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 H₂. We show how the rate of reversible heterolytic cleavage of H₂ can be controlled, spanning 4 orders of magnitude at 25 °C, from 2.1 × 10³ s^-1 to ≥10⁷ s^-1. Bifunctional Mo complexes, [CpMo(CO)(κ³-P₂N₂)]⁺ (P₂N₂ = 1,5-diaza-3,7-diphosphacyclooctane diphosphine ligand with alkyl/aryl groups on N and P), have been developed for heterolytic cleavage of H₂ 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 H₂, Mo hydride complexes bearing protonated amines, [CpMo(H)(CO)(P₂N₂H)]⁺, were characterized by spectroscopic studies and by X-ray crystallography. Variable-temperature¹H,¹⁵N, and 2-D¹H-¹H ROESY NMR spectra indicated rapid exchange of the proton and hydride. The exchange rates are in the order [CpMo(H)(CO)(P^Ph ₂N^Ph ₂H)]⁺ > [CpMo(H)(CO)(P ^tBu ₂N^Ph ₂H)]⁺ > [CpMo(H)(CO)(P^Ph ₂N^Bn ₂H)]⁺ > [CpMo(H)(CO)(P ^tBu ₂N^Bn ₂H)]⁺ > [CpMo(H)(CO)(P ^tBu ₂N ^tBu ₂H)]⁺. The pK_a 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)(P₂N₂H)]⁺ and [CpMo(H₂)(CO)(P₂N₂)]⁺ isomers, providing a design principle for controlling heterolytic cleavage of H₂