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Increasing Magnetic Coupling through Oxidation of a Ferrocene Bridge.

Ferrocene is an interesting coupler for designing magnetic molecules because of its rich chemistry and controllable oxidation state. In this work we have calculated the exchange spin coupling of a ferrocene-coupled nitronyl nitroxide diradical in its neutral and oxidized state (in which an additional spin center is introduced on the metallocene subunit). We do so by carrying out spin-unrestricted Kohn-Sham density functional theory (KS-DFT) calculations with different approximate exchange-correlation functionals and basis sets. We find that the neutral complex is weakly ferromagnetically coupled (in contrast to experimental results on single crystals), whereas the spin centers in the cationic complex are strongly antiferromagnetically coupled, resulting in an overall ferrimagnetic arrangement of the spins. Our calculations suggest that the magnetic exchange occurs through a spin alternation mechanism and that the lowest unoccupied molecular orbital (LUMO) plays an important role. The ferromagnetic behavior of the neutral complex is very sensitive to rotating one Cp ring versus the other. In the case of the cationic complex, the magnetic coupling is nearly independent of such structural changes. Thus, oxidation allows for switching between a weakly coupled and a strongly coupled, robust overall ferrimagnetic spin arrangement.