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.
Highlights
From its humble beginning with Pedersen’s discovery of the first crown ether in 1960,1 the domain of supramolecular chemistry is one that has grown in many directions, with chemists’ ability to manipulate non-covalent interactions underpinning developments in research fields from synthetic biology to materials science
In this review we describe how this simple variation in geometric isomerism between otherwise similar metal centres in the cages underpins both the control of heteronuclear cage assembly, and the ability of some of the cages to act as hosts for both anionic and neutral guests
The flexibility of the ligands associated with the CH2 ‘hinges’ allows them to adopt a conformation which maximises inter-ligand π-stacking,2c with every central phenyl ring of a bridging ligand lies parallel to, and overlapping with, an pyrazolyl-pyridine unit of another ligand coordinated to a Ru(II) ion, forming alternating stacks of electron-rich and electron-deficient components
Summary
Geometric isomerism in coordination cages based on tris-chelate vertices: a tool to control both assembly and host/guest chemistry 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. 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 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
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