Conformational control of Pd2L4 assemblies with unsymmetrical ligands.

James E M Lewis,Andrew Tarzia,Kim E Jelfs,Andrew J P White

doi:10.1039/c9sc05534g

James E M Lewis, Andrew Tarzia + Show 2 more

Open Access

https://doi.org/10.1039/c9sc05534g

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Abstract

With increasing interest in the potential utility of metallo-supramolecular architectures for applications as diverse as catalysis and drug delivery, the ability to develop more complex assemblies is keenly sought after. Despite this, symmetrical ligands have been utilised almost exclusively to simplify the self-assembly process as without a significant driving foa mixture of isomeric products will be obtained. Although a small number of unsymmetrical ligands have been shown to serendipitously form well-defined metallo-supramolecular assemblies, a more systematic study could provide generally applicable information to assist in the design of lower symmetry architectures. Pd2L4 cages are a popular class of metallo-supramolecular assembly; research seeking to introduce added complexity into their structure to further their functionality has resulted in a handful of examples of heteroleptic structures, whilst the use of unsymmetrical ligands remains underexplored. Herein we show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd2L4 metallo-supramolecular assemblies with high fidelity. In this manner it is possible to access Pd2L4 cage architectures of reduced symmetry, a concept that could allow for the controlled spatial segregation of different functionalities within these systems. The introduction of steric directing groups was also seen to have a profound effect on the cage structures, suggesting that simple ligand modifications could be used to engineer structural properties.

Highlights

Non-covalent interactions are utilised by Nature to meticulously control self-assembly processes, ensuring that components combine in speci c ratios and orientations to function effectively
The symmetry of the 1H NMR spectrum suggested that the product could not be isomer B, whilst cross-peaks observed in the ROESY NMR spectrum (Fig. S84†) between the methyl group (Hl) of one pyridine ring and the exohedral proton ortho to the pyridyl nitrogen atom of the other (Hb) ruled out isomer A
Despite the signi cant advances made in the chemists' toolbox of techniques for designing these systems, symmetrical ligands still tend to be employed to simplify the self-assembly process

Summary

Introduction

Non-covalent interactions are utilised by Nature to meticulously control self-assembly processes, ensuring that components combine in speci c ratios and orientations to function effectively. We show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd2L4 metallosupramolecular assemblies with high fidelity.

Results

Conclusion