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

The complexation of Ni2+ and Cu2+ by the tripodal amine phenol ligands 1,2,3-tris(2-hydroxy-5-sulfobenzylamino)propane (H(6)TAPS) and 1,1,1-tris(2-hydroxy-5-sulfobenzylaminomethyl)ethane (H(6)TAMS) 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 degrees C (I = 0.16 M NaCl): for Cu2+ (Ni2+) with H(6)TAPS 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 H(6)TAMS 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 H(6)TAPS and H(6)TAMS, 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 H(6)TAPS the ligand uses an N3O2 donor set to bind Cu2+ and the sixth site is occupied by either hydroxide or phenolate oxygen; with H(6)TAMS, the ligand uses either an N2O3 or an N3O2 donor set to bind Cu2+, and hydroxide co-ordinates in the sixth position.