Abstract
Coordination cages are well-known to act as molecular containers that can bind small-molecule guests in their cavity. Such cavity binding is associated with interactions of the guests with the surrounding set of surfaces that define the cavity; a guest that is a good fit for the cavity will have many favourable interactions with the interior surfaces of the host. As cages have exterior as well as interior surfaces, possibilities also exist for ‘guests’ that are not well-bound in the cavity to interact with the exterior surface of the cage where spatial constraints are fewer. In this paper, we report a combined solid-state and solution study using an octanuclear cubic M8L12 coordination cage which illustrates the occurrence of both types of interaction. Firstly, crystallographic studies show that a range of guests bind inside the cavity (either singly or in stacked pairs) and/or interact with the cage exterior surface, depending on their size. Secondly, fluorescence titrations in aqueous solution show how some flexible aromatic disulfides show two separate types of interaction with the cage, having different spectroscopic consequences; we ascribe this to separate interactions with the exterior surface and the interior surface of the host cage with the former having a higher binding constant. Overall, it is clear that the idea of host/guest interactions in molecular containers needs to take more account of external surface interactions as well as the obvious cavity-based binding.
Highlights
Functions such as catalysis [6,7,8,9,10,11,12], substrate transport [13,14,15,16], sensing [17] and enantioselective recognition and separation [18] are all based around guest binding inside the host cage, and a huge amount of work exists on the study of cage-based host/guest interactions
This includes the synthesis of cages that are engineered with interior binding sites to facilitate guest binding [19,20]; quantification of guest binding constants and analysis of the factors responsible for guest binding [21,22,23,24] including co-encapsulation of multiple guests [25,26,27]; and studies of how specific guests act as templates for host cage assembly [28,29]
Given that guest binding inside the cavities of these cages is expected to provide efficient quenching because of the proximity of the guest to all members of the chromophore array [39,40], we suggest that the second component is associated with the disulfide guest binding inside the cage cavity in the manner shown by the crystal structure above
Summary
As part of the study of coordination cages—hollow, self-assembled metal-ligand capsules—there is a great deal of emphasis on guest binding in the internal space [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] Functions such as catalysis [6,7,8,9,10,11,12], substrate transport [13,14,15,16], sensing [17] and enantioselective recognition and separation [18] are all based around guest binding inside the host cage, and a huge amount of work exists on the study of cage-based host/guest interactions. Nitschke [32] and Raymond [33] have likewise demonstrated interaction of small molecules or counter-ions with cage exterior surfaces
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