Synthesis and Characterization of Silver(I), Gold(I), and Gold(III) Complexes Bearing Amino-Functionalized N-Heterocyclic Carbenes

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) 2 Ag][AgCl 2 ] complexes (NHC = N-heterocyclic carbene, Me ( 2a ), i -Pr ( 2b )). Upon treatment of 2a with HBF 4, crystals of the unprecedented, NHC-stabilized silver cluster [(NHC) 4 Ag 10 Cl 10 ] ( 5 ) were obtained and characterized by X-ray diffraction. The crystal structure reveals that the carbene carbon atom exists in the rare μ 2 -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 (R 2 S)AuCl leads to the respective Au(I) complexes 3a, b, which are further oxidized with Br 2 to (NHC)AuBr 2 Cl ( 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.