66 research outputs found
Single-molecule magnet properties of a monometallic dysprosium pentalene complex
The pentalene-ligated dysprosium complex [(8-Pnâ )Dy(Cp*)] (1Dy) (Pnâ = [1,4-(iPr3Si)2C8H4]2â) and its magnetically dilute analogue are single-molecule magnets, with energy barriers of 245 cmâ1. Whilst the [Cp*]â ligand in 1Dy provides a strong axial crystal field, the overall axiality of this system is attenuated by the unusual folded structure of the [Pnâ ]2â ligand
Magnetic hysteresis up to 80 kelvin in a dysprosium metallocene single-molecule magnet
Single-molecule magnets (SMMs) containing only one metal center may represent the lower size limit for molecule-based magnetic information storage materials. Their current drawback is that all SMMs require liquid-helium cooling to show magnetic memory effects. We now report a chemical strategy to access the dysprosium metallocene cation [(CpiPr5)Dy(Cp*)]+ (CpiPr5 = penta-iso-propylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl), which displays magnetic hysteresis above liquid-nitrogen temperatures. An effective energy barrier to reversal of the magnetization of Ueff = 1,541 cmâ1 is also measured. The magnetic blocking temperature of TB = 80 K for this cation overcomes an essential barrier towards the development of nanomagnet devices that function at practical temperatures
Identification of oxidation state +1 in a molecular uranium complex
The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(Ρ5-C5iPr5)2] and the uranium(III) metallocene [IU(Ρ5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(Ρ5-C5iPr5)2]- (1) (C5iPr5 = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f3(7s/6dz2)1(6dx2-y2/6dxy)1 configuration. The metal-ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides
Synthesis and single-molecule magnet properties of a trimetallic dysprosium metallocene cation
The dimetallic fulvalene-bridged dysprosium complex [{Dy(Cp*)(Îź-BH4)}2(Fvtttt)] (1, Cp* = C5Me5) is converted into the trimetallic borohydride-bridged species [{Dy(Cp*)(Fvtttt)}2Dy(Îź-BH4)3] (2). In turn, 2 is reacted with a silylium electrophile to give [{Dy(Cp*)(Îź-BH4)(Fvtttt)}2Dy][B(C6F5)4] ([3][B(C6F5)3]), the first trimetallic dysprosocenium cation. Compound [3][B(C6F5)3] can also be formed directly from 1 by adding two equivalents of the electrophile. A three-fold enhancement in the effective energy barrier from 2 to 3 is observed and interpreted with the aid ofab initiocalculations
Dominance of Cyclobutadienyl Over Cyclopentadienyl in the crystal field splitting in dysprosium single-molecule magnets
Replacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single-molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(Ρ4-Cbâ˛â˛â˛â˛)Dy(Ρ5-C5Me4tBu)(BH4)]â (1), [(Ρ4-Cbâ˛â˛â˛â˛)Dy(Ρ8-Pnâ )K(THF)] (2) and [(Ρ4-Cbâ˛â˛â˛â˛)Dy(Ρ8-Pnâ )]â (3) leads to larger energy barriers to magnetization reversal (Cbâ˛â˛â˛â˛=C4(SiMe3)4, Pnâ =1,4-di(tri-isopropylsilyl)pentalenyl). Short distances to the Cbâ˛â˛â˛â˛ ligands and longer distances to the Cp ligands in 1â3 are consistent with the crystal field splitting being dominated by the former. Theoretical analysis shows that the magnetic axes in the ground Kramers doublets of 1â3 are oriented towards the Cbâ˛â˛â˛â˛ ligands. The theoretical axiality parameter and the relative axiality parameter Z and Zrel are introduced to facilitate comparisons of the SMM performance of 1â3 with a benchmark SMM. Increases in Z and Zrel when Cbâ˛â˛â˛ replaces Cp signposts a route to SMMs with properties that could surpass leading systems
Isolation of a perfectly linear uranium(II) metallocene
Reduction of the uranium(III) metallocene [(Ρ 5 âC 5 i Pr 5 ) 2 UI] ( 1 ) with potassium graphite produces the 'secondâgeneration' uranocene [(Ρ 5 âC 5 i Pr 5 ) 2 U] ( 2 ), which contains uranium in the formal divalent oxidation state. The geometry of 2 is that of a perfectly linear bis(cyclopentadienyl) sandwich complex, with the groundâstate valence electron configuration of uranium(II) revealed by electronic spectroscopy and density functional theory to be 5f 3 6d 1 . Appreciable covalent contributions to the metalâligand bonds were determined from a computational study of 2 , including participation from the uranium 5f and 6d orbitals. Whereas three unpaired electrons in 2 occupy orbitals with essentially pure 5f character, the fourth electron resides in an orbital defined by strong 7sâ6d z 2 mixing
Discovery of a dysprosium metallocene single-molecule magnet with two high-temperature Orbach processes
Magnetic bistability in single-molecule magnets (SMMs) is a potential basis for new types of nanoscale information storage material. The standard model for thermally activated relaxation of the magnetization in SMMs is based on the occurrence of a single Orbach process. Here, we show that incorporating a phosphorus atom into the framework of the dysprosium metallocene [(CpiPr5)Dy(CpPEt4)]+[B(C6F5)4]- (CpiPr5 is penta-isopropylcyclopentadienyl, CpPEt4 is tetraethylphospholyl) leads to the occurrence of two distinct high-temperature Orbach processes, with energy barriers of 1410(10) and 747(7) cm-1, respectively. These barriers provide experimental evidence for two different spin-phonon coupling regimes, which we explain with the aid of ab initio calculations. The strong and highly axial crystal field in this SMM also allows magnetic hysteresis to be observed up to 70 K, using a scan rate of 25 Oe s-1. In characterizing this SMM, we show that a conventional Debye model and consideration of rotational contributions to the spin-phonon interaction are insufficient to explain the observed phenomena
Primary Prevention of Gestational Diabetes Mellitus and Large-for-Gestational-Age Newborns by Lifestyle Counseling: A Cluster-Randomized Controlled Trial
In a cluster-randomized trial, Riitta Luoto and colleagues find that counseling on diet and activity can reduce the birthweight of babies born to women at risk of developing gestational diabetes mellitus (GDM), but fail to find an effect on GDM
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