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.