Christina Taouss

Packing principles for urea and thiourea solvates: structures of urea : morpholine (1 : 1), urea : 1,4-dioxane (1 : 1), thiourea : morpholine (4 : 3) and thiourea : 1,4-dioxane (4 : 1)

The solvents 1,4-dioxane and morpholine have been employed to synthesize solvates of urea and thiourea. The structures confirm the tendency of urea to form more rigid systems of hydrogen bonds in the plane of the N2CO moiety, thus forming layer structures with close complementarity of the donors and acceptors, whereas the more flexible sulfur acceptor of thiourea can also accept hydrogen bonds from donors that lie far from the N2CS plane, forming three-dimensional packing patterns with much more variable parameters. A database investigation confirms these tendencies. The solvate urea:morpholine (1:1) crystallizes in Pbcm with Z = 4. The complete urea molecule lies in the mirror plane, as do the heteroatoms of the morpholine molecule. The molecular packing is a layer structure. The solvate urea:1,4-dioxane (1:1) crystallizes in P2/c with Z = 2. The CO bond of the urea molecule lies along a twofold axis, whereas the dioxane molecule lies across an inversion centre. The molecules form a layer structure analogous to that of the morpholine solvate. The thiourea solvates are more complex, and both involve a more irregular hydrogen bonding geometry at sulfur. The solvate thiourea:morpholine (4:3) crystallizes in P21/c with Z = 2. The asymmetric unit contains two independent molecules of thiourea, one morpholine on a general position, and one morpholine disordered over an inversion centre. The thiourea molecules combine to form an open framework with a series of channels, in which the morpholine molecules are attached. The solvate thiourea:1,4-dioxane (4:1) crystallizes in P21/n with Z = 2. The asymmetric unit contains two independent molecules of thiourea and one molecule of dioxane across an inversion centre. One thiourea molecule and the dioxane combine to form a layer structure. The second thiourea molecule links these layers in the third dimension.

Thermodynamic and structural relationships between the two polymorphs of 1,3-dimethylurea

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Perez-Folch et al. (1997). J. Chem. Cryst. 27, 367-369; Marsh (2004). Acta Cryst. B60, 252-253], and as polymorph (II), P2(1)2(1)2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A, 113, 5998-6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd2 is confirmed rather than the original choice of Cc. The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C(2v) symmetry, and a trans orientation of the H-N-C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C(4)[R(2)(1)(6)] via bifurcated hydrogen-bond systems C(N-H)(2)···O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent-induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).

Methylthioureas and their morpholine and dioxane adducts; hydrogen-bonding patterns

We have redetermined the known structures of (i) methylthiourea (MTU) and (ii) 1,1-dimethylthiourea (1,1-DMTU), and investigated the structure of 1,3-dimethylthiourea (1,3-DMTU), which was however severely disordered. We report the structures of crystalline adducts of (iii) MTU with morpholine (1:1), (iv) 1,1-DMTU with 1,4-dioxane (2:1) and (v) 1,3-DMTU with 1,4-dioxane (2:1). Finally, (vi) we determined the structure of tetramethylthiourea (TetMTU). (i) In both independent molecules of MTU, the methyl group is trans to the C=S group across the C-N bond. The two molecules are connected to form an R2(2)(8) dimer by mutual N-H...S=C interactions. The packing involves six N-H...S=C interactions and is three-dimensional. (ii) The packing of the MTU-morpholine adduct is a layer structure, within which both molecules form linear aggregates parallel to the b axis. (iii) The packing of 1,1-DMTU involves N-H...S=C hydrogen bonds forming a corrugated layer structure. (iv) In the 2:1 adduct between 1,1-DMTU and 1,4-dioxane, the DMTU molecule occupies a general position whereas the dioxane molecule lies across an inversion centre. The crystal packing involves chains of alternating 1,1-DMTU R2(2)(8) dimers and dioxanes, both across inversion centres. (v) In the 2:1 adduct between 1,3-DMTU and dioxane, the 1,3-DMTU molecule occupies a general position, while the dioxane molecule lies across an inversion centre. One methyl group of the DMTU is trans and one cis to the sulfur across the corresponding C-N bond. The molecules form chains of alternating 1,3-DMTU R2(2)(8) dimers and dioxanes, both across inversion centres. Crystals of the 2:1 adduct between 1,3-DMTU and morpholine were also obtained, and were isotypic with the dioxane adduct. The morpholine molecule is disordered across the inversion centre. (vi) The molecule of TetMTU displays crystallographic twofold symmetry. Significant distortions reflect the steric pressure between methyl groups trans to sulfur. The packing of TetMTU involves only a weak hydrogen bond, C-Hmethyl...S, which connects the molecules to form layers.

Lutidine adducts of urea: molecular mechanisms for twinning effects on cooling

The structures of three adducts between urea and 2,6- or 3,5-lutidine have been determined: urea : 2,6-lutidine (1 : 1) (1), urea : 2,6-lutidine (2 : 1) (2) and urea : 3,5-lutidine (2 : 1) (3). Adducts 1 and 3 each crystallize as different polymorphs depending on the temperature. In each of the five structures, the lutidine nitrogen acts as a hydrogen bond acceptor for both NH groups of a urea molecule, and the lutidine molecules form infinite stacks. The known structure of 1 in space group C2/c (1m), previously known only from photographic data in projection, was redetermined at 200 K. Both residues display crystallographic twofold symmetry. The urea substructure is a ribbon of [R22(8)] rings. Further cooling results in a reduction in symmetry to P (structure 1t); crystals are non-merohedrally twinned by 180° rotation about (b*–c*), which is the same direction as the b axis of 1m. The qualitative structure remains almost unchanged, but the lutidine rings tilt slightly away from the twofold axis (which is lost on twinning). The three structures involving 2 : 1 adducts all display a urea substructure in the form of a corrugated layer, from which urea molecules project and form hydrogen bonds to the lutidines. The urea layers are formed from ribbons of [R22(8)] rings that are linked by lateral hydrogen bonds. However, the linkages are in all three cases subtly different. Adduct 2 crystallizes in Pnma with the lutidine and both ureas displaying mirror symmetry. Adduct 3 crystallizes in space group Abm2 (3o) at 173 K, with two lutidines and four ureas all displaying crystallographic mirror symmetry; on cooling to 100 K, the space group changes to Cc (3m) and the crystals form reticular pseudo-merohedral twins by 180° rotation about a*, probably as a consequence of the mutual displacement of the urea ribbons, parallel to their direction of propagation, to accommodate a change in hydrogen bonding.

Single crystals that spontaneously spawn other single crystals: a ternary and a binary adduct of thiourea and 2,5-dimethylpyrazine

Liquid diffusion of n-pentane into a solution of thiourea in 2,5-dimethylpyrazine led to a crystalline 4 : 3 adduct (1), in which corrugated thiourea layers are crosslinked with pyrazines. Attempts to obtain adducts with other stoichiometries, by crystallizing thiourea from a mixture of 2,5-dimethylpyrazine and methanol, formed the ternary 1 : 1 : 1 adduct 2 instead. Adduct 2 displays a layer structure in which parallel thiourea ribbons are linked on the one side by pyrazines and on the other side by methanol and pyrazines, leading to repeating crosslink sequences (⋯thiourea⋯methanol⋯pyrazine⋯methanol⋯thiourea⋯pyrazine⋯); the ribbons within a layer are thus unequally spaced. In inert oil, individual single crystals of 2 spontaneously convert to several smaller crystals, some single, of 1. The process may be regarded as a single-crystal to single-crystal transformation, although not in the usual sense. The 4 : 3 adduct (3) of thiourea with 2-methylpyrazine is isotypic to 1. In all three structures, the preponderant secondary interactions are classical hydrogen bonds.

Phosphanchalkogenide und ihre Metallkomplexe. IV. Halogenierungsprodukte der Gold(I)halogenidkomplexe einiger Diphosphanmonochalkogenide

Abstract Diphosphanegold(I) complexes of the form dppmEAuX [dppm = bis(diphenylphosphano)methane, E = S, Se; X = Br, I], dppeEAuX [dppe = 1,2-bis(diphenylphosphano)ethane; E = O, S; X = Br, I] and dppbzEAuX [dppbz = 1,2-bis(diphenylphosphano)benzene; E = S, Se, X = Br, I] were treated with elemental X2. With dppm, the three products [dppmEAuX2]+X3– (E = S, X = Br (1), I (2); E = Se, X = I (3) were obtained in quantitative yield. These are gold(III) complexes involving a five-membered ring . With dppe, the only related product was [dppeEAuBr2]+Br3– (4), in which the central ring is six-membered with two carbon atoms. These dppe systems are very sensitive to oxidation/hydrolysis of the ligand, and several such unintended products were isolated and identified. The reaction of dppbzSAuBr with bromine leads to [dppbzS]2+[AuBr4]–Br– (5), the dication of which is formally 1,1,3,3-tetraphenylbenzo[d]-2-thia-1, 3-diphosphol-1,3-diium and contains a central five-membered ring . The dications are associated with the bromide anions via S…Br contacts of ca. 3.1 Å to form inversion-symmetric S2Br2 rings. The halogenation of the dppbzSe derivatives leads to loss of selenium and formation of dppbzAuBr3 (6), with [4+1] coordination at gold, or the known compound [dppbzAuI2]+I3– (7). All products 1–6 were subjected to X-ray diffraction analyses, as were four hydrolysis products 4a–d and two further by-products [5(thtBr+)·2Br3–·3(AuBr4–)] (1a) and (tht)AuBr3 (1b). Compound 1a displays unusually short Br…Br contacts of 3.2398(8) Å between neighbouring tetrabromidoaurate(III) ions.

Phosphanchalkogenide und ihre Metallkomplexe. III. Halogenierungsprodukte der Gold(I)komplexe Ph3PEAuX (E = S oder Se; X = Cl, Br oder I)

Abstract The complexes Ph3PEAuI (E = S, Se; 1, 2) were obtained from the reaction of Ph3PEAuCl with KI; they are appreciably less stable than their chloro and bromo analogues. The X-ray structures were determined, whereby 1 proved to be contaminated by a small amount of Ph3PS·I2. Oxidation of 1 and 2 with elemental iodine led to the adducts Ph3PEAuI·0.5I2 (3 and 4), but X-ray investigation of a crystal initially assumed to be 3 proved it to be a 1:1 mixture of 3 with the adduct Ph3PS·1.5I2, while in 4 the iodine molecule was severely disordered, which prevented successful refinement of the structure. Decomposition of 4 by loss of gold led to Ph3PSeI2·1.5I2 4a. Complexes Ph3PEAuX (E = S, Se; X = Br, Cl) were oxidized by elemental bromine (X = Br) or PhICl2 (X = Cl) to Ph3PEAuX3 (5, 6, 9, 10); none of these X-ray structures could be refined satisfactorily because of diffuse scattering phenomena. Further oxidation led to the ionic compounds [Ph3PEX]+ [AuX4]– (X = Br, E = S, Se: 7, 8; X = Cl, E = S, 11) or [Ph3PSeCl]+ 0.5[Au4Cl10Se2]2– (12), containing the novel groupings P–E–X. X-ray structures confirmed the nature of all four of these compounds, which display longer P–E bonds than the gold(I) starting materials and short X···X and/or E···X contacts between cations and anions.

Imidazolidin-2-one: pseudosymmetry and twinning.

The title compound, C3H6N2O, crystallizes with imposed twofold symmetry in the space group I4(1)/a. The five-membered ring displays a half-chair conformation. N-H···O hydrogen bonds connect the molecules to form R2(2)(8) rings and thence ribbons parallel to the a and b axes. These intersect via O2H2 rings involving longer H···O contacts. The crystal was merohedrally twinned. Preliminary indications of the higher symmetry space group I4(1)/amd, which would require the ring to be planar, proved to be incorrect. A previous brief report of the structure in Fdd2 is also probably incorrect.

Adducts of urea with pyrazines

Abstract The adducts urea:2,3-dimethylpyrazine (1:1) (1), urea:2-methylpyrazine (2:1) (2), urea:2,6-dimethylpyrazine (2:1) (3), urea:2,5-dimethylpyrazine (2:3) (4) and urea:2,5-dimethylpyrazine (2:1) (5), together with the related adduct methylthiourea:2-methylpyrazine (1:1) (6), were prepared and their structures determined. In all cases, the basic motif of the packing is a urea (or thiourea for 6) ribbon consisting of linked R22

Structure of the adducts methylthiourea: 1,4-dioxane (2:1) and 1,1-dimethylthiourea: morpholine (1:1)

{\rm{R}}_2^2