http://www.rsc.org/suppdata/CC/b7/b700082k/b700082k.pdf

Supramolecular synthon polymorphism in 2 : 1 co-crystal of 4-hydroxybenzoic acid and 2,3,5,6-tetramethylpyrazine.

A series of seven 2 : 1 molecular complexes of urea (U) and methyl ureas with di-carboxylic acids (A) are reported, along with the results of their study by variable temperature diffraction. These all contain short, strong O–H⋯O hydrogen bonds and a recurring acid⋯amide heterodimer forming U–A–U synthons. Despite differences in the degree of saturation of the linking C–C groups of the di-carboxylic acids and the single or double methyl substitution of one of the N atoms of the urea, the packing arrangements are remarkably similar in five of the complexes; the exceptions being N-methylurea oxalic acid and N,N-dimethylurea fumaric acid. The five similar molecular complexes all show contraction of one unit cell parameter on increasing temperature due to rearrangements of the weaker interactions which hold together the U–A–U units. The strength of the short, strong O–H⋯O hydrogen bond is shown to be linked both to the length of the connecting bridge between the carboxylic acid groups of the acid, and to the ΔpKa values between the two components.

/pdf/engineering-short-strong-hydrogen-bonds-in-urea-di-18fqh85fhb.pdf

Engineering short, strong hydrogen bonds in urea di-carboxylic acid complexes

The phenomenon of solid-state proton migration within molecular complexes containing short hydrogen bonds is investigated in two dimethylurea-oxalic acid complexes. Extensive characterisation by both X-ray and neutron diffraction shows that proton migration along the hydrogen bond can be induced in these complexes as a function of temperature. This emphasises the subtle features of the hydrogen bond potential well in such short hydrogen bonded complexes, both intrinsically and in the effect of the local crystalline environment. Based on these findings, the synthesis and analysis of a series of solid-state molecular complexes is shown to be a potential route to designing materials with tuneable proton migration effects.

Temperature dependent solid-state proton migration in dimethylurea–oxalic acid complexes

http://journals.iucr.org/b/issues/1979/09/00/a17761/a17761.pdf

Konformation von 2,3,4-Tri-O-benzoyl-β-d-xylopyranosylfluorid und 2,3,4-Tri-O-acetyl-β-d-xylopyranosylfluorid im Kristall

2(C6H5)3PO · (COOH)2 kristallisiert aus einer Losung von Oxalsaure und (C6H5)3PO in Methanol. Kristalldaten: Raumgruppe P21/c (monoklin) mit a = 907,4(2), b = 1035,4(3), c = 1797,9(8) pm, β = 75,20(1)° und Z = 2; d (ber./gef.) = 1,27/1,31 g · cm−3; VZelle = 1633,1 · 106 pm3. Die Struktur wurde anhand von 3006 unabhangigen Reflexen direkt bestimmt und nach der Kleinsten-Quadrate-Methode (volle Matrix) bis zu einem R-Wert von 0,049 verfeinert. In der Verbindung ist ein Molekul trans-Oxalsaure mit zwei symmetrisch abhangigen (C6H5)3PO-Einheiten uber kurze O…H–O-Brucken verknupft. Abstande: P–O(1) 149,6(3); O(1)…O(2) 255,0(4); O(2)–C(19) 128,5(5); O(3)–C(19) 120,9(5) pm. Winkel: P–O(1)…O(2) 130,6(2)°;O(1)…O(2)–C(19) 116,2(2)°.



Preparation, Crystal and Molecular Structure of 2 (C6H5)3PO · (COOH)2



2(C6H5)3PO · (COOH)2 crystallizes from a solution of oxalic acid and (C6H5)3PO in methanol. Crystal data: space group P21/c (monoclinic) with a = 907.4(2), b = 1035.4(3), c = 1797.9(8) pm, β = 75.20(1)° and Z = 2; d (calc./obs.) = 1.27/1.31 g cm−;3; Vcell = 1633.1 × 106 pm3. The structure was determined by direct methods from 3006 independent reflections and has been refined by full matrix least squares to R = 0.049. In the compound one molecule of trans-oxalic acid and two symmetrically dependent (C6H5)3PO units are linked by short O…H–O bridges distances and angles see above.

Darstellung sowie Kristall‐ und Molekülstruktur von 2 (C6H5)3PO · (COOH)2

/pdf/the-crystal-structure-and-thermal-expansion-of-two-111v6bw9gk.pdf

H.J.G. Meutstege

Papers

The crystal structure and thermal expansion of two modifications of N-methylurea–oxalic acid (2:1)