Ligand design for heterometallic 3d metallacrown complexes
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Abstract
Two decades of research on single molecule magnets (SMMs) led to multiple concepts to reach one of the most ambitious goals in molecular magnetism, a high energy barrier (Ueff ) for the magnetization reversal of a single molecule. This energy barrier is dependent on the spin ground state (S) and the axial magnetoanisotropy (D) of the molecule. Previous research projects led to complexes with remarkably high S or D values but only few combining both. Up to now, a systematical manipulation and control of S and D on a molecular scope has not been accomplished yet. Herein, a systematic approach to obtain a high spin ground state accompanied with high molecular magnetic anisotropy is presented using the versatile compound class of metallacrowns (MCs). The magnetic director approach has proven to effectively yield 12-MC-4 structures with high spin ground states. In addition, the planar crown scaffold represents an ideal framework to align the Jahn-Teller axes of ring forming metal ions with high single ion anisotropy. Prerequisites for an effective use of both concepts are the ability of the central guest ion to take part in strong magnetic exchange coupling and a tuneable anisotropic electron distribution of the ring forming metal ions. Hence, 3d transition metal ions express themselves as the optimal choice. Systematic synthesis of heterometallic MCs requires varying selectivity of the central and the ring coordination pockets. To fulfill this condition, two distinct ligand optimizations were followed in this work. The donor atoms on apical coordination sites of the crown metal ions were varied yielding to a different binding affinity according to Pearson’s HSAB concept. Moreover, additional coordinating functionalities were introduced in established MC ligand systems, like amidoximes, in order to adjust the whole coordination environment of the crown forming metal ions. According to these approaches, several new 3d MC compounds were synthesized and analyzed using infrared spectroscopy, single crystal X-ray diffraction and SQUID magnetometry. Synthesis and the use of 2-methylmercaptobenzohydroxamic acid led to the isolation of the very first cadmium(II) metallacrown opening new perspectives for an improvement of the luminescence properties of this compound class. To highlight some of the results, a mixed valent cobalt(II/III) MC, double decker and a discrete Cu(II) MC and even two manganese double decker MCs are reported in this work.