Inducing Single-Molecule Magnetism in a Family of Loop-of-Loops Aggregates: Heterometallic Mn40Na4 Clusters and the Homometallic Mn-44 Analogue

The syntheses, crystal structures, and magnetic properties of a new family of heterometallic Mn40Na4 and homometallic Mn-44 loop-of-loops aggregates are reported. The reactions of [Mn3O(O2CMe)(6)(py)(3)]center dot py with 1,3-propanediol (pdH(2)) and 2-methyl-1,3-propanediol (mpdH(2)) in the presence of NaN3 afforded [Mn10Na(mu(3)-O)(2)(O2CMe)(13)(pd)(6)(py)(2)](4) (1)(4) and [Mn10Na(mu(3)-O)(2)(O2CMe)(13)(mpd)(6)(py)(H2O)](4) (2)(4), respectively. Mn40Na4 complexes (1)(4) and (2)(4) consist of four Mn-10 loops linked through Na+ ions to give a supramolecular aggregate with a saddle-like topology. Magnetic characterization of compound (1)(4) showed that each Mn-10 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 Mn-10 units of (1)(4) through the diamagnetic Na+ ions, and also intermolecular interactions between different Mn40Na4 aggregates. The magnetically discrete Mn-44 analogue was targeted with high priority and finally prepared from the reaction of [Mn3O(O2CMe)(6)(py)(3)]center dot py with pdH(2) in the presence of Mn(ClO4)(2)center dot 6H(2)O. The loop-of-loops structure of [Mn-44(mu(3)-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 Mn-44 cluster. In accord with this description and the stronger exchange coupling between the four Mn-10 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.