Determination of the stability constants of mercury(ii) complexes of mixed donor macrobicyclic encapsulating ligands

Abstract

The reactions of mercury(II) with the mixed donor encapsulating ligands 3,6,16-trithia-6,11,19-triazabicyclo[6.6.6]icosane (AMN₃S₃sar) and 1-amino-8-methyl-6,19-dithia-3,10,13,16-tetraazabicyclo[6.6.6]icosane (AMN₄S₂sar) have been studied. NMR ligand–ligand competition experiments with the ligands 1,4,8,11-tetraazacyclotetradecane ([14]aneN₄), 1-thia-4,7,10-triazacyclododecane ([12]aneN₃S) and ethylenediaminetetraacetic acid (EDTA) with AMN₃S₃sar and Hg(II) indicated that [14]aneN₄ would be an appropriate competing ligand for the determination of the Hg(II) stability constant. Calculations indicated the ratio of concentrations of AMN₃S₃sar, [14]aneN₄ and Hg(II) required for the determination of the stability constant ranged from 1 ∶ 1 ∶ 1 to 1 ∶ 5 ∶ 1. Refinement of the titration curves yielded log₁₀K[Hg(AMN₃S₃sar)]²⁺ = 17.7. A similar competition titration resulted in the determination of the stability constant for the AMN₄S₂sar system as log₁₀K[Hg(AMN₄S₂sar)]²⁺ = 19.5. The observed binding constants for the mixed N/S donor systems and the hexaaza analogues sar (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) and diamsar (1,8-diamino-3,6,10,13,16,19-hexazabicyclo[6.6.6]icosane (log₁₀K[Hg(diamsar)]²⁺ = 26.4; log₁₀K[Hg(sar)]²⁺ = 28.1) differ by approximately ten orders of magnitude. The difference is ascribed not to a cryptate effect but to a mismatch in the Hg–N and Hg–S bond lengths in the N/S systems.