Speciation, stability constants and structures of complexes of copper(II), nickel(II), silver(I) and mercury(II) with PAMAM dendrimer and related tetraamide ligands

The acid-base properties and Cu(II), Ni(II), Ag(I) and Hg(II) binding abilities of PAMAM dendrimer, L, and of the simple model compounds, the tetraamides of EDTA and PDTA, L¹, were studied in solution by pH-metric methods and by ¹H NMR and UV-Vis spectroscopy. PAMAM is hexabasic and six pK_a values have been determined and assigned. PAMAM forms five identifiable complexes with copper(II), CuLH₄]⁶⁺, CuLH₂]⁴⁺, CuLH]³⁺, CuL]²⁺ and CuLH_-1]⁺ in the pH range 2-11 and three with nickel(II), NiLH]³⁺, NiL]²⁺ and NiLH_-1]⁺ in the pH range 7-11. The complex CuLH₄]⁶⁺, which contains two tertiary nitrogen and three amide oxygen atoms coordinated to the metal ion, is less stable than the analogous EDTA and PDTA tetraamide complexes CuL¹]²⁺, which contain two tertiary nitrogen and four amide oxygen atoms, due to ring size and charge effects. With increasing pH, CuLH₄]⁶⁺ undergoes deprotonation of two coordinated amide groups to give CuLH₂]⁴⁺ with a concomitant change from O-amide to N-amidate coordination. Surprisingly and in contrast to the tetraamide complexes CuL¹]²⁺, these two deprotonation steps could not be separated. As expected the nickel(II) complexes are less stable than their copper(II) analogues. The tetra-N-methylamides of EDTA, L¹(b), and PDTA form mononuclear and binuclear complexes with Hg(II). In the case of L¹(b) these have stoichiometries HgL¹(b)Cl₂, HgL¹(b)H₋₂Cl₂]²⁻, Hg₂L¹(b)Cl₂]²⁺, Hg₂L¹(b)H₋₂Cl₂ and Hg₂L¹(b)H₋₅Cl₂]³⁻. Based on ¹H NMR and pH-metric data the proposed structure for HgL¹(b)Cl₂, the main tetraamide ligand containing species in the pH range <3-6.5, contains L¹(b) coordinated to the metal ion through the two tertiary nitrogens and two amide oxygens while the structure of HgL¹(b)H₋₂Cl₂]²⁻, the main tetraamide ligand species at pH 7.5-9.0, contains the ligand similarly coordinated but through two amidate nitrogen atoms instead of amide oxygens. The proposed structure of Hg₂L¹(b)Cl₂]²⁺, a minor species at pH 3-6.5, also based on ¹H NMR and pH-metric data, contains each Hg(II) coordinated to a tertiary amino nitrogen, two amide oxygens and a chloride ligand while that of Hg₂L¹(b)H₋₅Cl₂]³⁻, contains each Hg(II) coordinated to a tertiary amino nitrogen, two amidate nitrogens, a chloride and a hydroxo ligand in the case of one of the Hg(II) ions. The parent EDTA and PDTA amides only form mononuclear complexes. PAMAM also forms dinuclear as well as mononuclear complexes with mercury(II) and silver(I). In the pH range 3-11 six complexes with Hg(II) i.e. HgLH₄Cl₂]⁴⁺, HgLH₃Cl₂]³⁺, Hg₂LCl₂]²⁺, Hg₂LH₋₁Cl₂]⁺, HgLH₋₁Cl₂]⁻ and HgLH₋₂Cl₂]²⁻ were identified and only two with Ag(I), AgLH₃]⁴⁺ and Ag₂L]²⁺. Based on stoichiometries, stability constant comparisons and ¹H NMR data, structures are proposed for these species. Hence HgLH₄Cl₂]⁴⁺ is proposed to have a similar structure to CuLH₄]⁶⁺ while Hg₂LCl₂]²⁺has a similar structure to Hg₂L¹(b)H₋₅Cl₂]³⁻.