Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets

Abstract

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO 3 ) (Br 4 Cat)]•X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br 4 Cat 2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d 24 )(NO 3 )(Br 4 Cat)]•CH 3 CN-d 3 (1-Ln D ; Ln = La, Tb, Dy) have been synthesised, with 1-Tb D and the diamagnetic analogue 1-La D measured by INS. The dynamic magnetic properties of 1-Tb D and 1-Dy D have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO 3 )(Br 4 Cat)]•X (2-Ln: X = CH 2 Cl 2 ; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-Tb D by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-Tb D , while a previously reported anomalous phonon bottleneck effect in the 1-Dy D analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices.

Elucidation of LMCT Excited States for Lanthanoid Complexes: A Theoretical and Solid-State Experimental Framework

“…In previous work, we reported the magnetic properties of analogues of [Ln­(18- c -6)­(X 4 Cat)­(NO 3 )]·MeCN (Ln = La, Ce, Nd, Gd, Tb and Dy; 18- c -6 = 18- crown -6; X = Cl (tetrachlorocatecholate, 1-Ln ) or Br (tetrabromocatecholate, 2-Ln )). , As part of that work, we observed that the europium analogues with both tetrahalocatecholate ligands exhibit a very different color to the other analogues and corresponding broad absorption bands across the visible region (see Figure ). These bands are ascribed to LMCT transitions.…”

“…Nowadays, multireference methods based on complete active space self-consistent field (CASSCF) wave functions of different spin symmetries, coupled with nonperturbative spin–orbit coupling, are rather successful in describing the magnetic levels of Ln III complexes arising from the ground-state spin–orbit multiplet, , as well as providing important information about the ligand excited states, which can be used to estimate the energy transfer efficiency. , Experimentally observed multiconfigurational states have also been successfully modeled in this manner, notably for cyclopentadienyl complexes of Yb II/III . − The ab initio wave functions are usually optimized (i) for the metal-based states perturbed by the crystal field (CF) generated by the ligands, to study the magnetic properties of the compound and (ii) for the lanthanoid luminescence, or the ligands first excited levels, to study the ligand-based spectroscopic properties and energy transfer process. However, the modeling of a transition to a LMCT state has remained a significant challenge because it requires an accurate description of both the ground state, which is usually a magnetic level and the LMCT excited state, which involves a change of (oxidation) state for the metal and one or more ligands.…”

Modulation of Slow Magnetic Relaxation in Gd(III)‐Tetrahalosemiquinonate Complexes

“…This could lead to changes in the slow magnetic relaxation due to the spectrum of phonon modes that allow slow magnetic relaxation. This mechanism should affect only the dynamic magnetic properties, and not the static properties, as observed recently for solvatomorphs of the parent 2‐Cat analogues [29] . However, the difference in the crystal packing also leads to different intermolecular interactions in the two compounds.…”

“…This mechanism should affect only the dynamic magnetic properties, and not the static properties, as observed recently for solvatomorphs of the parent 2-Cat analogues. [29] However, the difference in the crystal packing also leads to different intermolecular interactions in the two compounds. This is observed as the difference in the low temperature magnetic susceptibility data, which was modelled with a mean-field interaction parameter J MF that is of the same magnitude, but notably of different signs in compounds 1 and 2.…”

“…[23] Of the readily available ligands that can be present as a stable radical, dioxolene and tetraoxolene ligands have found widespread use in transition metal molecular magnetism. [24,25] In Ln(III) molecular chemistry, tetraoxolene ligands have been used to bridge Ln(III) ions in dinuclear species, with the tetraoxolene in both the diamagnetic and radical forms, [26][27][28] while diamagnetic catecholate ligands have been utilized in both Ln(III) SMM [29][30][31] and other Ln compounds. [32][33][34][35] One group of compounds for which the Ln(III)-dioxolenes are isolable in their semiquinonate redox form are the family of compounds [Ln(Tp) 2 (DBSQ)] (Tp À = hydro-tris(1-pyrazolyl)borate, DBSQ• À = 3,5-di-tertbutyl-1,2-semiquinonate).…”