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

Abstract

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, L 1, were studied in solution by pH-metric methods and by 1H NMR and UV–Vis spectroscopy. PAMAM is hexabasic and six p K a values have been determined and assigned. PAMAM forms five identifiable complexes with copper(II), [Cu LH 4] 6+, [Cu LH 2] 4+, [Cu LH] 3+, [Cu L] 2+ and [Cu LH -1] + in the pH range 2–11 and three with nickel(II), [Ni LH] 3+, [Ni L] 2+ and [Ni LH -1] + in the pH range 7–11. The complex [Cu LH 4] 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 [Cu L 1] 2+, which contain two tertiary nitrogen and four amide oxygen atoms, due to ring size and charge effects. With increasing pH, [Cu LH 4] 6+ undergoes deprotonation of two coordinated amide groups to give [Cu LH 2] 4+ with a concomitant change from O-amide to N-amidate coordination. Surprisingly and in contrast to the tetraamide complexes [Cu L 1] 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, L 1( b), and PDTA form mononuclear and binuclear complexes with Hg(II). In the case of L 1 ( b) these have stoichiometries Hg L 1 ( b)Cl 2, [Hg L 1 ( b)H −2Cl 2] 2−, [Hg 2 L 1 ( b)Cl 2] 2+, Hg 2 L 1 ( b)H −2Cl 2 and [Hg 2 L 1 ( b)H −5Cl 2] 3−. Based on 1H NMR and pH-metric data the proposed structure for Hg L 1 ( b)Cl 2, the main tetraamide ligand containing species in the pH range <3–6.5, contains L 1 ( b) coordinated to the metal ion through the two tertiary nitrogens and two amide oxygens while the structure of [Hg L 1 ( b)H −2Cl 2] 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 [Hg 2 L 1 ( b)Cl 2] 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 [Hg 2 L 1 ( b)H −5Cl 2] 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. [Hg LH 4Cl 2] 4+, [Hg LH 3Cl 2] 3+, [Hg 2 LCl 2] 2+, [Hg 2 LH −1Cl 2] +, [Hg LH −1Cl 2] − and [Hg LH −2Cl 2] 2− were identified and only two with Ag(I), [Ag LH 3] 4+ and [Ag 2 L] 2+. Based on stoichiometries, stability constant comparisons and 1H NMR data, structures are proposed for these species. Hence [Hg LH 4Cl 2] 4+ is proposed to have a similar structure to [Cu LH 4] 6+ while [Hg 2 LCl 2] 2 +has a similar structure to [Hg 2 L 1 ( b)H −5Cl 2] 3−.