Synthesis and Characterization of Self‐Assembled Chiral Fe^II₂L₃ Cages

Abstract

We present here the synthesis of chiral BINOL‐derived (BINOL=1,1′‐bi‐2‐naphthol) bisamine and bispyridine‐aldehyde building blocks that can be used for the self‐assembly of novel chiral FeII 2L3 cages when mixed with an iron(II) precursor. The properties of a series of chiral cages were studied by NMR and circular dichroism (CD) spectroscopy, cold‐spray ionization MS, and molecular modeling. Upon formation of the M2L3 cages, the iron corners can adopt various isomeric forms: mer, fac‐Δ, or fac‐Λ. We found that the coordination geometry around the metal centers in R‐Cages 1 and 2 were influenced by the chiral BINOL backbone only to a limited extent, as a mixture of cages was formed with fac and mer configurations at the iron corners. However, single cage species (fac‐RR‐Cage and fac‐RS‐Cage) that are enantiopure and highly symmetric were obtained by generating these chiral M2L3 cages by using the bispyridine‐aldehyde building blocks in combination with chiral amine moieties to form pyridylimine ligands for coordination to iron. Next to consistent NMR spectra, the CD spectra confirm the configurations fac‐(Λ,Λ) and fac‐(Δ,Δ) corresponding to RR‐ and RS‐Cage, respectively.

Chirality‐Driven Self‐Assembly of Discrete, Homochiral FeII2L3 Cages

“…With all four diastereomers isolated, we investigated the handedness of the iron(II) centers using circular dichroism (CD) spectroscopy (Figure 2, Section S5). The CD spectra of Λ 2 ‐( R ) 3 ‐( R ) 6 ‐ 1 and Δ 2 ‐( S ) 3 ‐( S ) 6 ‐ 1 (Figure 2, left) are mirror images, displaying a pair of cage enantiomers [10a,11a] . The same holds for Λ 2 ‐( S ) 3 ‐( R ) 6 ‐ 1 and Δ 2 ‐( R ) 3 ‐( S ) 6 ‐ 1 (Figure 2, right).…”

“…With these ligands in hand, cage formation was performed following a typical procedure for the synthesis of Fe(pyridylimine)‐type cages [6,11a] . In general, stoichiometric amounts of BINOL‐based ligand, (a)chiral amine and iron(II) bis(triflimide) [Fe(NTf 2 ) 2 ] (3 : 6 : 2) were combined in dry acetonitrile under an inert atmosphere.…”

“…Further analysis with CD spectroscopy allows the assignment of the handedness of the metal centers. Based on a positive or negative first Cotton effect in the MLCT region of the spectrum, they can be assigned as Δ or Λ, respectively, and for enantiomeric pairs of homochiral cages, mirror images are expected [7,8,10a,b,11a,21] …”

“…Chiral vertices are typically generated by introducing enantiopure subcomponents proximal to the metal center, which enables the selective formation of either fac ‐(Λ) or fac ‐(Δ) coordination complexes within the cage [10,11a,12] . Ligands that can control the stereochemistry of the metal centers often bear (axial) chirality in the backbone [13] .…”

Subtle Stereochemical Effects Influence Binding and Purification Abilities of an FeII4L4 Cage

“…The stereochemistry of metal–organic cages can be influenced by enantiopure counterions and guests, through templation during cage formation or postassembly resolution of racemic cage mixtures. , More frequently, enantiopure components, i.e., ligands and metal complexes, are used to control the stereochemistry of self-assembled structures, whereby the resulting metal–organic cages are enantiopure. , In cases where the metal ions, particularly those from the d-block or f-block, have octahedral or pseudotricapped trigonal prismatic geometry, stereochemical information from the ligands can transfer to the metal vertices to produce either a preferred Δ or Λ handedness during higher-order self-assembly. Based upon this strategy, examples of the diastereoselective formation of homochiral cages, with precise control of the handedness of both metal vertices and the final assembled structure, have been reported. ,, …”