Overclocking Nitronyl Nitroxide Gold Derivatives in Cross-Coupling Reactions

Toward New Horizons in Verdazyl-Nitroxide High-Spin Systems: Thermally Robust Tetraradical with Quintet Ground State

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but examples reported to date exhibit limited stability and processability. In this work, we designed the first tetraradical based on an oxoverdazyl core and nitronyl nitroxide radicals and successfully synthesized it using a palladium-catalyzed cross-coupling reaction of an oxoverdazyl radical bearing three iodo-phenylene moieties with a gold(I) nitronyl nitroxide-2-ide complex in the presence of a recently developed efficient catalytic system. The molecular and crystal structures of the tetraradical were confirmed by single crystal X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼125 °C in an inert atmosphere; in a toluene solution upon prolonged heating at 90 °C in air, no decomposition was observed. The resulting unique verdazyl-nitroxide conjugate was thoroughly studied using a range of experimental and theoretical techniques, such as SQUID magnetometry of polycrystalline powders, EPR spectroscopy in various matrices, cyclic voltammetry, and high-level quantum chemical calculations. All collected data confirm the high thermal stability of the resulting tetraradical and quintet multiplicity of its ground state, which makes the synthesis of this important paramagnet a new milestone in the field of creating high-spin systems.

A Nitronyl Nitroxide-Substituted Benzotriazinyl Tetraradical

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium-catalyzed cross-coupling reaction of a triiodo-derivative of the 1,2,4-benzotriazinyl radical with gold(I) nitronyl nitroxide-2-ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at -0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high-level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl-nitronylnitroxide tetraradical is a new milestone in the field of creating high-spin systems.

Catalytic System for Cross-Coupling of Heteroaryl Iodides with a Nitronyl Nitroxide Gold Derivative at Room Temperature

A simple and highly effective methodology for the cross-coupling of heteroaryl iodides with NN-AuPPh3 at room temperature is reported. The protocol is based on a novel catalytic system consisting of Pd2(dba)3·CHCl3 and the phosphine ligand MeCgPPh having an adamantane-like framework. The present protocol was found to be well compatible with various heteroaryl iodides, thus opening new horizons in directed synthesis of functionalized nitronyl nitroxides and high-spin molecules.

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.