Reaction of nickel(II) perchlorate with the title ligand (Hpahqs) in aqueous solution followed by standing for about six months produces a complex having the formulation [Ni4(pahqs)4(H2O)4]·20H2O. A single crystal X-ray analysis revealed that the complex contained the four nickel(II) and four ligand molecules and showed a novel structure of oxygen bridged tetranuclear nickel(II) complex. Crystal data: triclinic, space group P\bar1, a=17.884(7), b=22.071(8), c=12.819(8) Å, α=100.77(4)°, β=92.55(4)°, γ=71.46(4)°, Z=2. Except one N atom in each azo-group, all the other donor atoms in the ligand were fully used in coordination to nickel(II), forming either N,N,O- or N,O-chelate rings, simultaneously. The geometry around each of the four nickel(II) was distorted octahedron. The bond lengths of N1–N(N atoms in N,N,O-terdentate moieties) were elongated compared with those of the corresponding complex, and the lengths of N1–N(N atoms in quinoline rings) were kept almost similar values to those of the Ni(II)-8-hydroxyquinoline complex. The bond lengths of Ni–O were elongated compared with those of the nickel(II) complexes with either N,N,O- or N,O-chelate rings, resulting in the contraction of the angles of N–Ni–N or N–Ni–O. The angles of Ni–O–Ni around four nickel(II) sites were ca. 140°. The antiferromagnetic exchange interaction was operating between the four nickel(II) via bridging oxygen atoms.

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