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

Shaoguang Zhang, Aaron Appel, R Morris Bullock

Research output: Contribution to journalArticlepeer-review

27 Citations (Scopus)

Abstract

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)(PPh 2NPh 2H)]+ > [CpMo(H)(CO)(P tBu 2NPh 2H)]+ > [CpMo(H)(CO)(PPh 2NBn 2H)]+ > [CpMo(H)(CO)(P tBu 2NBn 2H)]+ > [CpMo(H)(CO)(P tBu 2N tBu 2H)]+. The pKa 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

Original languageEnglish
Pages (from-to)7376-7387
Number of pages12
JournalJournal of the American Chemical Society
Volume139
Issue number21
DOIs
Publication statusPublished - May 31 2017

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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