Catalytic reactions in a Co12 cuboctahedral cage arising from guest encapsulation and cage-based redox activation

A Co-12 coordination cage with a cuboctahedral architecture, and incorporating a mixture of tritopic (face-capping) and ditopic (edge-bridging) ligands, shows strong guest binding of large aromatic fluorophores (fluorescein and its derivatives 6-carboxyfluorescein and Eosin-Y) with 1 : 1 binding constants in water in the range log K = 6.7-7.9; its large central cavity (>1000 angstrom(3)) facilitates binding of much larger guests than was possible with the smaller Co-8 cage that we have reported previously. Guest binding is accompanied by catalysed reactions of bound guests because the high positive charge on the cage surface (24+) also attracts anions, allowing the organic guests and anionic reaction partners to be co-located, resulting for example in cage-catalysed hydrolysis of phosphate esters (the insecticides Me-paraoxon and Et-paraoxon) and conversion of diacetyl fluorescein to fluorescein. In addition, we demonstrate a new type of cage-based catalysis which relies on the redox activity of the Co(ii)/Co(iii) couple in the cage to activate the peroxymonosulfate (PMS) anion by converting it to the highly reactive SO4- radical ion, which bleaches cavity-bound fluorescein by complete oxidation. This is an example of an 'advanced oxidation process' in which the host cage not only brings the fluorescein and the PMS together via orthogonal hydrophobic and electrostatic interactions, but also provides redox activation of the PMS via a Co(ii)/Co(iii) couple, with the cage taking an active role in the catalytic process rather than acting simply as a passive reaction vessel.

Automated summary

This article describes a Co-12 coordination cage with a cuboctahedral architecture that exhibits strong guest binding of large aromatic fluorophores. The cage also catalyzes reactions of the bound guests and demonstrates a new type of cage-based catalysis that relies on Co(ii)/Co(iii) redox activity. These findings highlight the active role of the cage in catalytic processes, going beyond its passive function as a reaction vessel.