Abstract
Three families of tetraoxolene-bridged dinuclear rare earth (RE) complexes have been synthesised and characterised, with general formula [((HB(pz)3)2RE)2(μ-tetraoxolene)] (HB(pz)3- = 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 (Me2-dhbq2-) has also afforded the two byproducts [((HB(pz)3)(MeOH)(B(OMe)4)Y)2(μ-Me2dhbq)] (4-Y) and [{((HB(pz)3)(MeOH)Y)2(μ-B(OMe)4)}2(μ-Me2dhbq)2]Cl2 (5-Y), with the B(OMe)4- ligands arising from partial decomposition of HB(pz)3-. 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.
Highlights
The rare earth (RE) complexes [((HB( pz)3)2RE)2(μ-ca)] (1-RE) with RE = Y, Eu, Gd, Tb, Dy, Ho, Er and Yb were synthesised following modification of the literature procedure to enhance the purity of the product.[36]
The tetraoxolene ligands are in the diamagnetic dianionic form
Solution electrochemical studies of dinuclear complexes confirm the redox-activity of the tetraoxolene ligands, with the electrochemical reversibility of a tetraoxolene-based one-electron reduction suggesting the possibility of accessing an analogue with a trianionic radical bridge following chemical reduction
Summary
The last few years have seen impressive advances in the field of single-molecule magnets (SMMs), with slow magnetic relaxation that arises from intrinsic molecular properties reported at unprecedentedly high temperatures.[1,2,3,4,5] Two recent spectacular examples are mononuclear Dy complexes with high axial symmetry, each with experimentally measured energy barriers to magnetisation reversal (Ueff ) over 1200 cm−1 (1700 K).[1,2] A salt of the complex [(Cpttt)Dy]+ (Cpttt = 1,2,4-tri(tertbutyl)cyclopentadienide) exhibits magnetisation hysteresis up to 60 K.1. The practical target of molecules that can act as magnets at liquid nitrogen temperatures for applications, The energy barrier to magnetisation reversal for lanthanoid (Ln) SMMs arises from crystal field (CF) splitting of the ground spin–orbit coupled J state of the Ln(III) ion into microstates. Quantum tunnelling of the magnetisation (QTM) between degenerate microstates is an efficient relaxation pathway for many Ln-SMMs,[13,14]
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