Aurophilic interactions in cationic gold complexes with two isocyanide ligands.: Polymorphic yellow and colorless forms of [(cyclohexyl isocyanide)2AuI](PF6) with distinct luminescence

Crystallographic studies of yellow and colorless forms Of [(C6H11NC)(2)Au-1](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 (AuAu)-Au-... contact (3.1822(3) Angstrom) 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 (AuAu)-Au-... contacts of 2.9803(6), 2.9790(6), 2.9651(6), and 2.9643(6) Angstrom. However, in the related compound, [(CH3NC)(2)Au-1](PF6), each cation is surrounded by six hexafluorophosphate ions and there is no close (AuAu)-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, lambda(max): 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)(2)Au-1](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.