Transformation of Ethylzinc Species to Zinc Acetate Mediated by O₂ Activation: Reactive Oxygen‐Centered Radicals Under Control

Abstract

The mechanisms of O 2 activation by main-group metal alkyl compounds and the character of reactive intermediate oxygen species, along with the origins of reaction outcomes, have been a challenge to understand since the Franklands pioneering studies.[1] For decades, these oxygenation reactions have been commonly considered as difficult to control owing to their radical-chain character.[2] Metal alkyl peroxide complexes have been proposed as intermediates in these reactions, [3,4] and the formation of the most commonly observable alkoxides as final products has traditionally been explained by s bond metathesis involving an alkyl peroxide intermediate and the starting metal alkyl complex. [2, 4a] Our recent systematic investigations have not only advanced a plausible hypothesis concerning the mechanism of O 2 activation by organometallic compounds, [4b,c,k,l, 5] but also convincingly demonstrated a high tendency of incipient zinc peroxide species to form relatively stable adducts with the parent zinc alkyl complex.[4m, 5] Moreover, we have revealed a long overlooked decomposition pathway of zinc alkyl peroxides via homolysis of the O À O bond, which is responsible for the formation of oxo complexes. [4l,m, 5] To date, the formation of alkyl peroxide (path a, Scheme 1), alkoxide (path b), and oxo species (path c) in the oxygenation of ZnÀR species has been well-documented. To our knowledge, homolytic MO À OR bond cleavage to give the corresponding metal carboxylate species (path d, scheme 1) has not been considered as a mode of decomposition of both zinc and other metal alkyl peroxides. [6] Our previous studies demonstrated that N,N-donor pyrroloimines, HL 1 , are versatile supporting ligands that can have diverse bonding modes to metal centers. [4m, 7] Moreover, the controlled oxygenation of [RZn(L 1 )] complexes provides a novel zinc alkyl peroxide or a zinc oxo-encapsulated cluster, the formation of which was mediated by the nature of the zinc-bonded alkyl substituents.[4m] Herein we present a novel extension of the latter investigations, which involves 2,5-bis[(2,6-diisopropylphenyl)aldimino]pyrrole (HL 2 ) as a N,N,N-donor supporting ligand, and the oxygenation of the corresponding ethylzinc derivative [EtZn (L 2 )] leading to the zinc acetate species.The reaction of Et 2 Zn with one equivalent of HL 2 in toluene affords the alkyl zinc complex [{EtZn(L 2 )} n ] (1) in quantitative yield. Although we were not able to obtain single crystals of 1, the 1 H NMR data (see below) are fully consistent with the anticipated formula. Molecular weight measurements revealed that 1 occurs predominantly as a dimeric species in a benzene solution. In the next step, a solution of 1 in toluene at À20 8C was treated with an excess of dry dioxygen, the reaction mixture was stirred for 10 minutes, and then the excess O 2 was removed. From this oxygenation reaction, we expected new zinc alkyl peroxide or oxo aggregates. To our surprise, colorless crystals of the zinc carboxylate [{(L 2 )Zn 2 (m-O 2 CMe) 3 } 2 ] (2) deposited...

Superior activity of Ce‐HZSM‐5 catalyst for catalytic oxidation of arsine at low oxygen

“…The conversion of Ce 4+ and Ce 3+ is a cyclic process and requires the participation of chemisorbed oxygen, which leads to a decrease in the amount of chemisorbed oxygen . During the catalytic reaction, the gaseous oxygen was constantly adsorbed on the catalyst, and transformed into lattice oxygen by reacting with the surface ions to obtain electrons . Accordingly, it is speculated that the increase in content of lattice oxygen is connected with the catalytic reaction of AsH 3 removal.…”

Oxygenation Chemistry of Magnesium Alkyls Incorporating β‐Diketiminate Ligands Revisited

“…Since then systematic studies by our group have been focused on the factors that control the facile activation of dioxygen by organometallics with nonredox active centers . For example, we demonstrated for the first time the essential role of the coordination state of the organometallic species as well as convincingly proved the formation of oxo, hydroxide, and carboxylate species as the result of the homolytic cleavage of the alkylperoxide O−O bond. Based on these systematic studies, we have proposed an inner‐sphere electron transfer (ISET) mechanism for the oxygenation of organometallics with nonredox active centers, which strongly contradicts the commonly accepted radical‐chain mechanism .…”

Reaching Milestones in the Oxygenation Chemistry of Magnesium Alkyls: towards Intimate States of O2 Activation and the First Monomeric Well‐Defined Magnesium Alkylperoxide

“…Moreover, despite the structural diversity of isolated metal alkylperoxides with non‐redox‐active centres, knowledge of their modes of further transformation still appears to be fragmentary. Our group's systematic studies on the reactivity of zinc alkyls towards O 2 have demonstrated that not only zinc alkylperoxides and alkoxides, but also zinc peroxide, hydroxide, oxo and even carboxylate species can be isolated from the controlled oxygenation of organozincs. Furthermore, we have provided compelling evidence that a variety of oxygenation products result from the decomposition of (L)ZnOOR species mediated by the O−O bond scission.…”