Spectroscopic Determination of the Electronic Structure of a Uranium Single-Ion Magnet.

Early actinide ions have large spin-orbit couplings and crystal field interactions, leading to large anisotropies. The success in using actinides as single-molecule magnets has so far been modest, underlining the need for rational strategies. Indeed, the electronic structure of actinide single-molecule magnets and its relation to their magnetic properties remains largely unexplored. A uranium(III) single-molecule magnet, [UIII {SiMe2 NPh}3 -tacn)(OPPh3 )] (tacn=1,4,7-triazacyclononane), has been investigated by means of a combination of magnetic, spectroscopic and theoretical methods to elucidate the origin of its static and dynamic magnetic properties.

A Mononuclear Uranium(IV) Single-Molecule Magnet with an Azobenzene Radical Ligand.

A tetravalent uranium compound with a radical azobenzene ligand, namely, [{(SiMe2 NPh)3 -tacn}U(IV) (η(2) -N2 Ph2 (.) )] (2), was obtained by one-electron reduction of azobenzene by the trivalent uranium compound [U(III) {(SiMe2 NPh)3 -tacn}] (1). Compound 2 was characterized by single-crystal X-ray diffraction and (1) H NMR, IR, and UV/Vis/NIR spectroscopy. The magnetic properties of 2 and precursor 1 were studied by static magnetization and ac susceptibility measurements, which for the former revealed single-molecule magnet behaviour for the first time in a mononuclear U(IV) compound, whereas trivalent uranium compound 1 does not exhibit slow relaxation of the magnetization at low temperatures. A first approximation to the magnetic behaviour of these compounds was attempted by combining an effective electrostatic model with a phenomenological approach using the full single-ion Hamiltonian.

[U(Tp(Me2))2(bipy)]+: a cationic uranium(III) complex with single-molecule-magnet behavior.

The addition of 2,2'-bipyridine to [U(Tp(Me2))(2)I] (1) results in the displacement of the iodide and the formation of the cationic uranium(III) complex [U(Tp(Me2))(2)(bipy)]I (2). This compound was isolated as a dark-green solid in good yield and characterized by IR and NMR spectroscopies, and its molecular structure was determined by single-crystal X-ray diffraction. Studies of its magnetic properties revealed a frequency dependence of magnetization with a blocking temperature of 4.5 K and, at lower temperatures, a slow relaxation of magnetization with an energy barrier of 18.2 cm(-1), characteristic of single-molecule-magnet behavior.

Zero-field slow magnetic relaxation in a uranium(III) complex with a radical ligand.

[U(Tp(Me2))2(bipy˙)], a uranium(III) complex with a radical bipyridine ligand which has magnetic properties with contributions from both the ligand and the metal, presents slow relaxation of the magnetisation at low temperatures, already under zero static magnetic field, and energy barriers slightly above the non-radical analogues.

Single-ion magnet behaviour in [U(Tp(Me2))2I].

[U(Tp(Me2))(2)I] exhibits at low temperatures single molecule magnet (SMM) behaviour comparable to its bipyridine derivative and related single ion U(III) complexes recently reported as SMMs. The trend of variation of the energy barrier for the magnetic relaxation in these compounds is well reproduced by quantum chemistry calculations.