A low-temperature oxyl transfer to carbon monoxide from the Zn-oxyl site in a zeolite catalyst

• Published in: Inorganic chemistry frontiers Vol. 8; no. 2; pp. 319 - 328
• Main Authors: Oda, Akira, Kumagai, Jun, Ohkubo, Takahiro, Kuroda, Yasushige
• Format: Journal Article
• Language: English
• Published: 26.01.2021
• ISSN: 2052-1553
• DOI: 10.1039/d0qi01112f

An atomic O radical anion bound to a metal ion (metal-oxyl) is a key intermediate in a variety of oxidative reactions. Understanding its structure-reactivity relationship is highly desirable for the rational design of challenging oxidative transformation processes. However, due to the difficulty of analysis, even the identification of an oxyl is a challenging subject especially in the research field of heterogeneous catalysts. Here, we report for the first time a low-temperature oxyl transfer to CO from the Zn II -oxyl bond isolated in a zeolite catalyst. Zeolite matrix isolation of this novel Zn II -oxyl bond allows us to observe the unique spectroscopic probes of the oxyl: a vibronically-resolved spectrum and ESR signatures. Using the oxyl-selective spectroscopic probes, we successfully demonstrated that the Zn II -oxyl bond has the capability of transferring the oxyl to CO even at 150 K with the generation of a single Zn I &z.rad; species. The superhyperfine interaction of the Zn I &z.rad; species with the framework Al atom, observed during the oxyl-transfer reaction, provided direct experimental evidence that the oxyl-functionality emerged at the framework Al site. DFT calculations showed that the Zn II -oxyl bond, which is constrained by the zeolite lattice ligation, acts as a superior electron donor toward CO at the rate-determining step of the oxyl-transfer reaction and effectively reduces the barrier to be <5 kJ mol −1 . Based on the results obtained in the present study as well as the previous work, we further deepen the understanding of why the abnormal Zn II -oxyl bond having exceptional reactivities is formed by the zeolite lattice ligation. We demonstrated that the Zn II -oxyl bond specifically formed by the zeolite lattice ligation has the capability of transferring the oxyl to CO even at 150 K with the generation of a single Zn I &z.rad; species.