Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes

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 ( D_SS ∼ 1 cm^-1), but drastically differ in the spin-orbit coupling parameter (from D_SOC = 0.087 cm^-1 for 1a to D_SOC = 0.765 cm^-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of D_SOC 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 D_SOC. 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.