posted on 2018-10-25, 00:00authored byDenis V. Korchagin, Alexander V. Akimov, Anton Savitsky, Sergei V. Chapyshev, Sergey M. Aldoshin, Eugenii Ya. Misochko
Previously unknown
the steric heavy atom effect on magnetic anisotropy
parameters of triplet phenyl nitrenes is reported. The heavy bromine
atom effect is revealed by W-band EPR and theoretical investigations
of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents
in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a (D = 0.9930 cm–1, E = 0.0261 cm–1), 1c (D = 1.244 cm–1, E = 0.030 cm–1), and 1d (D = 1.369
cm–1, E = 0.093 cm–1), generated by the photolysis of the corresponding azides in frozen
methylcyclohexane solution at 5 K, were determined from the W-band
EPR spectra. To clarify the origin of considerable differences in
the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF
calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The
calculations show that all nitrenes have nearly the same magnitudes
of the spin–spin interactions (DSS ∼ 1 cm–1), but drastically differ in the
spin–orbit coupling parameter (from DSOC = 0.087 cm–1 for 1a to DSOC = 0.765 cm–1 for 1f). Comprehensive analysis of various computational data
showed that the magnitude of DSOC of nitrenes 1a–f is the function of the N···Br
distance between the nitrene nitrogen and the neighboring bromine
atoms. The more bulky substituents are located in positions 3 and
5 of nitrenes 1a–1f, the smaller
the N--Br distance and the larger DSOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from
the through-space rather than through-bond electronic interactions
between the bromine atoms and the nitrene unit.