Interactions of Small-Molecule Guests with Interior and Exterior Surfaces of a Coordination Cage Host

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.

Inside or outside the box? Effect of substrate location on coordination-cage based catalysis

“…There is no point in over-analysing the large number of weak CH⋯O hydrogen-bonds that are involved, but observation of this ‘external’ guest is relevant to the discussion about external surface catalysis below, and we note that we have seen other examples of guests that lie in the space between cubic cage molecules in the crystal, interacting with the cage external surfaces. 16 …”

Application of Crystalline Matrices for the Structural Determination of Organic Molecules

“…coordination cages) can be thought of as analogous to cages in porous coordination polymers except without being interconnected by coordinative bonds. MOPs have long been investigated for guest binding applications. − Coordination cages as preformed crystalline matrices have recently been used for host–guest studies consistent with the crystalline sponge method . When Ward et al encountered difficulty in obtaining host–guest structures using a [Co 8 L 12 ]­(BF 4 ) 16 cage with naphthyl-based bis-bidentate bridging ligands via cocrystallization, they used preformed crystals of the cage for soaking, where neat cycloundecanone (with minimal MeOH to prevent crystal desolvation) was trapped and observed.…”

“…When Ward et al encountered difficulty in obtaining host–guest structures using a [Co 8 L 12 ]­(BF 4 ) 16 cage with naphthyl-based bis-bidentate bridging ligands via cocrystallization, they used preformed crystals of the cage for soaking, where neat cycloundecanone (with minimal MeOH to prevent crystal desolvation) was trapped and observed. Other studies with a similar cage led to the observation of adamantane-1-carboxylic acid and alkyl-phosphonate chemical warfare agent simulants via SC-XRD. , In 2020, Ward et al used their Co-based coordination cages to demonstrate that various fused bicyclic aliphatic and aromatic guests including 4-methoxycoumarin ( 24 ) were incorporated into the cage in a 1:1 or 2:1 ratio (Figure ), and to probe cavity-based binding and external crystal surface interactions . In these studies, crystals were soaked in neat liquid target or in a concentrated MeOH solution of target over 2 d, providing another way to analyze solids that require dissolution in polar protic solvents.…”

Metal–Organic Polyhedra as Building Blocks for Porous Extended Networks

“…[ 34 ] Furthermore, the better defined the reactivity of these porous building blocks, the greater the precision at which they can be interconnected for specific applications without disturbing their core functionalities. [ 35 ]…”