A Sodium Sodate as Precursor for Lanthanide Bis(4-R-benzoxazol-2-yl)methanide Single-Molecule Magnets

Abstract

From the sodium sodate precursor [(Na­(thf)6]­[Na­{(4-Me-NCOC6H3)2CH}2] (1) three isostructural dinuclear lanthanide complexes [(μ-Cl)­LnIII{(4-MeNCOC6H3)2CH}2]2 with Ln = Gd (2), Dy (3), and Er (4) based on the N,N′-chelating monoanionic bis­(4-methylbenzoxazol-2-yl)­methanide ligand (titled “Mebox”) were synthesized and characterized by X-ray diffraction and magnetic measurements. The sodium precursor 1 was analyzed via X-ray diffraction and diffusion-ordered NMR spectroscopy experiments (DOSY-NMR) in order to investigate its aggregation in solution and the solid state. The sodium analog [(thf)3Na­(NCOC6H4)2CH] (1′) based on the bis­(benzoxazol-2-yl)-methanide ligand (titled “box”) was prepared and analyzed for comparison reasons. From the lanthanide derivatives 2–4, the DyIII complex 3 displays slow relaxation of magnetization at zero field, with a relaxation barrier of U = 315.7 cm–1. The coupling strength between the two lanthanide centers was estimated with the GdIII equivalent 2, giving a weak antiferromagnetic coupling of J = −0.035 cm–1.

Synthesis and Magnetic Properties of Polyimidosulfur- and Bis(benzoxazolyl)methanide-ligated Complexes

“…We expect a 3 F ground term from Russell-Saunders schemata that splits in a 3 A2g ligand field term. Broadly speaking, an A term does not display first-order SOC, but the derived excited states, 3 T1g and 3 T2g, can mix with the ground state.…”

“…𝜏 −1 = 𝐴𝐻 𝑛 1 𝑇 + 𝐶𝑇 𝑛 2 + 𝜏 0 −1 exp (− 𝑈 eff 𝑘 B 𝑇 ) + 𝑄 (1)(2)(3)(4) where T devotes to the specific temperature dependency and 𝐴, 𝐶, 𝜏 0 −1 , and 𝑄 are treated as freeto-fit parameters since their determination is highly challenging. The four contributing terms of the relaxation processes are outlined below in detail.…”

“…Equation (1)(2)(3)(4)(5)(6) combines the free-to-fit parameter 𝐴, the magnetic field 𝐻 and 𝑛 1 = 2 for a non-Kramers ion and 𝑛 1 = 4 for a Kramers ion. One can perform field-dependent measurements to identify the direct process, which usually leads to a decay of the relaxation times 𝜏 −1 for high fields (𝐻).…”

“…𝜏 −1 = 𝑄 (1)(2)(3)(4)(5)(6)(7)(8) QTM is a free-to-fit parameter, it is often excluded from determining the energy barrier, especially for field-induced SMMs, where its impact is usually low. [21] Previous attempts to minimize the QTM contribution to the relaxation of SMMs are presented in chapter 1.5.…”

“…[73] To distinguish between a regular paramagnet and an SMM, one must search for maxima in the outof-phase susceptibility χ''. A paramagnet should not display slow relaxation as χT = χS, which results in a zero value for χ'' according to equation (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). Further, its χ' value is constant and refers directly to χS as the second part of the equation (1-10) is zero.…”