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

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 ion 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. Rare examples of three‐coordinate aluminum cations of the form [(NHC) ⋅ AlMes2]+[B(C6F5)4]− and their etherate analogues [(NHC) ⋅ AlMes2(OEt2)]+[B(C6F5)4]−are presented. The Lewis‐acidity of the cations was evaluated theoretically and experimentally to be very high. These cationic Lewis‐acids form a frustrated Lewis‐pair (FLP) upon addition of the phosphine PCy3, which reacts with small molecules such as CO2, ethene, and 2‐butyne.