The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes₂ ]⁺ [B(C₆ F₅ )₄ ]^- (NHC=IMe^Me 4, IiPr^Me 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes₂ (NHC=IMe^Me 1, IiPr^Me 2, IiPr 3) using [Ph₃ C]⁺ [B(C₆ F₅ )₄ ]^- in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) ⋅ AlMes₂ (OEt₂ )]⁺ [B(C₆ F₅ )₄ ]^- 7-9 (NHC=IMe^Me 7, IiPr^Me 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMe^Me ) ⋅ AlMes₂ ]⁺ cation is the acidity larger than that of B(C₆ F₅ )₃ and of similar magnitude as reported for Al(C₆ F₅ )₃ . The reaction of [(IMe^Me ) ⋅ AlMes₂ ]⁺ [B(C₆ F₅ )₄ ]^- 4 with the sterically less demanding, basic phosphine PMe₃ afforded a mixed NHC/phosphine stabilized cation [(IMe^Me ) ⋅ AlMes₂ (PMe₃ )]⁺ [B(C₆ F₅ )₄ ]^- 10. Equimolar mixtures of 4 and the sterically more demanding PCy₃ gave a frustrated Lewis-pair (FLP), i.e., [(IMe^Me ) ⋅ AlMes₂ ]⁺ [B(C₆ F₅ )₄ ]^- /PCy₃ FLP-11, which reacts with small molecules such as CO₂ , ethene, and 2-butyne.

Janis Walpuski

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