Dimerized p-Semiquinone Radical Anions Stabilized by a Pair of Rare-Earth Metal Ions

Tuning the slow magnetic relaxation with the substituents in anilate bridged bis(dysprosium) complexes

Dinuclear lanthanide complexes [((HB(pz)3)2Dy)2(μ-Th2An)] (1Dy) and [((HB(pz)3)2Dy)2(μ-ClCNAn)] (2Dy), based on the hydrotris(pyrazol-1-yl)borate (HBpz3-) scorpionate capping ligand and anilate (An2-) bridging linkers, namely homosubstituted dithiophene- and heterosubstituted chlorocyanoanilate, bearing electron-donating and withdrawing substituents at the 3,6-positions of the benzoquinone core, are reported. 1Dy shows an octacoordinated {N6O2} DyIII ion within a D4h distorted square antiprismatic coordination, an ideal geometry for Single-Molecule Magnet (SMM) behavior, given its oblate nature, whereas in 2Dy the octacoordinated DyIII ion adopts a D2d triangular dodecahedron geometry, while maintaining the same {N6O2} coordination sphere. Both complexes show field-induced single molecule magnet (SMM) behaviour, with tuning of the slow magnetic relaxation as a function of the nature of the substituents at the 3,6-positions of the anilate moiety. A comparison of the Arrhenius fitting parameters for 1Dy and 2Dy supports the hypothesis that square antiprismatic DyIII complexes, as 1Dy, exhibit higher energy barriers. This interpretation is supported by ab initio calculations that also shed light on the crucial role of intermolecular dipolar interactions.

Lanthanide-radical single-molecule magnets: current status and future challenges

In the field of molecular magnetism, the lanthanide-radical (Ln-Rad) method has become one of the most appealing tactics for introducing strong magnetic interactions and has spurred on the booming development of heterospin single-molecule magnets (SMMs). The article is a timely retrospect on the research progress of Ln-Rad heterospin systems and special attention is invested on low dimensional Ln-Rad compounds with SMM behavior, primarily concerning with nitrogen-based radicals, semiquinone and nitroxide radicals. Rational design, molecular structures, magnetic behaviors and magneto-structural correlations are highlighted. Meanwhile, particular attention is focused on the influence of exchange couplings on the dynamic magnetic properties, with the purpose of helping to guide the design of prospective radical-based Ln-SMMs.

Cerium-quinone redox couples put under scrutiny

Homoleptic cerous complexes Ce[N(SiMe₃)₂]₃, [Ce{OSi(OtBu)₃}₃]₂ and [Ce{OSiⁱPr₃}₃]₂ were employed as thermally robust, weakly nucleophilic precursors to assess their reactivity towards 1,4-quinones in non-aqueous solution. The strongly oxidizing quinones 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or tetrachloro-1,4-benzoquinone (Cl₄BQ) readily form hydroquinolato-bridged ceric complexes of the composition [(Ce^IVL₃)₂(μ₂-O₂C₆R₄)]. Less oxidising quinones like 2,5-di-tert-butyl-1,4-benzoquinone (tBu₂BQ) tend to engage in redox equilibria with the ceric hydroquinolato-bridged form being stable only in the solid state. Even less oxidising quinones such as tetramethyl-1,4-benzoquinone (Me₄BQ) afford cerous semiquinolates of the type [(Ce^IIIL₂(thf)₂)(μ₂-O₂C₆Me₄)]₂. All complexes were characterised by X-ray diffraction, ¹H, ¹³C{¹H} and ²⁹Si NMR spectroscopy, DRIFT spectroscopy, UV-Vis spectroscopy and CV measurements. The species putatively formed during the electrochemical reduction of [Ce^IV{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆H₄) could be mimicked by chemical reduction with Co^IICp₂ yielding [(Ce^III{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][Co^IIICp₂]₂.

Semiquinone radical-bridged M2 (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me6tren)2MII 2(C6H4O2 2-)]2+ (Me6tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me6tren)2MII 2(C6H4O2 -˙)]3+. Single-crystal X-ray diffraction shows that the Me6tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone-transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < -600 cm-1, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal-semiquinone exchange constants of J = -144(1) and -252(2) cm-1, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm-1, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear NiII complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.