Structural distortions in the spin-gap regime of the quantum antiferromagnet SrCu2(BO3)2

We report the first crystallographic study within the low-temperature (<40 K) spin-gap region of the two-dimensional frustrated antiferromagnet SrCu₂(BO₃)₂. The crystal system does not deviate from the tetragonal I-42m space group symmetry. However, our high-resolution neutron powder diffraction measurements uncover subtle structural modifications below 34 K, concomitant to the formation of the dimer singlet ground state. Intimate spin-lattice coupling leads to negative thermal expansion of the tetragonal structure, which reflects into particular local lattice adjustments. The extracted structural parameters suggest the reduction of the buckling found in the copper-borate planes and the strengthening of the leading, in-plane intra-dimer superexchange interaction. The observed contraction along the c-axis, associated with the inter-dimer exchange in adjacent layers, indicates the involvement of weaker three-dimensional interactions in the magnetic properties. The rules posed by the crystal symmetry do not preclude Dzyaloshinsky-Moriya interactions, which therefore remain as an important source of spin anisotropy necessary to rationalise the ground state behaviour.     

Reactions of titanocene dihalides with N,N′,N″-chelates: Preparation,X-ray structure and characterization of bis(chloro){2,6-bis[(3,5-dimethyl)pyrazol-1-yl]pyridine}(η-peroxo)titanium(IV)

The reaction of [Ti^IV(Cp)₂Cl₂] (Cp⁻ = η⁵-C₅H₅⁻) and 2,6-bis(3,5-dimethylpyrazolyl-1-yl)pyridine (bdmpp) in Me₂CO has afforded complex [Ti^IVCl₂(O₂)(bdmpp)] (1). The complex can also be prepared from the 1:1:1 [Ti(Cp)₂Cl₂]/bdmpp/H₂O₂ reaction mixture in various solvents. Single-crystal, X-ray crystallography has revealed that the Ti^IV center is in a distorted pentagonal bipyramidal environment in both of the two, crystallographically independent molecules that are present in the asymmetric unit; the equatorial positions are occupied by the two O atoms of the side-on (η²) O₂²⁻ group and the three N-atoms of the tridentate chelate, while the two chloro ligands are on the axial positions. IR, Raman, electronic and ¹H NMR data are discussed in terms of the known structure and the coordination modes of the peroxo and bdmpp ligands.     

Dinuclear versus trinuclear complex formation in zinc(II) benzoate/pyridyl oxime chemistry depending on the position of the oxime group

The initial employment of pyridine-3-carbaldehyde oxime, (3-py)C(H)NOH, and pyridine-4-carbaldehyde oxime, (4-py)C(H)NOH, in zinc(II) carboxylate chemistry is reported. The syntheses, crystal structures and IR characterization are described for [Zn₃(O₂CPh)₆{(3-py)C(H)NOH}₂] (1) and [Zn₂(O₂CPh)₄{(4-py)C(H)NOH}₂] (2). The trinuclear molecule of 1 has a linear structure, with one monoatomically bridging η¹:η²:μ and two syn, syn-η¹:η¹:μ benzoate groups spanning each pair of Zn^II ions; the terminal metal ions are each capped by one (3-py)C(H)NOH ligand coordinating through its pyridyl nitrogen. Complex 2 exhibits a dinuclear paddle–wheel structure with a Zn···Zn distance of 2.990(2) Å; each Zn^II ion has a square pyramidal geometry with four carboxylate oxygens in the basal plane and the pyridyl nitrogen of one monodentate (4-py)C(H)NOH ligand at the apex. Both complexes form 1D architectures by virtue of hydrogen bonding interactions involving the free oxime group as donor and the oxime nitrogen (1) or carboxylate oxygens (2) as acceptors. IR data are discussed in terms of the known structures and coordination modes of the ligands.     

A mononuclear complex and a cubane cluster from the initial use of 2-(hydroxymethyl)pyridine in nickel(II) carboxylate chemistry

The reactions of 2-(hydromethyl)pyridine, hmpH, with Ni(O₂CMe)₂·4H₂O in H₂O, in the absence of counterions, have been investigated. The synthetic study has led to the two new complexes [Ni(O₂CMe)₂(hmpH)₂] (1) and [Ni₄(O₂CMe)₄(hmp)₄(H₂O)₂] (2). Complex 1 can also be transformed into 2 by reacting with an excess of NaOH in H₂O. The structures of 1 and 2·2.25H₂O·0.5(1,4-dioxane) have been solved by single-crystal, X-ray crystallography. The octahedral Ni^II center in centrosymmetric 1 is coordinated by two 1.10 (Harris notation) MeCO₂⁻ groups and two N,O-chelating (1.11) hpmH ligands. The tetranuclear cluster molecule of 2·2.25H₂O·0.5(1,4-dioxane) possesses a distorted cubane {Ni₄(μ₃-OR′)₄}⁴⁺ core [R′ = (2-pyridyl)CH₂–] with the Ni^II ions and the oxygen atoms from the 3.31 hmp⁻ ligands occupying alternate vertices of the cube. Two 2.11 MeCO₂⁻ groups cap two opposite faces of the cube, while two 1.10 MeCO₂⁻ ions and two aqua ligands complete the octahedral coordination sphere of the metal centers. Characteristic IR bands for the two complexes are discussed in terms of the nature of bonding and the structures of the two complexes. The variable-temperature magnetic properties of 2 have been modeled with two J values, and reveal antiferromagnetic exchange interactions between the four Ni^II ions to give a diamagnetic ground state.     

Cosmological singularities and analytical solutions in varying vacuum cosmologies

We investigate the dynamical features of a large family of running vacuum cosmologies for which \(\Lambda \) evolves as a polynomial in the Hubble parameter. Specifically, using the critical point analysis we study the existence and the stability of singular solutions which describe de-Sitter, radiation and matter dominated eras. We find several classes of \(\Lambda (H)\) cosmologies for which new analytical solutions are given in terms of Laurent expansions. Finally, we show that the Milne universe and the \(R_{h}=ct\) model can be seen as perturbations around a specific \(\Lambda (H)\) model, but this model is unstable.     

Novel Co5 and Ni4 Metal Complexes and Ferromagnets by the Combination of 2-Pyridyl Oximes with Polycarboxylic Ligands

The use of 2-pyridyl oximes in metal complexes chemistry has been extensively investigated in the last few decades as a fruitful source of species with interesting magnetic properties. In this work, the initial combination of pyridine-2-amidoxime (pyaoxH₂) and 2-methyl pyridyl ketoxime (mpkoH) with isonicotinic acid (HINA) and 3,5-pyrazole dicarboxylic acid (H₃pdc) has provided access to three new compounds, [Ni₄(INA)₂(pyaox)₂(pyaoxH)₂(DMF)₂] (1), [Co₅(mpko)₆(mpkoH)₂(OMe)₂(H₂O)](ClO₄)₆ (2), and [Co₅(OH)(Hpdc)₅(H₂pdc)] (3). 1 displays a square-planar metal topology, being the first example that bears simultaneously HINA and pyaoxH₂ in their neutral or ionic form. The neighbouring Ni₄ units in 1 are held together through strong intermolecular hydrogen bonding interactions, forming a three-dimensional supramolecular framework. 2 and 3 are mixed-valent Co₄^IIICo^II and Co₂^IIICo^II₃ compounds with a bowtie and trigonal bipyramidal metal topology, accordingly. Direct current and alternate current magnetic susceptibility studies revealed that the exchange interactions between the Ni^II ions in 1 are ferromagnetic (J = 1.79(4) cm⁻¹), while 2 exhibits weak AC signals in the presence of a magnetic field. The syntheses, crystal structures, and magnetic properties of 1–3 are discussed in detail.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

Acid-catalyzed addition of indoles to hydroxyquinones

Hydroxyquinones react smoothly with substituted indoles in the presence of a catalytic amount of p-toluenesulfonic acid to afford a variety of addition products. The main products are either the corresponding p-quinone or the corresponding hydroquinone, resorcinol and benzenetriol mono- and diindolyl derivatives, depending on the nature of the hydroxyquinone and the substituents on the indole ring.     

Synthesis,spectroscopy and thermal analysis of nickel(II), palladium(II) and copper(II) complexes with N,N′-bis(2-hydroxyethyl)-oxamide

Summary The new complexes K₂[Ni(H₂heo)₂]·3H₂O, K₂[Pd-(H₂heo)₂], K₂[Cu(heo)]·3H₂O and [Cu₂(heo)·2H₂O] {heo = [(CONCH₂CH₂O)₂]^4-} were prepared and characterized by chemical analyses, conductivity measurements, thermogravimetry, magnetic susceptibility and i.r. and u.v. spectroscopy.Monomeric square planar structures with MN₄ coordination spheres are found for the nickel and palladium complex anions.For copper, two different complexes were identified. In slightly alkaline media, a green insoluble nonelectrolyte [Cu₂(heo)·2H₂O] was prepared; the two copper(II) centres are bridged by the tetra-deprotonated ligand in a trans-planar configuration with the H₂O molecules strongly coordinated to the copper(II). The copper(II) monomer has been prepared in strong alkaline media, this anion also contains the tetra-deprotonated ligand in a planar trans form.     

Resin-bound aminothiols: synthesis and application

Aminothiols were attached through their thiol group onto the 4,4′-dimethoxytrityl (Dmt)-, 4-methoxytrityl (Mmt)-, 4-methyltrityl (Mtt)-, trityl (Trt)- and 2-chlorotrityl (Clt)-resins. The new resins were used in the solid-phase synthesis of aminothiol containing peptides utilizing N-Fmoc amino acids. The synthesized peptides were cleaved from the resins by treatment with trifluoroacetic acid (TFA) solutions using triethylsilane (TES) or ethanedithiol (EDT) as scavengers.