Using Redox-Active π Bridging Ligand as a Control Switch of Intramolecular Magnetic Interactions

Intramolecular
magnetic interactions in the dinuclear complexes
[(tpy)­Ni­(tphz)­Ni­(tpy)]n+ (n = 4, 3, and 2; tpy, terpyridine; tphz, tetrapyridophenazine) were
tailored by changing the oxidation state of the pyrazine-based bridging
ligand. While its neutral form mediates a weak antiferromagnetic (AF)
coupling between the two S = 1 Ni­(II), its reduced form, tphz•–, promotes
a remarkably large ferromagnetic exchange of +214(5) K with Ni­(II)
spins. Reducing twice the bridging ligand affords weak Ni–Ni
interactions, in marked contrast to the Co­(II) analogue. Those experimental
results, supported by a careful examination of the involved orbitals,
provide a clear understanding of the factors which govern strength
and sign of the magnetic exchange through an aromatic bridging ligand,
a prerequisite for the rational design of strongly coupled molecular
systems and high TC molecule-based magnets.