Lothar Fink

Influence of the co-ligand on the magnetic and relaxation properties of layered cobalt(II) thiocyanato coordination polymers.

Reaction of Co(NCS)2 with 1,2-bis(4-pyridyl)-ethane (bpa) leads to the formation of [Co(NCS)2(bpa)2]n, which, on heating, transforms into the new layered coordination polymer [Co(NCS)2(bpa)]n. This compound can also be prepared in solution, but because no reasonable single crystals are available, its crystal structure was determined from X-ray powder data from scratch. In the crystal structure of [Co(NCS)2(bpa)]n, the cobalt(II) cations are coordinated by two S-bonded and two N-bonded thiocyanato anions and two N atoms of the bpa co-ligands in a distorted octahedral geometry. The cobalt(II) cations are linked into chains by pairs of μ-1,3 bridging thiocyanato anions. These chains are further connected into layers by the 1,2-bis(4-pyridyl)-ethane ligand. The compound was magnetically characterized, and, for comparative purposes, the complementary magnetic study of a known and very similar compound, [Co(NCS)2(bpe)]n (bpe = 1,2-bis(4-pyridyl)-ethylene), was also undertaken. The compounds differ in their interchain interactions, which are antiferromagnetic but significantly greater for [Co(NCS)2(bpe)]n. Magnetic measurements indicate that [Co(NCS)2(bpa)]n is a canted antiferromagnet with Néel temperature TN = 3.1 K and that Co(NCS)2(bpe) is an antiferromagnet with TN = 4.0 K. Both compounds show a metamagnetic transition with a critical field HC ∼ 40 Oe and ∼ 400 Oe, respectively. Magnetic relaxations were studied by means of dc and ac methods and analyzed using the Argand diagrams. Except for the thermally activated single chain and domain wall relaxations observed for both compounds, temperature-independent slow relaxations were observed for [Co(NCS)2(bpa)]n.

Exploration and synthesis of condensed coordination networks with modified magnetic properties

Eight new coordination polymers based on Ni(NCS)2 and 2-methylpyrazine were discovered, structurally characterized and most of them investigated for their thermal and magnetic properties. In the crystal structures of Ni(NCS)2(2-methylpyrazine)4 (1), Ni(NCS)2(2-methylpyrazine)4·2-methylpyrazine solvate (2) and Ni(NCS)2(2-methylpyrazine)4·ethanol solvate (3), the nickel(II) cations are always octahedral coordinated by two terminal N-bonded thiocyanato anions and four N-bonded 2-methylpyrazine ligands into discrete complexes. The crystal structures of Ni(NCS)2(2-methylpyrazine)2(H2O)2 (4) and Ni(NCS)2(2-methylpyrazine)2(CH3OH)2 (5) also consist of discrete complexes, but in contrast to compounds 1–3 two terminal bonded 2-methylpyrazine ligands are exchanged by two water and methanol molecules, respectively. In [Ni(NCS)2(2-methylpyrazine)3(H2O)]·[Ni(NCS)2(2-methylpyrazine)4] (6) two different complexes are observed, in which one nickel(II) cation is coordinated by two terminal N-bonded thiocyanato anions, three 2-methylpyrazine ligands and one water molecule, whereas the second cation is coordinated by two terminal N-bonded thiocyanato anions and four 2-methylpyrazine ligands. Compounds 2, 3, 4 and 5 transformed into a 2-methylpyrazine deficient 1 : 2 (1 : 2 = ratio between metal and 2-methylpyrazine) compound with the composition Ni(NCS)2(2-methylpyrazine)2 (7) on heating. On further heating this compound subsequently transformed into a 1 : 1 compound with the composition [Ni(NCS)2(2-methylpyrazine)]n (8). Compound 7 can only be prepared by thermal decomposition and therefore, its structure was solved ab initio from X-ray powder data. It consists of [Ni(NCS)]2 dimers that are μ-1,3-bridged by single thiocyanato anions into layers. Single crystal structure analysis of compound 8 proves that nickel(II) cations are linked by the thiocyanato anions into Ni–(NCS)2–Ni double chains, which are further connected by the 2-methylpyrazine ligands into layers. Magnetic measurements on compounds 2, 3, 4 and 5 show only Curie or Curie–Weiss paramagnetism, whereas in the ligand-deficient compound 7 metamagnetic behavior is observed. The results of these investigations are discussed and compared with those obtained for related coordination compounds with e.g. pyrazine and pyridine as neutral co-ligands.

Determination of the physical state of norethindrone acetate containing transdermal drug delivery systems by isothermal microcalorimetry, X-ray diffraction, and optical microscopy

Transdermal drug delivery systems (TDDS) enable a controlled drug delivery to the skin. The low permeability of the stratum corneum necessitates a high drug concentration of the polymeric matrix and often requires supersaturation. This, however, promotes crystallisation of supersaturated systems. Isothermal microcalorimetry at 25 degrees C, polarisation light microscopy, and X-ray powder diffraction (XRPD) were used to characterise the crystal growth of norethindrone acetate (NEA). The solubility of NEA in the patches determined by these methods is about 4%. The crystallisation process could be measured reliably and with a high accuracy by microcalorimetry and microscopy. XRPD was considerably less sensitive but was the only method allowing a semi-quantitative determination of the amounts of crystals formed. The drug-associated heat measured by microcalorimetry increased proportionally with increasing NEA concentration in the concentration range of 4-10% demonstrating a constant crystallisation rate. At a higher supersaturation, such as 12% drug content, the crystallisation process was accelerated. The application of Johnson-Mehl-Avrami kinetics for the analysis of nucleation and crystal growth of the punched patches indicated a site-saturated nucleation mechanism and a one-dimensional crystal growth. The crystallisation enthalpy of NEA was -22.8+/-2.6 kJ/mol. The most specific method to observe the crystal formation is polarisation light microscopy. However, the microscopic analysis requires much longer storage times than microcalorimetry to detect crystallisation.

Crystallisation of estradiol containing TDDS determined by isothermal microcalorimetry, X-ray diffraction, and optical microscopy

Transdermal drug delivery systems (TDDS) enable a controlled delivery of drugs to the skin. However, it is still a problem to achieve a stable and prolonged constant drug release. To attain high permeation rates across the skin, the concentrations of the drug dissolved have to be high and often create supersaturated, thermodynamically metastable, or unstable systems that possess a high tendency to crystallise. In the present study, microcalorimetry as well as polarisation microscopy and X-ray powder diffraction (XRPD) were used to characterise the growing crystal germs of estradiol (E2) hemihydrate. Polarisation microscopy enabled the observation of crystals with two different morphologies of E2 in the polymeric acrylic transdermal patch matrix. Crystal formation and growth were also detected by XRPD. The diffraction pattern corresponded to estradiol hemihydrate. The intensity of the observed reflections was proportional to the crystal quantities and increased during storage. A high supersaturation resulted in high peak intensities caused by a high crystallisation rate. Since precipitation is generally accompanied by heat evolution, crystal germ formation, and crystal growth could easily be detected early by isothermal microcalorimetry. Much lower amounts of crystal were detected by this method than with the significantly less sensitive XRPD method. Microscopy was equally sensitive to but much more time-consuming than microcalorimetry.

Thermally induced crystal-to-crystal transformations accompanied by changes in the magnetic properties of a CuII-p-hydroquinonate polymer

In the CuII-p-hydroquinonate coordination polymer 1, the major pathway for antiferromagnetic exchange coupling runs along the hydroquinonate linker. Upon heating, 1 looses its supporting DMF ligands in a two-step sequence; the antiferromagnetic CuII–CuII interaction in the final product 3 is now mediated by two bridging oxygen atoms which results in an increase of the J value by two orders of magnitude.

Predicted and experimental crystal structures of ethyl-tert-butyl ether

Possible crystal structures of ethyl-tert-butyl ether (ETBE) were predicted by global lattice-energy minimizations using the force-field approach. 33 structures were found within an energy range of 2 kJmol(-1) above the global minimum. Low-temperature crystallization experiments were carried out at 80-160 K. The crystal structure was determined from X-ray powder data. ETBE crystallizes in C2/m, Z = 4, with molecules on mirror planes. The ETBE molecule adopts a trans conformation with a (CH(3))(3)C-O-C-C torsion angle of 180°. The experimental structure corresponds with high accuracy to the predicted structure with energy rank 2, which has an energy of 0.54 kJmol(-1) above the global minimum and is the most dense low-energy structure. In some crystallization experiments a second polymorph was observed, but the quality of the powder data did not allow the determination of the crystal structure. Possibilities and limitations are discussed for solving crystal structures from powder diffraction data by real-space methods and lattice-energy minimizations.

Electron diffraction, X-ray powder diffraction and pair-distribution-function analyses to determine the crystal structures of Pigment Yellow 213, C23H21N5O9

The crystal structure of the nanocrystalline alpha phase of Pigment Yellow 213 (P.Y. 213) was solved by a combination of single-crystal electron diffraction and X-ray powder diffraction, despite the poor crystallinity of the material. The molecules form an efficient dense packing, which explains the observed insolubility and weather fastness of the pigment. The pair-distribution function (PDF) of the alpha phase is consistent with the determined crystal structure. The beta phase of P.Y. 213 shows even lower crystal quality, so extracting any structural information directly from the diffraction data is not possible. PDF analysis indicates the beta phase to have a columnar structure with a similar local structure as the alpha phase and a domain size in column direction of approximately 4 nm.

Predicted crystal structures of tetramethylsilane and tetramethylgermane and an experimental low-temperature structure of tetramethylsilane

No crystal structure at ambient pressure is known for tetramethylsilane, Si(CH(3))(4), which is used as a standard in NMR spectroscopy. Possible crystal structures were predicted by global lattice-energy minimizations using force-field methods. The lowest-energy structure corresponds to the high-pressure room-temperature phase (Pa3, Z = 8). Low-temperature crystallization at 100 K resulted in a single crystal, and its crystal structure has been determined. The structure corresponds to the predicted structure with the second lowest energy rank. In X-ray powder analyses this is the only observed phase between 80 and 159 K. For tetramethylgermane, Ge(CH(3))(4), no experimental crystal structure is known. Global lattice-energy minimizations resulted in 47 possible crystal structures within an energy range of 5 kJ mol(-1). The lowest-energy structure was found in Pa3, Z = 8.

Use of isothermal heat conduction microcalorimetry, X-ray diffraction, and optical microscopy for characterisation of crystals grown in steroid combination-containing transdermal drug delivery systems.

The combined application of the steroids estradiol (E2) hemihydrate and norethindrone acetate (NEA) is desirable for hormone replacement therapy. Transdermal drug delivery systems (TDDS) enable a controlled delivery of these drugs to the skin. However, in order to attain high skin permeation rates the concentration of the dissolved drugs in the TDDSs has to be high. This often results in supersaturated systems with a high crystallisation tendency. The combination of NEA and E2-hemihydrate in the acrylic matrix of patches yields crystals that are different from single drug systems. A new crystal phase showing additional X-ray powder diffraction peaks and a new feather-like crystal shape appeared. The crystal formation was considerably accelerated and enhanced by increasing E2 contents in the patches. The new crystal phase seems to be kinetically favoured compared with crystals appearing from pure E2-hemihydrate or NEA. A crystallisation enthalpy of -7.9+/-0.95 kJ/mol in the matrix containing a 1:3 mixture of E2-hemihydrate and NEA was determined by isothermal microcalorimetry. The crystallisation rate increased with higher drug concentrations. In addition, the influence of patch pre-treatment at 80 degrees C prior to storage on crystallisation was investigated. This treatment enabled a slight reduction of the crystallisation in the TDDSs. Microcalorimetry enabled the classification of various additives according to their influence on the crystallisation process.

catena-Poly[[dipyridine­nickel(II)]-trans-di-μ-chlorido] from powder data

The asymetric unit of the title compound, [NiCl2(C5H5N)2]n, contains two NiII ions located on different twofold rotational axes, two chloride anions and two pyridine rings in general positions. Each NiII ion is coordinated by two pyridine rings, which form dihedral angles of 33.0 (2) and 11.0 (2)° for the two centers, and four chloride anions in a distorted octahedral geometry. The chloride anions bridge NiII ions related by translation along the short b axes into two crystallographically independent polymeric chains.