A Family of Externally-Functionalised Coordination Cages

New synthetic routes are presented to derivatives of a (known) M8L12 cubic coordination cage in which a range of different substituents are attached at the C-4 position of the pyridyl rings at either end of the bis(pyrazolyl-pyridine) bridging ligands. The substituents are (i) -CN groups (new ligand L-CN), (ii) -CH2OCH2-CCH (containing a terminal alkyne) groups (new ligand L(C)); and (iii) -(CH2OCH2)(3)CH2OMe (tri-ethyleneglycol monomethyl ether) groups (new ligand L-PEG). The resulting functionalised ligands combine with M2+ ions (particularly Co2+, Ni2+, Cd2+) to give isostructural [M8L12](16+) cage cores bearing 24 external functional groups; the cages based on L-CN (with M2+ = Cd2+) and L(C) (with M2+ = Ni2+) have been crystallographically characterised. The value of these is twofold: (i) exterior nitrile or alkene substituents can provide a basis for further synthetic opportunities via 'Click' reactions allowing in principle a diverse range of functionalisation of the cage exterior surface; (ii) the exterior -(CH2OCH2)(3)CH2OMe groups substantially increase cage solubility in both water and in organic solvents, allowing binding constants of cavity-binding guests to be measured under an increased range of conditions.

Automated summary

New synthetic routes are used to introduce various substituents into a M8L12 cubic coordination cage, allowing the formation of functionalized cage cores with metal ions. This allows for a diverse range of synthetic opportunities and increased solubility of the cages.