Synthesis and characterization of high-spin cobalt(II)-based Inorgainc Tennis Ball

The cavities in coordination boxes, cages, and capsules are attracting much attention because of their encapsulation, catalytic reaction, and stereoselective transformation properties. “Inorganic Tennis Ball” (“ITB”) was named to the supramolecule obtained from the self-assembly of Cu(BF4)2 and (dach)Pt(BETMP) (dach = trans-1,2-diaminocyclohexane and BETMP = bisethylthiomethylidenepropanedioate). The cavity and hydrophobic pocket in the ITB have shown recognition properties for anions and pyridine molecules. It is essential to modify the metal sites in order to induce novel magnetism, encapsulation or reaction properties from ITB. We have devoted efforts to find out other transition metals that can replace copper(II) in the ITB. We now report the synthesis and characterization of a new cobalt(II)-based ITB and its magnetic properties. BETMP is a multidentate ligand with two sulfur atoms and two carboxylate groups. A diamineplatinum(II) moiety, (dach)Pt(II), was reported to be coordinated to the ligand exclusively by two sulfur atoms while copper(II) ion, on the other hand, by carboxylate oxygens. To reveal the coordination property of cobalt(II) ion to the ligand, cobalt(II) sulfate and potassium sulfate was reacted with two equivalents of Ba[BETMP] in water. After stirring for 2 h, the precipitate (BaSO4) was removed by filtration, and recrystallization in methanol/ether of the crude product obtained by evaporation of the filtrate afforded the crystalline compound K2Co(BETMP)2(HOCH3)2 (1) (Figure 1). Two carboxylates and two methanols coordinate cobalt(II), where the distance from cobalt(II) to methanol oxygen is 2.165(4) A. Compound 1 was reacted with two equivalent (dach)Pt(NO3)2 in methanol. After removal of KNO3 by filtration, crystalline compound [[{(dach)Pt(BETMP)}2Co(OHCH3)]2(NO3)][NO3]3 (2·[NO3]3) was obtained by slow evaporation of the filtrate (Scheme 1). ESI-MS spectrum of the methanol solution of 2·[NO3]3 detected the ions of m/z = 784.7 and 1207.4 which corresponds to [2-(OHCH3)2] and [(2-(OHCH3)2)·NO3], respectively, unique ions proving the existence of the tennis ball structure in the solution. Figure 2 shows the crystal structure of of 2. Compound 2 is a dimer composed of two homo copies. Hydrogen bonding interactions between NH of one copy and carboxylate oxygens of the other (the distances of N...O range from 2.97 to 3.03 A) are important driving forces for the formation of the dimer. The close contacts between two sulfurs in different copies (these distances range from 3.62 to 3.75 A) seem to also contribute to the gathering of the two copies. A NO3− anion is centered at the cavity inside the dimer. The distance between Co(II) and methanol oxygen is 2.129(18) A. This distance, along with that of 1, is similar to those often observed in other cobalt complexes. Figure 3 shows the electron paramagnetic resonance (EPR) spectrum of the compound 2·[NO3]3 in frozen solution state. The broad EPR feature from 75 to 360 mT Figure 1. ORTEP drawing of the anionic part of 1. The molecule has inversion symmetry. Bond distances (A): Co(1)-O(1), 2.042(3); Co(1)-O(4), 2.076(3); Co(1)-O(5), 2.165(4).