Polyamine macrocycles incorporating a phenolic function: Their synthesis, basicity, and coordination behavior toward metal cations. Crystal structureof a binuclear nickel complex

The synthesis and characterization of two new polyazamacrocycles, 1,4,7,10- tetraaza[12](2,6)phenolphane(L1) and 1,4,7,10,13-pentaaza[15](2,6)phenolpha ne (L2), are reported. Both ligands incorporate the 2,6-phenolic unit withi n the cyclic framework. The basicity behavior and the ligational properties of L1 and L2 toward Ni(II), Zn(II), and Cu(II) were determined by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I = 0.15 mol dm(-3)). UV spectra were used to understand the role of the phenolic f unction in the stabilization of the cations. L1 and L2 behave as pentaproti c bases under the experimental conditions used. The UV spectra showed that the deprotonation of the phenolic function occurs at low pH values for both ligands, giving rise to the simultaneous presence of positive and tone) ne gative charges on the macrocycle. While L1 forms only mononuclear complexes , L2 can also form binuclear species with all the metal ions investigated. In the mononuclear species of both ligands, one nitrogen atom close to the phenol remains unbound. The UV spectra reveled that the phenol, bridging th e two metal ions in phenolate form, plays an important role in the stabiliz ation of the binuclear complexes of L2. The coordination sphere of the two metals is completed by adding a secondary ligand such as water molecules or OH-, in any case preferring substrates able to bridge the two close metal ions. These results are confirmed by the crystal structure of [Ni-2(C16H28O N5)(H2O)(2)Cl-2]Cl . H2O . CH3OH (space group P2(1)/a, a = 14.821(5) Angstr om, b = 10.270(4) Angstrom, c = 17.663(6) Angstrom, beta = 108.87(3)degrees , V = 2544(2) Angstrom(3), Z = 4, R1 = 0.0973, wR2 = 0.2136). This structur e displays a Ni(II) binuclear complex of L2 in which the phenolic oxygen an d a chlorine ion bridge the two close Ni(II) ions.