Cindy Döring

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).

Aminkomplexe des Goldes, Teil 7: Pseudosymmetrie bei Aminkomplexen des Gold(I)-cyanids [1] / Amine Complexes of Gold, Part 7: Pseudosymmetry in Amine Complexes of Gold(I) Cyanide

The reaction between (tht)AuCl (tht = tetrahydrothiophene) and KCN leads to gold(I) cyanide. This can be treated with liquid amines or azaaromatics L to give crystalline molecular complexes LAuCN, the first complexes of the type (amine)cyanogold(I): L = cyclohexylamine, isobutylamine, isopropylamine, diethylamine, morpholine, piperidine, pyrrolidine, 2,4-lutidine, 3,5-lutidine, and 4- picoline. The cyclohexylamine complex was also obtained as the adduct LauCN L and the pyrrolidine complex in the ionic form [L2Au]+ [Au(CN)2]-. Two polymorphs of the 3,5-lutidine complex were obtained. Ethylenediamine gave the 2:1 complex L(AuCN)2. Several of the structures were difficult to refine because of pseudosymmetry (e. g. polar axes). The packing diagrams were analyzed in terms of Au-Au contacts and NH....N hydrogen bonds. As proof of the principle that the method is also suitable for other ligands, the complex with t-butyldiisopropylphosphane was synthesized and structurally characterized Graphical Abstract Aminkomplexe des Goldes, Teil 7: Pseudosymmetrie bei Aminkomplexen des Gold(I)-cyanids [1] / Amine Complexes of Gold, Part 7: Pseudosymmetry in Amine Complexes of Gold(I) Cyanide

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.

Two polymorphs of 4-hydroxypiperidine with different NH configurations

4-Hydroxypiperidine 1 exists in two crystal forms, tetragonal 1t, space group P21c and orthorhombic 1o, space group Fdd2, both with one molecule in the asymmetric unit. The latter form was obtained only rarely and in small quantities. In form 1t, the NH hydrogen is axial, whereas in 1o it is equatorial; the OH group is equatorial in both structures. The packing of both forms involves one hydrogen bond N–H⋯O and one O–H⋯N. In solution, NMR spectra indicate the presence of separate axial and equatorial forms (with respect to the OH group) below ca. −53 °C; however, not even at −104 °C, the lowest temperature reached, could any freezing out of the inversion at nitrogen be observed, implying that the energy barrier for this process is (as expected) small. We were unable to convert 1t, which appears to be the more stable form over the whole temperature range up to the melting point, to 1o by heating or via melting and re-cooling (or by any other method), perhaps because the hydrogen-bonding pattern is resistant to change. The crystalline forms 1t and 1o, despite being polymorphs of 1 with different NH configurations, should not be described as “configurational polymorphs” because of the facile interconversion in solution.

Aminkomplexe des Golds, Teil 8: Zwei Pyridinderivate des Gold(I)-thiocyanats [1] / Amine Complexes of Gold, Part 8: Two Pyridine Derivatives of Gold(I) Thiocyanate

Abstract The reaction between (tht)AuCl (tht = tetrahydrothiophene) and KSCN leads to a mixture possibly consisting of gold(I) thiocyanate and [Au(tht)2]+ [Au(SCN)2]-. This can be treated with pyridine to give the ionic derivative [Au(py)2]+ [Au(SCN)2]- (1), for which the structures of two polymorphs were determined. Orthorhombic polymorph 1a (space group Pbcn, Z = 4) consists of infinite chains of alternating cations and anions with all gold atoms lying on a twofold axis, whereas triclinic polymorph 1b (space group P1̅, Z = 2) contains isolated tetranuclear units cation···anion···anion···cation with inversion symmetry. In both patterns the gold atoms are linked by short aurophilic contacts. Attempts to recrystallize 1b from dichloromethane-diethyl ether led to small quantities of tetrapyridinium bis(dithiocyanatogold(I)) sulfate (2), (pyH)4[Au(SCN)2]2(SO4), which was identified by crystal structure analysis. The cations are associated with the sulfate ions by classical hydrogen bonds; aurophilic interactions between the anions lead to dimers, which further associate to chains by Au···S contacts.

Aminkomplexe des Goldes, Teil 9: Gold(I)-halogenid-Komplexe mit primären und azyklischen sekundären Aminen und ihre Oxidation zu Gold(III)-Derivaten

Abstract The reaction of (tht)AuX (X=Cl or Br; tht=tetrahydrothiophene) with various primary amines L leads to products of the form [L2Au]+X−. Packing diagrams of the corresponding structures are dominated by N–H···X hydrogen bonds and (in some cases) aurophilic contacts. The cyclohexylamine derivative was already known as its dichloromethane ⅔-solvate; we have isolated the solvent-free compound and its pentane ¼-solvate, which all show different packing patterns. With acyclic secondary amines, the products are more varied; LAuX and [L2Au]+[AuX2]− were also found. These gold(I) products were generally formed in satisfactory quantities. The attempted oxidation to Au(III) derivatives with PhICl2 or Br2 proved impossible for the primary amine derivatives [although isopropylamine-trichloridogold(III) was obtained unexpectedly from the corresponding cyanide] and unsatisfactory for the secondary amine derivatives. Products LAuX3 and [L2AuX2]+[AuX4]− were identified but were formed in disappointing yields. In isolated cases protonated products (LH)+[AuCl4]−, (LH+)3[AuCl4]−(Cl−)2 or [(Et2N)2CH]+[AuBr4]− were formed, presumably by involvement of the dichloromethane solvent and/or adventitious water. Here also the yields were poor, and some products arose as mixtures. Direct reaction of amines with AuCl3 or (tht)AuX3 was also unsuccessful. All products were characterized by X-ray structure analysis.

Aminkomplexe des Goldes, Teil 10: Gold(I)-thiocyanat-Komplexe mit Tetrahydrothiophen, Dimethylsulfid, Ammoniak, Aminen und Azaaromatena

Abstract The reaction of (tht)AuCl (tht=tetrahydrothiophene) with KSCN leads to a mixture of gold(I) thiocyanate AuSCN and [(tht)2Au]+ [Au(SCN)2]− 1. The compounds were separated and the X-ray structure of 1 confirmed as an alternating chain of anions and cations linked by aurophilic contacts. Either pure AuSCN or the mixture was used to synthesize further derivatives of AuSCN, all of which were investigated by X-ray methods. Most products were of limited stability when removed from their mother liquor. The dimethyl sulfide derivative 2 is molecular, (Me2S)AuSCN; the ammonia derivative 3 is ionic, [(NH3)2Au]+ [Au(SCN)2]−. The reaction with 2,2-bipyridyl leads (presumably by involvement of the solvent or of atmospheric moisture) to [bipy-H]+ [Au(SCN)2]− 13. All other products involve amines or azaaromatics as ligands L. The primary amine tert-butylamine forms an ionic product [L2Au]+ (SCN)− 4. The secondary amines piperidine and dibenzylamine lead to molecular structures LAuSCN (5 and 6), whereas pyridine-based azaaromatics lead to ionic products [L2Au]+ [Au(SCN)2]− with L=2-, 3- or 4-picoline (7–9), 2,4-, 3,4- or 3,5-lutidine (10–12). The 3,4-lutidine derivative 11 forms two polymorphs that tend to form mixed crystals. The dominant features of the crystal packing for 7–12 are short aurophilic interactions.

Silber(I)-cyanid-Komplexe mit Aminen und Azaaromaten

Abstract Silver cyanide can be treated with liquid amines or azaaromatics L to give crystalline complexes of various compositions, among them complexes of the simple type cyanido(amine)silver(I): L=isobutylamine and 4-picoline. Other AgCN:L ratios obtained were: 1:2 (benzylamine and 4-benzylpiperidine), 2:1 (2,4-lutidine), 2:3 (morpholine and 3,4-lutidine) and 3:4 (3,5-lutidine). The packing diagrams were analyzed in terms of Ag–Ag and Ag–CN contacts and N–H···N hydrogen bonds. The contacts often give rise to chains, which are sometimes linked to layers by hydrogen bonds.

3,4-Lutidinium salts with the di­iodido­aurate(I) anion: structure of [(3,4-lut)2H]+·[AuI2]− and of two polymorphs of [(3,4-lut)2H+]2·[AuI2]−·I−

Reactions between potassium tetraiodidoaurate(III) and pyridine (py, C5H5N) or 3,4-lutidine (3,4-dimethylpyridine, 3,4-lut, C7H9N) were tested as possible sources of azaaromatic complexes of gold(III) iodide, but all identifiable products contained gold(I). The previously known structure dipyridinegold(I) diiodidoaurate(I), [Au(py)2]+·[AuI2]-, (3) [Adams et al. (1982). Z. Anorg. Allg. Chem. 485, 81-91], was redetermined at 100 K. The reactions with 3,4-lutidine gave three different types of crystal in small quantities. 3,4-Dimethylpyridine-3,4-dimethylpyridinium diiodidoaurate(I), [(3,4-lut)2H]+·[AuI2]-, (1), consists of an [AuI2]- anion on a general position and two [(3,4-lut)2H]+ cations across twofold axes. Bis(3,4-dimethylpyridine-3,4-dimethylpyridinium) diiodidoaurate(I) iodide, [(3,4-lut)2H+]2·[AuI2]-·I-, (2), crystallizes as two polymorphs, each forming pseudosymmetric inversion twins, in the space groups P21 and Pc (but resembling P21/m and P2/c), respectively. These are essentially identical layer structures differing only in their stacking patterns and thus might be regarded as polytypes.

Structures of the adducts urea:pyrazine (1:1), thiourea:pyrazine (2:1) and thiourea:piperazine (2:1)

Abstract The adducts urea:pyrazine (1:1) (1), thiourea:pyrazine (2:1) (2), and thiourea:piperazine (2:1) (3) were prepared and their structures determined. Adduct 1 forms a layer structure, in which urea chains of graph set C(4)[ R21