Several salts of protonated amines and aza-aromatics with [AuCl 4 ]and [AuBr 4 ]anions contain two-dimensional ("square") anionic networks that display short halogen···halogen contacts. The Au 4 930 quadrilaterals formed by neighboring anions of the networks are to a good approximation squares, with sides of around 7.5 Å for tetrachloridoaurates and 8 Å for tetrabromidoaurates. Scheme 1. Formulae of the structures. Results and DiscussionCompound 1 crystallizes in space group P2 1 /c with one formula unit in the asymmetric unit ( Figure 1, Table 1). Both hydrogen atoms of the NH 2 group are involved in hydrogen bonds to metal-bonded chlorine, [8,9] but these contacts are not the focus of interest here. Instead, we concentrate on the Cl···Cl interactions between anions. These result in a "square" network parallel to the ab plane ( Figure 2). Here, as for the other compounds (see below), the description "square" is a major oversimplification, because the individual anions are significantly rotated and tilted with respect to their neighbors to form an irregular and rather corrugated network. However, the individual Au 4 quadrilaterals are to a reasonable approximation square, with side lengths 7.23 Å, 7.63 Å, and angles 84°, 96°.

The title compound, C(4)H(10)N(2)O, crystallizes with two independent molecules in the space group Cmc2(1). The first molecule lies in a crystallographic mirror plane with x = 0, whereas the second molecule lies on a general position with x is approximately equal to 1/3. Each molecule forms classical N-H...O=C hydrogen bonds, thereby leading to chains of molecules parallel or antiparallel, respectively, to the polar c axis.

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-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).

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez‐Folch et al. (1997). J. Chem. Cryst.27, 367–369; Marsh (2004). Acta Cryst. B60, 252–253], and as polymorph (II), P21212 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 C2v 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)[R21(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).

The structure of the 2,2′-bipyridyl adduct poly[(μ2-2,2′-bipyridyl-κ3 N,N′:N)di-μ3-cyanido-κ6 C:N:N-gold(I)potassium(I)], [AuK(CN)2(C10H8N2)] n , (1) (space group P21), has been redetermined [previous determination: Jones et al. (1980). Acta Cryst. B36, 160–162]. The bipyridyl ligands coordinate only the potassium ion, which has a coordination sphere consisting of seven N-atom donors; gold(I) remains in the form of linear dicyanidoaurate(I) ions. The extended structure consists of layers in which the AuI atoms form chains parallel to the short a axis, with Au...Au contacts of 3.7286 (1) Å, whereas the chains of potassium ions, which are also parallel to a, are bridged by bipyridyl and dicyanidoaurate residues. The analogous 1,10-phenanthroline adduct, namely poly[di-μ3-cyanido-κ6 C:N:N-(μ2-1,10-phenanthroline-κ3 N,N′:N)gold(I)potassium(I)], [AuK(CN)2(C12H8N2)] n , (2), crystallizes as nonmerohedral twins in the space group C2/c. The packing is closely related to that of (1), but the chains are now parallel to the short b axis and the layers are parallel to (10\overline{1}). The two independent AuI atoms occupy special positions on inversion centres and twofold axes; the Au...Au contacts are 3.6771 (2) Å.