Tetraanionic arachno-Carboranyl Ligand Imparts Strong Axiality to Terbium(III) Single-Molecule Magnets

A family of fully sandwiched arachno-lanthanacarborane complexes formulated as {6-[µ-1,2-[o-C6H4(CH2)2]-1,2-C2B10H10]2Ln}{Li5(THF)10} (Ln = Tb, Dy, Ho, Er, Y) is successfully synthesized, where the “carbons-adjacent” carboranyl ligand (arachno-C2B10H104−) bears four negative charges and coordinates to the central lanthanide ions using the hexagonal 6­C2B4 face. Thus, the central lanthanide cations are pseudo-twelve-coordinate and have an approximate local D6h symmetry or hexagonal-prismatic geometry. As the crystal field effect imparted by this geometry is still unknown, we thoroughly investigated the magnetic properties of this series of complexes and found the crystal field imposed by this ligand causes a relation of Tb > Dy > Ho > Er for the energy gaps between the ground and the first excited states, which is of striking resemblance to the ferrocenophane and phthalocyanine ligands despite that the latter two ligands give disparate local coordination geometries. Moreover, the effective energy barrier to magnetization reversal of 445(10) K, observable hysteresis loop up to 4K and the relaxation time of the yttrium diluted sample reaching 193(17) seconds at 2 K under an optimized field for the Tb analogue of this family of arachno-lanthanacarborane complexes, render a new benchmark for Tb3+ based single-molecule magnets.