Aluminum(III) Cations [(NHC) ⋅ ArMes2]+: Synthesis, Characterization, and Application in FLP-Chemistry

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) & sdot; AlMes(2)](+)[B(C6F5)(4)](-) (NHC=IMe(Me)4, IiPr(Me)5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) & sdot; AlHMes(2) (NHC=IMe(Me)1, IiPr(Me)2, IiPr 3) using [Ph3C](+)[B(C6F5)(4)](-) in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) & sdot; AlMes(2)(OEt2)](+) [B(C6F5)(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 [(IMeMe) & sdot; AlMes(2)](+) cation is the acidity larger than that of B(C6F5)(3) and of similar magnitude as reported for Al(C6F5)(3). The reaction of [(IMeMe) & sdot; AlMes(2)](+)[B(C6F5)(4)](-)4 with the sterically less demanding, basic phosphine PMe3 afforded a mixed NHC/phosphine stabilized cation [(IMeMe) & sdot; AlMes(2)(PMe3)](+)[B(C6F5)(4)](-)10. Equimolar mixtures of 4 and the sterically more demanding PCy3 gave a frustrated Lewis-pair (FLP), i.e., [(IMeMe) & sdot; AlMes(2)](+)[B(C6F5)(4)](-)/PCy3FLP-11, which reacts with small molecules such as CO2, ethene, and 2-butyne.

Automated summary

This study synthesized aminoborate complexes using N-heterocyclic carbene ligands and assessed their potential applications in catalytic CO2 conversion.