Maurice O. Okoth

Cephalexin: a channel hydrate

The antibiotic cephalexin [systematic name: D-7-(2-amino-2-phenylacetamido)-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid] forms a range of isomorphic solvates, with the maximum hydration state of two water molecules formed only at high relative humidities. The water content of the structure reported here (C(16)H(17)N(3)O(4)S.1.9H(2)O) falls just short of this configuration, having three independent cephalexin molecules, one of which is disordered, and 5.72 observed water molecules in the asymmetric unit. The facile nature of the cephalexin solvation/desolvation process is found to be facilitated by a complex channel structure, which allows free movement of solvent in the crystallographic a and b directions.

Two New Structures of 5-Nitrouracil

The structure of monoclinic anhydrous 5-nitrouracil, C4H3N3O4, and of the solvate 5-nitrouracil dimethyl sulfoxide, C4H3N3O4.C2H6OS, are presented and compared with the previously known structures of the orthorhombic anhydrous form and the monohydrate.

Hydration studies of a simple molecular solid

This paper presents novel information on the thermal dehydration and rehydration of the molecular solid, Oxalic Acid Dihydrate. Although the procedure of the overall decomposition process is well-defined, the structural basis and mechanism of the dehydration process has been poorly studied. We show that the dehydration occurs in a planar manner, with a resultant semi-topotactic relationship between hydrated and dehydrated structures, reflected in the molecular packing. During rehydration, the reconstruction of the phase can be seen to occur at a 3-dimensional phase boundary reaction front, i.e. a recrystallisation of the dihydrate on the surface of the reactant product, with the topotactic relationship leading to texturing and possibly epitaxial relationships between partially dehydrated and rehydrated structures. The proposed mechanism is shown to be consistent with the measured kinetics of the process.

Dehydration mechanism of a small molecular solid: 5-nitrouracil hydrate

Previous studies of the dehydration of 5-nitrouracil (5NU) have resulted in it being classified as a “channel hydrate” in which dehydration proceeds principally by the exit of the water molecules along channels in the structure. We have re-examined this proposal and found that in fact there are no continuous channels in the 5NU structure that would contribute to such a mechanism. Product water molecules would be immediately trapped in unlinked voids in the crystal structure and would require some additional mechanism to break loose from the crystal. Through a detailed structural analysis of the macro and micro structure of the 5NU as it dehydrates, we have developed a model for the dehydration process based on the observed development of structural defects in the 5NU crystal and the basic crystallography of the material. The model was tested against standard kinetic measurements and found to present a satisfactory account of kinetic observations, thus defining the mechanism. Overall, the study shows the necessity of complementing standard kinetic studies with a parallel macro and micro examination of the dehydrating material when evaluating the mechanisms of dehydration and decomposition processes.

catena-Poly[[(nitrito-kappa O-2,O ')silver(I)]-mu-1,2-bis[1-(pyridin-4-yl)ethylidene]hydrazine-kappa N-2:N ']

The asymmetric unit of the title compound, [Ag(NO2)(C14H14N4)]n, contains half of the repeating formula unit (Z′ = 1/2). The AgI ion lies on a twofold rotation axis. The primary structure consists of a one-dimensional coordination polymer formed by the AgI ions and the bipyridyl azine ligand in which there is an inversion center at the mid-point of the N—N bond. The nitrite anion interacts with the AgI ion through a chelating μ2 interaction involving both O atoms. In the crystal, the coordination chains are parallel and interact through Ag⋯π [3.220 (2) Å] and π–π [3.489 (3) Å] interactions.

5,7,7,12,14,14-Hexamethyl-4,8-diaza-1,11-diazo-niocyclotetra-deca-4,11-diene diiodide dihydrate.

The asymmetric unit of the title compound, C16H34N4 2+·2I−·2H2O, contains one half-cation, one iodide anion and one water molecule. The cation has crystallographically imposed centrosymmetric symmetry. Despite some differences in the unit-cell dimensions, packing analysis on a cluster of 15 cations and a comparison of the hydrogen bonding suggests that this compound is isostructural with its bromide analogue. Intermolecular hydrogen bonding forms eight-membered [H—O—H⋯I]2 and [H—N—H⋯I]2 rings and creates a sheet structure.

4-(Benz­yloxy)benzaldehyde

The title compound, C14H12O2, has an essentially planar conformation with the two aromatic rings forming a dihedral angle of 5.23 (9)° and the aldehyde group lying in the plane of its aromatic group [maximum deviation = 0.204 (3) Å]. Weak intermolecular C—H⋯O contacts are found to be shortest between the aldehyde O-atom acceptor and the H atoms of the methylene group.

The cobalt(II) salt of the azo dye Orange G

Crystallizing the cobalt(II) salt of the azo dye Orange G from water was found to give the solvent-separated ion-pair species hexaaquacobalt(II) 7-oxo-8-(2-phenylhydrazin-1-ylidene)-7,8-dihydronaphthalene-1,3-disulfonate tetrahydrate, [Co(H2O)6](C16H10N2O7S2)·4H2O. The asymmetric unit of the cobalt(II) salt contains three independent octahedral [Co(OH2)6]2+ cations, three azo anions, all with similar configurations, and 12 uncoordinated water molecules. The structure is closely related to that of one of the known magnesium analogues. Both structures have Z′ = 3, feature nearly planar azo anions [maximum displacement of azo-N atoms from the plane of the phenyl ring = 0.058 (7) Å] in their hydrazone tautomeric form, form layer structures with hydrophilic and hydrophobic layers alternating along the b-axis direction, and are stabilized by an extensive network of hydrogen bonds..

2-Aminoethanaminium iodide

The title salt, [NH3CH2CH2NH2]+·I−, has an array structure based on strong intermolecular N—H⋯N hydrogen bonding formed between the ammonium and amine groups of adjacent cations. This interaction gives a helical chain of cations that runs parallel to the b axis. The four remaining NH group H atoms all form hydrogen bonds to the iodide anion, and these iodide anions lie in channels parallel to the cation–cation chains.

1-Benzyloxy-4-(2-nitroethenyl)benzene

The title compound, C15H13NO3, crystallizes with three independent molecules per asymmetric unit (Z′ = 3). One of these molecules is found to have a configuration with a greater twist between its two aromatic rings than the other two [compare 70.26 (13) and 72.31 (12)° with 84.22 (12)°]. There are also differences in the number and nature of the weak intermolecular C—H⋯O contacts formed by each of the three molecules.