Influence of Constitution and Charge on Radical Pairing Interactions in Tris-radical Tricationic Complexes

Chuyang Cheng, Tao Cheng, Hai Xiao, Matthew D. Krzyaniak, Yuping Wang, Paul R. McGonigal, Marco Frasconi, Jonathan C. Barnes, Albert C. Fahrenbach, Michael R. Wasielewski, William A. Goddard, J. Fraser Stoddart*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The results of a systematic investigation of trisradical tricationic complexes formed between cyclobis(paraquat-p-phenylene) bisradical dicationic (CBPQT2(•+)) rings and a series of 18 dumbbells, containing centrally located 4,4′-bipyridinium radical cationic (BIPY•+) units within oligomethylene chains terminated for the most part by charged 3,5-dimethylpyridinium (PY+) and/or neutral 3,5-dimethylphenyl (PH) groups, are reported. The complexes were obtained by treating equimolar amounts of the CBPQT4+ ring and the dumbbells containing BIPY2+ units with zinc dust in acetonitrile solutions. Whereas UV-Vis-NIR spectra revealed absorption bands centered on ca. 1100 nm with quite different intensities for the 1:1 complexes depending on the constitutions and charges on the dumbbells, titration experiments showed that the association constants (Ka) for complex formation vary over a wide range, from 800 M-1 for the weakest to 180 000 M-1 for the strongest. While Coulombic repulsions emanating from PY+ groups located at the ends of some of the dumbbells undoubtedly contribute to the destabilization of the trisradical tricationic complexes, solid-state superstructures support the contention that those dumbbells with neutral PH groups at the ends of flexible and appropriately constituted links to the BIPY•+ units stand to gain some additional stabilization from C-H⋯π interactions between the CBPQT2(•+) rings and the PH termini on the dumbbells. The findings reported in this Article demonstrate how structural changes implemented remotely from the BIPY•+ units influence their non-covalent bonding interactions with CBPQT2(•+) rings. Different secondary effects (Coulombic repulsions versus C-H⋯π interactions) are uncovered, and their contributions to both binding strengths associated with trisradical interactions and the kinetics of associations and dissociations are discussed at some length, supported by extensive DFT calculations at the M06-D3 level. A fundamental understanding of molecular recognition in radical complexes has relevance when it comes to the design and synthesis of non-equilibrium systems.

Original languageEnglish
Pages (from-to)8288-8300
Number of pages13
JournalJournal of the American Chemical Society
Volume138
Issue number26
DOIs
Publication statusPublished - 6 Jul 2016

Bibliographical note

Funding Information:
This research is supported by National Science Foundation (NSF) under grant no. CHE-1308107. T.C., H.X., and W.A.G. thank the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award DESC0004993. This work was also supported by the NSF under grant no. CHE-1565925 (M.R.W.). The authors acknowledge the Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University for NMR and HRMS characterizations. We thank Dr. Amy Sarjeant and Charlotte C. Stern for solving the single-crystal X-ray structures. C.C. thanks the Chinese Scholarship Council for the Award for Outstanding Self-Financed Students Abroad and Foresight Institute for the Distinguished Student Award. A.C.F. acknowledges support from the Earth-Life Science Institute. J.C.B. acknowledges postdoctoral support from the Howard Hughes Medical Institute through the Life Sciences Research Foundation.

Publisher Copyright:
© 2016 American Chemical Society.

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