Isolation of the Triplet Ground State Aminyl Diradical

Abstract

Nitrogen-centered (aminyl) radicals are typically short-lived, and have been detected as intermediates in a variety of chemical and biological processes. [1,2] To the best of our knowledge, only two known stable aminyl radicals have been reported (more than 30 years ago), [3,4] while a few others have recently been isolated as aminyl metal complexes and as heteroatom (e.g., O, S) stabilized aminyls. [5][6][7] Aminyl diradicals, in which the radical centers are connected through m-phenylene to form a planar crossconjugated p system devoid of stabilizing heteroatoms, are predicted by computational studies to possess a strong preference for a high-spin, triplet (S = 1) ground state, that is, a singlet-triplet energy gap (DE ST ) that is an order of magnitude greater than the thermal energy (RT) at room temperature. [8,9] Such a nitrogen-centered diradical with large DE ST values and persistence at room temperature may be relevant to the design of strongly paramagnetic relaxation reagents [10,11] as well as the development of magnetic materials and devices. [12,13] To date, the two known examples of aminyl diradicals have been examined only at very low temperature. Platz and coworkers first reported an aminyl diradical in a 2-methyltetrahydrofuran (2-MeTHF) matrix at about À195 8C. [14] Recently, we prepared an aminyl diradical with an S = 1 ground state that was persistent in solution at about À100 8C. [9] Herein we report the isolation of an S = 1 ground state aminyl diradical with a large DE ST value and persistence at room temperature. The synthesis of the aminyl diradicals 1 and 2 demonstrates the balance between maintaining an effective 2p p -2p p overlap that enhances electron spin-spin interactions (exchange coupling), and the adequate protection of the carbon and nitrogen atoms with significant spin densities within the diazapentacene backbone in order to achieve the stability required for isolation. This result is significant as the 2p p -2p p overlap is generally severely perturbed by the increased steric bulk that is required for achieving radical stability, and thus a combination of a large DE ST value and persistence at room temperature is not commonly accomplished. In addition, we detected an unexpected self-associated dimer of the S = 1 aminyl diradical 2 in 2-MeTHF solution; this species appears to resemble a p dimer. Notably, characterization of the dimer is facilitated by its unique zero-field splitting tensor (D). p Dimers of neutral organic radicals in solution are relatively rare and the reports of such phenomena are limited to those of S = 1 = 2 radicals. [15] Diradicals 1 and 2 were prepared from the corresponding diamines 5 and 6, which were obtained from diamine 3 (Scheme 1). [16] The structure of diamine 6 was confirmed by X-ray crystallography, and shows overcrowding of the bulky tert-undecyl groups so that the C À C bond axes that connect the two outer pendant benzene rings to the backbone bend outward (Figure 1). [17] This bending creates more space for the pendant benzene rings in order...

Desaturation via Redox-Neutral Hydrogen Transfer Process: Synthesis of 2-Allyl Anilines, Mechanism and Applications

“…These experiments indicated that the visible light facilitates the reaction pathway via the radical process. On basis of the literature reports (Boraty nski et al, 2010;Gallagher et al, 2019;Hommelsheim et al, 2019;Jurberg and Davies, 2018;Snyder and Dougherty, 1989;Zhang et al, 2019b) and the EPR signal, we deduced that the radical process may be involved under the visible light conditions.…”

Chiral π-Conjugated Double Helical Aminyl Diradical with Triplet Ground State

“…Aminyl diradicals rac-, (PP)-and (MM)-2 2• -C2 are generated from the corresponding tetraamines using our established two-step procedure: [46][47][48][49] (1) the reaction of tetraamine with n-BuLi (2+ equiv) in THF/hexane provides the corresponding dianion, and (2) following solvent exchange at low temperature to 2-MeTHF, the dianion is oxidized to the diradical by addition of I2 via vacuum transfer, typically, with subsequent monitoring of reaction progress by EPR spectroscopy (Scheme 2). Scheme 2.…”

Recent advances in the syntheses of radical-containing macromolecules

“…Unfortunately, high‐spin molecules with large Δ E ST also tend to be much less kinetically stable than those with lower Δ E ST . Thus, while aminyl radicals display relatively high Δ E ST , continuing effort has been placed in order to increase the stability of these molecules such that they may be persistent at ambient conditions …”

“…These molecules were incredibly unstable, only persistent in solution at −195 °C. In order to improve the stability of such systems, a number of synthetic schemes were employed to stabilize and sterically shield the spin centers . In an example synthesis, diisopropenyl‐1,3‐dibromobenzene and 4‐ tert ‐butylaniline were coupled by Pd(OAc) 2 catalyzed CN coupling in the presence of sodium tert ‐butoxide ( t ‐BuONa) and tri( tert ‐butyl) phosphate ( t ‐Bu 3 P) .…”

“…The resulting diamine was annelated by H 3 PO 4 via electrophilic aromatic substitution, ultimately forming a planarized diamine. From here, the diamine was brominated by N ‐bromosuccinimide (NBS), and the bromine was then utilized in a Suzuki coupling reaction to attach additional steric groups . In one case, 4‐ tert ‐butylphenylboronic acid was bound by reacting with catalytic Pd(PPh 3 ) 4 and sodium carbonate.…”