Coordination behavior of theophylline with Au(III) and electrochemical reduction of the complex

Highlights: • Theophylline (THP) is a novel complexant for cyanide-free gold electroplating. • The coordination behavior between THP and Au(III) was determined. • The stability constant of THP-Au(III) ion was obtained by voltammetric curve. • The reduction of THP-Au(III) ion is in irreversible with diffusion control. -- Abstract: Coordination behavior between theophylline (THP) molecule and Au(III) ion as well as electrochemical reduction of THP-Au(III) complexing ion were studied in detail. UV–vis absorption spectroscopy clarifies that Au(III) is coordinated with THP in a 1:2 M ratio. Raman spectroscopy indicates that it is N in THP chemically bonding to Au(III), and density functional theory (DFT) together with time-dependent density functional theory (TD-DFT) further reveal that the specific N atom is N9 in the imidazole ring (N7H form). Based on the invented formula of cyanide-free gold electroplating with THP as a complexant, the electrode behavior of THP-Au(III) ion on the gold electrode was studied. The results show that the stability constant K_f of THP-Au(III) ion is 2.7 × 10³⁵, and the reduction of THP-Au(III) ion is in irreversible with diffusion control.