Inducing Single-Molecule Magnetism in a Family of Loop-of-Loops Aggregates: Heterometallic Mn_(40)Na_4 Clusters and the Homometallic Mn_(44) Analogue

摘要

The syntheses, crystal structures, and magnetic properties of a new family of heterometallic Mn_(40)Na_4 and homometallic Mn_(44) loop-of-loops aggregates are reported. The reactions of [Mn_3O(O_2CMe)_6(py)_3]-py with 1,3-propanediol (pdH_2) and 2-methyl-1,3-propanediol (mpdH_2) in the presence of NaN_3 afforded [Mn_(10)Na(μ_3-O)_2(O_2CMe)_(13)(pd)_6(py)_2]_4 (1)_4 and [Mn_(10)Na(μ_3-O)_2(O_2CMe)_(13)(rnpd)6(py)(H_(2O))]_4 (2)_4, respectively. Mn_(40)Na_4 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 Mn_(40)Na_4 aggregates. The magnetically discrete Mn_(44) analogue was targeted with high priority and finally prepared from the reaction of [Mn_3O(O_2CMe)_6(py)_3]·py with pdH_2 in the presence of Mn(CIO_4)_2·6H_2O. The loop-of-loops structure of [Mn_(44)(μ_3-O)_8(O_2CMe)_(52)(pd)_(24)(py)_8](ClO_4)(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 Mn_~(2+) 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 Mn~(2+) 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.