PEGylated, Water-Soluble, Stable Aminyl Radical

From Stable Radicals to Thermally Robust High-Spin Diradicals and Triradicals

Stable radicals and thermally robust high-spin di- and triradicals have emerged as important organic materials due to their promising applications in diverse fields. New fundamental properties, such as SOMO/HOMO inversion of orbital energies, are explored for the design of new stable radicals, including highly luminescent ones with good photostability. A relation with the singlet-triplet energy gap in the corresponding diradicals is proposed. Thermally robust high-spin di- and triradicals, with energy gaps that are comparable to or greater than a thermal energy at room temperature, are more challenging to synthesize but more rewarding. We summarize a number of high-spin di- and triradicals, based on nitronyl nitroxides that provide a relation between the experimental pairwise exchange coupling constant J/k in the high-spin species vs experimental hyperfine coupling constants in the corresponding monoradicals. This relation allows us to identify outliers, which may correspond to radicals where J/k is not measured with sufficient accuracy. Double helical high-spin diradicals, in which spin density is delocalized over the chiral π-system, have been barely explored, with the sole example of such high-spin diradical possessing alternant π-system with Kekulé resonance form. Finally, we discuss a high-spin diradical with electrical conductivity and derivatives of triangulene diradicals.

Facile light-initiated radical generation from 4-substituted pyridine under ambient conditions

A facile photogeneration of a stable radical species from a 4-substituted pyridine derivative under ambient conditions is achieved. The radical generation reaction accompanies visible colour change into green and is repeatable multiple times.

Redox Cascades and Making of a C-C Bond: 1,2-Benzodiazinyl Radicals and a Copper Complex of a Benzodiazine

Two 1,2-benzodiazinyl radicals, cinnolinyl radicals by name, were successfully isolated by cascade routes using 1,4-naphthoquinone as a precursor. Reaction of 1,4-naphthoquinone with hydrazine hydrate promotes a (5e + 5H⁺) redox cascade affording benzo[ g]naphtho[1,2- c]cinnolinyl-7,12,14-trione (Cn^•) in 69% yields, while the similar reaction with 2-hydrazinopyridine is a (7e + 7H⁺) oxidative cascade and furnishes N-pyridinecinnolinyl radical (^PyCn^•). The cascades are composed of C-N and C-C bond making reactions. The neutral even alternate arenes are always diamagnetic; thus, the isolation of Cn^• and ^PyCn^• is a breakthrough. The Cn^•/Cn^- and ^PyCn^•/^PyCn^- redox couples are reversible, and the reaction of Cn^• with [Cu^I(PPh₃)₃Cl] in the presence of hydrazine hydrate and Et₃N affords a Cn^- complex of copper(I), [(Cn^-)Cu^I(PPh₃)₂] (1). Similar to phenalenyl radical, ^PyCn^• exists in three redox states, ^PyCn⁺, ^PyCn^•, and ^PyCn^-, in a smaller potential range (-0.30 V to -0.60 V vs Fc⁺/Fc couple) and can be used as an oxidant as well as a reductant. ^PyCn^• acts as a catalyst for the oxidative cleavages of benzil to benzoic and 2,2'-pyridil to picolinic acids in methanol in the presence of air. The molecular and electronic structures of Cn^•, ^PyCn^•, and 1·1/2MeOH were confirmed by single crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.