Synthesis and molecular structure of biologically significant bis(1,3-dimesityl-4,5-naphthoquinoimidazol-2-ylidene)gold(I) complexes with chloride and dichloridoaurate counter-ions

Diffraction-quality single crystals of two gold(I) complexes, namely bis(1,3-dimesityl-4,5-naphthoquinoimidazol-2-ylidene)gold(I) chloride benzene monosolvate, [Au(C₂₉H₂₆N₂O₂)₂]Cl·C₆H₆ or [(NQMes)₂Au]Cl·C₆H₆, 2, and bis(1,3-dimesityl-4,5-naphthoquinoimidazol-2-ylidene)gold(I) dichloridoaurate(I) dichloromethane disolvate, [Au(C₂₉H₂₆N₂O₂)₂][AuCl₂]·2CH₂Cl₂ or [(NQMes)₂Au][AuCl₂]·2CH₂Cl₂, 4, were isolated and studied with the aid of single-crystal X-ray diffraction analysis. Compound 2 crystallizes in a monoclinic space group C2/c with eight molecules in the unit cell, while compound 4 crystallizes in the triclinic space group P-1 with two molecules in the unit cell. The crystal lattice of compound 2 reveals C-H...Cl^- interactions that are present throughout the entire structure representing head-to-tail contacts between the aromatic (C-H) hydrogens of naphthoquinone and Cl^- counter-ions. Compound 4 stacks with the aid of short interactions between a naphthoquinone O atom of one molecule and the mesityl methyl group of another molecule along the a axis, leading to a one-dimensional strand that is held together by strong π-η² interactions between the imidazolium backbone and the [AuCl₂]^- counter-ion. The bond angles defined by the Au^I atom and two carbene C atoms [C(carbene)-Au-C(carbene)] in compounds 2 and 4 are nearly rectilinear, with an average value of ∼174.1 [2]°. Though 2 and 4 share the same cation, they differ in their counter-anion, which alters the crystal lattice of the two compounds. The knowledge gleaned from these studies is expected to be useful in understanding the molecular interactions of 2 and 4 under physiological conditions.