A dynamic tetranuclear gold(i)-cyclophane – gold(i)-centred chirality and fluxionality arising from an intramolecular shift of Au–S bonds

Abstract

A tetranuclear gold(I) complex [Au₄(μ-PAnP)₂(μ-L)₂] (PAnP = 9,10-bis(diphenylphosphino)anthracene and L = benzene-1,2-dithiolate) has been synthesized and characterised by multinuclear NMR and X-ray crystallography. The molecule has a cyclophane-like structure which can be considered to be composed of two [Au₂(μ-PAnP)(μ-L)] units held together by Au–S bonds and aurophilic interactions (Au–Au = 3.0712(2) Å). L acts as a chelating and bridging ligand with one of its S atoms bonded to two Au ions as sulfonium ions and there are two Au₂S₂ cores on each side of the cyclophane. A sulfur atom in each Au₂S₂ core is a chiral sulfonium ion, being bonded to two chemically distinct Au ions. Two Au ions are bonded to four atoms (2S, P and Au) in an asymmetric environment, making them a rare example of gold(I)-centred chirality. The two Au₂S₂ cores have R_Au, R_S and S_Au, S_S configurations, and the chiralities of the sulfonium ion and the gold ion are correlated. Variable-temperature NMR spectroscopy showed that the metallacyclophane undergoes rapid exchange in solution. A bond shift mechanism involving simultaneous cleavage and formation of Au–S bonds is proposed for the exchange.