1/2Et₂O: Crystals were grown from a dilute Et₂O solution of 6 for three days at -40 ^oC. The systematic absences of: h0l: h+l = 2n+1, 0k0: k = 2n+1 uniquely determine the space group to be P21/n (#14). Non-hydrogen atoms were refined anisotropically, except for those of the solvating Et₂O which was found to be disordered and refined isotropically.

For 7: Crystals were grown by slow vapor diffusion of MeI into a benzene solution of 5 for 6 days, at room temperature. The systematic absences of h0l: h+l = 2n+1 and 0k0: k = 2n+1, uniquely determine the space group to be P21/n (#14).

For 9: Crystals were grown from an Et₂O solution of 9 for one week at -40 ^oC. The systematic absences of: h0l: l = 2n+1 and 0k0: k = 2n+1 uniquely determine the space group to be P21/c (#14). Hydrogen atoms were included at calculated positions but not refined, except for H(1) which was refined with a fixed thermal parameter.

For 12: Crystals were grown by slow vapor diffusion of pentane into a concentrated benzene solution of 12, for 10 days, at room temperature. Inspection of the reflections collected revealed the presence of a second, smaller crystal, with the same unit cell parameters. The set of reflections belonging to that crystal were not included in the unit cell refinement and subsequent structure solution. Based on a statistical analysis of intensity distribution, and the successful solution and refinement of the structure, the space group was determined to be P-1(#2). The hydrogen atoms were included in calculated positions and not refined, except for H(1) and H(41), which were refined isotropically.

Kinetic measurements. Reactions were monitored by ¹H NMR spectroscopy, with a Bruker AMX300 spectrometer, using 5 mm Wilmad NMR tubes, equipped with J. Young Teflon screw caps. The samples were frozen in liquid N₂ immediately after preparation, and defrosted just before being placed in the preshimmed probe, which was preheated at the required temperature. The probe temperature was calibrated using an ethylene glycol sample,⁸⁸ and monitored with a thermocouple. Repeated calibration showed that the temperature was maintained within +/- 0.2 ^oC of the set value. Single scan spectra were acquired automatically at preset time intervals. The peaks were integrated relative to ferrocene as an internal standard. Rate constants were obtained by non-weighted linear least-squares fits of the integrated first-order rate law in logarithmic form, lnC = lnC₀ - kobst. When several peaks from the same species were monitored, separate plots of lnC (as calculated from each signal) vs. t were produced, and the rate constants (typically within two standard deviations from each other) were averaged.

Kinetic study of the addition of PhSiH₃ and PhSiD₃ to 5. Samples of 5 (9.5 - 18.7 mg) and Cp₂Fe (2.0-5.0 mg) were weighed in a 1.00 +/- 0.01 mL volumetric flask. The solids were dissolved in a small amount of benzene-d₆, and known volumes of PhSiH₃ (64-200 uL) or PhSiD₃ (80-200 uL) were added using a 100 uL Hamilton syringe. The exact amount of silane added was determined by weighing the volumetric flask before and after the addition. The solution was diluted with benzene-d₆ to 1.00 mL, mixed quickly, and transferred to the NMR tube. The reaction was followed at 35.0 ^oC, and the rate of disappearance of 5 was monitored by integrating the C₅(CH₃)₅ and Si(CH₃)₃ peaks relative to Cp₂Fe. Rate constants were calculated using data from the first 1.1-2.9 t_1/2, after which period the accumulating decomposition products caused increased scatter in the data (although linearity was preserved up to 3-4 half-lives).

Kinetic study of the addition of (CH₂)₃SiH₂ to 5. A 1.15 M solution of (CH₂)₃SiH₂ was prepared by diluting 83.2 mg of the silane with benzene-d₆ up to 1.00 mL. Samples of 5 (1.8 - 5.0 mg) and Cp₂Fe (0.5 - 1.9 mg) were weighed in a 1.00 +/- 0.01 mL volumetric flask. After dissolving the solids in a small amount of benzene-d₆, a known volume (10 - 50 uL) of the silane solution was added, the mixture diluted to 1.00 mL, homogenized, and transferred to the NMR tube. (Although the samples were frozen in liquid N₂ immediately, the reaction had typically advanced to 10 - 20% conversion before data acquisition could begin.) The reaction was followed at 35.0 ^oC, and the rate of disappearance of 5 was monitored for up to 1.5-3.3 t_1/2 by integrating the C₅(CH₃)₅ and Si(CH₃)₃ peaks relative to Cp₂Fe.

Kinetic study of the elimination of trimethylsilane from 9 and 9-d₃. Samples of 9 (2.8-22.1 mg) or 9-d₃ (10.3-28.0 mg) and Cp₂Fe (2-4 mg) were weighed into NMR tubes and dissolved in 0.6-0.8 mL of benzene-d₆. To determine the exact volume of the solution, the NMR tubes were calibrated by adding known volumes of C₆H₆ with a Hamilton microliter syringe and measuring the resulting height of the solvent. The rate of disappearance of the starting material was followed at 60.6 ^oC, by integrating the biphenyl CH₃, TaCH₃, and Si(CH₃)₃ signals relative to the Cp₂Fe standard. Initial rates were determined by processing data from the first 2000-3000 s (0.3-0.5 t_1/2) only, where the deviation from linearity (see Discussion) was negligible. The activation enthalpy and entropy for the reaction were determined by weighted, linear least squares fit of ln(k/T) vs. 1/T at five different temperatures. Only the biphenyl CH₃ and TaCH₃ peaks were used at temperatures lower than 60 ^oC, due to the overlap of the Si(CH₃)₃ and HSi(CH₃)₃ peaks. Two kinetic runs were performed at each temperature other than 60 ^oC. The standard deviations in k and T at each data point were calculated based on the random error of the multiple measurements combined with 5% estimated systematic error.

Equilibrium study of the elimination of trimethylsilane from 9 and 9-d₃. Samples of 9 (27.8 mg) or 9-d₃ (23.6 mg) and Cp₂Fe (3-4 mg) were weighed into a 1.00 mL volumetric flask and dissolved in benzene-d₆. The solutions were heated in J.Young NMR tubes in the spectrometer probe at 60.6 ^oC for up to 50 000 s. The concentrations of the starting material and the products were calculated based on the initial concentration of 9 (9-d₃), relative to the ferrocene internal standard. The biphenyl CH₃ and TaCH₃ peaks were used to measure [11], and the HSi(CH₃)₃ peak was used to determine [HSi(CH₃)₃]. The equilibrium constant was calculated as the average value approached by the ratio [11][HSiMe₃]/[9] after 25 000 s.

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