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Solution-Phase MechanisticStudy and Solid-State Structureof a Tris(bipyridinium radical cation) Inclusion Complex

Fahrenbach, AlbertC.,Barnes, Jonathan C.,Lanfranchi, Don Antoine,Li, Hao,Coskun, Ali,Gassensmith, Jeremiah J.,Liu, Zhichang,Bení,tez, Diego,Trabolsi, Ali,Goddard, William A.,Elhabiri, Mourad,Sto American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.6
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The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT2(•+)) ring to forminclusion complexes with 1,1′-dialkyl-4,4′-bipyridiniumradical cationic (BIPY•+) guests has been investigatedmechanistically and quantitatively. Two BIPY•+ radicalcations, methyl viologen (MV•+) and a dibutynylderivative (V•+), were investigated as guests forthe CBPQT2(•+) ring. Both guests form trisradicalcomplexes, namely, CBPQT2(•+)⊂MV•+ and CBPQT2(•+)⊂V•+, respectively.The structural details of the CBPQT2(•+)⊂MV•+ complex, which were ascertained by single-crystalX-ray crystallography, reveal that MV•+ is locatedinside the cavity of the ring in a centrosymmetric fashion: the 1:1complexes pack in continuous radical cation stacks. A similar solid-statepacking was observed in the case of CBPQT2(•+) byitself. Quantum mechanical calculations agree well with the superstructurerevealed by X-ray crystallography for CBPQT2(•+)⊂MV•+ and further suggest an electronicasymmetry in the SOMO caused by radical-pairing interactions. Theelectronic asymmetry is maintained in solution. The thermodynamicstability 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 kineticsof 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 scanrate cyclic voltammetry (CV), and the experimental data were compareddigitally with simulated data, modeled on the proposed mechanism usingthe thermodynamic and kinetic parameters obtained from ITC, UV/vis,and stopped-flow spectroscopy. In particular, the electrochemicalmechanism of association/dissociation involves a bisradical tetracationicintermediate 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 detectedand quantified by variable scan rate CV. All the experimental observationslead to the speculation that the CBPQT(2+)(•+) ringof the bisradical tetracation complex might possess the unique propertyof being able to recognize both BIPY•+ radical cationand π-electron-rich guests simultaneously. The findings reportedherein lay the foundation for future studies where this radical–radicalrecognition motif is harnessed particularly in the context of mechanicallyinterlocked molecules and increases our fundamental understandingof BIPY•+ radical–radical interactions insolution as well as in the solid-state.Graphic Abstract <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-6/ja2089603/production/images/medium/ja-2011-089603_0004.gif'><A href='http://pubs.acs.org/doi/suppl/10.1021/ja2089603'>ACS Electronic Supporting Info</A>

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