Identification of a Stable Zn^II–Oxyl Species Produced in an MFI Zeolite and Its Reversible Reactivity with O₂ at Room Temperature

Abstract

Although a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidation reactions, such as O-O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable Zn -oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O at room temperature. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chemical calculations, namely analyses of the vibronic Franck-Condon progressions and the ESR signal features of both Zn -oxyl and Zn -ozonide species formed during this reversible process. Molecular orbital analyses revealed that the reversible reaction between a Zn -oxyl species and an O molecule proceeds via a radical O-O coupling-decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O-O bond generation process.

How to Constrain Metal–Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)–Oxyl and Ga(III)–Oxyl Bonds Isolated in Zeolite Matrix

“…Oxyl bound to a Zn II ion has been recently identified in the zeolite catalyst. 27,28 Despite the poor ESR sensitivity due to the fast relaxation behavior, the Zn II −oxyl bond shows a vibronic progression that provides an electronic structure description of the Zn II −oxyl σ bond. 27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms.…”

“…27,28 Despite the poor ESR sensitivity due to the fast relaxation behavior, the Zn II −oxyl bond shows a vibronic progression that provides an electronic structure description of the Zn II −oxyl σ bond. 27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms. 27,28 Considering the charge balance, it has been suggested that a monovalent ion-exchangeable site (Scheme 1, left) acts as the anchor site to constrain the monovalent Zn II −oxyl bond.…”

“…27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms. 27,28 Considering the charge balance, it has been suggested that a monovalent ion-exchangeable site (Scheme 1, left) acts as the anchor site to constrain the monovalent Zn II −oxyl bond. 27,28 In this study, we newly observe an isovalent Ga III −oxyl bond and define its geometric and electronic structures based on the vibronically resolved spectrum coupled with the DFT calculations.…”

“…27,28 Considering the charge balance, it has been suggested that a monovalent ion-exchangeable site (Scheme 1, left) acts as the anchor site to constrain the monovalent Zn II −oxyl bond. 27,28 In this study, we newly observe an isovalent Ga III −oxyl bond and define its geometric and electronic structures based on the vibronically resolved spectrum coupled with the DFT calculations. It was shown that the formation of the Ga III −oxyl bond requires a specific site, namely, the divalent ion-exchangeable site (Scheme 1, Photoreaction of a metal−hydride with O 2 was recently found to be an effective means of synthesis of the oxygen radical anions bound to the metal site in the zeolite.…”

Revisiting Alkane Hydroxylation with m‐CPBA (m‐Chloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes

“…They also reported that the generated Zn II À O * species can oxidize methane (BDE C-H = 105 kcal/mol) to methanol, demonstrating extremely high HAA reactivity of the oxylcomplex of late-transition metal ions. [21] Moreover, Schwarz and co-workers demonstrated that NiO + (formally [Ni II À O * ] + ) is an efficient oxidant for methane hydroxylation in the gas phase reaction. [7] Judging from those reports, we propose that not only the aroyloxyl radical (ArC(O)O * ) but also (L)Ni II À O * species generated by the OÀ O homolysis of the nickel(II)-m-CPBA adduct a is a reactive oxidant in the catalytic alkane oxidation reaction is 1,1,3,3tetramethylguanidine.…”