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.