Spectrophotometric study of the equilibria between nickel(II) Schiff-base complexes and alkaline earth or nickel(II) cations in acetonitrile solution

Adduct formation between M(ClO4)2 and nickel(II) complexes of tetradentate Schiff-base ligands ([NiL]) have been investigated in acetonitrile by means of UV−vis spectrophotometric analysis (M2+= Mg2+, Ca2+, Sr2+, Ba2+, Ni2+; [NiL] = [Ni(salen)] = [N,N‘-ethylenebis(salicylideneiminato)]nickel(II), [Ni(salphen)] = [N,N‘-o-phenylenebis(salicylideneiminato)]nickel(II), [Ni(salpren)] = [N,N‘-propylenebis(salicylideneiminato)]nickel(II)). [NiL] complexes act as bidentate oxygen-donor ligands forming adducts of the type [(NiL)M]2+ and [(NiL)2M]2+ whose general order of increasing stability was correlated to the dimensions of the M2+ cations. Complexes of general composition 2[Ni(salen)]·M(ClO4)2 were isolated as powders in the solid state (M = Sr, Ca, Mg). X-ray structures of the crystalline complexes [{Ni(salen)}2Ba(ClO4)2(thf)] (1) (thf = tetrahydrofuran) and [{Ni(salen)}2Ni(H2O)2](ClO4)2 (2) are reported. 1 crystallizes in the space group P21/c (a = 10.995 Å; b = 20.830 Å; c = 19.567 Å, β = 116.94°) and 2 in C2/c (a = 10.3020 Å; b = 21.818 Å; c = 17.796 Å; β = 118.98°). Two [Ni(salen)] units act as oxygen bidentate ligands in both the compounds. The barium is coordinated to eight oxygen atoms in a distorted square antiprismatic geometry. An 8-fold coordination is completed by a molecule of tetrahydrofuran and two perchlorate anions acting as bi- and monodentate O-ligands, respectively. In 2, the nickel atom, coordinated to two [Ni(salen)] complexes, has a distorted octahedral geometry and completes its coordination by linking two molecules of water.