Highly planar ground triplet biradical 5-trifluoromethyl-1,3-phenylene bis(tert-butyl nitroxide) testing the limit of stabilization

Abstract

The magnetic study on newly developed 5-trifluoromethyl-1,3-phenylene bis(tert-butyl nitroxide) clarified the ground triplet state. Although it can be isolated under ambient conditions, a diamagnetic dimerized product was given, after the benzene rings underwent a [3 + 3] cycloaddition. The ability of complex formation was proven with Gd³⁺, giving a metal-ligand alternating chain compound. The DFT calculation work clearly indicates that this biradical is a robust triplet ligand. The much low-lying triplet state originates in the planar molecular skeleton, thanks to the less hindered m-trifluoromethyl group. From the empirical viewpoint, CF₃ has an unexpectedly stronger stabilizing effect than Ph but weaker than t-Bu, when located at the m-position with respect to the nitroxide group.