| Summary: | The mononuclear copper complexes [Cu{NH=C(OR)NC(OR)=NH}<sub>2</sub>] with alkoxy-1,3,5-triazapentadiene ligands that have different substituents (R = Me (<b>1</b>), Et (<b>2</b>), <i><sup>n</sup></i>Pr (<b>3</b>), <i><sup>i</sup></i>Pr (<b>4</b>), CH<sub>2</sub>CH<sub>2</sub>OCH<sub>3</sub> (<b>5</b>)) were prepared, characterized (including the single crystal X-ray analysis of <b>3</b>) and studied as catalysts in the mild oxidation of alkanes with H<sub>2</sub>O<sub>2</sub> as an oxidant, pyridine as a promoting agent and cyclohexane as a main model substrate. The complex <b>4</b> showed the highest activity with a yield of products up to 18.5% and turnover frequency (TOF) up to 41 h<sup>−1</sup>. 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 <b>4</b>, having tertiary C−H bonds in its <i><sup>i</sup></i>Pr substituents, showed the fastest alteration rate, which can be significantly suppressed by using the CD<sub>3</sub>CN solvent instead of CH<sub>3</sub>CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex <b>4</b>-d<sub>32</sub> was prepared and showed higher stability under the same conditions. The complexes <b>1</b> and <b>4</b> showed different reactivity in the formation of H<sub>2</sub><sup>18</sup>O from <sup>18</sup>O<sub>2</sub> in acetonitrile solutions.
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