Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines

[(BDI)Mg ][B(C F ) ] (1; BDI=CH[C(CH )NDipp] ; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph C ][B(C F ) ]. Addition of 3-hexyne gave [(BDI)Mg ⋅(EtC≡CEt)][B(C F ) ]. Single-crystal X-ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg (C≡CPh)] ⋅2 [B(C F ) ] (2, 70 %) and [(BDI-H)Mg (C≡CSiMe )] ⋅2 [B(C F ) ] (3, 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg C(Ph)=C(Ph)C[C(Me)=NDipp] } ⋅ 2 [B(C F ) ] (4, 53 %) and {Mg C(Ph)=C(Me)C[C(Me)=NDipp] } ⋅2 [B(C F ) ] (5, 73 %), in which the Mg center is N,N,C-chelated. The (BDI)Mg cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of [(BDI)Mg ][B(C F ) ] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg ⋅PPh ][B(C F ) ] (6), [(BDI)Mg ⋅PCy ][B(C F ) ] (7), and [(BDI)Mg ⋅ PtBu ][B(C F ) ] (8). The bulkier phosphine PMes (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg ][B(C F ) ] and phosphines did not result in addition to the triple bond in 3-hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh , for which an FLP-type mechanism is tentatively proposed.