Record Alkali Metal Intercalation by Highly Charged Corannulene

Article Cite This: Acc. Chem. Res. 2018, 51, 1541−1549

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Record Alkali Metal Intercalation by Highly Charged Corannulene Alexander V. Zabula,*,†,‡ Sarah N. Spisak,‡ Alexander S. Filatov,‡ Andrey Yu. Rogachev,*,§ and Marina A. Petrukhina*,‡ †

Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemistry, University at Albany, State University of New York, Albany, New York 12222, United States § Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States

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‡

CONSPECTUS: The need for advanced energy storage technologies demands the development of new functional materials. Novel carbon-rich and carbon-based materials of different structural topologies attract significant attention in this regard. Attractive systems include a unique class of bowlshaped polycyclic aromatic hydrocarbons that map onto fullerene surfaces and are thus often referred to as fullerene fragments, buckybowls, or π-bowls. Importantly, carbon bowls are able to acquire multiple electrons in stepwise reduction reactions producing sets of successively reduced carbanions. The resulting negatively charged π-bowls exhibit unique supramolecular assembly and metal intercalation patterns that only recently have begun to be uncovered. First, we have resolved the long-standing mystery behind the supramolecular structure formed by a highly reduced fullerene fragment called corannulene (C20H104−) with multiple lithium ions, using X-ray crystallography coupled with NMR spectroscopy and theoretical calculations. This work provided a new paradigm for lithium ion intercalation between the curved carbon π-surfaces and facilitated understanding of the lithium ion storage mechanism in carbonaceous matrices. Next, we have initiated a new research direction, an investigation of the mixed alkali metal reduction reactions using bowl-shaped corannulene as a remarkable multielectron reservoir and unique ligand with open convex and concave π-surfaces. As a result, we have revealed the cooperative effect of lithium with heavier Group 1 metals in reduction and self-assembly processes of corannulene. Moreover, we have discovered a new class of organometallic supramolecules having heterometallic cores with high nuclearity and charge such as Li3M36+ and LiM56+ (M = K, Rb, and Cs) sandwiched between two tetrareduced corannulene decks. The resulting triple-decker supramolecular assemblies, fully characterized by X-ray diffraction and spectroscopic methods, were found to exhibit a record ability of the highly charged corannulene π-surfaces to be fully engaged in intercalation of multiple metal ions. Based on this unique ability, curved π-ligands with extended carbon frameworks are expected to show remarkable potential for alkali metal storage compared to flat polycyclic arenes. Notably, a previously unseen mode of internal lithium binding revealed in the heterobimetallic sandwiches is accompanied by unprecedented negative shifts (up to −25 ppm) in 7Li NMR spectra. Based on in-depth analysis of NMR data, augmented by DFT calculations, we have rationalized the observed experimental trends and proposed the mechanism of stepwise alkali metal substitution reactions. Furthermore, we have correlated the origin of the record 7Li NMR shifts with unique electronic structures of these novel supramolecular aggregates. Herein we present comprehensive analysis of unusual structural and electronic features of remarkable heterometallic selfassemblies formed by tetrareduced corannulene, using a wealth of our recent experimental and computational results. This work uncovers unique potential of highly negatively charged bowl-shaped π-ligands for new supramolecular chemistry and materials chemistry applications. with fullerenes and nanotubes8 but to their unique coordination chemistry,9 novel packing motifs in the solid state10 and on surfaces,11 selective host−guest supramolecular recognition abilities,12,13 interesting optoelectronic properties,14,15 and unusual reactivity.6,16,17 The utilization of curved PAHs as tunable building blocks for the preparation of novel organic and metal−organic materials has greatly expanded this field and opened it for new applications.14,18 The recently developed large scale production of corannulene19 brought this molecule from a lab curiosity to the most

1. INTRODUCTION Study of unique chemical and physical properties of nonplanar polycyclic aromatic hydrocarbons (PAHs) was triggered 60 years ago by the pioneering work of Barth and Lawton on the isolation of corannulene (1, Figure 1a,b).1,2 The discovery of this small π-bowl, often considered as 1/3 of the C60-fullerene (Figure 1c), predated by decades the tremendous development of the modern research field on new carbon allotropes having nonplanar frameworks such as fullerenes3 and nanotubes.4 Nowadays, substantial interest has been directed to both computational and experimental studies of nonplanar PAHs having different structural topologies.5−7 The renaissance of curved PAHs is attributed not only to their structural relevance © 2018 American Chemical Society

Received: March 28, 2018 Published: June 6, 2018 1541

DOI: 10.1021/acs.accounts.8b00141 Acc. Chem. Res. 2018, 51, 1541−1549

Article

Accounts of Chemical Research

Figure 2. Preparation of 14− (a), previously suggested model (b), and the newly established sandwich structure formed by 14− with lithium ions (c).

Figure 1. Corannulene (1), top and side views (a, b), the overlay of the corannulene core on C60−fullerene (c), and frontier orbitals for 1 (d).

in contrast to a broad resonance at δ = −4.5 ppm corresponding to the externally bound lithium ions.31 Based on the experimental NMR observations and semiempirical calculations it was postulated that two bowls of 14− host four lithium cations between their carbon surfaces with the four remaining metal ions coordinated to the sandwich exterior (Figure 2b). The proposed sandwich structure became for years a canonical model for describing the aggregation behavior of 1 and other bowl-shaped PAHs featuring corannulene fragments,32,33 as well as for the highly reduced corannulenefullerene combinations.34 Lack of solid-state structural studies for the above-mentioned systems, however, precluded their full understanding and thus thwarted rational design of prospective materials derived from bowl-shaped PAHs with superior electron acceptor and metal intercalation properties. Inspired by the high electron capacity of 1 and self-assembly with multiple lithium ions observed in solution, our group initiated a new research direction focused on comprehensive investigation of supramolecular aggregates formed by highly reduced bowl-shaped PAHs with alkali metals, using a synergistic combination of X-ray diffraction, spectroscopic, and computational tools.

utilized bowl-shaped PAH for the fabrication of new materials with a broad range of properties and applications. Importantly, corannulene represents a remarkable small-size reservoir for reversible acquisition of up to four electrons due to its doubly degenerate lowest unoccupied molecular orbitals (Figure 1d).20 The electron capacity of corannulene (five carbon atoms per one additional electron, 5C/e−) is higher compared to a planar analogue of corannulene, coronene (12C/e−),21 pristine graphite (6C/e−),22 and even twice higher than that for the C60-fullerene (10C/e−).23 Moreover, our recent studies revealed unprecedented ability of the highly reduced corannulene to store a record amount of alkali metal ions between its curved carbon surfaces. Therefore, materials assembled from corannulene-based building blocks should exhibit superior electron acquisition properties coupled with high alkali metal ion intercalation capacities, thus attracting special interest for their in-depth exploration. In this Account, we survey unique structural, electronic and coordination properties of the highly reduced corannulene tetraanion (14−), emphasizing the X-ray crystallographic and spectroscopic study of its supramolecular aggregation patterns with alkali metal ions. This work culminated in the discovery of a novel class of heterobimetallic supramolecules, revealing unique synergy of mixed-metal reduction and intercalation reactions.

2.1. All-Lithium Sandwiches

We found that the lithium salt of corannulene tetraanion can be isolated from a THF solution in the form of air- and moisturesensitive crystals with an overall formula of [Li(thf)4]+[{Li(thf)2}{14−/Li5/14−}{Li(thf)3}]− (2), according to the X-ray diffraction study (Figure 3a).35 Thus, the sandwich-type aggregation of 14− with multiple Li+ ions has been confirmed. However, in contrast to the previously proposed model ([14−/ Li4/14−]4−, Figure 2b),31 the presence of five lithium ions entrapped between two tetrareduced bowls (Figure 2c) has been revealed. In the [14−/Li5/14−]3− sandwich, all ten sixmembered rings of two corannulene tetraanions are involved in binding of five interior lithium ions, which are symmetrically located between the convex faces of 14−. Two partially solvated Li+ ions are coordinated to the exterior of the sandwich, while the remaining lithium ion is separated from the supramolecular aggregate by four coordinated THF molecules. The 7Li NMR study of the crystals of 2, dissolved in THF, showed the chemical shifts for sandwiched and external Li+ ions at δ =

2. NEW MODEL: STRUCTURAL STUDIES Corannulene can be reduced stepwise with alkali metals ranging from Li to Cs to form mono-, di- or trianions.24−29 However, the final reduction step to the tetraanion is accessible only upon the reaction of 1 with Li metal or the combination of Li with other alkali metals.20,30 Thus, the binding of lithium ions to the tetrareduced corannulene is a key factor for electrostatic and coordination stabilization of the highly charged 14−, in addition to its unique “annulene-within-an-annulene” electronic structure (Figure 2a).20 Back in 1994, Scott and co-workers investigated the in situ reduction of 1 with lithium metal using variable-temperature 7Li NMR spectroscopy. The observed sharp resonance signal at δ = −11.7 ppm was assigned to the Li+ ions sandwiched between corannulene decks 1542


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Figure 3. Structural investigation of the [14−/Li5/14−]3− supramolecular aggregate in 2−4. Molecular structure of 2 (a). Top and side views for [14−/ Li5/14−]3− (b). 7Li NMR spectra of the dissolved and in situ generated 2 (c, THF-d8, −80 °C). Molecular structure of 3 (d) and the 1D polymeric structure (e, [Li(thf)4]+ cations are not shown). Molecular structure of 4 (f).

−11.7 and −4.3 ppm (−80 °C), respectively (Figure 3c). These values are identical to those previously reported for in situ generated corannulene tetraanion,31 but the intensities of both signals are measured at 5:3, illustrating the presence of five sandwiched lithium ions in the product. Our 7Li NMR investigation of the in situ generated 14− showed the identical supramolecular structure compared to that prepared by dissolving crystals of 2 (Figure 3c), proving that the [14−/ Li5/14−]3− sandwich exists in solution and the intercalated lithium ion content remains the same as in the solid-state structure. Further evidence of the stability of the [14−/Li5/ 14−]3− aggregate was provided by the addition of strong chelating agents such as diglyme36 or 12-crown-4.35 Crystallization of 2 in the presence of diglyme afforded [Li(thf)2(diglyme)]+[{Li2(diglyme)2} {14−/Li5/14−}]− product (3, Figure 3d). In contrast to the monomeric structure observed in 2, the Li5-sandwiches are linked together through the {Li2(diglyme)2}2+ building blocks into 1D polymeric chains in 3 (Figure 3e). Interestingly, the addition of 2 equiv of the lithium trapping agent, 12-crown-4, to a solution of 2 resulted in the decoordination of the exterior lithium ions from the sandwich.35 The resulting product, [Li(12-crown-4)(thf)]+2[Li(thf)4]+[14−/Li5/14−]3− (4), features a remarkable “naked” trianionic [14−/Li5/14−]3− supramolecular structure (Figure 3f). Since the sandwich core is not affected by exterior metal binding, 4 serves as a unique model for the inspection of geometrical perturbation of the corannulene bowl upon acquisition of four additional electrons. The 7Li NMR spectrum of 4 (−80 °C, THF-d8) consists of resonances at δ = 0.2 and −3.8 ppm assigned to [Li(thf)4]+ and [Li(12-crown-4)(thf)]+, and a sharp signal at δ = −11.7 ppm corresponding to the sandwiched lithium ions. The respective intensities of these resonances of 1:2:5 are in full agreement with the X-ray crystal structure of 4. The most substantial geometric perturbation of the corannulene bowl observed in 2−4 is its core planarization. The bowl depth, d1 (Table 1), of 14− (0.242(2)−0.353(2) Å) is almost 3 times smaller than that in neutral C20H10 (0.875(2) Å).37 Furthermore, the comparative analysis of the C−C bond lengths in 14− vs 10 supports the formation of a previously proposed “annulene-within-an-annulene” electronic structure for the corannulene tetraanion (Figure 2a).20 The Li··· C 6(centroid) distances for [1 4−/Li 5/1 4−] 3−, ranging from 1.924(3) to 2.094(3) Å, are slightly longer than those in the lithium intercalated graphite, LiC6 (1.853 Å).38

The reduction of corannulene with an excess of Li metal in dimethoxyethane (DME) initially leads to the generation of the [14−/Li5/14−]3− aggregate.39 However, in contrast to THF, prolonged reaction time in DME results in Li-induced solvent fragmentation, producing polynuclear lithium alkoxides. The latter can be entrapped between the tetraanions of 1 to form unprecedented multilayered supramolecular products with 12 encapsulated lithium ions, [14−/Li6/(OR)6/Li6/14−]2− (Figure 4), as revealed crystallographically. The incorporation of the large [Li6/(OR)6/Li6]6+ unit moved the two corannulene bowls as far apart as 5.93 Å. Reproducible observation and structural characterization of several nanosize aggregates of the [14−/Li6/ (OR)6/Li6/14−]2− type opened up new supramolecular chemistry for corannulene tetraanions and indicated possible decomposition pathways for a solid−electrolyte interphase (SEI) in lithium-ion batteries. The generation of similar polynuclear lithium-alkoxo clusters can be expected in SEIs of the fullerene- or nanotube-based anode materials upon electrolyte degradation. 2.2. Li3M3 Sandwiches

The only vacant intercalation site within the [14−/Li5/14−]3− sandwich is located between the two central five-membered rings of 14− (Figure 3b). The separation between these rings (ca. 3.51 Å, Table 1) is smaller than that in LiC6 (3.7 Å)22 and in Li-Cp metallocene complexes (3.7−4.1 Å),40−43 thus making this site not suitable for insertion of an additional lithium ion. In order to provide access to this interior cavity, the distance between the five-membered rings within the sandwich has to be increased by tilting or moving apart the two bowls. This can be achieved if alkali metal cations larger than Li+ are sandwiched between the peripheral six-membered rings of 14− (Figure 5). Both strategies, tilting and moving apart the tetrareduced corannulene decks, have been employed by us using combinations of lithium with other alkali metals for the chemical reduction of 1. The cooperativity of lithium and heavier Group 1 metals such as potassium,44 rubidium45 or cesium30 in reduction of 1 resulted in the preparation of the first mixed-metal supramolecular products of the [14−/Li3M3/ 14−]2− type. All attempts to utilize the Li−Na combination in synthesis of new heterobimetallic products have been unsuccessful so far. For Li−M (M = K, Rb, Cs), the maximum amount of six metal ions entrapped between two corannulene bowls is achieved (Figure 6). Various crystallization techniques and solvent combinations were utilized for the isolation of the series of hexanuclear [14−/Li3M3/14−]2− aggregates with M = 1543


a

1544

−11.7

1.25(4) 0.77(16)

0.281(2), 0.329(2) 3.512(2) 3.815(2), −3.873(2)

0.664(3) 3.824(4) 3.980(4) 4.788(3) 5.120(4) 19.04(8) 14.5(3) −24.5

M = K (5) 0.703(6), 0.706(7) 3.820(7) 3.921(7), 3.993(7) 4.817(7), 4.896(7) 5.183(7) 21.03(14) 15.6(5) −23.9 −6.2

M = Rb (7) 0.744(7), 0.776(7) 3.833(8) 3.946(7), 3.986(7) 4.901(8), 4.962(8) 5.347(8) 23.31(14) 17.8(5) −22.0 −7.0

M = Cs (9) 0.693(8) 3.834(12) 3.814(10) 4.864(10) 5.465(15) 26.9(7) 21.1(7)

M = Rb (8)

[14−/Li3M2/14−]3−

0.95, 0.99 4.15

5.12−5.27 1.7 3.7 −21.5

4.850(3), −5.034(3) 0.90(6), 1.96(6) 0.8(2), 4.3(2) −22.4

M = Rb (11)

[14−/LiM5/14−]2− 0.855(3), −0.865(3) 4.008(3), 4.032(3)

M = K (10)

The distance between the centroids of the corresponding rings. bThe angle between the planes passing through hub (φ2) or rim (φ1) carbon atoms.

bowl depth, d1 C5···C5, d2a C6···C6, d3a C6···C6, d4a C6···C6, d5a φ1b φ2b δ(7Li)C5···C5 δ(7Li)C6···C6

[14−/Li5/14−]3− (4)

[14−/Li3M3/14−]2−

−20.4

M = Cs

Table 1. Selected Experimental Geometrical Parameters (in Å and deg) and Chemical δ(7Li) Shifts (in ppm) for [14−/Li5/14−]3−, [14−/Li3M3/14−]2−, [14−/Li3M2/14−]3−, and [14−/LiM5/14−]2− Sandwiches

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Figure 4. Formation of multilayered [14−/Li6/(OR)6/Li6/14−]2− sandwiches (a) and space-filling model of the [14−/Li6/O6/Li6/ 14−]2− aggregate (b).

Figure 7. Structures of [14−/Li3K3/14−]2− aggregates in 5 and 6. Molecular structure of 5 (a); top and side views of the polymeric chain in 6 (b, c, [Li(dme)(12-crown-4)]+ ions are omitted).

flexibility and coordination versatility of the tetrareduced corannulene for the first time. In the solid-state structure of 5, the coordination of five THF molecules to three potassium ions in the dianionic [14−/Li3K3/ 14−]2− unit precludes their aggregation with the neighboring sandwiches. In contrast, 6 exhibits further association of the sandwich units through intermolecular K···C contacts into a 1D polymer in the crystal structure (Figure 7b,c). Two types of mixed-metal products were crystallized from the reduction reaction of 1 with the Li−Rb metal combination, namely [Li-14−/Li3Rb2/14−]2− and [14−/Li3Rb3/14−]2− (Figure 8).45 The latter was obtained in the form of [{Rb(diglyme)}2{14−/Li3Rb3(diglyme)2/14−}] product (7, Figure 8a), showing an identical alkali metal ion intercalation pattern with the potassium analogues 5 and 6.

Figure 5. Strategies for accessing the internal cavity of the sandwich for additional metal intercalation.

Figure 6. Side and top views of [14−/Li3M3/14−]2− (M = K, Rb, Cs) supramolecular aggregates.

K, Rb, and Cs, followed by their successful X-ray crystallographic characterization. For the Li−K combination, the change of crystallization solvents allowed the isolation of monomeric [Li(thf)(12crown-4)]2+[14−/Li3K3/14−(THF)5] (5) and polymeric [Li(dme)(12-crown-4)]2+[14−/Li3K3/14−(dme)] (6) products (Figure 7a-c).44 Notably, the variation of coordination environment around the sandwich exterior showed no significant impact on the geometry of the sandwich core in 5 and 6. Based on the X-ray structural data, the separation between two central rings of corannulene hosting a lithium cation was measured at 3.824(4) Å for 5 (d2, Table 1). The d3 distances between six-membered rings involved in the Li-binding are slightly elongated compared to those in 4 (3.980(4) Å vs 3.815(2)-3.873(2) Å). As expected, the intercalation of three potassium cations leads to the tilting of the two corannulene decks in 5. The φ1 and φ2 angles, describing tilting for the hub and rim sides of 14−, are much larger in 5 (19.04(8) and 14.5(3)°) than in the [14−/Li5/14−]3− sandwich (1.25(4) and 0.77(16)°). Notably, the bowl depth of 14− in 5 (0.664(3) Å) is significantly greater compared to that in 4 (0.281(2) and 0.329(2) Å). This comparison fully reveals the structural

Figure 8. Structures of [14−/Li3Rb3/14−]2− and [Li-14−/Li3Rb2/14−]2− aggregates in 7 and 8. Molecular structure of 7 (a). Top view of the 1D polymeric chain in 7 (b, external [Rb(diglyme)]+ ions are omitted), 2D network in 7 (c). Molecular structure of 8 (d). Top view of [Li14−/Li3Rb2/14−]2− (e). Superposition of [14−/Li3Rb3/14−]2− and [Li14−/Li3Rb2/14−]2− (f, blue and red colors, respectively); 2D network in 8 (g). 1545


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The first bimetallic LiK5-aggregate was obtained in the form of [{K2(THF)(diglyme)}{14−/LiK5(diglyme)2/14−}] product (10, Figure 10). In 10, all coordination sites of the corannulene tetraanions are engaged in the binding of five K+ ions and one centrally entrapped Li+ ion.

In contrast to the empty concave cavities of 14− in the Li3K3core aggregates in 5 and 6, two Rb+ ions occupy these coordination sites in 7. Together with the sandwiched metal ion content it gives a degree of metal intercalation of MC5 that is greater than the highest possible metal saturation for graphite (MC6). The intermolecular Rb···C contacts involving entrapped rubidium cations link neighboring sandwiches into a 1D polymeric chain where all [14−/Li3Rb3/14−]2− units are oriented in one direction (Figure 7b). The external Rb+ ions further link these chains into a 2D network over additional Rb···C and Rb···O interactions (Figure 7c). A unique sandwich having a different core structure, [Li-14−/ Li3Rb2/14−]2−, was crystallized in the form of [{Rb(thf)}2{Li(thf)}{14−/Li3Rb2/14−}] (8, Figure 8d). Interestingly, its X-ray diffraction study revealed the presence of a vacant coordination site between two six-membered rings of 14−. Furthermore, a more pronounced tilting of corannulene decks is observed in 8 compared to 7 (d5 is 5.5 vs 5.2 Å, respectively) (Figure 8e,f). The side-bound {Li(thf)}+ cation that forms a Li4-rhombus with three sandwiched lithium ions may be responsible for this extra tilting. The intermolecular Rbsandw···C contacts lead to the 2D arrangement of the [Li-14−/Li3Rb2/14−]2− aggregates (Figure 8g). Additional Rb···C contacts between external {Rb2(thf)2}2+ ions and concave faces of 14− further link the resulting 2D sheets into a 3D network. Most recently, the first structural characterization of the sandwich having two alkali metals with the greatest ion size difference, [14−/Li3Cs3/14−]2−, has been accomplished.30 This product, crystallized in the form of [{Cs(diglyme)}+2{14−/ Li3Cs3/14−(diglyme)2}] (9), showed a similar hexanuclear bimetallic core structure and similar crystal packing (Figure 9a,b) as the rubidium analogue 7. The external cesium cations

Figure 10. Molecular structure (a) along with the 1D (b) and 2D (c) polymeric networks in 10.

The bowl depth of 14− in 10 (0.855(3)−0.865(3) Å) is substantially larger than in Li5- or Li3M3-products and close to the value found in neutral corannulene (0.875(2) Å).37 The distance d2 between the centroids of the five-membered rings in 10 (4.008(3) and 4.032(3) Å) is longer than in the Li3K3sandwich 5 (3.824(4) Å), thus weakening the Li···C 5 interactions. The distances between the centers of the sixmembered rings (4.850(3)−5.034(3) Å) are close to those measured in 5. The coordination environment of the two sandwiched potassium ions in 10 is saturated by chelating diglyme molecules. Three remaining encapsulated K+ ions interact with the anionic bowls of the neighboring sandwiches to form a polymeric chain in the solid state (Figure 10b). The {K2(THF)(diglyme)}2+ dicationic units bridge these chains into a 2D network (Figure 10c). The formation of the analogous lithium−rubidium product, [14−/LiRb5/14−]2−, has also been confirmed crystallographically,45 while the related cesium analogue was detected by 7Li NMR spectroscopy (vide supra) upon reduction of 1 with the Li−Cs mixture. These remarkable sandwich-type assemblies illustrate the unique ability of tetrareduced corannulene to encapsulate the highly charged hexanuclear alkali metal units, LiM56+, in between their bowl-shaped π-surfaces. 2.4. 7Li NMR Studies: New Records and Trends

The 7Li NMR spectroscopy monitoring of the reduction reaction of 1 with dual combinations of lithium and heavier alkali metals revealed unprecedented resonance signals and unique trends. In the case of the Li−Na mixture (under similar reaction conditions as for other binary metal reductants), the formation of the [14−/Li5/14−]3− self-assembly was observed, as evidenced by the detection of the only characteristic resonance signal (δ = −11.7 ppm, −80 °C) in the region of sandwiched lithium ions. This indicates that the binding affinity and the size of the sodium ion are not sufficient for replacing the intercalated lithium ions. In contrast, the 7Li NMR spectra measured for the reduction reaction of 1 with a Li−K mixture showed the appearance of previously unseen high-field shifted resonance signals at δ = −22.4 and −24.4 ppm, corresponding to the lithium cations sandwiched between the five-membered rings in LiK5 and Li3K3 assemblies.44 These values represent new record chemical shifts in solution 7Li NMR spectroscopy, as the previously known 7Li NMR resonances of various organolithium products appear in the range from δ = 0 ppm to

Figure 9. Molecular structure (a) and 2D polymeric network in 9 (b).

are symmetrically dished up inside the concave cavities of 14− due to their perfect steric complementarity with the endo surface of the corannulene bowl.46 This tight binding may explain the observation that the bowl depth of 14− in 9 is the largest in the series of analogous [14−/Li3M3/14−]2− aggregates. 2.3. LiM5 Sandwiches

Two interior lithium ions sandwiched between six-membered rings in the Li3M3-aggregates can be further substituted by larger alkali metals to give the C5-symmetric LiM5 internal core (M = K, Rb, Cs).44,45 Notably, at this point, the central Li ion is trapped inside of the sandwich and is not replaced. The resulting LiM5-aggregates represent the last step in the series of dynamical transformations observed in solution starting from [14−/Li5/14−]3− and involving the cascade of subsequent lithium substitution steps by heavier alkali metal ions. 1546


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Accounts of Chemical Research −16 ppm.47 The chemical shifts of the lithium cations jammed between six-membered rings in the [1 4− /Li 3 K 3 /1 4− ] 2− aggregate were detected at −23.9 ppm at −40 °C. Remarkably, for the solution featuring the LiK5-core sandwich a sharp 7Li NMR resonance at δ = −22.4 ppm was observed even at room temperature. This illustrates the absence of any dynamic processes involving the central lithium ion after its entrapment by the K5-belt in 10. Analogous mixed-metal core products have been identified in solution of Li−Rb and Li−Cs systems based on the presence of high-field shifted 7Li NMR resonances. The corresponding signals of internal Li+ ion in the Li3M3 (M = Rb and Cs) aggregates are found at −23.9 and −22.0 ppm, while those of LiM5 are shifted to −21.5 and −20.4 ppm, respectively. Notably, a clear trend in the shift of internal 7 Li NMR signals is observed for the series of LiM5-aggregates (Figure 11), correlating with the increasing size of the encapsulated alkali metal units.

internal Li+ ion migration. This finding explains why in our numerous chemical reduction experiments only sandwich aggregates with a high content of potassium (starting from Li2K3 and up) have been observed. A similar reaction pathway was later confirmed for the reduction processes of 1 with the Li−Rb and Li−Cs dual mixtures.45 However, while for the α-isomer of Li2Rb3 the activation energy was found to be similar (Ea = +15 kcal/mol) to that of the Li2K3-analogue, calculations for the β-Li2Rb3 aggregate revealed a barrierless process. These results indicate that an increase of the alkali metal ion size leads to systems that are substantially more dynamic. In full agreement with the NMR data, calculations also confirmed that the LiM5-core products represent the final substitution step in the dynamic transformations observed in solution. 3.2. 7Li NMR Chemical Shifts

The record high negative 7Li NMR signals characteristic of the heterobimetallic sandwiches pointed to their unique electronic structures that create a special internal environment for the centrally intercalated lithium ion. To get the first insights on these experimental observations, we carried out theoretical modeling that revealed strong coupling between the tetrareduced corannulene bowls within the sandwich systems.45 As shown by explicit calculations of shielding tensor followed by its subsequent decomposition into paramagnetic and diamagnetic components, a shared area of highly concentrated electronic density is formed between the two 5-membered rings where the internal Li+ center is residing (Figure 13). This results in the record high shielding effect detected by 7Li NMR spectroscopy.

Figure 11. 7Li NMR spectra for the prolonged reduction of 1 with dual Li-M combinations (M = K−Cs, THF-d8, −60 °C).

3. COMPUTATIONAL STUDIES 3.1. Intercalation Mechanism

With the help of theory, we have rationalized the formation of unique heterobimetallic Li−K sandwiches based on a stepwise substitution mechanism and “clamshell” sandwich opening effect.44 In full agreement with experimental observations, the homometallic Li5-aggregate was found to be inactive in the process of internal migration of Li+ from the site between the six-membered rings to the interior cavity of the sandwich, as evidenced by high activation barrier (Ea) and absence of driving force (Erel) (Figure 12a). However, replacement of one Li+ by

Figure 13. MOs that contribute most to the shielding tensor for LiM5 supramolecular aggregates (M = K, Rb, Cs).

Therefore, it can be expected that the coupling of two 14− anions strongly depends on the bowl separation and thus can be tuned in a controlled fashion by changing the size of internal alkali metal units. Indeed, the set of analogous LiM5-core complexes with an increased separation between the two bowls from K to Cs exhibits the down-shifting of the 7Li NMR signals (Figure 14) due to the reduced shielding. This can be illustrated by the above molecular orbitals (Figure 13), which clearly indicate the coupling between bowls as a function of alkali metal size. Interestingly, the same effect was recently observed in the open-shell corannulene-based supramolecular aggregates, where magnetic coupling between two 13•− radicals was fine-tuned by the size of alkali metal cations jammed inbetween.29

Figure 12. Schematic representation of migration mechanism as a function of replacement of small Li+ by large K+ ions. All numbers are in kcal/mol.

large K+ ion to form the Li4K-core product immediately resulted in a notable decrease of Ea and the appearance of a small driving force (Figure 12b). The second replacement may lead to two possible isomers (α-Li3K2 and β-Li3K2), from which only one was responsible for the migration process. Indeed, the α-isomer shows similar energetics as the Li4K-sandwich, whereas β-Li3K2 exhibits a substantial driving force (−13 kcal/mol) and almost no barrier (∼ +1 kcal/mol) for the

4. CONCLUSIONS The first X-ray crystallographic elucidation of the supramolecular structure formed by highly reduced corannulene with lithium ions, [14−/Li5/14−]3−, has provided a new 1547


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Sarah N. Spisak received her PhD in Inorganic Chemistry from the University at Albany in 2013 (Prof. M. A. Petrukhina). She is currently a Visiting Assistant Professor at Hamilton College, Clinton, NY. Alexander S. Filatov received his PhD in Inorganic Chemistry from the University at Albany in 2008 (Prof. M. A. Petrukhina). He is currently a Director of X-ray research facilities at the University of Chicago, IL. Andrey Yu. Rogachev is currently an Assistant Professor of Chemistry in Illinois Institute of Technology (IIT), Chicago. His interests in theoretical chemistry cover many areas with special focuses on physics and chemistry of planar and curved polyaromatic systems, multireference approach to spectroscopic, magnetic and catalytic properties. Figure 14. 7Li NMR trend for the family of Li3M3-core sandwiches.

Marina A. Petrukhina received her PhD in Inorganic Chemistry from Lomonosov Moscow State University and is currently a Professor at the University at Albany, NY. Her research interests cover broadly synthetic and structural inorganic, organometallic and supramolecular chemistry with special focus on structures and reactivity of nonplanar polyarenes and metal clusters.

paradigm for lithium ion intercalation and storage between nonplanar carbon surfaces. It also opened up new supramolecular and organometallic chemistry of the negatively charged bowl-shaped polycyclic aromatic ligands. Specifically, a remarkable family of heterobimetallic sandwiches with different core compositions has been isolated in our group, revealing unique ability of the tetrareduced corannulene for record encapsulation of alkali metal ions. The sandwich-type molecular architectures allowed unique electronic coupling of the bowlshaped anions 14− that is responsible for the record shielding effects observed for the first time in 7Li NMR spectra. Comprehensive analysis of the novel self-assemblies with different internal metal combinations revealed an interesting trend, namely, that an increase in separation of the two corannulene decks is accompanied by the downfield shift of the 7 Li NMR signals from the internally encapsulated lithium ions. As a result, 7Li NMR spectroscopy can serve as an effective tool for estimation of the coupling strength of charged carbon surfaces and shielding of the encapsulated lithium ions in new supramolecular aggregates. Overall, these results demonstrate an outstanding and unmatched ability of electron-rich bowlshaped carbanions for maximum intercalation of multiple alkali metal ions in between their nonplanar surfaces. An understanding of these effects should facilitate the design of new multimetallic supramolecular products with superior properties and stimulate practical applications of alkali metal-doped carbon-rich materials.

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ACKNOWLEDGMENTS M.A.P. thanks the National Science Foundation (CHE1212441 and CHE-1608628) for support of this work. Financial support from Illinois Institute of Technology (startup funds) and the 381688-FSU/ChemRing/DOD-DOTC is acknowledged by A.Yu.R.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alexander S. Filatov: 0000-0002-8378-1994 Marina A. Petrukhina: 0000-0003-0221-7900 Notes

The authors declare no competing financial interest. Biographies Alexander V. Zabula is a Senior Research Chemist at ExxonMobil Chemical Company. He received his PhD from Münster Universität (Prof. F. E. Hahn). His research interests cover the structure and reactivity of organometallic compounds, carbenes and their heavier analogues, and curved polyaromatic systems. 1548


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Record Alkali Metal Intercalation by Highly Charged Corannulene

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