CYCLIC TRINUCLEAR GOLD(I) COMPOUNDS: SYNTHESIS, STRUCTURES AND SUPRAMOLECULAR ACID-BASE π-STACKS

Trinuclear 9-membered rings can be formed by gold(I) ions with exobidentate C,N or N,N monoanionic ligands. They are generally slightly irregular and puckered unless the metallocycle is imposed by intramolecular crystallographic symmetry. Gold-gold intramolecular interactions are always present and the complexes exhibit a roughly D_3h symmetry. Crystal structures of these trinuclear complexes show individual complexes, dimers supramolecular columnar packing or more complex supramolecular aggregates. Dimers and supramolecular structures are held together by aurophilic intermolecular gold-gold interactions. Bulky substituents on the ligands can prevent intermolecular metal-metal interactions or the formation of supramolecular architectures. It is well established that the Au(I) center in many linear 2-coordinate Au(I) complexes displays electrophilic tendencies in reaction chemistry while also often accepting electron donors to expand the coordination. Trinuclear Au(I) pyrazolates, carbeniates and benzylimidazolates are well known and undergo electron loss through oxidative addition forming first Au(I,III) mixed valence species and ultimately (with the carbeniates, TR(carb), the benzylimidazolates, TR(bzim) and the chlorinated pyrazolate TR(Cl-pz) trinuclear Au(III) species. Surprisingly, these Au(I) carbeniates and benzylimidazolates also are excellent bases for the metals cations Tl(I) and Ag(I). The acidic, neutral [Hg(C₆F₄)₃]₃ interacts with TR(carb) and TR(bzim) as seen by X-ray solid state and solution NMR measurements. Recently it has been shown that the neutral pi acids C₆F₆ and TCNQ also form stacked pi-acid, pi-base solid state products with TR(carb) and TR(bzim). The TR(bzim) and TR(carb) products are luminescent as solids but C₆F₆ intercalation quenches the luminescence of the p-tol, ethoxy TR(carb), which is a dimer in the solid state. It is interesting that pi-acid, pi-base stacking can involve either a ABAB pattern or an ABBABBA pattern of the molecules, where B is base, TR(carb) or TR(bzim). The B units are aurophilically bonded to each other by two Au…Au linkages. DFT calculations demonstrate that the basicity of the BB dimer is increased relative to the molecular species.