Aurophilic Interactions in Cationic Gold Complexes with Two Isocyanide Ligands. Polymorphic Yellow and Colorless Forms of [(Cyclohexyl Isocyanide)₂Au^I](PF₆) with Distinct Luminescence

Crystallographic studies of yellow and colorless forms of [(C6H11NC)2AuI](PF6) show that they are polymorphs with differing, but close, contacts between the gold atoms which form extended chains. In the colorless polymorph the gold cations form linear chains with a short Au···Au contact (3.1822(3) Å) indicative of an aurophilic attraction. The structure of the yellow polymorph is more complicated with four independent cations forming kinked, slightly helical chains with very short Au···Au contacts of 2.9803(6), 2.9790(6), 2.9651(6), and 2.9643(6) Å. However, in the related compound, [(CH3NC)2AuI](PF6), each cation is surrounded by six hexafluorophosphate ions and there is no close Au···Au contact despite the fact that the isocyanide ligand has less steric bulk. The crystalline colorless and yellow polymorphs are both luminescent at 298 K, λmax: 424 nm (colorless) or 480 nm (yellow). Colorless solutions of the two polymorphs have identical absorption spectra and are nonluminescent at room temperature. Freezing solutions of [(C6H11NC)2AuI](PF6) produces intense luminescence which varies depending upon the solvent involved. Each polymorph melts to give a colorless but luminescent liquid which reverts to the yellow polymorph upon cooling.