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

Journal of the American Chemical Society

5 years ago

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

Shaoguang Zhang, Aaron M. Appel, R. Morris Bullock

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-temperature ¹H, ¹⁵N, and 2-D ¹H–¹H ROESY NMR spectra indicated rapid exchange of the proton and hydride. The exchange rates are in the order [CpMo(H)(CO)(P^Ph₂N^Ph₂H)]⁺ > [CpMo(H)(CO)(P^{^tBu}₂N^Ph₂H)]⁺ > [CpMo(H)(CO)(P^Ph₂N^Bn₂H)]⁺ > [CpMo(H)(CO)(P^{^tBu}₂N^Bn₂H)]⁺ > [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.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b03053

DOI: 10.1021/jacs.7b03053