Slow magnetisation relaxation in tetraoxolene-bridged rare earth complexes

Abstract

Three families of tetraoxolene-bridged dinuclear rare earth (RE) complexes have been synthesised and characterised, with general formula [((HB(pz))RE)(μ-tetraoxolene)] (HB(pz) = hydrotris(pyrazolyl)borate; tetraoxolene = chloranilate (1-RE), the dianionic form of 2,5-dihydroxy-1,4-benzoquinone (2-RE), or its 3,6-dimethyl analogue (3-RE)). In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and species with selected lanthanoid(iii) ions from Eu(iii) to Yb(iii) have been obtained, as well as the diamagnetic Y(iii) analogues. Use of the 3,6-dimethyl substituted tetraoxolene ligand (Me-dhbq) has also afforded the two byproducts [((HB(pz))(MeOH)(B(OMe))Y)(μ-Medhbq)] (4-Y) and [{((HB(pz))(MeOH)Y)(μ-B(OMe))}(μ-Medhbq)]Cl (5-Y), with the B(OMe) ligands arising from partial decomposition of HB(pz). Electrochemical studies on the soluble 1-RE and 3-RE families indicate multiple tetraoxolene-based redox processes. Magnetochemical and EPR studies of 3-Gd indicate the negligible magnetic coupling between the two Gd(iii) centres through the diamagnetic tetraoxolene bridge. Alternating current magnetic susceptibility studies of 1-Dy and 3-Dy reveal slow magnetic relaxation, with quantum tunnelling of the magnetisation (QTM) dominant in the absence of an applied dc field. The application of a dc field suppresses the QTM and relaxation data are consistent with an Orbach relaxation mechanism playing a major role in both cases, with effective energy barriers to magnetisation reversal determined as 47 and 24 K for 1-Dy and 3-Dy, respectively. The different dynamic magnetic behaviour evident for 1-Dy and 3-Dy arises from small differences in the local Dy(iii) coordination environments, highlighting the subtle structural effects responsible for the electronic structure and resulting magnetic behaviour.

Series of Chloranilate-Bridged Dinuclear Lanthanide Complexes: Kramers Systems Showing Field-Induced Slow Magnetic Relaxation

“…The IR spectra of 1-5 provide complementary structural feature information to that obtained by SCXRD analysis ( Figure S1). The IR vibrations associated with the bridging Cl 2 An 2− ligand in the five complexes are typified by predominant C-Cl and C-O vibrations at~850 and 1530 cm −1 , respectively [22][23][24]. These results are a promising indication that the Cl 2 An 2− ligand in complexes 1-5 is in a bi-separated delocalized form [22][23][24]31].…”

“…The series of Cl 2 An 2− bridged neutral dinuclear Ln III complexes 1-5 were prepared by a slight modification of the original method reported by Kaizaki et al [21]; some of these complexes have been previously reported [22][23][24]. All the complexes were isolated as X-ray quality single crystals through several recrystallizations from a concentrated dichloromethane solution layered with hexane.…”

“…On the other hand, the Kramers systems (complexes 4 and 5) clearly displayed a slow magnetic relaxation under applied dc fields, suggesting field-induced single-molecule magnets that occur through Orbach and Raman relaxation processes. Magnetochemistry 2019, 5, 30 2 of 12 of the representative cluster-type SMM [Mn III 8 Mn IV 4 O 12 (CH 3 COO) 16 (H 2 O) 24 ]·2CH 3 COOH·4H 2 O, which is based on first-row transition metal ions [19,20].Recently, chloralilate (Cl 2 An 2− ) bridged dinuclear Ln III complexes of the formula [{Ln III (Tp) 2 } 2 (µ-Cl 2 An)]·2CH 2 Cl 2 (Tp − = hydrotris(pyrazolyl)borate) have been reported in the literature [21][22][23][24]. In these complexes, the Dy III analogue displays a slow magnetic relaxation under small applied dc magnetic fields, thus behaving as a field-induced SMM [22][23][24].…”

“…Magnetochemistry 2019, 5, 30 2 of 12 of the representative cluster-type SMM [Mn III 8 Mn IV 4 O 12 (CH 3 COO) 16 (H 2 O) 24 ]·2CH 3 COOH·4H 2 O, which is based on first-row transition metal ions [19,20].Recently, chloralilate (Cl 2 An 2− ) bridged dinuclear Ln III complexes of the formula [{Ln III (Tp) 2 } 2 (µ-Cl 2 An)]·2CH 2 Cl 2 (Tp − = hydrotris(pyrazolyl)borate) have been reported in the literature [21][22][23][24]. In these complexes, the Dy III analogue displays a slow magnetic relaxation under small applied dc magnetic fields, thus behaving as a field-induced SMM [22][23][24]. This paper reports the syntheses, structures, and magnetic properties of a series of Cl 2 An 2− bridged dinuclear Ln complexes of the formula [{Ln(Tp) 2 } 2 (µ-Cl 2 An)]·2CH 2 Cl 2 [Ln = Gd (1), Tb (2), Ho (3), Er (4), and Yb (5)] to systematically investigate the magnetic properties in other Ln III analogues of [{Ln III (Tp) 2 } 2 (µ-Cl 2 An)]·2CH 2 Cl 2 .…”

“…Recently, chloralilate (Cl 2 An 2− ) bridged dinuclear Ln III complexes of the formula [{Ln III (Tp) 2 } 2 (µ-Cl 2 An)]·2CH 2 Cl 2 (Tp − = hydrotris(pyrazolyl)borate) have been reported in the literature [21][22][23][24]. In these complexes, the Dy III analogue displays a slow magnetic relaxation under small applied dc magnetic fields, thus behaving as a field-induced SMM [22][23][24].…”