Conformational aspects of polymorphs and phases of 2-propyl-1H-benzimidazole.

This paper reports on the polymorphism of 2-propyl-1H-benzimidazole (2PrBzIm) induced by temperature change. Upon heating, an irreversible reconstructive-type phase transition at T = 384 K from the ordered form I (P212121) to a new polymorph, form II HT (Pcam), was observed. The structural transformation between forms I and II involves significant changes in the crystal packing, as well as a key conformational variation around the propyl chain of the molecule. After the first irreversible phase transition, the II HT form undergoes two further (reversible) phase transitions upon cooling at 361 K (II RT) and 181 K (II LT). All three phases (forms II HT, II RT and II LT) have almost identical crystal packing and, given the reversibility of the conversions as a function of temperature, they are referred to as form II temperature phases. They differ, however, with respect to conformational variations around the propyl chain of 2PrBzIm. Energy calculations of the gas-phase conformational energy landscape of this compound about its flexible bonds allowed us to classify the observed conformational variations of all forms into changes and adjustments of conformers. This reveals that forms I and II are related by conformational change, and that two of the form II phases (HT and RT) are related by conformational adjustment, whilst the other two (RT and LT) are related by conformational change. We introduce the term 'conformational phases' for different crystal phases with almost identical packing but showing changes in conformation.

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.

Nanoporosity, Inclusion Chemistry, and Spin Crossover in Orthogonally Interlocked Two-Dimensional Metal-Organic Frameworks.

[Fe(tvp)2 (NCS)2 ] (1) (tvp=trans-(4,4'-vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two-dimensional grids. This uncommon supramolecular conformation defines square-sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest-loaded framework 1@guest displays complete thermal spin-crossover (SCO) behavior with the characteristic temperature T1/2 dependent on the guest molecule, whereas the guest-free species 1 is paramagnetic whatever the temperature. For the benzene-guest derivatives, the characteristic SCO temperature T1/2 decreases as the Hammet σp parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic-phase transition when the guest is benzonitrile or acetonitrile/H2 O.

Bridging, terminal, free, anti and gauche BPA ligands in a 1D hybrid organic-inorganic Mn(II)-cyanate system.

A hybrid organic-inorganic chain compound of the family Mn(II)-bpa-(NCO) is presented. It contains, unusually, five different dispositions for the bpa [1,2-bis(4-pyridyl)ethane] ligand. Water molecules and free bpa ligands occupy the voids of the structure. The compound shows significant antiferromagnetic-type interactions as a consequence of different exchange pathways.

Erratum: [Bis(2-pyrid-yl)amine-N,N'](nitrato-O,O')cobalt(II) nitrate. Corrigendum.

The chemical name and formula in the paper by Castillo, Luque, De la Pinta & Román [Acta Cryst. (2001), E57, m384-m386] is corrected.[This corrects the article DOI: 10.1107/S1600536801012909.].

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 ß = 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 ß = 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 ß = 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 ß = 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.

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

In the title compound, [Ni(C(24)H(16)N(6))(2)](NCS)(2)·2H(2)O, the central Ni(II) ion is octahedrally coordinated by six N atoms of two tridentate 2,3,5,6-tetra-2-pyridyl-pyrazine ligands (tppz). Two thio-cyanate anions act as counter-ions and two water mol-ecules act as solvation agents. O-H⋯N hydrogen bonds are observed in the crystral structure.

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

In the title compound, [Mn(C(2)N(3))(NO(3))(C(24)H(16)N(6))(H(2)O)], the central manganese(II) ion is hepta-coordinated 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 mol-ecule. The structure contains isolated neutral complexes, which are linked by O(water)-H⋯N hydrogen bonds generating chains along [010].