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A Comparative Study of the Catalytic Behaviour of Alkoxy-1,3,5-Triazapentadiene Copper(II) Complexes in Cyclohexane Oxidation

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
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Inorganics 2019, 7(7), 82; https://doi.org/10.3390/inorganics7070082
Received: 31 May 2019 / Revised: 22 June 2019 / Accepted: 26 June 2019 / Published: 29 June 2019
(This article belongs to the Special Issue First-Row Transition Metal Complexes)
The mononuclear copper complexes [Cu{NH=C(OR)NC(OR)=NH}2] with alkoxy-1,3,5-triazapentadiene ligands that have different substituents (R = Me (1), Et (2), nPr (3), iPr (4), CH2CH2OCH3 (5)) were prepared, characterized (including the single crystal X-ray analysis of 3) and studied as catalysts in the mild oxidation of alkanes with H2O2 as an oxidant, pyridine as a promoting agent and cyclohexane as a main model substrate. The complex 4 showed the highest activity with a yield of products up to 18.5% and turnover frequency (TOF) up to 41 h−1. Cyclohexyl hydroperoxide was the main reaction product in all cases. Selectivity parameters in the oxidation of substituted cyclohexanes and adamantane disclosed a dominant free radical reaction mechanism with hydroxyl radicals as C–H-attacking species. The main overoxidation product was 6-hydroxyhexanoic acid, suggesting the presence of a secondary reaction mechanism of a different type. All complexes undergo gradual alteration of their structures in acetonitrile solutions to produce catalytically-active intermediates, as evidenced by UV/Vis spectroscopy and kinetic studies. Complex 4, having tertiary C–H bonds in its iPr substituents, showed the fastest alteration rate, which can be significantly suppressed by using the CD3CN solvent instead of CH3CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex 4-d32 was prepared and showed higher stability under the same conditions. The complexes 1 and 4 showed different reactivity in the formation of H218O from 18O2 in acetonitrile solutions. View Full-Text
Keywords: copper alkoxy-1,3,5-triazapentadiene complexes; alkane functionalization; protection by deuteration; 18O isotopic labelling copper alkoxy-1,3,5-triazapentadiene complexes; alkane functionalization; protection by deuteration; 18O isotopic labelling
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Nesterova, O.V.; Kopylovich, M.N.; Nesterov, D.S. A Comparative Study of the Catalytic Behaviour of Alkoxy-1,3,5-Triazapentadiene Copper(II) Complexes in Cyclohexane Oxidation. Inorganics 2019, 7, 82.

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