Functional Trimethylphosphine Derivatives. 24. (Phosphinomethyl)aluminum Compounds: Bis- and Tris(dimethylphosphino)methyl Ligands in Tetrahedral and Octahedral Aluminum Phosphine Complexes and X-ray Structure of AI[(PMe₂)₂C(SiMe₃)]₃

The bis- and tris(phosphino)methanides Li[CX(PMe₂)2] (5a,b) react with Me₂AICl to give the tetrahedral, monomeric phosphine complexes Me₂Al[(PMe₂)₂CX] (6a,b) (a, X = PMe₂; b, X = SiMe₃) with the four-membered chelate rings A1PCP. 6a,b disproportionate in solution to AlMe₃and the homoleptic complexes Al[(PMe₂)₂CX]₃(7a,b) with hexacoordinated aluminum (A1P₆). 7a,b are also obtained directly from A1C1₃and 5a,b. In contrast, the sterically less hindered bis(phosphino)methanide Li[CX(PMe₂)₂] (2, X = H) yields {Me₂Al[CX(PMe₂)₂])_n(4) with Me₂AlCl. 4 exists in solution as an equilibrium mixture of dimeric (six-membered ring, A1CPA1CP, n = 2) and monomeric (three-membered ring, A1CP, n = 1) species. 4 forms aluminates with LiMe ([Me₃AlCH(PMe₂)₂Li(TMEDA)_3/2] (8)) and 2 ({Me₂Al[CH-(PMe₂)₂]₂Li(OEt₂)₂) (9)). In 8, the aluminate acts as a monodentate phosphine ligand to Li, whereas in 9 each of the two bis(phosphino)methyl groups acts as a monodentate ligand to Li, thus forming a six-membered ring (LiPCAlCP), The characterization of the compounds is based on¹H,¹³C,³¹P, and²⁷A1 NMR as well as on an X-ray structure determination of 7b. Crystals of 7b are rhombohedral of space group R3c with a = b = 12.964 (2) Å, c = 43.575 (5) Å, a = β= 90°, ϒ = 120°, V = 6342.3 A³, and d_calcd= 1.019 g/cm³for z = 6 at 22 °C. Crystals of 7b are rhombohedral of space group R3c with a = b = 12.964 (2) Å, c = 43.575 (5) Å, a = β = 90°, ϒ = 120°, V = 6342.3 ų, and d_calcd= 1.019 g/cm³for Z = 6 at 22 °C. Refinement converged at R_w= 0.069 for 67 refined parameters and 898 observables (F₀≥ 4.0 σ(F₀)). 7b is a trischelate with the ligands bound exclusively by Al-P bonds (Al-P = 2.495 (2) Å). Crystallographic D₃symmetry is imposed on the molecule. The A1P₆coordination geometry is almost halfway between octahedral and trigonal prismatic as manifested by a trigonal twist angle of 35.9° which compares with 60° for a regular octahedron and 0° for regular TP geometry. The observed distortion is induced by the small ligand bite.