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

By Fahrenbach, Albert C.; Barnes, Jonathan C.; Lanfranchi, Don Antoine; Li, Hao; Coskun, Ali; Gassensmith, Jeremiah J.; Liu, Zhichang; Benítez, Diego; Trabolsi, Ali; Goddard, William A.; Elhabiri, Mourad & Stoddart, J. Fraser
Published in Journal of the American Chemical Society NULL 2012

Abstract

The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT2(•+)) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY•+) guests has been investigated mechanistically and quantitatively. Two BIPY•+ radical cations, methyl viologen (MV•+) and a dibutynyl derivative (V•+), were investigated as guests for the CBPQT2(•+) ring. Both guests form trisradical complexes, namely, CBPQT2(•+)⊂MV•+ and CBPQT2(•+)⊂V•+, respectively. The structural details of the CBPQT2(•+)⊂MV•+ complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV•+ 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 CBPQT2(•+) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT2(•+)⊂MV•+ 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 CBPQT2(•+)⊂MV•+ complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 104 M -1 and (7.9 ± 5.5) × 104 M -1, respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a kf and kb of (2.1 ± 0.3) × 106 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+)(•+)⊂V•+ inclusion complex; in the case of the V•+ guest, the rate of disassociation (kb = 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+)(•+) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY•+ radical cation and π-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•+ radical -radical interactions in solution as well as in the solid-state.

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