Ferromagnetic superexchange coupling through a diamagnetic iron(II) ion in a mixed-valent iron(III, II, III) meso-helicate.

meso-Helicates [Ni(3)(pzNTR)(6)].4CH(3)CN and [Fe(3)(pzNTR)(6)](ClO(4))(2).CH(3)OH were prepared (HpzNTR stands for N-tert-butyl-alpha-3-pyrazolylnitrone). The former showed antiferromagnetic coupling between neighboring ions with J/k(B) = -28.2(2) K, which can be reasonably interpreted in terms of the superexchange mechanism. On the other hand, the latter exhibited ferromagnetic coupling with J/k(B) = +0.292(8) K between the two terminal S = 5/2 Fe(3+) ions through the intervening diamagnetic Fe(2+) ion. The admixture of low-lying excited states with the ground state stabilizes the ferromagnetically coupled state, like the Prussian blue magnet.