Octanuclear zinc(II) and cobalt(II) clusters produced by cooperative tetrameric assembling of oxime chelate ligands

We have synthesized an octanuclear zinc(II) cluster [L4Zn8(H2O)(3)] by the complexation of 3-hydroxysalamo (H4L) with zinc(II) acetate. The complex crystallizes in the triclinic system, space group P (1) over bar, with unit cell parameters a) 18.233( 10) angstrom, b = 20.518(11) angstrom, c) 21.366(11) angstrom, alpha = 98.7557(2)degrees, beta = 99.191(11), degrees, gamma = 108.309(10)degrees, and Z = 4. The crystallographic analysis revealed the S-4 symmetrical assembling of four ligands and that the tetrameric complex has three water molecules in an unsymmetrical fashion. Spectroscopic analysis of the complex strongly suggests that the octanuclear cluster also exists in solution and maintains a conformation similar to that in the crystal structure, although exchange of the coordinating water molecules presumably takes place. In addition, the formation process of the octanuclear complex is highly cooperative. A high coordinating ability of the [(salamo)Zn] unit as well as the catecholato(2-) moieties probably stabilizes the octanuclear assembly and makes the complexation process cooperative. The corresponding octanuclear cobalt(II) cluster [L4Co8(EtOH)(3)] was prepared in a similar manner. Complex [L4Co8(H2O)(2)X] (X = H2O or EtOH) was obtained by the recrystallization from chloroform/ hexane. The complex crystallizes in the triclinic system, space group P (1) over bar, with unit cell parameters a = 15.2359(10) angstrom, b = 16.9625(12) angstrom, c = 18.9325(13) angstrom, alpha = 101.9710(10),degrees beta = 105.5410(10)degrees, gamma = 97.1290( 10)degrees, and Z = 2. Temperature dependence of magnetic susceptibility showed a continuous decrease in the chi T-M value with decreasing temperature, suggesting antiferromagnetic interaction among cobalt( II) ions. The magnetic susceptibility above 40 K obeys the Curie - Weiss law with a Weiss constant theta of - 39 K and a Curie constant C of 19.7 cm(3) K mol(-1).