Aluminum(III) Cations [(NHC) ⋅ AlMes ] : Synthesis, Characterization, and Application in FLP-Chemistry.

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