Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride.

Controlling the heterolytic cleavage of the H-H bond of dihydrogen is critically important in catalytic hydrogenations and in the catalytic oxidation of H. We show how the rate of reversible heterolytic cleavage of H can be controlled, spanning 4 orders of magnitude at 25 °C, from 2.1 × 10 s to ≥10 s. Bifunctional Mo complexes, [CpMo(CO)(κ-PN)] (PN = 1,5-diaza-3,7-diphosphacyclooctane diphosphine ligand with alkyl/aryl groups on N and P), have been developed for heterolytic cleavage of H into a proton and a hydride, akin to frustrated Lewis pairs. The H-H bond cleavage is enabled by the basic amine in the second coordination sphere. The products of heterolytic cleavage of H, Mo hydride complexes bearing protonated amines, [CpMo(H)(CO)(PNH)], were characterized by spectroscopic studies and by X-ray crystallography. Variable-temperature H, N, and 2-D H-H ROESY NMR spectra indicated rapid exchange of the proton and hydride. The exchange rates are in the order [CpMo(H)(CO)(PNH)] > [CpMo(H)(CO)(PNH)] > [CpMo(H)(CO)(PNH)] > [CpMo(H)(CO)(PNH)] > [CpMo(H)(CO)(PNH)]. The pK values determined in acetonitrile range from 9.3 to 17.7 and show a linear correlation with the logarithm of the exchange rates. This correlation likely results from the exchange process involving key intermediates that differ by an intramolecular proton transfer. Specifically, the proton-hydride exchange appears to occur by formation of a molybdenum dihydride or dihydrogen complex, resulting from proton transfer from the pendant amine to the metal hydride. The exchange dynamics are controlled by the relative acidity of the [CpMo(H)(CO)(PNH)] and [CpMo(H)(CO)(PN)] isomers, providing a design principle for controlling heterolytic cleavage of H.