title: High-spin S = 2 ground state aminyl tetraradicals.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Rajca
authors.forename: Arnon
authors.initials: A
authors.lastname: Olankitwanit
authors.forename: Ying
authors.initials: Y
authors.lastname: Wang
authors.forename: Przemysław J
authors.initials: PJ
authors.lastname: Boratyński
authors.forename: Maren
authors.initials: M
authors.lastname: Pink
authors.forename: Suchada
authors.initials: S
authors.lastname: Rajca