Homoleptic Gold Acetonitrile Complexes with Medium to Very Weakly Coordinating Counterions: Effect on Aurophilicity?

A series of gold acetonitrile complexes [Au(NCMe)(2)](+)[WCA](-) with weakly coordinating counterions (WCAs) was synthesized by the reaction of elemental gold and nitrosyl salts [NO](+)[WCA] in acetonitrile ([WCA] = [GaCl4](-), [B(CF3)(4)](-), [Al(ORF)(4)](-); R-F = C(CF3)(3)). In the crystal structures, the [Au(NCMe)(2)](+) units appeared as monomers, dimers, or chains. A clear correlation between the aurophilicity and the coordinating ability of counterions was observed, with more strongly coordinating WCAs leading to stronger aurophilic contacts (distances, C-N stretching frequencies of [Au(NCMe)(2)](+) units). An attempt to prepare [Au(L)(2)](+) units, even with less weakly basic solvents like CH2Cl2, led to decomposition of the [Al(ORF)(4)](+) anion and formation of [NO(CH2Cl2)(2)](+)[F(Al(ORF)(3))(2)](-). All nitrosyl reagents [NO](+) [WCA](-) were generated according to an optimized procedure and were thoroughly characterized by Raman and NMR spectroscopy. Moreover, the to date unknown species [NO](+) [B(CF3)(3)CN](-) was prepared. Its reaction with gold unexpectedly produced [Au(NCMe)(2)](+)[Au(NCB(CF3)(3))(2)](-), in which the cyanoborate counterion acts as an anionic ligand itself. Interestingly, the auroborate anion [Au(NCB(CF3)(3))(2)](-) behaves as a weakly coordinating counterion, which becomes evident from the crystallographic data and the vibrational spectral characteristics of the [Au(NCMe)(2)](+) cation in this complex. Ligand exchange in the only room temperature stable salt of this series, [Au(NCMe)(2)](+)[Al(ORF)(4)](-), is facile and, for example, [Au(PPh3)(NCMe)](+)[Al(ORF)(4)](-) can be selectively generated. This reactivity opens the possibility to generate various [(AuLL2)-L-1](+)[Al(ORF)(4)](-) salts through consecutive ligand-exchange reactions that offer access to a huge variety of Au-I complexes for gold catalysis.