Synthesis and Characterization of Four- and Five-Coordinate Organoaluminum Complexes Incorporating the Amido Diphosphine Ligand System N(SiMe₂CH₂PPri₂)₂

The preparation of new four- and five-coordinate aluminum amido diphosphine complexes is reported. The reaction of the potentially tridentate ligand precursor LiN(SiMe 2 CH 2 PPr i 2 ) 2 with AlCl 3 in toluene at 25 °C leads to the formation of AlCl 2 [N(SiMe 2 CH 2 PPr i 2 ) 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 R 2 Mg (R = Me, CH 2 Ph) leads to the formation of bis(hydrocarbyl) derivatives of the formula AlR 2 [N(SiMe 2 CH 2 PPr i 2 ) 2 ]. The X-ray structure of Al(CH 2 Ph) 2 [N(SiMe 2 CH 2 PPr i 2 ) 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 AlMe 2 [N(SiMe 2 CH 2 PPr i 2 ) 2 ] and AlEt 2 [N(SiMe 2 CH 2 PPr i 2 ) 2 ] are consistent with overall C 2 v symmetry. These observations are supported by 27 Al NMR studies. Attempts to generate the monoalkyls Al(R)X[N(SiMe 2 CH 2 PPr i 2 ) 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(SiMe 2 CH 2 PPr i 2 ) 2 with RAlCl 2 (R = Me, Et) produces mixtures containing predominantly Al(R)X[N(SiMe 2 CH 2 PPr i 2 ) 2 ] (R = Me, Et). Treatment of the monoethyl derivative with excess AlCl 3 affords AlCl 2 [N(SiMe 2 CH 2 PPr i 2 ) 2 ]· AlCl 3 . 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 AlCl 3 molecule to the aluminum center, followed by coordination of a second molecule to a free phosphine and subsequent elimination of EtAlCl 2 . This species is also generated by the addition of 3 equiv AlCl 3 to the starting lithium salt LiN(SiMe 2 CH 2 PPr i 2 ) 2.