Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α-SiMe₃ Abstraction by an Amide Ligand

Abstract

Reaction of TaCl2(NSiMe3)­[N­(SiMe3)2] (1) with alkylating reagents form the alkyl amide imide complexes TaR2(NSiMe3)­[N­(SiMe3)2] (R = Me (2), CH2Ph (3)) and mixed amide imide compounds Ta­(NR′2)2(NSiMe3)­[N­(SiMe3)2] (R′ = Me (4), Et (5)). The reaction of 2 and 0.5 equiv of O2 leads to preferential oxygen insertion into one Ta–Me bond, yielding the alkoxy-bridged alkyl dimer Ta2(μ-OMe)2Me2(NSiMe3)2[N­(SiMe3)2]2 (6) as cis and trans isomers. Crystallization of the cis -6 and trans -6 mixture gave only crystals of trans -6. When the crystals of trans -6 were dissolved in benzene-d 6, conversion of trans -6 to cis -6 occurred until the trans -6 ⇌ cis -6 equilibrium was reached with K eq = 0.79(0.02) at 25.0(0.1) °C. Kinetic studies of the exchange gave the rate constants k = 0.018(0.002) min–1 for the trans -6 → cis -6 conversion and k′ = 0.022(0.002) min–1 for the reverse cis -6 → trans -6 conversion at 25.0(0.1) °C. Complex 6 reacts with additional O2, forming the dialkoxy dimer Ta2(μ-OMe)2(OMe)2(NSiMe3)2[N­(SiMe3)2]2 (7) as cis and trans isomers. Solid-state structures of 3 and trans -6 have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta­(NR′2)2(NSiMe3)­[N­(SiMe3)2] (R′ = Me (4), Et (5)) have also been prepared by salt metathesis reactions employing TaCl3[N­(SiMe3)2]2 (8). The pathway from 8 to 4 and 5 eliminates Me3Si–NR′2 (R′ = Me, Et), converting the amide N­(SiMe3)2 ligand to the imide NSiMe3 ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and α-elimination involving the mixed amide species TaCl2(NMe2)­[N­(SiMe3)2]2 (9) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of 1–6 and 8 show that only the alkoxy-bridged cis- 6 and trans -6 are dimers in benzene-d 6 solution at 25 °C.

Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

“…The tantalum core adopts a pseudo-tetrahedral geometry (τ4 = 0.95) which is typical for 4coordinate d 0 Ta(V) complexes. 50,51 As expected, the Ta=Calkylidene bond length (1.90(1) Å) is significantly shorter than the Ta-Calkyl distances (2.14(1) and 2.156(1) Å), and falls in the expected range. 23,[36][37][38][39][40]52 The Rh centre adopts a square-planar coordination geometry (τ4 = 0.09) which is characteristic for 4-coordinate 16valency electrons Rh(I) species.…”

NMR spectroscopic study of the adduct formation and reactivity of homoleptic rare earth amides with alkali metal benzyl compounds, and the crystal structures of [Li(TMEDA)2][Nd{N(SiMe3)2}3(CH2Ph)] and [{Li(TMP)}2{Li(Ph)}]2

“…This follows literature precedent for the reaction of [Y(N″)3] with n BuLi [30]. There is also a general lack of literature examples for benzyl ligands in the presence of silyl amide ligands indicating their incompatibility, with only examples of Sc, Ti, Al or Ta benzyls in the presence of N″,[36,[63][64][65] as well as Zr or Th benzyls with tris(silylamido)amines,[66,67] known.Reactions of [M(CH2Ph)] [M = Li(TMEDA), Na, K] with [Ce(TMP)3] in C6D6 were attempted. However, large amounts of solid material precipitated which we have been unable to characterise.…”

“…The 1:1 reaction of [Li(Ph)] with [Li(TMP)] in benzene at 80 °C for 2 hours afforded a brown suspension, wherein only some of the lithium phenyl was observed to dissolve. Upon cooling, pale yellow crystals suitable for X-ray diffraction studies were grown, which revealed this complex to be the hexanuclear (11) 18 (14), Li(1) 65 (10), N(1)-Li(2)-C (19) 124. 50 (11), N(1)-Li(2)-C(19A) 125.65 (10), C(19)-Li(2)-C(19A) 109.85 (9).…”

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

“…13 shows the 1 H DOSY spectrum of (Ph . Similar approaches have also proved successful for tantalum [217] and could prove useful for studies of other coinage metals (copper [218] and silver [219]). American Chemical Society.…”