Inducing Single-Molecule Magnetism in a Family of Loop-of-Loops Aggregates: Heterometallic Mn40Na4 Clusters and the Homometallic Mn44 Analogue

The syntheses, crystal structures, and magnetic properties of a new family of heterometallic Mn40Na4 and homometallic Mn44 loop-of-loops aggregates are reported. The reactions of [Mn3O(O2CMe)6(py)3]·py with 1,3-propanediol (pdH2) and 2-methyl-1,3-propanediol (mpdH2) in the presence of NaN3 afforded [Mn10Na(μ3-O)2(O2CMe)13(pd)6(py)2]4 (1)4 and [Mn10Na(μ3-O)2(O2CMe)13(mpd)6(py)(H2O)]4 (2)4, respectively. Mn40Na4 complexes (1)4 and (2)4 consist of four Mn10 loops linked through Na+ ions to give a supramolecular aggregate with a saddle-like topology. Magnetic characterization of compound (1)4 showed that each Mn10 loop has an S = 4 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac susceptibility signals. It also exhibits hysteresis loops that, however, are not typical of single-molecule magnets (SMMs) due to the existence of interloop interactions between the neighboring Mn10 units of (1)4 through the diamagnetic Na+ ions, and also intermolecular interactions between different Mn40Na4 aggregates. The magnetically discrete Mn44 analogue was targeted with high priority and finally prepared from the reaction of [Mn3O(O2CMe)6(py)3]·py with pdH2 in the presence of Mn(ClO4)2·6H2O. The loop-of-loops structure of [Mn443-O)8(O2CMe)52(pd)24(py)8](ClO4)(OH)3 (3) is essentially identical to those of (1)4 and (2)4, with the most significant difference being that the four Na+ ions of (1)4 and (2)4 have been replaced with Mn2+ ions. Compound 3 is thus best described magnetically as a Mn44 cluster. In accord with this description and the stronger exchange coupling between the four Mn10 loops expected through the connecting Mn2+ ions, magnetic susceptibility measurements revealed that 3 has an S = 6 ground-state spin and displays frequency-dependent in-phase and out-of-phase ac signals. Magnetization vs dc field sweeps on single-crystals of 3 displayed scan rate- and temperature-dependent hysteresis loops confirming that complex 3 is a new SMM, and is thus the second largest Mn cluster and SMM reported to date.