Subcomponent Self-Assembly of a Cyclic Tetranuclear FeII Helicate in a Highly Diastereoselective Self-Sorting Manner.

Jana Anhäuser,Rakesh Puttreddy,Arne Lützen,Kari Rissanen,Lukas Glanz,Andreas Schneider,Marianne Engeser

doi:10.1002/chem.201903164

Jana Anhäuser, Rakesh Puttreddy + Show 5 more

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https://doi.org/10.1002/chem.201903164

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Abstract

An enantiomerically pure diamine based on the 4,15‐difunctionalized [2.2]paracyclophane scaffold and 2‐formylpyridine self‐assemble into an optically pure cyclic metallosupramolecular Fe4L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self‐assembly process. The cyclic assembly results from steric strain that prevents the formation of a smaller linear dinuclear triple‐stranded helicate, and hence, leads to the larger strain‐free assembly that fulfils the maximum occupancy rule. Interestingly, use of the racemic diamine also leads to a racemic mixture of the homochiral cyclic helicates as the major product in a highly diastereoselective narcissistic chiral self‐sorting manner given the fact that the assembly contains ten stereogenic elements, which can in principle give rise to 149 different diastereomers. The metallosupramolecular aggregates could be characterized by NMR, UV/Vis and CD spectroscopy, mass spectrometry, and X‐ray crystallography.

Highlights

An enantiomerically pure diamine based on the 4,15-difunctionalized [2.2]paracyclophane scaffold and 2formylpyridine self-assemble into an optically pure cyclic metallosupramolecular Fe4L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self-assembly process
As was already nicely shown by Lehn and co-workers in 1991 when they reported the diastereoselective self-assembly of an enantiomerically pure tris(bipyridine) ligand into enantiomerically pure double-stranded trinuclear linear helicates upon coordination to silver(I) or copper(I) ions,[5] this approach usually gives rise to excellent stereoselectivity if the ligands are properly designed in a way that the formation of the helicate does not impose too much steric strain on the assembly, which could overcompensate for the energetical differences between the possible stereoisomers.[3p] the synthetic effort to access such well behaving ligands is usually quite substantial, and this approach has mainly been restricted to di- and trinuclear double- or triple-stranded linear helicates to date
There are only two studies up to now that involve enantiomerically pure bis(chelating) ligands that could be demonstrated to undergo diastereoselective self-assembly to enantiomerically pure cyclic helicates.[13c,14] In both of these, the individual metal centres are bridged by single ligand strands, which have stereogenic elements in their outer periphery

Summary

Introduction

An enantiomerically pure diamine based on the 4,15-difunctionalized [2.2]paracyclophane scaffold and 2formylpyridine self-assemble into an optically pure cyclic metallosupramolecular Fe4L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self-assembly process. This is quite an achievement in view of the fact, that the self-assembly of enantiomerically pure or racemic ligands to a trinuclear triple-stranded helicate can, in principle, result in the formation of six or twelve different diastereomers, respectively.

Results

Conclusion