Slow magnetic relaxation and strong magnetic coupling in the nitroxyl radical complexes of lanthanide(<scp>iii</scp>) with diamagnetic ground state (Ln = Lu, Eu)

Radical lanthanide complexes are appealing platforms to investigate the possibility to engineer relevant magnetic couplings between the two magnetic centers by exploiting the strongly donating magnetic orbitals of the radical. In this paper, we report a spectroscopic and magnetic study on [LnRad(NO₃)₃], where Ln = Eu³⁺ or Lu³⁺ and Rad is the tridentate tripodal nitroxyl radical 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl. A thorough magnetic investigation by Electron Paramagnetic Resonance (EPR) spectroscopy and magnetometry, fully supported by <i>ab initio</i> calculations, allowed us to unravel an unprecedentedly large antiferromagnetic coupling between the Eu³⁺ and the radical (<i>J</i> ₁₂ = +19.5 cm^-1, +<i>J</i> ₁₂ <i>S</i> ₁ <i>S</i> ₂ convention). Remarkably, both europium and lutetium complexes showed slow magnetization dynamics below 20 K. The field and temperature dependent relaxation dynamics, dominated by Raman and direct processes were modelled simultaneously, allowing us to assess that the Raman process is field dependent.