A comprehensive study on two novel cyclic tetranuclear Co(II) and Ni(II) bis(salamo)-based oligo(N,O-donor) complexes

A bis(salamo)-based oligo(N,O-donor) ligand H3L was synthesized and characterized, its two polynuclear cobalt(II) and nickel(II) bis(salamo)-based complexes with novel configurations were successfully constructed by introducing M(OAc)2·4H2O (M=Co(II) & Ni(II)) into H3L and reacting with the ligand (L)3- in self-assembly. Surprisingly, the coordination involves the salamo unit's oxime N atom, and the bidentate-coordinated acetate anions coordinate the metal(II) ions so that the structures of the self-assemblied complexes are all cyclic compounds. Where the cobalt(II) complex, [Co4(L)2(µ2-OAc)2(MeOH)2]⋅2MeOH, possesses a novel centrosymmetric cyclic tetranuclear structure, and the unit cell includes two chemically identical but crystallographically independent tetranuclear complex (A and B molecules). The nickel(II) complex, [{Ni4(L)2(µ2-OAc)2(H2O)2}{Ni4(L)2(µ2-OAc)2(EtOH)2}]⋅6EtOH⋅2(Me)2CO, also possesses a novel centrosymmetric cyclic tetranuclear structure, and the unit cell is comprised of chemically inequivalent and crystallographically independent tetranuclear complex (A and B molecules). Ni1 (Ni1#2) in the A molecule is coordinated to the water molecule, while Ni3 (Ni3#1) in the B molecule is coordinated to the ethanol molecule, and both Ni(II) atoms are hexa-coordinated in octahedral geometrical configurations. UV-visible and infrared spectra were used to qualitatively analyze the ligand H3L and its complexes, and the results of the analytical mapping proved how the metal(II) ions and the ligand H3L coordinate. Some related theoretical calculations such as DFT, IRI & BVS have been performed, and the fluorescence properties were also investigated.