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 Co₂(L^p)₃ dinuclear triple helicate (based on the E ligand isomer) to a C ₃-symmetric Co₄(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 Co₄(L^m)₆ complexes, of which Co₄(E-L^m)₆ was crystallographically characterised and consists of a pair of Co₂(L^m)₂ double helicates connected by an additional two bridging ligands which span the pair of helicate units, giving a cyclic Co₄ array in which one and then two bridging ligands alternate around the periphery. A set of Co₄(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 Co₄(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 Co₂L₃ (helicate) and (ii) Co₄L₆ (cage) assemblies with L^p, and the interconversion between a series of Co₄L₆ assemblies Co₄(Z-L^m)_n(E-L^m)_6-n with L^m, constitute significant advances in the field of photoswitchable supramolecular assemblies.