Lattice Solvent Engineering Improves the Stability of a Cobalt Pyrenylnitronylnitroxide Ferrimagnetic Chain.

Reaction of 2-(1'-pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) with [Co(hfac)2(H2O)2] (hfac = hexafluoroacetylacetonate) in n-heptane solvent (hep) with a small amount of bromoform (CHBr3 = bf) gives the 1D ferrimagnetic complex [Co(hfac)2PyrNN]n·0.5bf·0.5hep (Co-PyrNN·bf). This chain exhibits slow magnetic relaxation with magnetic blocking below 13.4 K, presenting a magnetic hysteresis with high coercive field (51 kOe at 5.0 K) as a hard magnet. It also shows frequency-dependent behavior consistent with one dominant relaxation process with an activation barrier of Δτ/kB = (365 ± 24) K. The compound is an isomorphous variant of a previously reported ambient unstable chain made by using chloroform (CHCl3 = cf), [Co(hfac)2PyrNN]n·0.5cf·0.5hep (Co-PyrNN·cf). This shows that the variation of a magnetically inactive lattice solvent can improve the stability of analogous, void space containing single-chain magnets.

High-Frequency EPR Studies of New 2p-3d Complexes Based on a Triazolyl-Substituted Nitronyl Nitroxide Radical: The Role of Exchange Anisotropy in a Cu-Radical System.

Using the 1-(m-tolyl)-1H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) (TlTrzNIT) radical and metal ß-diketonate complexes [M(hfac)2(H2O)2], where hfac is hexafluoroacetylacetonato, three new 2p-3d heterospin complexes were synthesized. Their structures were solved using single crystal X-ray diffraction data, and magnetic investigation was performed by DC and AC measurements and multifrequency EPR spectroscopy. Compounds 1 and 2 are isostructural complexes with molecular formula [M3(TlTrzNIT)2(hfac)6] (MII = Mn or Cu) while compound 3 is the mononuclear [Co(TlTrzNIT)(hfac)2] complex. In all complexes, the radical acts as a bidentate ligand through the oxygen atom of the nitroxide moiety and the nitrogen atom from the triazole group. Furthermore, in compounds 1 and 2, the TlTrzNIT is bridge-coordinated between two metal centers, leading to the formation of trinuclear complexes. The fitting of the static magnetic behavior reveals antiferromagnetic and ferromagnetic intramolecular interactions for complexes 1 and 2, respectively. The EPR spectra of 1 are well described by an isolated ferrimagnetic S = 13/2 (= 5/2 - 1/2 + 5/2 - 1/2 + 5/2) ground state with a biaxial zero-field splitting (ZFS) interaction characterized, respectively, by 2nd order axial and rhombic parameters, D and E, such that E/D is close to the maximum of 0.33. Meanwhile, EPR spectra for 2 are explained in terms of a ferromagnetic model with weakly anisotropic Cu-radical exchange interactions, giving rise to an isolated S = 5/2 (= 5 × 1/2) ground state with both an anisotropic g tensor and a weak ZFS interaction. Complex 2 represents one of only a few examples of Cu-radical moieties with measurable exchange anisotropy.

SMM Behavior Tuned by an Exchange Coupling LEGO Approach for Chimeric Compounds: First 2p-3d-4f Heterotrispin Complexes with Different Metal Ions Bridged by One Aminoxyl Group.

Coordination compounds containing three different spin carriers (2p, 3d, and 4f), with the general formula [MIIDyIII(LH)(hfac)5] (M = Co, Ni, Mn, Zn), have been obtained using Mannich ligands decorated with a nitronyl-nitroxide fragment. The synthetic approach is general and leads to binuclear 3d-4f complexes, the two metal ions being bridged by one aminoxyl group and by one oxygen atom arising from a hfac- ligand. Triangular spin topology affords significant 2p-3d, 3d-4f, and 2p-4f exchange interactions. For the [CoIIDyIIIRad] derivative obtained using a nitronyl-nitroxide chiral ligand, a high energy barrier (∼200 cm-1) and a slow relaxation behavior below 30 K were found and rationalized by ab initio calculations. The improvement of magnetic properties comes from the synergy of optimal single ions properties and exchange couplings contributions where the CoII-Rad interaction becomes the leading one. The role played by this interaction is clearly proved by the investigation of the magnetic properties of the [ZnIIDyIIIRad] derivative, with a much lower energy barrier (12.7 cm-1) and by the lack of SMM behavior of the previously reported [CoIIDyIIIRad] compound ( Chem. Commun . 2017 , 53 , 6504 ), with a linear topology of spin carriers and a negligible CoII-Rad interaction.

Observation of Tunneling-Assisted Highly Forbidden Single-Photon Transitions in a Ni_{4} Single-Molecule Magnet.

Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni_{4} single-molecule magnet in which a single photon can induce the spin to change by several times ℏ, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard Δm=±1 selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon Δm=±1 transitions.

Thermodynamics of spin crossover in a bis(terpyridine) cobalt(II) complex featuring a thioether functionality.

In this contribution, a terpyridine-based ligand bearing a thioether functionality is used to prepare a new cobalt(II) spin crossover complex: [Co(TerpyPhSMe)2](PF6)2 (1), where TerpyPhSMe is 4'-(4-methylthiophenyl)-2,2':6',2''-terpyridine. Its structure, determined by single crystal X-ray diffraction, reveals a mer coordination of the tridentate terpyridine ligands, leading to a tetragonally compressed octahedron. Intermolecular interactions in the crystal lattice freeze the complex in the high spin state in the solid state at all temperatures, as indicated by magnetometry and Electron Paramagnetic Resonance (EPR) spectra. When dissolved in acetonitrile, however, temperature dependent electronic, 1H-NMR and EPR spectra highlight an entropy-driven spin crossover transition, whose thermodynamics parameters have been determined. This is the first report of a cobalt(II) SCO complex featuring a thioether group, allowing its implementation in chemically grown bistable monolayers and may open important perspectives for the use of such systems in molecular spintronics.

Lanthanide chains containing the naphthalenyl nitronyl nitroxide radical.

In this contribution we report the synthesis, structure and magnetic properties of a family of lanthanide-based one dimensional compounds [Ln(hfac)3(NaphNN)] n , where LnIII = Gd (1), Dy(2), Tb(3) and NaphNN is the nitronyl nitroxide (NN) radical 2-(1'-naphthalenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-Imidazole-3-oxide-1-oxyl. The crystal structure reveals well isolated chains with a twofold helical axis. Magnetic investigation of the gadolinium(iii) chains showed relevant intrachain interactions between Gd-NN nearest neighbors and also Gd-Gd and NN-NN next nearest neighbors. The magnetic interaction parameters were obtained by fitting the data with a six membered ring model. The stronger antiferromagnetic interaction between NN radicals was confirmed by investigation of the mononuclear complex [Y(hfac)3(NaphNN)2] (4) with a similar coordination environment. The dynamic magnetic properties of 2 and 3 were investigated by using the temperature and frequency dependence of the magnetic susceptibility evidencing single chain magnet dynamics under a zero dc field.