Evidence for the Intervention of Different C−H Activating Intermediates in the Irradiation of (η5-C5Me5)(PMe3)IrH2 and the Reaction of (η5-C5Me5)(PMe3)Ir(H)(Cl) with Strong Base. Detection and Spectroscopic Characterization of (η5-C5Me5)(PMe3)Ir(Li)(Cl), an Intermediate in the Dehydrohalogenation Reaction

Reaction of (η 5 -C 5 Me 5 )(PMe 3 )Ir(H)(X) (X = Cl, Br) with tert -butyllithium in hydrocarbon solvent results in dehydrohalogenation of the iridium center and subsequent C−H bond activation of solvent to give (η 5 -C 5 Me 5 )(PMe 3 )Ir(R)(H) (R = Ph, cyclohexyl, cyclooctyl). Low-temperature 1 H, 31 P, and 7 Li NMR studies indicate that the dehydrohalogenation reaction occurs via the formation of the intermediate (η 5 -C 5 Me 5 )(PMe 3 )Ir(Li)(X). Competition experiments involving C−H bond activation in benzene−cyclohexane−cyclooctane mixtures have allowed for the determination of a relative intermolecular selectivity scale for these substrates. The selectivities (reported on a per hydrogen basis) for benzene, cyclooctane, and cyclohexane C−H bond activation were found to be 4.98:0.74:1, respectively, and are significantly different from those obtained via photoinduced dihydrogen elimination from (η 5 -C 5 Me 5 )(PMe 3 )IrH 2 . Further, when Brønsted bases other than tert -butyllithium were employed, the intermolecular selectivities in the base-promoted dehydrohalogenation reaction were found to be dependent on the alkali metal, but not the counteranion, of the base, with benzene/cycloalkane selectivity increasing in the order K > Na > Li. These results provide strong evidence that the selectivity-determining steps in the C−H bond activations by dehydrohalogenation of (η 5 -C 5 Me 5 )(PMe 3 )Ir(H)(X) and the photoinduced dihydrogen elimination in (η 5 -C 5 Me 5 )(PMe 3 )IrH 2 involve different reactive intermediates. In the base-induced reaction, we postulate that the eliminated salt remains coordinated to the iridium, primarily through the alkali metal, in the C−H activation transition state.