Controlling the Transmission of Stereochemical Information through Space in Terphenyl-Edged Fe₄L₆ Cages

Abstract

A series of terphenyl-edged Fe(4)L(6) cages were synthesized from substituted 4,4''-diamino-p-terphenyls, 2-formylpyridine, and iron(II). For the parent diaminoterphenyl, all three possible diastereomers, with T, S(4), and C(3) point symmetries, were formed in nearly equal amounts, as determined by (1)H and (13)C NMR. When 2,2″-dimethylterphenylenediamine was used, the T-symmetry diastereomer was observed to predominate. The use of 2',3',5',6'-tetramethylterphenylenediamine generated predominantly the S(4) cage diastereomer, whereas 2',5'-dimethylterphenylenediamine produced the C(3)-symmetric cage to a greater degree than the other two diastereomers. The factors contributing to the transfer of chiral information between metal vertices were analyzed, and the general principles underlying the delicately balanced thermodynamics were determined.

Stereochemical Communication within Tetrahedral Capsules

“…52 We have exploited this method to control the stereochemistry of a series of terphenyl-edged [Fe 4 (L) 6 ] 8+ cages 1316 ( Figure 5). 52 Cage 13 prepared from p-terphenyldiamine (9) exists in solution as a roughly equal mixture of the three cage diastereomers (illustrated in Figure 2). This deviation from the statistical distribution reflects the presence of only slight stereochemical coupling between metal centers.…”

Quantification of Stereochemical Communication in Metal–Organic Assemblies

“…In addition to analyzing the transmission of stereochemical information in all‐Δ or all‐Λ cages, we have also applied our model to a set of racemic Fe II 4 L 6 cages ( 9 – 12 ; Figure 1) that have been previously observed to form heterochiral species 6b. These assembled from ditopic achiral ligands (that is, with f 1 =0) based on terphenyl linkers with different methylation patterns.…”

“…The enthalpic and entropic components of f 2 can be extracted by a Van't Hoff type analysis and can be shown by our model to relate to all three of the pairwise equilibria6b between the T , S 4 , and C 3 states (see Section S6 in the Supporting Information). The generally good agreement between the separately determined entropies and enthalpies and those derived from our model further validates the applicability of the model, providing a unifying understanding of the equilibria.…”

Symmetry breaking in self-assembled M 4 L 6 cage complexes

“…Because each vertex is defined by a metal stereocenter (Δ or Λ), an M 4 L 6 structure can have T (ΔΔΔΔ/ΛΛΛΛ, homochiral), C 3 (ΔΔΔΛ/ΛΛΛΔ, heterochiral), or S 4 (ΔΔΛΛ, achiral) symmetry (36), with the homochiral cage being most commonly observed (33,(37)(38)(39)(40)(41)(42)(43). When the Tsymmetric configuration is adopted, all metal-to-metal distances are the same, avoiding strain potentially incurred by differences in these distances present in the C 3 -or S 4 -symmetric configurations (44). The geometries and steric properties of the ligands can nonetheless overrule the preference for T symmetry.…”

“…The geometries and steric properties of the ligands can nonetheless overrule the preference for T symmetry. For example, in cages constructed from pyridyl-imine ligands via subcomponent self-assembly (45), S 4 symmetry, rather than T, was found to be the lowest-energy stereochemical configuration when the two terminal phenylene rings within the ligand spacer are forced to be coplanar (44), a configuration that is optimal for the four syn ligands that an S 4 -symmetric cage requires to connect metal centers with opposite stereochemistry.…”