Gold(III) Complexes with 18-Crown-6, 1-Aza-18-Crown-6, and Cryptands 22 and 222: Stability and Structure

The growing demand for gold in various high-technology applications necessitates the development of efficient and selective methods for its recovery and analysis, which can be achieved using such macrocyclic ligands as crown esters and their aza-substituted derivatives. The present paper reports on the equilibrium constants for the formation of gold(III) complexes with 18-crown-6, 1-aza-18-crown-6, 1,10-diaza-18-crown-6, and the cryptand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (Kryptofix 222) in aqueous solution at T = 298.2 K, p = 0.1 MPa, I → 0. The equilibrium constants (log β) for the substitution of chloride ions by macrocycles were determined to be 4.52 ± 0.04, 9.15 ± 0.03, 9.08 ± 0.07, and 11.51 ± 0.08, respectively. Equilibrium constants for protonated and polyligand species are also provided. The complexation mechanism was elucidated using a combination of spectroscopic techniques. UV-Vis and IR spectroscopy confirm the substitution of chloride ligands by the nitrogen donor atoms of the aza-macrocycles within the tetrachloroaurate(III) ion. Furthermore, 1H NMR analysis reveals that the diaza-substituted ligands can form both inclusion complexes, where the gold cation is encapsulated within the macrocyclic cavity, and exclusion complexes. These findings provide a quantitative foundation for the design of novel macrocycle-based extractants and sensors for gold(III).