Stability and kinetics of acid- and anion-assisted dissociation reactions of hexaamine macrobicyclic mercury(II) complexes

The H+- and Cl--assisted dissociation kinetics and the stabilities of the complexes [Hg(sar)](2+) and [Hg((NH2)(2)-sar)](2+) (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane and (NH2)(2)-sar = 1,8-diamino-sar) were determined. The Hg2+ 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 less than or equal to [H+] less than or equal to 1.0 M, 0.01 less than or equal to [Cl-] less than or equal to 1.0 M, I = 2.0 M (NaO3SCF3), 25.0 degrees C) the observed rate law is upsilon(-Hg2+) = (a + b[Cl-])[H+][Hg(sar)(2+)]/(1 + c[Cl-]), with a = 35(3) M-1 s(-1), b = 2.9(4) x 10(3) M-2 s(-1), and c = 33(5) M-1. For the [Hg((NH3)(2)-sar)](4+) system (0.001 less than or equal to [H+] less than or equal to 1.0 M, 0.01 less than or equal to [Cl-] less than or equal to 1.0 M, I = 1.0 M (LiClO4), 25.0 degrees C) the observed rate law is upsilon(-Hg2+) = (a + b[H+] + c[H+](2))[Cl-][Hg((NH3)(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) x 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 HeI3- and HgI42- the stability constants at 25.0 degrees C were determined for [Hg(sar)](2+) (10(28.1(1)) M-1), [Hg(sar)I](+) (10(29.1(1)) M-2), [Hg((NH2)(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 (NaClO4) and for [Hg((NH2)(2)-sar)](2+) (10(26.4(3)) M-1) with [OD-] = 0.1 M, I = 0.1 M (NaOD)