Efficient Room‐Temperature Oxidation of Hydrocarbons Mediated by Tricopper Cluster Complexes with Different Ligands

Abstract

Six tricopper cluster complexes of the type [CuICuICuI(L)]1+ supported by a series of multidentate ligands (L) have been developed as oxidation catalysts. These complexes are capable of mediating the facile oxygen‐atom transfer to hydrocarbon substrates like cyclohexane, benzene, and styrene (C6H12, C6H6 and C8H8) upon activation by hydrogen peroxide at room temperature. The processes are catalytic with high turnover frequencies (TOF), efficiently oxidizing the substrates to their corresponding alcohols, aldehydes, and ketones in moderate to high yields. The catalysts are robust with turnover numbers (TON) limited only by the availability of hydrogen peroxide used to drive the catalytic turnover. The TON is independent of the substrate concentration and the TOF depends linearly on the hydrogen peroxide concentration when the oxidation of the substrate mediated by the activated tricopper complex is rapid. At low substrate concentrations, the catalytic system exhibits abortive cycling resulting from competing reduction of the activated catalyst by hydrogen peroxide. This behaviour of the system is consistent with activation of the tricopper complex by hydrogen peroxide to generate a strong oxidizing intermediate capable of a facile direct “oxygen‐atom” transfer to the substrate upon formation of a transient complex between the activated catalyst and the substrate. Some substrate specificity has also been noted by varying the ligand design. These properties of the tricopper catalyst are characteristic of many enzyme systems, such as cytochrome P450, which participate in biological oxidations.

Transition Metal Complexes and the Activation of Dioxygen

“…A variety of multicopper catalysts for hydrocarbon oxidations have been described [366,367], but while stable Cu(I) or Cu(II) precursors or 'resting states' are often identified, the nature of the multicopper-oxygen intermediates responsible for attacking the C-H bonds are usually only speculated on the basis of indirect and/or ambiguous spectroscopic or theoretical evidence [368][369][370]. In studies of dioxygen activation by more than two Cu(I) sites that target possible intermediates, tricopper species often comprise the bis(μ-oxo)tricopper(II,II,III) core (114, Figure 53).…”

Development of the Tricopper Cluster as a Catalyst for the Efficient Conversion of Methane into MeOH

“…Analysis of the products of the catalytic oxidation of C 6 H 6 , C 8 H 8 , and C 6 H 12 by H 2 O 2 mediated by various tricopper complexes in MeCN at room temperature. [38] . A) Evolution of the methane-oxidation reaction; inset shows a pseudo-first-order kinetic plot with rate constant k 1 = 0.065 min À1 (C 0 represents the initial concentration of the fully reduced tricopper complex and C represents the concentration of MeOH that is produced at any given time), with the best straight-line fit to the data.…”

“…Some substrate specificity in the oxidation has been noted by varying the ligand, as shown in Figure 10. [38] . [37] 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim…”

“…Typically, the TOF was 1 10 À2 s À1 and it was more or less independent of the nature of the substrate. [37,38] The catalytic cycle shown in Figure 8 has been verified by determining the quantity of H 2 O 2 that was consumed to yield the total amounts of the products that were formed over the time course of the experiments. [37] Typically, about three molecules of H 2 O 2 were required per O-atom-transfer or oxidation step, consistent with the hypothesis that H 2 O 2 was acting as a reductant to convert the "spent" catalyst back into the Cu I Cu I Cu I complex.…”

Die direkte katalytische Oxidation von Methan zu Methanol – eine kritische Beurteilung

“…[143] AufG rundlage von Redoxpotentiometrie und EPR-Spektroskopie wurde zunächst angenommen, dass das aktive Zentrum ein ferromagnetisch gekoppelter Trikupfercluster sei. [148,149] Es konnte schließlich gezeigt werden, dass ein pMMO-abgeleiteter Trikupfer-Peptid-Komplex aktiv fürd ie Umwandlung von Methan in Methanol ist. [146] Balasubramanian et al postulierten anschließend, dass pMMO ein Kupferzentrum mit einem aktiven zweikernigen Kupferzentrum sei (Abbildung 7b).…”