Gold(I) complexes bearing N-heterocyclic carbenes (NHC) of the type (NHC)AuBr (3a/3b) [NHC = 1-methyl-3-benzylimidazol-2-ylidene (= MeBnIm), and 1,3-dibenzylimidazol-2-ylidene (= Bn 2 Im)] are prepared by transmetallation reactions of (tht)AuBr (tht = tetrahydrothiophene) and (NHC)AgBr (2a/2b). The homoleptic, ionic complexes [(NHC) 2 Au]Br (6a/6b) are synthesized by the reaction with free carbene. Successive oxidation of 3a/3b and 6a/6b with bromine gave the respective (NHC)AuBr 3 (4a/4b) and [(NHC) 2 AuBr 2 ]Br (7a/7b) in good overall yields as yellow powders. All complexes were characterized by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. Reactions of the Au(III) complexes towards anionic ligands like carboxylates, phenolates and thiophenolates were investigated and result in a complete or partial reduction to a Au(I) complex. Irradiation of the Au(III) complexes with UV light yield the Au(I) congeners in a clean photo-reaction.

1-[2-(Dialkylamino)ethyl]-3-methylimidazolium salts (alkyl = Me (1a), i-Pr (1b)) have been prepared and used as precursors for the synthesis of the corresponding [(NHC)2Ag][AgCl2] complexes (NHC = N-heterocyclic carbene, Me (2a), i-Pr (2b)). Upon treatment of 2a with HBF4, crystals of the unprecedented, NHC-stabilized silver cluster [(NHC)4Ag10Cl10] (5) were obtained and characterized by X-ray diffraction. The crystal structure reveals that the carbene carbon atom exists in the rare μ2-coordination pattern, bridging two Ag(I) atoms with further stabilization of the cluster by numerous argentophilic interactions and a coordination of the amino nitrogen donor to one of the silver atoms. Transmetalation of 2a,b with (R2S)AuCl leads to the respective Au(I) complexes 3a,b, which are further oxidized with Br2 to (NHC)AuBr2Cl (4a,b). In red crystals of 4a the gold atom is coordinated in the unusual square-pyramidal geometry with the amine nitrogen atom in the axial position. Upon dissolution in wet organic solvents the amino group is protonated and the color changes to yellow. In square-planar Au(III) halide complexes electronic absorption spectra are dominated by LMCT absorption bands, but in the case of a square-pyramidal coordination sphere the d z 2 orbital is destabilized, becoming the HOMO and causing a low-energy d–d absorption. This interpretation is supported by DFT calculations.

The reaction of meta- and para-bromomethylene-azobenzenes with 1-methyl-imidazole yields the respective meta-/para-functionalized azobenzenes tagged with an imidazolium group. Similar reactions of ortho- and para-bromo-azobenzene with imidazole and successive quaternation with benzylbromide give the analogues, with an imidazolium group in ortho/para substituted azobenzenes. With the exception of the ortho derivative, all imidazolium salts could be transformed into their respective silver(i) complexes by reaction with Ag2O. Transmetallation of these silver(i) complexes with (Me2S)AuCl gives the azobenzene-containing complexes (NHC)AuCl. Two of these formed crystals suitable for X-ray diffraction, which revealed the typical linear coordination geometry of the NHC-Au-Cl moiety. All gold complexes feature E→Z photo-isomerisation upon irradiation with UV light. The thermal back reaction to the E-isomers is relatively slow and comparable to that of other azobenzene compounds.

3‐Methyl‐1‐(2‐picolyl)‐imidazolium chloride (1) has been synthesized and used as a precursor for the preparation of the (NHC)AgCl complex [NHC = 3‐methyl‐1‐(2‐picolyl)imidazol‐2‐ylidene] (2). Transmetalation of 2 with (tht)AuBr (tht = tetrahydrothiophene) yields the corresponding (NHC)AuBr complex 3, which is further oxidized by Br2 to give the (NHC)AuBr3 compound 4. Treatment of 4 with one equivalent of solid AgBF4 affords the [(NHC)AuBr2][BF4] congener 5 as a pale green, crystalline powder. The structure of 4 and 5 were determined by single‐crystal X‐ray diffraction, revealing for 5 a κ‐N‐coordination of the AuIII atom by the picolyl nitrogen atom. Further attempts to exchange all bromides by carboxylates results in the reduction to the dimeric AuI compound [(NHC)2Au2][BF4]2, 6.

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