Alkyl−Silyl Complexes Free of Anionic π Ligands. Synthesis and Characterization of (Me3ECH2)3MSi(SiMe3)3

Group 4 Metal and Lanthanide Complexes in the Oxidation State +3 with Tris(trimethylsilyl)silyl Ligands

A number of paramagnetic silylated d¹ group 4 metallates were prepared by reaction of potassium tris(trimethylsilyl)silanide with group 4 metallates of the type K[Cp₂MCl₂] (M = Ti, Zr, Hf). The outcomes of the reactions differ for all three metals. While for the hafnium case the expected complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was obtained, the analogous titanium reaction led to a product with two Si(H)(SiMe₃)₂ ligands. The reaction with zirconium caused the formation of a dinuclear fulvalene bridged complex. The desired [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of Cp₂Zr{Si(SiMe₃)₃}₂ with potassium. In related reactions of potassium tris(trimethylsilyl)silanide with some lanthanidocenes Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃Ln Si(SiMe₃)₃]⁻ with either [18-crown-6·K]⁺ or the complex ion [18-crown-6·K·Cp·K·18-crown-6] as counterions were obtained. Due to d¹ or fⁿ electron configuration, unambiguous characterization of all obtained complexes could only be achieved by single crystal XRD diffraction analysis.

Reactions of (Me₃Si)₃SiK with K[Cp₂MCl₂] (M = Ti, Zr, Hf) gave different paramagnetic silylated d¹ Group 4 metal ate complexes. For Hf the complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was formed, but for Ti a product with two Si(H)(SiMe₃)₂ ligands was isolated. With Zr a dinuclear fulvalene bridged complex formed but [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of the neutral complex. In reactions of (Me₃Si)₃SiK with Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃LnSi(SiMe₃)₃]⁻ were observed.

Non-Innocent Behaviour of Imido Ligands in the Reactions of Silanes with Half-Sandwich Imido Complexes of Nb and V: A Silane/Imido Coupling Route to Compounds with Nonclassical SiH Interactions

Reactions of imido complexes [M(Cp)(=NR')(PR''3)2] (M=V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and nitrogen and the steric properties of the phosphine. The main products are [M(Cp)(=NR')(PR3)(H)(SiRnCl3-n)] (M=V, Nb; R'=2,6-diisopropylphenyl (Ar), 2,6-dimethylphenyl (Ar')), [Nb(Cp)(=NR')(PR''3)(H)(SiPhR2)] (R2=MeH, H2), [Nb(Cp)(==NR')(PR''3)(Cl)(SiHRnCl2-n)] and [Nb(Cp)(eta 3-N(R)SiR2--H...)(PR''3)(Cl)]. Complexes with the smaller Ar' substituent at nitrogen react faster, as do more acidic silanes. Bulkier groups at silicon and phosphorus slow down the reaction substantially. Kinetic NMR experiments supported by DFT calculations reveal an associative mechanism going via an intermediate N-silane adduct [Nb(Cp){=N(-->SiHClR2)R'}(PR''3)2] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a Si--H or Si--Cl bond activation process. DFT calculations show that this imido-silane adduct is additionally stabilized by a Si--HM agostic interaction. Si--H activation is kinetically preferred even when Si--Cl activation affords thermodynamically more stable products. The niobium complexes [NbCp(=NAr)(PMe3)(H)(SiR2Cl)] (R=Ph, Cl) are classical according to X-ray studies, but DFT calculations suggest the presence of interligand hypervalent interactions (IHI) in the model complex [Nb(Cp) (==NMe)(PMe3)(H)(SiMe2Cl)]. The extent of Si--H activation in the beta-Si--HM agostic complexes [Cp{eta 3-N(R')SiR2--H}M(PR''3)(Cl)] (R''=PMe3, PMe2Ph) primarily depends on the identity of the ligand trans to the Si--H bond. A trans phosphine leads to a stronger Si--H bond, manifested by a larger J(Si--H) coupling constant. The Si--H activation diminishes slightly when a less basic phosphine is employed, consistent with decreased back-donation from the metal.

Amide−Silyl Ligand Exchanges and Equilibria among Group 4 Amide and Silyl Complexes

M(NMe2)4 (M = Zr, 1a; Hf, 1b) and the silyl anion (SiButPh2)- (2) in Li(THF)2SiButPh2 (2-Li) were found to undergo a ligand exchange to give [M(NMe2)3(SiButPh2)2]- (M = Zr, 3a; Hf, 3b) and [M(NMe2)5]- (M = Zr, 4a; Hf, 4b) in THF. The reaction is reversible, leading to equilibria: 2 1a (or 1b) + 2 2 <--> 3a (or 3b) + 4a (or 4b). In toluene, the reaction of 1a with 2 yields [(Me2N)3Zr(SiButPh2)2]-[Zr(NMe2)5Li2(THF)4]+ (5) as an ionic pair. The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in (Me2N)3Zr-N(SiMe3)2 (6a) to give 3a and [N(SiMe3)2]- (7) in reversible reaction: 6a + 2 2 <--> 3a + 7. The following equilibria have also been observed and studied: 2M(NMe2)4 (1a; 1b) + [Si(SiMe3)3]- (8) <--> (Me2N)3M-Si(SiMe3)3 (M = Zr, 9a; Hf, 9b) + [M(NMe2)5]- (M = Zr, 4a; Hf, 4b); 6a (or 6b) + 8 <--> 9a (or 9b) + [N(SiMe3)2]- (7). The current study represents rare, direct observations of reversible amide-silyl exchanges and their equilibria. Crystal structures of 5, (Me2N)3Hf-Si(SiMe3)3 (9b), and [Hf(NMe2)4]2 (dimer of 1b), as well as the preparation of (Me2N)3M-N(SiMe3)2 (6a; 6b) are also reported.

A Kinetic Study of the Silyl Substitution in Tantalum Amide Silyl Complex (Me2N)3Ta[Si(SiMe3)3]2

Kinetic studies have been performed for the substitution of the first silyl ligand in (Me(2)N)(3)Ta[Si(SiMe(3))(3)](2) (1) by Li(THF)(3)SiButPh(2) at 233 K (THF = tetrahydrofuran). In the presence of excess Li(THF)(3)SiButPh(2), these studies reveal that the reaction likely follows a dissociative pathway. THF, a polar solvent, is found to promote the substitution, and the order of the reaction with respect to THF is 1.7(0.3).

Zirconium, Hafnium, and Tantalum Amide Silyl Complexes: Their Preparation and Conversion to Metallaheterocyclic Complexes via γ-Hydrogen Abstraction by Silyl Ligands

New transition metal silyl amide complexes (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1) and (Me(2)N)M[N(SiMe(3))(2)](2)(SiPh(2)Bu(t)) (M = Zr, 2a, and Hf, 2b) were found to undergo gamma-H abstraction by the silyl ligands to give metallaheterocyclic complexes (3) and (M = Zr, 4a, and Hf, 4b), respectively. The conversion of 1 to 3 follows first-order kinetics with DeltaH() = 23.6(1.6) kcal/mol and DeltaS() = 3(5) eu between 288 and 313 K. The formation of 4a from (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a) and Li(THF)(2)SiPh(2)Bu(t) (6) involves the formation of the intermediate 2a, followed by gamma-H abstraction. Kinetic studies of these consecutive reactions, a second-order reaction to give 2a and then a first-order gamma-H abstraction to give 4a, were conducted by an analytical method and a numerical method. At 278 K, the rate constants k(1) and k(2) for the two consecutive reactions are 2.17(0.03) x 10(-)(3) M(-)(1) s(-)(1) and 5.80(0.15) x 10(-)(5) s(-)(1) by the analytical method. The current work is a rare kinetic study of the A + B --> C --> D (+ E) consecutive reactions. Kinetic studies of the formation of a metallaheterocyclic moiety have, to our knowledge, not been reported. In addition, gamma-H abstraction by a silyl ligand to give such a metallaheterocyclic moiety is new. Theoretical investigations of the gamma-H abstraction by silyl ligands have been conducted by density functional theory calculations at the Becke3LYP (B3LYP) level, and they revealed that the formation of the metallacyclic complexes through gamma-H abstraction is entropically driven. X-ray crystal structures of (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1), (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a), and (M = Zr, 4a, and Hf, 4b) are also reported.