Solution-phase mechanistic study and solid-state structure of a tris(bipyridinium radical cation) inclusion complex

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Organic Chemistry

Background:

• The cyclobis(paraquat-p-phenylene) (CBPQT(2(•+))) is a diradical dicationic ring.
• 1,1′-dialkyl-4,4′-bipyridinium radical cations (BIPY(•+)) are common guests in host-guest chemistry.

Purpose of the Study:

• To mechanistically and quantitatively investigate the formation of inclusion complexes between CBPQT(2(•+)) and BIPY(•+) guests.
• To elucidate the structural, thermodynamic, and kinetic properties of these trisradical complexes.

Main Methods:

• Single-crystal X-ray crystallography to determine complex structures.
• Quantum mechanical calculations to support structural findings.
• Isothermal titration calorimetry (ITC) and UV/vis spectroscopy for thermodynamic stability.
• Stopped-flow spectroscopy for kinetic studies of association and dissociation.
• Variable scan rate cyclic voltammetry (CV) for electrochemical mechanistic details.

Main Results:

• Two trisradical complexes, CBPQT(2(•+))⊂MV(•+) and CBPQT(2(•+))⊂V(•+), were formed.
• X-ray crystallography revealed a centrosymmetric arrangement of MV(•+) within the CBPQT(2(•+)) cavity, forming continuous radical cation stacks.
• Quantum mechanical calculations indicated electronic asymmetry in the SOMO due to radical-pairing interactions, persisting in solution.
• ITC and UV/vis spectroscopy yielded binding constants for CBPQT(2(•+))⊂MV(•+) of (5.0 ± 0.6) × 10^4 M⁻¹ and (7.9 ± 5.5) × 10^4 M⁻¹, respectively.
• Stopped-flow spectroscopy determined association (k_f = (2.1 ± 0.3) × 10^6 M⁻¹ s⁻¹) and dissociation (k_b = 250 ± 50 s⁻¹) rates for CBPQT(2(•+))⊂MV(•+).
• Electrochemical studies revealed a bisradical tetracationic intermediate, with slow dissociation (k_b = 10 ± 2 s⁻¹) for the V(•+) guest.

Conclusions:

• The CBPQT((2+)(•+)) ring exhibits unique recognition capabilities for both BIPY(•+) radical cations and potentially π-electron-rich guests.
• These findings establish a foundation for utilizing radical-radical recognition in mechanically interlocked molecules.
• The study enhances fundamental understanding of BIPY(•+) radical-radical interactions in both solution and solid states.