Cerium‐Complex‐Catalyzed Oxidation of Arylmethanols under Atmospheric Pressure of Dioxygen and Its Mechanism through a Side‐On μ‐Peroxo Dicerium(IV) Complex

Abstract

A new Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)2] (1), bearing a dianionic pentadentate ligand with an N3O2 donor set, has been prepared by treating (NH4)2Ce(NO3)6 with the sodium salt of ligand L1. Complex 1 in the presence of TEMPO and 4 Å molecular sieves (MS4 A) has been found to serve as a catalyst for the oxidation of arylmethanols using dioxygen as an oxidant. We propose an oxidation mechanism based on the isolation and reactivity study of a trivalent cerium complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)(THF)] (2), its side-on μ-O2 adduct [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(μ-η(2):η(2)-O2) (3), and the hydroxo-bridged Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(μ-OH)2 (4) as key intermediates during the catalytic cycle. Complex 2 was synthesized by reduction of 1 with 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diazacyclohexadiene. Bubbling O2 into a solution of 2 resulted in formation of the peroxo complex 3. This provides the first direct evidence for cerium-catalyzed oxidation of alcohols under an O2 atmosphere.

Dicerium letterbox-shaped tetraphenolates: f-block complexes designed for two-electron chemistry

“…We have previously reported the synthesis of their U III/III and U IV/IV complexes, and others have reported the use of pTP ligands to support catalysis by both partially deprotonated potassium salts [K 2 H 2 (pTP R )], 30 or V V and Mo VI -imido complexes which have shown catalytic reactivity for the ring-opening polymerisation of ε-caprolactone. 26,31 In the case of the potassium complexes, the authors attributed the remarkable stability of the doubly deprotonated salt [K 2 H 2 (pTP R )] to the formation of potassium-arene interactions with the central arene of the platform, and capacity for the remaining protons to bridge the two aryloxide O atoms on each side. 30 In our hands, the tetra-potassium salt 1 R , ([K 4 (pTP R )] 2 (THF) 11 , R = Me), is readily isolated from the reaction between H 4 (pTP Me ) with four equivalents of KN″ (N″ = N(SiMe 3 ) 2 ) in THF at room temperature, Scheme 1, although we note that it is extremely sensitive to hydrolysis.…”

“…However, it has been shown by EXAFS spectroscopy that the active form of the classical cerium oxidant, aqueous Ce(IV), is dinuclear, 24 so the development of robust and well-defined molecular [Ce] 2 complexes that can combine two readily accessible Ce IV states is a potentially important target for developing catalytic cerium oxidation chemistry. [25][26][27][28][29] Here we report the use of a tetrakis(aryloxide) ligand platform that makes the first robust molecular [Ce] 2 complexes ligated by aryloxides and shows how the letterbox structures strongly favour the Ce IV oxidation state. We are only able to observe two-electron separated redox states in the system, a feature not usually achievable in molecular f-block chemistry.…”

Europium‐Catalyzed Aerobic Oxidation of Alcohols to Aldehydes/Ketones and Photoluminescence Tracking

“…[16] Both Eu(II) and Eu(III) salts are commercially available from multiple chemical suppliers. [21,22] The redox cycling of europium should occur in the presence of an external oxidant since the standard reduction potential of europium (E red : Eu 3 + /Eu 2 + ) is an adequate value (À0.35 V versus NHE, and it depends on anion, ligand, and solvent). Recently, a variety of europium organometallic complexes with Eu 2 + and/or Eu 3 + have been intensively studied because of their unique photochemical properties, [16,17] and the oxidation state changes of europium were successfully applied to catalytic organic transformations.…”

“…[21] Moreover, for the lanthanides, Ce-based oxidation cycle for aerobic oxidation have been recently studied by Mashima and co-workers. [21,22] The redox cycling of europium should occur in the presence of an external oxidant since the standard reduction potential of europium (E red : Eu 3 + /Eu 2 + ) is an adequate value (À0.35 V versus NHE, and it depends on anion, ligand, and solvent). [17,23] Herein, we have developed a europium-catalyzed aerobic oxidation of alcohols to corresponding aldehydes or ketones with external catalytic oxidants.…”

Pyrazine Radical Cations as a Catalyst for the Aerobic Oxidation of Amines

“…The redox‐active pyrazine core can also be useful as an efficient antibiotic, as hole‐transporting material in electroluminescent diodes, or as single electron transfer (SET) reagent . Especially the latter methodology has recently gained much attention as 1,4‐DHP proved to be an efficient single‐electron reducing agent in a number of applications such as catalysis,[9b], reduction of organic compounds, redox‐active ligands, and so on.…”