Abstract
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 C4 position of the pyridyl rings at either end of the bis(pyrazolyl-pyridine) bridging ligands. The substituents are (i) –CN groups (new ligand LCN), (ii) –CH2OCH2–CCH (containing a terminal alkyne) groups (new ligand LCC); and (iii) –(CH2OCH2)3CH2OMe (tri-ethyleneglycol monomethyl ether) groups (new ligand LPEG). 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 LCN (with M2+ = Cd2+) and LCC (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)3CH2OMe 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.
Highlights
IntroductionThe ability of self-assembled coordination cages—hollow, pseudo-spherical, metalligand assemblies [1,2,3,4,5,6,7]—to bind small-molecule guests in their central cavities has resulted in a wide range of potential applications such as transport and release of ‘cargoes’ [8,9,10,11]including drug molecules; catalysed reactions of cavity-bound guests whose reactivity is altered [12,13,14,15,16,17,18,19,20,21]; and analysis or sensing of species whose binding in the cavity triggers an optical response [22]
Chosen substituents attached to cage exterior surfaces can control solubility [29,30]; provide functional groups which can interact with surfaces [31] or proteins [32]; and provide functionality such as redox [33] or photophysical [34] properties that supplement the properties of the cage/guest assembly
The ability to provide a range of externally-directed functional groups around the cage surface is an important development as this strongly influences how the cage hosts interact with the outside world: it provides quite a different research focus from the inward-looking aspects of host/guest complex formation based on the central cavity
Summary
The ability of self-assembled coordination cages—hollow, pseudo-spherical, metalligand assemblies [1,2,3,4,5,6,7]—to bind small-molecule guests in their central cavities has resulted in a wide range of potential applications such as transport and release of ‘cargoes’ [8,9,10,11]including drug molecules; catalysed reactions of cavity-bound guests whose reactivity is altered [12,13,14,15,16,17,18,19,20,21]; and analysis or sensing of species whose binding in the cavity triggers an optical response [22]. The ability of self-assembled coordination cages—hollow, pseudo-spherical, metalligand assemblies [1,2,3,4,5,6,7]—to bind small-molecule guests in their central cavities has resulted in a wide range of potential applications such as transport and release of ‘cargoes’ [8,9,10,11]. Guests bound inside coordination cages span a huge range from simple anions [23] or solvent molecules [24] via fullerenes [25,26] to small proteins [27,28]. The main focus on coordination cage chemistry has been guest binding in the central cavity which is a very well-developed area. Chosen substituents attached to cage exterior surfaces can control solubility [29,30]; provide functional groups which can interact with surfaces [31] or proteins [32]; and provide functionality such as redox [33] or photophysical [34] properties that supplement the properties of the cage/guest assembly.
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