(Phosphinomethyl)alumlnum Compounds: The Role of Coligands and X-ray Structure Determinations of the Heterocycles (TMEDA)LlCIAIMe₂C(SiMe₃)₂PMe₂ and (Me)(CI)AlC(SIMe₃)₂PMe₂SiMe₂C(SiMe₃)PMe₂

From the 1:1 reaction of R₂AlCl (a, R = CI; b, R = Me) with {Li[C(SiMe₃)₂(PMe₂)]}₂, six-membered heterocycles [R₂AlC(SiMe₃)₂(PMe₂)]₂ (1a,b) are obtained. The 1:2 reaction for R = CI involves a facile methyl migration from the silicon to aluminum and leads to the novel heterocycle (Me)(Cl)AlC-(SiMe₃)₂PMe₂SiMe₂C(SiMe₃)PMe2 (2), exhibiting a number of unusual structural features, as shown by the X-ray structure determination of 2, which crystallizes from pentane (colorless crystals, triclinic, space group P1, a = 9.161 (1), b = 15.280 (2), and c = 11.832 (1) Å, α = 109.62 (1), ß = 74.57 (1), and γ = 98.39 (1)°, V = 1500.9 ų, Z = 2, R = 0.035 for 283 refined parameters and 4841 observables). A formal [3 + 2] cycloaddition is observed in the same 1:1 reaction in the presence of tetramethylethylenediamine (TMEDA) to give five-membered heterocycles (TMEDA)LiClAlR₂C(SiMe₃)₂PMe₂ (3a,b), incorporating five different elements within the ring. 3b crystallizes from pentane (colorless crystals, monoclinic, space group P2₁/n, a = 8.915 (1), b = 18.887 (2), and c = 16.558 (2) Å, α = 90.0 (1), ß = 92.52 (1), γ = 90.0 (1)°, V = 2785.3 ų, Z = 4, R = 0.064 for 268 refined parameters and 3830 observables). An X-ray structure determination shows the ambidentate phosphinomethanide ligand to be bound to the lithium atom via phosphorus and to the aluminum atom via carbon. 3b thermally is converted to lb by loss of TMEDA, whereas 3a is stable to thermolysis. In the presence of tetrahydrofuran (THF), the 1:1 reaction for R = Me gives rise to Me₂(THF)Al[C(SiMe₃)₂(PMe₂)] (4), which is also obtained from 3b and THF. As a byproduct, due to some traces of HCl, in the 1:1 reaction (R = CI) the ylide complex Cl₃AlC(SiMe₃)₂PMe₂H (5) is identified spectroscopically and eventually can be isolated. 5 either is converted by further addition of HCl to [HC(SiMe₃)₂PMe₂H][AlCl₄] (6) or rearranges to its tautomer Cl₃AlPMe₂C(SiMe₃)₂H (7), thus constituting a novel example for an ylide ⇋phosphane diadic relationship. 7 is also obtained directly from A1C1₃ and HC(SiMe₃)₂(PMe₂).