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 (eta (5)-CsMe5)(PMe3)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 (eta (5)-C5Me5)(PMe3)Ir(R)(H) (R = Ph, cyclohexyl, cyclooctyl). Low-temperature H-1, P-31, and Li-7 NMR studies indicate that the dehydrohalogenation reaction occurs via the formation of the intermediate (eta (5)-C5Me5)(PMe3)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 (eta (5)-C5Me5)(PMe3)IrH2. Further, when Bronsted 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 (eta (5)-C5Me5)(PMe3)Ir(H)(X) and the photoinduced dihydrogen elimination in (eta (5)-C5Me5)(PMe3)IrH2 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.