We acknowledge Laurent Maron (Université Paul Sabatier, Toulouse) for discussions at the early stage of this project. We acknowledge Maciej Korzynski (ETHZ) for a help with preparation of the manuscript. We acknowledge Elizaveta A. Suturina (University of Bath) and Aleksander Jaworski (Stockholm University) for the advices concerning quantum chemistry calculations in lanthanide complexes. We thank Martin Bröring (Institut für Anorganische und Analytische Chemie) at TU Braunschweig for providing access to the SQUID magnetometer.