Solution-Phase Mechanistic Study and Solid-State Structure of a Tris(bipyridinium radical cation) Inclusion Complex

The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT(2(center dot+))) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY center dot+) guests has been investigated mechanistically and quantitatively. Two BIPY center dot+ radical cations, methyl viologen (MV center dot+) and a dibutynyl derivative (V center dot+), were investigated as guests for the CBPQT(2(center dot+)) ring. Both guests form trisradical complexes, namely, CBPQT(2(center dot+))subset of MV center dot+ and CBPQT(2(center dot+))CV(center dot+), respectively. The structural details of the CBPQT(2(center dot+))subset of MV center dot+ complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV center dot+ is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT(2(center dot+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT(2(center dot+))subset of MV center dot+ and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT(2(center dot+))subset of MV center dot+ complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 +/- 0.6) X 10(4) M-1 and (7.9 +/- 5.5) X 10(4) M-1, respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k(f) and k(b) of (2.1 +/- 0.3) x 10(6) M-1 s(-1) and 250 +/- 50 s(-1), respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT((2+)(center dot+))subset of V center dot+ inclusion complex; in the case of the V center dot+ guest, the rate of disassociation (k(b) = 10 +/- 2 s(-1)) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT((2+)(center dot+)) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY center dot+ radical cation and pi-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical-radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY center dot+ radical-radical interactions in solution as well as in the solid-state.