Stability and Kinetics of Acid- and Anion-Assisted Dissociation Reactions of Hexaamine Macrobicyclic Mercury(II) Complexes

Abstract

The H + - and Cl - -assisted dissociation kinetics and the stabilities of the complexes [Hg(sar)] 2+ and [Hg((NH 2 ) 2 -sar)] 2+ (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane and (NH 2 ) 2 -sar = 1,8-diamino-sar) were determined. The Hg 2+ dissociation rates depend on both the proton and the chloride ion concentrations. H + competes with the metal ion for dissociated amine groups, and Cl - competes with the amine for vacant coordination sites. The rate laws are complicated. For the [Hg(sar)] 2+ system (0.1 ≤ [H + ] ≤ 1.0 M, 0.01 ≤ [Cl - ] ≤ 1.0 M, I = 2.0 M (NaO 3 SCF 3 ), 25.0 °C) the observed rate law is v - Hg 2+ = ( a + b [Cl - ])[H + ][Hg(sar) 2+ ]/(1 + c [Cl - ]), with a = 35(3) M - 1 s - 1, b = 2.9(4) × 10 3 M - 2 s - 1, and c = 33(5) M - 1 . For the [Hg((NH 3 ) 2 -sar)] 4+ system (0.001 ≤ [H + ] ≤ 1.0 M, 0.01 ≤ [Cl - ] ≤ 1.0 M, I = 1.0 M (LiClO 4 ), 25.0 °C) the observed rate law is v - Hg 2+ = ( a + b [H + ] + c [H + ] 2 )[Cl - ][Hg((NH 3 ) 2 -sar) 4+ ])/((1 + d [Cl - ])(1 + e [H + ])), with a = 0.056(6) M - 1 s - 1, b = 8(3) M - 2 s - 1, c = 5(3) M - 3 s - 1, d = 1.3(4) M - 1, and e = 1.1(5) × 10 2 M - 1 . Intimate mechanisms for the dissociation reactions are proposed. Using iodide ion or sar ligand as competing ligands and the reported values for the stabilities of HgI 3 - and HgI 4 2 - the stability constants at 25.0 °C were determined for [Hg(sar)] 2+ (10 28.1(1) M - 1 ), [Hg(sar)I] + (10 29.1(1) M - 2 ), [Hg((NH 2 ) 2 -sar)I] + (10 28.5(1) M - 2 ), and [Hg(cyclam)I] + (10 30.8(1) M - 2 ) (cyclam = 1,4,8,11-tetraazacyclotetradecane) with [OH - ] = 0.1 M, I = 0.5 M (NaClO 4 ) and for [Hg((NH 2 ) 2 -sar)] 2+ (10 26.4(3) M - 1 ) with [OD - ] = 0.1 M, I = 0.1 M (NaOD).