Noelia De la Pinta

Defective Dicubane-like Tetranuclear Nickel(II) Cyanate and Azide Nanoscale Magnets

Four tetrameric nickel(II) pseudohalide complexes have been synthesized and structurally, spectroscopically, and magnetically characterized. Compounds 1-3 are isostructural and exhibit the general formula [Ni(2)(dpk·OH)(dpk·CH(3)O)(L)(H(2)O)](2)A(2)·2H(2)O, where dpk = di-2-pyridylketone; L = N(3)(-), and A = ClO(4)(-) for 1, L = NCO(-) and A = ClO(4)(-) for 2, and L = NCO(-) and A = NO(3)(-) for 3. The formula for 4 is [Ni(4)(dpk·OH)(3) (dpk·CH(3)O)(2)(NCO)](BF(4))(2)·3H(2)O. The ligands dpk·OH(-) and dpk·CH(3)O(-) result from solvolysis and ulterior deprotonation of dpk in water and methanol, respectively. The four tetramers exhibit a dicubane-like core with two missing vertexes where the Ni(II) ions are connected through end-on pseudohalide and oxo bridges. Magnetic measurements showed that compounds 1-4 are ferromagnetic. The values of the exchange constants were determined by means of a theoretical model based on three different types of coupling. Thus, the calculated J values (J(1) = J(2), J(3), and D) were 5.6, 11.8, and 5.6 cm(-1) for 1, 5.5, 12.0, and 5.6 cm(-1) for 2, 6.3, 4.9, and 6.2 cm(-1) for 3, and (J(1), J(2), J(3), and D) 6.9, 7.0, 15.2, and 4.8 cm(-1) for 4.

Structural analysis, spectroscopic, and magnetic properties of the 1D triple-bridged compounds [M(dca)2(bpa)] (M = Mn, Fe, Co, Zn; dca = dicyanamide; bpa = 1,2-bis(4-pyridyl)ethane) and the 3D [Ni(dca)(bpa)2]dca·6H2O.

The family of compounds [Mn(dca)(2)(bpa)] (1), [Fe(dca)(2)(bpa)] (2), [Co(dca)(2)(bpa)] (3), [Zn(dca)(2)(bpa)] (4), and [Ni(dca)(bpa)(2)]dca·6H(2)O (5), with dca = dicyanamide and bpa = 1,2-bis(4-pyridyl)ethane, has been synthesized. These compounds have been characterized by single crystal (1, 2, 4, and 5) and powder X-ray diffraction (3), by Fourier transform infrared (FTIR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, and by magnetic measurements. Compound 1 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.757(6), b = 9.692(3), and c = 13.073(4) Å, and β = 123.02(2)°; Compound 2 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.588(5), b = 9.661(3), c = 12.970(5) Å, and β = 123.16(3)°; Compound 4 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 16.519(2), b = 9.643(2), c = 12.943(2) Å, and β = 123.15(1)°; Compound 5 crystallizes in the monoclinic C2/c space group, Z = 4, with a = 18.504(4), b = 19.802(3), and c = 8.6570(18) Å, and β = 99.74(2)°. The compounds 1-4 are isostructural and show a one dimensional (1D) disposition, with the metal(II) ions bridged by double μ(1,5) dca ligands and unusually by a third bridge consisting of the bpa ligand, which adopts a very low torsion angle to accommodate in the structure. This kind of structure is unusual, even considering the voluminous bpa bridge. The compound 5 shows a 3D structure with layers of Ni-bpa joined by single dca bridges. Magnetic susceptibility measurements show antiferromagnetic couplings, increasing for 1-3. Compound 5 shows very slight antiferromagnetic interactions.

Trinuclear Nickel(II) Complex through a 2,3,5,6-Tetrakis(2-pyridyl)pyrazine Ligand with a Linear Exchange Pathway

The compound [Ni(3)(tppz)(2)(NCS)(2)(H(2)O)(4)](NO(3))(4).4H(2)O [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and NCS = thiocyanate ligand] consists of trimeric units, where the three Ni(II) cations are linked by two bis-tridentate tppz ligands. This compound crystallizes in the monoclinic space group C2/c, with Z = 4, a = 25.1116(12) A, b = 10.8127(5) A, c = 25.2294(13) A, and beta = 116.856(5) degrees . The crystal structure is in good agreement with the antiferromagnetic interactions because of unidirectional coupling through the tppz ligands.

Magnetostructural characterisation of two M–NCO–bpa polymers (M = Co, Mn and bpa = 1,2-bis(4-pyridyl)ethane)

The combination of bpa with the pseudohalide cyanate (NCO−) leads to the preparation of two new isomorphous compounds exhibiting the formula [M(NCO)2(bpa)2]n·bpa where M = Co (1), Mn (2). Both of them have been magnetostructurally characterised by means of X-ray diffraction analysis, TG measurements, IR and ESR spectroscopies and measurements of the magnetic susceptibility. Both compounds consist of M–(gauche-bpa)2–M chains connected through H-bonds. The resulting network shows pseudo-channels (8.5 A × 9.3 A). The structure also exhibits anti-bpa groups performing as crystallisation molecules that are perpendicularly disposed to the pseudo-channels. Both compounds exhibit antiferromagnetic interactions through metallic centres.

DPDS–DPS in situ transformation at room temperature via a 1,2-nucleophilic addition mechanism

A cleavage–reorganization reaction at room temperature has been detected in three metal organic coordination compounds synthesized from a DPDS [di(4-pyridyl)disulphide] ligand: Mn(NCS)2(DPS)4 (1), [Fe(NCS)2(DPS)2]·2H2O (2) and Zn(NCO)2(DPS) (3), [DPS = di(4-pyridyl)sulphide]. The in situ reorganization process is explained by a 1,2-nucleophilic addition mechanism.

Aqua-(dicyanamido-κN)(nitrato-κO,O')(2,3,5,6-tetra-2-pyridylpyrazine-κN,N,N)manganese(II).

In the title compound, [Mn(C2N3)(NO3)(C24H16N6)(H2O)], the central manganese(II) ion is heptacoordinated to a tridentate 2,3,5,6-tetra-2-pyridylpyrazine ligand (tppz), a bidentate nitrate ligand, a terminal monodentate dicyanamide ligand (dca) and a water molecule. The structure contains isolated neutral complexes, which are linked by O(water)—H⋯N hydrogen bonds generating chains along [010].

Ordered vacancy distribution in 2/1 mullite: a superspace model

A mullite single crystal with composition Al4.84Si1.16O9.58 (2) exhibiting sharp satellite reflections was investigated by means of X-ray diffraction. For the refinement of a superspace model in the superspace group Pbam(α0½)0ss different scale factors for main and satellite reflections were used in order to describe an ordered mullite structure embedded in a disordered polymorph. The ordered fraction of the mullite sample exhibits a completely ordered vacancy distribution and can be described as a block structure of vacancy blocks (VBs) that alternate with vacancy-free blocks (VFBs) along a and c. The incommensurate nature of mullite originates from a modulation of the block size, which depends on the composition. The displacive modulation is analyzed with respect to the vacancy distribution and a possible Al/Si ordering scheme is derived, although the measurement itself is not sensitive to the Al/Si distribution. An idealized, commensurate approximation for 2/1 mullite is also presented. Comparison of the ordered superspace model with different preceding models reconciles many key investigations of the last decades with partly contradicting conclusions, where mullite was usually treated as either ordered or disordered instead of considering simultaneously different states of order.

Bis(2,3,5,6-tetra-2-pyridyl-pyrazine-κN,N,N)nickel(II) dithio-cyanate dihydrate.

In the title compound, [Ni(C24H16N6)2](NCS)2·2H2O, the central NiII ion is octahedrally coordinated by six N atoms of two tridentate 2,3,5,6-tetra-2-pyridylpyrazine ligands (tppz). Two thiocyanate anions act as counter-ions and two water molecules act as solvation agents. O—H⋯N hydrogen bonds are observed in the crystral structure.

[Bis(2-pyrid­yl)amine-N,N′](nitrato-O,O′)cobalt(II) nitrate. Corrigendum

Corrigendum to Acta Cryst. (2001), E57, m384–m386.

Mullite – towards a unified superspace model

Mullite Al4+2xSi2−2xO10−x x ( = vacancy) is a well known material in the ceramics industry with a broad range of applications, but from a fundamental point of view its crystal structure is not completely understood. The dependence of the modulation wave vector q = (α 0 γ) on the composition lacks an explanation and it is not known why for vacancy concentrations x > 0.5 a lowering of the symmetry is observed [1]. A recent superspace model (SSM) described in Pbam(α01⁄2)0ss results in a brick pattern of alternating vacancy blocks and vacancy-free blocks [2]. Assuming that, independent of the composition, the brick pattern is maintained and intermeshing of vacancy blocks is avoided by the structure, the relation α = (1−x)/2 is derived from the SSM. It allows to predict the vacancy distribution of any mullite with x ≤ 0.5 including the cases of the prominent 3/2 mullite (x = 0.25), 2/1 mullite (x = 0.4) and 5/2 mullite (x = 0.5). Models derived from the SSM are depicted in the Figure below. The limit results from one of the atomic domains which has a negative length for x > 0.5, i.e. a new SSM is required. This border coincides with an observed change of the symmetry from orthorhombic to monoclinic because γ then deviates from 1⁄2. As the deviation is very small, the nature of the modulation seems to be the same for the complete solid solution range and the dependence of q on the composition is a result from the block pattern that determines the vacancy distribution. The SSM can be easily adapted to monoclinic symmetry with superspace group P21/a(α0γ)0s to describe a monoclinic brick pattern of alternating vacancy blocks and vacancy-free blocks. These predictions are strongly supported by HRTEM observations and image simulations [1]. However, the suggested vacancy ordering scheme is expected to be only present when higher order satellite reflections are present in the reciprocal space. In many samples this is not the case and first order satellite reflections appear as maxima in a characteristic diffuse scattering pattern [3], i.e. that the long-range vacancy ordering is not present and vacancies are rather midrange ordered. Nevertheless the dependency of q on x is maintained independent of the diffuseness, which indicates that the disordered vacancy distribution is strongly related to the structure of the highly ordered SSM. In [2] it is also shown that a disordered SSM of [3] and the ordered SSM are indeed strongly related. It means that although mullite samples usually exhibit a lower ordering degree than described by the model, it establishes a basic vacancy ordering pattern from which structural models that take diffuse scattering into account must originate. This work is supported by the Basque Government (No. IT-779-13 & predoctoral grant) and FEDER (MAT2015-66441-P). A travel grant by the Master and Doctoral School (UPV/EHU) is highly appreciated.