Geometric isomerism in coordination cages based on tris-chelate vertices: a tool to control both assembly and host/guest chemistry

Abstract

This 'Perspective' article summarises recent work from the authors' research group on the exploitation of the simple fac/mer geometric isomerism of octahedral metal tris-chelates as a tool to control the chemistry of coordination cages based on bis(pyrazolyl-pyridine) ligands, in two different respects. Firstly this geometric isomerism plays a major role in controlling the guest binding properties of cages because a fac tris-chelate arrangement of pyrazolyl-pyridine chelates around a metal ion vertex results in formation of a convergent set of inwardly-directed C-H protons in a region of high positive electrostatic potential close to a metal cation. This collection of δ+ protons therefore provides a charge-assisted hydrogen-bond donor site, which interacts with the electron-rich regions of guest molecules that are of the correct size and shape to occupy the cage cavity, and the strength of this hydrogen-bonding interaction plays a major role in guest recognition in non-aqueous solvents. Secondly the ability to prepare mononuclear complexes with either a fac or mer arrangement of ligands provides an entry into the controlled, stepwise assembly of heterometallic cages based on a combination of kinetically inert and kinetically labile metal ions at different sites. This has allowed introduction of useful physical properties such as redox activity or luminescence, commonly associated with inert metal ions which are not amenable to participation in thermodynamic self-assembly processes, to be incorporated in a predictable way into the superstructures of coordination cages at specific sites.

One Guest or Two? A Crystallographic and Solution Study of Guest Binding in a Cubic Coordination Cage

“…Two important results from these binding studies were that (i) these guests illustrated 1:1 binding behaviour in MeCN ( H ) or aqueous ( H w ) solution as shown by NMR or fluorescence titrations; and (ii) in MeCN guest binding is in part driven by a hydrogen‐bonding interaction between an electron rich part of the guest (e.g. a carbonyl or pyridine‐ N ‐oxide O atom) and a collection of convergent CH bonds (arising from methylene CH 2 and naphthyl CH units) on the cage interior surface, which lie close to a metal ion and are therefore in a region of positive electrostatic potential . We estimated that the collection of CH bonds in these H‐bond donor pockets—there are two such pockets at opposite ends of the long diagonal of the cage superstructure—was comparable to a phenol group in terms of its overall hydrogen‐bond donor strength .…”

“…a carbonyl or pyridine‐ N ‐oxide O atom) and a collection of convergent CH bonds (arising from methylene CH 2 and naphthyl CH units) on the cage interior surface, which lie close to a metal ion and are therefore in a region of positive electrostatic potential . We estimated that the collection of CH bonds in these H‐bond donor pockets—there are two such pockets at opposite ends of the long diagonal of the cage superstructure—was comparable to a phenol group in terms of its overall hydrogen‐bond donor strength . In the solid state, small guests such as DMMP or solvent molecules clearly show this hydrogen‐bonding interaction between the two guests and the H‐bond donor sites on the host interior surface .…”

“…The similarity of all of the H·G 2 structures in this series-over a range of guest sizes and degrees of saturation-suggestst hat this structural behaviour is general such that other bicyclic guestso ft his general size and shapes hould give the same type of structure. This provedt ob et he case using the guests indan-2-one (10), isoquinoline-N-oxide (11;aparticularly good H-bond acceptor due to the high partial negative charge on the exocyclic oxygen atom), [7] naphthoquinone (12;aquencher of the excited state in two isostructural cages which incorporate either metal-based [8e] or ligand-based [8f] luminophores in the superstructure), and 1-naphthaldehyde (13). The disposition of the pair of guestsi nthe cage cavity,a nd the main Hbondingi nteractions with the H-bond donorp ockets on the cage interior surface, are summarised in Figure 8a Of these structures the one with 12 as guest ( Figure 8) is noteworthy as it is bifunctional (like 2 reported above),a lthoughi nt his case the distance between two carbonyl groups is too short to allow the quinone unit to span the cavity and interact with both H-bond donor pockets.…”

Highly selective CO2vs. N2 adsorption in the cavity of a molecular coordination cage

“…[49][50][51] Here we report an investigation into the gas sorption capability of these materials, demonstrating a high selectivity for CO 2 uptake over N 2 in the solid state, which we ascribe to the presence of the same H-bond donor sites on the cage interior surface that facilitate guest binding in solution. 47,53 We 49 both of which have been reported before. The compounds were prepared as methanol solvates, and then dried and thermally desolvated.…”

“…Such examples of gas sorption into the cavities of molecular cages -in contrast to porous network materials -are very rare. 61,62 On the basis of the structural model based on CS 2 , we suggest that this arises because of favourable polar interactions between the CO 2 guest and charge-assisted hydrogen-bond donor sites on the interior surface of the cage host; 47,53,54 these same structural features also result in particularly high selectivity for binding of CO 2 compared to non-polar N 2 . We thank EPSRC for financial support (grants EP/N031555/1 and EP/K503812/1).…”