Synthesis, Characterization, and Reactivity of Niobium and Tantalum Complexes Bearing Metal−Nitrogen Bonds. X-ray Molecular Structure of [Nb(C₅H₄SiMe₃){NH(CH₂)₂-η-NH₂}Cl₃] and the Novel Tetranuclear Niobium Oxo Derivative [{Nb(C₅H₄SiMe₃)Cl(μ₂-O)}₄(Cl)₂(μ₃-O)]

Abstract

The reaction of the cyclopentadienyl-silyl-amido titanium compound [Ti{η5-C5H4SiMe2-η-N(CH2)2-η-NH2}Cl2] with group 5 metal monocyclopentadienyl complexes [MCpRCl4] (M = Nb, Ta; CpR = C5H4SiMe3 (Cp‘), C5Me5 (Cp*)) afforded the heterobimetallic complexes [TiCl2{η5-C5H4SiMe2-η-N(CH2)2-κ-NH2}MCpRCl4] (M = Nb, CpR = Cp‘, 2a; Cp*, 2b; M = Ta, CpR = Cp‘, 3a; Cp = Cp*, 3b). Compounds 2 evolve at room temperature to give a three-component mixture, the chlorosilyl-substituted cyclopentadienyl titanium compound [Ti(η5-C5H4SiMe2Cl)Cl3], the corresponding mononuclear amido-amino, [NbCpR{NH(CH2)2-η-NH2}Cl3] (CpR = Cp‘, 4a; Cp*, 4b), and the dinuclear imido niobium complexes [{NbCpRCl2}2(μ-N(CH2)2-η-N)] (CpR = Cp‘, 5a; Cp*, 5b). In contrast, the analogue tantalum complexes thermally degraded, when CpR = Cp‘ at normal temperature, whereas when CpR = Cp* at temperatures higher than 50 °C, they gave a unique tantalum complex, the corresponding mononuclear amido-amino derivative [TaCpR{NH(CH2)2-η-NH2}Cl3] (CpR = Cp‘, 6a; Cp*, 6b). Alternatively, these group 5 complexes were prepared by direct reaction of the monocyclopentadienyl derivatives with the corresponding organic diamine, in the appropriate proportion and reaction conditions. Tantalum amino adducts were observed as intermediate species in the course of such reactions, and even the dinuclear derivative [{TaCp*Cl4}2(μ-NH2(CH2)2NH2)] (8) could be isolated. Hydrolysis of the dinuclear imido complex 5a yielded the tricyclic tetranuclear niobium oxo derivative [{NbCp‘Cl(μ2-O)}4(Cl)2(μ3-O)] (9), which displays an asymmetrical structure as a consequence of the triply connected oxygen site. These compounds were characterized by elemental analysis and NMR spectroscopy, and the crystal structures of 4a and 9 were determined by X-ray diffraction methods.

Suitable Approach to Prepare N‐Substituted Niobium Complexes – Study of the Factors Controlling the Process

“…In 2 , the methoxyethylamine ligand is clearly coordinated to the niobium center through the amine end, whereas the methoxy moiety remains uncoordinated and is located far from the niobium atom. The Nb(1)–N(1) bond length [2.290(6) Å] lies within the range observed for a typical Nb–N coordination bond (2.20–2.51 Å), and corroborates the Nb–amine adduct formulation. Accordingly, the nitrogen atom shows a tetrahedral geometry, as expected for sp 3 hybridization.…”

“…In 6 , the amine framework is clearly linked to the cyclopentadienyl ring by the nitrogen atom to produce the dianionic tridentate ligand η 5 ‐C 5 H 4 SiMe 2 ‐κN(CH 2 ) 2 ‐κOMe, which adopts a mer disposition in the coordination sphere of the niobium center. The Nb(1)–O(1) bond length [2.323(3) Å] is similar to the Nb(1)–N(1) bond length [2.290(6) Å] for 2 and matches Nb–OR 2 bond lengths with no Nb–O π‐bonding contributions,, an observation supported by the pyramidal environment shown by the oxygen atom, which is consistent with sp 3 hybridization. Moreover, the Nb(1)–N(2) bond length [1.982(4) Å] is notably shorter than that found for 2 and comparable to those of Nb–N amido bonds with appreciable multiple interaction character (1.935–2.102 Å) .…”

Monocyclopentadienyl Phenoxido–Amino and Phenoxido–Amido Titanium Complexes: Synthesis, Characterisation, and Reactivity of Asymmetric Metal Centre Derivatives

“…Similar behaviour has been observed for analogous cyclopentadienyl titanium derivatives. [26][27][28] Such altered behaviour is due to the different trans influence derived from the presence of a chloride or a cyclopentadienyl ligand located in the trans position with respect to the amino functionality in 6a-1 and 6a-2, respectively.…”

Cyclopentadienyl–Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide

“…The exceptional catalytic behavior displayed by these complexes relies on the robustness and stability provided by the unprecedented cyclopentadienyl–silsesquioxanate (CpPOSS) ligand . The formation of the CpPOSS ligand occurs at the coordination sphere of the titanium atom through the condensation of both the silsesquioxanate fragment and the Cp ring . Such a reaction proceeds via a corner‐capped Cp intermediate, detected in the case of Ti(η 5 ‐C 5 Me 4 SiMe 2 Cl)( i Bu 7 Si 7 O 12 ‐κ 3 O 3 ) (complex 3a , Scheme ), which rapidly isomerizes to form the corresponding tetramethylcyclopentadienyl–silsesquioxane complex Ti(η 5 ‐C 5 Me 4 SiMe 2 OR 7 Si 7 O 11 ‐κ 2 O 2 )Cl (R = i Bu, 4a ; Ph, 4b , Scheme ).…”