Taming a monomeric [Cu(η6-C6H6)]+ complex with silylene.

Previous theoretical and experimental endeavors suggested that [Cu(C6H6)]+ prefers the η1/η2 mode over the η6 mode due to the augmented repulsion between the benzene ring and metal d-electrons. Nevertheless, the use of silylene as a neutral ligand has led to the isolation of the first monomeric copper cation, [{PhC(NtBu)2SiN(SiMe3)2}Cu(η6-C6H6)]+[SbF6]- (3), where a copper atom is bound to the benzene ring in an unsupported η6 fashion. However, the use of IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in place of silylene results in the formation of [IPr·Cu(η2-C6H6)]+[SbF6]- (6), where the copper atom is bound to the benzene ring in the η2 mode. The discrepancy in hapticities is also reflected when hexamethylbenzene is employed as the arene ring. The silylene supported copper cation continues to bind in the η6 mode in 2 while the NHC copper cation displays an η3 bonding mode in 5. DFT calculations are carried out to understand how the use of silylene led to the η6 binding mode and why IPr afforded the η2 binding mode.