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.