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 language | English |
|---|---|
| Pages (from-to) | 7376-7387 |
| Number of pages | 12 |
| Journal | Journal of the American Chemical Society |
| Volume | 139 |
| Issue number | 21 |
| DOIs | |
| Publication status | Published - May 31 2017 |
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ASJC Scopus subject areas
- Catalysis
- Chemistry(all)
- Biochemistry
- Colloid and Surface Chemistry
Cite this
Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes : Controlling the Dynamics of Exchange Between Proton and Hydride. / Zhang, Shaoguang; Appel, Aaron; Bullock, R Morris.
In: Journal of the American Chemical Society, Vol. 139, No. 21, 31.05.2017, p. 7376-7387.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes
T2 - Controlling the Dynamics of Exchange Between Proton and Hydride
AU - Zhang, Shaoguang
AU - Appel, Aaron
AU - Bullock, R Morris
PY - 2017/5/31
Y1 - 2017/5/31
N2 - 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
AB - 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
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UR - http://www.scopus.com/inward/citedby.url?scp=85019977327&partnerID=8YFLogxK
U2 - 10.1021/jacs.7b03053
DO - 10.1021/jacs.7b03053
M3 - Article
AN - SCOPUS:85019977327
VL - 139
SP - 7376
EP - 7387
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 21
ER -