Ligand-Controlled Magnetic Interactions in Mn4 Clusters

A method is presented to design magnetic molecules in which the exchange interaction between adjacent metal ions is controlled by electron density withdrawal through their bridging ligands. We synthesized a novel Mn4 cluster in which the choice of the bridging carboxylate ligands (acetate, benzoate, or trifluoroacetate) determines the type and strength of the three magnetic exchange couplings (J 1, J 2, and J 3) present between the metal ions. Experimentally measured magnetic moments in high magnetic fields show that, upon electron density withdrawal, the main antiferromagnetic exchange constant J 1 decreases from −2.2 K for the [Mn4(OAc)4] cluster to −1.9 K for the [Mn4(H5C6COO)4] cluster and −0.6 K for the [Mn4(F3CCOO)4] cluster, while J 2 decreases from −1.1 K to nearly 0 K and J 3 changes to a small ferromagnetic coupling. These experimental results are further supported with density-functional theory calculations based on the obtained crystallographic structures of the [Mn4(OAc)4] and [Mn4(F3CCOO)4] clusters.