Low dimensional magnetic solids and single crystal elpasolites: Need for improved crystal growing techniques

The need for extensive crystal growing experiments to develop techniques for preparing crystals suitable for magnetic anisotropy measurements and detailed X-ray and neutron diffraction studies is rationalized on the basis of the unique magnetic properties of the materials and their hydrogen bonded structures which have many features in common with metalloenzyme and metalloprotein active sites. Single crystals of the single and mixed lanthanide species are prepared by the Bridgeman technique of gradient solidification of molten samples. The effects of crystal imperfections on the optical properties of these materials are an important part of the projected research. A series of a-amido acid complexes of first row transition metals were prepared which crystallize as infinite linear chains and exhibit low dimensional magnetic ordering (one or two) at temperature below 40 K

Synthesis, Structure, and Dynamics of Tris(η5-cyclopentadienyl)lanthanides and Bis(η5-cyclopentadienyl)[bis(trimethylsilyl)amido]cerium(III)

The crystal structures of tris(η5-cyclopentadienyl)lanthanides (Ln = Ce, Dy, Ho) have been determined using different X-ray diffraction methods. Cp3Ce and Cp3Ho (Cp = cyclopentadienyl) crystal data needed special solution and refinement methods, due to the occurrence of intrinsic twinning in these species. Our results do not agree with the previously published cell constants of Cp3Ho. The space group and unit cell parameters of Cp3Dy have been derived from powder diffraction experiments. High-resolution 13C solid-state NMR data of Cp3La are presented, giving evidence of the dynamics and bonding situation of the Cp ligands. Cp3Ce turned out to be a reactive reagent for the synthesis of bis(η5-cyclopentadienyl)[bis(trimethylsilyl)amido]cerium(III)

Utilization of mononuclear trivalent lanthanide complexes for the assembly of heteronuclear (d-f) metal complexes

Lanthanides are known for their distinctive magnetic properties and have been utilized for the design of multinuclear single-molecule magnets. Mononuclear trivalent lanthanide complexes were prepared from the reaction of tripodal amido ligands [P(CH2NHArR)3] and Ln[N(SiMe 3)2] (ArR = C6H5, 3,5-Me 2 and 3,5-(CF3)2 and Ln = Y, Tb, Dy, Ho, Er, Tm and Yb) in the presence of THF. These mononuclear lanthanide complexes were then further utilized for the syntheses of d-f heteronuclear compounds, using various transition metal complexes such as Pt(cyclooctadiene)Me 2, Ni(acetylacetonate)2 and Co-porphyrin. Mononuclear trivalent lanthanide complexes, prepared using 2-methyl anthranilate, contained a rigid chelate ring with six proton environments. The 31 p{1H} NMR spectra demonstrated a through-space interaction between the minor lobe of phosphine lone pair and the yttrium metal. Binding of a paramagnetic cobalt metal complex to the unbound phosphine lone pair provided heterodinuclear d-f metal complexes. The EPR spectra and the magnetic study of heterodinuclear complexes indicated the through-space antiferromagnetic coupling between unpaired electrons of gadolinium and cobalt centers. Magnetic anisotropy of lanthanide complexes with more than C2 symmetry can be easily measured by their NMR shifts due to the presence of dipolar contribution. According to Bleaney, temperature dependence of the magnetic anisotropy of lanthanide complexes should be proportional to T–2 and the crystal field parameter (α 20).[special characters omitted] McGarvey later expanded the temperature dependence of the anisotropy by including a term that is proportional to T–3 and other crystal field parameters (equation 2).[special characters omitted] From our calculations, we demonstrated the dependence of higher terms (\u3eT–2) for the calculation of magnetic anisotropy near room temperature. These higher terms showed the contribution of 20-90% of the T–2 term. Estimation of crystal field parameters (related with magnetic properties) generally requires low temperature optical spectroscopy or a SQUID magnetometer. Our trivalent mononuclear lanthanide complexes have C3 symmetry, which required 6 crystal field parameters, B20, B40, B60, B43, B63 and B66. Here, we utilized variable temperature NMR spectra to calculate the set of crystal field parameters. A best set of crystal-field parameters were then obtained by comparing experimental and theoretical magnetic anisotropies. In the future, these parameters can be further utilized for the electronic structure of lanthanide complexes

The application of chiroptical spectroscopy (circular dichroism) in quantifying binding events in lanthanide directed synthesis of chiral luminescent self-assembly structures

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(III) and Eu(III)) was studied in CH3CN, showing the successive formation of M:L, M:L2 and M:L3 stoichiometric species in solution. The europium complexes EuL3 were also synthesised, structurally characterised and their photophysical properties probed in CH3OH and CH3CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral selfassemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))3 and Eu(1(R))3 as enantiomers, with high luminescence dissymmetry factors (glum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes

Scandium complexes bearing bis(oxazolinylphenyl)amide ligands: an analysis of their reactivity, solution-state structures and photophysical properties

The coordination chemistry of scandium supported by bis(oxazolinylphenyl)amide (R-BOPA) ligands is reported. The R-BOPA ligand is too sterically demanding to afford bis(amide) complexes [Sc(R-BOPA){N(SiMe3)2}2], but reaction of the protio-ligand with [Sc{N(SiMe3)2}2Cl(THF)] (1) afforded the mixed amido-chloride complexes [Sc(R-BOPA){N(SiMe3)2}Cl] (2). The selective reaction of the amido and chloride co-ligands in 2 has been investigated; whilst the chloride ligand can be removed cleanly by metathesis, protonation of the N(SiMe3)2 ligand results in competitive protonation of the R-BOPA ligand. The complexes [Sc(R-BOPA)(CH2SiMe2Ph)2] (5) have been synthesised. Each R-BOPA-containing complex exists in two isomeric forms. The equilibrium has been investigated both experimentally and computationally, and the data suggest that a concerted rotation of the phenyl rings interconverts the two diastereomeric isomers. All of the R-BOPA complexes were found to be luminescent; an analysis of the photophysics, aided by TD-DFT calculations, suggests ligand-centred luminescence with distinct emission lifetimes for each isomer

Synthesis and Reactivity of Bis(silylene)-Coordinated Calcium and Divalent Lanthanide Complexes

Divalent lanthanide complexes of Eu (1) and Yb (2) coordinated by a chelating pyridine‐based bis(silylene) ligand were isolated and fully characterized. Compared to the Eu$^{II}$ complex 1, the Yb$^{II}$ complex 2 presents a lower thermal stability, resulting in the activation of one Si$^{II}$−N bond and formation of an Yb$^{III}$ complex (3), which features a unique silylene‐pyridyl‐amido ligand. The different thermal stability of 1 and 2 points towards reduction‐induced cleavage of one Si$^{II}$−N bond of the bis(silylene) ligand. Successful isolation of the corresponding redox‐inert bis(silylene) Ca$^{II}$ complex (5) was achieved at low temperature and thermal decomposition into a Ca$^{II}$ complex (4) bearing the same silylene‐pyridyl‐amido ligand was identified. In this case, the thermolysis reaction proceeds through another, non‐redox induced, mechanism. An alternative higher yielding route to 4 was developed through an in situ generated silylene‐pyridyl‐amine proligand

Development of highly selective ligands for separations of actinides from lanthanides in the nuclear fuel cycle

This account summarizes recent work by us and others on the development of ligands for the separation of actinides from lanthanides contained in nuclear waste streams in the context of a future European strategy for nuclear waste management. The current status of actinide/lanthanide separations worldwide is briefly discussed, and the synthesis, development, and testing of different classes of heterocyclic soft N- and S-donor ligands in Europe over the last 20 years is presented. This work has led to the current benchmark ligand that displays many of the desirable qualities for industrial use. The improvement of radiolytic stability through ligand design is also discussed

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes : A Mechanistic View

Altres ajuts: acords transformatius de la UABHydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included

Low-coordinate dinuclear dysprosium(III) single molecule magnets utilizing LiCl as bridging moieties and tris(amido)amine as blocking ligands

A low-coordinate dinuclear dysprosium complex {[Dy(N3N)(THF)][LiCl(THF)]}2 (Dy2) with a double bridging ‘LiCl’ moiety and tris(amido)amine (N3N)3− anions as a blocking ligand is synthesized and characterized structurally and magnetically. Thanks to the use of the chelating blocking ligand (N3N)3− equipped with large steric –SiMe3 groups, the coordination sphere of both DyIII ions is restricted to only six donor atoms. The three amido nitrogen atoms determine the orientation of the easy magnetization axes of both DyIII centers. Consequently, Dy2 shows slow magnetic relaxation typical for single molecule magnets (SMMs). However, the effective energy barrier for magnetization reversal determined from the AC magnetic susceptibility measurements is much lower than the separation between the ground and the first excited Kramers doublet based on the CASSCF ab initio calculations. In order to better understand the possible influence of the anticipated intramolecular magnetic interactions in this dinuclear molecule, its GdIII-analog {[Gd(N3N)(THF)][LiCl(THF)]}2 (Gd2) is also synthesized and studied magnetically. Detailed magnetic measurements reveal very weak antiferromagnetic interactions in Gd2. This in turn suggests similar antiferromagnetic interactions in Dy2, which might be responsible for its peculiar SMM behavior and the absence of the magnetic hysteresis loop