C−H versus Ir−X (X = H, Cl) Reactivity in a Tropylium PCP Pincer Iridium Complex¹

The cationic tropylium [(PCP‘)Ir(CO)(H)(Cl)]⁺ complex (4) (PCP‘ = [2,7-(CH₂P^tBu₂)₂C₇H₄]⁺) is generated from the neutral cycloheptatriene pincer complex (PCP)Ir(CO)(H)(Cl) (1) (PCP = 2,7-(CH₂P^tBu₂)₂C₇H₅) by a hydride abstraction from the CH−Ir fragment with 1 equiv of trimethylsilyl trifluoromethanesulfonate. Interestingly, the tropylium ligand backbone in complex 4 is deprotonated by a base to give the neutral Ir(III) compound (PCP‘ ‘)Ir(CO)(H)(Cl) (5) (PCP‘ ‘ = 2-(CHP^tBu₂)-7-(CH₂P^tBu₂)C₇H₄) with a π-system that extends into one of the phosphine bridges. Finally treatment of 5 with a further equivalent of a base removes HCl from the iridium center, forming the Ir(I) complex 6 with the same ligand backbone as in 5. The HCl elimination and the deprotonation reactions are reversible. Thus addition of 2 equiv of HCl to 6 gives at first 5 then the tropylium complex 7, which differs from 4 only by the counterion. The seven-membered aromatic tropylium system in 4 coordinates to the Mo(CO)₃ fragment, generating the bimetallic complex [(CO)₃Mo(η⁷-PCP‘)Ir(CO)(H)(Cl)]⁺ (8). Quantum chemical calculations at various levels of theory illustrated the relative energetic stabilities of all iridium complexes.