Crystal structures of -di-bromido-bis-(3,5-lutidine)gold(III) tribromide and three 3,5-lutidinium salts containing tetra-bromido-aurate(III) anions (including three cases of polymorphism).

-Di-bromido-bis-(3,5-lutidine)gold(III) tribromide, AuBr(CHN) or (3,5-Lut)AuBr, , polymorph , crystallizes in the space group 1 with  = 1. The gold atom and the central bromine of the tribromide ion lie on inversion centres. Polymorph crystallizes in 222 with = 4. The gold atom, the nitro-gen atoms and the ring atoms at the 4-position of the lutidine ligands, and the central bromine of the tribromide ion all lie on twofold axes. The formula units of and are closely similar (including the relative orientations of anion and cation). 3,5-Lutidinium tetra-bromido-aurate(III), (CHN)[AuBr] or (3,5-LutH)[AuBr], , crystallizes as a new polymorph in 3 with = 3. Bis(3,5-lutidinium) tetra-bromido-aurate(III) bromide, (CHN)[AuBr]Br or (3,5-LutH)[AuBr]Br, , polymorph , crystallizes in 2/ with = 4. The gold atom lies on an inversion centre and the bromide ion on a twofold axis. Polymorph crystallizes in 2/ with = 8 (' = 2) and all atoms on general positions. Tris(3,5-lutidinium) bis-[tetra-bromido-aurate(III)] bromide, (CHN)[AuBr]Br or (3,5-LutH)[AuBr]Br, , crystallizes in 1 with = 2. One gold atom lies on a general position and two on inversion centres. The main inter-est centres on the crystal packing patterns. In structure , a short Au⋯Br contact, presumably a 'coinage bond', combines with a 'weak' hydrogen bond H ⋯Br and a further contact Br⋯Br to form a layer structure parallel to the plane. Polymorph shows similar Au⋯Br and H ⋯Br contacts, resulting in a chain of residues parallel to the axis. The most striking feature of the previous polymorph of compound was a topologically square, but distorted, network of tetra-bromido-aurate ions. The packing of the new polymorph of involves three-centre hydrogen bonds Br⋯H⋯Br', an axial coinage bond Au⋯Br, and two Br⋯Br contacts, one of which completes an unusual AuBr three-centre system. The extended packing shows three one-dimensional arrays of residues parallel to the threefold axis, linked by one Br⋯Br contact to form a layer. In polymorph of compound , the packing is based on a dimeric unit with twofold symmetry, centred on the free bromide ion, which is involved in two hydrogen bonds and two Br⋯Br inter-actions. The dimers are connected further Br⋯Br contacts to form a zigzag chain parallel to the axis. Polymorph displays two hydrogen-bonded (3,5-LutH⋯)Br groupings to the two free bromides, together with two tetra-bromido-aurate ions linked by a Br⋯Br contact; the second anion is also connected to a free bromide. The residues thus linked form a broad band parallel to the axis. There are also infinite stacks of planar residues with the repeating sequence (⋯[AuBr]⋯lutidinium⋯lutidinium⋯) parallel to the axis. In compound , all three cations are hydrogen bonded to the free bromide. The anions at two gold centres (Au1/Au2) form a chain parallel to the axis Br⋯Br contacts. The anions at Au3 combine with the free bromide to form a chain of AuBr rings parallel to the axis, Au⋯Br and Br⋯Br contacts, and these chains link with those at Au1 another Br⋯Br contact to form a broad ribbon of residues. The ribbons are in turn linked by a Br⋯π contact.