Element specific determination of the magnetic properties of two macrocyclic tetranuclear 3d–4f complexes with a Cu3Tb core by means of X-ray magnetic circular dichroism (XMCD)

Abstract

We apply X-ray magnetic circular dichroism to study the internal magnetic structure of two very promising star shaped macrocyclic complexes with a Cu^II₃Tb^III core. These complexes are rare examples prepared with a macrocyclic ligand that show indications of SMM (Single Molecule Magnet) behavior, and they differ only in ring size: one has a propylene linked macrocycle, [Cu^II₃Tb^III(L^Pr)(NO₃)₂(MeOH)(H₂O)₂](NO₃)·3H₂O (nickname: Cu₃Tb(L^Pr)), and the other has the butylene linked analogue, [Cu^II₃Tb^III(L^Bu)(NO₃)₂(MeOH)(H₂O)](NO₃)·3H₂O (nickname: Cu₃Tb(L^Bu)). We analyze the orbital and spin contributions to the Cu and Tb ions quantitatively by applying the spin and orbital sum rules concerning the L₂ (M₄)/L₃ (M₅) edges. In combination with appropriate ligand field simulations, we demonstrate that the Tb(III) ions contribute with high orbital magnetic moments to the magnetic anisotropy, whereas the ligand field determines the easy axis of magnetization. Furthermore, we confirm that the Cu(II) ions in both molecules are in a divalent valence state, the magnetic moments of the three Cu ions appear to be canted due to 3d–3d intramolecular magnetic interactions. For Cu₃Tb(L^Pr), the corresponding element specific magnetization loops reflect that the Cu(II) contribution to the overall magnetic picture becomes more important as the temperature is lowered. This implies a low value for the 3d–4f coupling.