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

<i>meso</i>-Helicates [Ni<sub>3</sub>(pzNTR)<sub>6</sub>]·4CH<sub>3</sub>CN and [Fe<sub>3</sub>(pzNTR)<sub>6</sub>](ClO<sub>4</sub>)<sub>2</sub>·CH<sub>3</sub>OH were prepared (HpzNTR stands for <i>N-tert</i>-butyl-α-3-pyrazolylnitrone). The former showed antiferromagnetic coupling between neighboring ions with <i>J</i>/<i>k</i><sub>B</sub> = −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 <i>J</i>/<i>k</i><sub>B</sub> = +0.292(8) K between the two terminal <i>S</i> = 5/2 Fe<sup>3+</sup> ions through the intervening diamagnetic Fe<sup>2+</sup> ion. The admixture of low-lying excited states with the ground state stabilizes the ferromagnetically coupled state, like the Prussian blue magnet.