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

The preparation of new four- and live-coordinate aluminum amido diphosphine complexes is reported. The reaction of the potentially tridentate ligand precursor LiN(SiMe(2)CH(2)PPr(2)(i))(2) with AlCl3 in toluene at 25 degrees C leads to the formation of AlCl2[N(SiMe(2)CH(2)PPr(2)(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 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(2)(i))(2)]. The X-ray structure of Al(CH(2)Ph)(2)[N(SiMe(2)CH(2)PPr(2)(i))(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)PPri(2))(2)] and AlEt(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)] are consistent with overall C-2 upsilon symmetry. These observations are supported by Al-27 NMR studies. Attempts to generate the monoalkyls Al(R)X[N(SiMe(2)CH(2)PPri(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(2)(i))(2) with RA1Cl(2) (R = Me, Et) produces mixtures containing predominantly Al(R)X[N(SiMe(2)CH(2)PPr(2)(i))(2)] (R = Me, Et). Treatment of the monoethyl derivative with excess AlCl3 affords AlCl2[N(SiMe(2)CH(2)PPr(2)(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 of EtAlCl(2). This species is also generated by the addition of 3 equiv AlCl3 to the starting lithium salt LiN(SiMe(2)CH(2)PPr(2)(i))(2).