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

Abstract

The reaction between two equivalents of the Schiff base ligands N,N′-bis(3-methoxysalicylidene)ethylenediamine (H2L1) or enantiopure N,N′-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}n, where X = ClO4- (1), PF6- (2), or BF4- (3), and {[Ce(NO3)2(L2)2]X·CH3CN}n, where X = ClO4- (4), PF6- (5), or BF4- (6), as well as the isostructural reference compound {Nd(NO3)2(L1)2]BF4·CH3CN}n (3Nd). A combination of static and dynamic magnetic measurements demonstrates the good isolation of the CeIII 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.