Abstract
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.
Highlights
Selective transformation of inactive C–H bonds into functional groups is a challenging class of reactions with great potential for fine organic synthesis, late stage drug functionalization and other fields [1,2,3,4,5]
In the case of alkane functionalization, C–H-attacking species may be active enough to react with a catalyst causing its degradation and limiting the turnover numbers (TONs)
We have studied the chemistry and catalytic behaviour of the copper coordination compounds [Cu{NH=C(OR)NC(OR)=NH}2 ] with a bidentate alkoxy-1,3,5-triazapentadiene ligands having various substituents
Summary
Selective transformation of inactive C–H bonds into functional groups is a challenging class of reactions with great potential for fine organic synthesis, late stage drug functionalization and other fields [1,2,3,4,5]. Due to the high inertness of C–H bonds, especially sp ones, their activation often requires harsh conditions and/or strong oxidizing agents [6]. The stability of coordination compounds (catalysts) under the catalytic conditions is a critical parameter for achieving high efficiency of the catalytic system [7]. In the case of alkane functionalization, C–H-attacking species may be active enough to react with a catalyst causing its degradation and limiting the turnover numbers (TONs). Nature found a solution to this problem by continuous regeneration of the catalysts (enzymes). Such an approach could be hardly implemented in the artificial catalytic systems, leaving the preparation of stable and robust catalysts as an obvious alternative
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