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/kB = −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/kB = +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.