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

Inclusion of photoswitchable azobenzene units as spacers into ditopic bridging ligands L m and L p , 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 2 (L p ) 3 dinuclear triple helicate (based on the E ligand isomer) to a C 3 -symmetric Co 4 (L p ) 6 assembly, assumed to be an edge-bridged tetrahedral cage, based on the Z ligand isomer. Another is the preparation of a series of Co 4 (L m ) 6 complexes, of which Co 4 ( E -L m ) 6 was crystallographically characterised and consists of a pair of Co 2 (L m ) 2 double helicates connected by an additional two bridging ligands which span the pair of helicate units, giving a cyclic Co 4 array in which one and then two bridging ligands alternate around the periphery. A set of Co 4 (L m ) 6 complexes could be prepared containing different ratios of Z  :  E ligand isomers (0 : 6, 2 : 4, 4 : 2 and 6 : 0) of which Co 4 ( Z -L m ) 2 ( E -L m ) 4 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 Co 2 L 3 (helicate) and (ii) Co 4 L 6 (cage) assemblies with L p , and the interconversion between a series of Co 4 L 6 assemblies Co 4 ( Z -L m ) n ( E -L m ) 6- n with L m , constitute significant advances in the field of photoswitchable supramolecular assemblies.