Three pentacoordinate Co^II complexes were synthesized using a common tridentate ligand and varying the halide/pseudohalide ligand. The effect of the latter on the geometry and magnetic properties of the three complexes has been analysed.

The first simultaneous redox and solvato-magnetic switching was achieved. The dinuclear complex [Dy₂(hfac)₆(H₂SQ)]·CH₂Cl₂ (Dy₂H₂SQ) (where hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate and H₂SQ = 2,2′-benzene-1,4-diylbis(6-hydroxy-4,7-di-tert-butyl-1,3-benzodithiol-2-ylium-5-olate) was reversibly oxidized into the dinuclear complex [Dy₂(hfac)₆(H₂O)₂(Q)] (Dy₂Q) (where Q = 2,2′-cyclohexa-2,5-diene-1,4-diylidenebis(4,7-di-tert-butyl-1,3-benzodithiole-5,6-dione)) inducing the reversible coordination of water molecules to the Dy^III ion. Magnetic susceptibility measurements and ab initio CASSCF/SI-SO calculations, confirmed by Cantilever Torque Magnetometry measurements, demonstrated that Dy₂H₂SQ is a Single-Molecule Magnet with a magnetic relaxation 7000 times slower than Dy₂Q (at 3 K) allowing a “ON–OFF” switching of the magnetic bistability.

We hereby report a dinuclear Dy(III) complex, [Dy(LH₃)Cl₂]₂·2Et₂O (1) (LH₄ = 2,3-dihydroxybenzylidene)-2-(hydroxyimino)propanehydrazide where both the metal centres are in a pentagonal bipyramidal (PBP) geometry with the axial positions being occupied by negatively charged Cl⁻ ions. The complex as well as it's 10% diluted analogue (1₁₀) do not show zero-field SMM behaviour. However, in the presence of small optimum dc fields the slow relaxation of magnetization was displayed. The effective energy barrier for 1₁₀ at 800 Oe of applied field was extracted as 83(17) K with τ₀ = 2(4) × 10⁻¹² s. Through a combined experimental and ab initio electronic structure calculations studies we have thoroughly analysed the role of the ligand field around the Dy(III), present in pentagonal bipyramidal geometry, in contributing to the slow relaxation of magnetization.

The reaction between the 2,2’-benzene-1,4-diylbis(6-hydroxy-4,7-di-tert-butyl-1,3-benzodithiol-2-ylium-5-olate triad (H₂SQ) and the metallo-precursor [Yb(hfac)₃]⋅2H₂O led to the formation of a dinuclear coordination complex of formula [Yb₂(hfac)₆(H₂SQ)]⋅0.5CH₂Cl₂ (H₂SQ-Yb). After chemical oxidation of H₂SQ in 2,2’-cyclohexa-2,5-diene-1,4-diylidenebis(4,7-di-tert-butyl-1,3-benzodithiole-5,6-dione (Q), the latter triad reacted with the [Yb(hfac)₃]⋅2H₂O precursor to give the dinuclear complex of formula [Yb₂(hfac)₆(Q)] (Q-Yb). Both dinuclear compounds have been characterized by X-ray diffraction, DFT optimized structure and electronic absorption spectra. They behaved as field-induced Single-Molecule Magnets (SMMs) nevertheless the chemical oxidation of the semiquinone to quinone moieties accelerated by a factor of five the relaxation time of the magnetization of Q-Yb compared to the one for H₂SQ-Yb. The H₂SQ triad efficiently sensitized the Yb^III luminescence while the chemical oxidation of H₂SQ into Q induced strong modification of the absorption properties and thus a quenching of the Yb^III luminescence for Q-Yb. In other words, both magnetic modulation and luminescence quenching are reached by the oxidation of the protonated semiquinone into quinone.

The two mononuclear complexes of the formula [Dy(tta)₃(L)] (1) and [Dy(hfac)₃(L)] (2) (where tta^- = 2-thenoytrifluoroacetylacetonate and hfac^- = 1,1,1,5,5,5-hexafluoroacetylacetonate) were obtained from the coordination reaction of the Dy(tta)₃·2H₂O or Dy(hfac)₃·2H₂O units with the 1,10-phenantroline-5,6-dione ligand (L). Their structures have been determined by X-ray diffraction studies on single crystals, and they revealed a supramolecular assembly of tetramers through σ-π interactions. Both complexes displayed a Single-Molecule Magnet (SMM) behavior without an external applied magnetic field. Magnetic relaxation happened through Orbach, Raman and Quantum Tunneling of the Magnetization (QTM). Wavefunction theory calculations were realized to rationalize the magnetic properties.