Diradicals antiferromagnetic coupling

While the structural and electronic properties of the dianions are consistent with the indicated Nin-enedithiolate description, two and four formulations are conceivable for the monoanion and the neutral complex, respectively. For the latter, one is a ligand diradical whose spins are antiferromagnetically coupled, consistent with diamagnetism. Numerous members of the series have been isolated and/or generated in solution by chemical or electrochemical reactions. Because redox potentials are markedly dependent on the nature of the R substituent, certain members of a given series have not been isolated in substance. 

Di-/-butyl-6-aminothiophenol gives complex (497) under strictly anaerobic conditions. In the presence of air, deep blue-violet solutions are obtained from which mixtures of solid deep blue-black cis/trans-(498) can be isolated.1345 For the square planar Ni complex, the cis-trans isomerization rate is fast. (498) is best described as a singlet diradical due to strong antiferromagnetic coupling of the benzosemiquinonate 7r-radicals.19, 134S... 

The monomeric diradical [SSSNC-CNSSS][AsF6]2, like 7[AsF6] (R=CF3), is paramagnetic in the solid state. A broad maximum in % at 60 K is indicative of short-range antiferromagnetic coupling.94... 

SOMO and SOMO, too. The delocalized spin on the donor site and the localized spin on the radical site in these cation diradicals are oriented in a parallel manner. As will be seen in Chapter 7 (Section 7.4), such a spin orientation results in ferromagnetic coupling (triplet interaction) between the spins. In contrast, an antiparallel type of mutual orientation of the spins results in antiferromagnetic coupling. The parallel type is crucial in the design of organic materials with magnetic properties. 

In a later article, complexes of Ni(II), Cu(II), Pd(II), and V02+ ions with the same tetra-substituted porphyrin were reported. Stepwise oxidation of these complexes gave products for which the authors proposed quinonoid, monoradical, and diradical structures. The most prolonged oxidations yielded the diradical products, which were isolated as dark purple crystals, relatively stable in air (40). The monoradical vanadyl complex was observed to be diamagnetic, suggesting antiferromagnetic coupling between the phenoxyl radical and unpaired electron on vanadium, whereas in the copper complex no such coupling was observed. More detailed studies of these systems seem warranted. 

Fig. 13 Diradicals (a) 1,3-phenylene-based, (b) 3,3 -biphenylene-based, and (c) 3,4 -biphenylene-based and diradical with antiferromagnetic coupling scheme.

Analogouss studies of conformationally restricted cyclophane-based diradical 19 gave singlet (S — 0) ground state, with much greater antiferromagnetic coupling,... 

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