Polynuclear Gold Complexes of the Carbodiimide Unit [N=C=N]^2-

The reaction of N,N′-bis(trimethylsilyl)carbodiimide with equimolar quantities of tris[(triphenylphosphine)gold]oxonium tetrafluoroborate in dichloromethane at -40°C gives high yields of a trinuclear complex {[(Ph₃P)Au]₂NCN[Au(PPh₃)]}⁺ BF₄^-(1). According to variable temperature NMR data the cation is fluxional in solution with a rapid site exchange of the [(Ph₃P)Au] units, but has an unsymmetrical (N,N,N′) low temperature ground state configuration. A single crystal X-ray diffraction study has shown that in the crystal lattice these cations are associated further into cyclic, hexanuclear dimers through two short head-to-tail Au-Au contacts (auriophilicity). Treatment of complex 1 with one equivalent of [(Ph₃P)Au]⁺ BF₄^- in tetrahydrofuran affords a tetranuclear complex {[(Ph₃P)Au]₂NCN[Au(PPh₃)]₂} ²⁺ 2BF₄^- (2). The ambient temperature NMR spectra of 2 suggest a fluxional, pseudo-symmetrical structure, but at low temperature a set of three ³¹P signals (1:2:3 intensity) indicates an aggregation into higher nuclearity species with non-equivalent [Au(PPh₃)] units.