Intramolecular aurophilic interactions in dinuclear gold(I) complexes with twisted bridging 2,2'-bipyridine ligands

Elimination of the chloride ion from the [(PPh₃)AuCl] complex using silver triflate (AgOTf) in the presence of 2,2'-bipyridine R₂bpy (the Au : R₂bpy molar ratio is 2 : 1) in dichloromethane at room temperature leads to dinuclear gold(I) complexes [(PPh₃Au)₂(μ-R₂bpy)](OTf)₂ (R₂bpy = bpy (1), dbbpy (2), CH₃Obpy (3), 3-CO₂CH₃bpy (4), 4-CO₂CH₃bpy (5)) in high yields. The crystal structures for all compounds were determined using X-ray diffraction analysis. In all structures, gold ions are in a typical linear environment, and the bipyridine molecule is twisted, which allows intramolecular aurophilic interactions. Relatively short Au(I)⋯Au(I) contacts (3.1262 (2)-3.400 (1) Å) are found in structures 3-5. DFT calculations show the presence of bond critical points (3, -1) for aurophilic interactions in these structures. In structures 1 and 2, the Au(I)⋯Au(I) distances are noticeably larger and equal to 4.479 (1) and 4.589 (1) Å respectively; there are no bond critical points (3, -1) for aurophilic interactions. All complexes show photoluminescence in solid state at room temperature when excited at 300 nm in a wide spectral range: from blue or blue-green emission (400-460 nm) for 1-4 to orange emission (580 mn) for 5. The lifetimes of the excited state are in the microsecond range which is characteristic of phosphorescence. TD-DFT calculations reveal that electronic transitions of different nature are responsible for the photoluminescence of these compounds.