Reactivity of a Cobalt(III)–Hydroperoxo Complex in Electrophilic Reactions

Abstract

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [CoIII(Me3-TPADP)(O2)]+ (Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [CoII(Me3-TPADP)(CH3CN)2]2+ with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+. The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm−1 for 16O2H samples (803 cm−1 for 18O2H samples) and its Co-O vibrational energy was observed at 571 cm−1 for 16O2H samples (551 cm−1 for 18O2H samples; 568 cm−1 for 16O22H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density function theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals

“…The deuterated analogue [Co III (TBDAP)­(O 2 D)­(MeCN)] 2+ displayed a 6 cm –1 downshift and a 3 cm –1 upshift of the 560 and 856 cm –1 features, respectively. The result is quite similar to those of previously reported M­(III)–O 2 H species . These results support the formation of hydroperoxide ligand in 4 under strong Lewis acidic and Brønsted acidic conditions.…”

Selective oxygenation of C–H and CC bonds with H2O2 by high-spin cobalt(ii)-carboxylate complexes

“…This kind of electrophilic species has been obtained after the oxidation of a cobalt complex. [76][77][78][79][80] The Eyring plot, obtained from kinetic analysis of the reaction of complex 2 with 10 equiv. of H 2 O 2 at variable temperature, gives the activation parameters of ΔH ‡ = 47.29 kJ mol −1 and ΔS ‡ = −147.98 kJ mol −1 K −1 (Fig.…”

Experimental and Computational Investigation of the Aerobic Oxidation of a Late Transition Metal-Hydride

“…While reactions of metal-hydrides with oxygen are typically proposed to proceed via insertion of oxygen into the metal-hydride to generate metal-hydroperoxo species, it is often the case that the metal-hydroperoxo species is not the final product or even observed under the reaction conditions. Metal-hydroperoxo complexes can be quite reactive, particularly under acidic conditions, and relatively few such species have been isolated and characterized. ,− The following reactivity has been commonly described for hydroperoxo groups (Scheme ): (a) O atom transfer from MOOH (to oxidize a substrate) resulting in a metal-hydroxo which is then protonated to release water, − (b) protonation of the MOOH α-oxygen resulting in a nonoxygenated metal species and hydrogen peroxide, − and (c) protonation of the hydroperoxo β-oxygen giving rise to a high-valent metal oxo and water (Scheme ). − In the reaction of 1 with O 2 , an Ir–OOH species is a proposed intermediate on the pathway to the final product 2 (Scheme ).…”