Competitive coordination aggregation for V-shaped [Co3] and disc-like [Co7] complexes: synthesis, magnetic properties and catechol oxidase activity

Abstract

Unique dependence on the nature of metal salt and reaction conditions for coordination assembly reactions of varying architecture and nuclearity have been identified in V-shaped [Co₃L₄] and planar disc-like [Co₇L₆] compounds: [Co^II₃L₂(μ-L)₂(μ-OH₂)₂(CF₃CO₂)₂] (1) and [Co^II₇(μ-L)₆(μ-OMe)₆]Cl₂ (2) (HL = 2-{(3-ethoxypropylimino)methyl}-6-methoxyphenol). At room temperature varying reaction conditions, cobalt-ligand ratios and use of different bases allowed unique types of coordination self-assembly. The synthetic marvel lies in the nature of aggregation with respect to the two unrelated cores in 1 and 2. Complex 1 assumes a V-shaped arrangement bound to L⁻, water and a trifluoroacetate anion, while 2 grows around a central Co^II ion surrounded by a {Co^II₆} hexagon bound to methoxide and L⁻. Magnetic measurements revealed that the intermetallic interactions are antiferromagnetic in nature in the case of complex 1 and ferromagnetic in the case of 2 involving high spin cobalt(II) ions with stabilization of the high-spin ground state in the latter case. In MeCN solutions complexes 1 and 2 showed catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCH₂) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) in air. The kinetic study in MeCN revealed that with respect to the catalytic turnover number (k_cat) 2 is more effective than 1 for oxidation of 3,5-DTBCH₂.