Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides

Two new iridium hydride complexes, Cp*Ir­(2-phenylpyridine)­H (Cp* = pentamethylcyclopentadienyl) and Cp*Ir­(benzo­[h]­quinoline)­H, and their rhodium analogues Cp*Rh­(2-phenylpyridine)H and Cp*Rh­(benzo­[h]­quinoline)H have been prepared from the corresponding chlorides. The X-ray structures of Cp*Ir­(2-phenylpyridine)H and Cp*Rh­(2-phenylpyridine)­H have been determined. The electrochemistry of all four hydride complexes and the corresponding chlorides has been studied by cyclic voltammetry; all exhibit irreversible M­(III/IV) (M = Ir, Rh) oxidations. The hydride complexes are more easily oxidized than their chloride analogues, and the rhodium hydrides are more easily oxidized than their iridium analogues. The hydride complexes transfer H^– to the N-carbophenoxypyridinium cation at room temperature, giving mixtures of the 1,2- and 1,4-dihydropyridine products. In CD₃CN all four hydrides give these products in nearly the same ratio, which results from kinetic control; the thermodynamic ratio of the products has been calculated, and isomerization in that direction has been observed. In weakly coordinating solvents the cations left after H^– transfer catalyze this isomerization. Acetonitrile can trap these cations, slowing isomerization substantially. The X-ray structures of [Cp*Ir­(2-phenylpyridine)­(CH₃CN)]­[PF₆] and [Cp*Rh­(2-phenylpyridine)­(CH₃CN)]­[PF₆] have also been determined.