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

• Published in: Chemistry : a European journal 2016-10, Vol.22 (42), p.15085-15094
• Main Authors: Engesser, Tobias A., Friedmann, Christian, Martens, Arthur, Kratzert, Daniel, Malinowski, Przemysław J., Krossing, Ingo
• Format: Article
• Language: English
• Subjects: Acetonitrile; Anions; auroborates; aurophilicity; Catalysis; Cation exchanging; Cations; Chains; Chemical synthesis; Chemistry; Coordination compounds; Correlation; Crystal structure; Crystallography; Decomposition; Dimers; Gold; Ligands; Magnetic resonance spectroscopy; Monomers; nitrosyl salts; Nitrosyls; NMR spectroscopy; Reagents; Salts; Solvents; Spectra; Stretching; Temperature effects
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201602797

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]−; RF=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)3CN]− 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 [AuL1L2]+[Al(ORF)4]− salts through consecutive ligand‐exchange reactions that offer access to a huge variety of AuI complexes for gold catalysis. Gold(I) variations: Oxidation of gold by [NO]+[WCA]− in acetonitrile led to structurally diverse [Au(NCMe)2]+[WCA]− salts ([WCA]−=weakly coordinating anion; [BF4]−, [GaCl4]−, [B(CF3)4]−, or [Al(OC(CF3)3)4]−). Monomers, dimers, and chains of [Au(NCMe)2]+ cations form depending on the WCA. [Au(NCMe)2]+ [Al(OC(CF3)3)4]− is the only salt to be stable at room temperature; it is a versatile reagent for further chemistry, and reacts directly to give the [Au(PPh3)(NCMe)]+ complex.