17O-ESR Evidence for Zeolite Matrix Isolation of a Square Planar ZnO3 Ring Radical with C2v Symmetry

Abstract

Atomic-level insights into the geometric and electronic states of the metal–ozonide sequence remain scarce. We have recently found that a zeolite catalyst has a local environment to isolate the extremely rare mononuclear Zn^II–ozonide species. In the present study, each atomic oxygen radical of the Zn^II–ozonide species was selectively labeled with ¹⁷O, and each local environment was analyzed by electron spin resonance (ESR) spectroscopy at an X-band frequency, computer simulations of ESR spectra, density functional theory (DFT) calculations, and ab initio molecular dynamics (AIMD) simulations. The two types of ¹⁷O hyperfine structures assignable to the C_2v geometry of a square planar ZnO₃ ring radical were obtained experimentally. The DFT calculations ascertained the ground-state C_2v geometry of the Zn^II–ozonide adduct. This model provides ¹⁷O-hyperfine coupling constants that correspond well with the experimental parameters, supporting the generation of the C_2v ZnO₃ ring radical. The C_2v ZnO₃ ring radical is stable even at around room temperature, as evidenced by the similarity in the ESR spectra collected at 300 and 4 K. Such an unusual stability was supported by AIMD simulations, where C_2v geometry was preserved at least for 50 ps at 300 K. Molecular orbital analyses showed that the ozonide species is stabilized via the highly polarized Zn^II–(O₃^–·) bonds. These interactions led to the polarization of two O–O bonds in the ozonide adduct, through which both side oxygens becomes anionic states, but the central oxygen becomes a cationic state. The zeolite lattice plays a pivotal role in constraining the effective charge of Zn close to (+2) and thereby stabilizing such a highly polarized Zn^II–(O₃^–·) bond. These novel findings suggest that the zeolite lattice has the potential as the ligand to create reactive metal–oxygen radicals with an unprecedented shape and ionic states.