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 T_C molecule-based magnets.