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

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posted on 2005-04-11, 00:00 authored by Angelika M. Winter, Klaus Eichele, Hans-Georg Mack, William C. Kaska, Hermann A. Mayer
The cationic tropylium [(PCP‘)Ir(CO)(H)(Cl)]+ complex (4) (PCP‘ = [2,7-(CH2PtBu2)2C7H4]+) is generated from the neutral cycloheptatriene pincer complex (PCP)Ir(CO)(H)(Cl) (1) (PCP = 2,7-(CH2PtBu2)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-(CHPtBu2)-7-(CH2PtBu2)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.