Photoswitching of Co(ii)-based coordination cages containing azobenzene backbones.

Inclusion of photoswitchable azobenzene units as spacers into ditopic bridging ligands L and L, containing two chelating pyrazolyl-pyridine termini, allows formation of metal complex assemblies with Co(ii) that undergo a range of light-induced structural transformations. One notable result is the light-induced conversion of a Co(L) dinuclear triple helicate (based on the ligand isomer) to a -symmetric Co(L) assembly, assumed to be an edge-bridged tetrahedral cage, based on the ligand isomer. Another is the preparation of a series of Co(L) complexes, of which Co(-L) was crystallographically characterised and consists of a pair of Co(L) double helicates connected by an additional two bridging ligands which span the pair of helicate units, giving a cyclic Co array in which one and then two bridging ligands alternate around the periphery. A set of Co(L) complexes could be prepared containing different ratios of  :  ligand isomers (0 : 6, 2 : 4, 4 : 2 and 6 : 0) of which Co(-L)(-L) was particularly stable and dominated the speciation behaviour, either during light-induced switching of the ligand geometry in pre-formed complexes, or when ligand isomers were combined in different proportions during the preparation. These examples of (i) interconversion between CoL (helicate) and (ii) CoL (cage) assemblies with L, and the interconversion between a series of CoL assemblies Co(-L)(-L) with L, constitute significant advances in the field of photoswitchable supramolecular assemblies.