Thermally resistant air-stable organic triradicals with a quartet ground state and a large energy gap between spin states are still unique compounds. Moreover, stable triradicals with bridging units of the ethylene-1,1-diyl type and ferromagnetic coupling are limited to the family of nitroxides. In this work, for the first time, we designed and prepared the triradical having a quartet ground state based on oxoverdazyl and nitronyl nitroxide radical fragments. The triradical and appropriate triplet diradical precursor were synthesized via a palladium-catalyzed cross-coupling reaction of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both the di-and triradical are air-stable and possess good thermal stability with decomposition onset at ∼160 °C in an inert atmosphere. X-ray diffraction analysis of single crystals confirmed the presence of verdazyl and nitroxide radical centers. In the diradical, the verdazyl and nitronyl nitroxide centers showed fully reversible redox waves. In case of the triradical, the electrochemical processes occur practically at the same redox potentials but become quasi-reversible for the nitroxide moieties. Magnetic properties of the di-and triradical were characterized by a SQUID magnetometry of polycrystalline powders and by EPR spectroscopy in different matrices. Collected data analyzed using of the highlevel quantum chemical calculations confirmed that the di-and triradical have high-spin ground states. Unique high stability of prepared verdazyl-nitronylnitroxyl triradical opens new perspectives for further functionalization and design of high-spin systems with four or more spins.
Precise nanoscale distance measurements by pulsed electron paramagnetic resonance (EPR) spectroscopy play a crucial role in structural studies of biomolecules. The properties of the spin labels used in this approach determine the sensitivity limits, attainable distances, and proximity to biological conditions. Herein, we propose and validate the use of photoexcited fullerenes as spin labels for pulsed dipolar (PD) EPR distance measurements. Hyperpolarization and the narrower spectrum of fullerenes compared to other triplets (e.g., porphyrins) boost the sensitivity, and superior relaxation properties allow PD EPR measurements up to a near‐room temperature. This approach is demonstrated using fullerene–nitroxide and fullerene–triarylmethyl pairs, as well as a supramolecular complex of fullerene with nitroxide‐labeled protein. Photoexcited triplet fullerenes can be considered as new spin labels with outstanding spectroscopic properties for future structural studies of biomolecules.
Dedicated to Professor Usein M. Dzhemilev on the occasion of his 65 th birthday
AbstractIt is shown that S N H approach opens new possibilities in the synthesis of polyfunctional nitronyl and imino nitroxides. It is found that the interaction of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl lithium salt Li1 with 3,6-diaryl-1,2,4-triazines leads to formation of the corresponding triazines bearing nitronyl nitroxide or imino nitroxide substituent at position 5 of the heterocycle. The reaction of Li1 with pyridazine-N-oxide gives rise to nitroxide with buten-3-ynyl substituent 5. Spin-labeled 5 could be readily transformed by the use of 1,3-dipolar and nucleophilic addition reactions, as well as oxidative coupling, that gives a large group of new paramagnets: 2-(1H-pyrazol-5-yl)vinyl-, 2-ethynylcyclopropyl-, 2-(3-(ethoxycarbonyl) isoxazol-5-yl)vinyl-, 1-(pyrrolidin-1-yl)but-3-ynyl-substituted nitronyl nitroxide and a diradical -2,2′-
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