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Self-Assembly of Charged Supramolecular Sandwiches Formed by Corannulene Tetraanions and Lithium Cations Alexander V. Zabula, Sarah N. Spisak, Alexander S. Filatov, and Marina A. Petrukhina* Department of Chemistry, University at Albany, State University of New York, 1400 Washington Avenue, Albany, New York 12222-0100, United States S Supporting Information *

ABSTRACT: The reduction of corannulene (C20H10) with excess lithium metal in a strong chelating O-donor solvent, diglyme, leads to the formation of the highly reduced C20H104− anion. However, in contrast to the formation of the sandwich-type supramolecular aggregate [Li5(C20H104−)2]3− observed in THF, corannulene tetraanions and lithium counterions in diglyme form only contact ion pairs according to 7Li NMR spectroscopy. Furthermore, the slow dissociation of the premade sandwich [Li5(C20H104−)2]3− in neat diglyme has been demonstrated by multinuclear NMR spectroscopy. In contrast, the [Li5(C20H104−)2]3− sandwich can be crystallized from the THF/diglyme mixture as the new crystalline product [Li(THF)2(diglyme)]+[Li2(THF)(diglyme)//Li5(C20H104−)2]−, showing a complex 1D hybrid architecture according to the single-crystal X-ray diffraction study.

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INTRODUCTION Nonplanar polycyclic hydrocarbons that map onto the surface of fullerenes and nanotubes (buckybowls) are the subject of intense research owing to their intriguing physical and chemical properties.1 Buckybowls demonstrate an enhanced capacity for acquisition of multiple electrons compared to planar polyarenes and even fullerenes.2 Upon reduction of buckybowls with alkali metals, the resulting highly charged species are capable of forming remarkable supramolecular aggregates with metal counterions.3 Such assemblies can represent the structural motifs of charged anode materials in prospective rechargeable batteries fabricated from carbon allotropes with curved aromatic frameworks.4 Previously, the study of aggregation of bowl-shaped carbanions with alkali metal ions was limited to in situ multinuclear NMR spectroscopy owing to their exceptional sensitivity toward air and moisture.3,5 Recently, we developed successful procedures to access solid crystalline products based on multicharged buckybowls.4,6 This allowed us to revise the long-standing model5 for aggregation of tetraanions of bowlshaped corannulene (C20H10, Scheme 1) with lithium ions using a synergistic combination of single-crystal X-ray diffraction and 7Li NMR investigations.4 The formation of the fascinating supramolecular [Li5(C20H104−)2]3− sandwich, in which five Li+ ions are jammed between two tetrareduced corannulene decks (Scheme 1), has been revealed. However, it remained unclear whether the preparation of the above supramolecular dimer can be accomplished in coordinating solvents other than THF. The variation of donor solvents is known to serve as a useful tool for tuning the metal coordination modes and supramolecular networks in the case of planar polyaromatic anions with alkali metal ions.7 In contrast, the effect of solvents on the self-assembly processes of bowl-shaped carbanions has not yet been investigated. © 2012 American Chemical Society

Therefore, in this work we set up a study of products formed by the highly reduced corannulene in the presence of a strong chelating O-donor such as diglyme.

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RESULTS AND DISCUSSION Upon reduction of C20H10 with lithium metal in diglyme, at least three distinctive reduction steps can be recognized. The formation of a green monoanion is followed by a purple dianion and, finally, by the appearance of a brown corannulene tetraanion. These color changes closely follow the transformations observed during stepwise lithium reduction of C20H10 in THF.3b,8 However, in contrast to a rather fast (up to 12 h) reduction in THF, the fully reduced C20H104− anions are formed in diglyme only after prolonged reaction times (up to 4 days). The reaction progress can be monitored by UV−vis spectroscopy, as the tetrareduced corannulene exhibits absorption maxima at 460, 605, and 714 nm in diglyme. These values are bathochromically shifted compared to the absorption maxima of C20H104−, generated in THF (429, 575, and 710 nm).8 Such differences in optical parameters may indicate different aggregation patterns of the highly charged C20H104− species in diglyme and THF, which can be related to the strong chelating properties of the former. Notably, the 7Li NMR spectrum of the mixture of C20H10 with an excess of lithium metal (≈ 10 equiv) in diglyme (Figure 1, blue line, −40 °C) does not show resonances of the sandwiched lithium ions (δ = −10.0 to −16.0 ppm). Instead, the resonance signals are observed in the range characteristic for contact ion pairs (δ = −6.0 to −9.0 ppm) and solventseparated Li+ ions (δ = 1.5 ppm).9 This observation indicates Received: June 7, 2012 Published: July 18, 2012 5541

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Scheme 1. Generation of Corannulene Tetraanions and Their Aggregation

which is also consistent with the presence of different contact ion pairs in solution. To investigate the influence of a strong donor solvent on the sandwich stability, a solution of the premade [Li5(C20H104−)2]3− dimer was monitored by 7Li NMR spectroscopy. The probe was prepared by dissolving the crystals of the freshly prepared [Li(THF)4]+[{Li(THF)2}// Li5(C20H104−)2//{Li(THF)3}]− salt (1)4 in diglyme. The 7Li NMR spectrum, measured at −40 °C immediately after the preparation, shows a sharp resonance signal at δ = −11.7 ppm (Figure 1, black lines). This value corresponds well to that previously detected for the solution of [Li5(C20H104−)2]3− in THF (δ = −11.7 ppm).4,5 Upon prolonged standing of this NMR probe at 25 °C, the intensity of this resonance for sandwiched lithium ions fades, whereas the signals of contact ion pairs in the range from δ = −6.5 to −9.0 ppm increase in intensity. This NMR investigation demonstrates that the preformed [Li5(C20H104−)2]3− sandwich completely disappears in diglyme solution in three days. Although the inability of C20H104− to form a supramolecular dimer with Li+ ions in neat diglyme was proven in this work, the [Li5(C20H104−)2]3− sandwich can be crystallized in the presence of diglyme by layering a THF solution with a hexanes/diglyme (v/v 10:1) mixture under strictly anaerobic conditions. Under these conditions, a new product of overall formula [Li(THF) 2 (diglyme)] + [Li 2 (THF)(diglyme)// Li5(C20H104−)2]− (2) is formed, as confirmed by single-crystal X - r ay d i ff r a c t i o n ( F i g u r e 2 a ) . I m p o r t a n t l y , t h e [Li5(C20H104−)2]3− dimer core is preserved in 2. Furthermore, these dimers form a complex 1D assembly through exterior binding to the [Li2(THF)(diglyme)]2+ cations serving as connecting building blocks. Notably, incorporation of

Figure 1. 7Li NMR spectra (diglyme/THF-d8 = 40:1, v/v, −40 °C) of the reaction mixture of corannulene with lithium (blue) and the solution of salt 1 (black).

that the presence of large amounts of diglyme prevents the sandwiching of lithium ions between the tetrareduced corannulene decks. The presence of several 7Li NMR signals in the range δ = −6.0 to −9.0 ppm shows that Li+ ions bind to C20H104− in different fashions. The remarkable coordination flexibility of corannulene anions toward various alkali metal ions has been recently revealed by us.6 The 1H NMR spectrum (−40 °C) of the same system demonstrates several proton resonance signals for C20H104− in the range 6.58−7.47 ppm,

Figure 2. Molecular structure (a), space-filling model of a 1D polymer (b), and top and side views of the supramolecular sandwich (c and d) for 2. 5542

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significant flattening of its carbon framework: the bowl depth is reduced to 0.214(3) and 0.404(3) Å in 2 compared to neutral corannulene [0.88 Å]12 and its mono- and dianions [0.85 and 0.79 Å, respectively].6 A substantial reduction of bowl depth of C20H104− has been seen earlier (Table 1),4 although the difference between the two crystallographically independent corannulene bowls was notably smaller (Δ = 0.19 Å in III vs 0.05 and 0.09 Å in I and II, respectively). This shows that reduction of the hydrocarbon core depth lessens the geometric strain and makes the curved surface more adaptable to external conditions, allowing it to vary more widely depending on coordination environment and other crystal-packing effects. The hub C−C bond lengths of C20H104− [1.396(3)− 1.400(3) Å] in 2 are slightly shorter than in C 20 H 10 0 [1.411(2)−1.417(2) Å]. The equalization of rim and flank C−C bond distances (Table 1) and shrinking of the central five-membered ring in C20H104− illustrate the formation of an annulene-within-an-annulene structure, proposed by Scott et al. (Scheme 1).13 Crystals of 2 were redissolved in THF-d8 to show that the chemical shifts of the sandwich, detected in 1H and 13C NMR spectra, are very close to those reported previously.13 The resonances of coordinated diglyme molecules appear as three sharp signals at room temperature in the 1H NMR spectrum of 2. Cooling the above solution results in the significant broadening of diglyme proton resonances, whereas the signal of C20H104− remains almost unchanged in the range +20 to −80 °C (Figure S1). This observation may indicate the association of [Li(diglyme)n(THF)m]+ cations with the sandwich through C−H(diglyme)···π contacts at low temperatures (loose ion pair). The variable-temperature 7Li NMR study for 2 shows one broad lithium resonance at δ = −8.4 ppm at 20 °C. Upon cooling below −30 °C, this signal is split into two resonances at −3.8 and −11.7 ppm, corresponding to the exterior and sandwiched lithium ions, respectively. Such temperaturedependent behavior and δ(7Li) chemical shifts are close to the previously reported data for the in situ generated sandwich5 and for salt 1.4 The slightly different value of chemical shifts for exterior Li+ ions in a solution of 2 (δ = −3.8 ppm) compared to 1 (δ = −4.3 ppm) is attributed to the coordination of diglyme molecules to lithium ions. Importantly, the intensity ratio of resonance signals for exterior and interior Li+ ions of 3:5 is in full agreement with the aggregation model previously proposed by us.4

[Li2(solvent)x]2+ moieties between two concave surfaces of πbowls has not been observed.10b The resulting supramolecular chains propagate in the crystallographic a direction and have additional solvent-covered [Li(THF)2(diglyme)]+ cations on one side (Figure 2b). Two exterior lithium ions are coordinated to the sandwich in an η 3 -fashion. The corresponding Li···C separations [2.215(4)−2.679(4) Å] are close to the related contacts observed in 1 [2.316(3)−2.704(3) Å] (Table 1).4 The THF Table 1. Ranges for Key Distances in [Li5(C20H104−)2]3− in Different Coordination Environments

C20H100 hub spoke flank rim bowl depth Li···Cintra

1.411(2)− 1.417(2) 1.376(2)− 1.381(2) 1.441(2)− 1.450(2) 1.377(2)− 1.387(2) 0.875(2)

Ia 1.391(5)− 1.403(4) 1.424(5)− 1.432(5) 1.429(5)− 1.443(5) 1.453(5)− 1.467(5) 0.283(5), 0.329(5) 2.240(6)− 2.662(7)

Li···Cinter

Li···Li

3.044(7)− 3.083(7)

II (salt 1)

III (salt 2)

1.391(2)− 1.399(2) 1.421(2)− 1.433(2) 1.429(2)− 1.447(2) 1.448(3)− 1.490(2) 0.259(2)− 0.353(2) 2.226(3)− 2.718(4) η3: 2.316(3)− 2.704(3) η6: 2.203(3)− 2.735(3) 3.048(4)− 3.100(4)

1.396(3)− 1.400(3) 1.421(3)− 1.430(3) 1.424(3)− 1.445(3) 1.444(3)− 1.491(3) 0.214(3), 0.404(3) 2.215(4)− 2.663(4) η3: 2.215(4)− 2.679(4)

3.037(4)− 3.172(4)

a

For the [Li(THF)4]+[Li(12-crown-4)(THF)]2+[Li5(C20H104−)2]3− salt.4

molecule in the [Li2(THF)(diglyme)]2+ dication acts as a bidentate ligand coordinating to two Li+ ions. The corresponding Li···OTHF distances [2.101(4) and 2.119(4) Å] are slightly elongated compared to the Li···O diglyme bond lengths [1.992(4)−2.045(4) Å]. The supramolecular dimer in the mixed THF-diglyme salt 2 shows essentially close geometrical parameters compared to that in 1 and the “naked” [Li5(C20H104−)2]3− sandwich (Table 1).4 Each sandwiched Li+ ion in 2 sits between the benzene rings of two corannulene decks with a slight shift to the outer edge of C20H104− (Figure 2c), as previously observed. The Li···Li distances within an almost planar Li5 cluster [3.037(4)− 3.172(4) Å] are slightly elongated in comparison with lithium metal (3.04 Å).11 The Li···C and Li···C6(centroid) distances within the sandwich fall in the ranges 2.215(4)−2.663(4) and 1.937(4)−2.027(4) Å, respectively. These separations compare well with those previously measured in the polyarene adducts with sandwiched or contact Li+ ions.7c,10 The corannulene bowls in 2 are found to be almost ideally eclipsed and arranged in a convex-to-convex fashion (Figure 2c,d). The acquisition of four electrons by C20H10 results in the

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CONCLUSIONS In summary, the supramolecular assembly processes of highly charged nonplanar polyarene, C20H104−, have been investigated in strongly chelating diglyme to compare it with commonly used THF. The effect of solvent media on the outcome of these reactions has been clearly demonstrated. Plus, the ability of the sandwich-type aggregate, [Li5(C20H104−)2]3−, to function as the nanoscale building block in the formation of a complex 1D hybrid organometallic architecture has been revealed. This illustrated the stability of the [Li5(C20H104−)2]3− dimer and allowed us to evaluate its structural parameters in various coordination environments, ranging from discrete units to an extended network. A substantial variation of the bowl depth of the tetrareduced corannulene has been experimentally observed. Study of supramolecular aggregation of highly reduced curved polyaromatic surfaces in different solvent media with various alkali metal ions is currently underway. 5543

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EXPERIMENTAL DETAILS

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ASSOCIATED CONTENT

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General Procedures. All manipulations were carried out using break-and-seal14 and glovebox techniques under an atmosphere of argon. Solvents (THF and hexanes) were dried over Na/ benzophenone and distilled prior to use. Diglyme and THF-d8 were dried over NaK2 alloy and vacuum-transferred. Lithium was purchased from Strem Chemicals. Corannulene was prepared as described previously15 and sublimed at 175 °C prior to use. The NMR spectra were measured on a Bruker AC-400 spectrometer at 400 MHz for 1H, 100.6 MHz for 13C, and 155.5 MHz for 7Li. The NMR spectra were referenced to the solvent signals for 1H and 13C and 0.1 M LiCl in THF-d8 for 7Li. The UV−vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. Synthesis of 2. THF (3 mL) was added to a flask containing excess Li metal (4 mg, 0.6 mmol) and C20H10 (15 mg, 0.06 mmol). The resulting deep green solution was stirred at room temperature for 12 h to afford a dark brown color of C20H104−. The mixture was filtered, layered with hexanes/diglyme (10:1, 3 mL), and kept at 10 °C to afford a brown crystalline solid in 36 h. This product was collected, washed several times with hexanes, and dried shortly in vacuo. The Xray quality crystals were obtained by layering the THF solution with hexanes/diglyme at 10 °C (80%). 1H NMR (400 MHz, THF-d8, 25 °C, ppm): δ 6.90 (C20H104−). 7Li NMR (400 MHz, THF-d8, 25 °C, ppm): δ −8.4. 7Li NMR (400 MHz, THF-d8, −80 °C, ppm): δ −3.8 (3Li), −11.7 (5Li). 13C NMR (400 MHz, THF-d8, 25 °C, ppm): δ 112.3, 95.1, 87.3. UV−vis (THF, nm): λmax 424, 572, 712. Crystal Structure Determinations and Refinement. Data collection was performed on a Bruker SMART APEX CCD-based X-ray diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at T = 100(2) K. Data were corrected for absorption effects using the empirical method SADABS.16 The structure was solved by direct methods and refined using the Bruker SHELXTL (Version 6.14) software package.17 Hydrogen atoms were included at idealized positions using the riding model. Crystal data for 2: C64H72Li8O9, M = 1040.74, monoclinic, P21/n, a = 11.2041(11) Å, b = 22.758(2) Å, c = 21.031(2) Å, V = 5359.0(9) Å3, Z = 4, T = 100(2) K, μ(Mo Kα) = 0.081 mm−1, 46 462 reflection measured, 12 552 unique, Rint = 0.0525, full-matrix least-squares refinement on F2 converged at R1 = 0.0572 and wR2 = 0.0911 for 734 parameters and 8469 reflections with I > 2σ(I) (R1 = 0.0946, wR2 = 0.1041 for all data) and a GOF of 1.019. S Supporting Information *

NMR and UV spectra for 2. Crystallographic data for 2 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 518 442 3462. Phone: +1 518 442 4406. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work from the National Science Foundation Award (CHE-1212441) is gratefully acknowledged. We thank the University at Albany for supporting the X-ray center at the Department of Chemistry. S.S. also thanks the International Centre for Diffraction Data (ICDD) for the 2012 Ludo Frevel Crystallography Scholarship.

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REFERENCES

(1) (a) Scott, L. T.; Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.; Hagen, S. Pure Appl. Chem. 1999, 71, 209. 5544

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(15) (a) Mehta, G.; Panda, G. Tetrahedron Lett. 1997, 38, 2145. (b) Scott, L. T.; Cheng, P.-C.; Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1997, 119, 10963. (c) Sygula, A.; Xu, G.; Marcinow, Z.; Rabideau, P. W. Tetrahedron 2001, 57, 3637. (16) SADABS; Bruker AXS, 2001. (17) (a) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (b) SHELXTL Version 6.14; Bruker AXS, 2000.

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