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

The results of a systematic investigation of trisradical tricationic complexes formed between cyclobis(paraquat-p-phenylene) bisradical dicationic (CBPQT^2(•+)) 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 (K_a) 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 CBPQT^2(•+) 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 CBPQT^2(•+) 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.