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

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 k f and k b 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 (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.

Mechanostereochemistry and the mechanical bond

“…The recently solved solid-state structure (figure 13b) of the tris-radical complex 61 formed between CBPQT 2+· and MV +· reveals significant radical-radical interactions (Fahrenbach et al 2012). (Fahrenbach et al 2012). This new departure for CBPQT 4+ -well used as a host for electron-rich aromatic guests for over 20 years-as a radical recognition unit will no doubt lead to many further MIMs endowed with novel switching properties.…”

“…Combining this source of radical templation with light-induced oxidation and copper-free 1,3-dipolar azide-alkyne cycloaddition, [2]rotaxanes such as 60 (figure 13a) can be synthesized (Li et al 2010a), which show, upon aerobic oxidation to their non-radical ground states, little or no recognition between the thread and ring components. The recently solved solid-state structure (figure 13b) of the tris-radical complex 61 formed between CBPQT 2+· and MV +· reveals significant radical-radical interactions (Fahrenbach et al 2012). (Fahrenbach et al 2012).…”

Donor-acceptor rotaxanes with tetracationic cyclophane ring

“…In investigations of rotaxanes [90][91][92][93][94][95] a growing attention is paid recently to radical-radical rotaxanes; 13,14,16,24,[96][97][98][99][100] one may hope that this area of radical-pairing interaction will become a topic of intense study. Owing to the constantly increasing amount of work concerning rotaxane chemistry, it would be impossible to cover all published reports; the present review is only a fragment of a wide research.…”

“…15,16 Two kinds of rotaxanes containing tetracationic cyclophane rings exist, namely the donoracceptor rotaxanes described in this paper, and a new kind, i.e. radical-radical rotaxanes 13,14 which are not discussed here.…”

“…10 In view of the presence of pyridinium quaternary groups in their structure, rotaxanes may be divided into those incorporating pyridinium units in the thread 11,12 and those with a tetracationic cyclophane ring. 13,14 The title rotaxanes containing a tetracationic cyclophane ring are a large, rapidly developing class of the rotaxane family, and are important species useful in many fields such as construction of molecular switches and machines. One should point out the enormous contribution of the J.Fraser Stoddart School to this fascinating area of supramolecular chemistry.…”