1.2.4.5- Tetrazine anion-radicals

The observation of very small substituent splittings in a series of tetrazine anion-radicals (55), (R = Me, Et, i-Pr, cyclo-Pr, Ph2CH, and Ph) confirms that the singly occupied orbital is as in 54 the heterocycle, and not the nitrile groups, remains the primary site for the unpaired electron in 55 (R = CN).141-144 By contrast, principal foci of spin density are outside the... 

Tetrazines, investigated by cyclic voltammetry in acetonitrile, give an uncomplicated, reversible reduction to an anion-radical, which is stable on the time scale of CV.347... 

Successive substitution of carbon by nitrogen lowers the energy of the LUMO. As a consequence, the ease of reduction and the stability of radical anions are increased from benzene to pyridine, diazines, triazines, and tetrazines. 

ESR spectra of radical cations and anions of 1,2,4,5-tetrazines were recorded and showed that the anions have a re-SOMO concentrated on the four ring nitrogen atoms whilst all but the 3,6-(NMe2)2 derivative form cr-cations, again with the SOMO confined to the four nitrogen atoms.451,452 The observed energy difference between tetrazines and their corresponding radical anions are well correlated with the results of CNDO and self-consistent field re-electron calculations.256 258... 

Dimethyl-l,2,4,5-tetrazine (6) and its dihydro derivative 7 form a black crystalline charge transfer complex 8. Addition of base to the aqueous solution of 8 affords the radical anion 9 while addition of acid transforms 8 to the radical cation 10.254... 

Various 1,2,4,5-tetrazines yield persistent radical anions by electrolytic reduction.451 453 by disproportionation in deoxygenated solvents containing strong bases,454-456 or by using a one-electron reducing agent.457-459... 

Section 6.21.5.9 on redox reactions discusses the formation of radical cations and radical anions from tetrazines, dihydrotetrazines, and tetrahydrotetrazines. The literature cited there offers all information on relevant ESR investigations in this field. Earlier literature is to be found in Neun-hoeffer s review <84CHEC-i(3)540>. 

Heterocyclic Compounds. All the heterocyclic compounds, unsubstituted by electroactive groups, from which radical ions have been obtained by electrochemical generation are nitrogen heterocycles. Among the numerous systems involved radical anions have been obtained from pyrazine [118], pyridazine [126], and tetrazine [126], and radical cations have been obtained from dihydropyrazine [126-128] and dihydrotetramethylpyrazine [128]. 

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