Towards a Generalized Synthetic Strategy for Variable Sized Enantiopure M4L4 Helicates

Stephanie A Boer,Winnie Cao,Bianca K Glascott,David R Turner

doi:10.3390/chemistry2030038

Stephanie A Boer, Winnie Cao + Show 2 more

Open Access

https://doi.org/10.3390/chemistry2030038

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Abstract

The reliable and predictable synthesis of enantiopure coordination cages is an important step towards the realization of discrete cages capable of enantioselective discrimination. We have built upon our initial report of a lantern-type helical cage in attempts to expand the synthesis into a general approach. The use of a longer, flexible diacid ligand results in the anticipated cage [Cu4(L1)4(solvent)4] with a similar helical pitch to that previously observed and a cavity approximately 30% larger. Using a shorter, more rigid ligand gave rise to a strained, conjoined cage-type complex when using DABCO as an internal bridging ligand, [{Co4(L2)4(DABCO)(OH2)x}2 (DABCO)]. The expected paddlewheel motif only forms for one of the Co2 units within each cage, with the other end adopting a “partial paddlewheel” with aqua ligands completing the coordination sphere of the externally facing metal ion. The generic approach of using chiral diacids to construct lantern-type cages is partially borne out, with it being apparent that flexibility in the core group is an essential structural feature.

Highlights

The formation of enclosed cages, capable of guest encapsulation, has been a mainstay of the supramolecular field since the early work on carcerands by Cram [1]
There has been particular attention given to the use of metal ions as structural agents in the formation of coordination cages, with their relatively predictable coordination geometries allowing for a good degree of control over the self-assembling synthesis [2,3,4,5,6,7,8]
Tetrahedral metallo-supramolecular cages, with metal ions situated at the corners of the cage, can have tris-chelated octahedral metal centres that, by definition, have either Λ or ∆ chirality with local C3 symmetry [13,14,15]

Summary

Introduction

The formation of enclosed cages, capable of guest encapsulation, has been a mainstay of the supramolecular field since the early work on carcerands by Cram [1]. The use of cages as agents for guest discrimination is prevalent in the literature, with the internal cavities providing excellent size and shape discrimination between analytes. Such discrimination should lend itself well to separations of racemic mixtures and there is growing literature regarding the synthesis of chiral self-assembled cages [9,10,11,12]. Rather than local chirality around metal centres, it is desirable that the chirality is communicated to the overall cage, including the interior cavity Strategies towards this include the use of chiral ligands to connect the metals together [21,22,23] or using supplementary non-bridging. One or two small single crystals formed, preventing bulk analysis. m/z (ES−): 955.9 ([Co4(L2)4(DABCO)(H2O)3(OH)]NO3)2−, calculated for C70H59Co4N11O392−, 956.0; 1841.9 ([Co4(L2)4(DABCO)]NO3)-, calculated for C70H52Co4N11O35−, 1842.0

X-ray Crystallography

Design and Synthesis of Ligands

Conclusions