Aluminum(III) Cations [(NHC) ⋅ AlMes2]+: Synthesis, Characterization, and Application in FLP-Chemistry

Abstract

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes2]+[B(C6F5)4]− (NHC=IMeMe 4, IiPrMe 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes2 (NHC=IMeMe 1, IiPrMe 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) ⋅ AlMes2(OEt2)]+ [B(C6F5)4]− 7–9 (NHC=IMeMe 7, IiPrMe 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMeMe) ⋅ AlMes2]+ 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) ⋅ AlMes2]+[B(C6F5)4]− 4 with the sterically less demanding, basic phosphine PMe3 afforded a mixed NHC/phosphine stabilized cation [(IMeMe) ⋅ AlMes2(PMe3)]+[B(C6F5)4]− 10. Equimolar mixtures of 4 and the sterically more demanding PCy3 gave a frustrated Lewis-pair (FLP), i.e., [(IMeMe) ⋅ AlMes2]+[B(C6F5)4]−/PCy3 FLP-11, which reacts with small molecules such as CO2, ethene, and 2-butyne.