Complexation of Ni2+ and Cu2+ by tripodal amine phenol ligands in aqueous solution†

Abstract

The complexation of Ni²⁺ and Cu²⁺ by the tripodal amine phenol ligands 1,2,3-tris(2-hydroxy-5-sulfobenzylamino)propane (H₆TAPS) and 1,1,1-tris(2-hydroxy-5-sulfobenzylaminomethyl)ethane (H₆TAMS) has been studied by potentiometry and UV/VIS spectrophotometry. Both metals form [M(H₃L)]^–, [M(H₂L)]^2–, [M(HL)]^3– and [ML]^4– complexes and in addition Cu²⁺ forms [ML(OH)]^5– complexes. The complex formation constants for these species have been measured at 25 °C (I = 0.16 M NaCl): for Cu²⁺ (Ni²⁺) with H₆TAPS log K{[M(H₃L)]^–} = 41.73 (36.88), log K{[M(H₂L)]^2–} = 38.53 (31.86), log K{[M(HL)]^3–} = 34.45 (25.79), log K{[ML]^4–} = 28.07 (17.53) and log K{[ML(OH)]^5-} = 18.96. The corresponding values for H₆TAMS are 40.20 (36.96), 35.99 (31.91), 29.40 (26.33), 20.20 (18.82) and 9.68. The co-ordination number and geometry of these complexes was investigated by variable pH UV/VIS spectrophotometry; Ni²⁺ and Cu²⁺ show differing complexation behaviour. With both H₆TAPS and H₆TAMS, Ni²⁺ is co-ordinated by all six ligand donor atoms, probably in an octahedral manner. In the Cu²⁺ complexes the axial sites are only weakly co-ordinating: with H₆TAPS the ligand uses an N₃O₂ donor set to bind Cu²⁺ and the sixth site is occupied by either hydroxide or phenolate oxygen; with H₆TAMS, the ligand uses either an N₂O₃ or an N₃O₂ donor set to bind Cu²⁺, and hydroxide co-ordinates in the sixth position.