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((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 ≤ [H+] ≤ 1.0 M, 0.01 ≤ [Cl-] ≤ 1.0 M, I = 2.0 M (NaO3SCF3), 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) × 103 M-2 s-1, and c = 33(5) M-1. For the [Hg((NH3)2-sar)]4+ system (0.001 ≤ [H+] ≤ 1.0 M, 0.01 ≤ [Cl-] ≤ 1.0 M, I = 1.0 M (LiClO4), 25.0 °C) the observed rate law is v - Hg 2+ = (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) × 102 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 HgI3 -and HgI4 2- the stability constants at 25.0 °C were determined for [Hg(sar)]2+ (1028.1(1) M-1), [Hg(sar)I]+ (1029.1(1) M-2), [Hg((NH2)2-sar)I]+ (1028.5(1) M-2), and [Hg(cyclam)I]+ (1030.8(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+ (1026.4(3) M-1) with [OD-] = 0.1 M, I = 0.1 M (NaOD).

Copper(II) Complexes of a Hexadentate Mixed‐Donor N3S3 Macrobicyclic Cage: Facile Rearrangements and Interconversions

“…Such a tetragonal distortion opposes the threefold symmetry of sar-type ligands, and indeed this lowering of symmetry has been observed and reported on a number of occasions with different cage ligands in complexes with Cu II . [11,30,31] Removal of labile divalent Cu [32] and Hg [33] from ligands that belong to the sar family is remarkably slow in comparison with the processes that occur on nonoctahedrally encapsulating polyamines, [34,35] but it is able to be observed and studied under less extreme conditions than those required for demetalation of Co cage complexes. The rate of demetalation is also dependent on the nature of the apical substituents.…”

Molecular imaging with copper-64 in the drug discovery and development arena

“…shows that the latter is ∼ 10 3 -fold more stable than the former. Hence, it is not unreasonable to expect the Cu 2+ complex at hexaaza cage, diamsar would be significantly more stable than the analogous complex with cyclam (reported to have log K values of ≈ 27 -28 [45,46] ) under similar conditions. More critical to the effective application of these ligands in molecular imaging is the kinetics or the rate of formation and dissociation of the Cu 2+ complex formed.…”

“…No log K value for the Cu 2+ complex of the hexaaza cage [Cu(II)-diamsar] 2+ has been reported. This is because once formed, the [Cu(II)-diamsar] 2+ is inert to dissociation under the conditions described above [46] . shows that the latter is ∼ 10 3 -fold more stable than the former.…”

Molecular imaging with copper-64

“…Unfortunately, the log K value of the hexa-aza cage [Cu(II)-diamsar] 2+ complex could not be determined since it is inert to dissociation [95]. No free Cu 2+ ion could be detected under the above conditions.…”

“…No free Cu 2+ ion could be detected under the above conditions. However, a comparison of the log K value of the Hg 2+ complex of cyclam [log K = 23.0 at 25°C and I = 0.1 M] and that of diamsar [log K = 26.3 at [OH À ] = 0.1M, I = 0.5M NaClO 4 at 25°C] shows the latter to be $10 3 -fold more stable than the former [90,95]. This arises from the constraints introduced by the additional ring system to form the cage.…”