INTRAMOLECULAR MAGNETIC COUPLING BETWEEN 2-NITRENE OR 2-NITROXIDE UNITS THROUGH 1,1-DIPHENYLETHYLENE CHROMOPHORES - ISOMERIC DINITRENES AND DINITROXIDES RELATED IN CONNECTIVITY TO TRIMETHYLENEMETHANE, TETRAMETHYLENEETHANE, AND PENTAMETHYLENEPROPANE

Isomeric vinylidenebis(phenylnitrenes) (N) and 1,1-bis[(N-oxy-tert-butylamino)phenyl]-2-methylpropenes (O) have been prepared, and their EPR fine structures and/or effective magnetic moments have been determined over a wide temperature range. The data were analyzed in terms of Bleaney-Bowers-type equations to give the energy gaps between the quintet and singlet states for N and the triplet and singlet states for 0. They were >>0 cm-1, -42.0 cm-1 (-502 J/mol, a negative value representing a singlet ground state), and -26.2 cm-1 (-313 J/mol), and 10.6 cm-1 (127 J/mol), -1.8 cm-1 (-22 J/mol), and -3.4 cm-1 (-41 J/mol) for p,p'-, m,p' and m,m'-dinitrenes N and dinitroxides O, respectively. A consistent reduction in the effective intramolecular exchange integrals Js between the two open-shell centers in the latter series is ascribed to the lower spin polarization on the phenyl rings of the dinitroxides relative to the dinitrenes. Whereas the ferro- and antiferromagnetic couplings in the p,p' and m,p'-topology are consistent not only with the Ovchinnikov's but also Borden-Davidson's rules, the antiferromagnetic interaction in the m,m'-substitution pattern is contradictory to the former and consistent with the latter theory. Since the p,p', m,p', and m,m' isomers are related to trimethylenemethane (4), tetramethyleneethane (5), and pentamethylenepropane (6), respectively, the results obtained in this work are consistent with the triplet ground state of 4. 5 and 6 are suggested to have ground singlet states. Radical centers X should be all at the para position for designing super-high-spin polyacetylenes 1 and polydiacetylenes 2.