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] − = [GaCl 4 ] − , [B(CF 3 ) 4 ] − , [Al(OR F ) 4 ] − ; R F = C(CF 3 ) 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 CH 2 Cl 2 , led to decomposition of the [Al(OR F ) 4 ] − anion and formation of [NO(CH 2 Cl 2 ) 2 ] + [F(Al(OR F ) 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(CF 3 ) 3 CN] − was prepared. Its reaction with gold unexpectedly produced [Au(NCMe) 2 ] + [Au(NCB(CF 3 ) 3 ) 2 ] − , in which the cyanoborate counterion acts as an anionic ligand itself. Interestingly, the auroborate anion [Au(NCB(CF 3 ) 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(OR F ) 4 ] − , is facile and, for example, [Au(PPh 3 )(NCMe)] + [Al(OR F ) 4 ] − can be selectively generated. This reactivity opens the possibility to generate various [AuL 1 L 2 ] + [Al(OR F ) 4 ] − salts through consecutive ligand‐exchange reactions that offer access to a huge variety of Au I complexes for gold catalysis.