Role of Anions and Mixtures of Anions on the Thermochromism, Vapochromism, and Polymorph Formation of Luminescent Crystals of a Single Cation, [(C₆H₁₁NC)₂Au]⁺

Abstract

Noncoordinating anions, which generally play a subordinate role in coordination chemistry, alter the structure, the luminescence, as well as the thermochromic and vapochromic behaviors of salts of the two-coordinate cation, [(C₆H₁₁NC)₂Au]⁺. Thus whereas the yellow polymorphs of [(C₆H₁₁NC)₂Au](PF₆) and [(C₆H₁₁NC)₂Au](AsF₆) contain single chains of cations and are vapochromic, yellow [(C₆H₁₁NC)₂Au](SbF₆) does not form the same polymorph and is not vapochromic but contains two distinct chains of cations connected through aurophilic interactions. Mixed crystals such as [(C₆H₁₁NC)₂Au](PF₆)_0.50(AsF₆)_0.50 have been prepared by adding diethyl ether to a dichloromethane solution containing equimolar amounts of [(C₆H₁₁NC)₂Au](PF₆) and [(C₆H₁₁NC)₂Au](AsF₆). The initial (kinetic) product for the three combinations of anions ((PF₆)^-/(AsF₆)^-, (PF₆)^-/(SbF₆)^-, and (AsF₆)^-/(SbF₆)^-) was a precipitate of fine yellow needles with a green emission, which were gradually transformed at rates that depended on the anions present into colorless crystals with a blue emission. Whereas neither polymorph of [(C₆H₁₁NC)₂Au](PF₆) nor [(C₆H₁₁NC)₂Au](SbF₆) is thermochromic, the colorless mixed crystal [(C₆H₁₁NC)₂Au](PF₆)_0.50(SbF₆)_0.50 is thermochromic and converts from blue-emitting to green-emitting at 87-95 °C. The temperature required to transform a crystal of the type [(C₆H₁₁NC)₂Au](PF₆)_<i>n</i>(AsF₆)_1-<i>n</i> from colorless (blue-emitting) to yellow (green-emitting) increases as the fraction of hexafluorophosphate ion in the crystal increases. The yellow crystals of [(C₆H₁₁NC)₂Au](PF₆)_0.75(AsF₆)_0.25, [(C₆H₁₁NC)₂Au](PF₆)_0.50(AsF₆)_0.50, and [(C₆H₁₁NC)₂Au](PF₆)_0.25(AsF₆)_0.75 are vapochromic, whereas the yellow crystals of [(C₆H₁₁NC)₂Au](PF₆)_0.50(SbF₆)_0.50 and [(C₆H₁₁NC)₂Au](AsF₆)_0.50(SbF₆)_0.50 are not.