Four‐Center Two‐Electron Bond in 3,6‐Di(tert‐butyl)naphthalene‐1,8‐diyl Bis(tert‐butyl Nitroxide)

Three polymorphs of the title compound were found, showing a head‐to‐tail (N−O)₂ four‐membered ring. The α and β phases underwent a reversible structural phase transition across ca. 140 K, while the γ phase was independent. The interatomic O ⋅ ⋅ ⋅ N distances are 79–81 % of the sum of the van der Waals radii, consistent with the diamagnetic properties observed in the magnetic and ESR studies for the α phase. A thermally accessible zero‐field splitting pattern was recorded in solid‐state ESR, and applying a singlet‐triplet (ST) model afforded 2J_obs/k_B=−1.87(8)×10³ K (H=−2JS₁ ⋅ S₂). The SQUID results showed 2J_obs/k_B=–2.159(5)×10³ K. Thanks to the introduction of two tert‐butyl groups on the naphthalene core, the generation of triplet species is reversible below 430 K. The residual electron density map from the crystallographic data and the QTAIM analysis indicate a four‐center two‐electron σ‐bond nature in (N−O)₂. Two DFT‐optimized structures were given, featuring 2J_calc/k_B=−8864 K as a covalent model and −974 K as a biradical model. Thus, we can propose a mechanism involving two‐step equilibria; a chemical equilibrium between the covalent and biradical forms and a subsequent spin equilibrium between the S and T states in a biradical.