An investigation into catalysed xanthene-based dye oxidation by a family of coordination cages

Abstract

The ability of a family of M₄, M₈ and M₁₂ coordination cages to effect catalytic oxidative degradation of a family of xanthene-based dyes using peroxymonosulfate (PMS) has been investigated in water. The M₁₂ cages bind one dye molecule inside the central cavity; the M₈ cages bind multiple anionic dye molecules around the external cage surface; the smallest M₄ cages do not interact strongly with the dyes. Three separate sets of experiments showed that octanuclear Co₈ was the most effective catalyst due to a combination of (i) its ability to bind multiple dye molecules around its surface in solution, and (ii) the Co(II)/Co(III) redox couple which activates the PMS anion by reducing it to the reactive species SO₄˙⁻ close to the cage-bound substrates. Control experiments showed that replacing Co(II) by Fe(II), Ni(II) or Zn(II) in isostructural M₈ cages removed catalytic activity, which specifically requires the Co(II)/Co(III) couple; and the effectiveness of the catalysis is guest-dependent according to parameters such as charge, hydrophobicity and inductive effect of substituents on the xanthene core. Overall the Co₈ cage fulfils three functions of (i) binding the guest, (ii) activating the PMS using the Co(II)/Co(III) couple, and (iii) accumulating the SO₄˙⁻ anions around the cationic cage surface close to bound guests.