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

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 over nearly four 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 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)(PPh2NPh2H)]+ > [CpMo(H)(CO)(PtBu2NPh2H)]+ > [CpMo(H)(CO)(PPh2NBn2H)]+ > [CpMo(H)(CO)(PtBu2NBn2H)]+ > [CpMo(H)(CO)(PtBu2NtBu2H)]+. 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. Thus the exchange dynamics are controlled through the relative acidity of the [CpMo(H)(CO)(P2N2H)]+ and [CpMo(H2)(CO)(P2N2)]+ isomers, providing amore » design principle for controlling heterolytic cleavage of H2.« less

Authors:
ORCiD logo; ORCiD logo; ORCiD logo
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1364010
Report Number(s):
PNNL-SA-125042
Journal ID: ISSN 0002-7863; KC0302010
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society; Journal Volume: 139; Journal Issue: 21
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; hydrogen; heterolytic; kinetics

Citation Formats

Zhang, Shaoguang, Appel, Aaron M., and Bullock, R. Morris. Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride. United States: N. p., 2017. Web. doi:10.1021/jacs.7b03053.
Zhang, Shaoguang, Appel, Aaron M., & Bullock, R. Morris. Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride. United States. doi:10.1021/jacs.7b03053.
Zhang, Shaoguang, Appel, Aaron M., and Bullock, R. Morris. Thu . "Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride". United States. doi:10.1021/jacs.7b03053.
@article{osti_1364010,
title = {Reversible Heterolytic Cleavage of the H-H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride},
author = {Zhang, Shaoguang and Appel, Aaron M. and Bullock, R. Morris},
abstractNote = {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 over nearly four 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 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)(PPh2NPh2H)]+ > [CpMo(H)(CO)(PtBu2NPh2H)]+ > [CpMo(H)(CO)(PPh2NBn2H)]+ > [CpMo(H)(CO)(PtBu2NBn2H)]+ > [CpMo(H)(CO)(PtBu2NtBu2H)]+. 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. Thus the exchange dynamics are controlled through 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.},
doi = {10.1021/jacs.7b03053},
journal = {Journal of the American Chemical Society},
number = 21,
volume = 139,
place = {United States},
year = {Thu May 18 00:00:00 EDT 2017},
month = {Thu May 18 00:00:00 EDT 2017}
}
  • Kinetics studies were done of the homolytic and heterolytic cleavage reactions of the chromium-carbon bonds in the complexes RCr(L)A{sup n+} (where L = (15)aneN{sub 4} = 1,4,8,12-tetraazacyclopentadecane; A = H{sub 2}O, OH{sup {minus}}). Activation parameters for homolysis of RCrL(H{sub 2}O){sup 2+} are as follows: 111 {plus minus} 2, 54 {plus minus} 6 (R = p-CH{sub 3}C{sub 6}H{sub 4}CH{sub 2}); 103 {plus minus} 2, 28 {plus minus} 5 (C{sub 6}H{sub 5}CH{sub 2}); 101 {plus minus} 3, 22 {plus minus} 9 (p-BrC{sub 6}H{sub 4}CH{sub 2}); 110 {plus minus} 3, 62 {plus minus} 6 (i-C{sub 3}H{sub 7}). The {Delta}H{sup {double dagger}} and {Delta}S{supmore » {double dagger}} parameters are considerably smaller than those for homolysis of (H{sub 2}O){sub 5}CrR{sup 2+} analogues. Primary alkyl macrocyclic complexes do not undergo homolysis. The complexes RCrL(OH){sup +} slowly hydrolyze for R = n-C{sub 3}H{sub 7} and i-C{sub 3}H{sub 7} whereas those for which R = ArCH{sub 2} do not. The activation parameters for hydrolysis are 78 {plus minus} 1, {minus}53 {plus minus} 2 (R = i-C{sub 3}H{sub 7}) and 83 {plus minus} 3, {minus}46 {plus minus} 9 (n-C{sub 3}H{sub 7}). This pathway shows no solvent deuterium isotope effect. The complexes RCrL(H{sub 2}O){sup 2+} are not subject to acidolysis by H{sub 3}O{sup +} or H{sub 2}O, unlike their (H{sub 2}O){sub 5}CrR{sup 2+} analogues. 23 refs., 3 figs., 2 tabs.« less
  • A balance of metal electrophilicity and ligand steric influences is required for facile, reversible H–H heterolytic cleavage in Mn complexes with pendant amines.
  • The reversibility of the reaction has allowed a quantitative assessment of the kinetic lability of the cobalt-rhodium bond through detailed variable temperature NMR studies. (PEt/sub 3/)/sub 2/(CO)Rh-Co(CO)/sub 4/ was prepared by the dropwise addition of a THF solution of K(Co(CO)/sub 4/) to a THF solution of RhCl(CO)(PEt/sub 3/)/sub 2/, under a nitrogen atmosphere. After stirring the mixture at 50/sup 0/C for 24h, the THF was removed and the residue extracted with petroleum ether. Cooling the extract to -65/sup 0/C produced a 77% yield of the complex. The solid state structure is illustrated; values for relative bond angles and bond lengthsmore » are given. Structural data indicate that the complex is coordinately unsaturated and is best formulated as possessing a polar-metal Rh(I) and Co(-I) center. The (Co(CO)/sub 4/)/sup -/ group behaves as a pseudohalide, the Rh-Co interaction is relatively weak, easily displaced by acetonitrile and THF. The rate of metal bond cleavage is extremely fast. 2 figures.« less
  • Use of hydrogen as a fuel by [FeFe]-hydrogenase enzymes in nature requires heterolytic cleavage of the H-H bond into a proton (H+) and hydride (H-), a reaction that is also a critical step in homogeneous catalysts for hydrogenation of C=O and C=N bonds. An understanding of the catalytic oxidation of H2 by hydrogenases provides insights into the design of synthetic catalysts that are sought as cost-effective alternatives to the use of the precious metal platinum in fuel cells. Crystallographic studies on the [FeFe]-hydrogenase enzyme were critical to understanding of its reactivity, but the key H-H cleavage step is not readilymore » observed experimentally in natural hydrogenases. Synthetic biomimics have provided evidence for H2 cleavage leading to hydride transfer to the metal and proton transfer to an amine. Limitations on the precise location of hydrogen atoms by x-ray diffraction can be overcome by use of neutron diffraction, though its use is severely limited by the difficulty of obtaining suitable crystals and by the scarcity of neutron sources. Here we show that an iron complex with a pendant amine in the diphosphine ligand cleaves hydrogen heterolytically under mild conditions, leading to [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4-, [PtBu2NtBu2 = 1,5-di(tert-butyl)-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane; ArF = 3,5-bis(trifluoromethyl)phenyl]. The Fe-H- - - H-N moiety has a strong dihydrogen bond, with a remarkably short H • • • H distance of 1.489(10) Å between the protic N-Hδ+ and hydridic Fe-Hδ-. The structural data for [CpC5F4NFeH(PtBu2NtBu2H)]+ provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes, with the pendant amine playing a key role as a proton relay. The iron complex [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4- is an electrocatalyst for oxidation of H2 (1 atm) at 22 °C, so the structural data are obtained on a complex that is a functional model for catalysis by [FeFe]-hydrogenase enzymes. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.« less