The halogen–π (X–π) interaction is an intermolecular interaction between the electron-poor region of bonded halogen atoms and aromatic rings. We report an experimental evaluation of the halogen–π (X–π) interaction using liquid chromatography with carbon-material coated columns providing strong π interactions in the normal phase mode. A C 70 -fullerene (C70)-coated column showed higher retentions for halogenated benzenes as the number of halogen substitutions increased as a result of X–π interactions. In addition, the strength of the X–π interaction increased in the order of F < Cl < Br < I. Changes to the UV absorption of C70 and the brominated benzenes suggested that the intermolecular interaction changed from the π–π interaction to X–π interaction as the number of bromo substitutions increased. Computer simulations also showed that the difference in dipole moments among structural isomers affected the strength of the π–π interaction. Furthermore, we concluded from small peak shifts in 1 H NMR and from computer simulations that the orbital interaction contributes to the X–π interactions. Finally, we succeeded in the one-pot separation of all isomers of brominated benzenes using the C70-coated column by optimizing the mobile phase conditions.
β-Trioxopyrrocorphins: pyrrocorphins of graded aromaticity
Octaethyltrioxopyrrocorphins unexpectedly show macrocycle-aromatic properties, even though they contain the macrocyclic π-system of the non-aromatic pyrrocorphins (hexahydroporphyrins). Two of the four possible triketone regioisomers were first reported in 1969 by one-pot oxidation of octaethylporphyrin but remained essentially unexplored since. We detail here the targeted preparation of the remaining two triketone isomers and the optical and NMR spectroscopic properties of all isomers. All four regioisomers possess unique electronic properties, including broadly varying degrees of diatropicity that were experimentally determined using 1 H NMR spectroscopy and computationally verified. Structural patterns modulating the aromaticity were recognized. These differences highlight the regioisomerically differentiated influences of the three β-oxo-functionalities. We also present the solid state structure of the two most common isomers (in their free base form or as zinc complexes), allowing further conclusions to be made about the resonance structures present in these triketones. Remarkably, also, the halochromic properties of the triketones differ sharply from those of regular (hydro)porphyrins, providing further support for the proposed 16-membered, 18 π-electron aromatic ring-current. The work conceptually expands the understanding of tris-modified hydroporphyrinoid analogues and the factors that enable and control porphyrinoid aromaticity.
- Award ID(s):
- 1800361
- Publication Date:
- NSF-PAR ID:
- 10337698
- Journal Name:
- Chemical Science
- Volume:
- 12
- Issue:
- 37
- Page Range or eLocation-ID:
- 12292 to 12301
- ISSN:
- 2041-6520
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/10.1039/D1SC03403K
