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

Abstract

[(BDI)Mg+ ][B(C6 F5 )4 - ] (1; BDI=CH[C(CH3 )NDipp]2 ; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph3 C+ ][B(C6 F5 )4 - ]. Addition of 3-hexyne gave [(BDI)Mg+ ⋅(EtC≡CEt)][B(C6 F5 )4 - ]. 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 Me3 SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg+ (C≡CPh)]2 ⋅2 [B(C6 F5 )4 - ] (2, 70 %) and [(BDI-H)Mg+ (C≡CSiMe3 )]2 ⋅2 [B(C6 F5 )4 - ] (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 }2 ⋅ 2 [B(C6 F5 )4 - ] (4, 53 %) and {Mg+ C(Ph)=C(Me)C[C(Me)=NDipp]2 }2 ⋅2 [B(C6 F5 )4 - ] (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(C6 F5 )4 - ] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg+ ⋅PPh3 ][B(C6 F5 )4 - ] (6), [(BDI)Mg+ ⋅PCy3 ][B(C6 F5 )4 - ] (7), and [(BDI)Mg+ ⋅ PtBu3 ][B(C6 F5 )4 - ] (8). The bulkier phosphine PMes3 (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg+ ][B(C6 F5 )4 - ] 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 HPPh2 , for which an FLP-type mechanism is tentatively proposed.