posted on 2021-02-25, 15:11authored byAkira Oda, Jun Kumagai, Kyoichi Sawabe, Takahiro Ohkubo, Yasushige Kuroda, Atsushi Satsuma
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 ZnII–ozonide species.
In the present study, each atomic oxygen radical of the ZnII–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 C2v geometry of a square planar ZnO3 ring radical
were obtained experimentally. The DFT calculations ascertained the
ground-state C2v geometry of the ZnII–ozonide adduct. This model provides 17O-hyperfine coupling constants that correspond well with the experimental
parameters, supporting the generation of the C2v ZnO3 ring radical. The C2v 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 C2v 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 ZnII–(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 ZnII–(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.