What Controls the Magnetic Exchange and Anisotropy in a Family of Tetranuclear {Mn2IIMn2III} Single-Molecule Magnets?

Twelve heterovalent, tetranuclear manganese(II/III) planar diamond or butterfly complexes, 1-12, have been synthesized and structurally characterized, and their magnetic properties have been probed using experimental and theoretical techniques. The 12 structures are divided into two distinct classes. Compounds 1-8 place the Mn(III), S = 2, ions in the body positions of the butterfly metallic core, while the Mn(II), S = 5/2, ions occupy the outer wing sites and are described as Class 1. Compounds 9-12 display the reverse arrangement of ions and are described as Class 2. Direct current susceptibility measurements for 1-12 reveal ground spin states ranging from S = 1 to S = 9, with each complex displaying unique magnetic exchange parameters (J). Alternating current susceptibility measurements found that that slow magnetic relaxation is observed for all complexes, except for 10 and 12, and display differing anisotropy barriers to magnetization reversal. First, we determined the magnitude of the magnetic exchange parameters for all complexes. Three exchange coupling constants (J(bb), J(wb), and J(ww)) were determined by DFT methods which are found to be in good agreement with the experimental fits. It was found that the orientation of the Jahn-Teller axes and the Mn-Mn distances play a pivotal role in determining the sign and strength of the J(bb) parameter. Extensive magneto structural correlations have been developed for the two classes of {(Mn2Mn2III)-Mn-II} butterfly complexes by varying the Mn-b-O distance, Mn-w-O distance, Mn-b-O-Mn-b angle (alpha), Mn-b-OMnb-O dihedral angle (gamma), and out-of-plane shift of the Mn-w atoms (beta). For the magnetic anisotropy the DFT calculations yielded larger negative D value for complexes 2, 3, 4, and 6 compared to the other complexes. This is found to be correlated to the electron-donating/withdrawing substituents attached to the ligand moiety and suggests a possible way to fine tune the magnetic anisotropy in polynudear Mn ion complexes.