Structural Flexibility in Metal-Organic Cages.

Andrés E Martín Díaz,James E M Lewis

doi:10.3389/fchem.2021.706462

Abstract

Metal-organic cages (MOCs) have emerged as a diverse class of molecular hosts with potential utility across a vast spectrum of applications. With advances in single-crystal X-ray diffraction and economic methods of computational structure optimisation, cavity sizes can be readily determined. In combination with a chemist’s intuition, educated guesses about the likelihood of particular guests being bound within these porous structures can be made. Whilst practically very useful, simple rules-of-thumb, such as Rebek’s 55% rule, fail to take into account structural flexibility inherent to MOCs that can allow hosts to significantly adapt their internal cavity. An often unappreciated facet of MOC structures is that, even though relatively rigid building blocks may be employed, conformational freedom can enable large structural changes. If it could be exploited, this flexibility might lead to behavior analogous to the induced-fit of substrates within the active sites of enzymes. To this end, in-roads have already been made to prepare MOCs incorporating ligands with large degrees of conformational freedom. Whilst this may make the constitution of MOCs harder to predict, it has the potential to lead to highly sophisticated and functional synthetic hosts.

Highlights

Metal-organic cages (MOCs) are self-assembled structures derived from carefully selected combinations of metal ions and ligands (Cook and Stang, 2015; Debata et al, 2019; Pilgrim and Champness, 2020)
Porous MOCs of myriad shape and size are capable of binding guest molecules in their cavities (Rizzuto et al, 2019), leading to applications in catalysis (Yoshizawa et al, 2009; Sinha and Mukherjee, 2018), storage of reactive species (Galan and Ballester, 2016), molecular separations (Zhang et al, 2021), and drug delivery (Casini et al, 2017) amongst others
Coincident with this, easier access to and improvements within SCXRD and the advancement of computational power for theoretical investigations allow researchers to gain rapid and in-depth analysis of these systems. This has led to some remarkable applications for MOCs, in the area of catalysis

Summary

Introduction

Metal-organic cages (MOCs) are self-assembled structures derived from carefully selected combinations of metal ions and ligands (Cook and Stang, 2015; Debata et al, 2019; Pilgrim and Champness, 2020). In this mini review we highlight examples from the literature that demonstrate the inherent structural flexibility of rigid metal-organic hosts, both in solution and the solid-state, and examine the limited number of examples wherein units with high degrees of conformational freedom have More recently Wu and co-workers (Cui et al, 2012) observed a similar change in the structure of a triply-stranded, dinuclear structure, assembled from bis-bidentate ligand 5 and Fe(II), with different anions bound in its cavity (Figure 1C).

Results

Conclusion