Synthesis of octanuclear mixed-valence cobalt(II, III) propionate and butyrate with the Étagère-like core

Abstract

The mixed-valence cobalt carboxylates attract attention due to the role that they play in the catalytic processes of industrial liquid-phase oxidation of hydrocarbons [1][2][3]. However, only poor information is available about the octanuclear mixed-valence cobalt complexes with a metal containing framework of the étagère-shapedtype [4][5][6]. We obtained and characterized with the X-ray diffraction two new cobalt carboxylate complexes with such framework: [Co 4 II Co 4 III (μ 4 -O) 4 · (μ 3 -OMe) 4 (μ-O 2 CR) 6 (O 2 CR) 2 (H 2 O) 6 ]·4H 2 O, where R = Et (I) and Pr (II).Compounds I and II crystallize in the monoclinic system. I, space group Cc, a = 17.373 (4), b = 15.987(3), c = 20.544(4) Å, β = 97.24 (3)º, Z = 4, R = 2.90% [I > 2σ(I)]. II, space group C2/c, a = 17.891(4), b = 16.283(3), c 20.862(4) Å, β = 94.599(4)º, Z = 4, R = 4.30% [I > 2σ(I)].Complexes I and II have the same metal-containing framework of the étagère-shaped type: the framework includes three cubane fragments {Co 4 O 4 } with two common faces. Around the framework there are six bidentate bridging carboxylate ligands forming a double helix motif. In the crystals of complexes I and II the enantiomeric components (right and left spirals) are in equal amount, that is, these compounds are racemates. The complexes I and II are valencetrapped: the Co III atoms are located in the central cubane fragment, while the Co II atoms are on the molecule periphery [7]. [Co 8 O 4 (MeO) 4 (EtCOO) 8 (H 2 O) 6 ]·4H 2 O (I). A weighed portion of cobalt(II) propionate dihydrate [8](2.89 g, 12.0 mmol) was dissolved in 45 ml of methanol. After cooling the solution to -10ºC to it was added dropwise with stirring 0.44 ml of 40% aqueous hydrogen peroxide (6.0 mmol). The solution color changed from purple to dark green and there was a slight evolution of oxygen. At the stepwise evaporation of the solvent, on the walls of the beaker a light-green precipitate began to form, and then formed dark green crystalline precipitate of compound I. Before complete evaporation of the solvent, the solution color changed initially to brown, then to crimson, indicating the predominance of the original cobalt(II) propionate. After evaporation of the solvent, to the residue was added methanol in small portion, each time decanting the formed brown solution until the solution remained