Ni(II), Cu(II), and Zn(II) Dinuclear Metal Complexes with an Aza−Phenolic Ligand: Crystal Structures, Magnetic Properties, and Solution Studies

The basicity behavior and ligational properties of the ligand 2-((bis(aminoethyl)amino)methyl)phenol (L) toward
Ni(II), Cu(II), and Zn(II) ions were studied by means of potentiometric measurements in aqueous solution
(298.1 ± 0.1 K, I = 0.15 mol dm-3). The anionic L-H- species can be obtained in strong alkaline solution; this
species behaves as tetraprotic base (log K1 = 11.06, log K2 = 9.85, log K3 = 8.46, log K4 = 2.38). L forms mono-
and dinuclear complexes in aqueous solution with all the transition metal ions examined; the dinuclear species
show a [M2(L-H)2]2+ stoichiometry in which the ligand/metal ratio is 2:2. The studies revealed that two mononuclear
[ML-H]+ species self-assemble, giving the dinuclear complexes, which can be easily isolated from the aqueous
solution due to their low solubility. This behavior is ascribed to the fact that L does not fulfill the coordination
requirement of the ion in the mononuclear species and to the capacity of the phenolic oxygen, as phenolate, to
bridge two metal ions. All three dinuclear species were characterized by determining their crystal structures, which
showed similar coordination patterns, where all the single metal ions are substantially coordinated by three amine
functions and two oxygen atoms of the phenolate moieties. The two metals in the dinuclear complexes are at short
distance interacting together as shown by magnetic measurements performed with Ni(II) and Cu(II) complexes,
which revealed an antiferromagnetic coupling between the two metal ions. The [Cu2(L-H)2]2+ cation shows a phase
transition occurring by the temperature between 100 and 90 K; the characterization of the compounds existing at
different temperatures was investigated using X-ray single-crystal diffraction, EPR, and magnetic measurements.