Synthesis, acid-base and coordination properties towards Cu(II), Zn(II), and Cd(II) ions of two new polyamino-phenolic ligands, including the crystal structure of a fully protonated ligand

The synthesis of the two new ligands 2,6-bis{[bis-(2-aminoethyl)ethylamino]methyl}phenol (M) and bis{[2(2-aminoethylamino)ethylamino]methyl} phenol (L2) is reported. Both ligands have two diethylenetriamine subunits separated by two phenolic spacers; the simple phenol is present in the framework of L1, while the longer 4,4'-bis(1-phenol) is in L2. The acid-base behavior of both ligands in aqueous solution was studied by potentiometry (25 degreesC, I = 0.15 mol dm(-3), NaCl). As a neutral molecule, L1 behaves as hexaprotic base and as monoprotic acid, L2 behaves as hexaprotic base and as diprotic acid. The stability constants for their Cu(II), Zn(II), and Cd(II) complexes were determined by potentiometry and UV-Vis experiments under the same experimental conditions and compared. Both ligands form stable mono- and dinuclear complexes. The analysis of the data revealed a similar coordination arrangement in the mononuclear complexes, while a different arrangement is suggested for the dinuclear one. The role of the phenol and of the 4,4'-bis(1-phenol) as spacers of two amine functions in metal ion coordination is discussed. The crystal structure of the fully protonated species of L1 (L1.6HBr) is reported and discussed.