Aluminum(III) Cations [(NHC) ⋅ AlMes<sub>2</sub>]<sup>+</sup>: Synthesis, Characterization, and Application in FLP‐Chemistry
Luis Werner, Julika Hagn, Janis Walpuski, Udo Radius
Abstract
Abstract The three‐coordinate aluminum cations ligated by N ‐heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes 2 ] + [B(C 6 F 5 ) 4 ] − (NHC=IMe Me 4 , I i Pr Me 5 , I i Pr 6 , Mes=2,4,6‐trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes 2 (NHC=IMe Me 1 , I i Pr Me 2 , I i Pr 3 ) using [Ph 3 C] + [B(C 6 F 5 ) 4 ] − in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four‐coordinate aluminum etherate cations [(NHC) ⋅ AlMes 2 (OEt 2 )] + [B(C 6 F 5 ) 4 ] − 7 – 9 (NHC=IMe Me 7 , I i Pr Me 8 , I i Pr 9 ) were isolated. According to a theoretical and experimental assessment of the Lewis ‐acidity of the [(IMe Me ) ⋅ AlMes 2 ] + cation is the acidity larger than that of B(C 6 F 5 ) 3 and of similar magnitude as reported for Al(C 6 F 5 ) 3 . The reaction of [(IMe Me ) ⋅ AlMes 2 ] + [B(C 6 F 5 ) 4 ] − 4 with the sterically less demanding, basic phosphine PMe 3 afforded a mixed NHC/phosphine stabilized cation [(IMe Me ) ⋅ AlMes 2 (PMe 3 )] + [B(C 6 F 5 ) 4 ] − 10 . Equimolar mixtures of 4 and the sterically more demanding PCy 3 gave a frustrated Lewis ‐pair (FLP), i.e., [(IMe Me ) ⋅ AlMes 2 ] + [B(C 6 F 5 ) 4 ] − /PCy 3 FLP‐11 , which reacts with small molecules such as CO 2 , ethene, and 2‐butyne.