Crystal structures of three 4-methyl-piperidinium salts (one as three polymorphs) with tetrahalogenidoaurate(III), halide and (in one case) di-chloro-iodate(I) counter-anions.

Abstract

The structures of three 4-methyl-pyridinium tetra-halogenidoaurate(III) halides, one also including a di-chloro-iodate(I) anion, are presented. Bis(4-methyl-piperidinium) tetra-bromido-aurate(III) bromide, (4-Me-pipH)₂[AuBr₄]Br, 1, crystallizes in space group C2/c with Z = 4. The gold atom lies on the inversion centre 0.75, 0.75, 0.5 and the bromide ion on the twofold axis 0.5, y, 0.25. Bis(4-methyl-piperidinium) tetra-chlorido-aurate(III) chloride, (4-Me-pipH)₂[AuCl₄]Cl, 2, was obtained as three polymorphs, none of which is isotypic to 1. Polymorph 2a crystallizes in space group P2₁/c with Z = 4; all atoms lie on general positions. Polymorph 2b crystallizes in space group P2/c with Z = 8; two chloride ions lie on the twofold axes 0, y, 1/4 and 0.5, y, 0.25. Polymorph 2c crystallizes in space group P1 with Z = 10; all atoms lie on general positions. Hexa-kis-(4-methyl-piperidinium) tetra-kis-{tetra-chlorido-aurate(III)} di-chloro-iodate(I) chloride, (4-Me-pipH)₆[AuCl₄]₄(ICl₂)Cl, 3, crystallizes in space group P1 with Z = 1; two gold atoms occupy inversion centres, as do the iodine atom and one chloride. This is the first time that we have observed the chlorinating agent PhICl₂ to be 'non-innocent'. The main inter-est centres on the crystal packings, which involve hydrogen, halogen and coinage bonds, and display prominent substructures, one type involving the cations and the halides, and the other type involving the anions. A common feature in the packing of compounds 1 and 2 is a chain consisting of cations linked by halide ions. In 1, the chains are flanked by tetra-bromido-aurate ions via short Br⋯Br contacts. The anions form zigzag chains via Br⋯Br^-⋯Br groupings. In polymorph 2a, chains of cations and chloride ions connect to chains of tetra-chlorido-aurate ions to form a layer structure. In polymorph 2b, cation/chloride chains connect with chains of tetra-chlorido-aurate and chloride anions to form a layer structure; approximately linear Au⋯Cl^-⋯Au groupings are a striking feature (these are also seen in 2c and 3). The asymmetric unit of polymorph 2c is an ensemble ca 27 Å long. Cation/chloride chains are linked by tetra-chlorido-aurates to form one-dimensional polymers. The tetra-chlorido-aurate and chloride ions associate to form a layer structure. In the packing of compound 3 the chloride/tetra-chlorido-aurate substructure is dominant. Two tetra-chlorido-aurate ions associate with the free chloride to form a layer structure; the layers are linked in the third dimension via the third tetra-chloro-aurate and the di-chloro-iodate ions. There is no independent cation/chloride substructure; the cations may be considered as being inter-spersed in the spaces of the anionic substructure, forming hydrogen bonds (some multicentred) to chlorine atoms of the anions. A brief database survey of other structures involving aryl- or alkyl-ammonium cations, tetra-halogenidoaurates and halides is presented, including packing diagrams of selected examples.