Crystal structures of trans-di­bromido­bis­(3,5-lutidine)gold(III) tribromide and three 3,5-lutidinium salts containing tetra­bromido­aurate(III) anions (including three cases of polymorphism)

Mol­ecular structures and packing diagrams of four systems involving 3,5-lutidine and gold(III) centres are analysed in terms of hydrogen bonds, halogen bonds, coinage bonds and stacking inter­actions.

trans-Di­bromido­bis­(3,5-lutidine)gold(III) tribromide, [AuBr₂(C₇H₉N)₂](Br₃) or [(3,5-Lut)₂AuBr₂](Br₃), 1, polymorph a, crystallizes in the space group P1 with Z = 1. The gold atom and the central bromine of the tribromide ion lie on inversion centres. Polymorph b crystallizes in C222₁ with Z = 4. The gold atom, the nitro­gen 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 1a and 1b are closely similar (including the relative orientations of anion and cation). 3,5-Lutidinium tetra­bromido­aurate(III), (C₇H₁₀N)[AuBr₄] or (3,5-LutH)[AuBr₄], 2, crystallizes as a new polymorph in P3₂ with Z = 3. Bis(3,5-lutidinium) tetra­bromido­aurate(III) bromide, (C₇H₁₀N)₂[AuBr₄]Br or (3,5-LutH)₂[AuBr₄]Br, 3, polymorph a, crystallizes in C2/c with Z = 4. The gold atom lies on an inversion centre and the bromide ion on a twofold axis. Polymorph b crystallizes in P2₁/c with Z = 8 (Z′ = 2) and all atoms on general positions. Tris(3,5-lutidinium) bis­[tetra­bromido­aurate(III)] bromide, (C₇H₁₀N)₃[AuBr₄]₂Br or (3,5-LutH)₃[AuBr₄]₂Br, 4, crystallizes in P1 with Z = 2. One gold atom lies on a general position and two on inversion centres. The main inter­est centres on the crystal packing patterns. In structure 1a, a short Au⋯Br_anion contact, presumably a ‘coinage bond’, combines with a ‘weak’ hydrogen bond H_ortho⋯Br_anion and a further contact Br_cation⋯Br_anion to form a layer structure parallel to the ac plane. Polymorph 1b shows similar Au⋯Br_anion and H_ortho⋯Br_anion contacts, resulting in a chain of residues parallel to the c axis. The most striking feature of the previous polymorph of compound 2 was a topologically square, but distorted, network of tetra­bromido­aurate ions. The packing of the new polymorph of 2 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 AuBr₂ 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 a of compound 3, 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 inter­actions. The dimers are connected via further Br⋯Br contacts to form a zigzag chain parallel to the c axis. Polymorph b displays two hydrogen-bonded (3,5-LutH⋯)₂Br groupings to the two free bromides, together with two tetra­bromido­aurate 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 c axis. There are also infinite stacks of planar residues with the repeating sequence (⋯[AuBr₄]⁻⋯lutidinium⋯lutidinium⋯) parallel to the a axis. In compound 4, all three cations are hydrogen bonded to the free bromide. The anions at two gold centres (Au1/Au2) form a chain parallel to the a axis via Br⋯Br contacts. The anions at Au3 combine with the free bromide to form a chain of Au₂Br₄ rings parallel to the a axis, via Au⋯Br and Br⋯Br contacts, and these chains link with those at Au1 via another Br⋯Br contact to form a broad ribbon of residues. The ribbons are in turn linked by a Br⋯π contact.