220 research outputs found
Strong direct exchange coupling and single-molecule magnetism in indigo-bridged lanthanide dimers
The synthesis, structure and magnetic properties of the indigo-bridged dilanthanide complexes [{(η5-Cp*)2Ln}2(ÎŒ-ind)]nâ with Ln = Gd or Dy and n = 0, 1 or 2 are described. The gadolinium complexes with n = 0 and 2 show typically weak exchange coupling, whereas the complex bridged by the radical [ind]3â ligand shows an unusually large coupling constant of J = â11 cmâ1 (â2J formalism). The dysprosium complexes with n = 0 and 1 are single-molecule magnets in zero applied field, whereas the complex with n = 2 does not show slow magnetic relaxation
Allyl and pentadienyl carbanion complexes of alkali metals : metal- and functionality-directed structure and bonding
Five ansa-tris(allyl) complexes [(PhSi{C3H3(SiMe3)}3)(Li?tmeda)3] (2.1), [(MeSi{C3H3(SiMe3)}3)(Li?pmdeta)3] (2.2), [(MeSi{C3H3(SiMe3)}3)-(Na.tmeda)3] (2.3), [(PhSi{C3H3(SiMe3)}3)(Na?tmeda)2Na]2 [2.4]2 and [(MeSi-{C3H3(SiMe3)}3)(K?OEt2)2(KLi{OtBu})]2 [2.5]2 have been synthesised, and studied by X-ray crystallography and NMR spectroscopy. A collaboration was undertaken to study some of the complexes by DFT. Crystallographic studies have shown that the overall structure of the complex is dependent on a combination of several factors: the metal cation; the substituent on the central silicon atom for the ansa-tris(allyl) ligands; and the co-ligand, tmeda or pmdeta. (tmeda = N,N,N?,N?ÂŹ-tetramethylethylenediamine and pmdeta = N,N,N?,N?,N??-pentamethyldiethylene-triamine). Solution studies of the ansa-tris(allyl) complexes showed that the solid-state structures are maintained in solution. The first examples of donor-functionalised allyl pro-ligands have been synthesised and coordinated to a variety of s-block metals; [Li{(SiMe3)2C3H2(1-CH2C4H7O)}]2 [4.1]2, [Li{(SiMe3)2C3H2(1-CH2CH2OCH3)}]2 [4.2]2, [(thf)K{(SiMe3)2C3H2(1-CH2C4H7O)}]2 [4.5]? and [Mg{(SiMe3)2C3H2(1-CH2C4H7O)}2] (4.6). As with the ansa-tris(allyl) complexes, both X-ray crystallographic and NMR spectroscopy studies have been undertaken, and the structures of the donor-functionalised allyl complexes were found to be dependent on the metal cation, with each cation coordinated in a different manner by the allyl ligand. For the potassium allyl complex 4.5 there is complete delocalisation of the allyl negative charge, and it is ?3-coordinated in a polymeric structure. However for lithium complexes, [4.1]2 and [4.2]2, the donor-functionalised allyl ligand is ?2-coordinated, and the negative charge is only partially delocalised. The magnesium complex 4.6 has the allyl ligand coordinated via a ?-bond to the metal and the allyl has localised single and double bonds.Finally, the synthesis of the first two donor-functionalised pentadienyl ligands and their lithium complexes are reported. Complexes [(tmeda)Li{1,5-(SiMe3)2C5H4(CH2C4H7O)}] (6.1) and [(tmeda)Li{1,5-(SiMe3)2C5H4(CH2CH2OCH3)}] (6.2) are the first structurally characterised lithium pentadienyl complexes, and are the first donor-functionalised pentadienyl complex of any metal. As well as structural characterisation, complexes 6.1 and 6.2 have been investigated by NMR spectroscopy and collaborative DFT studies. X-ray crystallography revealed that both complexes have the W-conformation of the pentadienyl ligand ?2-coordinated to the lithium cation, as well as the ether oxygen atom and the tmeda nitrogen atoms. DFT studies showed that the most stable gas-phase structure of the 1,5-bis(trimethylsilyl)-pentadienyl anion is the W-conformation, but its lithium complex is most stable in the U-conformation. The [Li{1,5-(SiMe3)2C5H4(CH2CH2OCH3)}]? anion has the W-conformation and the U-conformation is isoenergetic, but the addition of tmeda gives the W-conformation as the most stable in both the gas-phase and in toluene. Finally NMR spectroscopy studies showed that in solution complexes 6.1 and 6.2 are either in the symmetrical U-conformation or in fluxional process with a very low activation energy.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Uranocenium: synthesis, structure and chemical bonding
Abstraction of iodide from [(5-C5iPr5)2UI] (1) produces the cationic uranium(III) metallocene [(5-C5iPr5)2U]+ (2) as a salt of [B(C6F5)4]â. The structure of 2 consists of unsymmetrically bonded cyclopentadienyl ligands and a bending angle of 167.82° at uranium. Analysis of the bonding in 2 shows that the uranium 5f orbitals are strongly split and mixed with the ligand orbitals, leading to non-negligible covalent contributions to the bonding. Studying the dynamic magnetic properties of 2 reveals that the 5f covalency leads to partially quenched anisotropy and fast magnetic relaxation in zero applied magnetic field. Application of a magnetic field leads to dominant relaxation via a Raman process
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
Interpreting molecular crystal disorder in plumbocene, Pb(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>: Insight from theory
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
Bimetallic synergy enables silole insertion into THF and the synthesis of Erbium singleâmolecule magnets
The potassium silole K2[SiC4â2,5â(SiMe3)2â3,4âPh2] reacts with [M(η8âCOT)(THF)4][BPh4] (M=Er, Y; COT=cycloâoctatetraenyl) in THF to give products that feature unprecedented insertion of the nucleophilic silicon centre into a carbonâoxygen bond of THF. The structure of the major product, [(ÎŒâη8 : η8âCOT)M(ÎŒâL1)K]â (1M), consists of polymeric chains of sandwich complexes, where the spiroâbicyclic silapyran ligand [C4H8OSiC4(SiMe3)2Ph2]2â (L1) coordinates to potassium via the oxygen. The minor product [(ÎŒâη8 : η8âCOT)M(ÎŒâL1)K(THF)]2 (2M) features coordination of the silapyran to the rareâearth metal. In forming 1M and 2M, silole insertion into THF only occurs in the presence of potassium and the rareâearth metal, highlighting the importance of bimetallic synergy. The lower nucleophilicity of germanium(II) leads to contrasting reactivity of the potassium germole K2[GeC4â2,5â(SiMe3)2â3,4âMe2] towards [M(η8âCOT)(THF)4][BPh4], with intact transfer of the germole occurring to give the coordination polymers [{η5âGeC4(SiMe3)2Me2}M(η8âCOT)K]â (3M). Despite the differences in reactivity induced by the group 14 heteroatom, the singleâmolecule magnet properties of 1Er, 2Er and 3Er are similar, with thermally activated relaxation occurring via the firstâexcited Kramers doublet, subject to effective energy barriers of 122, 80 and 91 cmâ1, respectively. Compound 1Er is also analysed by highâfrequency dynamic magnetic susceptibility measurements up to 106 Hz
Carbonyl back-bonding influences the rate of quantum tunnelling in a dysprosium metallocene single-molecule magnet
The isocarbonyl-ligated metallocene coordination polymers [Cp*2M(ÎŒ-OC)W(Cp)(CO)(ÎŒ-CO)]â were synthesized with M = Gd (1, L = THF) and Dy (2, no L). In a zero direct-current field, the dysprosium version 2 was found to be a single-molecule magnet (SMM), with analysis of the dynamic magnetic susceptibility data revealing that the axial metallocene coordination environment leads to a large anisotropy barrier of 557(18) cmâ1 and a fast quantum-tunnelling rate of âŒ3.7 ms. Theoretical analysis of two truncated versions of 2, [Cp*2Dy{(ÎŒ-OC)W(Cp)(CO)2}2]â (2a), and [Cp*2Dy(OC)2]+ (2b), in which the effects of electron correlation outside the 4f orbital space were studied, revealed that tungsten-to-carbonyl back-donation plays an important role in determining the strength of the competing equatorial field at dysprosium and, hence, the dynamic magnetic properties. The finding that a classical organo-transition-metal bonding scenario can be used as an indirect way of tuning the rate of quantum tunnelling potentially provides an alternative chemical strategy for utilizing the fast magnetic relaxation properties of SMMs.peerReviewe
Chemical modulation of autophagy as an adjunct to chemotherapy in childhood and adolescent brain tumors
Brain tumors are the leading cause of cancer-related death in children and are the most challenging childhood cancer in relation to diagnosis, treatment, and outcome. One potential novel strategy to improve outcomes in cancer involves the manipulation of autophagy, a fundamental process in all cells. In cancer, autophagy can be thought of as having a âJanusâ-like duality. On one face, especially in the early phases of cancer formation, autophagy can act as a cellular housekeeper to eliminate damaged organelles and recycle macromolecules, thus acting as tumor suppressor. On the other face, at later stages of tumor progression, autophagy can function as a pro-survival pathway in response to metabolic stresses such as nutrient depravation, hypoxia and indeed to chemotherapy itself, and can support cell growth by supplying much needed energy. In the context of chemotherapy, autophagy may, in some cases, mediate resistance to treatment. We present an overview of the relevance of autophagy in central nervous system tumors including how its chemical modulation can serve as a useful adjunct to chemotherapy, and use this knowledge to consider how targeting of autophagy may be relevant in pediatric brain tumors
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(η5-C5iPr5)2] and the uranium(III) metallocene [IU(η5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η5-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
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