Fahrenbach, Albert C. and Barnes, Jonathan C. and Lanfranchi, Don Antoine and Li, Hao and Coskun, Ali and Gassensmith, Jeremiah J. and Liu, Zhichang and Benítez, Diego and Trabolsi, Ali and Goddard, William A., III and Elhabiri, Mourad and Stoddart, J. Fraser (2012) Solution-Phase Mechanistic Study and Solid-State Structure of a Tris(bipyridinium radical cation) Inclusion Complex. Journal of the American Chemical Society, 134 (6). pp. 3061-3072. ISSN 0002-7863 http://resolver.caltech.edu/CaltechAUTHORS:20120418-075913493
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The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT^(2(•+))) 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 CBPQT^(2(•+)) ring. Both guests form trisradical complexes, namely, CBPQT^(2(•+))⊂MV^(•+) and CBPQT^(2(•+))⊂V^(•+), respectively. The structural details of the CBPQT^(2(•+))⊂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 CBPQT^(2(•+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT^(2(•+))⊂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 CBPQT^(2(•+))⊂MV^(•+) complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 10^4 M^(–1) and (7.9 ± 5.5) × 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) × 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+)(•+))⊂V^(•+) inclusion complex; in the case of the V^(•+) 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+)(•+)) 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.
|Additional Information:||© 2011 American Chemical Society. Received: September 23, 2011. Publication Date (Web): December 9, 2011. We (A.C.F., J.C.B., H.L., A.C., J.J.G., Z.L., and J.F.S.) at Northwestern University acknowledge support from the Air Force Office of Scientific Research (AFSOR) under the Multidisciplinary Research Program of the University Research Initiative (MURI) Award FA9550-07-1-0534 on “Bioinspired Supramolecular Enzymatic Systems” and the National Science Foundation (NSF) under the auspices of Award CHE-0924620. We (W.A.G and J.F.S.) also acknowledge support by the Microelectronics Advanced Research Corporation (MARCO) and its Focus Center Research Program (FCRP) on Functional Engineered NanoArchitectonics (FENA) as well as support from the Non-Equilibrium Energy Research Center (NERC), which is an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Basic Sciences (DOE-BES), under Award DE-SC0000989. W.A.G., J.F.S., A.C.F., and J.C.B. were supported by the WCU Program (NRF R-31-2008-000-10055-0) funded by the Ministry of Education, Science and Technology, Korea. A.C.F. acknowledges support from an NSF Graduate Research Fellowship. This work was also supported (M.E. and D.A.L.) by the Centre National de la Recherche Scientifique (CNRS) and the University of Strasbourg (UMR 7509 CNRS-UdS) in France.|
|Official Citation:||Solution-Phase Mechanistic Study and Solid-State Structure of a Tris(bipyridinium radical cation) Inclusion Complex Albert C. Fahrenbach, Jonathan C. Barnes, Don Antoine Lanfranchi, Hao Li, Ali Coskun, Jeremiah J. Gassensmith, Zhichang Liu, Diego Benítez, Ali Trabolsi, William A. Goddard, III, Mourad Elhabiri, and J. Fraser Stoddart Journal of the American Chemical Society 2012 134 (6), 3061-3072|
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|Deposited By:||Ruth Sustaita|
|Deposited On:||18 Apr 2012 16:25|
|Last Modified:||26 Dec 2012 15:04|
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