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

Abstract

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(OR^F )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 CH₂ Cl₂ , led to decomposition of the [Al(OR^F )₄ ]^- anion and formation of [NO(CH₂ Cl₂ )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(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(OR^F )₄ ]^- , is facile and, for example, [Au(PPh₃ )(NCMe)]⁺ [Al(OR^F )₄ ]^- can be selectively generated. This reactivity opens the possibility to generate various [AuL¹ L² ]⁺ [Al(OR^F )₄ ]^- salts through consecutive ligand-exchange reactions that offer access to a huge variety of Au^I complexes for gold catalysis.