Research output: Contribution to journal › Article › peer-review
Relevance of Protons in Heterolytic Activation of H2O2 over Nb(V) : Insights from Model Studies on Nb-Substituted Polyoxometalates. / Maksimchuk, Nataliya V.; Maksimov, Gennadii M.; Evtushok, Vasilii Yu et al.
In: ACS Catalysis, Vol. 8, No. 10, 05.10.2018, p. 9722-9737.Research output: Contribution to journal › Article › peer-review
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TY - JOUR
T1 - Relevance of Protons in Heterolytic Activation of H2O2 over Nb(V)
T2 - Insights from Model Studies on Nb-Substituted Polyoxometalates
AU - Maksimchuk, Nataliya V.
AU - Maksimov, Gennadii M.
AU - Evtushok, Vasilii Yu
AU - Ivanchikova, Irina D.
AU - Chesalov, Yuriy A.
AU - Maksimovskaya, Raisa I.
AU - Kholdeeva, Oxana A.
AU - Solé-Daura, Albert
AU - Poblet, Josep M.
AU - Carbó, Jorge J.
N1 - Publisher Copyright: Copyright © 2018 American Chemical Society.
PY - 2018/10/5
Y1 - 2018/10/5
N2 - Nb-monosubstituted Lindqvist-type polyoxometalates (POM), (Bu4N)4[(NbW5O18)2O] (1) and (Bu4N)3[Nb(O)W5O18] (2), catalyze epoxidation of alkenes with hydrogen peroxide and mimic the catalytic performance of heterogeneous Nb-silicate catalysts. Dimer 1 is more active than monomer 2, but the catalytic activity of the latter increases in the presence of acid. Kinetic and spectroscopic studies suggest a mechanism that involves generation of monomer (Bu4N)2[Nb(OH)W5O18] (3), interaction of 3 with H2O2 leading to a protonated peroxo niobium species, (Bu4N)2[HNb(O2)W5O18] (4), followed by oxygen transfer to a C=C bond in alkene. The previously unknown peroxo complex 4 has been isolated and characterized by elemental analysis; UV-vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopy; cyclic voltammetry; and potentiometric titration. The physicochemical techniques support a monomeric Lindqvist structure of 4 bearing one peroxo ligand attached to Nb(V) in a η2-coordination mode. While the unprotonated peroxo complex (Bu4N)3[Nb(O2)W5O18] (5) is inert toward alkenes under stoichiometric conditions, 4 readily reacts with cyclohexene to afford epoxide and 1,2-trans-cyclohexane diol, which proves the key role of protons for heterolytic activation of H2O2 over Nb(V). The IR, Raman, UV-vis, and 17O NMR spectroscopic studies along with DFT calculations showed that the activating proton in 4 is predominantly located at a Nb-O-W bridging oxygen. However, DFT calculations revealed that the protonated peroxo species "HNb(O2)" is present in equilibrium with a hydroperoxo species "Nb(η2-OOH)," which has a lower activation barrier for the oxygen transfer to cyclohexene and is, therefore, the main epoxidizing species. The calculations indicate that protonation is crucial to generating the active species and to increasing POM electrophilicity.
AB - Nb-monosubstituted Lindqvist-type polyoxometalates (POM), (Bu4N)4[(NbW5O18)2O] (1) and (Bu4N)3[Nb(O)W5O18] (2), catalyze epoxidation of alkenes with hydrogen peroxide and mimic the catalytic performance of heterogeneous Nb-silicate catalysts. Dimer 1 is more active than monomer 2, but the catalytic activity of the latter increases in the presence of acid. Kinetic and spectroscopic studies suggest a mechanism that involves generation of monomer (Bu4N)2[Nb(OH)W5O18] (3), interaction of 3 with H2O2 leading to a protonated peroxo niobium species, (Bu4N)2[HNb(O2)W5O18] (4), followed by oxygen transfer to a C=C bond in alkene. The previously unknown peroxo complex 4 has been isolated and characterized by elemental analysis; UV-vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopy; cyclic voltammetry; and potentiometric titration. The physicochemical techniques support a monomeric Lindqvist structure of 4 bearing one peroxo ligand attached to Nb(V) in a η2-coordination mode. While the unprotonated peroxo complex (Bu4N)3[Nb(O2)W5O18] (5) is inert toward alkenes under stoichiometric conditions, 4 readily reacts with cyclohexene to afford epoxide and 1,2-trans-cyclohexane diol, which proves the key role of protons for heterolytic activation of H2O2 over Nb(V). The IR, Raman, UV-vis, and 17O NMR spectroscopic studies along with DFT calculations showed that the activating proton in 4 is predominantly located at a Nb-O-W bridging oxygen. However, DFT calculations revealed that the protonated peroxo species "HNb(O2)" is present in equilibrium with a hydroperoxo species "Nb(η2-OOH)," which has a lower activation barrier for the oxygen transfer to cyclohexene and is, therefore, the main epoxidizing species. The calculations indicate that protonation is crucial to generating the active species and to increasing POM electrophilicity.
KW - DFT
KW - epoxidation
KW - hydrogen peroxide
KW - Lindqvist structure
KW - niobium
KW - peroxo complex
KW - polyoxometalate
KW - SILANOL-FUNCTIONALIZED POLYOXOTUNGSTATES
KW - SELECTIVE OXIDATION
KW - OLEFIN EPOXIDATION
KW - MOLECULAR-ORBITAL METHODS
KW - SOLVENT SOLUBLE FORMS
KW - MESOPOROUS NIOBIUM-SILICATES
KW - METAL-OXIDE ANALOGS
KW - CATALYTIC-OXIDATION
KW - HYDROGEN-PEROXIDE
KW - ALKENE EPOXIDATION
UR - http://www.scopus.com/inward/record.url?scp=85054353040&partnerID=8YFLogxK
U2 - 10.1021/acscatal.8b02761
DO - 10.1021/acscatal.8b02761
M3 - Article
AN - SCOPUS:85054353040
VL - 8
SP - 9722
EP - 9737
JO - ACS Catalysis
JF - ACS Catalysis
SN - 2155-5435
IS - 10
ER -
ID: 17035726