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The bipolar electrochemical properties of verdazyl radicals have long been known, and recently their application in symmetric redox flow batteries was investigated. However, the performance of aryl‐substituted Kuhn‐ and oxo‐verdazyl radicals in cycling experiments was unsatisfactory. In this work, N,N’‐dialkylated oxo‐verdazyl radicals were investigated as bipolar redox‐active organic molecules (ROM). With a higher theoretical cell voltage than that of Kuhn‐ and arylated oxo‐verdazyl radicals, these stable radicals revealed themselves a promising candidate for battery application. The initial tests of an N,N’‐diisopropyl‐verdazyl radical showed moderate cycling, stability and the formation of a new redox‐active species during the cycling between neutral and cationic states was observed. By postcycling analysis, the decomposition product could be identified as the radical lacking an isopropyl group. By subsequent molecular engineering, the performance of the N,N’‐dialkylated verdazyl radical could be improved by suppressing the decomposition pathway through variation of the N‐substituent. Overall, the optimized verdazyl radical possesses a theoretical cell voltage of greater than 1.6 V. H‐cell cycling experiments over 200 cycles showed an average Coulombic efficiency of 99.5% and a capacity fade of less than 0.1%/cycle for both oxidation and reduction. Finally, the best‐performing oxo‐verdazyl radicals were investigated under flow conditions. In this work, N,N’‐dialkylated oxo‐verdazyl radicals have been studied in redox flow batteries for the first time. Investigation of the degradation led to the identification of a decomposition product. By that, the molecular design and the cycling performance could be improved.

A nitronyl nitroxide derivative, 2-phenylethynyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide (1), and two verdazyl derivatives carrying a phenylacetylene unit, 1,5-diphenyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (2) and 1,5-diisopropyl-3-phenylethynyl-6-oxo-1,2,4,5-tetrazin-2-yl (3), were synthesized and their packing structures were studied by X-ray crystallographic analysis and magnetically characterized in the solid state. While 1 and 3 had an isolated doublet spin state, 2 formed an antiferromagnetically coupled pair (2J/kB = −118 K). Density functional theory (DFT) calculations reveal that the spin density polarized in the phenyl group decreases as the dihedral angle between the phenyl ring and radical plane increases.

New 3-aryl(hetaryl)-1-(benzo[ d ]thiazol-2-yl)-6-methyl-5-phenylverdazyls were synthesized and characterized by EPR and IR spectroscopies, mass spectrometry, cyclic voltammetry, and X-ray diffraction. The effect of substituents on the spectral and electrochemical properties was studied. It was shown that substituents at position 3 of the verdazyl tetrazine ring, regardless of their nature, cause a significant shift of the oxidation and reduction potentials to the anodic region. The results obtained suggest that the synthesized compounds can be used as an electroactive components in rechargeable organic batteries.

An optimized synthetic protocol toward the assembly of Kuhn verdazyls based on an azo coupling of arenediazonium salts with readily available hydrazones followed by the base-mediated cyclization of in situ formed formazans with formalin was developed. The scope and limitations of the presented method were revealed. Some new mechanistic insights on the formation of Kuhn verdazyls were also conducted. It was found that in contradiction with previously assumed hypotheses, the synthesis of verdazyls was accomplished via an intermediate formation of verdazylium cations which were in situ reduced to leucoverdazyls. The latter underwent deprotonation under basic conditions to generate corresponding anions which coproportionate with verdazylium cations to furnish the formation of Kuhn verdazyls. The spectroscopic and electrochemical behavior of the synthesized verdazyls was also studied. Overall, our results may serve as a reliable basis for further investigation in the chemistry and applications of verdazyls.

Data on the influence of neutral salts on the rates of unimolecular heterolyses of organic substrates, obtained mainly by the verdazyl method, are summarized here. It is assumed that heterolysis takes place with consecutive formation of four ion pairs: contact (CIP), cavity-separated (CSIP), one solvent moleculeseparated (SIP) and solvent-separated (SSIP). [Formula: see text] In the limiting step, the CIP interacts with a solvent cavity and the CSIP is formed, which converts quickly into the SIP and subsequently to the SSIP, which also quickly gives the reaction products. In the transition state, bonds between the molecules solvating the CIP are broken. In the absence of salt, the return from external ion pairs does not have much importance. The verdazyl indicator quickly and quantitatively reacts with the SSIP. The normal salt effect takes place due to the action of salt on the covalent substrate, which catalyses CIP formation. The special salt effect is caused by the association of the salt with the CIP, which prolongs the lifetime of the intermediate and increases the probability of its contact with a solvent cavity. The negative special salt effect is caused by association of the salt with the SIP or SSIP, which prolongs the lifetime of the intermediates and increases the probability of their contact with a solvent cavity to return to the covalent substrate. When the salt reacts with the SIP, the salt effect does not depend on the concentration and nature of verdazyl, but such a dependence takes place when the salt reacts with the SSIP. The site of the action of the salt is determined by the Hard and Soft Acids and Bases (HSAB) principle.

The synthesis and characterization of a new 6-phosphaverdazyl radical incorporated into a spirocyclic framework containing a cyclotriphosphazene ring is described. Reaction of 1,1,3,3-bis[spiro-2',2''-dioxy-1',1''-biphenyl]-5,5-bis(1-methylhydrazido)-1,3,5,2,4,6-cyclotriphosphazene ( 7 ) with trimethylorthobenzoate yielded 6-[2,2,4,4-bis(2,2'-dioxy-1,1'-biphenyl)-2,4,6,1,3,5-cyclotriphosphazen-6-yl]-1,2,5,6-tetrahydro-1,5-dimethyl-3-phenyl-1,2,4,5,6-tetrazaphosphorine ( 8 ), which was characterized spectroscopically and by X-ray diffraction. Oxidation of 8 yielded 6-[2,2,4,4-bis(2,2'-dioxy-1,1'-biphenyl)-2,4,6,1,3,5-cyclotriphosphazen-6-yl]-1,5-dimethyl-3-phenyl-6-phosphaverdazyl ( 5 ), which was characterized by EPR and ENDOR spectroscopy. Analysis of both spectra indicate that spin density from the phosphaverdazyl ring induces spin polarization into the phosphazene ring, as evidenced by hyperfine coupling to all three phosphorus nuclei.Key words: heterocycle, phosphorus–nitrogen compounds, phosphazenes, stable radicals, verdazyls.

A series of tetrahydro-1,2,4,5-tetrazinan-3-ones have been prepared by the reaction of carbonic acid bis (1-methylhydrazide) with aromatic aldehydes. These were oxidised to oxoverdazyl free radicals and used immediately for ESR spectroscopy studies that indicate that the unpaired electron is delocalised over the verdazyl ring. The ESR spectra can be very well simulated considering hyperfine couplings with the four nitrogen atoms of the verdazyl ring and the six hydrogen atoms of the two methyl groups bonded to it.

Stable radicals are of intense fundamental interest because they challenge conventional bonding paradigms, and they find a wide range of uses ranging from organic and polymer synthesis to biological and medicinal applications to materials science. Yet the directed synthesis and study of stable radicals for either fundamental or applied purposes are rarely pursued strategies. This Award Lecture describes my research group's efforts in exploratory and targeted research focusing on stable radical design.Key words: stable radicals, verdazyls, molecular magnetism.