Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering

Redox, spectroscopic and magnetic properties of C3-symmetric rare earth complexes featuring atypical ortho-dioxolene binding

The molecular symmetry in rare earth (RE) coordination chemistry is critically important for controlling the electronic structure of the RE ion and the resulting magnetic and photophysical properties. Here, we report a family of complexes with unusual C3-point symmetry: [REIII(Br4catH)3(tpa)] (Br4catH- = tetrabromocatecholate, tpa = tris(2-pyridylmethyl)amine). The synthesis and solid-state characterisation of eleven analogues (RE = Y, Sm to Lu) were performed, enabling a systematic investigation of the effect of symmetry on various physical properties across the RE series. The crystal structures reveal a unique cooperative coordination motif, featuring a cyclic hydrogen-bonding network between the atypical monodentate monoprotonated Br4catH- ligands. Electrochemical analysis reveals a single oxidation process that suggests a concerted three-electron oxidation of all tetrabromocatecholate ligands to semiquinonate. Furthermore, single-molecule magnet (SMM) behaviour was investigated, revealing unexpected in-field slow magnetic relaxation for both Dy and Yb analogues, which can be rationalised by the effect of C3-symmetry. Finally, luminescence measurements were performed to probe the CF splitting of the Yb analogue and quantify the error in the overall CF splitting predicted by ab initio calculations. The governing effects of C3-symmetry are consistent observations in all RE3+ metals studied in this work, manifesting in the concerted three-electron oxidation, SMM behaviour, ground state composition, and luminescence properties.

Trigonal Planar Heteroleptic Lanthanide(III) Bis(silyl)amide Complexes Containing Aminoxyl Radicals and Anions

Modulation of the crystal field (CF) in lanthanide (Ln) complexes can enhance optical and magnetic properties, and large CF splitting can be achieved with low coordination numbers in specific geometries. We previously reported that the homoleptic near-linear Sm2+ complex [SmII{N(SiiPr3)2}2] (1-Sm) is oxidized by the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO•) radical to give the heteroleptic, approximately trigonal planar Sm3+ complex, [SmIII{N(SiiPr3)2}2(TEMPO-)] (2-Sm). Here, we report the synthesis of homologous [LnIII{N(SiiPr3)2}2(TEMPO-)] (2-Ln; Ln = Tm, Yb) complexes by the oxidation of the parent [Ln{N(SiiPr3)2}2] (1-Ln; Ln = Tm, Yb) with TEMPO•; complexes 2-Ln all contain TEMPO- anions. The homoleptic bent Ln3+ complexes [LnIII{N(SiiPr3)2}2][B(C6F5)4] (3-Ln; Ln = Sm, Tm, Yb) were also treated with TEMPO• to yield the heteroleptic, approximately trigonal planar Ln3+ complexes [LnIII{N(SiiPr3)2}2(TEMPO•)][B(C6F5)4] (4-Ln; Ln = Sm, Tm, Yb); the cations of 4-Ln all contain TEMPO• radicals. We have compared the electronic structures of the two geometrically similar families of Ln3+ complexes with the TEMPO- anion (2-Ln) or TEMPO• radical (4-Ln) using a combination of UV-vis-NIR and EPR spectroscopy, magnetic measurements, and ab initio calculations. These studies revealed no single-molecule magnet behavior for 2-Yb despite evidence for sizable CF splitting and a high degree of purity of the ground stabilized mJ = |±7/2⟩ state, while the radical TEMPO• in 4-Yb did not significantly improve performance.

Isolation of Elusive Fluoflavine Radicals in Two Differing Oxidation States

Facile access and switchability between multiple oxidation states are key properties of many catalytic applications and spintronic devices yet poorly understood due to inherent complications arising from isolating a redox system in multiple oxidation states without drastic structural changes. Here, we present the first isolable, free fluoflavine (flv) radical flv(1-•) as a bottleable potassium compound, [K(crypt-222)](flv•), 1, and a new series of organometallic rare earth complexes [(Cp*2Y)2(μ-flvz)]X, (where Cp* = pentamethylcyclopentadienyl, X = [Al(OC{CF3}3)4]- (z = -1), 2; X = 0 (z = -2), 3; [K(crypt-222)]+ (z = -3), 4) comprising the flv ligand in three different oxidation states, two of which are paramagnetic flv1-• and flv3-•. Excitingly, 1, 2, and 4 constitute the first isolable flv1-• and flv3-• radical complexes and, to date, the only isolated flv radicals of any oxidation state. All compounds are accessible in good crystalline yields and were unambiguously characterized via single-crystal X-ray diffraction analysis, cyclic voltammetry, IR-, UV-vis, and variable-temperature EPR spectroscopy. Remarkably, the EPR spectra for 1, 2, and 4 are distinct and a testament to stronger spin delocalization onto the metal centers as a function of higher charge on the flv radical. In-depth analysis of the electron- and spin density via density functional theory (DFT) calculations utilizing NLMO, QTAIM, and spin density topology analysis confirmed the fundamental interplay of metal coordination, ligand oxidation state, aromaticity, covalency, and spin density transfer, which may serve as blueprints for the development of future spintronic devices, single-molecule magnets, and quantum information science at large.