A Radically Configurable Six-State Compound

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Science  25 Jan 2013:
Vol. 339, Issue 6118, pp. 429-433
DOI: 10.1126/science.1228429


Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

Typically, organic radicals are unstable species with fleeting lifetimes that make their isolation and characterization a demanding task. The few that are stable, from the triphenylmethyl radical discovered (1) by Gomberg in 1900 to the others isolated (28) in the intervening years, generally have structures that raise kinetic barriers (9), thus helping to curtail the rates of intermolecular dimerization and/or oxidation of the open-shell centers. Not long after Gomberg's discovery, Michaelis described (10, 11) the redox properties of N,N′-dialkyl-4,4′-bipyridinium (BIPY)–based compounds—also known as viologens (11, 12)—and their ability to form radical cations (BIPY•+) upon a one-electron reduction. The inter- and intramolecular behavior of these radical cationic BIPY•+ compounds was investigated further more than 40 years ago by Kosower (12, 13) and Hünig (14), who drew attention to their ability to dimerize in solution—and in so doing form diamagnetic (closed shell) species—a situation they referred to frequently as pimerization. In more recent times, this spin-pairing of radical cation dimers has been stabilized in the supramolecular settings of host-guest systems. For example, Kim (15) has demonstrated the enhanced stabilization of N,N′-dimethyl-4,4′-bipyridinium radical cations—or, more commonly, methyl viologen (MV•+) radical cations—inside the cavity of cucurbit[8]uril. Likewise, we have observed (16) the inclusion of MV•+ inside the cavity of the diradical dication cyclobis(paraquat-p-phenylene) (17) [CBPQT2(•+)] ring to form a tris(bipyridinium radical cation) inclusion complex.

In the context of mechanically interlocked molecules, and hard on the heels of discovering (18, 19) how both the mixed-valence and radical-cation dimers of tetrathiafulvalene can be stabilized inside the central rings of [3]catenanes, we have reported recently (20) on the phenomenon of BIPY-based radical-radical molecular recognition in a tristable [2]rotaxane and subsequently on the templation (21) of a [2]rotaxane where the BIPY unit of the respective dumbbell components function as a recognition unit for the CBPQT2(•+) ring. In all of these BIPY-based examples, the stability of the radical cationic state is low and the lifetimes of these radical viologen derivatives is short as a result of oxidation followed closely by decomplexation, as in the case of the [2]pseudorotaxane, or translational motion of the ring in the interlocked system to alleviate any strain that occurs as a consequence of Coulombic repulsion. In the investigation described here, a class of stable organic radicals is introduced as a readily obtainable homo[2]catenane (HC), wherein no relative movements involving the two mechanically interlocked rings occur at room temperature, regardless of the redox state, thus maintaining the ideal geometry for electron transfer in the face of large Coulombic forces. Consequently, the radical trapped within the heptacationic HC is so stable that the compound can withstand being purged with air for several weeks and still retain its radical character. The stability of the open-shell electronic structure is achieved by the close stacking of the BIPY units that is enforced by the mechanical bond formed between the two electron-deficient CBPQT rings immobilized with respect to one another. Indeed, the monoradical heptacation resists complete oxidation to the highly energetic 8+ redox state under ambient conditions as a result of the increased Coulombic repulsions concentrated within such a confined space of just over 1 cubic nanometer.

A recent addition (21) to the protocols employed in the template-directed syntheses of mechanically interlocked molecules involves taking advantage of favorable radical-radical interactions that exist between BIPY radical cation dimers. The extent of dimerization is easily controlled by enforcing an N2 atmosphere and also by moderating the concentrations of the species, typically in CH3CN solution when dealing with the PF6 salt. Beginning with the pairing of two dibromide precursors DB•+, after DB2+ has been reduced in the presence of an excess of Zn dust to give the (DB•+)2 dimer, we have employed (Fig. 1A) this template-directing strategy in the preparation of a homocatenane. Mechanical bond formation is envisaged to begin with the addition of one equivalent of 4,4′-bipyridine to the dimer, closing one ring, and thus producing the intermediate DB•+⊂CBPQT2(•+) as an inclusion complex. Addition of a second equivalent of 4,4′-bipyridine to this complex generates the HC, presumably as the tetraradical tetracation HC4(•+), which is the "as synthesized" purple product before the crude reaction mixture is removed from the N2 atmosphere.

Fig. 1

(A) The key posited steps in the synthetic route to HC4(•+), and ultimately HC8+, involve the reduction (Step 1) of a bisbromomethyl(bis-p-benzylbipyridinium) dication, DB2+ to DB•+, which spontaneously forms the (DB•+)2 dimer before its reaction (Step 2) with 4,4′-bipyridine to afford HC4(•+) through the 1:1 inclusion complex DB•+⊂CBPQT2(•+) as a templating intermediate. Addition of an aqueous NH4PF6 solution converts all of the counterions to PF6 ions before exposure of the reaction mixture to air, whereupon HC4(•+) is oxidized to HC•7+/HC2•6+, which then affords HC8+ upon treatment with tris(4-bromophenyl)aminium hexachloroantimonate (TBPA•SbCl6). (B) The 1H NMR spectrum (600 MHz) of HC8+, recorded in CD3CN at 298 K, reveals resonances for the α, β, HP and HCH2 protons associated with the outside BIPY2+ units in addition to resonances for the α, β, HP and HCH2 protons associated with the inside BIPY2+ units. The inset on the right shows the solid-state structure of HC•7+. The three large peaks resonating at ~6 parts per million (ppm) are attributed to NH4+ protons.

Upon exposure to air, HC4(•+) is oxidized and comes to rest as an HC•7+/HC2•6+ equilibrium mixture, stopping just short of reaching the fully oxidized HC8+. The purple solid was purified by preparative reverse-phase high pressure liquid chromatography (RP-HPLC) and its purity confirmed by analytical HPLC. When HC•7+/HC2•6+ was examined by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), peaks were observed for [M – PF6]+ from HC•7+ and [M – 2PF6]2+ from HC•7+ and HC2•6+. The resonances in the 1H nuclear magnetic resonance (NMR) spectrum of HC8+ (Fig. 1B) were obtained by treating HC•7+/HC2•6+ with an excess of tris(4-bromophenyl)aminium hexachloroantimonate (TBPA•SbCl6) (22) and the assignments were confirmed by two-dimensional 1H-1H correlated spectroscopy experiments. For more details of the HPLC (fig. S1), MS (fig. S17), and NMR experiments (fig. S11), see the supplementary materials.

The characterizations (Fig. 2) of the redox states of the HC from electrochemical experiments and electron paramagnetic resonance (EPR) spectroscopy go hand in hand. Figure 2A shows the plot of the current versus potential, which was obtained by conducting square-wave differential pulse voltammetry (DPV) experiments on the HC in a 1-mM dimethylformamide (DMF) solution [0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6); Ag/AgCl used as reference]. In total, there are five reversible redox processes and six redox states that are accessible (electro)chemically, whereas the remaining three redox states (HC•+, HC3•3+, and HC3•5+) have yet to be observed experimentally. Cyclic voltammetry (CV), which was conducted (fig. S18) at 200 mVs−1, confirmed that all of the redox processes are fully reversible. The electrochemical data highlight the small separation of the 6+ and 7+ reduction potentials in both the CV and DPV experiments, an observation that is consistent with their appearance in the mass spectrometric (fig. S17) and spectroelectrochemical (SEC) (fig. S10) data. The continuous wave (CW) EPR spectra (Fig. 2B) of the HC in DMF (1 mM, 0.1 M TBAPF6; Ag used as pseudoreference) are shown at different potentials during the electrolysis of a 1-mL sample. The lack of hyperfine splitting is indicative of an intramolecular spin exchange mechanism between BIPY•+ units (20) in the HC. The EPR trace when no potential was applied confirms that the starting state of the HC largely comprises the paramagnetic HC•7+. Electrolysis was performed by holding the displayed potentials for 30 min, beginning with –100 mV. Conversion to HC4(•+), which was achieved by applying a potential of –300 mV, confirms that it is virtually "EPR silent," indicating that the four radicals are spin-paired and the compound in this redox state is diamagnetic. Shifting the potential to +100 mV to obtain HC2•6+ resulted in little or no signal in the EPR trace, supporting the notion that the bisradical is also spin-paired. Moving the potential to +200 mV and holding it for 30 min led, however, to a dramatic increase in the intensity of the EPR signal as a result of the conversion to the paramagnetic HC•7+.

Fig. 2

(A) The six redox states of the HC identified by square-wave DPV. (B) EPR analysis was performed on a 1-mM sample of the HC, beginning with the starting state (top), where no potential was applied. Bulk electrolysis was then conducted on the sample using the displayed potentials (from top to bottom), and an aliquot was removed for EPR analysis immediately after each electrolysis step. The small signals observed through the sequence of reductions can be attributed to incomplete spin-pairing, deficient bulk electrolysis, or both. (C) A graphical representation of the redox switching mechanism between the paramagnetic and diamagnetic states. The paramagnetic HC•7+, corresponding to the "on" state, can be reduced (electro)chemically by adding either one or three electrons to form HC2•6+ or HC4(•+), respectively. Both of these states—considered as "off" states—are diamagnetic.

The difference in signal intensities between the initial and +200 mV EPR traces supports the notion that an equimolar mixture of HC2•6+ and HC•7+ exists in equilibrium in the starting state. This observation is, of necessity, a qualitative one, since it would be inappropriate to try to quantify the spin concentration on account of the varying degrees of spin exchange between the BIPY+ units for the different redox states of HC, not to mention the difficulty in generating exclusively, by electrochemical means, the monoradical state in solution. Increasing the potential in the positive direction to +400 mV resulted in the loss of signal in the EPR spectrum, indicating that HC•7+/HC2•6+ has been oxidized to HC8+. Hence, in the bisradical state, the two electrons are spin-paired and exist as a (BIPY•+)2 radical cation dimer. A solid-state CW EPR spectrum was also obtained (fig. S12) using single crystals of HC•7+. The result was close to identical with that obtained in solution for the monoradical, an observation that confirms that the paramagnetism persists in the solid state. The paramagnetic character of the monoradical crystalline material (HC•7+) was verified using superconducting quantum interference device magnetometry, wherein the magnetic susceptibility displayed Curie-like paramagnetic behavior (figs. S13 to S16). The spectro- and electrochemical data, as well as the magnetization results, provide a mechanistic basis for switching (Fig. 2C) between a paramagnetic (HC•7+) "on" state and diamagnetic (HC4(•+) and HC2•6+) "off" states.

Characterizing the location and delocalization of the radical electrons in the different redox states of the HC is of paramount importance in understanding the relationship between the (super)structure and function. Single-crystal x-ray diffraction analyses (Fig. 3) have been performed on the PF6 salts of (i) HC•7+ (Fig. 3A), (ii) HC2•6+ (Fig. 3D), and (iii) HC4(•+) (Fig. 3G), as well as (iv) the intermediate DB•+⊂CBPQT2(•+) (figs. S8 and S9). The overall charge balances in (i), (ii), (iii), and (iv) were established by the presence of seven, six, four, and three PF6 ions, respectively. In (i) to (iii), the plane-to-plane distances between each pair of stacked BIPY2+/•+ units are highlighted in Fig. 3, A, D, and G. Analysis of the intermediate DB•+⊂CBPQT2(•+) reveals that the three BIPY•+ units are evenly spaced at 3.40(1) Å apart and the BIPY•+ of DB•+ is centered within the cavity of the CBPQT2(•+) ring. The dihedral angles between the pyridinium rings, which are given below each BIPY•+ unit in fig. S8, are sufficiently low to deem the rings coplanar. This flattening effect (23, 24) is characteristic of BIPY•+ radical cations. This method of analysis serves as a means of identifying which BIPY units are participating in delocalization of the radical electrons. By applying this approach to the solid-state structures of HC•7+ (Fig. 3A), HC2•6+ (Fig. 3D), and HC4(•+) (Fig. 3G), it is possible to identify the location of electronic delocalization. In HC•7+ and HC2•6+, the inner BIPY2(•+)/•3+ plane-to-plane separation is 3.60(1) Å and the associated dihedral angles are small and similar within HC2•6+ [8.8(1)° and 9.1(1)°] and HC•7+ [1.5(1)° and 0.7(1)°]. These observations suggest that radical electrons are delocalized across the two inner BIPY units, resulting in a spin-paired (BIPY•+)2 radical cation dimer for HC2•6+ and a single unpaired spin (BIPY2)•3+ mixed-valence dimer for HC•7+, whereas the outer dicationic BIPY2+ units possess large dihedral angles (18° to 25°) and are participating very little, if at all, in radical delocalization. By contrast, although the solid-state structural analysis of HC4(•+) reveals (Fig. 3G) a similar plane-to-plane distance of 3.60(1) Å between the inside BIPY•+ units, the distances separating the inside and outside BIPY•+ units decrease to 3.20(1) Å—down from a range of 3.80(1) Å to 4.40(1) Å in the case of HC•7+ and HC2•6+—causing the width of the CBPQT2(•+) rings themselves to decrease in HC4(•+). Moreover, all of the BIPY•+ units are flattened in HC4(•+), with their torsional angles all under 5°, an observation that is indicative of tetraradical formation and the complete delocalization of the four radicals between the inside and outside BIPY•+ units. Additionally, although the superstructures (Fig. 3, B and C) of HC•7+ and HC2•6+ (Fig. 3, E and F) demonstrate a total lack of intermolecular BIPY•+∙∙∙BIPY•+ interactions—leading to the formation of small cubic crystals of HC•7+ (Fig. 3J) and HC2•6+ (Fig. 3K) as observed by scanning electron microscopy (SEM)—in the case of the superstructure (Fig. 3, H and I) of HC4(•+), the individual tetraradicals pack by dint of intermolecular BIPY•+∙∙∙BIPY•+ interactions, resulting in the formation of rectangular needles (Fig. 3L) upon crystallization of HC4(•+).

Fig. 3

Stick diagrams and space-filling representations of the x-ray crystal (super)structures of HC•7+ (A to C), HC2•6+ (D to F), and HC4(•+) (G to I), as well as SEM images (J to L) of the microscopic crystals that correspond to each redox state. The annotated plane-to-plane distances are those between adjacent BIPY2•+/•+ units. The dihedral angles between the pyridinium rings in the BIPY2+/•+ units are listed below the stick diagrams [(A), (D), and (G)]. The space-filling representations indicate the packing arrangements of HC•7+, HC2•6+, and HC4(•+). The PF6 counterions were removed for clarity.

The ability to control chemically the redox states, and consequently the morphologies of the crystals, augurs well for the use of the HC in device settings—for example, as semiconducting materials in organic field-effect transistors, wherein the individual HC4(•+) tetraradicals self-assemble on the surface of an SiO2 wafer prepatterned with electrodes. Moreover, these geometrical observations, along with EPR and ultraviolet/visible/near-infrared spectroscopic data (fig. S10), suggest that there are varying degrees of electronic coupling between the inside BIPY units in HC2•6+ and HC•7+. This type of through-space mixed-valence behavior has been observed and quantified (2528) in other organic-based systems. Further investigations into the potential for intramolecular electron transfer are currently under way in order to assign each mixed-valence species quantitatively according to Robin-Day classification (29).

Theoretical calculations were performed on the HC using density functional theory (DFT) (30) (M06 functional with the 6-311++G** basis set and using Poisson Boltzmann Finite continuum solvation). In the valence bond (VB) description, the tetraradical HC4(+) has a singly occupied VB orbital (SOVBO) on each of the four BIPY•+ units that combine to form two paired VBs, A-B and C-D, where the four BIPY•+ units are labeled A through D as shown in Fig. 3, D and G. With DFT, the four SOVBOs are combined to form two doubly occupied molecular orbitals (MOs), of which the highest occupied (HOMO) is shown in Fig. 4A, and two empty MOs, of which the lowest unoccuped (LUMO) is shown in Fig. 4A. The delocalization of the HOMO between A and B and between C and D indicates the bonding. All of the other eight states outside of HC4(+), ranging from HC0 to HC8+, can be visualized as adding or subtracting electrons from these four MOs. For HC2•6+, the electrons are removed from the SOVBO on A and D, making them doubly charged (stabilized by solvation) so that the doubly occupied HOMO localizes on B and C as in Fig. 4A, with the LUMO on A and D. Removing one more electron to form HC•7+ leads to the singly occupied MO (SOMO) in Fig. 4A, again on B and C, which becomes the LUMO for HC8+. Additional orbitals are shown in fig. S23.

Fig. 4

(A) Calculated LUMOs and HOMOs for HC4(•+), HC2•6+, and HC8+, along with the SOMO for HC•7+, reveal similarities in the orbital overlaps for the HOMO of the bisradical hexacation HC2•6+ and the SOMO of the monoradical heptacation HC•7+. In both cases, there are orbital overlaps involving the two inside BIPY•+ units, supporting the notion of delocalization of the radical electrons across these two units. Although the orbital interactions for the HOMO of HC4(•+) also indicate electronic delocalization over two BIPY•+ units, they occur across both pairs of inside and outside BIPY•+ units. (B and C) Histograms summarizing the calculated binding energies (kcal mol−1) between each ring at all of the (electro)chemically accessible redox states of the homo[2]catenane in CH3CN (B) and in the gas phase (C).

The theoretical energies (internal energies comparable to enthalpies reported in kcal mol–1) for the process of formation of the uncatenated rings into the (electro)chemically accessible states of the HC were calculated in CH3CN solution (Fig. 4B) and in the gas phase (Fig. 4C). The results of these calculations indicate that the 8+ redox state is the most energetically unfavorable state, with an energy of 67 kcal mol−1 in solution and 619 kcal mol−1 in the gas phase, whereas the 7+ redox state has an energy of 17 kcal mol−1 in solution and 427 kcal mol−1 in the gas phase. These large differences in energy provide some explanation as to why the molecule resists complete oxidation to HC8+, but instead stops at the thermodynamic mixture HC•7+/HC2•6+ under ambient conditions. It is important to note that the 4+ oxidation state is the most thermodynamically stable with an energy of –127 kcal mol−1 in CH3CN, an observation that is consistent with the formation of HC4(•+) in the proposed mechanism for the synthesis of the HC. Further discussion of the binding energies (table S1) for all of the redox states as well as an accurate prediction of the redox potentials (table S2) can be found in the supplementary materials.

The ease with which this radically configurable homo[2]catenane can be made and its solid-state morphologies controlled, combined with the fact that its monoradical state is stable under ambient conditions, gives this compound great potential for incorporation into organic radical frameworks (31, 32), electronic memory devices (3335), semiconductors (36), and energy storage devices (37).

Supplementary Materials

Materials and Methods

Supplementary Text

Schemes S1 and S2

Figs. S1 to S18

Tables S1 to S3

References (3843)

References and Notes

  1. Acknowledgments: The data reported in this paper are tabulated in the supplementary materials, and the crystallographic parameters of each single crystal were deposited into the Cambridge Crystallographic Data Centre, where they are freely available under reference numbers 855030, 855031, 855032, and 889233. We thank S. Shafaie for his expertise and assistance with high-resolution mass spectrometry, and the Integrated Molecular Structure Education and Research Center at Northwestern University for providing access to equipment for the relevant experiments. Molecular crystal images were produced using the UCSF Chimera package from the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco. The authors acknowledge our joint collaborators Turki S. Al-Saud and M. B. Alfageeh from the King Abdulaziz City of Science and Technology in Saudi Arabia. J.F.S. is supported by the Non-Equilibrium Energy Research Center, which is an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Basic Energy Sciences (DOE-BES) under award DESC0000989. R.C. is supported by the Argonne-Northwestern Solar Energy Research Center, which is an EFRC funded by DOE-BES under award DE-SC0001059. M.R.W. and S.M.D. are supported by the U.S. National Science Foundation under grant CHE-1012378. J.C.B. was supported by a National Defense Science and Engineering Graduate Fellowship from the U.S. Department of Defense (DOD) and gratefully acknowledges support from the Ryan Fellowship (as does D.C.) awarded under the auspices of the Northwestern University International Institute for Nanotechnology, as well as the DOD award W911NF-10-1-0510 and the DOE-BES under award DE-SC0005462. A.C.F. and D.C. are supported by a Graduate Research Fellowship from the National Science Foundation. M.A.G. was supported by a Summer Undergraduate Research Fellowship from the American Chemical Society. J.F.S., D.B., E.T., and W.A.G. are funded through the Focus Center Research Program Center on Functional Engineered Nano Architectonics. J.F.S. and A.M.Z.S. were supported under the auspices of an international collaboration supported in the United States by the NSF under grant CHE-0924620 and in the United Kingdom by the Engineering and Physical Sciences Research Council under grant EP/H003517/1. J.C.B., A.C.F., D.C., W.A.G., and J.F.S. are also supported by the World Class University program (R-31-2008-000-10055-0) funded by the Ministry of Education, Science and Technology, Republic of Korea.
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