Ligand-Driven Exchange Coupling in Mn4 Single-Molecule Magnets

Distorted cubane [Mn4+Mn33+(mu(3) - L2-)(3)(mu(3) - X-)(Z)(3-)(dbm)(3)(-)] (L = O; X = various; Z = (OAc)(3); dbmH = dibenzoyl - methane) single-molecule magnets (SMMs) have been studied by first-principles calculations. Our earlier studies showed that the basic mechanism of antiferromagnetic Mn4+ - Mn3+ coupling (J(AB)) is determined by the pi type hybridization among the d(z)(2) orbitals at the Mn3+ sites and the t(2g) orbitals at the Mn4+ site through the p orbitals at the mu(3) - L2- ions. This result allows us to predict that ferrimagnetic structure of Mn4+Mn33+ molecules will be the most stable with the Mn4+ - (mu(3) - L2-) - Mn3+ angle alpha approximate to 90 degrees, while synthesized Mn4+Mn33+ molecules have alpha approximate to 95 degrees. One approach is suggested to design new Mn4+Mn33+ SMMs having a much more stable ferrimagnetic state. This approach is controlling the Mn4+ - (mu(3) - L2-) - Mn3+ exchange pathways by rational variations in ligands to strengthen the hybridization between Mn ions. In this paper, by combining variations in the L and Z ligands, ten new high-spin [Mn4+Mn33+(mu(3) - L2-)(3)(mu(3) - F-)(3)(Z)(3-)(CH(CHO)(2))(3)(-)] (L = NCGeH5, NGeCH5, NGe2H5, NSiGeH5, or NGeSiH5; Z = (OAc)(3) or MeC (CH2NCOMe)(3)) molecules have been designed. Our calculated results show that these ten modelling Mn-4 molecules have alpha approximate to 90 degrees and J(AB)/k(B) in the range of (-231.6, -147.4) K, in which the molecule with [L, Z] = [NCGeH5, MeC(CH2NCOMe)(3)] has the highest J(AB)/k(B) of -231.6 K corresponding to alpha = 89.23 degrees. This value is over 3 times larger than that of synthesized Mn-4 SMMs. Our calculated results demonstrate combining variations in the L and Z ligands as an effective way to tailor intramolecular exchange coupling of the Mn-4 molecules. The results would give some hints for synthesizing new SMMs.