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Lanthanide single-molecule magnets.

The magnet-like behavior can beobserved by slow relaxation of the magnetization below theblocking temperature. Since the discovery of SMMs in theearly 1990s, this assumption has formed the basis for theunderstanding of the origin of the anisotropic barrier.However, in recent years the development of novel lantha-nide-only SMMs that challenge and defy this theory pose anumber of questions:

A Polynuclear Lanthanide Single-Molecule Magnet with a Record

Here we develop a general approach to calculating the energy spectrum and the wave functions of the low-lying magnetic levels of a lanthanoid ion submitted to the crystal field created by the surrounding ligands. This model allows us to propose general criteria for the rational design of new mononuclear lanthanoid complexes behaving as single-molecule magnets (SMMs) or acting as robust spin qubits. Three typical environments exhibited by these metal complexes are considered, namely, (a) square antiprism, (b) triangular dodecahedron, and (c) trigonal prism. The developed model is used to explain the properties of some representative examples showing these geometries. Key questions in this area, such as the chemical tailoring of the superparamagnetic energy barrier, tunneling gap, or spin relaxation time, are discussed. Finally, in order to take into account delocalization and/or covalent effects of the ligands, this point-charge model is complemented with ab initio calculations, which provide accurate information on the charge distribution around the metal, allowing for an explanation of the SMM behavior displayed by some sandwich-type organometallic compounds.

Rational design of single-ion magnets and spin qubits based on mononuclear lanthanoid complexes.

A redox-active luminescent ytterbium based single molecule magnet

A new 2D dysprosium layer compound has been successfully synthesized from reaction with 2-(3-pyridyl) pyrimidine-4-carboxylic acid (H3-py-4-pmc), in which the Dy3+ ions reside in square antiprismatic coordination environments and are connected by carboxylate/oxalate/hydroxyl bridges. Magnetic studies reveal ferromagnetic interactions between Dy3+ ions, slow magnetic relaxation with an effective energy barrier Ueff of 186 K under zero dc field and pronounced hysteresis loops at low temperatures. Further dilution magnetic study suggests that the slow magnetic relaxation originates from the single-ion magnetic behavior of Dy3+ ion and that magnetic coupling suppresses the quantum tunneling of magnetization at low temperature. In addition, theoretical calculation indicates strong Ising anisotropy of the Dy3+ ion that is due to the strong interaction between Dy3+ ions and hydroxyl groups.

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Slow magnetic relaxation in a novel carboxylate/oxalate/hydroxyl bridged dysprosium layer

A single molecule magnet behaviour in a D3h symmetry Dy(III) complex involving a quinone–tetrathiafulvalene–quinone bridge

Novel method for the synthesis of functionalized tetrathiafulvalenes, an acceptor–donor–acceptor molecule comprising of two o-quinone moieties linked by a TTF bridge

The paramagnetic derivatives of 4,4′,7,7′-tetra-tert-butyl-2,2′-bis-1,3-benzodithiol-5,5′,6,6′-tetraone (1), viz., radical anion salts of the alkali metals (Li, Na, K) and cobaltocenium cations, chelated mono-o-semiquinone complexes with different metal fragments (Tl, TlMe2, SnPh3, Mn(CO)4, Mn(PPh3)(CO)3), a number of copper(I) complexes with sterically hindered phosphines as well as binuclear heterometallic derivatives of triphenylantimony(V) o-semiquinone-catecholate with the analogous paramagnetic centers, were studied by ESR spectroscopy. The reaction of di-o-quinone 1 with sodium amalgam resulted in the formation of all reduced forms including quinone-semiquinone, disemiquinone, semiquinone-catecholate, and dicatecholate. A radical cation with the unpaired electron localized on the tetrathiafulvalene (TTF) fragment, which resulted from the oxidation of di-o-quinone 1, was detected by ESR spectroscopy.

ESR study of paramagnetic derivatives of sterically hindered di-o-quinone with the tetrathiafulvalene bridge

The new mono- and binuclear semiquinonato dimethylthallium complexes (Q-TTF-SQ)TlMe2 (1) and Me2Tl(SQ-TTF-SQ)TlMe2 (2) based on di-o-quinone with tetrathiafulvalene (TTF) bridge, 4,4′,7,7′-tetra-tert-butyl-2,2′-bis-1,3-benzodithiol-5,5′,6,6′-tetraone Q-TTF-Q, were synthesized by the reaction between corresponding mono- and di-sodium semiquinonates (Q-TTF-SQ)Na and Na(SQ-TTF-SQ)Na and one or two equivalents of Me2TlCl, respectively. The same products could be obtained by the interaction of Q-TTF-Q with one or two equivalents of Me3Tl. Complexes 1 and 2 were characterized by IR and electronic absorption spectroscopy, EPR, and magnetic measurements. The molecular structures of 1 and 2 were determined by single-crystal X-ray diffraction. It was found that mono-semiquinonato derivative 1 partially disproportionates into Q-TTF-Q and binuclear complex 2 in THF solution. According to variable temperature magnetic susceptibility measurements and EPR data, compound 1 reveals paramagnetic behavior with an S = 1/2 state in the range 50–300 K, whereas compound 2 has an S = 0 ground state as the consequence of antiferromagnetic coupling between semiquinonato moieties realized through the TTF-bridge.

Svetlana Klementieva

Papers

A single molecule magnet behaviour in a D3h symmetry Dy(III) complex involving a quinone–tetrathiafulvalene–quinone bridge

Novel method for the synthesis of functionalized tetrathiafulvalenes, an acceptor–donor–acceptor molecule comprising of two o-quinone moieties linked by a TTF bridge

ESR study of paramagnetic derivatives of sterically hindered di-o-quinone with the tetrathiafulvalene bridge

Mono‐ and Binuclear Dimethylthallium(III) Complexes with o‐Benzoquinone‐TTF‐o‐Benzoquinone Ligand; Synthesis, Spectroscopy and X‐ray Study