Ferromagnetic Superexchange Coupling through a Diamagnetic Iron(II) Ion in a Mixed-Valent Iron(III, II, III) meso-Helicate

meso-Helicates [Ni₃(pzNTR)₆]·4CH₃CN and [Fe₃(pzNTR)₆](ClO₄)₂·CH₃OH were prepared (HpzNTR stands for N-tert-butyl-α-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 Fe³⁺ ions through the intervening diamagnetic Fe²⁺ ion. The admixture of low-lying excited states with the ground state stabilizes the ferromagnetically coupled state, like the Prussian blue magnet.