A synthetic strategy for a new series of oxo-centered tricobalt complexes with mixed bridging ligands of acetate and pyrazolate anions

Abstract

A new series of oxo-centered tricobalt(III, III, III) complexes, containing mixed bridges of acetate and pyrazolate anions, has been synthesized based on a strategy consisting of two stages. In the first stage, the reaction of cobalt(II) acetate tetrahydrate with pyrazole in the presence of oxidants, followed by purification with silica-gel column chromatography, afforded six tricobalt complexes: [Co₃O(μ-OAc)₅(μ-pz)(Hpz)₃](PF₆) (mono), two isomers of [Co₃O(μ-OAc)₄(μ-pz)₂(Hpz)₃](PF₆) (bis-1 and bis-2), two isomers of [Co₃O(μ-OAc)₃(μ-pz)₃(Hpz)₃](PF₆) (tris-2 and tris-3), and one isomer of [Co₃O(μ-OAc)₂(μ-pz)₄(Hpz)₃](PF₆) (tetra-1). In the second stage, substitution reactions have been examined for the complexes obtained in the first stage. As a result, one acetate bridge in bis-1, bis-2, and tris-3 could be substituted with one pyrazolate site-selectively. Bis-1, bis-2, and tris-3 were converted to another isomer of [Co₃O(μ-OAc)₃(μ-pz)₃(Hpz)₃](PF₆) (tris-1), tris-2, and tetra-1, respectively. Single-crystal X-ray diffraction studies found that in every complex acetato and pyrazolato work as bridging ligands, while pyrazole (Hpz) acts as a terminal ligand. Moreover, the electrochemical properties of the seven new tricobalt complexes (mono, bis-1, bis-2, tris-1, tris-2, tris-3, and tetra-1) were investigated by cyclic voltammetry and it was found that (1) each complex shows reversible redox behaviour corresponding to the Co^IIICo^IIICo^III/Co^IIICo^IIICo^IV couple and (2) the redox potential decreases from 1.21 V vs. Fc/Fc⁺ in mono to 1.09 V vs. Fc/Fc⁺ in tetra-1, as the number of pyrazolate bridges increases.