Synthesis and characterization of four- and five-coordinate organoaluminum complexes incorporating the amido diphosphine ligand system N(SiMe2CH2PPri2)2

Article abstract of DOI:10.1021/om9604583

The preparation of new four- and five-coordinate aluminum amido diphosphine complexes is reported. The reaction of the potentially tridentate ligand precursor LiN(SiMe₂CH₂PPr₂ⁱ)₂ with AlCl₃ in toluene at 25°C leads to the formation of AlCl₂[N(SiMe₂CH₂PPr₂ ⁱ)₂]. The X-ray crystal structure shows it to be monomeric with a distorted-trigonal-bipyramidal geometry having the tridentate ligand meridionally bound. The solution NMR spectra are also consistent with this geometry. Addition of the alkyllithium reagents RLi (where R = Me, Et) or dialkyl magnesium reagents R₂Mg (R = Me, CH₂Ph) leads to the formation of bis-(hydrocarbyl) derivatives of the formula AIR₂[N(SiMe₂CH₂PPr₂ ⁱ)₂]. The X-ray structure of Al(CH₂Ph)₂[N(SiMe₂CH₂PPr ₂ⁱ)₂] shows that it is mononuclear in the solid state with a distorted-tetrahedral geometry in which coordination to only one phosphine is observed. Variable-temperature NMR studies are consistent with a rapidly fluxional molecule at ambient temperature. In solution, the NMR spectroscopic parameters of AlMe₂[N(SiMe₂CH₂PPr₂ ⁱ)₂] and AlEt₂[N(SiMe₂CH₂PPr₂ ⁱ)₂] are consistent with overall C_2v symmetry. These observations are supported by ²⁷Al NMR studies. Attempts to generate the monoalkyls Al(R)X[N(SiMe₂CH₂PPr₂ⁱ)₂] (R = Me, Et; X = Cl) by the addition of 1 equiv of the appropriate alkylating reagent results in equimolar mixtures of the corresponding dialkyl and dichloride; however, the reaction of the lithium salt LiN(SiMe₂CH₂PPr₂ⁱ)₂ with RAlCl₂ (R = Me, Et) produces mixtures containing predominantly Al(R)X[N(SiMe₂CH₂PPr₂ⁱ)₂] (R = Me, Et). Treatment of the monoethyl derivative with excess AlCl₃ affords AlCl₂[N(SiMe₂CH₂PPr₂ ⁱ)₂]. AlCl₃. The X-ray crystal structure of this compound shows it to be monomeric, with each phosphine bound to a tetrahedral Al center. Solution NMR studies are consistent with this geometry. This species is likely formed as the result of the coordination of an AlCl₃ molecule to the aluminum center, followed by coordination of a second molecule to a free phosphine and subsequent elimination OfEtAlCl₂. This species is also generated by the addition of 3 equiv AlCl₃ to the starting lithium salt LiN(SiMe₂CH₂PPr₂ⁱ)₂.