Correlating Axial and Equatorial Ligand Field Effects to the Single- Molecule Magnet Performances of a Family of Dysprosium Bis- Methanediide Complexes

Lewis Thomas-Hargreaves, Marcus Giansiracusa, Matthew Gregson, Emanuele Zanda, Felix O'Donnell, Ashley Wooles, Nicholas Chilton, Stephen Liddle

Research output: Contribution to journalArticlepeer-review

Abstract

Treatment of the new methanediide-methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh2S)2}2-) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)2][Dy(SCS)2(K(DME)2)2] (2Dy), [Dy(SCS)2][Na(DME)3] (3Dy) and [Dy(SCS)2][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)2][K(DB18C6)(THF)(Toluene)] (5Dy, NCN = {C(PPh2NSiMe3)2}2-, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy-5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (~14 Oe/s). Fitting the alternating current magnetic susceptibility data for 2Dy-5Dy gives energy barriers to magnetic relaxation (Ueff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy-4Dy join a priviledged group of SMMs with Ueff values of ~1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy-4Dy, the Dy-components can be grouped into 2Dy-cation/4Dy and 2Dy-anion/3Dy, where the former have almost linear C=Dy=C units with short average Dy=C distances, and the latter have more bent C=Dy=C units with longer average Dy=C bonds. Both Ueff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M-L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between Ueff barrier and axial linearity in structurally comparable systems.
Original languageEnglish
JournalChemical Science
Publication statusAccepted/In press - 1 Mar 2021

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