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

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 ZnII-oxyl bond isolated in a zeolite catalyst. Zeolite matrix isolation of this novel ZnII-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 ZnII-oxyl bond has the capability of transferring the oxyl to CO even at 150 K with the generation of a single ZnI species. The superhyperfine interaction of the ZnI 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 ZnII-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 ZnII-oxyl bond having exceptional reactivities is formed by the zeolite lattice ligation.