Crystals2014, 4(1), 53-63; doi:10.3390/cryst4010053 - published online 4 March 2014 Show/Hide Abstract
Abstract: Four novel organic salts of piperazine and 2-methylpiperazine with p-toluenesulfonic acid and chloroacetic acid have been synthesized and structurally characterized. The hydrogen-bonding ring synthons that exist between the cation/anion pairs are compared and contrasted alongside database results.
Crystals2014, 4(1), 42-52; doi:10.3390/cryst4010042 - published online 4 March 2014 Show/Hide Abstract
Abstract: Polyoxomolybdate inorganic-organic hybrid crystals were synthesized with 1-decyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium as ionic-liquid surfactants. Both hybrid crystals possessed alternate stacking of surfactant layers and octamolybdate (Mo8) monolayers, while the molecular structures of Mo8 were different depending on the surfactants and solvents employed for crystallization. Each Mo8 anion was connected by two sodium cations to form infinite one-dimensional chain. The surfactant chains in these crystals were arranged in a complicatedly bent manner, which will be induced by the weak C–H···O hydrogen bonds between the Mo8 anions and ionic-liquid surfactants.
Crystals2014, 4(1), 32-41; doi:10.3390/cryst4010032 - published online 26 February 2014 Show/Hide Abstract
Abstract: Two new Cu(II) complexes of the ligand 3-carboxy-5-(2-pyridyl)-1H-pyrazole, H2L1, have been prepared and structurally characterized and found to be comprised of a similar [M2L2] dimer motif. Subtle variation in the synthetic conditions allowed isolation of two metal complexes: [Cu2L12(MeOH)2],1, a discrete dimer linked by hydrogen bonding interactions in the solid state, and poly-[Cu2L12], 2, a polymeric material where the dimer motif is linked by carboxylate bridges to give an extended two-dimensional sheet. The selective isolation of each phase by careful synthetic control highlights the subtlety and importance of the underlying synthetic conditions.
Crystals2014, 4(1), 12-31; doi:10.3390/cryst4010012 - published online 26 February 2014 Show/Hide Abstract
Abstract: Covalently-bonded atoms of Groups IV–VII tend to have anisotropic charge distributions, the electronic densities being less on the extensions of the bonds (σ-holes) than in the intervening regions. These σ-holes often give rise to positive electrostatic potentials through which the atom can interact attractively and highly directionally with negative sites (e.g., lone pairs, π electrons and anions), forming noncovalent complexes. For Group VII this is called “halogen bonding” and has been widely studied both computationally and experimentally. For Groups IV–VI, it is only since 2007 that positive σ-holes have been recognized as explaining many noncovalent interactions that have in some instances long been known experimentally. There is considerable experimental evidence for such interactions involving groups IV and VI, particularly in the form of surveys of crystal structures. However we have found less extensive evidence for Group V. Accordingly we have now conducted a survey of the Cambridge Structural Database for crystalline close contacts of trivalent nitrogen, phosphorus and arsenic with six different types of electronegative atoms in neighboring molecules. We have found numerous close contacts that fit the criteria for σ-hole interactions. Some of these are discussed in detail; in two instances, computed molecular electrostatic potentials are presented.
Crystals2014, 4(1), 1-10; doi:10.3390/cryst4010001 - published online 8 January 2014 Show/Hide Abstract
Abstract: Single-crystal structures of 1,1′-methylenebis(imidazolidine-2,4-dione) and its sodium and dipotassium salts were determined. Powder X-ray diffraction was also employed to characterize the bulk materials and those phases which did not yield single-crystals. These compounds are of interest for intumescent coatings.