An interplay between metal-fullerene and metal-metal bonding in molecular magnetism of erbium metallofullerenes

Lanthanide-based endohedral metallofullerenes (EMFs) form a special class of molecular magnets, in which magnetic anisotropy and metal-metal interactions can be varied in a broad range by judicious choice of the structure of endohedral species. Nonetheless, the influence of metal-cage interactions on the magnetic anisotropy, which should pertain to all types of EMFs, has not been studied yet in necessary detail. In this work, an interplay between magnetic anisotropy, Er & ctdot;Er coupling, and bonding interactions in erbium mono- and dimetallofullerenes is studied by EPR, photoluminescence, SQUID magnetometry, and ab initio calculations. We analyzed the ligand field created solely by metal-fullerene interactions in the mono-EMFs Er@C80,82(CH2Ph) and showed how it changes when Er-Er bonding kicks in for di-EMFs Er2@C82 and Er2@C80(CH2Ph). X-band and Q-band EPR spectroscopy is used to precisely describe the ground-state doublets of Kramers mono-Er EMFs and Er2@C80(CH2Ph). For Er2@C82, which is a non-Kramers system, X-band measurement in the parallel mode was used. The EPR signals of di-Er EMFs were observed at large g-values exceeding 20, which unambiguously assigns them to the coupled states of Er2 dimers rather than single-ion transitions. Ab initio calculations revealed that the moderate cage-induced magnetic anisotropy of mono-EMFs is enhanced by the Er-Er bond in di-EMFs. Magnetization studies augmented with effective spin Hamiltonian modelling showed that the Er & ctdot;Er coupling in Er2@C82 with a two-electron Er-Er bond is weakly antiferromagnetic, while Er2@C80(CH2Ph) with a single-electron Er-Er bond features a strong ferromagnetic interaction between Er moments and unpaired electron spin. An interplay between magnetic anisotropy, Er & ctdot;Er coupling, and bonding interactions in erbium mono- and dimetallofullerenes is studied by EPR, photoluminescence, SQUID magnetometry, and ab initio calculations.