Issue 6, 2022

Selective oxygenation of C–H and C[double bond, length as m-dash]C bonds with H2O2 by high-spin cobalt(ii)-carboxylate complexes

Abstract

Four cobalt(II)–carboxylate complexes [(6-Me3-TPA)CoII(benzoate)](BPh4) (1), [(6-Me3-TPA)CoII(benzilate)](ClO4) (2), [(6-Me3-TPA)CoII(mandelate)](BPh4) (3), and [(6-Me3-TPA)CoII(MPA)](BPh4) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me3-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H2O2-dependent selective oxygenation of C–H and C[double bond, length as m-dash]C bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C–H bonds and epoxidation of alkenes. An electrophilic cobalt–oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)–acetonitrile complex [(6-Me3-TPA)CoII(CH3CN)2](ClO4)2 (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H2O2 followed by the oxidation of substrates by a metal-based oxidant.

Graphical abstract: Selective oxygenation of C–H and C [[double bond, length as m-dash]] C bonds with H2O2 by high-spin cobalt(ii)-carboxylate complexes

Supplementary files

Article information

Article type
Paper
Submitted
06 Jul 2021
Accepted
22 Dec 2021
First published
30 Dec 2021

Dalton Trans., 2022,51, 2480-2492

Selective oxygenation of C–H and C[double bond, length as m-dash]C bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes

I. Ghosh, B. Chakraborty, A. Bera, S. Paul and T. K. Paine, Dalton Trans., 2022, 51, 2480 DOI: 10.1039/D1DT02235K

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