Switchable molecular magnets

Recent developments in supramolecular complexes of azabenzenes containing one to four N atoms: synthetic strategies, structures, and magnetic properties

For the last couple of decades, azabenzene-based ligands have drawn much attention from inorganic chemists due to their ability to coordinate with different metal ions to form supramolecular clusters. These azabenzenes are weak σ donors and strong π acceptors and electron-deficient. Metallogrid complexes and non-grid oligomers are well-defined supramolecular clusters, formed by appropriate chelating ligands, and can show interesting optical, magnetic, and electronic properties. Self-assembly of [n × n] metallogrid complexes is dominated by the entropic factor while the formation of oligonuclear metal ion complexes is dominated by other effects like CFSE, electrostatic factors, ligand conformational characters, etc. Herein, the present article gives an overview of six-membered heterocyclic azine-based ligands and their potential for different metal ions to form polynuclear complexes. Moreover, their temperature-dependent magnetic properties and SCO phenomena are well described and tabulated.

Pre- and post-synthetic modulation of the ordering temperatures in a family of anilato-based magnets

We report the synthesis and characterization of six novel heterometallic molecule-based 2D magnets with the bromanilato ligand (C₆O₄Br₂^2- = 1,3-dibromo-2,5-dihydroxy-1,4-benzoquinone dianion) and six different benzene derivative molecules. The compounds, formulated as (NBu₄)[MnCr(C₆O₄Br₂)₃]·1.75C₆H₅Br (1), (NBu₄)[MnCr(C₆O₄Br₂)₃]·C₆H₅X with X = Cl (2), I (3) and CH₃ (4) and (NBu₄)[MnCr(C₆O₄Br₂)₃]·2C₆H₅X with X = CN (5) and NO₂ (6), present the classical hexagonal honeycomb-(6,3) lattice with alternating Mn(ii) and Cr(iii) ions. The layers are packed in an eclipsed way along the a direction giving rise to hexagonal channels where the benzene derivative molecules are located with π-π interactions between the benzene and anilato rings. The interlayer space contains the NBu₄⁺ cations needed to compensate the anionic charge of the [Mn^IICr^III(C₆O₄Br₂)₃]^- layers. The Mn-Cr exchange coupling through the bromanilato ligands is antiferromagnetic, leading to a long range ferrimagnetic order in the six compounds with ordering temperatures around 10 K. These ordering temperatures can be slightly modified in the range 9.5-11.4 K by simply changing the benzene-derivative solvent molecule. Here we discuss the possible structural and electronic reasons for this tuning effect of the solvent molecule and the important structural role played by the solvent molecules. We also show that it is possible to exchange the solvent molecules inside the hexagonal channels post-synthetically causing a tiny change in the ordering temperature and coercive field. Furthermore, we also show that it is possible to further change the ordering temperatures by simply removing the solvent molecules by heating the sample at low pressures to obtain a de-solvated phase.

Chloranilato-Based Layered Ferrimagnets with Solvent-Dependent Ordering Temperatures

We report the synthesis and the characterization of six new heterometallic chloranilato-based ferrimagnets formulated as (NBu4)[MnCr(C6O4Cl2)3]·nG with n = 1 for G = C6H5Cl (1), C6H5I (3), and C6H5CH3 (4); n = 1.5 for G = C6H5Br (2) and n = 2 for G = C6H5CN (5) and C6H5NO2 (6); (C6O4Cl2)2− = 1,3-dichloro,2,5-dihydroxy-1,4-benzoquinone dianion. The six compounds are isostructural and show hexagonal honeycomb layers of the type [MnCr(C6O4Cl2)3]− alternating with layers containing the NBu4+ cations. The hexagons are formed by alternating Mn(II) and Cr(III) connected by bridging bis-bidentate chloranilato ligands. The benzene derivative solvent molecules are located in the hexagonal channels (formed by the eclipsed packing of the honeycomb layers) showing π-π interactions with the anilato rings. The six compounds behave as ferrimagnets with ordering temperatures in the range 9.8–11.2 K that can be finely tuned by the donor character of the benzene ring and by the number of solvent molecules inserted in the hexagonal channels. The larger the electron density on the aromatic ring and the larger the number of solvent molecules are, the higher Tc is. The only exception is provided by toluene, where the formation of H-bonds might be at the origin of weaker π-π interactions observed in this compound.

A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines

Complexes [Co(L)₂](ClO₄)₂ (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a-c) containing three redox active centres (a Co²⁺ ion and two triaryl amine (Tara) units) have been synthesised. The order of oxidation steps in [Co(L)₂](ClO₄)₂ (L = 1a-c) was determined using cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. In acetonitrile solutions, at room temperature, the first oxidation is Co-centred followed by the Tara oxidation at more anodic potentials. The order of oxidation is inverted in solutions of the less polar solvent dichloromethane. The Co^3+/2+-centred redox event leads to a spin transition between the paramagnetic high-spin (HS) Co²⁺ and the diamagnetic low-spin (LS) Co³⁺ state, which was proven using ¹H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L)₂](ClO₄)₂, an equilibrium between the diamagnetic [Co³⁺(L)]³⁺ and paramagnetic [Co²⁺(L)(L⁺)]³⁺ state in [Co(L)₂]³⁺ (L = 1a-c) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between the [Co²⁺(L)₂]²⁺ and [Co³⁺(L)₂]³⁺ redox states mediated by [Co²⁺(L)(L⁺)]³⁺. Variable temperature vis-NIR spectroscopy of in situ generated [Co(L)₂]³⁺ revealed a temperature-dependent redox equilibrium between the [Co³⁺(L)₂]³⁺ and the [Co²⁺(L⁺)(L)]³⁺ states (L = 1a-c). Magnetic coupling between the HS-Co²⁺ ion and the Tara⁺ radical in [HS-Co²⁺(L⁺)(L)]³⁺ (L = 1a,c) was deduced from broad and undetectable lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co³⁺(L⁺)₂]⁵⁺ (L = 1a,c) leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.

Why Are Dithienylethene-Linked Biscobaltocenes so Hard to Photoswitch?

Attaching an organometallic unit to a dithienylethene (DTE) molecular switch can allow one to vary its switching and spectroscopic properties, and to create switchable magnetic properties. In this work, two different dithienylethene molecular switches are used as a bridge between two cobalt sandwich units. The only difference between the switching cores is in the size of the cycloalkene ring connecting both thiophene rings. The complexes present different oxidation states for the cobalt atoms, which are demonstrated to determine the switching reaction. The UV/Vis measurements show that while the Co(I) complexes undergo the switching reaction, the Co(II,III) complexes switch poorly. Kohn-Sham density functional theory calculations indicate diabatic ring-closure mechanisms and a large number of excited states hindering the cyclization reaction and favoring the relaxation to the open form of the molecular switch.

Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism

We report the synthesis and magnetic and photomagnetic behaviour of a novel valence tautomeric cobalt complex, [Co(3,5-dbbq)₂(μ-bpym)] (1) (3,5-dbbq = 3,5-di-tert-butyl-1,2-benzoquinone and μ-bpym = 2,2′-bipyrimidine). The synthesis is performed by reacting Co₂(CO)₈ and μ-bpym in the presence of the ligand 3,5-dbbq in a mixed solvent under inert atmosphere. The magnetic behavior clearly shows the presence of electron transfer from the catecholate ligand to the cobalt center, producing valence tautomers of [Co^II(SQ)₂] with a transition temperature (T_1/2) of 215 K. Photomagnetic studies, performed via both SQUID magnetometry and X-band electron paramagnetic resonance, show the clear presence of photoinduced valence tautomerism, at temperatures considerably higher than previous systems. A metastable charge distribution is observed, strengthening previous investigations on the character of mixed valence ligands. Entropy-driven valence tautomeric interconversion is observed, and drives the transition to the most stable charge distribution. The complex has the ability to coordinate and can be used as a photoswitchable building block, with the photomagnetic characterisation evidencing a metastable state lifetime of the photo-induced valence tautomeric process of ca. 2.9 × 10⁴ s below 20 K. The observed yields are higher than ones in similar systems, showing that tiny changes in the molecular structures may have a huge impact.

Interweaving spins with their environment: novel inorganic nanohybrids with controllable magnetic properties

We discuss the developments in the synthesis and characterization of magnetic nanohybrids made of molecular magnets and nanostructured materials.