C-H versus Ir-X (X = H, Cl) reactivity in a tropylium PCP pincer iridium complex

The cationic tropylium [(PCP')Ir(CO)(H)(Cl)](+) complex (4) (PCP'= [2,7-((CH2PBu2)-Bu-t)(2)C7H4](+)) is generated from the neutral cycloheptatriene pincer complex (PCP)Ir(CO)(H)(Cl) (1) (PCP = 2,7-((CH2PBu2)-Bu-t)(2)C7H5) 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-((CHPBu2)-Bu-t)-7-((CH2PBu2)-Bu-t)C7H4) with a pi-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)(3)Mo(eta(7) -PCP')Ir(CO)(H)(Cl)](+) (8). Quantum chemical calculations at various levels of theory illustrated the relative energetic stabilities of all iridium complexes.