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Slow magnetisation relaxation in tetraoxolene-bridged rare-earth complexes

Abstract

Three families of tetraoxolene-bridged dinuclear rare earth complexes of 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)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HB(pz)3 ligands completing the coordination. Specis 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-) 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 tetramethoxyborate ligands arising from partial decomposition of HB(pz)3. Electrochemical studies on the soluble 1-RE and 3-RE families indicated a reversible tetraoxolene-based one-electron reduction process for both families, although efforts to chemically reduce the parent complexes to isolate analogues with radical trianionic bridging ligands were not successful. Magnetochemical and EPR studies of spin-only 3-Gd confirm that there is 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 magnetisation quantum tunnelling (QTM) dominant in the absence of an applied dc field. The application of a dc field effectively 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 quite different dynamic magnetic behaviour evident for 1-Dy and 3-Dy, despite the similar local Dy(III) coordination environments, highlights the subtle structural effects that are responsible for the electronic structure and resulting magnetic behaviour.

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Publication details

The article was received on 09 Aug 2017, accepted on 13 Sep 2017 and first published on 13 Sep 2017


Article type: Paper
DOI: 10.1039/C7DT02932B
Citation: Dalton Trans., 2017, Accepted Manuscript
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    Slow magnetisation relaxation in tetraoxolene-bridged rare-earth complexes

    M. Dunstan, E. Rousset, M. Boulon, R. W. Gable, L. Sorace and C. Boskovic, Dalton Trans., 2017, Accepted Manuscript , DOI: 10.1039/C7DT02932B

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