Cerium-Based Metal–Organic Frameworks: Unveiling the Role of Terahertz Vibrations in the Spin Relaxation Dynamics

The reaction between two equivalents of the Schiff base ligands <i>N</i>,<i>N</i>'-bis(3-methoxysalicylidene)ethylenediamine (H₂L1) or enantiopure <i>N</i>,<i>N</i>'-bis(3-methoxysalicylidene)cyclohexane-1,2-diamine (H₂L2) with one equivalent of Ce(NO₃)₃·6H₂O in the presence of a bulky counteranion leads to the formation of chiral metal-organic frameworks (MOFs) whose channels encapsulate the counteranion, leading to the formation of compounds with the structural formulas {[Ce(NO₃)₂(L1)₂]X·H₂O}_<i>n</i>, where X = ClO₄^- (<b>1</b>), PF₆^- (<b>2</b>), or BF₄^- (<b>3</b>), and {[Ce(NO₃)₂(L2)₂]X·CH₃CN}_<i>n</i>, where X = ClO₄^- (<b>4</b>), PF₆^- (<b>5</b>), or BF₄^- (<b>6</b>), as well as the isostructural reference compound {Nd(NO₃)₂(L1)₂]BF₄·CH₃CN}_<i>n</i> (<b>3Nd</b>). A combination of static and dynamic magnetic measurements demonstrates the good isolation of the Ce^III centers and a field-induced slow relaxation of the magnetization. Correlations between the temperature and field-dependent magnetic relaxation data and ultralow-frequency Raman spectroscopy reveal the presence of a vibronic barrier driving magnetic relaxation. Theoretical calculations have been performed to elucidate the nonparticipation of the electronic excited states in the main relaxation processes.