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[(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 Bronsted) 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.

The synthesis of structurally diverse heterocycles for chemical space exploration was achieved via the cascade reactions of indigo with propargylic electrophiles. New pyrazinodiindolodione, naphthyridinedione, azepinodiindolone, oxazinoindolone and pyrrolodione products were prepared in one pot reactions by varying the leaving group (-Cl, -Br, -OMs, -OTs) or propargyl terminal functionality (-H, -Me, -Ph, -Ar). Mechanistic and density functional theory studies revealed that the unsaturated propargyl moiety can behave as an electrophile when aromatic terminal substitutions are made, and therefore competes with leaving group substitution for new outcomes. Selected products from the cascade reactions were investigated for their absorption and fluorescence properties, including transient absorption spectroscopy. This revealed polarity dependent excited state relaxation pathways, fluorescence, and triplet formation, thus highlighting these reactions as a means to access diverse functional materials rapidly.