The role of molecular oxygen in the iron(III)-promoted oxidative dehydrogenation of amines

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2015
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Saucedo-Vásquez, Juan Pablo
Sosa-Torres, Martha Elena
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Dalton Transactions. 2015, 44(12), pp. 5510-5519. ISSN 1477-9226. eISSN 1477-9234. Available under: doi: 10.1039/C4DT03606A
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A mechanistic study is presented of the oxidative dehydrogenation of the iron(III) complex [FeIIIL3]3+, 1, (L3 = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex [FeIIL4]2+, 2, (L4 = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(III) complex 1. Reduced oxygen species, e.g., superoxide, (O2˙) and peroxide (O22−), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, −d[(FeIIIL3)3+]/dt = kOD[(FeIIIL3)3+][EtO][O2], with kOD = 3.80 ± 0.09 × 107 M−2 s−1 (60 °C, μ = 0.01 M). The reduction O2 → O2˙ represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H2O2, replacing O2 as the oxidant, the experimental rate law depended on [H2O2]: −d[(FeIIIL3)3+]/dt = kH2O2[(FeIIIL3)3+][H2O2]), with kH2O2 = 6.25 ± 0.02 × 10−3 M−1 s−1. In contrast to the reaction performed under N2, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(III) complex 1 with O2. Under N2, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(II)-monoimine complex [FeIIL4]2+ and the iron(II)-amine complex [FeIIL3]2+ in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O2, with kEtO− = 3.02 ± 0.09 × 105 M−1 s−1 (O2) compared to kEtO− = 4.92 ± 0.01 × 104 M−1 s−1 (N2), documenting the role of molecular oxygen in the dehydrogenation reaction.

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ISO 690SAUCEDO-VÁSQUEZ, Juan Pablo, Peter M. H. KRONECK, Martha Elena SOSA-TORRES, 2015. The role of molecular oxygen in the iron(III)-promoted oxidative dehydrogenation of amines. In: Dalton Transactions. 2015, 44(12), pp. 5510-5519. ISSN 1477-9226. eISSN 1477-9234. Available under: doi: 10.1039/C4DT03606A
BibTex
@article{SaucedoVasquez2015molec-31184,
  year={2015},
  doi={10.1039/C4DT03606A},
  title={The role of molecular oxygen in the iron(III)-promoted oxidative dehydrogenation of amines},
  number={12},
  volume={44},
  issn={1477-9226},
  journal={Dalton Transactions},
  pages={5510--5519},
  author={Saucedo-Vásquez, Juan Pablo and Kroneck, Peter M. H. and Sosa-Torres, Martha Elena}
}
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    <dcterms:abstract xml:lang="eng">A mechanistic study is presented of the oxidative dehydrogenation of the iron(III) complex [Fe&lt;sup&gt;III&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;]&lt;sup&gt;3+, &lt;/sup&gt;1, (L&lt;sup&gt;3&lt;/sup&gt; = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex [Fe&lt;sup&gt;II&lt;/sup&gt;L&lt;sup&gt;4&lt;/sup&gt;]&lt;sup&gt;2+&lt;/sup&gt;, 2, (L&lt;sup&gt;4&lt;/sup&gt; = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(III) complex 1. Reduced oxygen species, e.g., superoxide, (O&lt;sub&gt;2&lt;/sub&gt;˙&lt;sup&gt;−&lt;/sup&gt;) and peroxide (O&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;2−&lt;/sup&gt;), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, −d[(Fe&lt;sup&gt;III&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;)&lt;sup&gt;3+&lt;/sup&gt;]/dt = k&lt;sub&gt;OD&lt;/sub&gt;[(Fe&lt;sup&gt;III&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;)&lt;sup&gt;3+&lt;/sup&gt;][EtO&lt;sup&gt;−&lt;/sup&gt;][O&lt;sub&gt;2&lt;/sub&gt;], with k&lt;sub&gt;OD&lt;/sub&gt; = 3.80 ± 0.09 × 107 M&lt;sup&gt;−2&lt;/sup&gt; s&lt;sup&gt;−1&lt;/sup&gt; (60 °C, μ = 0.01 M). The reduction O&lt;sub&gt;2&lt;/sub&gt; → O&lt;sub&gt;2&lt;/sub&gt;˙&lt;sup&gt;−&lt;/sup&gt; represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;, replacing O&lt;sub&gt;2&lt;/sub&gt; as the oxidant, the experimental rate law depended on [H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;]: −d[(Fe&lt;sup&gt;III&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;)&lt;sup&gt;3+&lt;/sup&gt;]/dt = kH&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;[(Fe&lt;sup&gt;III&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;)&lt;sup&gt;3+&lt;/sup&gt;][H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;]), with kH&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; = 6.25 ± 0.02 × 10&lt;sup&gt;−3&lt;/sup&gt; M&lt;sup&gt;−1&lt;/sup&gt; s&lt;sup&gt;−1&lt;/sup&gt;. In contrast to the reaction performed under N&lt;sub&gt;2&lt;/sub&gt;, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(III) complex 1 with O&lt;sub&gt;2&lt;/sub&gt;. Under N&lt;sub&gt;2&lt;/sub&gt;, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(II)-monoimine complex [Fe&lt;sup&gt;II&lt;/sup&gt;L&lt;sup&gt;4&lt;/sup&gt;]&lt;sup&gt;2+&lt;/sup&gt; and the iron(II)-amine complex [Fe&lt;sup&gt;II&lt;/sup&gt;L&lt;sup&gt;3&lt;/sup&gt;]&lt;sup&gt;2+&lt;/sup&gt; in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O&lt;sub&gt;2&lt;/sub&gt;, with k&lt;sub&gt;EtO&lt;/sub&gt;− = 3.02 ± 0.09 × 10&lt;sup&gt;5&lt;/sup&gt; M&lt;sup&gt;−1&lt;/sup&gt; s&lt;sup&gt;−1 &lt;/sup&gt;(O&lt;sub&gt;2&lt;/sub&gt;) compared to k&lt;sub&gt;EtO&lt;/sub&gt;− = 4.92 ± 0.01 × 104 M&lt;sup&gt;−1&lt;/sup&gt; s&lt;sup&gt;−1&lt;/sup&gt; (N&lt;sub&gt;2&lt;/sub&gt;), documenting the role of molecular oxygen in the dehydrogenation reaction.</dcterms:abstract>
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