Crystal structures of seven gold(III) complexes of the form L Au X 3 (L = substituted pyridine, X = Cl or Br)

Abstract

The structures of seven gold(III) halide derivatives of general formula LAuX ₃ (L = methyl-pyridines or di-methyl-pyridines, X = Cl or Br) are presented: tri-chlorido-(2-methyl-pyridine)-gold(III), [AuCl₃(C₆H₇N)], 1 (as two polymorphs 1a and 1b); tri-bromido-(2-methyl-pyridine)-gold(III), [AuBr₃(C₆H₇N)], 2; tri-bromido-(3-methyl-pyridine)-gold(III), [AuBr₃(C₆H₇N)], 3; tri-bromido-(2,4-di-meth-yl-pyridine)-gold(III), [AuBr₃(C₇H₉N)], 4; tri-chlorido-(3,5-di-methylpyridine)-gold(III), [AuCl₃(C₇H₉N)], 5; tri-bromido-(3,5-di-methyl-pyridine)-gold(III), [AuBr₃(C₇H₉N)], 6, and tri-chlorido-(2,6-di-methyl-pyridine)-gold(III), [AuCl₃(C₇H₉N)], 7. Additionally, the structure of 8, the 1:1 adduct of 2 and 6, [AuBr₃(C₆H₇N)]·[AuBr₃(C₇H₉N)], is included. All the structures crystallize solvent-free, and all have Z' = 1 except for 5 and 7, which display crystallographic twofold rotation symmetry, and 4, which has Z' = 2. 1a and 2 are isotypic. The coordination geometry at the gold(III) atoms is, as expected, square-planar. Four of the crystals (1a, 1b, 2 and 8) were non-merohedral twins, and these structures were refined using the 'HKLF 5' method. The largest inter-planar angles between the pyridine ring and the coordination plane are observed for those structures with a 2-methyl substituent of the pyridine ring. The Au-N bonds are consistently longer trans to Br (average 2.059 Å) than trans to Cl (average 2.036 Å). In the crystal packing, a frequent feature is the offset-stacked and approximately rectangular dimeric moiety (Au-X)₂, with anti-parallel Au-X bonds linked by Au⋯X contacts at the vacant positions axial to the coordination plane. The dimers are connected by further secondary inter-actions (Au⋯X or X⋯X contacts, 'weak' C-H⋯X hydrogen bonds) to form chain, double chain ('ladder') or layer structures, and in several cases linked again in the third dimension. Only 1b and 7 contain no offset dimers; these structures instead involve C-H⋯Cl hydrogen bonds combined with Cl⋯Cl contacts (1b) or Cl⋯π contacts (7). The packing patterns of seven further complexes LAuX ₃ involving simple pyridines (taken from the Cambridge Structural Database) are compared with those of 1-8.

Keywords: coinage bonds; crystal structure; gold; halogen bonds; polymorphs; pyridine; twinning.