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), CH
2CH
2OCH
3 (
5))
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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), CH
2CH
2OCH
3 (
5)) were prepared, characterized (including the single crystal X-ray analysis of
3) and studied as catalysts in the mild oxidation of alkanes with H
2O
2 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 CD
3CN solvent instead of CH
3CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex
4-d
32 was prepared and showed higher stability under the same conditions. The complexes
1 and
4 showed different reactivity in the formation of H
218O from
18O
2 in acetonitrile solutions.
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