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 (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.