High-Spin S = 2 Ground State Aminyl Tetraradicals

Aminyl tetraradicals with planar tetraazanonacene backbones have quintet (S = 2) ground states and do not show any detectable thermal population of the low-spin excited states up to the highest temperature investigated (100 K) in the 2-methyltetrahydrofuran (2-MeTHF) matrix. This indicates that the nearest electronic excited state (triplet) is at least ∼0.3 kcal mol–1 higher in energy, that is, the triplet–quintet energy gap, ΔE TQ > 0.3 kcal mol–1, which is consistent with the broken-symmetry-DFT-computed ΔE TQ of about 5 kcal mol–1. In concentrated (ca. 1–10 mM) solutions of tetraradical 4 in 2-MeTHF at 133 K, a fraction of tetraradicals form a dimer (association constant, K assoc ≈ 60 M–1), with a weak, antiferromagnetic exchange coupling, J/k ≈ −0.1 K ∼ 0.2 cal mol–1, between the S = 2 tetraradicals. This weak intradimer exchange coupling is expected for two tetraradicals at the distance of about 6 Å. The most sterically shielded tetraradical 5 in 2-MeTHF has a half-life of 1 h at room temperature; the product of its decay is the corresponding tetraamine, suggesting that the hydrogen atom abstraction from the solvent is primarily responsible for the decomposition of the tetraradical.