Tripodal Aminophenolate Ligand Complexes of Aluminum(III), Gallium(III), and Indium(III) in Water

Four water-soluble amine phenols have been prepared: tris(((2-hydroxy-5-sulfobenzyl)amino)ethyl)amine (H 6 TRNS), 1,1,1-tris(((2-hydroxy-5-sulfobenzyl)amino)methyl)ethane (H 6 TAMS), 1,2,3-tris((2-hydroxy-5-sulfobenzyl)amino)propane (H 6 TAPS), and cis, cis -1,3,5-tris((2-hydroxy-5-sulfobenzyl)amino)cyclohexane (H 6 TACS). Complex formation constants have been measured at 25 °C (μ = 0.16 M NaCl): log K [M(TRNS)] 3 - ([M(HTRNS)] 2 - ) M = Ga 28.55 (36.90), In 29.3 (34.9); log K [M(TAMS)] 3 - ([M(HTAMS)] 2 - ) M = Al 22.5 (29.3), Ga 31.83, In 28.49; log K [M(TAPS)] 3 - ([M(HTAPS)] 2 - ) M = Al 22.8 (29.0), Ga 31.54 (35.15), In 27.56 (31.93). The order of stability is Ga(III) > In(III) > Al(III) for H 6 TAMS and H 6 TAPS, while for H 6 TRNS it is Ga(III) ≈ In(III) > Al(III). The solution structures of the complexes have been probed by multinuclear NMR ( 1 H, 13 C, 27 Al, 71 Ga, 115 In) and UV spectroscopic studies, and these solution structures are compared with solid state structures for analogous complexes. The 1 H NMR spectrum in D 2 O establishes TAPS to be preorganized for metal ion binding. H 6 TACS exists in the wrong conformation for metal ion binding, and this results in slow complexation kinetics and relatively weaker binding.