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

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.