Synthesis and Structural Characterization of Five-Coordinate Aluminum Complexes Containing Diarylamido Diphosphine Ligands

A series of five-coordinate aluminum complexes supported by o-phenylene-derived amido diphosphine ligands, [N(o-C6H4PR2)2]− ([R-PNP]−; R = Ph, iPr) and [N(o- C6H4PPh2)(o-C6H4PiPr2)]− ([Ph-PNP-iPr]−), have been prepared and structurally characterized. Alkane elimination reactions of trialkylaluminum with H[Ph-PNP] (1a), H[iPr-PNP] (1b), and H[Ph-PNP-iPr] (1c) in toluene at −35 °C respectively produced the corresponding dialkyl complexes [Ph-PNP]AlR2, [iPr-PNP]AlR2, and [Ph-PNP-iPr]AlR2 (R = Me (2a−c), Et (3a−c), iBu (4a−c)) in high isolated yield. The dihydride complexes [Ph-PNP]AlH2 (6a), [iPr- PNP]AlH2 (6b), and [Ph-PNP-iPr]AlH2 (6c) were prepared in one-pot reactions of in situ prepared dichloride precursors (5a−c) with LiAlH4 in THF at room temperature. X-ray diffraction studies of 2a−c, 3b−c, 5b, and 6b revealed a distorted trigonal-bipyramidal structure for these molecules in which the two phosphorus donors are mutually trans. The solution structures of these organoaluminum complexes were all characterized by 1H, 13C, and 31P NMR spectroscopy. The NMR data are indicative of solution C2 symmetry for [Ph-PNP]− and [iPr-PNP]− complexes, whereas they are indicative of C1 for [Ph-PNP-iPr]− derivatives. The 1H NMR spectra of 3a−c and 4a−c revealed diastereotopy for the α-hydrogen atoms in these molecules.